Download Factors Driving Herbivores Consumption and

Factors Driving Herbivores Consumption and
Feeding Preferences across Different
Macrophytes Ecosystems
Candela Marco Méndez
DOCTORADO EN CIENCIAS DEL MAR Y BIOLOGÍA APLICADA
Factors Driving Herbivores Consumption and Feeding
Preferences across Different Macrophytes Ecosystems
Factores que Influyen en el Consumo y las Preferencias de los
Herbivoros a través de Distintos Ecosistemas de Macrófitos
Memoria presentada para optar al grado de
Doctor Internacional en la Universidad de Alicante por
CANDELA MARCO-MÉNDEZ
ALICANTE, Julio 2015
Dirgida por Dr. José Luis Sánchez Lizaso y Dra. Patricia Prado
“The sea, once it casts its spell, holds one
in its net of wonder forever”
Jacques Yves Cousteau
Acknowledgements / Agradecimientos
AGRADECIMIENTOS / ACKNOWLEDGEMENTS
En primer lugar quiero agradecer a la Mancomunidad de los Canales del Taibilla la
financiación aportada a través de los diferentes proyectos en los que hemos colaborado.
Gracias a ellos esta tesis ha sido posible.
A José Luis tengo que agradecerle muchas cosas. Él ha hecho posible que trabaje
dedicándome al mar, un sueño. Además de haber confiado en mí todos estos años para
diferentes proyectos, en los que he aprendido muchísimo, me dio la libertad de elegir mi
tema de tesis, un regalo. Ha apoyado cada una de las aventuras herbívoras que
componen esta tesis, desde Florida al Delta del Ebro y sus arrozales, y ha sido un apoyo
especialmente importante en esta recta final. Muchas gracias por todo.
A Pat he de agradecerle también que se embarcara en esta aventura conmigo. Gracias a
ti esta tesis despegó en Port St Joe. Me llevo de estos años todo lo que he aprendido
contigo de este mundo científico y anécdotas inolvidables de nuestras aventuras,
especialmente en el Delta del Ebro. Gracias por todo el trabajo dedicado a esta tesis.
I would like to especially thank to Ken Heck for his implication in this PhD thesis.
Your support and guidance have been really important to me all along this way. I will
always be eternally grateful for your confidence in me to lead the projects of Alabama.
Thanks to your stubbornness you finally took me back a second time, you never gave up
on me and I will never forget it. Thank you for becoming such an important part of my
life.
Este equipo de directores y mentores no sería posible sin Just Cebrián. Nos conocimos
gracias al proyecto de intercambio de estudiantes que JL y tú inicasteis hace unos años y
hoy eres parte fundamental de esta tesis. No olvido que gracias también a ti despegué y
me he ido formando como científica. Has hecho muchas cosas posibles a lo largo de
estos años, convirtiendote en alguien muy especial e importante para mí. Tu energía
mueve montañas Just. Gracias por tu confianza, tu apoyo y tu implicación en este
trabajo. Sólo espero que de alguna forma esta tesis os devuelva un poquito de vuestra
dedicación.
Gracias a la Universidad de South Alabama, al DISL y al I.R.T.A, en especial a Carles
Ibáñez, por hacer posible las diferentes estancias.
A mi familia quiero agradecerle su paciencia y compresión a lo largo de estos años de
ausencias. Especialmente a mi madre, quien sé que más acusa esta distancia. Gracias por
confiar en mí cuando te dije que quería dedicarme a esto, que el mar era mi sueño y que
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Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
me iba a estudiar a Galicia. Gracias por apoyarme en cada paso y nunca perder la
confianza en mí. A mi tío también tengo mucho que agradecerle pues nada de esto sería
posible sin él, gracias por tu apoyo incondicional. A mis hermanos, Israel y Oihana, a
quienes admiro profundamente, gracias por ser mi inspiración, por ser parte de mí y por
llenar de música y arte mi vida. A Loren por entrar en nuestras vidas. Y a mi tia Gloria y
a mi yayi centenaria Nati, por tu vitalidad, que espero perdure en cada viaje. Por ultimo,
gracias a nuestra recién llegada Lunita por cambiar nuestras vidas, espero que juntas
vivamos infinitas aventuras acuáticas.
Como en todo camino recorrido hay personas que dejan huella, y que de una u otra
forma han hecho posible que yo esté aquí. Empezando por Galicia, gracias a mis
hermanas y compañeras de aventuras Guiomar y Leticia, con quien compartí años
inolvidables. A Ari a Paola y a Pepelu, mi familia en la residencia. Y como no a los
“niños”: Ibaitxo, Fer, Iaguete, Protec, Victor y especialmente a Txinin, gracias por tu
apoyo y tu amistad durante esos años y ahora a pesar de la distancia que nos separa. A
mis amigos donostiarras, gracias por seguir siendo “my home”, mi lugar al que volver,
especialmente a Mart, Leire, Pauli, Maialen, Arantxi, Aroi, Maritxu, Nuri, Luis, Jorge
(y los caballitos de mar con los que empezó todo…), Iker, Ido, Pon, Peio…A Iñigo
quiero agradecerle el maravilloso camino recorrido juntos, gracias por tu apoyo durante
esos años y por seguir formando parte de mi vida. A Sau, gracias por implicarte en este
proyecto y hacer la portada de esta tesis, significa mucho para mí, pero sobretodo gracias
por estar en mi vida y pintarla cada día. A Lauri, gracias por ser mi norte, mi luz, mi
ángel, mi más… (+=+), you´re my Brown eyed girl. En Alicante, gracias especialmente a
mis niñas Alicia y Karen, por vuestro apoyo, vuestra amistad, vuestra luz e ilusión. A
Karen´s family, Angel S, Jarica, Barbara, Raquel, Paula, Vicky, Gabriela, Josele, los
botánicos y toda la gente entrañable que he conocido en Alicante… gracias a todos!! I
also would like to thank all people that I met in Dauphin Island Sea Lab, many of you
have earned a special place in my heart, thank you all!!!
Muchas gracias también a todos mis compañeros/profesores del departamento de
Ciencias del Mar y Biología Aplicada y del CIMAR. Especialmente tengo que dar las
gracias a Alfonso Ramos por abrirme las puertas del CIMAR y acogerme como un
miembro más en esa familia. Allí he podido llevar a cabo gran parte de esta tesis y ha
sido un espacio fundamental de retiro en esta última fase de escritura. Gracias por tu
inestimable ayuda en la identificación de epifítos y fragmentos minúsculos de algas de
los contenidos de mis erizos y salpas. Y sobretodo gracias por tu inagotable ilusión, por
tu apoyo y tus consejos desde el primer día que pisé esta Universidad. A Paqui, gracias
8
Acknowledgements / Agradecimientos
por apostar por mí y ofrecerme mi primera experiencia laboral en empresa recién salida
del master. A Juanma Ruiz por ese primer artículo juntos. A Pablo Sánchez por sus
consejos en estadística y por conseguirme esas salpas!! Y como no, a Tochín, por su
perseverancia e implicación a la hora de conseguir esos ejemplares; y a Pablo Arechavala,
por sus clases de disección y censos visuales, muchas gracias!!!! A Aitor también tengo
que agradecerle mucho el tiempo que ha dedicado a resolverme dudas estadísticas. A
Ángel Loya, Tito y Elena (mi gran e inigualable partenaire de aventuras marinas) tengo
que agradecerles su amistad y compañerismo especialmente importante los primeros
años en el departamento. Juntos hemos vivido momentos inolvidables, sin vosotros esto
no sería posible. Gracias por vuestro apoyo. Esther, tu has sido especialmente
importante para mí desde el primer día que pisé esta Universidad. Desde entonces te
convertiste en amiga, familia y compañera y he tenido la suerte de recorrer todo este
camino contigo. Tu apoyo ha sido fundamental en cada fase. Gracias por estar siempre
ahí. Y también, of course, a Anousky, Frantxo…
Pero todos los profesores y compañeros de este departamento habeis contribuído de
alguna forma en esta tesis, gracias por vuestra ayuda, por compartir vuestra experiencia
conmigo y por los buenos momentos dentro y fuera del trabajo. La lista es grande pero
espero no olvidarme de nadie: Tito, Ángel, Elena, José, Yolanda, Yoana, Arecha, Maite,
Tocho, Andrés, Irene, Aitor, Esther, Damián, Carlos V, Vicky, Kilian, Carlos S, Nuria,
Andrea, Vicent, Lute, Lydia, Agustín, José Miguel, Aurora, Marta D, Mercedes,
Antonio, Isidro, Celia, Elia, Bea, Cristina, Mohamed, Marouane, Ana F, Ana N, José
Vicente, Rosa, Marta, José Luis, Pablo S, Just B, Alfonso, Paqui, Paco, Zubcoff, Juan
Car, Willy…y también a nuestro querido Felio, para nosotros uno más de esta familia,
por tu gran espíritu marinero; y a tod@s los becari@s y estudiantes que han pasado por el
dpto. y cuya ayuda ha sido fundamental en muchos proyectos. Perdón si me olvido de
alguien. Muchísimas gracias a todos!
Por último, la parte más difícil y más importante, muchas gracias a Tito. El tiempo,
energía, ilusión y trabajo que has dedicado a esta tesis la hace también tuya. Has sido un
apoyo fundamental, el mejor compañero de batalla, en lo personal y en lo profesional y
lo mejor y más bonito que me ha pasado recorriendo este camino. Sin tí esto no sería
possible, pero nadie como Dylan para decírtelo: “If not for you; My sky would fall; Rain
would gather too; Without your love I’d be nowhere at all; I’d be lost if not for you; And you
know it’s true…” Y si aún no lo sabes, luego te la canto…
Portada: Alvaro Sau; Diseño y maquetación: Luis M. Ferrero-Vicente
English revision: Julie Smith & Guido Jones.
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Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
10
Table of contents / Tabla de contenidos
TABLE OF CONTENTS / TABLA DE CONTENIDOS
Resumen General ................................................................................................... 15
1. General Introduction ....................................................................................... 39
1.1. Importance of herbivory in macrophyte ecosystems .......................................... 41
1.2. Factors driving abundance and structure of macrophytes in aquatic ecosystems:
Top-down vs. Bottom-up control theories ........................................................ 61
1.2.1. Influence of herbivore abundances, distribution and feeding behavior ...... 63
1.2.2. Influence of nutritional content in food selectivity ................................... 67
1.2.3. Influence of chemical and structural deterrence in food selectivity ............. 68
1.2.4. Influence of flower and seed availability ................................................. 70
1.2.5. Influence of epiphyte availability ........................................................... 71
1.3. Methods employed in herbivory studies ............................................................ 72
1.3.1. Grazing intensity estimation: indirect vs. direct methods ........................ 72
1.3.2. Food choice experiments ......................................................................... 75
1.3.3. Dietary studies: gut content, stable isotope analyses (SIA) and mixing
models................................................................................................................... 76
1.4. Objectives ........................................................................................................... 77
2. Epiphytes mediate the trophic role of sea urchins in Thalassia
testudinum seagrass beds ...................................................................................... 79
2.1. ABSTRACT...................................................................................................... 81
2.2. INTRODUCTION .......................................................................................... 82
2.3. MATERIAL AND METHODS .................................................................... 84
2.3.1. Study site .............................................................................................. 84
2.3.2. Tethering experiments ........................................................................... 85
2.3.3. Food choice experiments ......................................................................... 86
2.3.4. Gut contents and stable isotope analyses .................................................. 86
2.3.5. Nutrient contents in seagrass and epiphytes ............................................ 87
2.3.6. Epiphytic community ............................................................................ 88
2.3.7. Data analyses........................................................................................ 88
2.4. RESULTS ......................................................................................................... 89
2.4.1. Tethering experiments ........................................................................... 89
2.4.2. Food choice experiments ......................................................................... 90
2.4.3. Gut contents and stable isotope analyses .................................................. 90
2.4.4. Nutrient contents in seagrass leaves and epiphytes................................... 91
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Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
2.4.5. Epiphytic community.............................................................................91
2.5. DISCUSSION...................................................................................................92
3. Epiphyte presence and seagrass species identity influence rates of
herbivory in Mediterranean seagrass meadows ..........................................97
3.1. ABSTRACT ......................................................................................................99
3.2. INTRODUCTION ........................................................................................100
3.3. MATERIAL AND METHODS...................................................................103
3.3.1. Study site ............................................................................................103
3.3.2. Herbivore abundances and feeding observations....................................104
3.3.3. Tethering experiments .........................................................................104
3.3.4. Food choice experiments ......................................................................105
3.3.5. Gut contents, stable isotope analyses and nutrient contents ....................106
3.3.6. Epiphytic community...........................................................................107
3.3.7. Data analyses......................................................................................107
3.4. RESULTS .......................................................................................................107
3.4.1. Herbivore densities and feeding observations ........................................107
3.4.2. Tethering experiments .........................................................................108
3.4.3. Food choice experiments .......................................................................108
3.4.4. Gut contents ........................................................................................109
3.4.5. Stable isotope analyses..........................................................................109
3.4.6. Nutrient contents in seagrass leaves and epiphytes .................................110
3.4.7. Epiphytic community...........................................................................111
3.5. DISCUSSION.................................................................................................112
4. Epiphytes and nutrient contents influence Sarpa salpa herbivory
on Caulerpa species vs. seagrass species in Mediterranean meadows
.......................................................................................................................................117
4.1. ABSTRACT ....................................................................................................119
4.2. INTRODUCTION ........................................................................................120
4.3. MATERIAL AND METHODS...................................................................123
4.3.1. Study site ............................................................................................123
4.3.2. Bottom characterization ......................................................................124
4.3.3. Fish abundances and feeding observations ............................................125
4.3.4. Tethering experiments .........................................................................126
4.3.5. Food choice experiments .......................................................................127
4.3.6. Gut contents, stable isotope analyses and nutrient contents ....................129
12
Table of contents / Tabla de contenidos
4.3.7. Epiphytic community .......................................................................... 130
4.3.8. Data analyses...................................................................................... 130
4.4. RESULTS ....................................................................................................... 133
4.4.1. Bottom characterization ...................................................................... 133
4.4.2. Fish abundances and feeding observations ............................................ 133
4.4.3. Tethering experiments ......................................................................... 134
4.4.4. Food choice experiments ....................................................................... 135
4.4.5. Gut contents........................................................................................ 135
4.4.6. Stable isotope analyses ......................................................................... 136
4.4.7. Nutrient contents in macrophytes and epiphytes .................................... 137
4.4.8. Epiphytic community .......................................................................... 137
4.5. DISCUSSION................................................................................................. 138
5. Seasonal effects of waterfowl grazing on submerged macrophytes:
The role of flowers ................................................................................................ 149
5.1. ABSTRACT.................................................................................................... 151
5.2. INTRODUCTION ........................................................................................ 152
5.3. MATERIAL AND METHODS .................................................................. 154
5.3.1. Study site ............................................................................................ 154
5.3.2. Waterfowl abundance and behavioral observations ............................... 155
5.3.3. Exclusion cage experiments .................................................................. 156
5.3.4. Tethering experiments ......................................................................... 157
5.3.5. Data analyses...................................................................................... 157
5.4. RESULTS ....................................................................................................... 158
5.4.1. Waterfowl abundance and behavioral observations ............................... 158
5.4.2. Exclusion cage experiments .................................................................. 159
5.4.3. Tethering experiments ......................................................................... 160
5.5. DISCUSSION................................................................................................. 160
5.5.1. Waterfowl abundance.......................................................................... 161
5.5.2. Seasonal herbivory impacts on macrophytes........................................... 162
5.6. CONCLUSION.............................................................................................. 166
6. Rice fields used as feeding habitats for waterfowl throughout the
growing season ....................................................................................................... 167
6.1. ABSTRACT.................................................................................................... 169
6.2. INTRODUCTION ........................................................................................ 170
6.3. MATERIAL AND METHODS .................................................................. 173
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Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
6.3.1. Study area...........................................................................................173
6.3.2. Waterfowl populations ........................................................................174
6.3.3. Exclusion cage experiments ..................................................................174
6.3.4. Tethering experiments .........................................................................175
6.3.5. Gut contents ........................................................................................176
6.3.6. Stable isotope and nutrient content analyses ..........................................177
6.3.7. Statistical analyses...............................................................................177
6.4. RESULTS .......................................................................................................179
6.4.1. Waterfowl populations ........................................................................179
6.4.2. Exclusion cages and tethering experiments ............................................179
6.4.3. Gut contents ........................................................................................181
6.4.4. Stable isotope analyses..........................................................................182
6.4.5. Nutrient contents in waterfowl and food resources ................................182
6.5. DISCUSSION.................................................................................................182
7. General Discussion .........................................................................................189
7.1. Pros and cons of using different methodological approaches in the study of
herbivore preferences and consumption rates across different ecosystems.......191
7.2. Grazing intensity estimation across different macrophyte ecosystems ............199
7.3. Main factors driving herbivory across different macrophyte ecosystems .........202
7.3.1. Influence of herbivore abundance and feeding behavior .........................202
7.3.2. Influence of food availability and food preferences: The role of epiphytes and
flowers ................................................................................................................206
7.3.3. Influence of nutritional content (C:N molar ratio) on food selectivity .....212
7.3.4. Potential influence of macrophyte chemical and structural defenses on food
selectivity ............................................................................................................216
7.4. Collected summary of the main driving factors identified across the different
ecosystems studied............................................................................................219
Conclusions / Conclusiones .............................................................................223
References ................................................................................................................229
14
Table of contents / Tabla de contenidos
15
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Photo: L.M. Ferrero-Vicente
16
Resumen General
Resumen General
1. Introducción
La herbivoría en los macrófitos
sumergidos ha sido frecuentemente
subestimada, considerándose que a
través de su consumo los herbívoros
retiraban tan solo una pequeña
cantidad de la producción primaria
(aproximadamente entre 10–15%,
Den Hartog 1970; Thayer et al.
1984). A partir de los primeros
estudios realizados en praderas
marinas, se estimó que, dependiendo
del lugar, aproximadamente entre el
3% y el 100% de la producción
primaria neta de los pastos marinos
entraba en las redes tróficas a través
del pastoreo de herbívoros. Este
rango tan amplio sugiere que la
herbivoría en las praderas marinas
varía mucho en el tiempo y en el
espacio, tal como sucede en otros
ecosistemas (Louda & Collinge
1992; Hacker & Bertness 1995,
1996), pero también que el efecto de
los herbívoros no es despreciable
(Heck & Valentine 2006).
Los efectos más importantes sobre
praderas marinas han sido atribuidos
a grandes herbívoros vertebrados, los
cuales fueron más abundantes y más
diversos de lo que lo son hoy en día.
Mientras que en sistemas tropicales
tortugas y manatíes han sido
señalados como los principales
herbívoros, en sistemas templados las
aves acuáticas (patos, gansos y cisnes)
parecen ejercer una presión selectiva
sobre los macrófitos, provocando un
impacto
importante
en
la
composición de especies de las
comunidades de plantas sumergidas
(Bortolus et al. 1998; Nolet et al.
2001).
Otros
estudios
han
demostrado que no sólo el pastoreo
de grandes herbívoros como las aves
acuáticas, sino también el ejercido
por macroherbívoros tales como
peces y erizos de mar pueden, en
algunos casos, ser también intensos,
afectando a la productividad y
densidad de los macrófitos (Lodge et
al. 1998; Valentine & Heck 1999).
Sin embargo, dada la alta variabilidad
reflejada en los estudios de
herbivoría, es necesario seguir
estudiando el impacto que las
distintas especies de herbívoros
17
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
pueden
tener
en
diferentes
ecosistemas, e identificar los factores
que
impulsan
las
complejas
interacciones planta-herbívoro que
podrían determinar la abundancia de
macrófitos y su distribución en estos
ecosistemas.
En el estudio de la ecología de los
ecosistemas
de
macrófitos
sumergidos los conceptos top-down y
bottom-up han sido utilizados
frecuentemente (Atkinson & Grigg
1984; Carpenter et al 1985) para
describir si es la acción de los
consumidores sobre la población de
alimentos
(top-down)
o
la
disponibilidad de los recursos para las
poblaciones (bottom-up) lo que regula
la estructura de las comunidades
acuáticas. El razonamiento se puede
resumir en que la oferta de recursos
puede establecer el nivel de
abundancia de una población, que a
su vez está modificada por la acción
de los consumidores (Valiela 1995).
Estos conceptos opuestos pero
complementarios
pueden
ser
particularmente
útiles
en
la
comprensión de las complejas
interacciones entre macrófitos y
herbívoros que tienen lugar en los
distintos ecosistemas.
18
El dilema surge porque varios
factores
vinculados
a
ambos
conceptos tales como la luz, la
temperatura o nutrientes (control
bottom-up) y la competencia y
herbivoría (control top-down) pueden
influir en la estructura y dinámica de
las comunidades de macrófitos. Estos
factores
actúan
sinérgicamente
(Burkepile & Hay 2006) y muestran
fuertes
variaciones
espaciales
(relacionadas con la profundidad o
diferencias
biogeográficas)
y
temporales (entre días o épocas). En
general, las macrófitos responden a
parámetros ambientales (control
bottom-up: temperatura, nutrientes,
luz) reflejándose en un patrón común
de la estacionalidad de la biomasa,
aumentando
en
primavera,
alcanzando máximos en verano,
disminuyendo en otoño, y con valores
mínimos en invierno (Ballesteros
1991, 1992; Alcoverro et al. 1997) y
desviaciones de este patrón común se
relacionan con diferencias en la
actividad de los herbívoros (control
top-down). El paradigma de la
herbivoría es entender el predominio
relativo de estos dos controles, el topdown y el bottom-up en el
funcionamiento de cada uno de los
ecosistemas de macrófitos. Aunque
Resumen General
muchos investigadores entienden que
ambos controles pueden actuar en
conjunto determinando la estructura
y funcionamiento de los ecosistemas
costeros (Lotze et al. 2006), es
necesario seguir investigando para
comprender mejor si ambos son
responsables de los cambios que
presentan las comunidades. Entre los
diversos factores asociados a ambos
efectos de control, durante esta tesis
nos hemos centrado en el estudio de
las interacciones macrófito-herbívoro
y en los factores potencialmente
implicados en el consumo y la
selectividad de los herbívoros.
La herbivoría puede ser muy variable
a través del espacio y el tiempo,
presentando diferentes patrones de
defoliación entre praderas y/o épocas
(Tomas et al. 2005a Prado et al.
2007a, 2010a; Steele et al. 2014).
Esta
variabilidad
ha
sido
parcialmente atribuida a cambios en
la abundancia y distribución de los
herbívoros, como consecuencia de la
interacción entre varios factores, tales
como eventos de migración (Lodge
et al. 1998), tasas de reclutamiento
(Camp et al. 1973), efectos de
depredación (McClanahan et al.
1994), la sobrepesca (Klumpp et al.
1993), la caza (Fox & Madsen 1997)
y los patrones de movimiento (Jadot
et al. 2002, 2006) asociados a cada
especie de herbívoro.
El tamaño del hábitat también puede
jugar un papel importante en las
interacciones
macrófito-herbívoro.
Las densidades de herbívoros y la
presión por herbivoría asociada
pueden aumentar o disminuir con la
disminución de la disponibilidad de
recursos. Dado que los recursos
alimenticios
generalmente
se
distribuyen de forma heterogénea
tanto espacial como temporalmente,
los herbívoros pueden mostrar
respuestas
flexibles
a
esta
heterogeneidad,
por
ejemplo,
recurriendo a un nuevo hábitat,
seleccionando un parche diferente
dentro de un mismo hábitat y/o
seleccionando diferentes recursos
dentro de un mismo parche
(Stephens & Krebs 1986; Hughes
1993; Sutherland 1996; Guillemain
& Fritz 2002).
La competencia también puede
desencadenar una respuesta conductual diferente: los individuos
pueden buscar alimento en parches
alternativos con menor abundancia
de recursos para evitar los efectos
negativos derivados de una alta
19
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
densidad de competidores en los
mejores parches (Guillemain & Fritz
2002).
También se ha demostrado que la
calidad nutricional de la planta (a
menudo expresado como contenido
de
nitrógeno
foliar)
puede
desempeñan un papel central
determinando los patrones de
alimentación de los herbívoros en
varias comunidades terrestres (Onuf
et al 1977; Slansky & Feeny 1977;
Kraft & Denno 1982; Coley 1983).
Varios estudios sugieren que existe
una relación significativa y positiva
entre el contenido de nitrógeno en
macrófitos y el pastoreo de
vertebrados (McGlathery 1995;
Preen 1995; Cebrián & Duarte
1998). En estos ecosistemas, debido a
que los epífitos y las macroalgas
tienen normalmente una relación
C:N más baja que las fanerógamas
(Duarte 1992, 1995; Alcoverro et al.
1997, 2000), se cree que podrían
mantener una presión de herbivoría
comparativamente mayor (Tomas et
al. 2005b, 2006). Sin embargo, dado
que los herbívoros no siempre tienen
la oportunidad de elegir entre plantas
con
diferente
contenido
en
nitrógeno, se cree que algunos de
ellos pueden haber desarrollado
20
adaptaciones o capacidades digestivas
para asegurar la ingesta de nitrógeno
cuando se alimentan de plantas de
baja calidad nutricional (Terra et al.
1987; Simpson & Simpson 1990;
Slansky 1993). Entre las estrategias
empleadas se incluye una mayor
duración de los episodios de
alimentación y/o un aumento de las
tasas de consumo para compensar la
escasez de nitrógeno en su forraje
(Price et al 1980; Clancy & Fenny
1987). En general, los estudios
sugieren que las diferentes especies
pueden verse afectadas de forma
diferente por las condiciones
nutricionales del alimento, y por
tanto, un bajo valor nutricional, por
sí solo, no siempre pueden ser una
defensa eficaz contra el pastoreo
(Boyd & Goodyear 1971; Mattson
1980; Moran & Hamilton 1980).
Además del contenido nutricional,
una estrategia clave de defensa
utilizada por las plantas para
minimizar la pérdida foliar por parte
de los múltiples herbívoros implica
adaptaciones que reducen su calidad
como alimento. Esto es debido a
componentes
químicos,
como
compuestos fenólicos (Mariani &
Alcoverro 1999; Verges et al.
2007a,b, 2011) o carbohidratos
Resumen General
estructurales en las fanerógamas
(celulosa), que pueden afectar a la
digestibilidad de los alimentos y a su
absorción (Klumpp & Nichols 1983;
Fritz & Simms 1992). Estos rasgos
no sólo varían mucho entre las
diferentes especies de plantas, sino
que
también
pueden
diferir
sustancialmente entre las diferentes
partes de una misma planta,
pudiendo influir drásticamente en las
interacciones
macrófito-herbívoro
(Raupp & Denno 1983; Poore 1994;
Orians et al. 2002; Taylor et al. 2002;
Vergés et al. 2007a,b; 2011).
En general, el comportamiento
alimenticio de los herbívoros implica
un conjunto de decisiones complejas
que pueden verse afectadas por todos
estos factores. Si bien estos
comportamientos, y los patrones de
distribución y de pastoreo resultantes,
se han explorado con frecuencia en
los ecosistemas terrestres, en el caso
de los ecosistemas acuáticos estos
patrones
son
mucho
menos
conocidos, por lo que se requieren
más estudios. Dada su influencia
potencial en la distribución y
abundancia de los macrófitos
(Steneck & Sala 2005; Valentine &
Duffy 2006), estos estudios deberían
incorporar
observaciones
del
comportamiento alimenticio para una
mejor
comprensión
de
las
interacciones
macrófito-herbívoro
que tienen lugar en los distintos
ecosistemas.
En cuanto a los métodos utilizados
para cuantificar la herbivoría, las
diferencias metodológicas entre los
distintos estudios pueden explicar, en
parte, la variabilidad en las tasas de
herbivoría
encontradas
en
la
literatura. Además, los primeros
ecólogos marinos emplearon métodos
indirectos (por ejemplo, contando
marcas de consumo, comparando la
densidad foliar o la biomasa en sitios
con y sin presión de herbívoros o
mediante estudios de ingestión en el
laboratorio), en lugar de estimar
directamente en el campo las tasas de
herbivoría en las hojas (ver Heck &
Valentine 2006). Estos métodos
indirectos
han
llevado
sistemáticamente a subestimar las
tasas de herbivoría, basándose todos
ellos en medidas estáticas de la
pérdida foliar por herbívoros hechas
una o varias veces al año (Cebrián et
al. 1996a,b).
Hoy en día algunos estudios han
revaluado las tasas de herbivoría
mediante métodos directos, como
21
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
por ejemplo los experimentos de
tethering, demostrando que la
herbivoría puede ser más importante
en algunos ecosistemas de macrófitos
(Kirsh et al. 2002; Tomás et al.
2005a; Prado et al. 2007a; Taylor &
Schiel 2010). Está técnica fue
desarrollada por Kirsch et al. (2002) y
se basa en estimar directamente la
cantidad de superficie foliar perdida
por consumo transcurrido un
determinado periodo de tiempo. Las
pérdidas en el área de tejido de la
hoja debidas al consumo (diferencia
entre área inicial y área final tras
aplicar correcciones debidas al
crecimiento) se comparan con la
cantidad producida. Tras las altas
discrepancias
encontradas
al
comparar las mediciones directas e
indirectas se ha sugerido que estas
últimas son inapropiadas para estimar
de forma precisa la presión por
herbivoría. A través de métodos
directos los estudios han demostrado
no sólo que la herbivoría puede en
algunos casos llegar a determinar la
estructura y distribución de los
macrófitos (Tomás et al. 2005a;
Taylor & Schiel 2010), sino que
también puede ser muy variable tanto
espacial
como
temporalmente,
presentando distintos patrones de
22
defoliación entre praderas y/o épocas
(Prado et al. 2007a, 2008a, 2010a;
Steele et al. 2014). Sin embargo, se
necesitan más estudios para volver a
evaluar y comparar la importancia de
la herbivoría en diferentes especies de
macrófitos y las implicaciones
ecológicas en el funcionamiento de
los diferentes ecosistemas.
Para comprobar en qué medida los
patrones de herbivoría previamente
observados en el campo (por ejemplo,
mediante tethering y observaciones
del comportamiento alimenticio)
podrían estar relacionados con
preferencias alimenticias de los
herbívoros (Vergés et al. 2007a,b;
2011), los estudios a menudo
emplean
experimentos
de
preferencias
con
distintas
combinaciones de alimentos. Esta
metodología permite aislar otros
factores (por ejemplo, la depredación)
que pueden interferir en el
comportamiento alimenticio, a la vez
que permite evaluar cómo influyen
los factores relacionados con la
calidad del alimento en la decisión de
los herbívoros. En los últimos años
estos experimentos han permitido
demostrar que las características
relacionadas con la calidad del
alimento, como los metabolitos
Resumen General
secundarios, las defensas estructurales
y el contenido nutricional de los
macrófitos, pueden influir en la
selección del alimento por parte de
los herbívoros de forma diferente
dependiendo de la especie (Vergés et
al. 2007a,b; Prado & Heck 2011).
Un seguimiento espacial y temporal
de los movimientos y la dieta de los
animales es también fundamental
para entender su ecología, aunque
implica dificultades metodológicas a
la hora de realizar mediciones. Entre
los métodos para cuantificar las
contribuciones de distintos alimentos
a una dieta, el análisis de los
contenidos estomacales es el más
preciso,
aunque
aporta
una
información dietética referente a un
período de tiempo corto y requiere
un tiempo de muestreo prolongado
(Legagneux et al. 2007). Además esta
técnica normalmente requiere el
sacrificio del animal, a menos que ya
haya muerto por otras causas (por
ejemplo, la caza).
Los análisis de isótopos estables son
otra técnica no destructiva que puede
proporcionar información de la dieta
y, a menudo, de la procedencia de las
fuentes de alimento referente a un
periodo de tiempo más largo
(Hobson & Clark 1992a,b, 1993;
Inger & Bearhop 2008). Puesto que
los consumidores se alimentan de
recursos con valores isotópicos
distintos, sus tejidos reflejan las
diferentes composiciones isotópicas
de sus dietas. Esto depende del
período de tiempo durante el cual el
consumidor adquiere la dieta, así
como de las tasas de síntesis de los
diferentes tejidos (Legagneux et al.
2007). Los diferentes tejidos
animales llevan asociados tiempos de
síntesis distintos y, por tanto, el valor
isotópico de un determinado tejido
refleja la dieta/hábitat del animal en
el momento de la síntesis del tejido
(Bearhop et al. 2002, Pearson et al.
2003, Hobson & Bairlein 2003,
Ogden et al. 2004). Los procesos de
ingestión, digestión y asimilación de
los consumidores llevan asociados
cambios en las relaciones isotópicas
(fraccionamiento o niveles de
enriquecimiento tróficos), de modo
que los valores isotópicos de la dieta y
de los consumidores difieren de una
manera
predecible.
Por
ello,
seleccionando cuidadosamente los
tejidos se puede inferir la dieta o el
hábitat preferente de un animal en
distintas escalas de tiempo y espacio.
En condiciones apropiadas los
23
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
modelos de mezcla mixing models
(por ejemplo: IsoSource o MixSir)
permiten utilizar las proporciones de
isótopos estables en los tejidos para
cuantificar la importancia relativa de
los diferentes alimentos en de la dieta
de un consumidor (Phillips & Gregg
2001, 2003; Moore & Seemens
2008; Inger & Bearhop 2008).
C:N), de los macrófitos, epífitos,
flores o semillas en las preferencias y
los patrones de herbivoría observados
en los distintos ecosistemas.
El objetivo principal de esta tesis es
estudiar diferentes interacciones
macrófito-herbívoro e identificar qué
factores median las preferencias
alimenticias y las tasas de consumo
de los herbívoros en distintos
ecosistemas de macrófitos. Para ello
se investigó la herbivoría combinando
diferentes metodologías que incluyen
métodos directos de cuantificación de
consumo (tethering), observaciones
del comportamiento alimenticio,
experimentos
de
preferencias
alimenticias o de jaulas de exclusión y
análisis de la dieta (contenidos
estomacales combinados con análisis
isotópicos) para obtener información
de la dieta integrada en el tiempo y
lograr una visión más amplia de las
interacciones
macrófito-herbívoro
que tienen lugar en cada caso de
estudio. Además tratamos de
dilucidar el papel mediador del
contenido nutricional (medido por
En el este del Golfo de México, las
praderas marinas están dominadas
normalmente
por
Thalassia
testudinum Banks ex König (Zieman
& Zieman 1989), siendo el erizo de
mar Lytechinus variegatus (Lam.) la
principal especie herbívora (Camp et
al. 1973; Greenway 1976; Macia &
Lirman 1999). Sin embargo, se ha
reportado que L. variegatus puede
alimentarse también de las hojas en
descomposición a tasas incluso más
altas que de las hojas verdes
(Montague et al. 1991). Además, se
ha
demostrado
que
pueden
manifestar un consumo preferente
por las hojas epifitadas (Greenway
1995). En este estudio el objetivo
principal fue evaluar el consumo de
las hojas en descomposición frente a
las hojas verdes de T. testudinum y
dilucidar el papel mediador de los
epífitos en los patrones de consumo
por parte del erizo en el noreste del
24
2. Los epífitos influyen en el
papel trófico de los erizos de
mar en las praderas de Thalassia
testudinum
Resumen General
Golfo de México. Las observaciones
previas
del
comportamiento
alimenticio realizadas dentro de la
pradera señalaron que el número de
individuos que se alimentaban de
material
detrítico
era
aproximadamente 5 veces mayor que
del que se alimentaba de las hojas
verdes, por lo que se planteó como
hipótesis que las tasas de consumo
también reflejarían este patrón. Más
específicamente, se evaluó: (1) las
tasas de consumo de hojas verdes y
detríticas (peso seco de haz por día:
mg PS haz-1 día-1) de T. testudinum
mediante experimentos de tethering; y
(2) la influencia de la descomposición
y de la presencia de los epífitos en las
preferencias del erizo de mar,
utilizando combinaciones pareadas de
hojas
verdes
y
hojas
en
descomposición, con y sin epífitos.
Estos experimntos de preferencias de
24 h de duración fueron realizados en
jaulas (n = 6) conteniendo cada una 4
individuos de L. variegatus. Además,
el contenido de nutrientes en los los
dos tipos de hojas y en los epífitos y
la biomasa de epífitos y su
composición fueron investigados
como posibles variables explicativas.
Por último, se llevaron a cabo análisis
de contenidos estomacales y análisis
de isótopos estables para medir la
importancia relativa de los distintos
recursos en las dietas del erizo de mar
y evaluar las distintas contribuciones
dietéticas a largo plazo.
Lytechinus variegatus registró tasas de
consumo más altas en las hojas en
descomposición (12,15 ± 1,3 mg PS
haz-1 día-1) en comparación con un
consumo indetectable de hojas verdes
de las praderas. Además, para los dos
tipos de hojas, manifestó un consumo
preferente por las hojas epifitadas. La
preferencia por la hojas en
descomposición
pudo
estar
relacionada con una biomasa de
epifitos significativamente mayor en
estas hojas (3,64 ± 0,28 mg PS cm-2)
en comparación con las hojas verdes
(2,11 ± 0,25 mg PS cm-2). Los
análisis de isótopos estables señalaron
a los epífitos y a las hojas verdes
como las principales fuentes de
nitrógeno y carbono en la dieta de L.
variegatus. Estos resultados sugieren
que las epífitos afectan las
interacciones tróficas entre erizos y
T. testudinum en las praderas. Por lo
tanto, los cambios en la biomasa de
epífitos y en la biomasa de hojas en
descomposición podrían regular el
forrajeo del erizo de mar y su impacto
25
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
en la dinámica trófica de estas
praderas.
3. La presencia de epífitos y la
identidad de las especies de
fanerógamas influyen en las
tasas de herbivoría de las
praderas del Mediterráneo
En el Mediterráneo, las praderas
marinas están dominadas por
Posidonia oceanica (L.) Delile (Prado
et al. 2011), mientras que Cymodocea
nodosa
(Ucria)
Ascherson
se
encuentra comúnmente ocupando
pequeños parches dentro de estas
praderas (Pérès & Picard 1964). Las
tasas de herbivoría sobre estas
especies son generalmente bajas en
comparación con otras especies de
fanerógamas marinas, aunque pueden
variar sustancialmente (por ejemplo,
2–57% de la productividad de P.
oceanica, Cebrián et al. 1996a; Prado
et al. 2007a; 1–50% de la
productividad de C. nodosa, Cebrián
et al. 1996b). Mediante el uso de
métodos directos se ha demostrado
que las tasas de herbivoría en estas
praderas son mayores de lo que
inicialmente se había estimado
(Kirsch et al. 2002; Tomás et al.
2005a; Prado et al. 2007a), no
26
obstante es necesario que nuevos
estudios
sigan
revaluando
y
cuantificando por métodos directos la
importancia de la herbivoría en el
funcionamiento ecológico de los
sistemas mediterráneos.
En el Mediterráneo occidental el
erizo de mar Paracentrotus lividus
(Lam.) y el pez Sarpa salpa (L.) son
los dos principales macroherbívoros,
los cuales habitan comúnmente en
praderas marinas poco profundas y
fondos rocosos (Verlaque 1990).
Según Prado et al. (2007a) S. salpa
puede ser responsable del 70% del
consumo foliar total en las praderas
de P. oceanica (aproximadamente 40%
de la producción foliar) siendo P.
lividus responsable del 30% restante
(aproximadamente el 17% de la
producción foliar), aunque la
intensidad de herbivoría puede variar
en gran medida a través del espacio y
el tiempo (Cebrián et al. 1996a,b;
Prado et al. 2007a, 2010a; Steele et
al. 2014). Se ha demostrado que
factores como la disponibilidad y la
accesibilidad del alimento, la calidad
nutricional, la presión humana sobre
las poblaciones de herbívoros, el
reclutamiento de herbívoros y el
riesgo de depredación influyen en la
intensidad de herbivoría de las
Resumen General
praderas (Prado et al. 2008a, 2009,
2010a). Además, los estudios
también sugieren que no sólo la
calidad nutricional, sino también la
composición de la comunidad epífita
pueden influir fuertemente en el
consumo de los herbívoros (Thacker
et al. 2001; Tomas et al. 2005b;
Marco-Méndez et al. 2012).
En este contexto, los objetivos de este
estudio
fueron
investigar
la
intensidad de herbivoría y las
preferencias alimenticias de P. lividus
y S. salpa, en un hábitat mixto del
Mediterráneo
occidental
con
presencia de roca, P. oceanica y C.
nodosa y dilucidar el papel mediador
de los epifitos en las preferencias de
los herbívoros. Para ello se evaluaron
las tasas de consumo por medio de
los experimentos de tethering y las
preferencias por medio de tethering y
experimentos de inclusión de erizos
en jaulas con distintas combinaciones
pareadas de C. nodosa y P. oceanica
con y sin epífitos. Estos experimentos
se
complementaron
con
observaciones del comportamiento
alimenticio en el medio. Además, la
contribución de cada fuente de
alimento a la dieta se estimó
mediante el análisis de contenidos
estomacales e isótopos estables. Por
último, se investigó el contenido
nutricional (ratio C:N) y la
comunidad epífita de las dos especies
de fanerógamas como posibles
variables explicativas del comportamiento herbívoro.
Encontramos que la preferencia por
C. nodosa fue débil para S. salpa, pero
muy evidente para P. lividus,
principal responsable del consumo
registrado para ambas especies de
fanerógamas. La preferencia por las
hojas epifitadas vs. sin epifitar así
como su mayor calidad nutricional
confirma que los epífitos influyen
fuertemente en las tasas de consumo
y en las preferencias (Alcoverro et al.
1997; Tomas et al. 2005b, 2006;
Marco-Méndez et al. 2012) debido a
su mayor valor nutritivo. La mayor
calidad nutricional de C. nodosa así
como la mayor cobertura y número
de taxones de epífitos en sus hojas
parece explicar la mayor herbivoría
observada en esta especie, al menos
por parte de P. lividus. Nuestro
estudio también evidencia que las
interacciones
macrófito-herbívoro
son complejas y sugiere que las tasas
de consumo finales en estas praderas
no sólo están determinadas por las
preferencias
alimenticias,
sino
también por factores que podrían
27
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
influir en el comportamiento del
herbívoro cambiando sus prioridades,
como el riesgo de depredación y/o el
rango de movilidad.
4. Los epífitos y el contenido
nutricional influyen en la
herbivoría de Sarpa salpa sobre
las especies de Caulerpa y las
fanerógamas presentes en las
praderas del Mediterráneo
Las praderas del Mediterráneo,
comúnmente
dominadas
por
Posidonia oceanica (L.) Delile (Prado
et al. 2011), con presencia de
Cymodocea nodosa (Ucria) Ascherson
en pequeños parches (Pérès & Picard
1964), además de estar sometidas a la
presión de los herbívoros también se
ven amenazadas por el aumento de la
colonización de especies de Caulerpa,
las cuales podrían llegar a sustituirlas
afectando al funcionamiento de estos
ecosistemas clave. Los objetivos de
este estudio fueron evaluar y
comparar la presión que el pez
herbívoro Sarpa salpa (L.) ejerce
sobre las especies de Caulerpa y las
especies nativas de fanerógamas
marinas en una pradera mixta, e
investigar si el patrón de herbivoría
observado responde a preferencias
28
alimenticias. Nuestra hipótesis es que
si S. salpa muestra una preferencia y
presión mayor sobre las especies de
Caulerpa, esto podría beneficiar a las
especies de fanerógamas presentes en
las praderas mixtas del Mediterráneo.
Con este fin se estudiaron los
patrones temporales y espaciales de la
herbivoría por parte de S. salpa en
una pradera mixta del Mediterráneo
occidental con presencia de P.
oceanica,
C.
nodosa,
Caulerpa
cylindracea
Sonder y Caulerpa
prolifera (Forsskål) JV Lamouroux,
durante el verano-otoño de 2012.
Además realizamos experimentos de
tethering con combinaciones pareadas
de las distintas especies de
fanerógamas y de Caulerpa para testar
las preferencias de S. salpa y el papel
mediador de los epífitos y/o
nutrientes.
Las
observaciones
realizadas en el campo y los
experimentos se combinaron con el
análisis de la dieta a corto y largo
plazo con el fin de investigar las
contribuciones dietéticas de los
diferentes recursos alimenticios.
Nuestro estudio pone en relieve la
importancia de C. nodosa y C.
prolifera en la dieta de S. salpa, y
sugiere que la herbivoría en praderas
mediterráneas puede ser muy variable
Resumen General
y estar mediada por múltiples
factores. En verano, cuando las
densidades de S. salpa fueron más
altas, C. nodosa fue el macrófito más
consumido (12,75 ± 3,43 mg de peso
húmedo al día), probablemente
influenciado por la mayor calidad
nutricional de sus hojas y sus epífitos,
así como por las diferencias en la
composición de la comunidad epífita
(Marco-Méndez et al. 2015a). Los
experimentos de preferencias, las
observaciones del comportamiento
alimenticio y los análisis del
contenido estomacal indican que C.
prolifera es el alimento preferido por
S. salpa. En contraste, la preferencia
de S. salpa por C. prolifera no se
mantuvo cuando las hojas de las
fanerógamas estaban epifitadas, lo
que sugiere que la presencia de
epifitos y, en consecuencia, las
diferencias
en
el
contenido
nutricional probablemente explican
los patrones de herbivoría observados
en la pradera mixta. De hecho, el
modelo de mezcla IsoSource (Phillips
& Gregg 2003) confirmó la
importancia de las especies de
Caulerpa en su dieta (lo que a tenor
de nuestros resultados parece
fundamentalmente atribuible a C.
prolifera) así como el papel destacado
de los epífitos en la dieta a largo
plazo. Aunque no se detectó
consumo de C. cylindracea durante el
estudio y S. salpa parece alimentarse
preferentemente de especies nativas,
el hecho de que se encontrara dentro
de los contenidos estomacales sugiere
que con el tiempo podría adaptarse a
consumir este nuevo recurso. Nuestro
estudio pone de manifiesto la
complejidad de las interacciones
macrófito-herbívoro y sugiere que las
tasas de consumo finales y las
diferencias en la dieta no sólo están
determinadas por las preferencias
alimenticias, sino también por el
rango de movilidad, así como por las
diferencias temporales y espaciales en
la
disponibilidad
de
recursos
alimenticios.
5. Efectos estacionales del
pastoreo de las aves acuáticas
sobre los macrófitos
sumergidos: El papel de las
flores
Los pocos estudios realizados en los
ecosistemas acuáticos mediterráneos
sugieren que, en general, el pastoreo
de las aves acuáticas no tiene un
fuerte efecto en la biomasa de la
vegetación sumergida, debido a la
29
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
alta productividad primaria (Mitchell
& Perrow 1998; Marklund et al.
2002; Sandsten et al. 2002). Sin
embargo, también se ha sugerido que
las aves acuáticas en zonas
mediterráneas pueden tener un fuerte
efecto cualitativo sobre la estructura
de las comunidades vegetales
seleccionando las especies más
apetecibles o comestibles o sus
estructuras reproductivas (RodríguezVillafañe et al. 2007; Gayet et al.
2012). Se ha sugerido que una
marcada preferencia de los herbívoros
por plantas con abundantes flores y/o
frutos en desarrollo puede conllevar a
la larga una reducción en el número
de semillas producidas por estas
plantas (Herrera et al. 2002), lo que
podría impactar fuertemente en el
éxito reproductivo de los macrófitos.
En este contexto, el objetivo general
de este estudio fue investigar si las
diferencias estacionales en las
poblaciones de las dos principales
especies de aves acuáticas (Anas
platyrhynchos L. y Fulica atra L.) y en
la abundancia de las dos especies
principales de macrófitos sumergidos
[Ruppia cirrhosa (Petagna) Grande y
Potamogeton pectinatus L.] pueden
explicar los patrones de consumo
observados
en
las
lagunas
30
mediterráneas. Además, se investigó
el potencial papel mediador de las
flores en las preferencias alimenticias
de las aves acuáticas y en los impactos
globales sobre la biomasa de
macrófitos. Con este fin se evaluaron
durante el verano y otoño de 2010 y
el invierno de 2011: (1) la abundancia
de A. platyrhynchos y F. atra; (2) los
impactos del pastoreo de estas aves
en ambas especies de macrófitos y sus
flores (sólo en verano) mediante el
uso de jaulas de exclusión; y (3) las
tasas de consumo de los macrófitos
por medio de experimentos de
tethering.
A pesar de la baja abundancia de aves
acuáticas registradas en verano, los
experimentos
de
exclusión
evidenciaron una intensa herbivoría
sobre la biomasa, la longitud foliar y
la abundancia de flores de R. cirrhosa
(̴ 8 veces mayor dentro de jaulas de
exclusión; 1015,7 ± 269,8 flores m-2).
En el caso de P. pectinatus, los
experimentos de exclusión no
detectaron consumo, a pesar de la
presencia de flores, lo que sugiere una
preferencia
por
los
tejidos
reproductivos de R. cirrhosa. Además,
la mayor abundancia de flores de R.
cirrhosa en comparación con P.
pectinatus (̴ 10 veces más dentro de
Resumen General
las jaulas de exclusión) podría haber
influido en la mayor herbivoría
registrada sobre la primera especie.
Aunque la abundancia de aves
acuáticas aumentó en otoño e
invierno, los experimentos no
detectaron efectos sobre la vegetación
durante ese período, posiblemente
debido a una mayor disponibilidad de
recursos alternativos y a una
disminución de la biomasa de la
planta y de la longitud de la hoja, lo
que podría reducir las tasas de
encuentro.
Nuestros resultados contrastan con
los impactos de herbivoría descritos
para fochas y patos en el norte de
Europa (Van Donk & Otte 1996;
Søndergaard et al. 1996), pero
concuerdan con otros estudios
mediterráneos (Rodrígez-Pérez &
Green 2006; Rodríguez-Villafañe et
al. 2007) en los que los principales
efectos de las aves acuáticas tuvieron
lugar durante el verano. Los
resultados de nuestro estudio
sugieren que los mayores impactos de
las aves acuáticas sobre la vegetación
sumergida dentro de lagunas salobres
del Mediterráneo no se producen
cuando la abundancia de individuos
es más alta, sino en verano, cuando la
disponibilidad de las plantas y flores
es mayor. A largo plazo, el aumento
de la presión de herbivoría sobre R.
cirrhosa y sus flores podría reducir el
éxito reproductivo de esta especie y
alterar la estructura general de la
comunidad
de
macrófitos
sumergidos. Nuestros resultados
sugieren
que
los
impactos
estacionales de las aves acuáticas no
siguen una norma general, sino que
dependen de una combinación del
número de individuos y de la
abundancia
de
los
recursos
alimenticios. Este estudio contribuye
a una mejor comprensión de las
interacciones entre las aves acuáticas
y los macrófitos en los ecosistemas
acuáticos mediterráneos a lo largo del
ciclo anual. La información aportada
podría ayudar a favorecer una mejor
conservación de estos hábitats
naturales y a la sostenibilidad a largo
plazo de su diversidad natural.
6. Uso de los campos de arroz
como hábitat de alimentación
de las aves acuáticas durante la
temporada de cultivo
En muchas regiones, los humedales
naturales han desaparecido y los
campos de arroz (Oriza sativa L.) se
han convertido en el hábitat principal
para las aves acuáticas durante la
31
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
invernada (Tréca 1994; Elphick &
Oring 1998), o la cría (Acosta et al.
1996; Lane & Fujioka 1998). En las
zonas del Mediterráneo, existe poca
información sobre el uso que las aves
acuáticas hacen de estos hábitats
(Tourenq et al. 2003) y además los
pocos estudios existentes se han
centrado sobretodo en el forrajeo de
las garzas (Fasola et al. 1996).
Los humedales mediterráneos como
el Delta del Ebro, la Albufera y el
Parque Nacional de Doñana son
algunas de las principales áreas de
invernada para las aves acuáticas de la
región
paleártica
occidental
(Sánchez-Guzmán et al. 2007;
Rendón et al. 2008). En algunos de
estos humedales, el ánade real (Anas
platyrhynchos L.) y las fochas (F. atra
L.) son las especies dominantes,
principalmente
en
invierno
(Martínez-Vilalta 1996; Mañosa et
al. 2001). Se ha reportado que estas
dos
especies
se
alimentan
principalmente de la vegetación
acuática sumergida de pantanos y
lagunas (Rodríguez-Villafañe et al.
2007; Marco-Méndez et al. 2015b).
Sin embargo, durante los últimos
siglos los humedales mediterráneos se
han reducido a un 10–20% de su área
original (Fasola & Ruiz 1997), lo que
32
aumenta la importancia relativa de
los campos de arroz como hábitat de
alimentación alternativo para las
especies de aves acuáticas (Fasola &
Ruiz 1996, 1997).
El principal objetivo de nuestro
estudio fue determinar la importancia
de los campos de arroz como hábitats
de alimentación alternativos para las
dos principales especies de aves
acuáticas del Delta del Ebro (noreste
de España), A. platyrhynchos y F. atra
durante la temporada de cultivo del
arroz. Para ello: (1) se investigaron
las abundancias estacionales de las
dos especies dentro de un campo de
arroz;
(2)
se
cuantificó
estacionalmente el impacto del
pastoreo de las aves acuáticas en la
biomasa de las plantas de arroz y en
las semillas usando experimentos de
jaulas de exclusión y (3) se cuantificó
el consumo estacional de las plantas
de arroz utilizando experimentos de
tethering. Las jaulas de exclusión y los
experimentos
de
tethering
se
realizaron dentro de un campo de
arroz al comienzo de la temporada de
cultivo (verano de 2010) y antes de la
cosecha (otoño de 2010). Además, se
estudió la dieta a través del análisis de
los contenidos estomacales y del
análisis de isótopos estables y se
Resumen General
investigó el papel del contenido
nutricional en la selectividad del
alimento. La hipótesis de partida
planteada en este estudio es que las
aves acuáticas usan los campos de
arroz como hábitats de alimentación
alternativos, especialmente durante la
temporada de cultivo, cuando hay
una disponibilidad cada vez mayor de
alimento y recursos vegetales en estos
campos.
En verano, las abundancias de aves
acuáticas fueron bajas, pero los
experimentos de jaulas de exclusión
detectaron efectos del pastoreo de las
aves en el arroz, mediante una
reducción significativa de la biomasa
vegetal en las jaulas abiertas (aunque
el consumo no pudo detectarse
mediante tethering). En otoño, la
abundancia de aves acuáticas
aumentó y los experimentos de
tethering detectaron consumo de las
plantas de arroz con semillas,
mientras que los experimentos de
jaulas de exclusión no detectaron
efectos del pastoreo. Los análisis de
los contenidos estomacales indicaron
que A. platyrhynchos es una especie
principalmente
granívora,
alimentándose principalmente de
semillas de R. cirrhosa y de arroz (O.
sativa), mientras que F. atra es
principalmente herbívora, alimentándose de hojas de las dos especies
de macrófitos. Sin embargo, los
análisis de isótopos estables y del
modelo de mezcla mostraron que, a
largo plazo, ambas especies parecen
adquirir la mayor parte de sus
necesidades alimenticias de las
plantas de arroz y de P. pectinatus.
Los análisis de la dieta confirmaron
la importancia del arroz en la dieta de
ambas especies, pero también
sugirieron que las aves acuáticas
pueden sufrir variaciones estacionales
en la dieta, en su mayoría influidas
por los cambios en la disponibilidad
de recursos alimenticios en la zona y
no por su calidad nutricional.
Nuestros resultados confirman que
en las regiones mediterráneas,
durante la temporada de cultivo los
arrozales actúan como un hábitat de
alimentación complementario para
las aves acuáticas. El uso de estos
campos por parte de las aves acuáticas
podría ayudar a mitigar la pérdida de
hábitats naturales en otras áreas
dominadas progresivamente por la
agricultura. Por esta razón, es
necesario aumentar la colaboración
entre investigadores, agricultores y
agrónomos para comprender mejor
cómo la gestión adecuada de los
33
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
campos de cultivo puede aumentar su
valor de conservación para las aves,
sin poner en peligro su viabilidad
económica.
7. Discusión
El objetivo principal de esta tesis
doctoral fue identificar qué factores
marcan las preferencias y las tasas de
consumo de los herbívoros en
distintos ecosistemas de macrófitos.
El uso combinado de métodos
permitió una visión más amplia de las
interacciones
macrófito-herbívoro
que tienen lugar en cada caso de
estudio. Sin embargo, cada método
lleva asociada algunas limitaciones
que deben tenerse en cuenta a la hora
de interpretar los resultados. Por ello,
los resultados obtenidos con cada uno
de los métodos fueron contrastados
entre sí con el fin de evitar sobre o
subestimaciones de la herbivoría. Por
ejemplo, las estimas obtenidas
mediante el tethering y las
observaciones del comportamiento
alimenticio pueden estar influidas por
varios factores que actúan de manera
diferente de una pradera a otra o
entre los tiempos estudiados incluso
para una misma especie, causando
alta variabilidad en las estimaciones.
Teniendo en cuenta los múltiples
34
factores que pueden estar influyendo
en el consumo de los herbívoros y su
comportamiento alimenticio en cada
caso de estudio, es posible que las
estimaciones de consumo no estén
reflejando
necesariamente
una
preferencia alimenticia. En contraste,
los experimentos de preferencias
permiten testar la selectividad de los
herbívoros aislándolos de los factores
que acontecen en su medio natural
(por ejemplo, la depredación, la
competencia...), lo que puede ayudar
a dilucidar cómo los factores
relacionados con la palatabilidad del
alimento influyen en la selectividad.
En cuanto a la dieta, los contenidos
estomacales aportan información de
la dieta a corto plazo que puede estar
limitada por la disponibilidad del
alimento en el momento de la
recogida del individuo y por la
digestibilidad. Por otro lado, los
análisis isotópicos dan información
relativa a un período más largo
aunque restringido al período de
síntesis del tejido investigado. En
consecuencia,
al
combinar
información y resultados obtenidos
por distintos métodos a veces
encontramos
resultados
contradictorios, tales como que el
alimento preferido (identificado por
Resumen General
experimentos de preferencia) no fue
siempre el más consumido en el
medio (en los experimentos de
tethering y en el comportamiento
alimenticio observado) o el alimento
más presente en el contenido
estomacal (dieta a corto plazo),
aunque pueda contribuir a la dieta a
largo plazo (análisis isotópicos). Esto
sugiere que los herbívoros no siempre
tienen la oportunidad de alimentarse
de su alimento preferido en el medio,
ya que bajo condiciones naturales
pueden estar influidos por otros
factores que actúan de manera
diferente en cada caso de estudio. Por
estas razones creemos que la
combinación de métodos es la mejor
manera de comprender los patrones
de herbivoría que tienen lugar en los
ecosistemas de macrófitos ya que
permiten identificar los principales
factores que intervienen y su
variabilidad.
Las tasas de consumo fueron muy
variables entre épocas, macrófitos y
especies de herbívoros. Sorprendentemente, a pesar de ser los herbívoros
más pequeños, los erizos de mar
registraron las tasas de consumo más
altas, L. variegatus en las hojas sueltas
en descomposición de T. testudinum y
P. lividus en C. nodosa en el verano de
2011. Esto confirma que herbívoros
marinos distintos de los grandes
vertebrados también pueden ejercer
un papel importante en los
ecosistemas de praderas marinas
(véase la revisión de Heck &
Valentine 2006). Además, también
demuestra la mayor presión por
herbivoría en el Mediterráneo no
siempre se debe principalmente a S.
salpa (Prado et al. 2007a; SánchezLizaso & Ramos-Esplá 1994), cuyo
consumo en nuestro estudio fue muy
bajo para P. oceanica e indetectable
para C. nodosa. Sin embargo, en el
estudio llevado a cabo en 2012
(Capítulo 4), C. nodosa fue el
macrófito más consumido por esta
especie, registrándose tasas de
consumo
en
verano
significativamente mayores que las
registradas para las otras especies
durante todo el estudio. Nuestros
resultados sugieren que la herbivoría
sobre C. nodosa (por ejemplo, 2,82 %
de macrófito consumido por P.
lividus en el Capítulo 3 y el 0,51%
por S. salpa en el Capítulo 4) también
puede ser importante y no debe ser
subestimada. A pesar del menor
consumo y la falta de diferencias
significativas, los resultados también
muestran que el consumo de C.
35
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
prolifera puede exceder (en verano) o
igualar (en otoño) el consumo de P.
oceanica detectado en nuestro estudio.
Después de C. nodosa, P. pectinatus
registró las tasas de consumo más
altas. En el humedal esta especie fue
el macrófito más consumido por las
aves acuáticas en toda la temporada
estudio (verano 2010, otoño 2010 e
invierno de 2011), seguido de R.
cirrhosa, y la planta de arroz con
semillas.
En
cuanto
a
los
experimentos de jaulas de exclusión,
los resultados señalaron a R. cirrhosa
como el macrófito más consumido.
De las diferencias encontradas entre
las jaulas control y exclusión se podría
estimar que transcurrido un mes, la
biomasa y la longitud foliar de R.
cirrhosa
se
redujo
en
aproximadamente un 38% y un 36%
respectivamente debido al pastoreo
de las aves acuáticas. Estos resultados
se encuentran dentro del rango
estimado del efecto que las aves
acuáticas tienen sobre otras especies
de macrófitos como Zostera japonica
Ascherson & Graebner o Z. marina
L. (Baldwin & Lovvorn 1994a;
Madsen 1988; Portig et al. 1994).
En general, estos resultados reflejan
una alta variabilidad en la intensidad
36
de la herbivoría detectada para las
diferentes especies de macrófitos y
herbívoros estudiados. Además,
apreciamos que en algunos casos, la
alta herbivoría cuantificada parecía
reflejar una situación particular
asociada a las características de un
determinado hábitat y a una alta y
poco frecuente abundancia de
herbívoros, mientras que en otros
casos, la alta movilidad de los
herbívoros parecía explicar la
variabilidad y la inconsistencia en las
tasas registradas. De hecho, gracias a
la combinación de metodologías se
pudo llevar a cabo una interpretación
más cautelosa de los resultados
permitiendo la identificación de los
posibles factores que intervienen en
cada caso de estudio.
Durante esta tesis se encontró que las
interacciones
macrófito-herbívoro
pueden estar influenciadas por varios
factores que actúan al unísono,
aunque algunos de ellos fueron
claramente
identificados
como
principales mediadores. A través de
nuestros estudios podemos ver que
los ecosistemas no responden
linealmente a un control top-down o
bottom-up puesto que los herbívoros y
los macrófitos responden de manera
diferente entre especies y las
Resumen General
interacciones son más complejas de lo
que en un principio se había
planteado (véase la Introducción).
De todos nuestros estudios podemos
afirmar que la variabilidad en la
disponibilidad de los recursos
alimenticios, probablemente influida
por las diferentes condiciones
abióticas (por ejemplo: mayor
disponibilidad de luz y mayor
temperatura en verano), fue en
general, el factor que más afectó al
consumo de macrófitos en los
diferentes ecosistemas (por ejemplo:
la disponibilidad de hojas en
descomposición, la mayor biomasa
epífitos o los cambios estacionales en
la
comunidad
epífita,
la
disponibilidad y abundancia de flores,
de planta de arroz o de semillas...).
En el primer estudio, la alta
disponibilidad en otoño de hojas de
T. testudinum en descomposición
altamente epifitadas, las cuales fueron
identificadas como el alimento
preferido de L. variegatus, pareció ser
el principal factor determinante de la
baja herbivoría observada sobre las
plantas vivas en la pradera. Sin
embargo, a lo largo de nuestros
estudios, a veces encontramos que los
herbívoros
manifestaban
una
preferencia por un determinado
recurso que, a pesar de estar
disponible en el hábitat estudiado, no
fue el más consumido (por ejemplo,
en el Capítulo 4). Aparentemente
esto parece contradictorio pero tiene
sentido si tenemos en cuenta que a
veces el alimento preferido puede ser
difícil de encontrar en condiciones
naturales y/o que el comportamiento
alimenticio de los herbívoros puede
verse afectado por otros factores
distintos
de
la
preferencia
alimenticia. Este fue el caso de P.
lividus, con una fuerte preferencia
por C. nodosa, probablemente debido
a su mayor calidad nutricional y una
mayor biomasa de epifitos. Sin
embargo, P. lividus normalmente
habita
en
refugios
rocosos
priorizando
estrategias
de
supervivencia (Ebling et al. 1966;
Boudouresque & Verlaque 2001,
2013), lo que tal vez explica por qué
en otras praderas donde no existen
refugios rocosos; los erizos son
escasos (Fernández & Boudouresque
1997) y en consecuencia no se han
sugerido como posibles herbívoros de
C. nodosa. En nuestro caso, la
combinación de la disponibilidad del
alimento preferido, C. nodosa, en un
hábitat rocoso que favorece altas
37
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
abundancias de erizos de mar (debido
a la alta disponibilidad de refugio
rocoso) influyó en el patrón de
herbivoría detectado. Una conclusión
similar se puede extraer del patrón de
herbivoría observado para S. salpa en
los diferentes estudios. En este caso,
aunque la disponibilidad de las
distintas especies de macrófitos no
difirió entre temporadas, esta especie
consumió grandes cantidades de C.
nodosa en verano y mostró una
marcada preferencia por C. prolifera
frente a las fanerógamas marinas sólo
en ausencia de epífitos. En este caso,
el patrón de herbivoría observado
parece responder a una combinación
de variabilidad estacional, temporal y
espacial natural en las abundancias de
peces, su distribución y su
comportamiento alimenticio (Peirano
et al. 2001; Jadot et al. 2002, 2006), y
a las diferencias en la biomasa de
epífitos, en la estructura de su
comunidad y en los contenidos
nutricionales de los distintos
recursos.
En los Capítulos 5 y 6 encontramos
una clara desconexión entre las
abundancias estacionales de las aves
acuáticas y su efecto estacional en los
macrófitos sumergidos de la laguna y
los campos de arroz. En ambos
38
estudios se detectaron importantes
efectos en verano, a pesar de que la
abundancia de aves acuáticas fueron
las más bajas en ambos hábitats
estudiados.
La
disponibilidad
estacional de flores en la laguna
explicó los principales efectos
detectados sobre R. cirrhosa en
verano. En el campo de arroz,
también se detectó un claro efecto en
verano sobre las plántulas, aunque
también se observó un consumo
importante de semillas de arroz en
otoño. En este estudio, el análisis de
la dieta fue crucial para entender que
las aves acuáticas cambian su dieta de
acuerdo a la variación estacional en la
disponibilidad
de
recursos
alimenticios (Guillemain & Fritz
2002; Guillemain et al. 2002;
Dessborn et al. 2011), y por tanto
este se presenta como el factor más
determinante en el pastoreo de las
aves acuáticas en los humedales
mediterráneos.
Se puede concluir que la abundancia
de herbívoros, su comportamiento
alimenticio, sus preferencias, los
cambios en la disponibilidad de
alimento y su contenido nutricional
fueron factores que influyeron en
mayor o menor medida en las
diferentes interacciones estudiadas.
Resumen General
39
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Photo: L.M. Ferrero-Vicente
40
CHAPTER 1: General Introduction
1. General Introduction
1.1. Importance of herbivory in
macrophyte ecosystems
Herbivory
on
submerged
macrophytes has been often thought
to account for a modest loss of leaf
production (by ca. 10–15%, Den
Hartog 1970; Thayer et al. 1984).
From early studies conducted in
seagrass meadows it was estimated
that, depending on the location,
somewhere between ∼3% and 100%
of seagrass net primary production
enters food webs via the grazing
pathway
(Fig.
1.1).
This
extraordinarily wide range suggests
that herbivory on seagrasses varies
greatly in time and space, as it does
in most other ecosystems (Louda &
Collinge 1992; Hacker & Bertness
1995, 1996), not that grazing on
seagrasses is by any means
inconsequential (Heck & Valentine
2006).
Here, I summarize the diversity of
herbivore species and associated plant
losses (leaves and rhizomes) after a
review of the available literature
(Table 1.1). Major effects have been
historically attributed to larger
vertebrate seagrass herbivores, which
were once very abundant and more
diverse than their modern-day
counterparts. Moreover, they relied
extensively on seagrass production to
meet
their
nutritional
needs
(Domning 2001; Jackson et al. 2001).
Since megagrazers such as marine
mammals and turtles were abundant
throughout most of the evolutionary
history of seagrasses (Jackson 1997),
seagrasses in the lower latitudes of
the Atlantic Ocean might have
undergone the same kind of intense
grazing pressure as their terrestrial
counterparts (Domning 2001). As a
result, it is likely that grazers played a
more important role in the
development of seagrasses and the
meadows they form than we are
familiar with today.
While seagrasses developed independently of terrestrial grasses (Larkum
& Den Hartog 1989; Les et al.
1997), they possess many of the same
traits considered to be grazer-derived
adaptations (Valentine & Duffy
2006), which for the terrestrial plants
41
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
are suggested to be the results of long
evolutionary history of interaction
with large vertebrate grazers.
Among these large herbivores,
waterfowl (ducks, geese and swans)
appear to have also exerted selective
pressures on macrophyte traits with
important impacts on the species
composition of submerged plant
communities (Bortolus et al. 1998;
Nolet et al. 2001; see Table 1.1). For
example, in Doñana Natural Park
(SW Spain) Rodriguez-Pérez &
Green (2006) showed that flamingos
and wildfowl (ducks and coots) can
have significant negative additive
effects on aboveground (leafs and
shoots) or belowground (roots) parts
of Ruppia maritima L. In Lake
Stigsholm, Denmark, Søndergaard et
Figure 1.1. Frequency distribution of % of net primary production consumed by herbivores for
vascular plants (top) and macroalgae (bottom). Data from terrestrial, freshwater and marine
environments are also shown. All values > 100% are shown in one category. From compilation in
Valiela (1984), Cry & Pace (1993) and Cebrián & Duarte (1994). Adapted from Valiela (1995).
42
CHAPTER 1: General Introduction
al. (1996) found that during the
growing season, macrophyte biomass,
shoot length, shoot number per
square meter and shoot height were
significantly
greater
in
exclosures/enclosures
protected
against grazing by waterfowl than in
unprotected
exclosures/enclosures.
Other large herbivores such as
dugongs are important grazers in east
Australia, able to reduce shoot
density, above and belowground
biomass by 65, 73 and 31%
respectively over 3.5 months (Preen
1995). Also, green turtles (Chelonia
mydas L.) are one of the major
herbivore groups inhabiting seagrass
meadows in south Florida and the
Caribbean Sea where they feed
actively on subtropical turtlegrass
Thalassia testudinum Banks ex König
(Bjorndal 1980; Zieman et al. 1984).
All these studies confirm that
waterfowls, sirenians (i.e., manatees
and dugongs) and sea turtles are
important macrophyte consumers
(Preen 1995; Ganter 2000; Moran &
Bjorndal 2005). In their absence,
herbivory has usually been considered
a marginal process in seagrass
ecosystems. Nevertheless, there is
increasing worldwide evidence that
pressure exerted by marine herbivores
other than these large vertebrates
Figure 1.2. Lytechinus variegatus in a Thalassia testudinum meadow in the northern Gulf of
Mexico (USA). Photo: P. Prado
43
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
may also play an important role in
the energetics and food web
interactions of seagrass ecosystems
(see review by Heck & Valentine
2006).
Studies over the last two decades
have shown that a number of sea
urchins and herbivorous fish species
can ingest large amounts of
aboveground seagrass biomass (see
Table 1.1). In the eastern Gulf of
Mexico, intense herbivory events in
which Lytechinus variegatus (Lam.)
removes 50 to 90% of the annual
aboveground primary production
(APP) of (T. testudinum) are not
uncommon and may be related to
dramatic increases in sea urchin
density (Camp et al. 1973; Greenway
1976; Valentine & Heck 1991; Heck
& Valentine 1995). According to
Valentine & Heck (1991) L.
variegatus can also regulate the
biomass and size of T. testudinum
meadows at some localities in the
northern Gulf of Mexico, when
present at commonly observed
densities (Fig. 1.2). Similarly,
tropical urchins in the Atlantic,
Pacific and Indian Oceans can
consume large amounts of seagrass
biomass (Keller 1983; Klumpp et al.
1993; Alcoverro & Mariani 2002)
and an Atlantic species (Diadema
antillarum Philippi) can produce
overgrazed,
unvegetated
‘halos’
around coral reefs (Ogden et al.
1973). Several species of herbivorous
fishes that shelter on reefs but forage
Figure 1.3. A. Sarpa salpa; B. Paracentrotus lividus and C. Mediterranean meadow of Posidonia
oceanica.
44
CHAPTER 1: General Introduction
in the surrounding seagrass meadows
can also contribute to the formation
of ‘halos’ around reefs (Randall 1965;
McAfee & Morgan 1996; Valentine
&
Duffy
2006).
In
the
Mediterranean, the sea urchins
Paracentrotus lividus (Lam.) and the
fish Sarpa salpa L. are the main
herbivores in seagrass meadows (Fig.
1.3), although early studies have
considered their herbivory low in
intensity in comparison to other
temperate and tropical species, with
wide variations in consumption
estimates (2–57% of Posidonia
oceanica L. leaf productivity, Cebrián
et al. 1996a, Prado et al. 2007a; 1–
50% of Cymodocea nodosa [Ucria]
Ascherson leaf productivity, Cebrián
et al. 1996b). Overall, these studies
proved that not only grazing by large
herbivores such as waterfowl but also
by macroherbivores like fishes and
sea urchins can also in some cases be
intense and control both macrophyte
productivity and density (Lodge et al.
1998; Valentine & Heck 1999).
However, given the wide variability
in herbivory rates, further studies are
needed to determine the relative
impact that different herbivore
species may have across macrophyte
ecosystems as well as to identify the
main factors driving plant-herbivore
interactions and their relative role in
determining macrophyte abundance
and distribution.
45
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Table 1.1. Summary of selected studies or reports of herbivory in seagrasses. W: waterfowl; U:
urchin; G: gastropod; C: crustacean; F: fish; R: reptile; M: mammal; SAV: submerged aquatic
vegetation. This table is updated from Valentine & Heck 1999 and Valentine & Duffy 2006 in
Larkum et al. 2006.
Seagrass, location,
study type
Description of results
Source
Anas acuta (W),
Anas carolinensis
(W), Branta
canadensis (W),
Margarites
helicinus (G),
Microcottus
sellaris (F),
Littorina sitkana
(G), Lacuna
variegata (G),
Telmessus
chierogonus (C)
Zostera marina. Izembek
Lagoon, Alaska. Sampling
and 13C:12C analysis.
Eelgrass was found to be incorporated
into the local food chain through
herbivory by at least 7 species.
McConaughey
& McRoy 1979
A. acuta (W),
Anas penelope
(W), Anas
platyrhynchos
(W), Branta
bernicla (W).
Zostera noltii and Z. marina.
Dutch Wadden Sea. Fieldbased bioenergetic study
and field experiment where
changes in the SAV)shoot
density, biomass and percent
cover were monitored.
An estimated 1426 kg DW of seagrass
( ̴50% of all SAV production) consumed,
mostly by A. acuta and A. penelope.
Jacobs et al.
1981
A. acuta (W), A.
crecca (W), A.
penelope (W), A.
platyrhynchos
(W), Aythya ferina
(W), Branta
bernicla (W),
Cygnus olor (W),
Fulica atra (W),
Idotea chelipes
(C)
Field based
study
and
experiment.
bioenergetic
laboratory
An estimated 7.5% of Z. marina
production consumed by waterfowl and
a single species of isopod.
Nienhuis &
Groenendijk
1986
A. acuta (W),
Aythya americana
(W), A.
platyrhynchos
(W), B. bernicla
(W)
Zostera japonica and Z.
marina. Boundary Bay,
British Columbia. Collections
and field based bioenergetic
study. Above and below
ground standing stock were
monitored. Waterfowl use
days were estimated. Some
birds were collected and
esophagus
contents
recorded.
Bird density positively correlated with
the SAV distribution. Dabbling ducks
and geese consumed some 362 t of Z.
japonica leaves and rhizomes (50% of
aboveground and 43% of belowground
biomass) at the study site. Lesser
amounts of Z. marina consumed
Baldwin &
Lovvorn 1994a
Grazer
46
CHAPTER 1: General Introduction
Grazer
Seagrass, location,
study type
Description of results
Source
A. acuta (W),
Anas clypeata
(W), A. penelope
(W), A.
platyrhynchos
(W), Anas
strepera (W), A
ferina (W), F. atra
(W), Netta rufina
(W),
Phoenicopterus
ruber (W)
Ruppia maritima. Doñana
Natural Park (SW Spain).
The effect of waterbirds on
the submerged macrophyte
was studied in eleven fish
ponds. Separate exclosure
designs excluded flamingos
or all waterbirds from 3 x 3 m
plots within the ponds to
compare them with control
plots. Four experiments were
conducted for three month
periods at different points of
the annual cycle, with
varying bird densities.
Flamingos and wildfowl (ducks and
coots) had significant negative additive
effects on the presence of aboveground
(leafs and shoots) or belowground
(roots) parts of Ruppia at all times of
the year. For plots where Ruppia was
present, aboveground biomass was
significantly higher in all bird exclosures
than in controls or flamingo exclosures.
Seasonal changes in seedbank
densities were consistent with
consumption by birds. The study refutes
previous suggestions that major effects
of waterbirds are limited to temperate
regions and to periods of early growth
or when major concentrations of
migratory wildfowl are formed in
autumn. Flamingos are important in
structuring shallow wetlands in the
Mediterranean, and possibly many
other regions.
RodriguezPérez & Green
2006
A. crecca (W), A.
penelope (W), B.
bernicla bernicla
(W)
Z. noltii and Z. marina.
Solent, England. Field study,
percent cover recorded at 5
stations. Exclosures used to
monitor seagrass change
due to grazing.
Large reduction in seagrass area
coverage attributed to Brent geese
feeding.
Tubbs & Tubbs
1983
A. crecca (W), A.
penelope (W), B.
bernicla (W)
Z. noltii and Z. marina. Dutch
Wadden Sea. Field surveys
plus an exclosure experiment
were used to quantify
impacts of wildfowl grazing
on seagrass biomass.
Brent geese and widgeon reduced
aboveground biomass 30% faster than
in areas where grazers were excluded.
Belowground biomass in grazed cages
was 48% lower than in ungrazed plots.
Madsen 1988
A. penelope (W),
B. bernicla hrota
(W).
Zostera sp., Strangford
Lough, Northern Ireland.
Field study. The impact of
grazers was documented by
monitoring
changes
in
seagrass biomass in the
study area, along with the
use of exclusion cages in
grassbeds with uniform
coverage
Grazing led to faster rates of seagrass
loss than that due to weathering in
ungrazed areas. Belowground biomass
was 48% lower in grazed plots than in
ungrazed.
Portig et al.
1994
Ampithoe spp. (C)
Syringodium isoetifolium. Fiji.
Laboratory determinations of
ingestion rates of manatee
grass
Initially amphipods fed at the top of the
leaf. One day later they made nests
from fragments of grazed grass.
Grazing rates ranged from 1.7 mg WW
ind-1 day-1 to 26.4 mg WW ind-1 day-1.
Mukai & Lijima
1995
47
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Grazer
Seagrass, location,
study type
Description of results
Source
Arrhamphus
sclerolepis krefftii
(F)
No specific species of
seagrass listed. South east
Queensland,
Australia.
Histological examinations of
A. sclerolepis krefftii were
conducted to determine how
garfish
can
assimilate
nutrients from ingested
seagrass leaves.
This study found mucosal cells in the
alimentary canal of this garfish could
assimilate seagrass.
Tibbetts 1997
Astropyga
magnifica (U)
Z. marina. Tomioka Bay,
Amakusa, Japan. Eelgrass
patch size, density and
biomass used to document
impact of a sea urchin
aggregation on seagrass
density. Urchin gut contents
were also recorded.
A seagrass patch was reduced from 71
to < 3 m2 in three months by grazing.
Urchin stomachs were completely full of
seagrass. No other plants were
observed. The seagrass standing crop
decreased from 7789 g DW to 375 g
DW.
Bak & Nojima
1980
Aythya americana
(W)
Halodule wrightii. Lower
Laguna Madre, Texas. Two
years of field collection and
one experiment at 3 sites
were used to assess impact
of redhead ducks on SAV
biomass.
Rhizome biomass was 75% lower in
grazed areas than where grazers were
excluded. When rhizome biomass was
grazed below 0.18 g DM core-1 (at 1/3
of the sites), grass did not recover.
Mitchell et al.
1994
A. americana (W)
H. wrightii, Chandeleur
Sound, Lousiana. Field
monitoring of seagrass
biomass to document the
impact of waterfowl grazing
on seagrass
Waterfowl grazing was found to reduce
aboveground
and
belowground
biomass by 90% and 49%, respectively.
Michot &
Chadwick 1994
Chelonia mydas
(T)
Thalassia testudinum, Great
Iguana, Bahamas. Field
based observations and
bioenergetic study. A 3 ha
area of turtlegrass was
impounded along with 12
turtles and changes in
seagrass biomass were
noted.
Turtles grazed grass blades by biting
the lower parts of the leaf and allowing
the upper portion to float away, creating
a patch of closely-cropped patches with
leaves averaging 2.5 cm in length. The
grazed areas were recropped while
adjacent stands of tall blades remained
untouched. There were no sharp
boundaries between grazed and
ungrazed areas.
Bjorndal 1980
Diadema
antillarum (U) and
C. mydas (T)
T. testudinum. St. Croix, US
Virgin Is. Field experiments
where changes in seagrass
growth and biomass were
recorded along grazing
gradient.
Turtle grazing had a significant negative
impact on seagrass production. Urchins
were ineffective in controlling the
abundance of seagrass. However,
urchin grazing did increase the rate at
which seagrass biomass turned over
within enclosures.
Zieman et al.
1984
48
CHAPTER 1: General Introduction
Grazer
Seagrass, location,
study type
Description of results
Source
Cygnus olor (W)
and F. atra (W)
Potamogeton crispus. Lake
Stigsholm, Denmark. They
study the impact of grazing
waterfowl on submerged
macrophytes. Two types of
experiment were conducted,
small-scale
exclosure
experiments (1 m2 plots) and
large-scale
enclosure
experiments (100 m2 plots).
In both experiments, shoots
of P. crispus L. were planted
in densities ranging from 1 to
8 m-2.
The herbivorous waterfowl community
foraging in the lake comprised mainly
coots, with densities ranging from 0 to 9
ind ha-1, and a few mute swans, 0.2 ind
ha-1. During the growing season,
macrophyte biomass, shoot length,
shoot number m-2 and shoot height
became significantly greater in
exclosures/enclosures
protected
against grazing by waterfowl than in
unprotected exclosures/ enclosures.
This study provided further evidence
that
waterfowl
may
suppress
macrophyte biomass in lakes with a low
abundance of submerged macrophytes.
Søndergaard et
al. 1996
Dugong
(M)
Halodule uninervis, Nang
Bay,
Moluccas.
East
Indonesia. Observation and
biomass monitoring.
Dugongs were found to remove some
75% of the belowground biomass in the
upper 4–5 cm of sediment. Vegetation
biomass recovered to nearby ambient
levels in just 4–5 months following
grazing during the wet season, no such
recovery was noted during the dry
season.
De Iongh et al.
1995
D. dugong (M)
Zostera capricorni, Halophila
ovalis and H. uninervis,
Moreton Bay, east Australia.
Aerial and boat surveys,
monitoring along with field
experiments to document
dugong grazing on seagrass
habitats.
Dugongs appear to spend most of their
time grazing. In one area, shoot
density, above and belowground
biomass was reduced by 65, 73 and
31% respectively over 3.5 months.
Grazing impacts were variable; in one
area shoot density was reduced by 85%
in 12 days, 95% in 17 days. In another
area, biomass was reduced by 96%
aboveground and 71% belowground.
Preen 1995
Heliocidaris
erythrogramma
(U)
Posidonia australis. Botany
Bay,
Australia.
Field
observations and mapping
from 1930 to 1985 were used
to document the impact of
urchin grazing on a seagrass
meadow.
Urchins completely denuded 20 ha of
seagrass from 1979–1982 before being
dispersed by a storm. Urchin
aggregations reappeared in late ‘82 and
an additional 25 ha of Posidonia was
lost from ‘82–‘84. Up to 1987 no
regrowth had occurred.
Larkum & West
1990
Hyporhampus
melanochir (F)
Zostera
muelleri
and
Heterozostera
tasmanica.
Crib Point, Western Port
Bay, and Duck Point, Corner
Inlet. Victoria, Australia. Field
sampling and stomach
contents.
During the day, green eelgrass tissue
was in the guts of 93% of the fish,
making up almost 70% of the total
volume. Insects, amphipods and shrimp
larvae made up most of the remaining
food. Amphipods were far more
important prey at night. Eelgrass tissue
was consumed by 1/3 of fish and was
only 18% of total volume at night.
Robertson &
Klumpp 1983
dugong
49
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Grazer
Seagrass, location,
study type
Description of results
Source
Hyporhampus
melanochir (F)
Nectocarcinus
inegrifons (C)
Posidonia australis. Victoria,
Australia. Gut content and
biochemical marker analyses
were used to establish the
importance of seagrass
production in the local food
web.
Lipid and sterol analyses found that
both N. inegrifons, and H. melanichir
were found to rely heavily on seagrass
production to meet their nutritional
needs.
Nichols et al.
1986
Hyporhampus
unifasciatus (F)
Ruppia
maritima
and
Halodule wrightii. Crystal
River, Florida. Shallow water
fish collected with a bag in
approximately 1 m of water.
Volume of SAV in gut ̴ 50% in large
halfbeaks.
Carr & Adams
1973
Lagodon
rhomboides (F)
Z. marina. Field sampling
and laboratory bioenergetic
and radioactive labeling
study. Pinfish (> 65 mm SL)
were fed diets of either
eelgrass or frozen grassshrimp.
Assimilation
efficiency for plants (either
eelgrass or algae) and
shrimp
and
labeled
seagrass.
Pinfish found to assimilate a substantial
portion of the organic material from
eelgrass, but with less efficiency than
shrimp. Specific growth rates of pinfish
fed on grass-shrimp partially substituted
with either eelgrass or digestible
carbohydrates were not significantly
different from growth rates when fed
solely on shrimp. Pinfish appeared to
increase feeding rates when offered low
caloric seagrass.
Montgomery &
Targett 1992
L. rhomboides
(F),
Stephanolepis
hispidus (F),
Nicholstina usta
(F) and
Lytechinus
variegatus (U)
T. testudinum, H. wrightii and
Syringodium filiforme. Gulf of
Mexico. They investigate
how consumers of marine
vascular plants discriminate
among
different
food
resources,
using
foodpreference assays carried
out in the laboratory with
seagrass
leaves
and
seagrass incorporated agar
diets of the 3 seagrass
species.
Results showed that S. filiforme was
preferred by all fish species (81, 60.2
and 59% of total leaf consumption of
pinfish,
filefish
and
parrotfish,
respectively), whereas sea urchins
consumed the highest amounts of H.
wrightii (71.2% of total). The study
concluded that structural plant features
(e.g., leaf manipulability and/or visual
recognition of resources) are the most
important factors driving discrimination
between seagrass species
by
omnivorous fish, whereas strict
herbivores make feeding decisions that
are highly influenced by nutritional
characteristics,
presumably
as
recognized by both olfaction and
gustation.
Prado & Heck
2011
L. variegatus (U)
T. testudinum. Offshore
grass beds of west Florida.
Field observations and
measurements.
An episodic settlement of sea urchins
led to significant reductions of seagrass
coverage. Grazing was found to denude
an estimated 20 ha area of seagrass
habitat.
Camp et al.
1973
50
CHAPTER 1: General Introduction
Grazer
Seagrass, location,
study type
Description of results
Source
Mixed grazers
assemblages
including: L.
variegatus (U),
Sparisoma
radians (F),
Archosargus
rhomboides (F),
Monocanthus
setiferus (F),
Acanthurus
chirurgus (F),
Sphaeroides
spengleri (F), and
Acanthostracion
quadricornis (F)
T. testudinum. Kingston
Harbour Jamaica. Laboratory
measurements,
field
sampling, stomach content
analysis
and
field
experimentation to estimate
herbivory in grazers on
seagrass.
Five species of fish found to feed on
both live and detrital seagrass along
with algae and crustaceans. Only the
sea urchins Lytechinus and the bucktooth parrotfish S. radians were found
to feed predominantly on seagrass.
Lytechinus was estimated to consume
some 49% of the SAV leaf tissue
produced each day. A small fraction of
this production was consumed by
fishes.
Greenway 1976
and 1995
L. variegatus (U)
T. testudinum. Card Sound
Florida. Field observations.
A large population of sea urchins
consumed all benthic plants in a several
hectare area of Card Sound
Bach 1979
L. variegatus (U)
T. testudinum. Miskito Cays,
Nicaragua.
Laboratory
measurements,
field
collections and observations.
Feeding
preferences
determined by turning over
urchins. Urchin gut contents
examined at one station.
Sea urchins were estimated to
consume some 0.5 g DW urchin-1 day-1
of seagrass. However, gut contents
indicated that 40% of this urchin´s diet
was detrital turtlegrass. Less than 5% of
the diet was live grasses.
Vadas et al.
1982
L. variegatus (U)
and Tripneustes
ventricosus (U)
T. testudinum. Discovery
Bay,
Jamaica.
Field
experimet tested for intraand interspecific competition
between two species of
urchins.
Aboveground
biomass within cages was
used to document the effects
of urchin manipulations.
Tripneustes grazing had a highly
significant effect on seagrass biomass
enclosure treatments. Lytechinus had a
moderate effect on seagrass biomass.
Keller 1983
L. variegatus (U)
T. testudinum. Biscayne Bay
Florida. Laboratory estimate
of sea urchin ingestion rates
and preference when fed live
seagrass and seagrass
detritus.
Urchins ingested decayed leaves at a
significantly higher rate than when fed
green leaves. No evidence of a
significant preference for decayed
leaves over green ones was found
Montague et al.
1991
51
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Grazer
Seagrass, location,
study type
Description of results
Source
L. variegatus (U)
T. testudinum. St. Joseph
Bay, Florida, northeastern
Gulf of Mexico. Grazing
experiments were conducted
quarterly from fall 1988 to
1989 using cages where sea
urchins were added at
treatment densities of 0, 10,
20, 40 and 80 individuals m-2.
Sea urchin treatment effects
were compared by collecting
three 0.01 m2 clip samples
(3% of cage area) of
aboveground biomass at 6day intervals for 18 days.
The study found that the lowest
densities of sea urchins required to
overgraze (i.e., completely defoliate)
turtlegrass occur during winter
(approximately 20 individuals m-2), while
higher densities (approx. 40 individuals
m-2) are required for overgrazing during
summer and fall. The study concluded
that sea urchin herbivory is important in
regulating
subtropical
seagrass
meadow biomass and size, particularly
where sea urchin densities exceed 20
individuals m-2.
Valentine &
Heck 1991
L. variegatus (U)
T. testudinum. St. Joseph
Bay, Florida, northeastern
Gulf of Mexico. A 4-month
enclosure experiment, using
high (40 ind m-2) sea urchin
densities during winterspring. The treatments were:
(1) continuous grazing for a
period of 4 months, (2)
intermittent grazing, in which
urchins were kept in the
enclosures for only 1 wk out
of 4 during each of the 4
months the experiment
lasted; and (3) no grazing,
consisting of two cages with
no urchins, for 4 months.
Effects were quantified by
clipping
aboveground
seagrass biomass monthly
from three haphazardly
placed 0.01 m2 quadrats
within each cage.
Sea urchin grazing dramatically
affected seagrass habitat structure for
long periods of time (> 3.5 yr).
Intermittent grazing produced significant
reductions in aboveground plant
biomass compared to controls, while
continuous
grazing
produced
apparently
permanent
loss
of
seagrasses.
Heck &
Valentine 1995
52
CHAPTER 1: General Introduction
Grazer
Seagrass, location,
study type
Description of results
Source
L. variegatus (U)
T. testudinum. Florida Keysnortheastern Gulf of Mexico.
In the first experiment, they
varied the duration of sea
urchin grazing bouts in order
to understand the impacts of
temporally varying herbivory
on seasonal changes in
turtlegrass
biomass
in
shallow water (< 2 m) near
Big Pine Key. In the second
experiment, they examined
the effects of chronic low
levels of grazing on seagrass
growth and biomass in a
deeper-water
seagrass
habitat (6 to 7 m) in Hawk
Channel.
The study suggests that the impacts of
sea urchin grazing are highly variable in
the Florida Keys. Depending on the
season, urchin grazing (at densities of
20 ind m-2) had both negative and
positive effects on seagrass biomass at
Big Pine Key. If grazing occurred during
spring, turtlegrass biomass was
significantly reduced by short bouts of
sea urchin herbivory. The impacts of
this spring grazing extended into early
summer. If grazing occurred in summer,
sea urchins reduced turtlegrass
biomass for only short periods of time,
after which urchin grazing stimulated
turtlegrass production. These findings
are similar to those of previous grazing
experiments conducted in St. Joseph
Bay. In contrast, they found little
evidence that ambient (0 to 8 ind m-2)
densities of sea urchins could control
either seagrass production or biomass
in the deeper waters of Hawk Channel.
Valentine et al.
2000
Mediterranean
herbivores
Cymodocea nodosa. The
study
investigated
the
magnitude and variability of
herbivory
(i.e.,
leaf
consumption and sloughing
caused by herbivore bites)
on the seagrass C. nodosa
along
the
Spanish
Mediterranean coast and test
the hypothesis that this is
higher in meadows growing
in sheltered bays than in
exposed, open zones.
Total leaf loss by herbivores varied by
about three orders of magnitude along
the Spanish Mediterranean coast, from
< 1 to 130 mg DW shoot-1 yr-1. These
differences were paralleled by a great
variation in the fraction of leaf
production lost by herbivores, which
ranged from < 1 to about 50%. Most
(75%) of the populations, however,
supported modest losses in leaf
production (< 10%). A significant
fraction (30%) of the variance in
herbivory was explained by meadow
exposure, the meadows growing in
sheltered bays suffering about five
times the losses encountered in open
sites.
Cebrián et al.
1996b
Meuschenia
freycinetti (F),
Meuschenia
trachylepsis (F)
Monacanthus
chinensis (F),
P. australis. Port Hacking,
N.S.W. Australia. Field
sampling and stomach
contents. The entire fish
community in a 400 m2 area
of Posidonia was collected
twice each in 2 seasons.
Stomachs
of
all
leatherjackets
were
dissected and the contents
identified.
The
relative
percentages of food items
were determined. Rectal
items
were
identified
microscopically to determine
which items were food.
Leatherjackets dominated the fish
community, averaging 26% of the
number and 34% of the biomass.
Seagrasses ingested were small pieces
of leaf material covered with epibionts.
M. freycinetti consistently bit off pieces
in neat semicircular bites. Juveniles of
all species fed principally on encrusting
animals listed above with little seagrass
being present. Microscopic rectal
contents from several inds. of each
species showed that Posidonia was
undigested.
Bell et al. 1978
53
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Grazer
Seagrass, location,
study type
Description of results
Source
M. chinensis (F)
P. australis. Quibray Bay,
Botany
Bay,
N.S.W.
Australia. Stomach analysis
and field sampling. 14C
labeled seagrass was used
to
assess
seagrass
assimilation by fishes.
Gut analysis showed fish ate SAV,
along with 5 spp of algae, crustaceans
and other invertebrates. Seagrass and
amphipods were most abundant in fish
guts. Microscopic examination of
ingested plant material suggested that
plants were untouched. (i.e., no cellwall
damage
observed).
However,
radioactive labeling showed that ̴ 22%
of labeled SAV was in the liver and gut
wall of the fish, 32–33% in the feces.
The remaining label may have been in
other tissues. This is significant as it
shows that microscopic examination of
the cell walls does not necessarily
provide a complete picture. The actual
% of SAV production removed was low.
Conacher et al.
1979
Monacanthus
ciliatus (F) and
Stephanolepsis
hispidus (F)
T. testudinum. Apalachee
Bay, FL. Field sampling and
stomach contents of Filefish
collected over a nine year
period.
These fish fed on a wide variety of prey;
however seagrass and invertebrates
accounted for 80% of the stomach
contents. As fish grew, the dietary
importance of seagrasses and
associated
epifauna
increassed.
Approximately 1/2 of the diet of larger
fishes was Thalassia. The pattern was
the same for both species of filefishes.
The incidense of SAV in the diets of
Monocanthus was greatest in late
summer and early fall coincident with
peak SAV productivity. The incidence of
SAV in Stephanolepsis increased
between summer and fall.
Clements &
Livingston 1983
Paracentrotus
lividus (U)
Posidonia
oceanica.
Mediterranean Sea. Field
experimentation used to
determine grazing impact on
seagrass biomass shoot
density and production.
Loss of seagrass biomass was directly
proportional to grazing intensity
Kirkman &
Young 1981
P. lividus (U),
Sarpa salpa (F)
P. oceanica. Southeastern
Spain, Mediterranean Sea.
Field
surveys,
measurements of plant
growth and ambient nutrients
prior to and after the onset of
aquaculture of a seagrass
habitat. Estimates of grazing
intensity were made based
on the frequency of leaf tips
damaged by urchins or fish.
Seagrass coverage diminished greatly
after commencement of aquaculture
activities. Herbivore grazing was found
to be intense near aquaculture facilities
and it was hypothesized that this
increased grazing played a significant
role in the reduction of seagrass
coverage. This elevated level of grazing
was attributed to aquaculture induced
improvements in the palatability of
Posidonia leaves for local herbivores.
Ruiz et al. 2001
54
CHAPTER 1: General Introduction
Grazer
Seagrass, location,
study type
Description of results
Source
P. lividus (U),
S. salpa (F)
P. oceanica. Medes Islands
Marine Reserve (northeast
coast of Spain, northwestern
Mediterranean Sea). They
quantified herbivore density
and
grazing
pressure
through both direct (tethering
experiment) and indirect
(through marks of herbivore
attacks) measurements.
The study found that grazing varied
greatly both temporally and spatially.
However the study also shows that
whereas consumption by the sea urchin
P. lividus was relatively minor, P.
oceanica was intensely grazed by the
fish S. salpa in summer. During this
period, fish are very abundant at a
depth of 5 m, with consumption rates
that temporarily exceed seagrass
production which was at its yearly
minimum. This imbalance causes the
appearance of mowed patches where
seagrass biomass was reduced by
50%. Through direct measurements,
the study revealed that P. oceanica
consumption by herbivores can be
substantial with respect to the total
annual production and much higher
than previously estimated through
indirect measurements.
Tomas et al.
2005a
P. lividus (U),
S. salpa (F)
P. oceanica. Medes Islands
Marine Reserve (northeast
coast of Spain, northwestern
Mediterranean Sea). They
evaluated
by
cage
experiment the effects of
macroherbivores (sea urchin
P. lividus and sparid fish S.
salpa) in a temperate
seagrass (P. oceanica)
meadow by means of 2
macroherbivore
density
manipulation experiments in
which several measurements
were carried out of plant and
epiphyte
vitality,
and
abundance.
The results show that P. oceanica can
withstand high densities of sea urchins
over long periods, as no significant
effect on shoot density, shoot size, leaf
growth or carbohydrate reserves was
detected during the time course of both
experiments. Epiphyte biomass was
greatly reduced when urchins were
present even at relatively low densities
(5 ind m–2), suggesting that epiphytes
are the most limiting food resource for
sea urchins in P. oceanica meadows.
Whereas epiphyte load was only
reduced 30% by the presence of fish
alone, sea urchins at low densities (5
ind m–2) decreased epiphyte load ca.
60%, reaching values as high as 80% in
high density treatments, in which case
the effect of fish was negligible.
Tomas et al.
2005b
55
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Grazer
Seagrass, location,
study type
Description of results
Source
P. lividus (U),
S. salpa (F)
P. oceanica. Sampling was
conducted once per season
in 10 shallow meadows of P.
oceanica (L.) Delile from the
continental
NW
Mediterranean
coast
covering a spatial scale of >
300 km. The rates of
seagrass defoliation exerted
by the herbivorous fish S.
salpa and the sea urchin P.
lividus
were
evaluated
through both direct (tethering
experiment) and indirect (bite
mark) methods.
Results indicated that a large proportion
(ca. 57%) of the annual leaf production
is lost to herbivory, yet with
considerable spatial variation. Patterns
of seagrass defoliation showed high
temporal variability, with a peak in
summer with values that exceeded
about 2.5 times those of leaf
production, with a minimum during the
winter period. On average, defoliation
exerted by S. salpa accounted for 40%
of leaf production (ca. 70% of total
annual losses to herbivory), while P.
lividus was also responsible for a
substantial 17% removal of leaf
production. This study provides
evidence that P. oceanica leaf losses to
herbivores are not marginal, but a
widespread process that occurs at
much higher rates than previously
estimated through indirect methods (ca.
2%), resetting the paradigm of the
negligible importance of herbivory in
temperate systems.
Prado et al.
2007a
P. lividus (U),
S. salpa (F)
P. oceanica. Sampling was
conducted in three healthy
shallow seagrass beds (5–7
m) in the Northeast Spanish
coast subjected to increasing
nutrient availability. They
assess the effect of nutrient
availability on the diet shifts
of
the
two
main
Mediterranean herbivores,
the Sparid fish S. salpa L.
and the sea urchin P. lividus
(Lam.) that feed mostly on
the seagrass P. oceanica L.
(Delile), epiphytes and
benthic macroalgae.
Results suggested that sea urchins
behave as facultative omnivores and
feed on plant or mixed diets depending
on the trophic status of the system. It is
unclear if this modification is behavioral
or the consequence of mere changes in
food item availability, as animal
epiphytes (hydrozoans, bryozoans,
ascidians etc.) can also become more
abundant on seagrass leaves under
increased nutrient conditions. In
contrast, adult fish appear to feed on
plant material independent of nutrient
availability in the ecosystem.
Prado et al.
2010b
56
CHAPTER 1: General Introduction
Grazer
Seagrass, location,
study type
Description of results
Source
S. salpa (F)
P. oceanica. The study was
from May 2003 to June 2004
at a depth of 8 m in a P.
oceanica meadow off the
northeast Spanish coast of
the Mediterranean Sea. They
conducted a field fertilization
experiment and measured
monthly seasonal changes in
fish herbivore behavior
following
alterations
in
qualitative (nutrient content,
species composition) and
quantitative
(biomass
availability) features of
primary producers (seagrass,
epiphytes and macroalgae)
as a result of nutrient
fertilization.
Nutrient addition induced changes in
both seagrass (enhanced plant N
content and leaf growth) and epiphytes
(enhanced N content, biomass load and
altered species composition) throughout
late spring and summer. This nutritional
alteration of food organoleptic
properties concurred within the
seasonal peak of herbivory by the fish
S. salpa and caused disproportionate
herbivore pressure on enriched plots,
until shoots were reduced to around
50% of their original leaf area. When
plant biomass on fertilized plots was
substantially reduced, herbivores fed
mostly on control plots until the end of
the summer period. During the rest of
the year (autumn and winter), moderate
nutrient additions could not disrupt the
seasonal inertia of the ecosystem. The
study concluded that the foraging
behavior of herbivorous fish is the result
of chemical and visual sensing of the
environment.
Prado et al.
2010a
S. salpa (F)
P. oceanica. The study
investigated macroherbivore
feeding preferences through
paired
feeding
assays
performed in the field (bay of
Fornells on the Balearic
Island of Menorca) and in
aquaria and determined the
plant traits that explained
their foraging strategies
Strong within-plant heterogeneity was
found in both seagrass susceptibility to
herbivory and chemical composition,
but different consumers exhibited
contrasting feeding choices. S. salpa
preferred the most nutritious and
chemically defended younger leaves,
suggesting a full adaptation to
consuming this macrophyte and a
greater impact of this herbivore on the
plant. In contrast, P. lividus consistently
preferred the older leaves covered by
epibionts. Artificial diet experiments
showed that morphology and fine-scale
structural defenses were the primary
determinant of urchin feeding choices,
with nutrient content and chemical
defenses of secondary importance.
Epibiosis did not strongly influence fish
feeding, but it did have a strong
‘shared-doom’ effect on urchin
consumption. This effect was driven by
a distinct preference towards a mixed
diet that included both host tissues and
their epibiotic community.
Vergés et al.
2011
57
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Grazer
Seagrass, location,
study type
Description of results
Source
Scarus spp. (F),
Sparisoma spp.
(F), Acanthurus
spp. (F)
T. testudinum, H. wrightii. US
Virgin
Islands.
Field
experiment,
stomach
contents and observation.
Three
separate
transplantations of mixed
plots.
Thalassia
and
Halodule were within a halo
zone next to a coral reef
used to assess the impact of
herbivores on seagrass
abundances. In addition, an
artificial reef was built in a
mixed
Turtlegrass
and
Halodule habitat.
Parrotfish totally consumed seagrass
patches transplanted into a halo zone
next to coral reef. Parrotfish (Scarus
and Sparisoma) all seem to feed to
some degree on the grass, S.
guacamaia had 95% of their gut volume
filled with Halodule. Acanthurus
chiurugis and A. bahamensis had 40
and 80% gut volume filled with
seagrass.
Randall 1965
Scarus
guacamaia (F)
and Sparisoma
radians (F)
T. testudinum. St. Croix, US
Virgin Is.: Seagrass leaves
were collected and fish bite
marks identified along a
transect running from the
base of a coral reef into an
adjacent seagrass habitat.
Leaves collected closest to a reef
showed bites resulting from a
population of large parrotfishes (S.
guacamaia) whereas the station 20 and
60 m from the reef had bites
characteristic of S. radians. The station
4 m from the reef showed mixed
feeding.
Ogden &
Zieman 1977
Scarus croicensis
(F), Sparisoma
aurofrenatum (F),
Acanthurus
chiurugus (F), A.
bahianus (F).
S.
filiforme
and
T.
testudinum. San Blas Island,
Panama. Field tethering
study measured both feeding
selectivity and intensity.
Each species of seagrass was heavily
grazed but herbivory on the grasses
was variable spatially.
Tribble 1981
Scarid and
siganid fishes
Enhalus
acoroides,
Thalassia
hemprichii,
Halodule
uninervis,
Cymodocea
rotundata,
Syringodium
isoetifolium.
Palau, Western Caroline Is.
Field based monitoring.
All samples of Thalassia and
Cymodocea had bite marks. At one site
approx. 30–40% of leaves of all species
except Enhalus, had at least one bite
taken. Enhalus had marks on at least
75% of blades.
Ogden &
Ogden 1982
Sparisoma
chrysopterum (F)
and Sparisoma
rubripinne (F)
T. testudinum. Carrie Bow
Cay, Belize. Field tethering
study using clean fleshly
collected pieces of Thalassia
blades with algal species
Each study found that herbivore fishes
readily consumed seagrass leaves but
the intensity varied according to coral
reef habitat and depth. Lewis also found
that tethered Thalassia was entirely
consumed by two parrotfish, S.
rubripinne and S. chrysopterum. Lewis
also found that Thalassia was among
the preferred sources of food during
trials.
Hay 1981;
Lewis 1985
58
CHAPTER 1: General Introduction
Grazer
Seagrass, location,
study type
Description of results
Source
Sparisoma
radians (F)
T. testudinum. Northern
Florida Keys. Field tethering.
Study that compared tissue
losses to bucktooth parrotfish
with
net
aboveground
turtlegrass production at
multiple sites.
This study found that grazing intensity
varied greatly both spatially and
temporally. Some 80% of annual net
aboveground turtlegrass production
was consumed.
Kirsch et al.
2002
S. radians (F)
T. testudinum. Florida Keys
National Marine Sanctuary.
Feeding
preference
experiments
for
the
turtlegrass T. testudinum
with high or low nitrogen
concentrations in both field
and laboratory experiments
using tethered shoots (Kirsch
et al. 2002).
In field choice experiments, they found
that S. radians consumed a greater
proportion of T. testudinum shoots with
high nitrogen (N) concentrations than
shoots with low N concentrations (68 vs
5%).
Similarly,
in
laboratory
experiments, S. radians consumed
more T. testudinum with high N than
with low N concentrations (25 vs 5%).
When they repeated laboratory
experiments using an agar mixture with
powdered T. testudinum leaves of
either high or low N concentration they
found that S. radians again showed a
significant preference for the high N
food source. They conclude that
mechanisms for choice of high N food
were most likely related to gustation
and/or olfaction of chemical cues (e.g.,
N or phenols).
Goecker et al.
2005
Tectura depicata
(G)
Z. marina. Monterey Bay,
California. Lab. Experiment.
Zostera transplanted into
plastic flower pots, at natural
densities of limpets were
maintained on eight plants
while 8 others kept grazer
free. Seagrass growth was
determined weekly along
with total leaf length. At the
end of the experiment, plants
were
harvested
and
analyzed for biomass (shoot,
rhizome, root) rates of leaf
photosynthesis, respiration
and sucrose. Enzymes were
measured in leaves and
shoots, plus protein and
sugar contents. Chlorophyll a
was extracted from leaf
segments.
Growth rates, carbon reserves, root
proliferation and net photosynthesis of
grazed plants were 50–80% lower than
on ungrazed plants. The carbon
allocated to the roots of ungrazed
plants was 800% higher for ungrazed
plants that for grazed plants. Limpet
grazing induced carbon limitation in
eelgrass growing in an otherwise light
replete environment.
Zimmerman et
al. 1996
59
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Grazer
Seagrass, location,
study type
Description of results
Source
Temnopleuris
michaelsenii (U)
Cockburn Sound, Warnbro
Sound, Australia. Field
sampling and seagrass
mapping were used to
document urchin denudation
of a seagrass habitat.
In Cockburn Sound, seagrasses were
grazed by locally abundant T.
michaelsenii. The most heavy damage
was localized. Where grazing was
heavy, plants had not recovered 2–4
years later. Urchins invaded a second
site reducing remnants of a once
healthy seagrass meadow to bare sand.
Intense grazing was noted in fall of
three different years. Outbreaks were
also reported from a third site, where
sea urchins removed all of the leaves in
deeper portions of a seagrass bed.
Cambridge et
al. 1986
Trichechus
manatus (M)
S. filiforme, Cape Canaveral,
Florida.
Field
experimentation.
Percent
cover and aboveground
biomass used to document
herbivore
impact
on
seagrass.
Grazing led to significant reductions in
coverage and biomass and leaf lengths
aggregated but their distribution was
positively correlated with Syringodium
and Halodule density
Provancha &
Hall 1991
Tripneustes
gratilla (U)
Halophila stipulacea. Sinai,
Northern Red Sea and the
Jordanian coast of the Gulf
of Aqaba. Field observation.
Heavy urchin grazing was recorded on
seagrass at depths ranging from 5 m to
9 m. This was subsequently verified by
gut content analysis.
Lipkin 1979;
Bouchon 1980;
Hulings &
Kirkman 1982;
Jafari &
Mahasnesh
1984
T. gratilla (U),
Salmacis
phaeroides (U)
Thalassia
hemprichii.
Bolinao, Philippines. Field
and
laboratory
measurements of sea urchin
consumption of seagrass
biomass. Food preferences
for several plant species also
examined.
Preference tests showed Tripneustes
chose live SAV alternative food
choices. Salmacis consumed equal
quantities of all plant species. Both
urchins efficiently digested and
absorbed seagrass (> 60%). Estimates
of total SAV consumption by both sea
urchins was 240–400 g DW m-2 day-1,
an average of ̴17% of SAV produced
with a range from 3–100% of SAV
production.
Klumpp et al.
1993
T. gratilla (U)
Thalassodendron ciliatum.
Mombassa lagoon, Kenya.
Multiple
sampling
approaches
and
mathematical estimates were
used to determine the
degree to which sea urchins
affect seagrass density and
coverage
Grazing urchins were found to exert a
controlling influence over T. ciliatum
density in Mombassa lagoon.
Alcoverro &
Mariani 2002
60
CHAPTER 1: General Introduction
1.2. Factors driving abundance
and structure of macrophytes in
aquatic ecosystems: Top-down
vs. Bottom-up control theories
In ecological studies of macrophyte
ecosystems the concepts ‘top-down’
and ‘bottom-up’ controls have long
been used (e.g., Atkinson & Grigg
1984; Carpenter et al. 1985), to
describe mechanisms where either
the action of consumers on their food
population (top-down) or resource
availability to populations (bottomup) regulate the structure of aquatic
communities (Fig. 1.4). This
reasoning can be summarized by
saying that resource supply may set
the overall potential level of
abundance of a population, which is
modified by the action of consumers
(Valiela 1995). These opposing but
complementary concepts can be
particularly useful in understanding
complex
macrophyte-herbivore
interactions taking place in different
ecosystems.
The dilemma arises because several
factors linked to both concepts such
as light, temperature or nutrients
(bottom-up) and competition and
herbivory (top-down) can influence
the structure and dynamics of
macrophyte communities. These
factors
act
synergically
(e.g.,
Burkepile & Hay 2006) and exhibit
strong spatial (e.g., depth-related,
biogeographical) and temporal (e.g.,
daily, seasonal) variations. In
temperate seas (Hruby 1975; Littler
1980) and also some tropical seas
(Croley & Dawes 1970; Mathieson
& Dawes 1975; Ngan & Price 1980),
seasonal patterns involving changes
in macrophyte species composition
and abundance have been observed.
In general, macrophyte responses to
environmental parameters (bottomup control: temperature, nutrients,
light) are reflected in a common
pattern of biomass seasonality,
increasing in spring, with a
maximum in the summer and
decreasing in autumn, with a
minimum in the winter (Ballesteros
1991, 1992; Alcoverro et al. 1997).
Deviations from this common
pattern have been related to
differences in herbivore activity (topdown control). The paradigm of
herbivory is to understand the
relative dominance of bottom-up vs.
top-down factors in driving the
functioning of each macrophyte
ecosystem.
61
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
A prevailing view is that the
increasing eutrophication of bays and
estuaries has indirectly triggered
global
reductions
of
seagrass
meadows via the overgrowth of
seagrasses by the nutrient-induced
proliferation of fast-growing algae
(Duarte 1995; Bricker et al. 1999;
Howarth et al. 2000; NAS 2000;
Hauxwell et al. 2001). This
explanation is most often proposed to
account for the loss of seagrasses in
North America (Orth & Moore
1983; Neundorfer & Kemp 1993;
Short et al. 1995; Tomasko et al.
1996), Europe (Giesen et al. 1990;
Den Hartog 1994), and Australia
(Cambridge & McComb 1984;
Shepherd et al. 1989).
In contrast, Heck & Valentine
(2007)
suggested
that
the
accumulation of plant biomass in
shallow benthic habitats is more
likely to be controlled by consumer
effects (top-down control) than by
HUMANS
(Fishing; Hunting)
-
-
+
-
PREDATORS
- +
GRAZERS
-
+
EPIPHYTES
+
+
MACROPHYTES
+
+
-
+
+
FLOWERS/FRUITS/SEEDS
+
LIGHT, TEMPERATURE, NUTRIENTS
Figure 1.4. Schematic illustration of simplified interaction in macrophytes aquatic ecosystems.
Plus and minus signs indicate positive and negative effects, respectively, that one class of
organism has on another, in the direction of the arrow and following bottom-up or top-down
cascade effects (adapted from Valentine & Duffy 2006 in Larkum et al. 2006).
62
CHAPTER 1: General Introduction
nutrients (bottom-up control). They
criticized that the paradigm of
nutrient-based
seagrass
decline
summarized by Duarte (1995) and
others (Bricker et al. 1999; Howarth
et al. 2000; NAS 2000; Hauxwell et
al. 2001) was largely based on data
from either observational studies or
experimental studies that did not
include manipulations of grazers in
their design. They pointed out that
when algal grazers (primarily
mesograzers
and
some
small
herbivorous fishes) were included in
study designs, grazing effects always
explained at least as much, or more,
of the variance in algal abundance
than did nutrient enrichment
(Neckles et al. 1993; Williams &
Ruckelshaus 1993; Lin et al. 1996;
Heck & Valentine 2006; Valentine
& Duffy 2006;).
This difference in emphasis is
another example of the on-going
debate among ecologists about the
relative importance of top-down and
bottom-up factors in controlling
ecosystem structure and function (see
Williams & Heck 2001; Valentine &
Duffy 2006 for recent discussions in
a marine context). Although it is
understood by many investigators
that both top-down and bottom-up
factors can act in concert to
determine the structure and function
of coastal ecosystems (Lotze et al.
2006), more research is needed to
better understand whether both
effects are responsible for the changes
in
macrophytes
presented
communities. Among the several
factors associated with both control
effects, during this thesis, we focused
on studying herbivore-macrophyte
interactions and potential factors
involved in consumption and
selectivity by herbivores
1.2.1. Influence of herbivore
abundances, distribution and
feeding behavior
Many studies have shown that
herbivory can be highly variable
through space and time, displaying
different patterns of defoliation
between meadows and/or seasons
(Tomas et al. 2005a; Prado et al.
2007a, 2008a, 2010a; Steele et al.
2014). This variability has been
partially attributed to changes in
herbivore abundance and distribution
due to interactions between factors
such as migration events (Lodge et
al. 1998), recruitment rates (Camp et
al.
1973),
predation
effects
(McClanahan
et
al.
1994),
63
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 1.5. Migratory concentration of coots in a wetland during winter
overfishing (Klumpp et al. 1993,
Prado et al. 2008a), hunting (Fox &
Madsen 1997) and movement
patterns (Jadot et al. 2002, 2006), all
depending on the herbivore species.
Migration
events
have
been
suggested as the main factor
influencing seasonal variation in
waterfowl abundances in aquatic
ecosystems (Lodge et al. 1998;
Rodriguez-Perez & Green 2006). It
has been suggested that in temperate
areas of North America, Europe and
New Zealand, waterfowl display
increasing population in autumn and
winter as a result of migratory
concentration, which can drive
important impacts on the submerged
vegetation (Lodge et al. 1998,
Marklund et al. 2002) (Fig. 1.5). In
seagrass ecosystems, populations of
macroherbivores such as fishes and
sea urchins seem to be mainly driven
by recruitment rates (Camp et al.
64
1973),
predation
effects
(McClanahan et al. 1994) or
overfishing (Klumpp et al. 1993). In
sea urchin populations, larval
availability
and
post-settlement
mortality are perhaps the most
important bottlenecks for the
abundance of adult echinoids
(Rumrill 1990; Lozano et al. 1995;
López et al. 1998). Predation seems
to play a major role; it can reduce the
densities of recently metamorphosed
individuals from hundreds to only a
few individuals per square meter
(Turón et al. 1995; López et al.
1998). Indeed, it has been suggested
that the abundance of the sea urchin
Paracentrotus lividus in shallow
habitats seem to be affected by
availability of refuges from predators,
with the highest densities recorded in
rocky habitats (Delmas & Régis
1986; Delmas 1992) (Fig. 1.6). In
seagrass beds, P. lividus abundances
CHAPTER 1: General Introduction
Figure 1.6. Paracentrotus lividus in a Mediterranean rocky habitat.
are commonly higher in Posidonia
oceanica (Boudouresque & Verlaque;
2001, 2013) than in Cymodocea
nodosa (Fernandez & Boudouresque
1997), which may be due to
variability in predation pressure
(Traer 1980), and/or to reduced
meadow recruitment in the absence
of significant rhizome structural
complexity (Prado et al. 2009).
Regarding the fish Sarpa salpa,
studies have shown that populations
benefit from fishing protection,
resulting in enhanced grazing
pressure over seagrass meadows
(Prado et al. 2008a).
Habitat size can also play an
important role in plant-herbivore
interactions. Herbivore densities and
associated grazing pressure can either
increase or decrease with decreasing
availability of plant resources. A
commonly observed pattern is that
herbivore densities increase with
increasing habitat size, potentially
driven by increasing resource
availability (Tahvanainen & Root
1972; Root 1973). This trend
assumes that herbivores are able to
move freely across the landscape
mosaic and can modify their foraging
behavior
and
distribution
to
maximize resource acquisition at the
landscape level.
65
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Food
resources
are
generally
distributed heterogeneously; both in
time and space, and foraging animals
show flexible responses to this
heterogeneity. This flexibility is
expressed at different levels: animals
can turn to a new habitat, select a
different patch within a habitat
and/or select different items within a
patch (reviews in Stephens & Krebs
1986; Hughes 1993; Sutherland
1996). In fact, changes in diet and
foraging
behavior
have
been
previously reported (Guillemain &
Fritz 2002; Guillemain et al. 2002;
Dessborn et al. 2011) as a way of
coping with seasonal changes in
abundance or availability of food
within the habitat (Rodríguez-Pérez
& Green 2006) or to respond to the
differential nutritional requirements.
For example waterfowl are known to
attain higher consumption of
carbohydrate-rich seeds in wintering,
possibly in order to face winter while
during breeding seasons they
increased feeding on protein-rich
food (Baldassarre & Bolen 1984;
Hohman et al. 1996). Other
herbivores such as S. salpa feed more
actively in summer, in order to
accumulate reserves for the winter
66
period, when they
(Peirano et al. 2001).
reproduce
Competition can also trigger
different
behavioral
responses:
individuals will turn to alternative
patches
with
lower
resource
abundance to avoid the negative
effects of interference at high
densities of competitors in the best
patches (Guillemain & Fritz 2002).
This crowding behavior is also
associated with reduced predation
pressure in small habitat fragments
(Kondoh
2003),
differential
migration behavior of herbivore
species (Bowman et al. 2002) and
distinctive sensorial developments to
detect the available vegetation
resources during the foraging process
(Bukovinszky et al. 2005).
Overall, foraging behavior is an
ensemble of complex feeding
decisions aimed at optimizing the
intake of energy and essential dietary
elements within the physical and
chemical
features
of habitats
(Stephens & Krebs 1986). In marine
organisms, foraging behaviors have
been most often investigated from
the perspective of predator species
that seek scattered and often mobile
prey of high nutritional value (Carr
CHAPTER 1: General Introduction
et al. 1996; Zimmer & Butman
2000; Weissburg et al. 2002). In
contrast,
comparatively
less
information is available on how
herbivores choose food resources,
possibly because they are believed to
face an apparent surplus of food and
to be mainly influenced by qualitative
plant characteristics (Prado & Heck
2011). While such behavior, and the
resulting distribution and grazing
patterns, have been frequently
explored in terrestrial ecosystems, less
is known concerning marine and
aquatic ecosystems. Given their
potential
influence
on
the
distribution and abundances of
macrophytes (see reviews by Steneck
& Sala 2005; Valentine & Duffy
2006), studies should incorporate
feeding behavior observations for a
better understanding of herbivoremacrophyte
interactions
across
different ecosystems.
1.2.2. Influence of nutritional
content in food selectivity
Plant nutritional quality (often
expressed as leaf nitrogen content)
has been shown to play a central role
in determining herbivore feeding
patterns
in
several
terrestrial
communities (e.g., Onuf et al. 1977;
Slansky & Feeny 1977; Kraft &
Denno 1982; Coley 1983). An
explicit assumption of most foraging
models is that consumers are able to
choose among potential diet sources,
depending on N and essential amino
acid
contents
(Tenore
1977;
Greenstone 1979).
Many insect herbivores preferentially
graze on nitrogen-rich leaves
(McNeill & Southwood 1978;
Schroeder 1986; Athey & Connor
1989) which confers them higher
survivorship, faster growth, and
higher levels of fecundity than
grazers on plants with lower leaf
nitrogen concentrations (Rausher
1981; Raupp & Denno 1983;
Schroeder 1986). These findings
have led researchers to hypothesize
that low leaf nitrogen concentration
is a form of plant defense against
grazing by specialist herbivores
(Feeny 1970; Schroeder 1986;
Augner 1995).
Several studies suggest that there is a
significant
positive
relationship
between leaf nitrogen content and
vertebrate grazing (McGlathery
1995; Preen 1995, but see Cebrián &
Duarte 1998). In those ecosystems,
since epiphytes and macroalgae have
67
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
typically lower C:N ratios than
seagrasses (Duarte 1992, 1995;
Alcoverro et al. 1997, 2000) they are
thought to sustain a comparatively
higher herbivory pressure (Tomas et
al. 2005b, 2006), which may be
additionally enhanced by nutrient
availability.
However,
some
herbivores such as certain parrotfish,
green turtles, and dugongs can show
a consistent preference for exclusive
seagrass grazing despite local
availability of other plant resources
(Bjorndal 1980; Williams 1988;
McGlathery 1995; Goto et al. 2004).
Since herbivores do not always have
the opportunity to choose between
plants of varying nitrogen content,
some of them may thus have evolved
morphological
adaptations
or
digestive capabilities to ensure
nitrogen intake while feeding on low
quality plants (Terra et al. 1987;
Simpson & Simpson 1990; Slansky
1993). Potential strategies include
increasing the length of feeding bouts
and/or
increasing
rates
of
consumption to compensate for
shortages of nitrogen in their forage
(Price et al. 1980; Clancy & Fenny
1987). This has been observed in L.
variegatus which feeds at higher rates
when offered seagrass leaves of lower
68
leaf nitrogen content (Valentine &
Heck 2001). Overall studies suggest
that different species may be
distinctively affected by nutrient
conditions; therefore low nutritive
value by itself may not always be an
effective defense against grazing
(Boyd & Goodyear 1971; Mattson
1980; Moran & Hamilton 1980).
1.2.3. Influence of chemical and
structural deterrence in food
selectivity
In addition to nutrient content, a key
defense strategy used by plants to
minimize leaf loss to multiple
consumers involves a group of traits
that reduce plant quality as food for
herbivores, due to the amount of
chemical components such as
phenolics (Mariani & Alcoverro
1999; Verges et al. 2007a,b, 2011),
which are known to deter feeding in
algae and terrestrial plants, or varying
levels of structural carbohydrates
(cellulose) in seagrass leaves, which
may affect food digestibility and
absorption (e.g., Klumpp & Nichols
1983; Fritz & Simms 1992). These
plant quality traits not only vary
greatly between different plant
species, but can also differ
substantially between different parts
CHAPTER 1: General Introduction
of the same plant and critically
mediate plant-herbivore interactions
(Raupp & Denno 1983; Poore 1994;
Orians et al. 2002; Taylor et al. 2002;
Vergés et al. 2007a,b, 2011).
This can also be applied to
macroalgae which are known to deter
herbivores by using chemical
defenses (Hay & Fenical 1988;
Erickson et al. 2006). For example,
caulerpenyne
is
a
secondary
metabolite synthesized by species of
the genus Caulerpa; which plays a
major role in their chemical defense
(Paul & Fenical 1986; Pohnert &
Jung 2003) against epiphytes and
herbivores (Erickson et al. 2006).
However, there are also examples in
which caulerpenyne does not deter
fish grazing (Meyer & Paul 1992),
indicating that the toxicity is not
intrinsic to the compound, but is a
result
of
metabolite-consumer
interactions (Paul 1992).
These findings suggest that marine
herbivores
may
have
evolved
morphological
adaptations
or
digestive capabilities that allow them
to obtain nutrients and energy from
marine vascular plants. In fact some
fishes have been shown to have low
gut pH, allowing them to digest
cellulose (Lobel 1981; Lobel &
Ogden 1981; Montgomery &
Targett 1992), while other fishes,
reptiles, and sirenians possess
microbial symbionts capable of
digesting cellulose in seagrass leaf
tissues (Bjorndal 1979; Thayer et al.
1984; Luczkovich & Stellway 1993).
In addition, many species of marine
invertebrates possess cellulase that
could allow them to digest seagrass
cellulose (Klumpp & Nichols 1983).
General
studies
confirm
the
defensive value of macrophyte
chemical and structural composition
in feeding decisions. However, they
also provide evidence that since
marine herbivores may have evolved
different morphological adaptations
to feeding on macrophytes it cannot
be generalized, as the compounds
that deter feeding in one consumer
may have no effect on another
(Vergés et al. 2007a,b, 2011).
Therefore the defensive value of the
chemical composition of a given
macrophyte species is a specific
function of both the secondary
metabolites present and the herbivore
species
coevolving
with
the
macrophyte.
69
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
1.2.4. Influence of flower and seed
availability
To date, studies on herbivory have
mainly focused on consumption of
vegetative tissues (see Heck &
Valentine 2006). However, there is
increasing evidence that herbivores
can also reduce the reproductive
success of seagrasses through direct
consumption
of
inflorescences
(Holbrook et al. 2000; Piazzi et al.
2000; Balestri & Cinelli 2003) and
through
post-dispersal
seed
predation (Fishman & Orth 1996;
Holbrook et al. 2000; Nakaoka 2002;
Orth et al. 2002).
A marked preference of herbivores
for plants bearing abundant flowers
and/or developing fruits (Fig. 1.7)
has been suggested as eventually
leading to a reduction in the number
of seeds produced by these plants
(Herrera et al. 2002) and could
strongly impact the reproductive
success of macrophytes. By selecting
the most palatable species or their
reproductive structures, herbivores
can have a strong qualitative effect on
the structure of plant communities
(Rodriguez-Villafañe et al. 2007) and
become the central force driving
species’ composition in some aquatic
ecosystems (Bonser & Reader 1995;
Rachich & Reader 1999).
Seed predation is also an important
process regulating the dynamics of
plant populations (Janzen 1971;
Wenny 2000). For terrestrial plants,
Figure 1.7. Flowers of A. Ruppia cirrhosa and B. Potamogeton pectinatus.
70
CHAPTER 1: General Introduction
seed predation can be significant,
reaching almost 100% of seed
production in some cases (Crawley
1992; Wenny 2000; Clarke &
Kerrigan 2002). In marine vascular
plants, for which seed dispersal is
facilitated by buoyancy and water
movement (Orth et al. 2006a), losses
due to predation in the habitats to
which seeds disperse have been
poorly documented (Orth et al.
2006b). Predation on aquatic
macrophyte seeds or dispersing
propagules has been demonstrated
for a few seagrass species, and has
been attributed to a variety of
crustacean taxa, to fishes (Fishman &
Orth 1996; Holbrook et al. 2000;
Nakaoka 2002; Orth et al. 2002) and
also to waterfowl, especially ducks
(see Kear 2005; Baldassarre & Bolen
2006). In general these suggest that
seed predation may be important in
governing
seed
survival,
and
ultimately seedling establishment
(Fishman & Orth 1996; Holbrook et
al. 2000; Lacap et al. 2002; Orth et
al. 2002). However, seed survival
may be also influenced by such
factors as macrophyte structural
complexity (e.g., shoot density, leaf
morphology) and density of predators
in different habitats.
1.2.5. Influence of epiphyte
availability
Seagrass leaves sustain an important
epiphytic community (Fig. 1.8),
which in the case of P. oceanica can
contribute up to 40% of the total
biomass of canopies (Mazzella & Ott
1984; Romero 1989), and consists
mainly of crustose red algae
Hydrolithon spp. and brown algae of
Figure 1.8. Epiphytized leaf of Cymodocea nodosa.
71
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
the genera Myrionema, Giraudia and
Cladosiphon
(Ballesteros
1987;
Romero 1989). Some animals such as
hydrozoans and bryozoans also
appear, especially in deeper waters
(Ballesteros 1987; Lepoint et al.
1999).
Epiphytes are important contributors
to the primary production of seagrass
ecosystems and exhibit significant
seasonal fluctuations in biomass and
productivity (Penhale 1977; Klumpp
et al. 1992; Mazzella et al. 1992).
Epiphyte growth is influenced by the
same abiotic factors that influence
the seagrass host (e.g., irradiance,
temperature and dissolved inorganic
nutrients: Harlin 1980; Penhale &
Thayer 1980; Borum 1985; Mazzella
et al. 1989; Williams & Ruckelshaus
1993). In addition, biotic factors play
a significant role in shaping the
epiphyte community, notably leaf
turnover of the seagrass host
(Romero 1989) and grazing (Neckles
et al. 1993; Williams & Ruckelshaus
1993), since algal epiphytes have
been found to be more attractive food
for herbivores than the seagrass
leaves themselves (Kitting et al.
1984).
72
In Mediterranean seagrass meadows,
the sea urchin P. lividus has been
reported to feed preferentially on
these epiphytes rather than on plant
material (Verlaque & Nédelec 1983;
Nédelec & Verlaque 1984; Shepherd
1987; Tomas et al. 2005b), which
could be attributed to their higher
nutritional quality (lower C:N ratios)
in comparison to seagrasses (Duarte
1990; Alcoverro et al. 1997 for
epiphytes, Alcoverro et al. 2000 for
seagrasses; Tomas et al. 2006). The
other main macroherbivore in P.
oceanica meadows, S. salpa, also feeds
on seagrass leaves and their
epiphytes, and likewise, epiphytes
appear to be an important part of its
diet (Verlaque 1981; Velimirov 1984;
Havelange et al. 1997). Based on
this, epiphytes have been suggested
to be a key factor in the relationship
between herbivores and seagrass
(Tomas et al. 2004, 2005b, 2006).
1.3. Methods employed in
herbivory studies
1.3.1. Grazing intensity estimation:
indirect vs. direct methods
Methodological differences among
the varying approaches used to
calculate rates of seagrass herbivory
CHAPTER 1: General Introduction
may explain the wide range of
grazing estimates in the literature
(Table 1.1). In addition, early
seagrass ecologists followed the lead
of their terrestrial counterparts and
relied predominantly on the use of
indirect approaches (e.g., counting
grazing marks on leaves, comparing
leaf density or biomass at grazed and
ungrazed sites or laboratory ingestion
studies), rather than direct field
estimates of grazing rates on seagrass
leaves (see revision by Heck &
Valentine 2006). Indirect approaches
to estimate grazing on seagrasses
have consistently led to low rates of
herbivory, all of them being based on
static measures of leaf loss to grazers
made once or a few times annually
(Cebrián et al. 1996a,b).
In one common indirect approach,
the estimates of leaf damage caused
by herbivores are made on
haphazardly selected shoots collected
once a year (Cebrián et al. 1996a).
These estimates are compared with a
real estimate of aboveground
production at the same site and the
ratio of the estimate of grazerinduced leaf tissue loss to production
is used to estimate grazing intensity.
Estimates of grazing using this
method are bound to be low as they
do not account for leaves completely
consumed by grazers (Lowman
1984), nor do they account for
grazer-induced shoot mortality.
In other cases, grazing estimates are
based on comparisons of the expected
lengths of undamaged leaves with the
actual lengths of leaves collected at a
given sample site. The difference
between the actual and expected
lengths of leaves is assumed to be the
amount of leaf tissue lost to grazing.
In this, as well as the previous
method, the numbers of herbivores
are not counted nor are the
proportions of leaf tissue that is lost
to disturbances (such as storms)
quantified (Cebrián et al. 1996b).
Such indirect calculations are also
unable to account for grazer-induced
increases in plant production or leaf
turnover rate (e.g., Zieman et al.
1984; McNaughton et al. 1996).
This is significant since increased
rates of aboveground production (via
either increased production by
surviving shoots or via increased
recruitment of new shoots) can
prevent accurate measurements of
production consumed from grazed
and ungrazed plots (Valentine &
Heck 1999).
73
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Nowadays some studies have reassessed herbivory rates through
direct methods such as tethering
experiments and demonstrated that
herbivory can be important in some
macrophyte ecosystems (e.g., Kirsh et
al. 2002; Tomas et al. 2005a; Prado
et al. 2007a; Taylor & Schiel 2010).
In the first study, Kirsch et al. (2002)
used a digital scanner and
commercially available software to
directly estimate the amount of
turtlegrass (T. testudinum) tissue lost
to small parrotfish (Sparisoma radians
[Valenciennes]) in the Florida Keys
(Fig. 1.9). The losses in leaf tissue
area were then compared to the
amount produced. From these
efforts, Kirsch et al. (2002) found
that, on average, fishes consumed
some 80% of the net aboveground
production of turtlegrass. Using
similar techniques, Tomas et al.
(2005a) provided evidence that
herbivory by S. salpa in undisturbed
P. oceanica meadows can be
substantial, with defoliation reducing
up to 50% of plant biomass in some
areas of the Mediterranean Sea. Also
by direct measurement, in a later
study employing direct measurement
Prado et al. (2007a) confirmed that a
large proportion (ca. 57%) of the
74
annual leaf production is lost to
herbivory, yet with considerable
spatial variation. On average,
defoliation by S. salpa accounted for
40% of leaf production (ca. 70% of
total annual losses to herbivory),
while P. lividus was also responsible
for a substantial 17% removal of leaf
production.
Wide discrepancies encountered
when comparing direct and indirect
measurements suggest that the latter
are inappropriate to achieve accurate
estimates of herbivory pressure.
Through direct methods, studies
show that in some cases herbivory
can be strong and determine the
structure
and
distribution
of
temperate macrophyte assemblages
(e.g., Tomas et al. 2005a; Taylor &
Schiel 2010) but also that herbivory
can be highly variable through space
and time, displaying different
patterns of defoliation on comparing
meadows and/or seasons (Prado et al.
2007a, 2008a, 2010a; Steele et al.
2014). Nevertheless, further studies
are required to re-evaluate the
comparative importance of herbivory
on different macrophyte species and
the ecological implications for the
functioning of different macrophyte
ecosystems.
CHAPTER 1: General Introduction
Figure 1.9. Example of method used by
Kirsch et al. (2002) to estimate the daily
amount of leaf tissue lost to grazers. Leaves
were digitally scanned before and after
deployment for 24 h, and the daily amount
of tissue lost was calculated.
1.3.2. Food choice experiments
Feeding
patterns
of
aquatic
herbivores can be further affected by
several factors such as competition or
predation, food availability or food
quality, although their relative
importance is often difficult to assess
(Jormalainen et al. 2001). In general,
herbivores search for energetic
compounds and nutrients, such as
proteins, nitrogen and carbohydrates,
and the availability of these primary
compounds is therefore expected to
strongly affect food selection
(Mattson 1980).
how food ingestion previously
observed in the field (e.g., by
tethering experiments and feeding
observation) could be related to food
selectivity or preference by herbivores
(Vergés et al. 2007a,b; 2011). This
methodology allows other factors to
be isolated (e.g., predation), that may
interfere with feeding behavior in the
field, and also enables testing how
factors related to food quality can
influence herbivore decisions (e.g.,
Tomas et al. 2011a). In recent years,
food choice experiments have
demonstrated that features related
with food quality such as secondary
metabolites, structural defenses and
nutritional content in seagrasses can
influence
feeding
decisions
differently, depending on the
herbivore species (Vergés et al.
2007a,b; Prado & Heck 2011).
These experiments may also help us
understand patterns of consumption
recorded by tethering experiments or
previous feeding behavior observed in
the field under natural conditions
(Tomas et al. 2011b).
Herbivory studies often employed
food choice experiments with
different diet combinations to test
75
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
1.3.3. Dietary studies: gut content,
stable isotope analyses (SIA) and
mixing models
Tracking the movement and diet of
animals in time and space is clearly
fundamental to understanding their
ecology, but they are also inherently
difficult to measure. Among methods
for quantifying dietary contributions,
stomach content analysis is the most
accurate, although it gives dietary
information relative to a short time
period and requires a long sampling
time (Legagneux et al. 2007). In
addition, this technique normally
requires sacrifice of the animal,
unless it has already died from other
causes (e.g., hunting).
Stable isotope analyses (SIA) are
another technique that can provide
non-destructive and time-integrative
information about the diet and often
provenance of food sources (Hobson
& Clark 1992a,b, 1993; Inger &
Bearhop 2008). Since consumers feed
on isotopically distinct food items,
their tissues reflect the different
stable isotopic compositions of their
Figure 1.10. Dual isotope plot showing compositional differences between marine and terrestrial
biomes, and rates of trophic enrichment between food sources and consumers. Adapted from:
https://silentwitnesss.wordpress.com/category/stable-isotopes-2/
76
CHAPTER 1: General Introduction
diets. This depends on the window
period when the consumer ate the
diet as well as the turnover rates of
the tissues (Legagneux et al. 2007).
Different
animal
tissues
are
synthesized and replaced at different
rates, and the isotopic make-up of
new tissues generally reflects the
diet/habitat of animals at the time of
tissue synthesis (Bearhop et al. 2002;
Pearson et al. 2003; Hobson &
Bairlein 2003; Ogden et al. 2004).
The processes of ingestion, digestion
and assimilation by consumers are
associated with a shift in isotopic
ratios (fractionation or trophic
enrichment factors; Fig. 1.10), so
that the isotopic values of diet and
consumers differ in a predictable
manner. Thus, by carefully selecting
among tissues we can infer an
animal’s diet or habitat preference
over a range of different time scales
and, if the animal is moving, spatial
scales.
Under
appropriate
conditions,
mixing models permit stable isotope
ratios in tissues to be used to quantify
the relative importance to a consumer
of different dietary items. The first
models were able to provide
proportional
values
for
each
component of the diet (with
associated
confidence
intervals)
(Phillips & Gregg 2001, 2003). Very
recently, Bayesian models (Moore &
Seemens 2008) allow the researcher
to incorporate uncertainty in source
isotope values and fractionation
factors within the models, and hence
the results are considerably more
robust than previous modeling
approaches (Inger & Bearhop 2008).
1.4. Objectives
The reviewed literature suggests that
herbivory pressure is highly variable,
partly due to important biases in the
methods historically employed and
partly to the complexity of the
seagrass-herbivore interactions. The
main purpose of this dissertation is to
study different macrophyte-herbivore
interactions and identify what factors
drive herbivore preferences and final
consumption rates across different
macrophyte ecosystems. For this aim
we
investigate
herbivory
by
combining different methodologies
such as direct quantification methods
(tethering experiments), feeding
observations,
food
choice
or
exclusion cage experiments and
dietary analyses (gut contents
combined with isotopes analyses) to
provide time-integrative information
77
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
about the diet and a broader vision of
the interactions taking place in each
case under study. Additionally we try
to elucidate the mediating role of
nutritional content (as measured by
C:N ratios), epiphytes, flowers or
seeds, in food selectivity and of
herbivory patterns observed in the
different ecosystems.
The specific objectives assessed
during the different studies were:
• To identify the main factors
driving herbivory across different
macrophyte ecosystems using a
combined methodological approach.
78
• To assess patterns of herbivory
on different macrophytes in order to
unify
consumption
rate
measurements
for
further
comparison.
• To investigate the mediating role
of nutritional content in the different
food sources (as measured by C:N
ratios),
epiphyte
loads
and
assemblages, and flowers and seeds
availability in herbivore feeding
decisions and final consumption
rates.
CHAPTER 1: General Introduction
79
Factors Driving Herbivores Consumption and Feeding Preferences Across Different Macrophytes Ecosystems
Published as:
Marco-Méndez C, Prado P, Heck KL Jr, Cebrián J, Sánchez-Lizaso
JL (2012) Epiphytes mediate the trophic role of sea urchins in
Thalassia testudinum seagrass beds. Mar Ecol Prog Ser 460:91–100
Photo: P. Prado
80
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
2. Epiphytes mediate the trophic role of sea urchins
in Thalassia testudinum seagrass beds
2.1. ABSTRACT
The trophic role of the sea urchin Lytechinus variegatus (Lam.) and
the importance of epiphytes as mediators of trophic interactions
were evaluated in a meadow of Thalassia testudinum Banks ex
König, St. Joseph Bay, Florida, in fall 2009. Tethering
experiments were deployed within the meadow to assess
consumption rates, and food choice experiments, with
combinations of different types of T. testudinum leaves, green
leaves and detached (decayed) leaves, both with and without
epiphytes, were carried out with individual urchins in 4 different
24 h feeding trials. Lytechinus variegatus had higher consumption
rates on decayed (12.15 ± 1.3 mg DW shoot−1 d−1; mean ± SE)
compared to an undetectable consumption of green seagrass leaves,
and consistently chose epiphytized leaves of both type. Therefore,
consumption rates were highest for decayed leaves with epiphytes.
This choice may be related to the significantly higher epiphytic
biomass on decayed (3.64 ± 0.28 mg DW cm−2) than on green
leaves (2.11 ± 0.25 mg DW cm−2). Stable isotope analyses pointed
to epiphytes and green leaves as the main sources of nitrogen and
carbon in L. variegatus diet. These results suggest that epiphytes
mediate trophic interactions between sea urchins and turtlegrass.
Therefore, changes in epiphytes and decayed leaf biomass can
regulate sea urchin foraging and its impact on the trophic
dynamics of the meadows.
81
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
2.2. INTRODUCTION
The role of herbivory in seagrass
ecosystems has recently received
much attention. Plant availability,
nutrient conditions, historical and
present harvesting of herbivore
populations,
recruitment
and
predation processes, among other
factors may interact to influence the
abundance of herbivore populations
and the consumption of seagrasses
(Heck & Valentine 2006; Prado et al.
2008a, 2009, 2010a,b). Many studies
have reported that < 10% of the
annual
aboveground
primary
productivity (APP) usually enters the
food web via grazing in seagrass beds
(see review by Cebrián & Duarte
1998),
with
the
remaining
productivity entering the detrital
food web through being buried in
sediments or exported to other
neighboring systems (Thayer et al.
1984; Zieman & Zieman 1989). Yet,
there is also a growing body of
literature
documenting
the
importance of seagrass herbivory in
many seagrass systems (see reviews by
Heck & Valentine 2006; Valentine
& Duffy 2006), particularly by
macroherbivores, such as fish and sea
urchins, that may consume > 50% of
the annual APP (Prado et al. 2007a).
82
Large consumption rates may be a
mechanism to compensate for the
low nutritional quality of seagrass
leaves (Mattson 1980; Goecker et al.
2005), which feature high C:N ratios
and structural constituents such as
cellulose that are not easily digested
by
many
marine
organisms
(Velimirov 1984; Frantzis et al.
1992). It has been suggested that
seagrass detritus fraction provides a
low-fiber and high-nitrogen food
after microbial colonization, which
renders it nutritious for animals after
the breakdown of tissues and plant
chemical defenses (Mann 1972;
Robertson et al. 1982). Large
numbers of invertebrates, including
echinoderms,
amphipods,
and
isopods, feed on decaying leaves and
organic matter from seagrass beds
(Fenchel 1970; Robertson et al.
1982). Abundant sea urchins in
seagrass
ecosystems,
such
as
Lytechinus variegatus (Lam.) (Camp
et al. 1973; Heck & Valentine 1995;
Valentine & Heck 1999) are
generalist consumers and feed both
on live and dead seagrass leaves
(Beddingfield & McClintock 2000;
Watts et al. 2007). Therefore, the
relative availability of green and
decaying leaves, as well as the
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
herbivore’s choice of leaf type, may
be important factors determining sea
urchin effects within seagrass
meadows. This issue has been
inadequately investigated, although it
can have important consequences for
the functioning of ecosystems,
depending
on
the
different
interactions between grazers and
seasonal changes in the standing
plant biomass, production (Duarte
1989; Kaldy & Dunton 2000), and
leaf loss dynamics (Francour 1990;
Romero et al. 1992). Previous studies
have suggested that sea urchins
preferentially feed on epiphytes
(Verlaque
&
Nédelec
1983;
Shepherd 1987; Tomas et al. 2005b;
Vergés et al. 2011), possibly due to
their C:N ratios and the absence of
refractory plant material. Also, the
composition
of
the
epiphyte
assemblage
may
influence
consumption through changes in the
abundance of certain species or
morphological groups (Gacia et al.
1999; Thacker et al. 2001; Prado et
al. 2007b, 2008b).
The multiplicity of factors involved
in seagrass trophic interactions
requires the use of analytical and
manipulative methods that integrate
temporal variability in resource
acquisition. Stable isotope analyses
have been useful in providing
integrative information on the
relative importance of dietary sources
(e.g., Tomas et al. 2006; Prado et al.
2010b), but they may not identify
minor
trophic
resources
and
deviations from conventional values
of fractionation due to variability in
taxonomic level, individual age, and
type of N excretion (see review by
Vanderklift & Ponsard 2003;
Crawley et al. 2007). In addition,
contributions from diet sources with
similar isotopic signatures (e.g.,
epiphytes or macroalgae) may be
difficult to distinguish and quantify.
Therefore, other methods, such as
analyses of stomach contents, provide
complementary information to test
and validate the results of stable
isotope models (e.g., Harrigan et al.
1989; Cocheret de la Morinière et al.
2003).
Additional
knowledge
obtained from measurements of
consumption rates of the different
food sources is also central to the
complete understanding of fluxes of
energy and matter to higher trophic
levels (Fry et al. 1984; Lepoint et al.
2000), as well as growth patterns of
populations. However, consumption
rates are often assessed using indirect
83
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
methods, such as quantification of
herbivore bite marks, which tend to
underestimate seagrass consumption
(Cebrián et al. 1996a,b) compared to
less frequently-used direct methods
such as tethering experiments (e.g.,
Kirsch et al. 2002; Tomas et al.
2005a; Prado et al. 2007a).
In the eastern Gulf of Mexico,
seagrass beds are typically dominated
by Thalassia testudinum Banks ex
König (Zieman & Zieman 1989),
and the sea urchin L. variegatus is the
main herbivore species, particularly
within the northern region (Camp et
al. 1973; Greenway 1976; Macia &
Lirman
1999).
However,
L.
variegatus has also been reported to
ingest decaying leaf material, at even
higher rates than green leaves
(Montague et al. 1991). In addition,
epiphytized leaves may be preferred
over
non-epiphytized
leaves
(Greenway 1995). In this study we
aimed to evaluate consumption of
detrital vs. green leaves of T.
testudinum and to elucidate the role
of epiphytes in mediating sea urchin
consumption in the northeastern
Gulf of Mexico. Preliminary feeding
observations conducted by randomly
collecting sea urchins (n = 40) within
the seagrass bed pointed to ca. 5
84
times more individuals feeding on
detrital material than on green leaves,
and
we
hypothesized
that
consumption rates would also reflect
this pattern. More specifically, we
assessed (1) consumption rates of
green and detrital leaves (mg DW
shoot−1 day−1) of T. testudinum using
tethering experiments; and (2) the
influence of decomposition and
epiphytes in sea urchin preferences,
using paired combinations of green
and decaying seagrass leaves with and
without epiphytes. In addition,
nutrient contents in seagrass and
epiphytes, as well as epiphyte
biomass and composition, were
investigated
as
potentially
explanatory variables, and gut
contents and stable isotope analyses
were used to measure the relative
importance of seagrass and epiphytes
in sea urchin diets and to assess
dietary contributions in the longer
term.
2.3. MATERIAL AND
METHODS
2.3.1. Study site
The study site was located at the
T.H. Stone Memorial Park in St.
Joseph Bay, Florida, in the northeast
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
Gulf of Mexico. We selected this
location because of low human
impact and low nutrient conditions,
and because it offered easy access to
lush seagrass meadows with high sea
urchin abundance. Salinity typically
ranges from 30 to 36‰ (Stewart &
Gorsline 1962; Folger 1972), and
seagrass species composition is
dominated
by
turtlegrass
T.
testudinum, with lesser amounts of
manatee grass Syringodium filiforme
Kützing and shoal grass Halodule
wrightii Ascherson (Iverson &
Bittaker 1986). Seagrass productivity
is highly seasonal, with leaf biomass
and density peaking near 150 g
AFDW m−2 and 3000 leaves m−2
from June through August (Iverson
& Bittaker 1986). Our work was
conducted in October 2009, and sea
urchin abundance in the area was 8.8
± 1.7 ind m−2, green leaves shoot
density was 637.78 ± 25.25 shoots
m−2, and seagrass cover was 81.17 ±
2.71%. Biomass of decayed leaves
was 44.9 ± 6.4 g DW m−2 and of
green leaves was ca. 33.7 ± 4.1 g DW
m−2. All results are mean ± SE, if not
stated otherwise.
2.3.2. Tethering experiments
The consumption rates of green and
decayed T. testudinum leaves were
estimated in 2 separated tethering
experiments. Each replicate (n = 20)
included 3 leaves from which
previous bite marks were removed
with scissors. We measured the leaf
area (length and width) and holepunched all the shoots at the base of
the leaves to control for growth rates
of tethers during the experimental
period (Prado et al. 2007a). Green
and decayed leaves were attached to
pickets and deployed haphazardly
within the turtlegrass bed. After 4
days, all the leaves were collected and
measured again for variation in
length and width whenever a grazing
bite mark was present. Additionally,
we collected 25 green and 25 decayed
leaves, removed epiphytes with a
razor blade and dried them to a
constant weight at 60°C for 24 h to
assess area-DW relationships (Green
leaves: y = 0.0054x − 0.0116, R2 =
0.76, df = 24, F = 73.35; Decayed
leaves: y = 0.0038x + 0.0071, R2 =
0.65, df = 24, F = 44.42), and we
expressed consumption rates as mg
DW d−1 lost to sea urchins.
85
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
2.3.3. Food choice experiments
Food choice experiments were used
to test the effect of leaf type (green
vs. decayed) and epiphytes (present
vs. absent) on consumption rates of
L. variegatus (expressed as leaf area
loss in cm2) by exposing individuals
to 4 different experiments based on
specific paired combinations of food
resources (n = 18 paired replicates in
each of the 4 food choice
experiments): (1) Decayed nonepiphytized
vs.
green
nonepiphytized (DNE vs. GNE); (2)
green
epiphytized
vs.
green
nonepiphytized (GE vs. GNE); (3)
decayed epiphytized vs. decayed nonepiphytized (DE vs. DNE); and (4)
green epiphytized vs. decayed
epiphytized (GE vs. DE). All the
experiments were carried out in six
50 × 50 cm2 cages, made of PVC and
plastic mesh (each containing 4 sea
urchins and 3 replicate shoots, with
the same area, of the different paired
combinations) haphazardly deployed
on an unvegetated bottom adjacent
to the shallow study bed (ca. 1 m
depth). In each experiment, replicates
(n = 18) were paired shoots, each
comprised of 3 leaves (an average of:
41.38 ± 1.24 cm2) of a paired
combination of choices (e.g., decayed
86
or green leaves, epiphytized or nonepiphytized). Previous bite marks
were removed with scissors and all
the replicates were previously
measured and hole-punched to
control for growth that might occur
during the experimental period
(Prado et al. 2007a). All L. variegatus
used in this study were mature
individuals (i.e., size > 30 mm; Serafy
1979) of ~50 mm test diameter.
After each 24 h experiment, samples
were collected for areal loss
measurements and sea urchins replaced with new ones from the
surrounding bed.
2.3.4. Gut contents and stable
isotope analyses
To confirm patterns from food
choice experiments, 12 sea urchins
were randomly collected from the
seagrass bed and their gut contents
and stable isotope signatures
determined. In the laboratory, the
muscle of the Aristotle’s lantern and
gut contents were extracted. Stomach
contents were separated by leaf types
(green and decayed leaves) under the
microscope and then each portion
was dried at 60°C to a constant
weight. The contribution of each leaf
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
type was expressed as a percentage of
total food content within the gut.
Shoots of T. testudinum and decayed
leaves were randomly collected for
nutrient contents (n = 12), and stable
isotope analyses. The lantern muscle
of L. variegatus (n = 12), green and
decayed leaves without epiphytes
(GNE and DNE respectively; n = 6
of each), and epiphytes of green and
decayed leaves (EG and ED; n = 6 of
each) were dried to a constant weight
(60°C, 24 h), and then ground to fine
powder for determination of isotopic
signatures (δ15N and δ13C‰) to
identify
their
corresponding
signatures. Analyses were carried out
with an EA-IRMS (Thermo
Finnigan) analyzer in continuous
flow configuration. An average
difference in isotopic composition
between the sample and reference
material (δsample-standard, expressed as
‰) is determined by the equation:
[(Rsample − Rstandard)/Rstandard] × 1000 =
δsample-standard
where Rsample is 13C/12C or 15N/14N in
the sample; Rstandard is 13C/12C or
15
N/14N in the working reference
(CaCO3 from a belemnite [PBD]
and atmospheric nitrogen standard
for δ13C and δ15N measurements,
respectively), which is calibrated
against an internal standard (i.e.,
Atropina, IAEA and/or UGS). The
analytical precision of the methods
used was 0.15‰. The analyses were
carried out by the Technical Unit of
Instrumental Analyses (University of
La Coruña).
2.3.5. Nutrient contents in seagrass
and epiphytes
Shoots of T. testudinum and decayed
leaves collected for stable isotope
analyses were also used for nutrient
contents. Analyses were conducted
for each type of leaf without
epiphytes (GNE and DNE), the
epiphytes (from green leaves, EG
and from decayed leaves, ED)
previously removed from the leaves
and, additionally, for green and
decayed leaves with epiphytes (GE
and DE) to identify nutritional
changes between leaf type and by the
presence of epiphytes (n = 6 of each).
All samples were dry weighed before
N and C content analyses, as
determined by a Carlo-Erba CNH
elemental auto analyzer.
87
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
2.3.6. Epiphytic community
The epiphytic community was
investigated on 10 green and decayed
leaves collected haphazardly in the
meadow (n = 10). The oldest leaf of
each green shoot was selected as
representative of the green shoots’
epiphytic and sessile epifaunal
assemblages (see Prado et al. 2007b,
2008b), which were scraped from the
leaves with a razor blade, and taxa
from green and decayed leaves were
identified to genus under the
microscope. After this, epiphytes
were dried to a constant weight
(60°C, 24 h), and epiphytic biomass
was expressed in mg DW cm−2.
2.3.7. Data analyses
Differences in consumption rates
between decayed and green leaves, in
the proportion of each leaf type in sea
urchin gut and in the number of
epiphyte taxa and biomass in each
leaf type, were tested with an
unpaired t-test for independent
samples. In all food choice
experiments, differences between the
different paired samples were
investigated with a paired t-test for
dependent samples.
88
Differences in the isotope signal
(δ15N and δ13C) of green and decayed
leaves (without epiphytes) and their
respective
epiphytes
were
investigated
with
a
one-way
ANOVA (4 levels). The MixSiR
isotope mixing model (Moore &
Semmens 2008) was then used to
identify the contributions of green,
decayed leaves and epiphytes to sea
urchin diet. The input parameters for
the model are the isotopic values of
consumer and trophic resources
(measured in this study) and an
overall rate of fractionation with its
SE. Usually, this fractionation rate
comes from the literature, but since
Prado et al. (2012) conducted an
experiment with L. variegatus that
specifically aimed at evaluating the
effect of diet in isotope fractionation
in different tissues (muscle, gonad,
gut, and test), we used some of their
results to feed the model. Since the
tissue investigated in this experiment
was muscle, we only used muscle
values and not results for the other
tissues. In addition, because results
by Prado et al. (2012), concluded
that there is a strong dietary effect on
fractionation
(i.e.,
seagrass,
macroalgae, and omnivorous diet
fractionations were different) and our
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
urchins were collected where all of
those diets were available (seagrass,
algal epiphytes, and epifauna), the
model was run with a mean
fractionation value that was the
average of the 3 diets (0.63 ± 0.29 for
δ15N and 2.49 ± 0.25 for δ13C; means
± SE). In spite of the limitations, we
used these values of fractionation
because we are convinced that the
mean values are more accurate than
assuming the theoretical 3.4‰
enrichment among trophic levels.
A one-way ANOVA (6 levels) was
used to investigate differences in the
nutritional content (%N and C:N
molar ratios) of different food
resources available to sea urchins (i.e.,
each type of leaf material,
epiphytized and non-epiphytized,
and their respective epiphytes). The
Student Newman-Keuls post-hoc
test (Zar 1984) was used to
investigate the presence of significant
groupings. ANOVA assumptions of
normality and homogeneity of
variance were assessed with the
Kolmogorov-Smirnov and Cochran’s
C-test,
respectively.
When
assumptions could not be met by
transformation, the significance level
was set at 0.01 to reduce the
possibility of a Type I error
(Underwood
1997).
Significant
patterns of variation in epiphytic
assemblages were investigated with
n-MDS
ordination
(presenceabsence transformation, Bray-Curtis
index), and the ANOSIM procedure
available from the PRIMER 6
software package (Clarke & Warwick
2001). Finally, SIMPER analyses
(Clarke & Warwick 1994) were used
to identify the species that primarily
account for observed differences in
epiphyte assemblages between green
and decayed leaves.
2.4. RESULTS
2.4.1. Tethering experiments
Lytechinus variegatus showed high
consumption rates of decayed leaves
(12.1 ± 1.3 mg DW shoot−1 d−1),
whereas consumption of green leaves
was undetectable (t = 8.97, df = 38, p
< 0.001). A slight growth was
recorded for green leaves (0.88 ± 0.32
mg DW shoot−1 d−1) but there was no
growth in the decayed leaves during
the period of the experiment. Local
seagrass biomass during the study
period was 33.8 ± 4.08 g DW m−2 of
green leaves and 44.9 ± 6.45 g DW
m−2 of decayed leaves (means ± SE).
89
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
2.4.2. Food choice experiments
No significant differences were
observed between consumption of
GNE and DNE during the food
choice experiments (t = 0.93, df = 17,
p = 0.364, Fig. 2.1A). In contrast, sea
urchins
consumed
significantly
higher amounts of DE than GE (t =
−2.49, df = 17, p < 0.05, Fig. 2.1B),
showing that when epiphytes were
present sea urchins chose decayed
over green leaves. Epiphytized leaves
suffered higher consumption than
non-epiphytized leaves in the 2 trials
where either green or decayed leaves
were offered to the sea urchins (DE
vs. DNE: t = 8.24, df = 17, p < 0.001,
Fig. 2.1C; GE vs. GNE: t = −3.14,
df = 17, p < 0.01, Fig. 2.1D).
2.4.3. Gut contents and stable
isotope analyses
Fragments of decayed leaves were
more abundant than fragments of
green leaves (84.5 ± 2.9% vs. 15.4 ±
2.9%; t = −58.75, df = 22, p < 0.001)
in the guts of sea urchins.
The
δ15N
signatures
showed
significant differences between leaves
and epiphytes but not between each
type of leaf or epiphytes (One-way
ANOVA, p < 0.001, Fig. 2.2, Table
90
Figure 2.1. Leaf loss (cm2) to Lytechinus
variegatus during paired food choice
experiments. A. Green non-epiphytized
leaves (GNE) vs. decayed non-epiphytized
leaves (DNE); B. Green epiphytized leaves
(GE) vs. decayed epiphytized leaves (DE);
C. DNE vs. DE; D. GNE vs. GE. Mean +
SE. *p < 0.05; **p < 0.01; ***p < 0.001; ns:
not significant.
2.1). In contrast, the δ13C signatures
displayed significant differences
between all the samples analysed
(One-way ANOVA, p < 0.001, Fig.
2.2, Table 2.1).
The Aristotle’s lantern muscle of L.
variegatus showed a δ15N signature of
8.9 ± 0.3‰ that was close to the
epiphyte signal and a δ13C signature
of −11.6 ± 0.3‰, intermediate to the
leaves and the epiphyte signal.
Results from MixSir indicated that
urchins acquired most of their dietary
requirements from epiphytes and a
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
lesser proportion from seagrass
leaves. Specifically, the model
estimated the contribution of each
food source in the sea urchins diet,
giving the value at different
percentiles. Our results point to
epiphytes as the most important food
followed by green leaves and decayed
leaves (e.g., each of these respective
values of contribution, at the
percentile 5%: 46.8% of epiphytes,
1.8% of green leaves and 1% of
decayed leaves; at the percentile 25%:
55.9%, 10.7% and 5.5%; at the
percentile 50%: 61%, 22% and
13.8%; at the percentile 75%: 65.8%,
30.5% and 28.9%, and at the
percentile 95%: 72.9%, 39.3% and
48.3%).
2.4.4. Nutrient contents in seagrass
leaves and epiphytes
There were significant differences for
both %N and C:N molar ratios
among the different types of samples
analyzed (One-way ANOVA, %N:
df = 5, p < 0.001; C:N: df = 5, p <
0.001; Fig. 2.3). The highest
nitrogen content was found in GNE
(2.86 ± 0.07% N), followed by GE
(2.13 ± 0.12% N). All the other
sample types showed lower values
that did not differ significantly from
one another (Fig. 2.3A). Similar but
inverse patterns were obtained for
C:N molar ratios; DNE showed the
highest values, followed by DE, ED
(epiphytes from decayed leaves), EG
(epiphytes from green leaves), GE
and GNE (see Fig. 2.3B).
2.4.5. Epiphytic community
There were no significant differences
in the mean number of epiphytic taxa
growing on the leaf surface of green
(0.29 ± 0.03 taxa cm−2) and decayed
leaves (0.28 ± 0.02 taxa cm−2) of T.
testudinum (df = 18, t = 0.20, p =
0.84). In contrast, cover by red
calcareous algae was lower on green
(60.3 ± 4.5%) than on decayed leaves
(71.8 ± 2.0%) (df = 18, t = 2.30, p =
0.03). Similarly, epiphytic biomass
was higher on decayed (3.6 ± 0.28
mg DW cm−2) than on green seagrass
leaves (2.1 ± 0.25 mg DW cm−2) (df
= 20, t = −5.16, p < 0.001).
n-MDS ordination of epiphytic taxa,
one-way ANOSIM (Global R =
0.005, p = 0.40), and SIMPER
analyses (average of dissimilarity:
31.75%) found no significant
differences between green and
decayed leaves in the composition of
their epiphytic assemblages.
91
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 2.2. δ15N and δ13C in the Aristotle’s
lantern muscle of Lytechinus variegatus and
in green and decayed non-epiphytized leaves
(GNE and DNE, respectively) and their
epiphytes (epiphytes of green leaves: EG and
epiphytes of decayed leaves: ED).
2.5. DISCUSSION
To our knowledge, this is the first
time that the consumption of detrital
and green leaves has been
simultaneously
compared
using
tethering
experiments.
Results
showed substantial consumption of
detrital leaves, whereas consumption
of green leaves was undetectable. Our
food-choice
experiments
demonstrated that these differences
in consumption were strongly
mediated by epiphytes; detrital leaves
were largely preferred over green
leaves but only when epiphytes were
present (Montague et al. 1991;
Greenway 1995). Our results confirm
previous evidence that decaying plant
material is an important dietary
92
resource for sea urchins (Lowe &
Lawrence 1976; Klumpp & Van der
Valk 1984), and further show that
this dietary choice is largely
dependent on the presence of
epiphytes. Our results also provide
new quantitative information on the
detritivorous role of L. variegatus in
T. testudinum beds during the fall
period. Events of intense herbivory in
which L. variegatus may remove up
to 50 to 90% of the APP of T.
testudinum, are not uncommon and
may be related to dramatic increases
in sea urchin density (Camp et al.
1973; Greenway 1976; Valentine &
Heck 1991; Heck & Valentine
1995).
However,
given
the
importance of detrital resources
evidenced in this experiment, part of
the large spatial and temporal
variability in sea urchin grazing
observed to date may be related to
differences in the biomass of
decaying leaves trapped within
meadows.
Results from gut contents of L.
variegatus are consistent with our
experiments and show that sea
urchins ate more detrital leaves than
green leaves. In the fall, the biomass
of detrital leaves is typically high and
often exceeds the biomass of green
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
leaves (in our case by ca. 25%).
Decomposition of seagrass leaves is
known to cause slight changes in the
δ13C signature (Zieman et al. 1984;
Vizzini et al. 2002). Our stable
isotope results also evidenced that
green leaves were significantly
depleted in δ13C compared to
decayed leaves (ca. 1.4‰) whereas
associated epiphytic communities
displayed opposite patterns (0.7‰
lower for communities on decayed
leaves). The δ15N signatures,
however, showed no effects of
decomposition,
and
significant
differences were only evident
between seagrass leaves and epiphytes
(Tomas et al. 2006; Prado et al.
2010b). Results from the MixSir
model indicate that L. variegatus diet
has a higher nitrogen contribution
from epiphytes than from seagrass
leaves, with comparatively higher
contributions from green leaves than
from decayed leaves (see also Tomas
et al. 2006 for similar contributions
to sea urchin diet). However, results
from mixing models need to be
interpreted with care. In recent
controlled experiments with L.
variegatus (Prado et al. 2012), muscle
fractionation of individuals fed on
green seagrass leaves was lower than
Figure 2.3. A. %N and B. C:N molar ratio
in leaves and epiphytes. EG: epiphytes from
green leaves; ED: epiphytes from decayed
leaves; other labels for leaf and epiphytes as
in Fig. 2.1. Mean + SE
for individuals fed on macroalgae
(Δ13C = −0.86 and 0.94, respectively);
and
for
nitrogen,
muscle
fractionation was higher for seagrass
than for algal diets, suggesting a
similar contribution from both
sources to the diet (Prado et al.
2012).
This apparent discrepancy between
gut contents, tethering results, and
fractionation-corrected
stable
isotopes results could be explained by
variability in the availability of
detrital leaves, green leaves, and
93
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
epiphytes. Shoot density and
standing plant biomass were high
(637.78 ± 25.25 shoots m−2 and 33.79
g DW m−2; means ± SE) at the time
of our study, which may favour the
trapping and accumulation of
decaying leaves shed within the
seagrass bed, this is also the period of
maximum epiphyte biomass (e.g.,
Tussenbroek 1995; Cebrián et al.
1997). During winter, however,
shoot density typically decreases
(Valentine & Heck 1991), and
remaining decaying leaves can be
more easily exported to neighbouring
systems (Mateo et al. 2006; K. Heck
& P. Prado pers. obs.). The isotopic
signatures measured during our
experiments
integrate
items
consumed by the urchin during
previous seasons. Hence, this
seasonality in the type and quantity
of dietary availability could explain
the apparent discrepancy in our
results: tethering and food choice
experiments showed higher selection
for epiphytized decayed leaves, while
isotope results suggested slightly
higher contributions of green leaves
to diet, compared to decayed leaves.
The
higher
consumption
of
epiphytized leaves at the time of
study may be explained by L.
94
variegatus using a large proportion of
its dietary carbon to build the test
(Prado et al. 2012), which can be in
part provided by calcareous red algae
(see James 2000 for species feeding
on rhodolith beds).
Differences
in
the
temporal
availability of food items can lead to
profound dietary changes as has been
reported many times (Boudouresque
& Verlaque 2001; Wernberg et al.
2003). The impacts of herbivores on
seagrass productivity and on the
dynamics of the bed may also be
related to the relative availability of
green vs. decayed leaves (e.g., Tubbs
& Tubbs 1982, 1983; Prado et al.
2008a). In winter, with less biomass
of green and decayed leaves, the
impacts of herbivory on the meadow
could be greater. Indeed, in previous
manipulative
experiments,
the
number of urchins required to
completely defoliate a seagrass bed
during summer and fall was
estimated to be twice that required
during the winter period (ca. 40 and
20 ind m−2; Valentine & Heck 1991).
High abundances of decayed leaves
could represent a refugium for green
leaves by reducing grazing by sea
urchins and regulating the nature and
CHAPTER 2: Epiphytes mediate the trophic role of sea urchins in Thalassia testudinum seagrass beds
epiphytes was not significantly
different. The lack of correlation
between the consumption rates and
the nutrient content of the diet is
also seen in the food-choice
experiments when one type of leaf
(either green or decayed), with or
without epiphytes, was offered to the
urchins. This suggests that nutrient
content itself did not determine food
selectivity by the sea urchins in our
study. Chemical and structural
defenses in living seagrass tissues also
appear to be unimportant for L.
variegatus (see Steele & Valentine
2010 for evidence of lack of chemical
deterrence in this seagrass species).
Higher nutrient contents have been
repeatedly proposed as a factor
triggering
preference
and
consumption of primary producers by
herbivores (e.g., McGlathery 1995;
Heck et al. 2000; Prado et al. 2010b).
magnitude of herbivore impacts on
seagrass beds.
Food-choice experiments showed a
preference for detrital leaves, but only
when epiphytes were present, thus
providing another example of the
importance of epiphytes in mediating
consumption
patterns
(e.g.,
Montague et al. 1991; Greenway
1995; Alcoverro et al. 1997; Vergés
et al. 2011). Detrital leaves had
approximately half the nitrogen
content of green leaves alone and a
nearly 2-fold higher C:N ratio, but
consumption rates of both types of
leaves without the epiphytes were not
significantly different. Similarly, the
urchins consumed decayed leaves
with epiphytes to a greater extent
than they did green leaves with
epiphytes, even though the total
nutrient content in leaves and
Table 2.1. One-way ANOVA testing for differences on δ15N and δ13C between different sample
types (non-epiphytized leaves and epiphytes). SNK = significant differences between investigated
groups: green non-epiphytized leaves (GNE), decayed non-epiphytized leaves (DNE), epiphytes
from green leaves (EG) and epiphytes from decayed leaves (ED). Significant differences are
indicated. ***p < 0.001. NT: no transformation.
Source of variation
δ 15N
δ 13C
df
MS
F
p
Food type
3.00
7.37
24.21
***
Residual
20.00
0.30
SNK
Transformation
ED=EG>GNE=DNE
NT
df
MS
F
3.00
43.50
222.21
20.00
0.20
p
***
ED>EG>GNE>DNE
NT
95
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
However, several authors have found
that increased nutrient content in
seagrass leaves does not necessarily
lead to higher consumption rates by
herbivores (for a review see Cebrián
2004).
Our results instead suggest that food
selectivity by the sea urchin in this
experiment is positively related to the
amount of epiphyte biomass. No
food preference was exhibited
between leaves with no epiphytes.
The selection of epiphytized decayed
leaves over epiphytized green leaves
corresponds to higher epiphyte
biomass on the decayed leaves. On
average, decayed leaves had 41.6%
more epiphytic biomass and 26.4%
higher cover of red calcareous algae
in comparison with green leaves,
possibly as a result increased epiphyte
accrual on older leaves (Casola et al.
1987; Cebrián et al. 1999), although
no
differences
in
epiphyte
community composition were found
between green and decayed leaves. In
addition, when either green or
decayed leaves with or without
epiphytes were offered to sea urchins,
epiphytized leaves were chosen.
To conclude, our results showed that
detrital epiphytized leaves were the
96
preferred food item of L. variegatus
and support its detritivorous role
within seagrass beds. At other times
of the year, field observations (K.
Heck & P. Prado pers. obs.) and
long-term stable isotope results
suggest that herbivory on green
leaves is greater when the abundance
of decayed leaves is low. Our results
suggest that sea urchins select the
highest possible values of epiphyte
biomass, which tend to be higher in
decayed leaves. In turn, this selection
could reduce the impact of the urchin
on photosynthetically active green
leaves (Valentine & Heck 1991) and
substantially modify the magnitude
of top-down effects within meadows.
At the ecosystem level, this
facultative trophic behavior of L.
variegatus may also speed up the
recycling of detritus during the fall by
enhancing turnover rates of carbon
and nitrogen and making them
available for plant growth (Cebrián
& Duarte 1995). In addition, by
enhancing fragmentation of detrital
leaves, urchins may favour the export
of carbon and nutrients contained in
the detrital fragments to less
productive systems such as adjacent
beaches and other unvegetated
habitats (Wernberg et al. 2005).
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean seagrass
meadows
97
Published as:
Marco-Méndez C, Ferrero-Vicente LM, Prado P, Heck KL Jr,
Cebrián J, Sánchez-Lizaso JL (2015) Epiphyte presence and
seagrass species identity influence rates of herbivory in
Mediterranean seagrass meadows. Estuar Coast Shelf S 154:94–101
98
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
3. Epiphyte presence and seagrass species identity
influence rates of herbivory in Mediterranean
seagrass meadows
3.1. ABSTRACT
Herbivory on Mediterranean seagrass species is generally low
compared to consumption of some other temperate and tropical
species of seagrasses. In this study we: (1) investigate the feeding
preference of the two dominant Mediterranean seagrass
herbivores, the sea urchin Paracentrotus lividus (Lam.) and the fish
Sarpa salpa (L.), on Posidonia oceanica (L.) Delile and Cymodocea
nodosa (Ucria) Ascherson and (2) elucidate the role of epiphytes in
herbivore choices. We assessed consumption rates by tethering
seagrass shoots, and preferences by food choice experiments with
the following paired combinations: (1) Epiphytized leaves of both
C. nodosa vs. P. oceanica (CE vs. PE); (2) Non-epiphytized leaves
of C. nodosa vs. P. oceanica (CNE vs. PNE); (3) Epiphytized vs.
non-epiphytized leaves of C. nodosa (CE vs. CNE) and (4)
Epiphytized vs. non-epiphytized leaves of P. oceanica (PE vs.
PNE). We found that preference for C. nodosa was weak for S.
salpa, but strong for P. lividus, the species responsible for most
consumption at our study. Overall both herbivores showed
preference for epiphytized leaves. The higher nutritional quality of
C. nodosa leaves and epiphytes together with the high coverage and
diversity of the epiphyte community found on its leaves help
explain the higher levels of herbivory recorded on epiphyted leaves
of C. nodosa. Other factors such as seagrass accessibility, herbivore
mobility and size, and behavioral responses to predation risks, may
also affect the intensity of seagrass herbivory, and studies
99
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
addressing the interactions with these factors are needed to
improve our understanding of the nature, extent and implications
of herbivory in coastal ecosystems.
3.2. INTRODUCTION
Herbivory on seagrasses has been
often thought to remove only a
modest amount of leaf production
(by approx. 10–15%, Den Hartog
1970; Thayer et al. 1984). However
prior estimates of leaf consumption
rates that were assessed using indirect
methods, such as quantifying
herbivore bite marks, are now known
to
underestimate
seagrass
consumption (e.g., Cebrián et al.
1996a) compared to the less
frequently-used estimates provided
by tethering experiments (e.g.,
Tomas et al. 2005a; Prado et al.
2007a).
In the Mediterranean, seagrass
meadows are dominated by Posidonia
oceanica (L.) Delile (Prado et al.
2011), while Cymodocea nodosa
(Ucria) Ascherson is commonly
found in small patches within these
meadows (Pérès & Picard 1964).
Herbivory rates on these seagrass
100
species are generally low in
comparison to some other seagrass
species, although they may vary
substantially (e.g., 2–57% of P.
oceanica leaf productivity, Cebrián et
al. 1996a; Prado et al. 2007a; 1–50%
of C. nodosa leaf productivity,
Cebrián et al. 1996b). Although a
few studies have used direct methods
to estimate rates of seagrass herbivory
(e.g., Kirsch et al. 2002; Tomas et al.
2005a; Prado et al. 2007a), further
reassessment of herbivory rates by
direct methods is needed to evaluate
the importance of herbivory for the
ecological
functioning
of
Mediterranean systems.
The sea urchin Paracentrotus lividus
(Lam.) and the fish Sarpa salpa (L.)
are the two main macroherbivores in
the western Mediterranean, and are
commonly observed in shallow
seagrass meadows and rocky bottoms
(Verlaque 1990). Prado et al. (2007a)
found that S. salpa accounted for 70%
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
of the total leaf consumption by
herbivores (approx. 40% of leaf
production)
with
P.
lividus
accounting for the remaining 30%
(approx. 17% of leaf production;
Prado et al. 2007a) in Posidonia
oceanica
meadows,
although
herbivory intensity can vary largely
through space and time (Cebrián et
al. 1996a,b; Prado et al. 2007a,
2010a; Steele et al. 2014). Factors
such as plant availability and
accessibility, plant nutritional quality,
human pressure on herbivore
populations,
and
herbivore
recruitment and predation risk have
all been shown to influence the
intensity of herbivory on seagrasses
meadows (Prado et al. 2008a, 2009,
2010a). In addition, it has been
shown that as many terrestrial plants
and
marine
algae,
seagrasses
chemically deter herbivores using
secondary metabolites, although
inhibition varies among consumers
(Vergés et al. 2007a,b, 2011).
Therefore
seagrass-herbivore
interactions are further complicated
and studies are needed to improve
our knowledge of the mechanisms
controlling feeding decisions and
consumption
rates
of
major
herbivores in seagrass ecosystems
(Heck & Valentine 2006).
The abundance of the sea urchin P.
lividus in shallow habitats seem to be
affected by refuge availability of
predation refuges, with the highest
densities recorded in rocky habitats
(> 50 to 100 ind m−2) (Delmas &
Régis 1986; Delmas 1992). In
seagrass beds, P. lividus abundances
are commonly higher in P. oceanica
(2–3 ind m−2; Boudouresque &
Verlaque 2001, 2013) than in C.
nodosa (0.7 ind m−2; Fernandez &
Boudouresque 1997), which may be
due to differing predation pressure
(Traer 1980), and/or to reduced
meadow recruitment in the absence
of significant rhizome structural
complexity (Prado et al. 2009).
Regarding S. salpa, studies have
shown similar densities in P. oceanica
meadows (2.5 ind m−2; Guidetti
2000; Guidetti & Bussotti 2000,
2002) and rocky habitats (2.3 ind
m−2; Guidetti 2000), but lower
densities appear to happen in C.
nodosa meadows (0.22 ind m−2;
Guidetti & Bussotti 2000, 2002).
Feeding behavioral patterns and food
preferences can also differ between
both herbivores (Prado et al. 2011)
101
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
causing different impacts on seagrass
meadows. S. salpa potentially feeds
on a wide range of macroalgae and
seagrasses
(Christensen
1978;
Havelange et al. 1997; Stergiou &
Karpouzi 2001) and may aim at
maintaining a diverse diet to achieve
the required nutrients (Goldenberg
& Erzini 2014). Factors such as
seagrass
abundance,
habitat
heterogeneity and patterns of
movement are known to influence
the grazing patterns of S. salpa
(Prado et al. 2008a, 2011). In
contrast, marine invertebrates living
on the sea bottom, such the sea
urchin P. lividus, are less mobile but
also have a diet commonly based on
macroalgae and seagrasses although
they are sometimes influenced by the
limitation
of
food
resources
(Fernandez 1990; Mazzella et al.
1992; Boudouresque & Verlaque
2001, 2013).
Preferences and higher feeding rates
of marine herbivores on diets with
high nitrogen and protein content or
with low amounts of structural
components have been reported by
some studies (Mariani & Alcoverro
1999; Goecker et al. 2005),
suggesting that herbivores will
maximize the consumption of food
102
items with higher nutritional and
energy contents (Hughes 1980).
Seagrasses leaves may also have
varying
levels
of
structural
carbohydrates (cellulose), which may
affect
food
digestibility
and
absorption (e.g., Klumpp & Nichols
1983). In fact, previous studies have
suggested that differences in the
nutritional quality among seagrass
species could result in different levels
of herbivory (Cebrián & Duarte
1998; Prado et al. 2010b). Epiphytes
growing on the seagrass leaves have
also been reported to have higher
nutritional quality than leaves (e.g.,
Alcoverro et al. 1997 for epiphytes,
Alcoverro et al. 2000 for seagrass),
which may lead to increased feeding
selectivity by herbivores (Heck et al.
2006; Marco-Méndez et al. 2012).
Indeed, P. lividus (Boudouresque &
Verlaque 2001, 2013; Tomas et al.
2005b) and S. salpa (Verlaque 1981,
1985) preferentially feed on the
epiphytic flora of P. oceanica leaves.
These studies also suggested that not
only the nutritional quality but also
the composition of the epiphytic
assemblages can strongly influence
herbivore consumption through
changes in the abundance of certain
taxa or morphological groups
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
(Thacker et al. 2001; Prado et al.
2008b; Marco-Méndez et al. 2012).
3.3. MATERIAL AND
METHODS
In this context, the aims of this study
were to investigate the intensity of
herbivory and food preferences by P.
lividus and S. salpa, in a mixed
seagrass-rocky habitat with the
presence of P. oceanica and C. nodosa
in the western Mediterranean and to
elucidate the mediating role of
epiphytes in determining herbivore
choices. Both food choices and
tethering experiments were carried
out and were complemented with
feeding behavioral observations in
the
field. In addition, the
contribution of each food source to
the diet was estimated by gut
contents analyses and stable isotopes.
Nutrient contents and epiphytic
community analyses of the two
seagrass species were investigated as
potential explanatory variables for
herbivore behavior. The results will
contribute
to
further
our
understanding on the feeding
behavior of these two herbivores as
well as their role on the trophic
functioning
of
Mediterranean
seagrass beds.
3.3.1. Study site
The study site was located at Cabo
de las Huertas (38°21.264′ N;
0°24.207′ W; Alicante, Spain; Fig.
3.1). The site features a mixed
seagrass-rocky habitat (depth range:
2–5 m; covered area: ∼0.30 km2)
composed by inter-mixed small
patches (of variable size) of
seagrasses, macroalgae, unvegetated
sandy substrate and unvegetated
rocky substrate. The work was
conducted in late summer, when
epiphyte biomass show maximum
values and herbivore pressure (as
frequency of attack marks on leaves)
by S. salpa and P. lividus is presumed
to be greatest (Alcoverro et al. 1997).
Bottom cover in the mixed seagrassrocky habitat of study during this
period was: 17 ± 3% Caulerpa
prolifera (Forsskål) JV Lamourox; 11
± 3% Posidonia oceanica; 10 ± 2%
Cymodocea nodosa; 7 ± 3%
unvegetated sandy substrate; 4 ± 1%
Padina pavonica (L.) Thivy and the
remaining 51 ± 4% rocky substrate,
which appears unvegetated or with
occasional presence of coralline red
103
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
macroalgae
such
as
maërl
(Lithothamnion
corallioides
and
Phymatolithon calcareum) and Jania
rubens (L.) JV Lamourox. Seagrass
shoot density was 2071 ± 291 shoots
m−2 for C. nodosa and 584 ± 29 shoots
m−2 for P. oceanica (mean ± SE).
3.3.2. Herbivore abundance and
feeding observations
Sea urchin abundance was counted in
40 cm x 40 cm quadrats randomly
deployed within the mixed seagrassrocky habitat of study (n = 20). We
further investigated feeding items
being consumed by sea urchins by
randomly collecting individuals (n =
86) from the mixed seagrass-rocky
habitat and noting the resources
attached to their oral side, that
contained bite marks (e.g., P.
oceanica, C. nodosa, J. rubens and C.
prolifera). Individuals of S. salpa were
counted within the study area by
divers using visual census within 50
m2 transects (n = 18). In each
transect we recorded number of
individuals within a school of fish,
the average size of individuals and
the food items consumed when
feeding.
3.3.3. Tethering experiments
Consumption rates of C. nodosa and
P. oceanica leaves were estimated with
tethering experiments in the mixed
seagrass-rocky habitat. We deployed
one tethering line for each seagrass
species within monospecific seagrass
patches.
Each
tethering
line
consisted of 20 replicates, with one
replicate corresponding to one 3-leaf
Figure 3.1. Map of the study area, Cabo de las Huertas (Alicante, Spain).
104
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
shoot. Prior to deployment leaves
were cut down to the same length to
remove previous herbivore marks,
their area measured, and tissues holepunched at the base of the leaf to
allow estimates of any growth during
the experiment (Prado et al. 2007a).
The shoots were attached to pins
using cable ties, and deployed in the
field for a week by securing the pins
to the bottom and tying them to a
thin rope. After this period, shoots
were collected and leaf area lost to
urchins and S. salpa consumption
(which were different in the shape of
scars produced by each species;
Boudouresque & Meisnez 1982) was
measured to estimate consumption
rates (cm2 leaf area consumed per
day).
3.3.4. Food choice experiments
Paired food choice experiments were
conducted for both P. lividus and S.
salpa. A total of 4 different food
choice experiments were carried out:
(1) C. nodosa epiphytized (CE) vs. P.
oceanica epiphytized (PE); (2) C.
nodosa non-epiphytized (CNE) vs. P.
oceanica non-epiphytized (PNE); (3)
C. nodosa epiphytized (CE) vs. C.
nodosa non-epiphytized (CNE); and
(4) P. oceanica epiphytized (PE) vs. P.
oceanica non-epiphytized (PNE). For
sea urchins, six 50 cm × 50 cm × 50
cm cages made of PVC and plastic
mesh were haphazardly deployed on
an unvegetated patch within the
study area (2–3 m depth), each
containing 3 sea urchin individuals
and 3 seagrass replicates (n = 18).
Each replicate of the different paired
combinations tested was constituted
by two different shoots, one of each
food choice (e.g., CE vs. PE), tied
together. All sea urchins were starved
during 24 h prior to being used in the
experiment. All individuals used were
mature (test diameter > 30 mm;
Serafy 1979), with approx. 50 mm
test diameter, and were replaced with
new individuals at the end of each
paired food preference test. For S.
salpa, we deployed tethering lines
containing the paired combinations
within an unvegetated patch (n = 18).
Food choice experiments ran for four
days and after this period, samples
were collected and transported to the
laboratory. Area lost from the tethers
was processed in the same way as for
the other tethering experiments.
105
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 3.2. Tethering results showing leaf
loss of each seagrass species (cm2 d-1) by P.
lividus and S. salpa herbivory. NS: not
significant.
3.3.5. Gut contents, stable isotope
analyses and nutrient contents
Individuals of P. lividus (test
diameter without spines: 4.8 ± 0.1
cm) and S. salpa (total length: 22.9 ±
0.6 cm) were collected randomly
from the mixed seagrass-rocky
habitat of study for gut contents (n =
10), nutrient content (n = 5) and
stable isotope analyses (SIA, n = 5).
In the laboratory, the Aristotle's
lantern in sea urchins and lateral
muscle in fish were isolated for SIA
and nutrient content analysis. Gut
contents were extracted and food
items separated under the microscope
(i.e., P. oceanica green leaves, P.
oceanica detritus, C. prolifera, J.
rubens, other macroalgae, and fauna).
Each fraction was dried to constant
mass at 60°C.
106
Shoots of P. oceanica and C. nodosa
were randomly collected for SIA and
nutrient content analysis. These
samples included P. oceanica and C.
nodosa epiphytized leaves, epiphyte
unscraped (PE and CE) and nonepiphytized leaves, scraped free of
epiphytes (PNE and CNE; n = 5), as
well as their respective epiphytes (EC
and EP; n = 5) which included both
epifauna (heterotrophic metazoans)
and epiflora (macroalgae). Samples
were dried to constant mass at 60°C
and grounded to fine powder for
determination of nutrient contents
(C:N) and isotopic signatures (δ15N
and δ13C). Analyses were carried out
with an EA-IRMS (Thermo
Finnigan) analyzer in continuous
flow configuration at the Technical
Unit of Instrumental Analyses
(University of La Coruña). The
average difference in isotopic
composition between the sample and
reference
material
(δsample-standard,
expressed in ‰) corresponds to:
[(Rsample - Rstandard)/Rstandard] x 1000 = δ
sample-standard
where Rsample is the 13C/12C or
15
N/14N ratio in the sample; R standard
is the 13C/12C or 15N/14N ratio for the
reference material (i.e., CaCO3 from
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
belemnite [PBD] for
atmospheric nitrogen
measurements) calibrated
internal standard (i.e.,
IAEA and/or UGS).
δ13C and
for δ15N
against an
Atropina,
3.3.6. Epiphytic community
For epiphytic community analysis we
collected the oldest leaves on shoots
of P. oceanica and C. nodosa (n = 8),
which
includes
the
epiphyte
community accumulated during the
entire life span of the leaf (Cebrián et
al. 1999; Prado et al. 2008b).
Epiphytic cover (%) of leaf surface
was estimated visually, and then,
organisms were scraped off gently for
identification to genus level under
the microscope. Finally epiphytes
were dried to a constant mass at 60°C
for biomass determination.
3.3.7. Data analyses
Results from food choice experiments
were analyzed with paired t-tests.
Differences in consumption rates
(i.e., tethering experiments), number
of epiphytic taxa and epiphyte cover
and biomass between P. oceanica and
C. nodosa leaves were analyzed with
standard t-tests. Differences in
isotopic signatures (δ15N and δ13C)
and nutrient contents (C:N molar
ratio) among food resources were
tested with a one-way ANOVA with
6 levels (PE, PNE, CE, CNE, EC
and EP). Student-Newman-Keuls
post-hoc tests were used to single out
significant groupings. ANOVA
assumptions of normality and
homogeneity of variance were
assessed with the Kolmogorov–
Smirnov and the Cochran's C-test,
respectively. When assumptions were
not met, the level of significance was
set at 0.001 in order to reduce the
possibility of committing Type I
error (Underwood 1997). Epiphytic
assemblages were investigated with
n-MDS
ordination
(presenceabsence transformation, Bray-Curtis
similarity index), ANOSIM and
SIMPER available in the PRIMERE v.6 software package (Clarke &
Warwick 1994, 2001).
3.4. RESULTS
3.4.1. Herbivore densities and
feeding observations
The average abundance of P. lividus
and S. salpa in the mixed seagrassrocky habitat was 19.1 ± 2.6 and 0.5
± 0.1 ind m−2, respectively. Food
items attached to the oral side of P.
lividus indicated that 23.3% of the
107
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
individuals were feeding on P.
oceanica, 15.1% on J. rubens, 8.1% on
P. pavonica, 7% on C. nodosa, 3.5%
on Caulerpa cylindracea (Sonder), and
3.5% on C. prolifera at the time of
collection, with the remaining 39.5%
not
feeding.
For
S.
salpa,
observations showed 44.9% of the
individuals were feeding on C.
prolifera, 41.4% on P. oceanica, and
2.2% on C. nodosa, and 11.4% were
not feeding at the time of
observation.
3.4.2. Tethering experiments
Leaf consumption rates measured
with tethering experiments did not
vary significantly between seagrass
species (t = −1.2; df = 34.2; p = 0.22,
Fig. 3.2). Paracentrotus lividus was
responsible for 96% of the leaf
consumption recorded for P. oceanica,
and S. salpa only contributed 4%. All
the consumption recorded for C.
nodosa was due to P. lividus.
3.4.3. Food choice experiments
Paracentrotus lividus displayed higher
consumption rates of CE vs. PE (t =
−3.575, df = 17, p = 0.002, Fig. 3.3A)
and CNE vs. PNE (t = −3.699, df =
17, p = 0.002, Fig. 3.3B). For both
seagrass species, consumption was
also higher in the presence of
epiphytes (CE vs. CNE: t = −3.660,
df = 17, p = 0.002, Fig. 3.3C; PE vs.
Figure 3.3. Leaf loss (cm2 d-1) to P. lividus (A, B, C, D) and S. salpa (E, F, G, H) during paired
food preference experiments. A, E. C. nodosa epiphytized (CE) vs. P. oceanica epiphytized (PE);
B, F. C. nodosa non-epiphytized (CNE) vs. P. oceanica non-epiphytized (PNE); C, G. C. nodosa
epiphytized (CE) vs. C. nodosa non-epiphytized (CNE); D, H. P. oceanica epiphytized vs. P.
oceanica non-epiphytized. Mean ± SE. *p < 0.05; **p < 0.01; NS = not significant.
108
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
PNE: t = 2.563, df = 17, p = 0.020,
Fig. 3.3D). The results for S. salpa
appeared to follow a similar pattern
to that observed for P. lividus, at least
qualitatively, although significant
differences were only found for the
experiment CNE vs. PNE (t = 2.426;
df = 17; p = 0.027, Fig. 3.3F). All
other
comparisons
were
not
significantly different (CE vs. PE: t =
2.087, df = 17, p = 0.052, Fig. 3.3E;
CE vs. CNE: t = 2.018, df = 17, p =
0.060, Fig. 3.3G; PE vs. PNE: t =
1.837, df = 17, p = 0.084, Fig. 3.3H).
3.4.4. Gut contents
Gut contents of P. lividus were
mostly much digested detritus which
was difficult to identify, decayed and
green leaves of P. oceanica, and a
small amount of algae and fauna. In
contrast, gut contents of S. salpa
mostly corresponded to P. oceanica
and C. prolifera with a low
contribution of other algal species
(Fig. 3.4).
3.4.5. Stable isotope analyses
Both isotopic signals showed
significant differences among food
items (One way ANOVA, p < 0.001,
Fig. 3.5A, Table 3.1). The δ15N
values recorded for the consumers (S.
salpa: 12.9 ± 0.5‰ and P. lividus
10.2 ± 0.1‰) were the highest
observed. These values lay much
closer to both types of epiphytes,
which included both macroalgae and
metazoans (EC: 6.8 ± 0.1‰; EP: 6.8
± 0.3‰) than to epiphytized (PE:
Figure 3.4. Percentage of the different food items found in the gut contents of P. lividus and
S. salpa (%).
109
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
4.5 ± 0.1‰; CE: 6.6 ± 0.1‰) and
non-epiphytized leaves (PNE: 4.4 ±
0.1‰; CNE: 9.6 ± 0.1‰) (Fig.
3.5A).
3.4.6. Nutrient contents in seagrass
leaves and epiphytes
1.07 ± 0.01; S. salpa: 1.06 ± 0.04).
For dietary items, there were
significant differences among C:N
molar ratios, with the highest values
found for seagrass leaves and the
lowest values for epiphytes (Fig.
3.5B, Table 3.1).
The two herbivores showed the
lowest C:N molar ratios (P. lividus:
Figure 3.5. A. δ15N and δ13C signatures of consumers (P. lividus and S. salpa) and food items,
including values for different leaves and epiphytes (CE, CNE, PE, PNE, EC and EP); B. C:N
molar ratios in consumers, leaves and epiphytes. Mean ± SE.
110
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
Figure 3.6. n-MDS ordination of epiphyte taxa found on C. nodosa (CE) and P. oceanica leaves
(PE).
3.4.7. Epiphytic community
No significant differences were
detected in leaf epiphyte biomass per
unit leaf area between the two
seagrass species (CE: 1.26 ± 0.22 mg
DW cm−2; PE: 0.79 ± 0.03 mg DW
cm−2; t = 1.805, df = 18, p = 0.088).
However, we found significant
differences in leaf epiphytic cover (t =
6.611, df = 14, p = 0.000) and in the
number of epiphytic taxa (t = 3.608,
df = 14, p = 0.003) between the two
species, with values being higher on
C. nodosa (67.5 ± 3.6%; 0.91 ± 0.16
taxa cm−2) than in P. oceanica (32.4 ±
3.9%; 0.32 ± 0.03 taxa cm−2).
n-MDS ordination of epiphytic taxa
also displayed distinctive groupings
for C. nodosa and P. oceanica
(ANOSIM: Global R = 0.623, p =
0.001, Fig. 3.6). SIMPER analyses
indicated an average of similarity
between C. nodosa (CE) and P.
oceanica (PE) of 69.49%, with
Myriactula gracilis van der Ben
(Archaeplastida)
Botryllus
sp.
(Metazoa) and Ectocarpus confervoides
Le
Jolis
(Stramenopiles)
contributing the most to this
111
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
dissimilarity (10.06%, 8.78%, and
7.92% respectively). The average
similarity among C. nodosa leaves was
59.33% and was mainly due to
Ceramium sp. (Archaeplastida) and
to Myriactula gracilis (Archaeplastida)
with contributions of 28.05% and
22.64%, respectively. The average of
similarity among P. oceanica leaves
was 48.27% mainly due to Botrillus
sp. (Metazoa) and to Cladophora sp.
(Archaeplastida) with contributions
of 17.56% and15.84%, respectively.
3.5. DISCUSSION
This study evaluated herbivory
impacts in a mixed shallow meadow
of P. oceanica and C. nodosa using
direct methods. Although P. oceanica
has been commonly reported to be
subjected to heavy herbivore pressure
in shallow meadows (Prado et al.
2007a, 2008a), our results showed
that herbivory on C. nodosa is also
important. Paracentrotus lividus was
responsible for the consumption of
both seagrasses at our study site,
which contrasts with previous studies
in P. oceanica meadows, where
herbivory was mostly due to S. salpa
(∼70% of total annual losses) and
lesser extent to P. lividus (the
remaining 30%) (Prado et al. 2007a).
112
This notable herbivory by P. lividus
evidenced in our study may be
explained by the high abundance of
sea urchins in this mixed seagrassrocky habitat (19 ind m−2) which
differs notably from the abundances
commonly reported in P. oceanica (2–
3 ind m−2; Boudouresque & Verlaque
2001, 2013) and C. nodosa meadows
(approx. 0.7 ind m−2; see Fernandez
& Boudouresque 1997). The high
availability of rocky substrate
(approx. 51% of coverage) that
provided shelter from predation
probably influenced these densities
(Ebling et al. 1966; Boudouresque &
Verlaque 2001, 2013). On the other
hand, the lower S. salpa densities
(0.49 ind m−2), which were more
typical of C. nodosa mixed meadows
(0.22 ind m−2; see Guidetti &
Bussotti 2002) than of those reported
for P. oceanica meadows or rocky
habitats (∼2.5 ind m−2; Guidetti
2000), explain the low observed
herbivory by this species.
Food choice experiments revealed a
slight preference for C. nodosa by S.
salpa whereas it was strongly evident
for P. lividus. In the presence of
epiphytes, P. lividus showed an
approx. 8 times higher consumption
of C. nodosa than P. oceanica, and
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
values also remained higher when
epiphytes were removed. Conversely
to the general pattern (Prado et al.
2007a, 2008a), S. salpa displayed
lower consumption rates than P.
lividus in all paired food choice tests,
and experiments were only clearly
conclusive about the preference for
C. nodosa when epiphytes were
removed. Although this preference
for C. nodosa has been previously
reported for P. lividus (Boudouresque
& Verlaque 2001, 2013) this is the
first time that preference for C.
nodosa has been reported for S. salpa.
Yet, multiple preference experiments
may be necessary to unravel food
preferences of consumers. For
instance, Goldenberg & Erzini
(2014) conducted an aquarium
experiment with adult S. salpa and
the seagrasses C. nodosa, Zostera
marina L. and Zostera noltii Hornem,
and their results pointed to Z. noltii
as the preferred food. For epiphytes,
the preference of P. lividus for coated
leaves confirms previous results
suggesting their influence on sea
urchin
food-preferences
and
consumption rates (e.g., Greenway
1995; Marco-Méndez et al. 2012).
Yet, the consistent selectivity of both
consumers for C. nodosa over P.
oceanica, even when epiphytes were
removed, indicates that not only
epiphytes but other factors inherent
to plant features are also involved.
Among these factors, differences
between seagrasses in nutrients,
chemical and structural defenses and
epibiotic
load,
can
influence
herbivore preferences (Verges et al.
2007a,b, 2011). In this study
chemical and structural defenses were
not measured although Verges et al.
(2011) showed that they can strongly
influence S. salpa and P. lividus
preferences.
Previous studies have also suggested
that higher herbivory on C. nodosa
over P oceanica could be related to
higher specific growth rates which, in
turn, could be due to enhanced
nutritional quality in fast-growing
species (Cebrián & Duarte 1998). In
our study, the higher C:N values
recorded in non-epiphytized seagrass
leaves of C. nodosa also suggests that
preference could be related to the
higher nutritional quality of this
species. In addition, C:N ratios in
both seagrass species (epiphytized
and non-epiphytized leaves) were
higher than in epiphytes supporting
the hypothesis that the presence of
epiphytes can increase seagrass
113
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Table 3.1. One-way ANOVA showing differences in δ15N and δ13C signatures and C:N ratio
among food items. Significant differences are indicated: ***p< 0.001. NT: no transformation.
Source of variation
Sample type
Residual
Total
SNK
Transformation
15δN
df
MS
5 6.8429
24 0.1367
29
F
50.07
p
***
df
MS
F
5 39.864 245.82
24 0.1622
29
p
***
C:N
df
MS
F
p
5 436.2017 138.59 ***
24 3.1475
29
EC=EP=CE=CNE>PE=PNE
CNE>CE>PE>PNE>EP>EC
PNE>PE=CNE>CE>EC>EP
NT
NT
NT
consumption rates and mediate
herbivores
preferences
(MarcoMéndez et al. 2012) because of their
higher
nutritional
value
(see
Alcoverro et al. 1997, 2000). In all,
epiphytized leaves of C. nodosa
showed higher nutritional value than
epiphytized leaves of P. oceanica due
to the higher quality of its leaves plus
the increased nutritional value by the
presence of epiphytes and may be
involved in the observed food
preferences.
In addition, the epiphytic community
structure also revealed important
differences between seagrass species
that might have further influenced
herbivore preferences. Cymodocea
nodosa displayed a higher epiphyte
coverage and mean number of
epiphytic leaf taxa. Variability in
epiphytic biomass and species
composition between seagrass species
has been indicated to be influenced
by multiple factors such as light
114
13δC
conditions
(Carruthers
1994),
nutrients (Prado et al. 2008b), and
grazing (Prado et al. 2007b), but
given that samples were collected
from the same location, differences
were more likely the resulting of
shoot morphology and leaf age on
patterns of epiphytic colonization
(Knowles & Bell 1998; Lavery &
Vanderklift 2002). Therefore, our
study confirms the role of epiphytes
in mediating seagrass consumption
and preferences (e.g., Greenway
1995; Marco-Mendez et al. 2012)
especially for P. lividus whose diet is
indicated to be greatly supported by
epiphytes (Tomas et al. 2005b).
In contrast to our results from food
choice experiments, which pointed to
C. nodosa as the most consumed and
preferred food, feeding observations
showed that both herbivores were
feeding most frequently on P.
oceanica. For P. lividus, most of the
individuals were hidden within rocky
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
shelters and passively caught drift
food items without venturing into
the meadow (Marco-Méndez, pers.
observ.). In fact, when we analyzed
gut contents of P. lividus we found a
high fraction of undetermined
detritus, with low contribution of P.
oceanica and algae, and surprisingly,
no presence of C. nodosa. Macroalgae
and seagrass are known to constitute
the common diet of P. lividus, but
some authors have suggested that
preference for certain taxa may be
altered by the relative availability of
food resources, especially when they
are limited (Fernandez 1990;
Mazzella et al. 1992; Boudouresque
& Verlaque 2001, 2013). Suspended
organic particles have also been
reported to be part of the diet of P.
lividus (Verlaque & Nedelec 1983;
Frantzis et al. 1988; Bulleri et al.
1999) which we suspect could
account for the ‘undetermined
detritus’ found in their gut contents.
Despite the limited number of guts
analyzed,
both
our
feeding
observations and gut contents
analyses suggest that the sea urchin
diet was conditioned by the
availability of drift material and low
mobility from rocky shelters. The
observed behavior suggests that sea
urchins prioritize staying within their
shelter to searching for their
preferred food (Boudouresque &
Verlaque 2001, 2013). In late
summer, when the experiment was
carried out, P. oceanica leaves detach
(Mateo et al. 2003) and floating
detached leaves may become more
available to sea urchins and constitute
an important part of their diet
(Boudouresque & Verlaque et al.
2001, 2013). On the other hand,
enhanced sea urchin abundances in
our study area might be the result of
higher availability of rocky shelters,
and higher levels of urchin herbivory
may be elevated by the proximity of
tethers to these shelters, which made
the preferred food available to
individuals.
For S. salpa, the lower herbivory rates
recorded (4% of P. oceanica and no
consumption of C. nodosa) were in
agreement with low fish abundance
(0.49 ind m−2). Analyses of fish
stomach contents and feeding
observations also pointed to P.
oceanica and Caulerpa prolifera as the
most important food items. These
results are coherent with previous
studies reporting S. salpa feeding on a
wide range of macroalgae and
seagrasses
(Christensen
1978;
115
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Havelange et al. 1997; Stergiou &
Karpouzi 2001). The large presence
of C. prolifera in gut contents
confirms Caulerpa spp. as an
important diet item (see also
Verlaque 1990; Tomas et al. 2011b).
However, these results contrast with
food choice experiments pointing to
C. nodosa as the preferred food, and
suggest that other factors were also
driving feeding patters. Among
potential
explanatory
variables,
seagrass
abundance,
habitat
heterogeneity and complexity, and
the type of foraging movements are
known to influence grazing impacts
by S. salpa (Prado et al. 2008a, 2011).
On the one hand, fish mobility across
other sites with higher abundance of
P. oceanica and C. prolifera within its
home range (approx. 1 ha according
to Jadot et al. 2002), may explain
enhanced presence of these species
within gut contents. On the other,
given the similar availability of both
seagrass species at the study site and
the low fish abundance in the area,
the slightly higher consumption of P.
oceanica tethers might be just due to
the arrival of a single school of fish,
and
detecting
differences
in
consumption rates between seagrass
116
species might have required longer
than a week period.
In conclusion, preference for C.
nodosa was weak for S salpa but
strongly evident for P. lividus which
was primarily responsible for the
consumption of both seagrasses.
Preference for leaves coated by
epiphytes
vs.
leaves
without
epiphytes, as well as their higher
nutritional quality (e.g., Alcoverro et
al. 1997, 2000) confirms previous
results that epiphytes strongly
influence consumption rates and
preferences (e.g., Marco-Méndez et
al. 2012) because of their higher
nutritional value. The higher
nutritional quality of C. nodosa and
the higher coverage and number of
epiphytic taxa on its leaves appears to
explain the higher herbivory of this
species, at least for P. lividus. Our
study also indicated the complexity of
seagrass-herbivore interactions and
suggested
that
final
seagrass
consumption rates are not only
determined by food preferences, but
also by factors that could influence
herbivore behavior by changing their
priorities such as predation risk
and/or home-range mobility.
CHAPTER 3: Epiphyte presence and seagrass species identity influence rates of herbivory in Mediterranean meadows
Submitted as:
Marco-Méndez C, Ferrero-vicente LM,, Prado P, Sánchez-Lizaso
JL Epiphytes and nutrient contents influence Sarpa salpa herbivory
on Caulerpa species vs. seagrass species in Mediterranean meadows.
117
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
.
Submitted as:
Marco-Méndez C, Ferrero-Vicente LM, Prado P, Sánchez-Lizaso
JL (2015) Epiphytes and nutrient contents influence Sarpa salpa
herbivory on Caulerpa species vs. seagrass species in Mediterranean
meadows
118
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
4. Epiphytes and nutrient contents influence Sarpa
salpa herbivory on Caulerpa species vs. seagrass
species in Mediterranean meadows
4.1. ABSTRACT
Nowadays, Mediterranean seagrass ecosystems are endangered by
increased colonization of Caulerpa species, which may replace
them affecting key ecosystem functions and services. The fish
Sarpa salpa (L.) is one of the main macroherbivores in the western
Mediterranean seagrass meadows which is also known to feed on a
wide range of macroalgae such as Caulerpa species providing a
certain resistance to invasion of native assemblages. In this study
we investigate temporal and spatial patterns of herbivory by S.
salpa and assess by tethering experiment consumption rates of
Posidonia oceanica (L.) Delile, Cymodocea nodosa (Ucria) Ascherson,
Caulerpa prolifera (Forsskål) JV Lamouroux and Caulerpa
cylindracea Sonder in a western Mediterranean mixed meadow
during summer-autumn 2012. Additionally we carried out food
choice experiments with different paired combination of seagrasses
and Caulerpa species to test S. salpa feeding preferences and the
mediating role of epiphytes and/or nutrients in herbivore choices.
In summer, C. nodosa was the most consumed macrophyte (12.75 ±
3.43 mg WW d-1), probably influenced by the highest densities of
S. salpa, the higher nutritional quality of its leaves and epiphytes,
and by interspecific and temporal differences in the epiphytic
community composition. Feeding observations, gut content
analyses and food choice experiments in the absence of epiphytes
pointed to C. prolifera as the macrophyte consistently selected by S.
salpa. In contrast, this preference for C. prolifera was not sustained
vs. epiphytized leaves, suggesting that epiphyte and nutritional
119
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
contents explain herbivory patterns on the mixed meadow. Stable
isotopic analyses supported these results highlighting the dietary
importance of epiphytes and Caulerpa species for S. salpa.
4.2. INTRODUCTION
Mediterranean seagrass meadows are
dominated by Posidonia oceanica (L.)
Delile (Den Hartog 1970; Thayer et
al. 1984), while Cymodocea nodosa
(Ucria) Ascherson is commonly
found in small patches within these
meadows (Pérès & Picard 1964).
Herbivory rates on these seagrass
species are generally low in
comparison to other temperate and
tropical species, although they may
vary substantially (2–57% of P.
oceanica leaf productivity, Cebrián et
al. 1996a; Prado et al. 2007a; 1–50%
of C. nodosa leaf productivity,
Cebrián et al. 1996b). This variability
in estimated herbivory has been
suggested to be partly a consequence
of the different methods employed
for quantification (Tomas et al.
2005a). Prior estimates of leaf
consumption rates were assessed
using indirect methods, such as
quantifying herbivore bite marks,
which
are
now
known
to
underestimate seagrass consumption
120
(e.g., Cebrián et al. 1996a) compared
to the less frequently-used estimates
provided by tethering experiments
(e.g., Tomas et al. 2005a; Prado et al.
2007a). Nowadays direct methods
have shown that, in some instances,
such grazing can be heavy and
determine
the
structure
and
distribution of temperate macrophyte
assemblages (e.g., Tomas et al.
2005a; Taylor & Schiel 2010). In
addition, these studies also provide
evidence that herbivory can be highly
variable through space and time,
displaying different patterns of
defoliation between meadows and/or
seasons (Prado et al. 2007a, 2010a;
Steele et al. 2014). Nevertheless,
further studies are required to reevaluate the comparative impact of
herbivory on different macrophyte
species and its ecological role in the
functioning
of
Mediterranean
seagrass meadows.
Nowadays, Mediterranean seagrass
ecosystems are endangered by
increased colonization by Caulerpa
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
species, which may replace them;
consequently affecting key ecosystem
functions and services (Hendriks et
al. 2010). Among the main Caulerpa
living in the Mediterranean, only the
chlorophyte
Caulerpa
prolifera
(Forsskål)
JV
Lamouroux,
is
autochthonous. It develops in
shallow subtidal waters, cohabiting
with the seagrasses C. nodosa and P.
oceanica (Vergara et al. 2012; MarcoMéndez et al. 2015a). The green alga
Caulerpa
cylindracea
(Sonder)
(formerly
Caulerpa
racemosa
[Forsskål] J Agardh var. cylindracea
[Sonder] Verlaque, Huisman &
Boudouresque;
[hereinafter,
C.
cylindracea, according to Belton et al.
2014; Marín-Guirao et al. 2015]),
originally
from
southwestern
Australia,
has
rapidly
spread
throughout
the
western
Mediterranean during the last 20
years (Verlaque et al. 2000, 2003).
The alga has successfully colonized a
wide variety of soft and hard
substrata, including dead P. oceanica
rhizomes or ‘matte’ (tough, lignified
roots and rhizomes admixed with
sediment; Boudouresque & Meisnez
1982) and C. nodosa meadows with
patchy distribution (Vázquez-Luis et
al. 2008). Commonly the lack of
herbivore pressure is related to the
low palatability of macrophytes,
owing to their secondary chemistry
(Paul et al. 1987), often considered
one of the main causes for such
invasion success (Sant et al. 1996). In
particular, Caulerpa species contain
caulerpenyne, a secondary metabolite
that acts as a feeding deterrent that
inhibits
the
growth
of
microorganisms, interferes with the
development of fertilized sea urchin
eggs, and is toxic to larvae and adults
of potential herbivores (Lemée et al.
1996; Ricci et al. 1999). However,
recent
studies
suggest
that
Mediterranean
herbivores
have
evolved a certain capacity to tolerate
this secondary metabolite (Cornell &
Hawkins 2003), allowing them to
consume large quantities of Caulerpa
species (Cebrián et al. 2011; Tomas
et al. 2011a,b; Marco-Méndez et al.
2015a). Since fish generally have
higher
mobility
and
greater
consumption rates than invertebrate
herbivores, they have been suggested
as a suitable biocontrol agent for
limiting the spread of introduced
algae (e.g., Weijerman et al. 2008;
Vermeij et al. 2009). According to
this, fish herbivory pressure on
Caulerpa species could eventually
121
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
benefit seagrass species by reducing
the proliferation of these species and
their negative impact on the
dynamics of Mediterranean seagrass
meadows (Ruitton et al. 2005).
The fish Sarpa salpa (L.) is one of the
main macroherbivores in the western
Mediterranean, and is commonly
observed
in
shallow
seagrass
meadows and rocky bottoms
(Verlaque 1990) feeding on a wide
range of macroalgae and seagrasses
(Havelange et al. 1997). This species
has been reported to account for 70%
of the total leaf consumption of P.
oceanica (Prado et al. 2007a) and is
known to ingest large quantities of
Caulerpa species such as C. prolifera
(Marco-Méndez et al. 2015a) and C.
cylindracea, providing a certain
resistance to invasion of native
assemblages (Tomas et al. 2011b). In
general, studies point to higher
feeding activity of S. salpa in summer
to accumulate reserves for the winter
period, when fish eat less and adults
prepare for reproduction (Peirano et
al. 2001). However, S. salpa
herbivory intensity seems to vary
greatly over space and time (Prado et
al. 2007a, 2010a; Steele et al. 2014).
It is also influenced by other factors
such as macrophyte availability and
122
accessibility, habitat heterogeneity,
nutritional quality, human pressure
on herbivore populations, herbivore
recruitment, predation and patterns
of movement (Prado et al. 2008a,
2011).
Preferences and feeding rates of
marine herbivores may be driven by
enhanced nitrogen and protein
content, epibiotic load, or lower
amounts of chemical and structural
components (Mariani & Alcoverro;
1999; Verges et al. 2007a,b, 2011).
Varying
levels
of
structural
carbohydrates in seagrass leaves
(cellulose),
may
affect
food
digestibility and absorption (e.g.,
Klumpp & Nichols 1983) and
differences in nutritional quality
among seagrass species or between
seagrasses and epiphytes could result
in different levels of herbivory
(Cebrián & Duarte 1998; Prado et al.
2010b). Furthermore, it has been
shown that secondary metabolites of
both macroalgae and seagrasses
chemically deter herbivores, although
inhibition varied between consumers
(Vergés et al. 2007a,b, 2011).
Given that several factors could be
involved in the complex seagrassherbivore interactions, studies require
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
combined experimental approaches
and dietary analyses integrating
temporal variability in resource
acquisition. Among methods used to
quantify
dietary
contributions,
stomach content analysis is the most
accurate, although it applies to very
short time periods and requires
extensive sampling (Legagneux et al.
2007). In contrast, more recent
techniques, such as stable isotopes
(reviewed in Kelly 2000), provide
useful complementary and timeintegrative methods in dietary studies
(Marco-Méndez et al. 2012), based
on the premise that consumers’
tissues will resemble the long-term
isotopic composition of the diet (Fry
& Sherr 1984; Minagawa & Wada
1984).
The aims of this study were to assess
and compare S. salpa herbivory
pressure on Caulerpa species and
seagrass species in a mixed meadow
and investigate whether the observed
herbivory pattern responds to
preferences. We hypothesize that if
S. salpa displays higher selectivity and
herbivory pressure on Caulerpa
species, this could eventually act in
favor of seagrasses in Mediterranean
mixed meadows. For this aim we
studied temporal and spatial patterns
of herbivory by S. salpa in a western
Mediterranean mixed meadow with
the presence of P. oceanica, C. nodosa,
C. prolifera and C. cylindracea, during
summer-autumn 2012. To this end, a
combination of field experiments and
dietary analyses were used to
investigate
consumption
rates,
dietary contributions, and feeding
preferences
for
the
different
macrophyte species, including the
role of epiphytes and nutrient
contents in mediating herbivory by S.
salpa.
4.3. MATERIAL AND
METHODS
4.3.1. Study site
The study site was located at Cabo
de Santa Pola (38°12'34.56''N,
0°30'31.55''W,
western
Mediterranean) in a mixed habitat
(depth range: 2–4 m; study area: ̴
0.75 km2) formed by intertwined
patches (variable size) of Posidonia
oceanica, Cymodocea nodosa, Caulerpa
prolifera, unvegetated sandy substrate,
and rocky substrate covered by
Caulerpa cylindracea and other
macrophyte species (e.g., Cystoseira
compressa
[Esper]
Gerloff
&
Nizamuddin; Dictyota sp; Ulva
123
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
compressa L.; Jania rubens [L.] JV
Lamouroux, Padina pavonica [L.]
Thivy and Halopteris scoparia [L.]
Sauvageau]). Caulerpa cylindracea was
first recorded in 2002 at a site located
around ten km north of the study
area, where it colonized soft
sediments and dead matte of P.
oceanica. Two months later, it was
detected on the rocky platform of our
study area (Pena-Martín et al. 2003).
Currently, this alien alga occurs in
extensive areas of ecologically
important rocky bottoms, as well as
on sandy and muddy substrates, and
on dead matte of P. oceanica (MarínGuirao et al. 2015). It also occurs
intermixed with C. nodosa in seagrass
meadows, with a patchy distribution
(Vázquez-Luis et al. 2008).
In this study we focus on herbivory
patterns of S. salpa on C. prolifera, C.
cylindracea and the seagrasses C.
nodosa and P. oceanica present in a
mixed meadow. In order to integrate
possible spatial and temporal
variability in macrophyte availability
and grazing activity, experiments
were carried out during summer 2012
(July-August) and autumn 2012
(September-October)
in
two
randomly selected locations (A and
B) 2-3 km apart (Fig. 4.1).
124
4.3.2. Bottom characterization
At each location (A and B) and study
time (hereafter, T1: summer; T2:
autumn) shoot density (number per
m2) was measured by counting shoots
in a 40 cm x 40 cm quadrant placed
in three randomly selected patches
within the mixed habitat. Bottom
cover was measured by visually
estimating
the
percentage
of
substrate occupied by macrophytes
along 25 m line transects (n = 3).
Because of the heterogeneity of this
mixed habitat, we also characterize
the macrophyte species assemblage
covering the rocky substrate present
in the study area. Three random
replicates were taken within a 20 cm
x 20 cm quadrant by scraping the
whole surface with a trowel. In the
laboratory, all macrophytes were
sorted, along with the larger
fragments of detritus. After species
identification, macrophytes and
detritus were dried for 24 h at 80ºC,
and weighed. We estimated the
percentage
covered
by
each
macrophyte species relative to the
total weight of sample scraped from
the rocky substrate. All the
techniques were conducted according
to standardized procedures (Romero
1985; Alcoverro et al. 1995;
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
Sánchez-Lizaso 1993; Ruitton et al.
2005).
When
during
visual
characterization two species were
highly mixed in the same patch, we
recorded it as: e.g., C. nodosa and C.
prolifera (CE-CP).
4.3.3. Fish abundances and feeding
observations
Individuals of S. salpa were counted
by scuba divers through visual census
(50 m2 line transects). Censuses were
carried out at the same time of day,
Figure 4.1. Map of the study area, Cabo de Santa Pola (Spain), showing the two study locations
(A and B).
125
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
two different days at each time (T1;
T2) and at each study location (A; B)
(n = 32 censuses per time and
location). In each transect we
recorded number of individuals
within the school and their average
size (ind m-2).
The feeding activity of S. salpa was
also
recorded
through
visual
observations. In this case, scuba
divers observed and recorded a total
of 18 schools of fish feeding at each
time and study location (each
observation during ca. 7 min). On
each occasion, we followed a school
of fish and recorded the number of
individuals within the school, their
average size, and if they were
swimming or feeding, in which case
the food items consumed were
recorded.
The
percentage
of
individuals swimming or feeding on
the different items was estimated
relative to the total of individuals
observed.
4.3.4. Tethering experiments
Temporal and spatial variation in
consumption rates of C. prolifera, C.
cylindracea, C. nodosa and P. oceanica
by S. salpa were estimated with
tethering
experiments deployed
126
within monospecific patches at the
two different times and locations of
study (one tethering per species, time
and location). Each tethering line
and controls consisted of 18
replicates, with similar amounts of
freshly collected algae and seagrass
biomass (collected the morning of
the experiment). On tethering lines,
we used floating replicates (by using
small buoys) to avoid benthic
invertebrate herbivores (e.g., sea
urchins). Floating replicates were tied
to a thin cord and deployed in the
field for a week, mixing them within
the
monospecific
macrophyte
patches. Each end of the line was
secured to the bottom with rebars.
Controls for changes in wet weight
unrelated to herbivory consisted of
identical portions of each macrophyte
species (individually protected from
herbivores by 0.5 cm2 mesh cages)
deployed in the field during the same
period. All replicates were cut down
to remove previous herbivore marks
and blotted dry of excess water before
measuring initial and final wet
weight (3 g wet weight per replicate).
After a week, tethering and control
replicates were collected and biomass
consumption by S. salpa, whose bite
marks are easily distinguishable (e.g.,
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
Tomas et al. 2005b), was estimated
as [(Hi x Cf/Ci) - Hf], where Hi and
Hf were initial and final wet weights
of tissue exposed to herbivores, and
Ci and Cf were initial and final
weights in controls (Parker & Hay
2005; Tomas et al. 2011a).
Macrophytes
consumption
was
expressed as mg of wet weight
consumed per day.
4.3.5. Food choice experiments
Given the high consumption of
Caulerpa spp by S. salpa detected in
previous studies (Tomas et al. 2011b;
Marco-Méndez et al. 2015a), we
conducted
paired
feeding
experiments to examine the relative
palatability of C. prolifera and C.
cylindracea vs. the two main
Mediterranean seagrass species: C.
nodosa and P. oceanica. In addition,
given the previously reported
importance of epiphytes in herbivore
feeding choices (Marco-Méndez et
al. 2012), paired experiments were
carried out with epiphytized and
non-epiphytized leaves only for
seagrass, as Caulerpa species were not
epiphytized.
Food
choice
experiments were conducted in
summer, when macroalgae and
epiphyte biomasses have maximum
Figure 4.2. Bottom coverage (%) with the
different macrophyte species at both
locations (A and B) and both sampling
times (T1: summer 2012; T2: autumn
2012). Labels: sandy substrate (Sand); dead
Posidonia oceanica matte (Dead matte);
Caulerpa prolifera (CP); Caulerpa cylindracea
(CC); Cymodocea nodosa (CE); Posidonia
oceanica (PE); mixed rocky substrate and C.
prolifera (Rock-CP) and mixed C. prolifera
and C. nodosa (CP-CE).
values and they undergo the highest
pressure from S. salpa (Alcoverro et
al.
1997).
Experiments
were
deployed in large sandy patches (ca.
2–4 m depth; at least 5 m away from
rocks, seagrass or macroalgae) to
ensure that no other surrounding
macrophytes could interfere with fish
feeding choices and that invertebrate
herbivores did not have access to
experimental setups. A total of 9
paired floating tethering experiments
were carried out with the following
paired combinations: (1) C. nodosa
epiphytized vs. C. prolifera (CE vs.
CP); (2) C. nodosa epiphytized vs. C.
cylindracea (CE vs. CC); (3) C. nodosa
non-epiphytized vs. C. prolifera
127
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
(CNE vs. CP); (4) C. nodosa nonepiphytized vs. C. cylindracea (CNE
vs. CC); (5) P. oceanica epiphytized vs
C. prolifera (PE vs. CP); (6) P.
oceanica epiphytized vs C. cylindracea
(PE vs. CC); (7) P. oceanica nonepiphytized vs. C. prolifera (PNE vs.
CP); (8) P. oceanica non-epiphytized
Figure 4.3. A. S. salpa abundances (ind m-2); B. Feeding observations (%): swimming (SW);
feeding on H. scoparia and C. prolifera (HS-CP); C. prolifera and P. oceanica (CP-PE); C.
prolifera and C. nodosa (CP-CE) and C. prolifera (CP) and C. Macrophyte consumption by S.
salpa (mg WW d-1) at both locations (A and B) and both times of sampling (T1: summer 2012;
T2: autumn 2012). Mean ± SE (in SNK, a and b indicate significant groupings).
128
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
vs. C. cylindracea (PNE vs. CC) and
(9) C. prolifera vs. C. cylindracea (CP
vs. CC). For each experiment, similar
amounts of freshly collected algal and
seagrass biomass were offered in pairs
(ca. 3 g wet weight). Replicate pairs
(n = 18) and their respective controls
(individually
protected
from
2
herbivores by 0.5 cm mesh cages)
were deployed at least 1 m apart and
collected
after
four
days.
Consumption was estimated as for
tethering experiments and expressed
as mg wet weight lost by S. salpa bite
marks.
4.3.6. Gut contents, stable isotope
analyses and nutrient contents
A total of 26 individuals of S. salpa
(average length: 22.9 ± 0.6 cm) were
randomly collected within the area
for a dietary study. Since individuals
were caught at two different times we
studied them separately (n = 13
individuals per group or school).
We used all individuals for gut
content analyses and 10 individuals
(n = 5 from each school) for nutrient
content and stable isotope analyses
(SIA). In the laboratory, lateral
muscles were isolated for SIA and
nutrient content analysis. Gut
contents were extracted and food
items separated under the microscope
(e.g., P. oceanica leaves, C. prolifera,
C.
cylindracea
and
epiphytic
macroalgae). Each fraction was dried
to constant weight at 60ºC.
Samples of Caulerpa and seagrass
species were randomly collected from
the study area for SIA and nutrient
content analyses. These samples
included: C. prolifera (CP), C.
cylindracea (CC), P. oceanica and C.
nodosa epiphytized leaves, epiphytes
not scraped off (PE and CE) and
non-epiphytized leaves, epiphytes
scraped off (PNE and CNE; n = 5),
as well as their respective epiphytes
(EC and EP; n = 5). These latter
included
both
epifauna
(heterotrophic
metazoans)
and
epiflora (macroalgae). Samples from
schools and food resources were dried
to constant weight at 60ºC and
ground to fine powder for
determination of nutrient contents
(C:N) and isotopic signatures (δ15N
and δ13C).
Analyses were carried out with an
EA-IRMS
(Thermo
Finnigan)
analyzer
in
continuous
flow
configuration at the Technical Unit
of Instrumental Analyses (University
129
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
of La Coruña, Spain). The average
difference in isotopic composition
between the sample and reference
material (δsample-standard, expressed in ‰)
corresponds to:
[(R sample - R
δsample-standard
)/R
standard
] x 1000=
standard
where Rsample is the 13C/12C or
15
N/14N ratio in the sample; R standard
is the 13C/12C or 15N/14N ratio for the
reference material (i.e., CaCO3 from
belemnite [PBD] for δ13C and
atmospheric nitrogen for δ15N
measurements), calibrated against an
internal standard (i.e., atropine,
IAEA and/or UGS).
4.3.7. Epiphytic community
For epiphytic community analyses we
collected the oldest leaves on shoots
of P. oceanica and C. nodosa (n = 10)
at the two study times (summer and
autumn
2012).
These
are
representative of the epiphyte
community during the entire life
span of the leaf (Prado et al. 2008b).
Epiphytic cover (%) on the leaf
surface was estimated visually, and
then organisms were scraped off
gently for identification to genus
level under the microscope. Finally,
epiphytes were dried to a constant
130
weight at 60ºC
determination.
for
biomass
4.3.8. Data analyses
Differences in bottom coverage,
percentage of rocky substrate covered
and consumption rates by S. salpa
among macrophyte species, times
(T1: summer; T2: autumn) and
locations (A and B) were investigated
with a three-way ANOVA design
with two fixed factors (‘Macrophyte’
and ‘Time’) and a random orthogonal
factor (‘Location’). The factor
‘Macrophyte’ had as many levels as
the number of species identified in
coverage and four levels for tethering
analyses (CP, CC, CE, PE). Factors
‘Time’ and ‘Location’ both had two
levels in all analyses.
Differences in the abundances of
S. salpa during the study were
analyzed with a two-way ANOVA
with ‘Time’ and ‘Location’ as fixed
and random factors, respectively.
Wilcoxon signed-ranks paired test
was
applied
to
food-choice
experiments, due to lack of normality
and homoscedasticity of data.
Differences in isotopic signatures (δ
15
N and δ13C) and nutrient content
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
(C:N molar ratio) among food
resources were tested through a oneway ANOVA with 8 levels (CP, CC,
PE, PNE, CE, CNE, EC and EP).
Differences between S.
salpa
individuals from the two different
schools were subjected to standard ttests. The IsoSource (Phillips &
Gregg 2003) isotope mixing model
was used to identify the contributions
of each food source to the diets of
school 1 and 2 separately. Since
results by Prado et al. (2012)
concluded that there is a strong
dietary effect on fractionation (i.e.,
seagrass,
macroalgae,
and
omnivorous diet fractionations were
different) and both schools of S. salpa
were collected where all those diets
were available, the model was run
with the means of the fractionation
values found for seagrass and
macroalgae diets (0.63 ± 0.29‰ for
δ15N and 2.49 ± 0.25‰ for δ13C;
means ± SE). Since seagrasses have
very low digestibility, those mean
values were considered more accurate
than assuming the theoretical 3.4‰
enrichment between trophic levels.
The input parameters for the model
were the isotopic values of the
consumer and trophic resources
(measured in this study) and the
overall fractionation rates (mean ±
SE). Since the sensitivity of the
mixing model is mostly affected by
the isotopic signature difference
between the sources (Phillips &
Gregg 2003) and no differences were
found between EC and EP isotopic
signals, averaged δ15N and δ13C
values were used to run the model.
We also decided to use averaged δ15N
and δ13C values for CR and CP, as
no significant differences were found
between their δ15N values, despite
δ13C values being significantly
different.
Differences in number of epiphyte
taxa, cover and biomass between P.
oceanica and C. nodosa leaves were
analyzed with standard t-tests.
ANOVA assumptions of normality
and homogeneity of variance were
assessed with the KolmogorovSmirnov and Cochran’s C tests,
respectively. When necessary, an
appropriate
transformation
was
performed before further analysis.
When assumptions were not met, the
level of significance was set at 0.01 to
reduce the possibility of committing
Type I errors (Underwood 1997).
131
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Student-Newman-Keuls
post-hoc
tests were used to single out
significant groupings. The statistical
tests were done using SPSS software
Figure 4.4. Consumption by S. salpa during paired food preference experiments (mg WW d-1):
A. C. nodosa non-epiphytized (CNE) vs. C. prolifera (CP); B. C. nodosa non-epiphytized (CNE)
vs. C. cylindracea (CC); C. C. nodosa epiphytized (CE) vs. C. prolifera (CP); D. C. nodosa
epiphytized (CE) vs. C. cylindracea (CC); E. P. oceanica non-epiphytized (PNE) vs. C. prolifera
(CP); F. P. oceanica non-epiphytized (PNE) vs. C. cylindracea (CC); G. P. oceanica epiphytized
(PE) vs. C. prolifera (CP); H. P. oceanica epiphytized (PE) vs. C. cylindracea (CC) and I. C.
prolifera (CP) vs C. cylindracea (CC). Mean ± SE. *p < 0.05; **p < 0.01; NS = not significant.
132
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
and GMAV 5 software (University
of Sydney, Australia).
n-MDS ordination (Bray-Curtis
similarity index), ANOSIM and
SIMPER
(available
in
the
PRIMER-E v.6 software package,
Clarke & Warwick 1994) were
applied
to
stomach
contents
(percentage)
and
epiphytic
assemblages
(presence-absence
transformation).
4.4. RESULTS
4.4.1. Bottom characterization
No significant differences were found
in the shoot density of P. oceanica and
C. nodosa either between times or
between locations (T1: C. nodosa
1029.2 ± 217.4 shoots m-2; P. oceanica
478.1 ± 66.6 shoots m-2; T2: C.
nodosa 875.0 ± 219.7 shoots m-2; P.
oceanica 367.7 ± 45.1 shoots m-2). In
contrast, significant effects were
found for bottom coverage but only
among ‘Macrophyte’ species (Three
way ANOVA, p < 0.01, Fig. 4.2).
The highest percentages of cover
were recorded for P. oceanica (T1:
46.2 ± 6.6%; T2: 34.6 ± 8.4%;
average of A and B) and C. prolifera
(T1: 19.7 ± 6.8%; T2: 27.5 ± 5.3%;
average of A and B). On rocky
substrates,
the
interaction
‘Macrophyte x Time’ was significant
(Three-way ANOVA, p < 0.001).
The highest percentage of rocky
substrate covered was recorded in
time 1 for C. prolifera (48.0 ± 22.2%)
while C. cylindracea and P. oceanica
coverage were similar during the
study, with values slightly higher at
time 1 (8.6 ± 1.7% and 8.9 ± 5.3%
respectively) than at time 2 (5.9 ±
3.5% and 4.6 ± 4.6%). The remaining
percentages corresponded to other
macroalgae species identified (C.
compressa; Dictyota sp; U. compressa; J.
rubens, P. pavonica and H. scoparia,
and detritus).
4.4.2. Fish abundances and feeding
observations
There was a significant ‘Time x
Location’
interaction
in
the
abundance of S. salpa. The highest
abundances reported during the
study were those recorded at time 1
and location A (0.56 ± 0.15 ind m-2)
(Two way ANOVA, p < 0.01, Fig.
4.3A).
Feeding observations showed that
individuals fed on a variety of species
during time 1, (H. scoparia, C.
prolifera, P. oceanica and C. nodosa)
133
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 4.5. Stomach contents analyses of S. salpa individuals from the two schools collected; A.
Percentage of food items found and B. n-MDS ordination. Labels as in previous figures.
but mainly on C. prolifera and C.
nodosa during time 2 (Fig. 4.3B).
4.4.3. Tethering experiments
Tethering
experiments
showed
significant differences for the
interaction ‘Macrophyte x Time’
(Fig. 4.3C, Table 4.1). Consumption
rates of C. nodosa were only
134
significantly
higher than
the
consumption recorded for the other
macrophyte species at time 1 (12.75
± 3.43 mg WW d-1, 0.51 ± 0.13% of
wet plant biomass per day). In
addition, consumption rates of C.
nodosa at time 1 were also
significantly higher than at time 2.
Despite the fact SNK analyses did
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
not detect further significant
differences between the rest of
macrophyte species or trough times,
consumption of C. prolifera tended to
be higher during time 1 (0.53 ± 0.21
mg WW d-1) than during time 2
(0.21 ± 0.12 mg WW d-1) while P.
oceanica
displayed
lower
but
consistent consumption rates and no
herbivory on C. cylindracea was
detected.
4.4.4. Food choice experiments
Sarpa
salpa
displayed
higher
consumption rates of CP vs. CNE (Z
= -2.525, p = 0.012, Fig. 4.4A) but
not of CC vs. CNE (Z = -1.604, p =
0.109, Fig. 4.4B). In presence of
epiphytes, no significant differences
were
found
either
in
the
consumption of CE vs. CP or in that
of CE vs. CC (Fig. 4.4C,D).
Regarding P. oceanica, a significant
higher consumption of CP vs. PNE
was recorded (Z = -2.023, p = 0.040,
Fig. 4.4E) but no consumption of
PNE vs. CC (Fig. 4.4F). In the
presence of epiphytes no significant
differences were found either in the
consumption of PE vs. CP or in PE
vs. CC (Fig. 4.4G,H). Finally, the
consumption of CP was significantly
higher than CC (Z = -3.185, p =
0.001, Fig. 4.4I). The highest
consumption rates for CP were
observed vs. CNE (2.58 ± 0.91 mg
WW d-1), followed by those observed
vs. CC and PNE (1.07± 0.49 mg
WW d-1; 0.34± 0.17 mg WW d-1
respectively).
4.4.5. Gut contents
Gut contents of S. salpa individuals
from school 1 comprised epiphytes
(6.7%), P. oceanica (39.7%) and C.
prolifera (53.5%), while school 2
samples showed a diet of P. oceanica
(0.5%), C. prolifera (31.8%) and C.
cylindracea (67.7%) (Fig. 4.5A).
n-MDS ordination of the gut items
showed different groupings between
individuals from schools 1 and 2
(Fig. 4.5B). ANOSIM results
confirmed that these two groupings
were significantly different (Global
R: 0.48; p = 0.001). The average
similarity among school 1 gut
contents was 42.87% and school 2
was
59.85%.
The
average
dissimilarity between the two schools
of fish was 80.81%, mostly due to C.
cylindracea (41.89%), C. prolifera
(29.44%) and P. oceanica (24.52%).
135
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
4.4.6. Stable isotope analyses
Both δ13C and δ15N signatures
showed significant differences among
food items (One-way ANOVA,
p < 0.001, Fig. 4.6A, Table 4.2). The
highest δ15N values were recorded for
CP and CC (7.25 ± 0.27‰ and 7.59
± 0.07‰ respectively) and the lowest
for PE and PNE (4.49 ± 0.07‰ and
4.36 ± 0.09‰ respectively). For δ13C,
the highest values were recorded for
CNE (-9.58 ± 0.01‰) and the
lowest for CC (-16.67 ± 0.11‰), and
the epiphytes from P. oceanica and C.
nodosa leaves (-17.06 ± 0.17‰; -5.93
± 0.23‰ respectively). Regarding
consumers, significant differences
were found in the δ15N values
between the two S. salpa schools
(t = 5.004, df = 7.527, p = 0.001) and
these
values
(school
1:
12.85 ± 0.46‰;
school
2:
9.95 ± 0.35‰) were closer to CC
and CP values. In contrast, no
significant differences were found for
δ13C signals between the two schools
(t = 2.620, df = 7.824, p = 0.31) and
values (school 1: -16.23 ± 0.42‰;
school 2: -17.90 ± 0.35‰) lay much
closer to CC, CP and epiphytes
(including both ‘macroalgae’ and
metazoans), than to seagrass values
Table 4.1. Three-way ANOVA showing differences in macrophyte consumption by S. salpa (mg
WW d-1) between macrophyte species (CP; CC; CE; PE), times (T1; T2) and locations (A; B).
Labels: C. prolifera (CP); C. cylindracea (CC); C. nodosa (CE) and P. oceanica (PE). Significant
differences are indicated: ** p < 0.01; NS: not significant. NT: no transformation. In SNK,
significant differences between investigated groups are indicated.
Source of variation
Consumption (mg WW d-1)
df
MS
F
p
Macrophyte (Mac)
3.00
694.95
31.35
**
Time (Ti)
Location (Loc)
MacXTi
1.00
1.00
3.00
749.47
13.79
718.19
56.05
0.51
29.67
NS
NS
**
MacXLoc
TiXLoc
MaXTiXloc
3.00
1.00
3.00
22.17
13.37
24.20
0.83
0.50
0.90
NS
NS
NS
272.00
287.00
26.85
RES
TOT
SNK
Transformation
136
CETi1>CETi2=CPTi1=CPTi2=CCTi1=CCTi2=PETi1=PETi2
NT
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
(Fig. 4.6A, Table 4.2).
Results from the IsoSource model
indicated that, in the long term, the
diet of both schools of S. salpa
consisted of Caulerpa spp, epiphytes
and seagrasses (school 1, P. oceanica:
20%; C. nodosa: 12%; epiphytes: 28%;
Caulerpa spp: 32%; school 2, P.
oceanica: 10%; C. nodosa: 16%;
epiphytes: 38%; Caulerpa spp: 30%;
both at the percentile 50%).
4.4.7. Nutrient contents in
macrophytes and epiphytes
There were significant differences
among C:N molar ratios of food
items, with the highest values found
for PNE and PE (34.06 ± 0.45;
29.93 ± 1.48 respectively) and the
lowest for both types of epiphytes
(EC: 14.66 ± 0.33; EP: 10.06 ± 0.98;
one-way ANOVA, p < 0.001, Fig.
4.6B, Table 4.2). For S. salpa, no
differences were found in the C:N
molar ratios between the two schools
(3.47 ± 0.01 for both; t = -0.188, df =
7.957, p = 0.855).
4.4.8. Epiphytic community
Significant differences were found in
the epiphytic biomass due to the
interaction ‘Macrophyte x Time’
(Two-way ANOVA, p < 0.001,
Table 4.3). Despite lower leaf area,
C. nodosa supported the most
epiphytic biomass and P. oceanica the
lowest, both values recorded at time
2 (EC: 0.23 ± 0.04 mg DW cm-2;
EP: 0.07 ± 0.02 mg DW cm-2; Table
4.3).
We
found
significant
differences in leaf epiphytic cover
with respect to ‘Macrophyte’ and
‘Time’ (two-way ANOVA, Table
4.3). During the study, the recorded
values were consistently higher for C.
nodosa vs. P. oceanica leaves and for
time 2 vs. time 1 (Table 4.3).
Concerning the number of epiphytic
taxa, significant differences were
found
for
the
interaction
‘Macrophytes x Time’ (Two-way
ANOVA, p < 0.01, Table 4.3). The
highest number of epiphytic taxa was
found on C. nodosa leaves at study
times and the lowest was recorded on
P. oceanica leaves at time 2.
n-MDS ordination of epiphytic taxa
displayed four distinctive groupings
considering times (T1: summer; T2:
autumn) and seagrass species (CE;
PE): T1-Sum-CE; T1-Sum-PE;
T1-Aut-CE and T1-Aut-PE (Oneway ANOSIM, four levels: Global R
= 0.749, p = 0.001, Fig. 4.7).
SIMPER analyses indicated that the
137
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Table 4.2. Differences in δ15N and δ13 C signatures and nutrient contents (C:N ratios) among
food items: C. cylindracea (CC); C. prolifera (CP); C. nodosa non-epiphytized (CNE); P. oceanica
epiphytized (PE); P. oceanica non-epiphytized (PNE) and epiphytes of C. nodosa (EC) and P.
oceanica (EP). Significant differences are indicated: *** p < 0.001. NT: no transformation was
carried out. In SNK, significant differences between investigated groups are indicated.
Source of
variation
15δN
13δC
Food item
df
7
MS
7.2754
Residual
32
0.1515
Total
39
SNK
CC=CP>EC=EP=CE=CNE>PE=PNE
CNE>CE>PE=PNE>=CP>EP=CC=EC
PNE>PE=CNE>CE>CC>CP>EC>EP
NT
NT
NT
Transformation
F
48.01
p
***
df
7
MS
35.1677
32
0.401
C:N
p
***
39
epiphytic community on C. nodosa
leaves displayed an average similarity
of 70.19% at time 1 (T1-Sum-CE)
and 82.97% at time 2 (T2-Aut-CE).
The epiphytes on P. oceanica had an
average similarity of 63.14% at time 1
(T1-Sum-PE) and 65.33% at time 2
(T2-Aut-PE).
The
average
dissimilarity between C. nodosa and
P. oceanica was 64.50% at time 1 (R =
0.85, p = 0.001) and 62.75% in time
2 (R = 0.99, p = 0.001), mainly due
to Myrionema magnusii (Sauvageau)
Loiseaux, Ceramium sp, Lyngbya sp
and Sphacelaria cirrosa (Roth) C
Agardh. The epiphytic community
on C. nodosa leaves showed an
average of dissimilarity between time
1 and time 2 of 53.79% mainly due
to S. cirrosa, Myriactula gracilis van
der Ben and Cladophora sp. (R =
0.90; p = 0.001) while P. oceanica
138
F
87.69
df
7
MS
345.571
32
2.596
F
133.12
p
***
39
epiphytic community did not display
significant dissimilarity between
times (32.55%, R = -0.092; p = 0.99).
The average dissimilarity between
T1-Sum-CE and T2-Aut-PE was
63.91% and between T2-Aut-CE
and T1-Sum-PE was 63.54%, due in
both cases mainly to M. magnusii (R
= 0.85, p = 0.002 and R = 0.92, p =
0.001 respectively).
4.5. DISCUSSION
This study points to green alga
Caulerpa prolifera and seagrass
Cymodocea nodosa respectively as the
most consumed and the most
preferred food species by the
Mediterranean fish Sarpa salpa.
Different nutritional contents and
epiphyte presence likely explain why
the preference of S. salpa for C.
prolifera was not sustained vs.
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
epiphytized leaves and therefore did
not deflect herbivory pressure on the
most epiphytized and nutritious
seagrass
C. nodosa,
the
‘most
consumed’ macrophyte in the mixed
meadow. Our results highlight the
possible mediating role of epiphytes
and nutrient contents in S. salpa
selectivity. Despite this, they also
show the complexity of macrophyteherbivore interactions and suggest
that final consumption rates and
dietary differences are not only
determined by food preferences, but
Figure 4.6. A. δ15N and δ13C signatures of S. salpa individuals from the two schools collected
and food items, including epiphytized and non-epiphytized seagrass leaves and their respective
epiphytes (CC, CP, CE, CNE, PE, PNE, EC and EP); B. C:N molar ratios in consumers and
food items. Mean ± SE.
139
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
also by multiple factors such as
feeding
behavior,
home-range
mobility and spatial and temporal
variability in fish abundances and
macrophytes availability. All these
factors could act synergically to
influence herbivory patterns in
Mediterranean seagrass meadows.
Tethering experiments showed that
C. nodosa was the most consumed
macrophyte, recording in summer
consumption
rates
significantly
higher than the reported for the
other macrophytes species during the
whole study (12.75 ± 3.43 mg
WW d-1; 0.51 ± 0.13% of plant per
day). Despite the fact analyses did
not detect further significant
differences among the rest of the
species, the consumption of C.
prolifera in summer tended to be
higher than in autumn (̴ 2.5 times),
while consumption of P. oceanica was
consistently low and no consumption
of C. cylindracea was detected during
the study. The high variability
reported in the present study concurs
with the high temporal and spatial
variability in the previous estimates
of S. salpa herbivory on P. oceanica
(Prado et al. 2007a; Tomas et al.
2005a). Although it is well known
that P. oceanica has been subjected to
140
heavy herbivore pressure in shallow
meadows (Prado et al. 2007a), our
results show that herbivory on C.
nodosa and C. prolifera can also be
important (see also Marco-Méndez
et al. 2015a, Tomas et al. 2011b) and
should be considered when studying
herbivory in Mediterranean seagrass
meadows.
The high variance in herbivory has
been partially attributed to changes
in
herbivore
abundance
and
distribution, which can be a
consequence of the interaction
among recruitment rates (Camp et al.
1973),
predation
effects
(McClanahan et al. 1994) or
overfishing (Klumpp et al. 1993). In
addition, the fish S. salpa displays
seasonal mobility patterns according
to its nutritional needs. This
accounts for massive schools of fish
feeding actively in summer on
seagrass meadows in order to
accumulate reserves for the winter
period, when fish eat less, migrate to
greater depths and prepare for
reproduction (Peirano et al. 2001).
This seasonal migration explains the
high temporal variability in the
abundances of S. salpa individuals
detected in our study, with the
highest fish densities during summer
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
(location A: 0.56 ± 0.15 ind m-2) but
decreasing during autumn (Tomas et
al. 2005a; Prado et al. 2007a). Since
the significantly higher abundance
recorded at location A was restricted
to summer, it was probaby related to
variability in the mobility pattern
within the home range of the species
(ca. 4.3 ha; Jadot et al. 2002, 2006),
rather than to spatial differences in
recruitment rates, predation or
overfishing. Accordingly, temporal
variability in fish abundance strongly
influenced
the
more
intense
herbivory in summer, especially on C.
nodosa, and the low consumption
rates of all macrophyte species during
the autumn (Ruitton et al. 2006;
Tomas et al. 2011b). Nevertheless,
these results contrast with a previous
study carried out in a differently
located mixed meadow (MarcoMéndez et al. 2015a), where
herbivory by S. salpa on C. nodosa in
late summer was not detected despite
similar fish densities and habitat
features. Furthemore, the lack of
consumption of C. cylindracea
throughout our study contrasts with
the findings of Tomas et al. (2011b),
where S. salpa consumed large
quantities of that invasive algae. This
apparent contradiction between
studies reinforces the theory that
herbivory varies strongly both
spatially and temporally (Tomas et al.
2005a; Prado et al. 2008a). It is not
only influenced by temporal changes
in fish abundances but probably also
by their home-range size, habitat
selection or variability in individual
behavior, (Jadot et al. 2002, 2006).
Plant availability and accessibility or
feeding preferences for some
macrophyte species could also be
mediating
herbivory
on
Mediterranean seagrasses meadows
(Prado et al. 2008a, 2009, 2010a).
Food choice experiments recorded
the highest consumption on C.
prolifera (2.58 ± 0.92 mg WW shoot1 -1
d in CP vs. CNE) and showed
that S. salpa individuals only
preferred to feed on C. prolifera vs. P.
oceanica
and
C. nodosa
when
epiphytes were removed, pointing to
the mediating role of epiphytes in
herbivore selectivity (Tomas et al.
2005b; Marco-Méndez et al. 2012,
2015a). In addition, their consistent
preference for C. prolifera vs. C.
cylindracea suggests that other factors
inherent to macrophyte features
could also be involved. Even though
some
inconclusive
experiments
showed no preferences for C.
141
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 4.7. n-MDS ordination of epiphyte taxa between the two study times (T1-Summer; T2Autumn) and seagrass species (CE: C. nodosa; PE: P. oceanica).
cylindracea vs. seagrasses (epiphytized
or non-epiphytized leaves), the
strong preference for C. prolifera and
the lack of consumption detected by
tethers suggest that S. salpa may
prefer feeding on native species. This
again contrasts with the reported
selectivity for this species vs. other
native species such as P. oceanica
(Tomas et al. 2011b). Studies suggest
that manifested preferences and
higher feeding rates of marine
herbivores on such diets respond to a
combination of high nitrogen and
protein content and epibiotic load, or
with low amounts of chemical and
142
structural components (Cebrián &
Duarte 1998; Mariani & Alcoverro
1999; Verges et al. 2007a,b).
In our study, differences in C:N
ratios among Caulerpa species,
seagrass species and epiphytes are
likely to have influenced the observed
patterns of herbivory and selectivity.
However, although both Caulerpa
species recorded lower C:N ratios
than seagrasses, preferences were
only manifested for C. prolifera. On
the one hand, lower C:N ratio values
are consistent with S. salpa's
preference for C. prolifera vs. C.
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
cylindracea. On the other, the
manifested preference for C. prolifera
vs. seagrasses, which was dissipated
in presence of epiphytes seem to
support general assumption that
macroalgae and epiphytes are
considered a better food source than
seagrasses due to their typically lower
C:N ratios (Duarte 1992, 1995;
Alcoverro et al. 1997, 2000). In fact,
C:N ratios values were ca. 2 times
higher in non-epiphytized leaves of
both seagrasses than in C. prolifera.
These nutritional differences were
slightly reduced when seagrasses were
epiphytized. Together with the
significantly lower C:N ratios and
higher nutritional content of
epiphytes compared to C. prolifera
(%N was ca. 3 times higher and %C
ca. 4 times higher), such differences
could explain why the preference for
C. prolifera vs. seagrasses is dissipated
in the presence of epiphytes. This
partly accounts for the herbivory
patterns in the mixed meadow (i.e.,
tethering results). It also confirms
that epiphytes and their higher
nutritional value (e.g., Alcoverro et
al. 1997, 2000) can mediate
herbivore
preferences
and
consumption rates (Marco-Méndez
et al. 2012; Tomas et al. 2005b;
2006). Furthermore, it seems that
because of the higher quality of its
leaves (lower C:N ratio), plus the
increased nutritional value resulting
from the presence of epiphytes,
higher herbivory was recorded for C.
nodosa than P. oceanica in the mixed
meadow (Marco-Méndez et al.
2015a).
Variability in epiphyte composition
has also been reported to influence
herbivore
consumption
and
preferences (Marco-Méndez et al.
2012, 2015a). In the present study,
the epiphytic community structure
revealed
important
differences
between seagrass species and times.
Such differences were probably
influenced by differences in light
shading (Carruthers 1994), and the
effects of shoot morphology and leaf
age on the surface area and timing of
epiphytic colonization (Lavery &
Vanderklift 2002). Cymodocea nodosa
leaves were found to have the highest
epiphytic biomass, cover and taxa,
which may account for it undergoing
more intense grazing than P.
oceanica. However, there was a
disconnection between the highest
epiphytic
biomass
and
cover
(autumn)
and
the
highest
consumption of C. nodosa (summer).
143
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Moreover, analyses of the epiphytic
community
showed
differences
between seagrass species but also
temporal
changes
in
species
composition which could also
influence consumption patterns in
the mixed meadow, confirming the
epiphytic community's mediating
role
in
herbivore-macrophyte
interaction.
Despite the fact differences in
nutritional contents and epiphyte
presence probably explain S. salpa
selectivity and herbivory patterns
observed in these Mediterranean
meadows, involvement of other
macrophyte features could not be
ruled out. Verges et al. (2011)
showed that chemical and structural
defenses can strongly influence
herbivore preferences, although in
this study they were not measured.
Among them, varying levels of
secondary metabolites, toughness
(Fritz & Simms 1992), energy
contents (Hughes 1980) or structural
carbohydrates (cellulose) affecting
food digestibility and absorption
(e.g., Klumpp & Nichols 1983) can
vary greatly between different
macrophyte
species,
critically
mediating
plant-herbivore
interactions (Orians 2002; Taylor et
144
al. 2002). We hypothesized that the
high level of structural carbohydrates
in seagrass leaves, that makes their
digestion less effective (Thayer et al.
1984; Cebrián & Duarte 1998),
could also have influenced selectivity
for C. prolifera vs. non-epiphytized
seagrass leaves. Indeed, these
substances have already been pointed
out as primarily responsible for the
lower herbivory on seagrasses
(Nienhuis & Groenendijk 1986;
Cebrián et al. 1996a,b). Besides
structural defenses, the limited
literature suggests that dominant
species with relatively slow growth
rates such as P. oceanica (Hemminga
& Duarte 2000), are chemically
defended whereas others like
Halophila minor (Zollinger) Den
Hartog, ephemeral pioneer species
with a fast growth rate (Vermaat et
al. 1995), shows no deterrence (Paul
et al. 1990). According to this, C.
nodosa and other fast-growing species
could be less defended chemically
and thus more highly grazed by
herbivores than slower growing
species like P. oceanica (Cebrián &
Duarte 1998). This seems coherent
with the differential consumption
rates between seagrasses found in our
studies. However, Verges et al.
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
Table 4.3. Differences in biomass (mg WW cm-2), coverage (%) and number of taxa (taxa cm-2)
between macrophyte species (CE; PE) and times (Ti1; Ti2). Significant differences are indicated:
** p < 0.01; *** p < 0.001; NS: not significant. NT: no transformation was carried out. In SNK,
significant differences between investigated groups are indicated.
Source of
variation
Biomass (mg WWcm-2)
Coverage (%)
df
MS
F
p
df
MS
F
p
Macrophyte
1 0.0386 2.4
NS 1 8293.0302 40.5 **
(M
Time) (Ti)
1 0.0012 0.08 NS 1 2609.2685 12.74 ***
MacXTi
1 0.2639 16.44 *** 1 41.1031
0.2 NS
Residual
76 0.0161
36 204.7503
Total
79
39
SNK
ECTi2>ECTi1=EPTi1>EPTi2
Ti2>Ti1; EC>EP
Transformation
NT
NT
(2011) also proved that S. salpa
preferred the most nutritious and
chemically
defended
leaves,
suggesting a full adaptation to
consuming
highly
chemically
defended species such as P. oceanica.
This was consistent with our results
and with Goecker et al. (2005), who
found that nitrogen was the key trait
mediating consumption of Thalassia
testudinum Banks ex König by the
bucktooth
parrotfish
Sparisoma
radians (Valenciennes) (although
phenolics may have also affected
feeding).
Concerning
Caulerpa
species, it has also been reported that
this
genus
can
synthesize
caulerpenyne, a secondary metabolite
that plays a major role in their
chemical defense (Pohnert & Jung
2003)
against
epiphytes
and
Taxa cm-2
MS
F
19.2495 138.11
0.1093 0.78
1.3705 9.83
0.1394
df
p
1
***
1
NS
1
**
36
39
ECTi1=ECTi2>EPTi1>EPTi2
Ln(X)
herbivores (Erickson et al. 2006).
Nevertheless, there are examples in
which caulerpenyne does not deter
fish grazing (Meyer & Paul 1992),
indicating that the toxicity is not
intrinsic to the compound, but is a
result
of
metabolite-consumer
interactions (Paul 1992). Studies also
suggest that the presence of the
native
C.
prolifera
in
the
Mediterranean may have allowed
Mediterranean herbivores to evolve a
certain capacity to tolerate this
metabolite (Cornell & Hawkins
2003), and thus make other species
like C. cylindracea more vulnerable to
herbivores.
However,
neither
consumption nor preferences were
detected for this species, despite
previous studies proving that S. salpa
indeed feeds on C. cylindracea
145
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
(Tomas et al. 2011b) and that it is
also consumed (Bulleri et al. 2009;
Cebrián et al. 2011) or preferred
(Tomas et al. 2011a) by the main
Mediterranean
invertebrate
herbivore,
the
sea
urchin
Paracentrotus lividus (L.). In our
study, the observed preference of S.
salpa for C. prolifera vs. seagrasses
(without epiphytes) suggests that this
fish could in fact have evolved some
tolerance
for
caulerpenyne.
Additionally, since lower levels of
caulerpenyne have been reported
(Jung et al. 2002) for the invasive C.
cylindracea (not consumed in this
study) compared to the non-invasive
C. prolifera (which was the preferred
Caulerpa species in this study)
chemical deterrence was unlikely to
be a factor determining the patterns
of S. salpa herbivory observed in the
mixed meadow. From such evidence,
it seems that the more intense
herbivory on C. nodosa and the
selectivity for C. prolifera must have
been mostly influenced by differences
in nutritional content rather than in
chemical compounds, which seem
not to inhibit S. salpa herbivory.
Although this could theoretically also
trigger higher selectivity for C.
cylindracea vs. seagrasses, our results
146
suggest that S. salpa choose to feed
on native species, which contrasts
with results obtained by Tomas et al.
(2011b).
During the whole study period,
feeding observations revealed that the
observed S. salpa individuals were
consistently feeding on a mix of C.
prolifera alga and C. nodosa seagrass,
in agreement with tethering results
and food choice experiments. They
are therefore apparently the most
important food items in their diet.
For C. prolifera, gut content analyses
were consistent with previous results,
confirming it as a ‘preferred food
item’ in the diet of the two schools of
fish sampled. In contrast, the absence
of C. nodosa coupled with the
presence of C. cylindracea did not
agree
with
tethering
results,
indicating
these
species
as
respectively the ‘most consumed’ and
the only ‘not consumed’ macrophytes
in the mixed meadow. This confirms
that herbivore-seagrass interactions
are complex and not only respond to
changes in fish abundance or feeding
preferences but to multiple factors
that could be acting synergistically. It
is known that seagrass abundance,
habitat heterogeneity and complexity,
and S. salpa foraging movements can
CHAPTER 4: Epiphytes and nutrient contents influence Sarpa salpa herbivory on Caulerpa species vs. seagrass species
also influence herbivory patterns in
Mediterranean seagrass meadows
(Prado et al. 2008a, 2010a, 2011).
On the one hand, P. oceanica, C.
prolifera and mixed patches of C.
nodosa-C. prolifera were consistently
present throughout the study without
temporal variation in the mixed
meadow, which was coherent with
feeding observations and gut content
analyses, except for C. nodosa. On the
other hand, C. cylindracea did not
display any temporal variation or
different availability on the rocky
substrate that could explain its high
presence in the guts of one school. In
contrast, we hypothesize that spatial
variability may be involved and that
S. salpa mobility across other sites
with lower abundance of C. nodosa,
or higher abundance of C. cylindracea
within its home range (ca. 4.3 ha
according to Jadot et al. 2002, 2006),
could account for the absence or
enhanced presence of these species
within gut contents, also explaining
the dietary differences between the
two schools. IsoSource mixing model
results showed that both seagrassses
as well as Caulerpa species and
epiphytes all contribute to the longterm diet of S. salpa, and highlights
the importance of Caulerpa species
(which seems to be mainly attributed
to the high consumption of C.
prolifera) and epiphytes in their diet.
This
analytical
contribution
ultimately reflects preferences and
consumption
patterns
observed
during the study and supports the
previously reported importance of
epiphytes (Marco-Méndez et al.
2012, 2015a) and Caulerpa species in
S. salpa herbivory (Ruitton et al.
2006, Tomas et al. 2011b).
In conclusion, our study highlights
the importance of C. nodosa and C.
prolifera in the diet of S. salpa, and
also that herbivory in Mediterranean
meadows can be highly variable and
mediated by multiple factors. In
summer, when densities of S. salpa
are highest, C. nodosa was the ‘most
consumed’ macrophyte, probably
influenced by the higher nutritional
quality of its leaves and epiphytes, as
well as by differences in the epiphytic
community composition (MarcoMéndez et al. 2015a). Food choice
and feeding observations, and gut
content analyses all pointed to C.
prolifera as a food consistently
selected by S. salpa. In contrast,
preference of S. salpa for C. prolifera
was not sustained vs. epiphytized
leaves, which suggests that epiphyte
147
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
presence and consequently different
nutritional contents probably explain
the herbivory patterns in the mixed
meadow. In fact, the IsoSource
mixing
model
confirms
the
importance of Caulerpa species,
which from our results seems to be
mostly attributable to the high
consumption of C. prolifera. Despite
this predominance, they also
highlight the role of epiphytes in the
long-term diet of S. salpa. Although
C. cylindracea consumption was not
observed during the study and S.
salpa seems to feed preferentially on
148
native species, the fact that it was
found within stomach contents
suggests that they may eventually
adapt to feeding on this new
resource. Our study illustrates the
complexity of macrophytes-herbivore
interactions and suggests that final
consumption rates and dietary
differences are not only determined
by food preferences, but also by
home-range mobility, as well as by
temporal and spatial differences in
the availability of food resources.
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
149
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Published as:
Marco-Méndez C, Prado P, Ferrero-Vicente LM, Ibáñez C,
Sánchez-Lizaso JL (2015) Seasonal effects of waterfowl grazing on
submerged macrophytes: The role of flowers. Aquat Bot 120: 275–
282
150
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
5. Seasonal effects of waterfowl grazing on
submerged macrophytes: The role of flowers
5.1. ABSTRACT
Seasonal effects of waterfowl (Fulica atra L. and Anas platyrhynchos
L.) grazing on submerged macrophytes (Ruppia cirrhosa [Petagna]
Grande and Potamogeton pectinatus L.) and the mediating role of
flowers on plant consumption were evaluated by exclusion cages
and tethering experiments deployed in a Mediterranean lagoon
throughout the annual cycle. Despite the low waterfowl abundance
recorded in summer, exclusion-cage experiments evidenced intense
herbivory on the biomass, canopy height and flowers of R. cirrhosa
(flowers abundance was ∼8 times higher inside exclusion cages;
1015.7 ± 269.8 flw m−2). For P. pectinatus, exclusion cage
experiments did not evidence waterfowl consumption, in spite of
the presence of flowers, which suggest preference for reproductive
tissues of R. cirrhosa. In addition, the higher abundance of R.
cirrhosa flowers compared to P. pectinatus (∼10 times higher inside
the exclusion cages) was likely influenced by more intense
herbivory on the former species. Although waterfowl abundance
increased in autumn and winter, experiments did not evidence
herbivory effects during that period, possibly because of enhanced
availability of alternative resources and decreased plant biomass
and canopy height reducing encounter rates. Hence, our results
suggest that waterfowl effects on submerged macrophytes in
Mediterranean aquatic ecosystems are strongly influenced by
seasonal changes in the availability of food resources and its
flowering events. The higher herbivory on R. cirrhosa and its
flowers in summer suggest that waterfowl grazing may be driven
by food preference for reproductive tissues, and could have a strong
151
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
effect on the community structure and abundance of submerged
macrophytes.
5.2. INTRODUCTION
In aquatic ecosystems such as coastal
lagoons and lakes, the submerged
aquatic vegetation (SAV) plays a vital
role: influencing nutrient dynamics
and water chemistry; modulating the
structure and dynamics of food webs;
and increasing habitat diversity (see
Jeppesen et al. 1998). These aquatic
ecosystems are habitats for many
herbivorous waterfowl that can also
heavily use aquatic macrophyte
resources during migratory stopovers
and/or
in
locations
hosting
permanent populations (e.g., Michot
& Nault 1993; Baldwin & Lovvorn
1994a,b). Several studies have
reported long term changes in
aquatic vegetation coinciding with
changes in waterfowl abundances
(Perrow et al. 1997; Søndergaard et
al. 1998; Mitchell & Perrow 1998;
Blindow et al. 2000). High densities
of SAV can attract waterfowl (by
providing food and shelter) that
cause
strong
qualitative
and
quantitative
effects
on
plant
communities through effects on
vegetation
structure,
species
152
composition and by reducing stand
biomass (Bortolus et al. 1998; Nolet
et al. 2001).
Most of these studies conducted in
temperate areas of North America,
Europe and New Zealand, suggest
that major impacts of waterfowl on
the SAV occur during the autumn
(Perrow et al. 1997; Mitchell &
Perrow 1998; Marklund et al. 2002)
and winter (Kiorboe 1980; Van
Donk 1998), when macrophyte
productivity is low and migratory
events result in increased abundance
of individuals (Søndergaard et al.
1996). Waterfowl herbivory is also
important in temperate lakes during
plant colonisation stages and at very
low vegetation densities (Marklund
et al. 2002; Körner & Dugdale 2003;
Hilt 2006). In contrast, the few
studies conducted in Mediterranean
aquatic ecosystems suggest that, in
general, waterfowl grazing does not
have a strong effect on the biomass of
submerged vegetation due to the
high level of primary production
(Mitchell & Perrow 1998; Marklund
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
et al. 2002; Sandsten et al. 2002).
However, it has also been suggested
that waterfowl in Mediterranean
areas can have a strong qualitative
effect on the structure of plant
communities by selecting the most
palatable species or their reproductive
structures (Gayet et al. 2012;
Rodríguez-Villafañe et al. 2007). A
marked preference of herbivores for
plants bearing abundant flowers
and/or developing fruits has been
suggested as eventually leading to a
reduction in the number of seeds
produced by these plants (Herrera et
al. 2002) and could strongly impact
the
reproductive
success
of
macrophytes.
Ruppia cirrhosa (Petagna) Grande,
Potamogeton pectinatus L. and
Zoostera spp have been reported as
the dominant macrophyte species in
Mediterranean lagoons, with a
seasonal cycle characterised by
declining biomasses in autumn and
winter —particularly R. cirrhosa
(Menéndez et al. 2002; RodríguezPérez & Green 2006)— and
flowering
event
in
summer
(Menéndez & Comín 1989; Prado et
al. 2013). The waterfowl community
in Mediterranean wetlands is
dominated by the duck Anas
platyrhynchos L. and the Eurasian
coot Fulica atra L. whose abundances
increase in autumn and winter, due
to migratory concentrations (Mañosa
et al. 2001; Hidding et al. 2009).
Anas platyrhynchos is considered to be
mostly granivorous (Arzel et al.
2007) and coots (F. atra) mainly
herbivorous, with both species having
long been recognised to feed on
submerged macrophytes such as
Potamogeton spp and Ruppia spp as
well as their seeds and flowers
(Tubbs & Tubbs 1983; Perrow et al.
1997; Figuerola et al. 2002, 2003;
Green et al. 2002). However,
ecological
interactions
between
waterfowl
and
aquatic
plant
communities
in
Mediterranean
lagoons need to be further
investigated for the conservation of
these natural habitats and the longterm sustainability of endangered
and/or economically valued animal
species, as well as the natural
diversity of ecosystems.
In this context, the general objective
of this study was to investigate
whether seasonal differences in the
two main populations of waterfowl
(A. platyrhynchos and F. atra) and in
the abundance of the two main
submerged macrophytes (R. cirrhosa
153
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
and P. pectinatus) can explain
patterns of plant consumption within
Mediterranean lagoons. In addition,
we investigated the potential role of
macrophytes’ flowers in mediating
waterfowl feeding preferences and
overall impacts on macrophytes’
biomass. With these aims, three
specific objectives were assessed
during three seasons: (1) waterfowl
abundances of A. platyrhynchos and F.
atra; (2) grazing impacts on both
macrophyte species and their flowers
(only in summer) by deploying
exclusion cage experiments; and (3)
plant consumption rates by tethering
experiments.
5.3. MATERIAL AND
METHODS
5.3.1. Study site
The study was conducted at the
Encanyissada coastal lagoon, located
within the Ebro Delta Natural Park
(Spain, NW Mediterranean), a
Natura 2000 wetland area of
recognised international importance
for waterbird conservation by the
Ramsar Convention and by BirdLife
International (Viada 1998) where ca.
70% of the total surface is devoted to
rice cultivation. The submerged
154
vegetation in the lagoons is
dominated by R. cirrhosa in high
salinity areas (12–27‰) and by P.
pectinatus in low salinity areas (3–
12‰).
Seasonal
variation
in
macrophytes’ biomasses within the
lagoon have been reported values
from 151.3 ± 16.6 g DW m−2 in
August to 21.6 ± 2.7 g DW m−2 in
February for R. cirrhosa and values
from 162.6 ± 24.4 g DW m−2 in
August to 54.8 ± 13 g DW m−2 in
February for P. pectinatus. Flowering
of R. cirrhosa has been reported in
August in the lagoon, although
flowers can start in June (personal
observation). For P. pectinatus,
flowering occurs in June to July
(personal observation) ending by
August, when only fruits (achenes)
were observed (Prado et al. 2013). In
summer, water conditions (mainly
increased water temperature and
nutrient supply from rice agriculture)
also contribute to the proliferation of
fast-growing species such as floating
macroalgae or epiphytic loads
(Valiela et al. 1997; Menéndez
2005). Waterfowl abundances in this
area are especially notorious during
autumn and winter, due to the
migratory events and the abundance
of wintering grounds, when ducks
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
and coots become the most
important species (Martínez-Vilalta,
1989, 1994, 1996). In this study, we
focused on the herbivory of A.
platyrhynchos and F. atra as both have
been
reported
to
feed
on
macrophytes as well as their seeds
and flowers (Tubbs & Tubbs 1983;
Perrow et al. 1997; Figuerola et al.
2002).
5.3.2. Waterfowl abundance and
behavioral observations
Monitoring
the
waterfowl
community was conducted on a
previously delimitated area of the
lagoon which included the two
experimental
areas
of
plots
deployment. Waterfowl abundance
was counted (using binoculars) from
a fixed point located approximately
Figure 5.1. Map of the Encanyissada lagoon showing the position of the two sampling sites
where experiments were deployed: R. cirrhosa and P. pectinatus sites (adapted from Prado et al.
2013).
155
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
100 m from each area. At each study
season (summer, autumn and winter)
and during the 30-day experimental
period, waterfowl were counted at
the same time of the day on 4
random days. The number of
individuals of F. atra and A.
platyrhynchos in each study area was
added to estimate total waterfowl
abundance.
Feeding on the submerged vegetation
and the possible disturbance of the
experimental area by other bird
species was also monitored by
deploying a camera for time-lapse
video recording (Day 6 Plotwatcher)
facing the tethering and exclusion
cage experiments at different days
throughout the study period.
5.3.3. Exclusion cage experiments
To evaluate the grazing effect by
waterfowl on macrophytes’ biomass,
six bird exclusion and six open cages
were deployed randomly in two
shallow areas of the lagoon (80–100
cm depth; separated ∼1.5 km); one
monospecific area with R. cirrhosa
and another with P. pectinatus. Each
plot (exclusion and control) covered
an area of 1.5 m2 and contained plant
biomass
(either
Ruppia
or
156
Potamogeton) that was representative
of the lagoon (Prado et al. 2013).
Exclusion plots consisted of a rigid,
plastic net above the canopy top (1
cm2 mesh size) tied to four poles (1.5
m long, 10 mm diameter) inserted
into the sediment, preventing the
entrance of birds and enabling water
circulation on the sides during
occasional storms (total experimental
area covered: ∼300 m2, see Fig. 5.1).
Cage experiments were deployed for
a 30-day period in three different
seasons: summer 2010 (from midJune to mid-July: when flowering
started and waterfowl abundances are
the lowest); autumn 2010 (from midSeptember to mid-October: when
flowers are no longer available and
waterfowl abundance increases); and
winter 2011 (from mid-February to
mid-March: when macrophytes
abundance is the lowest and
waterfowl abundance is the highest).
After this period, 3 corers of 16 cm
(Ø) were collected from central areas
(defined by a minimum security
margin of 0.3 m from each side) of
each plot. To assess a possible
shading effect by the cage mesh net,
3 additional corers were collected
from the edges of the exclusion cages
for further comparison (within the
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
0.3 m margin area). In each season,
exclusion cages were removed after
the 30-day experiment period to
avoid the destruction of the plots or
overlapping by repeated sampling. In
autumn, green macroalgal blooms
occurred in the experimental area
and, as they were attached to
macrophytes, their biomass was also
quantified. At each sampling event,
plants were placed into bags and
carefully washed in the laboratory to
remove attached sediments. We
measured the canopy height, counted
the number of flowers (in summer)
and
separated
the
attached
macroalgae (in autumn). All samples
were dried at 60°C to constant
weight and weighted to the nearest
0.1 g. The following macrophytes’
variables were assessed: canopy
height (cm); biomass (g DW m−2);
number of flowers per m2; and
attached biomass of macroalgae (g
DW m−2).
was
deployed
within
each
monospecific area during the three
different seasons studied (summer;
autumn and winter). Each tethering
line consisted of macrophytes’ shoots
previously weighted in the laboratory
(ca. 3 g WW; n = 6). Shoots were
attached to pickets using cable ties,
secured between them using a thin
rope and randomly deployed within
each monospecific experimental area
during 30 days. Tethering controls (n
= 6), consisting of equivalent plant
biomasses covered with a protective
mesh, were also placed in the
submerged macrophyte areas in order
to assess possible growth and/or
decomposition of tethered plants
during the experimental period. After
the 30-day period, all replicates were
collected
and
reweighted
for
variations in wet weight, and biomass
changes in control tethers were used
to correct consumption estimates,
expressed in terms of g WW lost d−1.
5.3.4. Tethering experiments
5.3.5. Data analyses
Tethering
experiments
were
conducted simultaneously to the
deployment of exclusion cages to
quantify consumption rates of
waterfowl in the lagoon. For each
macrophyte species a tethering line
Seasonal variation in the total
waterfowl
abundance
(A.
platyrhynchos and F. atra) was first
evaluated using a one-way ANOVA
with season as fixed factor (three
levels). Then, we investigated
157
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
seasonal differences in the abundance
of each waterfowl species using a
two-way ANOVA with season (three
levels) and waterfowl species (two
levels) as fixed factors.
The effects of waterfowl grazing on
each macrophyte over the study
period (i.e., cage experiments) were
investigated
with
a
two-way
ANOVA with season (three levels)
and treatment (two levels) as fixed
factors. The possible ‘shading effect’
by exclusion cages was first
investigated for each macrophyte
species using a two-way ANOVA
with season (three levels) and shade
(two levels) as fixed factors.
Waterfowl effects on abundance of
flowers and on the biomass of
attached macroalgae was also
evaluated using a two-way ANOVA
with macrophyte (two levels) and
treatment (two levels) as fixed
factors. Seasonal variation in
macrophytes
consumption
(i.e.,
tethering
experiments)
was
investigated using a two-way
ANOVA design with season (three
levels) and macrophyte species (two
levels) as fixed factors.
For all ANOVAs, assumptions of
normality and homogeneity of
158
variance were assessed with the
Kolmogorov-Smirnov and Cochran’s
C-test,
respectively.
When
assumptions could not be met by
variable
transformation,
the
significance level was set at 0.01 to
reduce the possibility of a Type I
error (Underwood 1997). The
Student Newman-Keuls post hoc test
(Zar 1984) was then used to
investigate the presence of significant
groupings.
5.4. RESULTS
5.4.1. Waterfowl abundance and
behavioral observations
The total waterfowl abundance in the
lagoon was significantly different
among seasons with increasing values
from summer to winter (One-way
ANOVA, p < 0.01, Table 5.1a). For
F. atra and A. platyrhynchos
abundances,
analyses
showed
significant effects of season and
waterfowl species. The abundance of
F. atra was significantly higher than
that of A. platyrhynchos, with higher
values in autumn and winter than in
summer (Two-way ANOVA, Fig.
5.2, see SNK in Table 5.1b). Feeding
observations recorded by the camera
proved that both species were grazing
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
on R. cirrhosa and P. pectinatus in the
experimental area.
5.4.2. Exclusion cage experiments
During the seasonal study, analyses
did not detect significant ‘shading
effects’ inside exclusion cages on the
biomass and canopy height of R.
cirrhosa and P. pectinatus (Two-way
ANOVA, p > 0.5).
Ruppia cirrhosa biomass displayed
significant
differences
between
seasons, with higher values in
autumn than in summer and winter,
with no effects for treatment (Fig.
5.3A, see SNK in Table 5.2). Yet, a
significant Season × Treatment
interaction was observed, caused by
significantly higher biomasses inside
exclusion cages during the summer
period (Fig. 5.3A, see SNK in Table
5.2). The highest biomass was
recorded in autumn control cages
(284.4 ± 19.8 g DW m−2) and the
lowest in winter exclusion cages (69.3
± 7.2 g DW m−2). For canopy height,
similar patterns were observed (i.e.,
season and Season × Treatment
effects), but there was also a
significant effect of treatment, with
higher heights within exclusion cages
(Fig. 5.3B, see SNK in Table 5.2).
Figure 5.2. Seasonal variability in the
abundance of A. platyrhynchos, F. atra, and in
the overall number of individuals ha−1.
The highest values were recorded in
summer exclusion cages (42.1 ± 5.4
cm) and the lowest in winter control
cages (13.1 ± 0.7 cm).
For P. pectinatus, analyses showed
that biomass and canopy height were
only significantly different between
seasons (Fig. 5.3C,D, Table 5.2).
The highest biomass was recorded in
autumn (538.9 ± 88.2 g DW m−2)
and the lowest in winter (76.8 ± 7.4 g
DW m−2). The highest canopy height
was recorded in summer (76.2 ± 4.7
cm) and the lowest in winter (27.8 ±
1.2 cm). The abundance of flowers in
summer and macroalgal biomass in
autumn showed a significant
Macrophyte × Treatment interaction
(see SNK in Table 5.3). The highest
flower abundance was recorded
inside R. cirrhosa exclusion cages
159
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Table 5.1. a. One-way ANOVA testing differences on Total Waterfowl population (including
individuals of An: A. platyrhynchos and F: F. atra); b. Two-way ANOVA testing for differences
on populations of waterfowl species (An: A. platyrhynchos and F: F. atra) among seasons (S:
summer; A: autumn; W: winter). In SNK, significant differences between investigated groups,
A. platyrhynchos population in summer (SAn), autumn (AAn) and winter (WAn), F. atra
population in summer (SF), autumn in (AF), and winter (WF) are indicated. Significant
differences are indicated as follows: ** p < 0.01; *** p < 0.001; NS: not significant.
Source of variation
Season (S;A;W)
Residual
Transformation
Source of variation
Season (S;A;W)
Waterfowl type (An;F)
Season X Waterfowl
Residual
SNK
Transformation
df
2.00
9.00
df
2.00
1.00
2.00
18.00
(1015.7 ± 269.8 flw m−2) and the
lowest inside P. pectinatus exclusion
cages (93.9 ± 33.8 flw m−2; see SNK
in Table 5.3). For attached
macroalgae, the highest biomass was
recorded in control cages of P.
pectinatus (405.7 ± 91.5 g DW m−2)
and the lowest in control cages of R.
cirrhosa (8.8 ± 3.4 g DW m−2) (see
SNK in Table 5.3).
5.4.3. Tethering experiments
Analyses showed that macrophyte
consumption was not significantly
different across seasons, but was
significantly higher in P. pectinatus
160
a. Total waterfowl Census (A+F)
MS
F
4.35
16.24
0.27
Ln x
b. Waterfowl Census (An and F)
MS
F
5.95
17.51
15.09
44.37
1.12
3.30
0.34
AFu = WFu > SFu = SAn = AAn = WAn
Ln x
p
**
p
***
***
NS
than in R. cirrhosa (Fig. 5.4, Table
5.4).
5.5. DISCUSSION
Contrary to previous findings in
Northern Europe, our study shows
that major herbivory impacts of
waterbirds in Mediterranean regions
are neither restricted to periods of
early growth, or to autumn when
macrophyte productivity is low and
wildfowl
form
migratory
concentrations. Our results show that
waterfowl
grazing
effects
on
submerged
macrophytes
in
Mediterranean aquatic lagoons were
influenced by the seasonal changes in
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
Figure 5.3. Seasonal trends on submerged vegetation during cage experiments: A. Biomass (g
DW m−2) of R. cirrhosa; B. Canopy height (cm) of R. cirrhosa; C. Biomass (g DW m−2) of P.
pectinatus; D. Canopy height (cm) of P. pectinatus. Mean ± SE. ** p < 0.01; NS = not significant.
the availability of food resources and
flowering events rather than by
waterfowl abundances. The higher
abundance of flowers recorded in R.
cirrhosa (∼10 times higher than P.
pectinatus inside exclusion cages)
concurred with higher waterfowl
consumption on this specie, and
appears to be a key factor controlling
herbivory pressure.
5.5.1. Waterfowl abundance
Total waterfowl abundance (F. atra
and A. Platyrhynchos) in the lagoon
increased from summer to winter
(from 0.51 ind ha−1 to 3.14 ind ha−1)
with F. atra being the most abundant
species in the entire lagoon
throughout the study. This seasonal
pattern has been previously reported
for coots and ducks in other
Mediterranean
wetlands,
with
abundances peaking in October–
November during the post-breeding
period, and with a mean density of
2.9 ind ha−1 (Rodríguez-Pérez &
Green 2006). Although the grazing
activity in Central and Northern
Europe takes place in late autumn
and winter due to populations’
increase (Van Donk & Otte 1996;
Søndergaard et al. 1996; Froelich &
Lodge
2000;
Santamaría
&
Rodríguez-Gironés 2002) our study
161
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
suggests that major effects of
waterbirds
on
submerged
macrophytes
in
Mediterranean
lagoons are not restricted to periods
of high waterfowl concentrations
(Rodríguez-Pérez & Green 2006).
5.5.2. Seasonal herbivory impacts
on macrophytes
Figure. 5.4. Seasonal consumption rates of
R. cirrhosa and P. pectinatus during tethering
experiments (g WW d−1).
Experiments with exclusion cages
showed that waterfowl grazing
effects on R. cirrhosa and P. pectinatus
were not driven by seasonal variations
in waterfowl abundance. In summer,
although waterfowl abundance was
lower, grazing effects were evident in
R. cirrhosa biomass, canopy height
and flowers which suffered the most
intense herbivory in open cages
(flowers abundance were ∼8 times
higher inside exclusion cages). The
higher abundance of flowers recorded
in R. cirrhosa (ca. 10 times higher
than P. pectinatus inside exclusion
cages) concurred with higher
waterfowl consumption on this
specie, which suggest that this may
be a key factor controlling herbivory
pressure. In fact, preferential
consumption of flowers has been
reported in previous exclusion
Table 5.2. Two-way ANOVA testing for differences on biomass (g DW m−2) and canopy height
(cm) among seasons (S: summer; A: autumn; W: winter) and treatments (C: control; E:
exclusion) in Ruppia cirrhosa and Potamogeton pectinatus. In SNK, significant differences between
groups are indicated, summer control (SC), summer exclusion (SE), autumn control (AC),
autumn exclusion (AE), winter control (WC), winter exclusion (WE) are indicated. Significant
differences are indicated as follows: *p < 0.05; ** p < 0.01; ***p < 0.001; NS: not significant. NT:
no transformation.
Source of
variation
df
Rupppia cirrhosa
Biomass
Canopy Height
MS
F
p df MS
F
p
Season (S)
2
575.41
55.58
***
2
575.41
55.58
Treatment (T)
1
11.17
1.08
NS
1
11.17
SxT
2
37.2
3.59
*
2
37.2
***
2
1.08
*
1
3.59
**
2
102
56986.79
Residual
SNK
Transformation
162
df
Potamogeton pectinatus
Biomass
Canopy Height
MS
F
p df
MS
F
1962923.62
p
34.45
***
2
17626.45
61.09
***
50.51
0
NS
1
643.38
2.23
NS
107705.44
1.89
NS
2
145.02
0.5
NS
102
288.52
AE=AC>SE>SC=WC=WE
SE>AC=AE=SC>WE=WC
AC=AE>SE=SC>WE=WC
SE=SC>AE=AC>WE=WC
Sqr (x+1)
NT
NT
NT
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
experiments
with
coots
in
Mediterranean lagoons featuring a
diverse community of macrophytes
(Rodríguez-Villafañe et al. 2007).
Yet, we did not observe higher
herbivory pressure on P. pectinatus
despite the presence of flowers and
the higher canopy height of this
species, which can also influence
waterfowl grazing (Hurter 1972).
Hence, our results suggest that
waterfowl have an important impact
on R. cirrhosa in summer, which is
likely influenced by preference for
flowers that are locally very abundant
during the summer period (as
previously described for this lagoon,
see Prado et al. 2013).
A marked preference of herbivores
for plants bearing abundant flowers
and/or developing fruits has been
suggested as eventually leading to a
reduction in the number of seeds
produced by these plants (Herrera et
al. 2002) and could strongly impact
the
reproductive
success
of
macrophytes. Rodríguez-Villafañe et
al. (2007) conducted a bird-exclusion
experiment in Lake Sentiz (Spain)
and
found
that
Potamogeton
gramineus L. only developed leaves
and flowers under waterfowl
exclusion, thus decreasing in
abundance
until
becoming
codominant
with
Myriophyllum
alterniflorum DC which also suffered
higher consumption of flowers
outside the cages. This suggests that
by selecting the most palatable
species
or
their
reproductive
structures waterfowl can have a
strong qualitative effect on the
structure of plant communities and
become the central force driving
species’ composition in some aquatic
ecosystems (Bonser & Reader 1995;
Rachich & Reader 1999). In
addition, it is possible that waterfowl
selectivity for R. cirrhosa influence
vegetative regrowth during the
following year, as has been reported
to occur with other macrophytes
species (Van Dijk et al. 1992;
Fishman & Orth 1996; Piazzi et al.
2000). Yet, some studies also suggest
that plants may have mechanisms to
compensate herbivory pressure such
as increasing the proportion of
female flowers (Howe & Westley
1988),
or
the
amount
of
belowground structures, which may
enhance substrate fixation and
facilitate lateral expansion (Orth
1977). These mechanisms of
compensatory growth are the main
drivers of evolutionary responses for
163
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
plant-animal
coexistence
(McNaughton 1983). Although we
did not measure how waterfowl
grazing affected macrophytes’ grow
rates,
a
previous
study
in
Mediterranean wetlands suggested
that a strong grazing effect on
macrophytes’ biomass and the
reproductive structures in one year
are likely to influence Ruppia growth
the following year (Rodríguez-Pérez
& Green 2006).
both R. cirrhosa and P. pectinatus may
be due to the enhanced availability of
other resources such as rice seeds
(due to the harvest season) or the
proliferation of floating macroalgal
mats, which have been commonly
reported to proliferate in spring and
summer due to higher water
temperature
and
irradiance
(Menéndez & Sánchez 1998;
Menéndez & Comín 2000). During
our experiment, floating macroalgae
ended up attached to macrophytes’
leaves, particularly in P. pectinatus,
possibly due to differences in water
salinity and/or nutrient availability
within the lagoon (Prado et al. 2013).
Yet,
conversely
to
enhanced
palatability
effects
commonly
reported
for
epiphytes
and
Despite the increased waterfowl
abundance in autumn and winter,
grazing effects on the submerged
macrophytes were negligible in both
seasons.
In
autumn,
flowers
disappeared, and the lack of effects
on biomass and canopy height of
Table 5.3. Two-way ANOVA testing for differences on flowering rates (No. flw m−2) and on
attached macroalgae biomass (g DW m−2) between macrophytes (R: R. cirrhosa; P: P. pectinatus)
and treatments (C: control; E: exclusion). In SNK, significant differences between investigated
groups, R. cirrhosa control cages (RC), R. cirrhosa exclusion cages (RE), P. pectinatus control
cages (PC) and P. pectinatus exclusion cages (PE) are indicated. Significant differences are
indicated as follows: *p < 0.05; ***p < 0.001; NS: not significant. NT: no transformation.
Source of variation
SUMMER
AUTUMN
Flowering rates
Macroalgal biomass
df
MS
F
p
df
MS
F
p
Macrophyte (M)
Treatment (T)
1
1
44.26
20.99
5.98
2.84
*
NS
1
1
1555865.55
184939.9
33.09
3.93
***
NS
MxT
1
44.57
6.02
*
1
190780.64
4.06
*
Residual
68
7.4
68
47018.54
SNK
Transformation
164
RE>RC=PC=PE
PC>PE=RE=RC
Ln (x+1)
NT
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
Table 5.4. Two-way ANOVA testing for differences on consumption (g WW d−1) between
seasons (S: summer; A: autumn; W: winter) and macrophyte species (R: R. cirrhosa; P: P.
pectinatus). In SNK, significant differences between investigated groups, summer in R. cirrhosa
(SR), autumn in R. cirrhosa (AR), winter in R. cirrhosa (WR), summer in P. pectinatus (SP),
autumn in P. pectinatus (AP), winter in P. pectinatus (WP) are indicated. Significant differences
are indicated as follows: **p < 0.01; NS: not significant. NT: no transformation.
Source of variation
Season (S;A;W)
Macrophyte type (R;P)
Season X Macrophyte
Residual
SNK
Transformation
Differences between R. cirrhosa and P. pectinatus in all the seasons
Consumption rates
df
MS
F
p
2.000
0.000
3.170
NS
1.000
0.010
8.100
**
2.000
0.000
0.430
NS
30.000
0.000
AP=SP=SR=AR=WP=WR
NT
macroalgae (Gayet et al. 2012;
Marco-Méndez et al. 2012),
increased algal biomass did not result
on preferential waterfowl grazing on
P. pectinatus. Conversely, given the
large accumulation of macroalgae, it
is possible that the availability of this
alternative
resource
decreased
waterfowl
effects
on
both
macrophytes species. Later in winter,
the lower biomass and canopy height
recorded for both macrophytes could
have made them a less accessible
resource and therefore, harder to be
found by waterfowl, particularly due
to the enhanced water turbidity
during this period. We hypothesise
that in this season, the reported
ability of ducks and coots switching
to feeding on invertebrates and seeds
may help them to persist within the
lagoon area, in spite of the scarcity of
submerged vegetation (RodríguezPérez & Green 2006).
Despite exclusion-cage experiments
not evidencing grazing effect on
macrophytes in some seasons, results
from tethering experiments suggests
that there is some consumption
through the year. However, the low
consumption rates recorded suggest
low encounter of tethers by
waterfowl, possibly resulting from
the high mobility of waterfowl in the
lagoon or the high abundance of
other resources. This could explain
the low consumption of R. cirrhosa
tethers during the summer period
despite strong waterfowl impacts in
165
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
cage experiments. It is likely that
tethering experiments underestimate
waterfowl consumption and these
results need to be interpreted with
caution. Yet, tethering results were
supported by the video camera
feeding observations, evidencing that
observed differences in plant biomass
during the study were due to waterfowl.
5.6. CONCLUSION
Our findings contrast with the
seasonal herbivory impacts described
for coots and ducks in Northern
Europe (Van Donk & Otte 1996;
Søndergaard et al. 1996), but are in
concordance
with
other
Mediterranean studies (RodríguezPérez & Green 2006; RodríguezVillafañe et al. 2007) reporting major
effects of waterbirds during the
summer period, when plant and
flowers’ availability is higher.
Overall, this suggests that seasonal
impacts of waterfowl are not a
166
general rule, but depend on a
regional combination of animal
numbers
and
compositional
abundance of food resources. To
conclude, the strongest waterfowl
impacts on the submerged vegetation
within
brackish
Mediterranean
lagoons do not occur when
abundance of individuals is higher,
but in summer when plants and
flowers are largely available. In the
long term, higher herbivory pressure
on R. cirrhosa and its flowers could
reduce the reproductive success of
this species and alter the overall
community structure of submerged
macrophytes. This study contributes
to a better understanding of the
interactions
between
waterfowl
herbivory and the SAV in aquatic
Mediterranean ecosystems along the
seasonal cycle, and may allow a better
conservation of natural habitats and
the long-term sustainability of the
natural diversity of ecosystems.
CHAPTER 5: Seasonal effects of waterfowl grazing on submerged macrophytes:The role of flowers
167
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Published as:
Marco-Méndez C, Prado P, Ferrero-Vicente LM, Ibáñez C,
Sánchez-Lizaso JL (2015). Rice fields used as feeding habitats for
waterfowl throughout the growing season. Waterbirds 38(3). In
press
168
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
6. Rice fields used as feeding habitats for waterfowl
throughout the growing season
6.1. ABSTRACT
The role of rice fields as feeding habitats for the two main
waterfowl herbivores, Eurasian Coot (Fulica atra L.) and Mallard
(Anas platyrhynchos L.), was investigated in a Mediterranean
wetland, the Ebro Delta of northeast Spain. Exclusion cages and
tethering experiments were deployed within a rice field at the
beginning of the growing season (summer 2010) and before
harvest (autumn 2010). In summer, waterfowl abundances were
low, but cage experiments detected rice field damage by waterfowl
grazing, through a significant reduction in plant biomass (although
consumption was undetectable using tethers). In autumn,
waterfowl abundance increased and tethering experiments detected
consumption of rice plants with developed seeds, whereas cage
experiments did not show grazing effects. Gut content analyses
indicate that A. platyrhynchos is mainly granivorous, feeding mostly
on seeds of Ruppia cirrhosa (Petagna) Grande and rice (Oryza
sativa L.), while F. atra is herbivorous, feeding mainly on
macrophyte leaves. However, stable isotope analyses and mixing
model results showed that in the long term both species seem to
acquire most of their dietary needs from rice plants and
Potamogeton pectinatus L. Dietary analyses confirm the importance
of rice in both species’ diets but also suggest that waterfowl may
undergo seasonal dietary variations. These are mostly influenced by
changes in the availability of food resources in the area rather than
by their nutritional quality. This study confirms the ecological
importance of rice fields as a complementary feeding habitat for
waterfowl during the growing season in Mediterranean areas. It
169
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
also highlights the importance of including these habitats in
wetland management for waterfowl conservation.
6.2. INTRODUCTION
Wetlands are vital habitats for many
waterbird species, but human
activities are increasing their loss
throughout the world in different
forms of: (1) complete loss (e.g.,
drainage for urbanization or dry
cultivation); (2) degradation in
quality (e.g. contamination or
alteration of hydrological regimes); or
(3) conversion of land to aquatic
agriculture, which changes the
structure and function of the habitat,
but retains the area as wetland
(Tourenq et al. 1999, 2000). Of the
latter, rice (Oryza sativa L.)
cultivation is by far the most
common and 40% of the world’s
population now depends on this food
source (Fasola & Ruiz 1997). As a
consequence, in many regions,
natural wetlands have now virtually
disappeared and rice fields have
become the primary wetland habitat
for waterbirds during wintering
(Tréca 1994; Elphick & Oring
1998), or for breeding (Acosta et al.
1996; Lane & Fujioka 1998).
170
Rice field systems, including
irrigation canals, are used by a variety
of waterbirds such as waders, gulls,
terns, ducks and herons, primarily as
feeding habitats (Elphick & Oring
1998; Bird et al. 2000). Damage to
rice fields by birds can be caused by
trampling, which can disturb the
grain or destroy seedlings, or by
ingurgitation
of
rice
seeds
(Hoffmann & Johnson 1991). The
majority of the studies concerning
rice field damage by waterfowl have
been conducted in North and South
America, Africa and Australia
(Stafford et al. 2006, 2010). In
Europe, this topic has been primarily
studied outside the growing season
(Tourenq et al. 2003; Ibáñez et al.
2010), even though the latter is the
period when rice fields become
important feeding and shelter
grounds for many species of
waterfowl (Pierluissi 2010). In
Mediterranean areas, there is a lack
of information on habitat use
(Tourenq et al. 2003) and many
studies have been solely focused on
foraging herons (Fasola et al. 1996).
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
Mediterranean wetlands such as the
Ebro Delta, Albufera and Doñana
National Park are some of the main
wintering areas for waterfowl within
the Western Palearctic region
(Sánchez-Guzmán et al. 2007;
Rendón et al. 2008). In some of these
wetlands, Mallard (Anas platyrhynchos
L.) and Eurasian Coot (Fulica atra
L.) are the dominant species,
especially in winter (Martínez-Vilalta
1996; Mañosa et al. 2001). Both
species have been reported to feed
mainly on the submerged aquatic
vegetation of marshes and lagoons
(Rodríguez-Villafañe et al. 2007;
Marco-Méndez et al. 2015b).
However, over the past few centuries
these Mediterranean wetlands have
been reduced to 10–20% of their
original area (Fasola & Ruiz 1997),
which
enhances
the
relative
importance of rice fields as an
alternative feeding habitat for
waterfowl species (Fasola & Ruiz
1996, 1997).
In Mediterranean countries, the rice
cropping cycle begins in April when
flooded fields are sown (Longoni
2010). Rice reaches its maximum
height in August and is usually then
harvested in September or October,
after fields have been drained. In the
Ebro Delta, fields remain flooded
until January in a new agroenvironmental initiative of the
European Union, to increase
wintering areas for migratory
waterbirds. In those rice fields,
waterfowl forage for the available
food resources: invertebrates, land
plants and water weeds, or rice seeds
(Elphick & Oring 1998; Manley et
al. 2004).
Waterfowl diet or selectivity can be
influenced by several factors such as
food availability (Petrie & Rogers
1996), high nutrient contents
(McGlathery 1995), high protein and
energy contents (Hughes 1980;
Sedinger & Raveling 1984; Murphy
& King 1987) or easier digestibility
(Charalambidou
&
Santamaría
2002), the last especially affecting
selectivity for seeds. Among methods
for quantifying dietary contributions,
stomach content analysis is the most
accurate, although it gives dietary
information relative to a short time
period and require an extended
sampling time (Legagneux et al.
2007). Stable isotope analyses are
another technique that can provide
non-destructive and time-integrative
information about the diet and often
provenance of food sources (Hobson
171
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
& Clark 1992a,b, 1993; Inger &
Bearhop 2008). Since consumers feed
on isotopically distinct food items,
their tissues reflect the different
stable isotopic compositions of their
diets. This depends on the window
period when the birds consumed the
diet as well as the turnover rates of
the tissues (Legagneux et al. 2007),
so it can be used to make inferences
about diet and the type of habitats in
which they live (Inger & Bearhop
2008).
The main objective of this study was
to elucidate the importance of rice
fields as alternative feeding habitats
for the two main waterfowl species of
the Ebro Delta in northeastern
Spain: A. platyrhynchos and F. atra
during the growing season. For this
purpose we: (1) investigated seasonal
Figure 6.1. Map of the Encanyissada lagoon showing the location of the rice field used in this
study (adapted from Prado et al. 2013).
172
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
abundances of both species within a
rice field; (2) quantified the seasonal
grazing impact of waterfowl on rice
plant biomass and seeds using
exclusion cage experiments; (3)
quantified seasonal consumption of
rice
plants
using
tethering
experiments; and (4) studied dietary
contributions through gut content
and
stable
isotope
analysis.
Additionally, we investigated the role
of nutrient content in food
selectivity. We hypothesized that
waterfowl use rice fields as alternative
feeding habitats especially during the
growing season, when there is an
increasing availability of vegetative
structure and food resources.
6.3. MATERIAL AND
METHODS
6.3.1. Study area
The Ebro Delta (Catalonia, Spain) is
a wetland area of international
importance
for
waterbird
conservation. The area holds a total
breeding waterbird population of
nearly 60000 pairs, including gulls
and terns (48%), ducks and coots
(25%), waders (14%) and herons
(13%).
The
ornithological
importance of the Ebro Delta is also
well-known during autumn and
winter, when the area is visited by
some 250000 birds, including ducks
and coots (50%), gulls and terns
(29%), waders (18%) and herons
(3%)
(Martínez-Vilalta
1996;
Mañosa et al. 2001). The Ebro Delta
is included in the list of the Ramsar
Convention and is considered an
Important Bird Area by BirdLife
International (Viada 1998). The area
supports well-developed rice farming
activity
on
which
waterfowl
conservation depends to a large
extent. As a consequence of
agricultural practices, coastal lagoons
and marshes have been reduced to
about 25% while rice fields occupy
about 65% of the total delta area.
Nowadays the Natural Park and
some neighboring wetlands and rice
fields, a total of about 12000 ha, are
included in the Natura 2000
Network (Ibáñez et al. 2010).
To carry out this study we selected an
area (24 ha) made up of the rice field
under study (surrounded by other
rice fields) and a flooded area we
consider to be representative of the
different habitats available for
waterfowl in the Ebro Delta (Fig.
6.1). The study area was 250 m away
from the Encanyissada lagoon (418
173
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
ha), the largest in the Ebro Delta,
where the submerged vegetation is
dominated by Ruppia cirrhosa
(Petagna) Grande in high salinity
areas (12–27‰) and by Potamogeton
pectinatus L. in low salinity areas (3–
12‰) (Prado et al. 2013).
Experiments were deployed within
one of the rice fields in summer 2010
(from mid-June to mid-July) when
rice was in the vegetative phase
(seedlings with 4–5 leaves) and
waterfowl abundance is expected to
be low, and in autumn 2010 (from
mid-September to mid-October)
when rice was in the ripening phase
(maximum plant biomass and
developed seeds) and waterfowl
abundance is expected to be high.
6.3.2. Waterfowl populations
From previous surveys by Ebro Delta
Park and observations during 20082010 (unpubl. data), A. platyrhynchos
and F. atra were normally the most
abundant species in the area
(summer: 50% and 23% respectively;
autumn: 45% and 55% respectively).
This also occurs in rice fields, where
despite a greater variety of species
visiting here, A. platyrhynchos are
often the most abundant species
(e.g., summer: 20%; autumn: 25%).
174
Due to this and their reported
grazing activity on the neighboring
lagoon (Marco-Méndez et al. 2015b)
we focused on these species.
During each study season (summer
and autumn 2010), four random days
were selected for visual census. Each
morning (around 09:00), individuals
were counted (using binoculars) by
the same person from a fixed point.
The abundance of A. platyrhynchos
and F. atra counted in the study area
was expressed as density (ind ha-1).
To elucidate the relative importance
of this habitat for each waterfowl
species, the percentage of individual
of A. platyrhynchos and F. atra was
estimated using the lagoon and the
rice fields in summer and autumn
2010. We estimated total abundances
using seasonal abundances in the rice
field and those recorded in the
lagoon in a simultaneous survey
(Marco-Méndez et al. 2015b) carried
out during the same study period.
6.3.3. Exclusion cage experiments
To evaluate the effects of waterfowl
grazing on rice fields, six bird
exclusion cages and six open control
cages (1.5 m2) were deployed
randomly within a rice field. The top
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
and sides of exclusion cages were
made of rigid plastic netting (1 cm2
mesh size) attached to four poles (1.5
m long, 1 cm diameter) driven into
the sediment, preventing the
entrance of birds.
Exclusion and open control cages
were deployed for a 30-day period in
summer and autumn 2010. In
summer (from mid-June to midJuly), rice plants consisted of 4–8
small leaves (seedlings), whereas in
autumn (from mid-September to
mid-October) plants were much
larger and had developed seeds. After
a 30-day period, one rice plant
sample was collected within each
control and exclusion cage using a 16
cm diameter PVC core (n = 12
samples per season). After each
sampling event, plants were kept
within labeled plastic bags and
transported to the laboratory where
they were washed to remove attached
sediments. All samples were dried at
60ºC to constant weight and the
following variables measured: total
biomass (g DW m-2), shoot density
(nº of shoots m-2) and also seed
biomass (g DW of seeds m-2) and
density (nº seeds m-2) in the harvest
season (autumn). Differences in
those variables between control and
exclusion cages were used to quantify
the impact of grazing on the rice
field.
6.3.4. Tethering experiments
Tethering
experiments
were
conducted simultaneously with the
deployment of exclusion cages during
the two study seasons to quantify
consumption rates of rice plants
through waterfowl herbivory. In each
season, tethering lines consisted of
replicates with similar amounts of
rice plants collected from the field
and weighed in the laboratory (g
WW; n = 20). Replicates were plants
with leaves (seedlings) in summer
and plants with ripe seeds in autumn,
as for cage experiments. Plants were
attached to pins using cable ties, and
randomly deployed within the rice
field for 30 days. Tethers were
separated from each other by
approximately 1.5 m and each picket
was considered an independent
sample (Prado et al.
2007a).
Controls for tethering experiments
were carried out simultaneously to
correct changes in plant mass
unrelated to herbivory (e.g., growth
and/or decomposition). Control
replicates consisted of identical
numbers of rice plants (n = 20) placed
175
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
in the rice field with a protective
mesh (1 cm2 mesh size) to prevent
consumption during the study
period. After that period, all
replicates were collected and
reweighed for variations in wet
weight as a proxy of leaf loss by
consumption (g WW d-1). Therefore,
we calculated the weight of plants
eaten by waterfowl (with apparent
bites) as the difference between
initial and final wet weight of tissue
exposed to herbivores, using biomass
variation in control tethers to correct
consumption estimates. In autumn,
we also counted the number of seeds
lost after 30 days (nº seeds shoot-1 d1
).
6.3.5. Gut contents
In order to support patterns from
exclusion and tethering experiments,
individuals of both waterbird species
were collected in the study area for
gut contents (n = 10 individuals per
species) and stable isotope analyses (n
= 5 individuals per species). For legal
reasons, individuals could only be
collected during the hunting season
(October-February), mainly during
the first two months, so gut contents
reflect the available food consumed
during our autumn experiments
176
(from mid-September to midOctober, which includes the end of
the harvest season) and winter (after
harvest). To integrate longer
temporal variability into resource
acquisition we also used stable
isotope analyses. Studies suggest that
animals can retain information about
previous feeding locations for periods
that depend upon the elemental
turnover rates for the tissue of
interest (Tieszen et al. 1983; Tieszen
& Boutton 1989; Hobson & Clark
1992a). Therefore, in our study
muscle tissue may reflect waterfowl
diet/habitat several weeks before
individuals were collected, from 12 to
28 days (Tieszen et al. 1983; Hobson
& Clark 1992a). This means that if
individuals were hunted in October
(most individuals, except two
individuals of F. atra that were
provided in late autumn-early winter)
they will likely reflect items
consumed in late summer-early
autumn.
Individuals were processed in the
laboratory. After extraction, muscle
was reserved for isotopic analyses (see
below) and guts were stored in 70%
alcohol until analyzed (Mouronval et
al. 2007). The content of each gut
was analyzed under the microscope,
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
and sorted into the different food
items found: leaf and stem fragments
of R. cirrhosa and P. pectinatus and
their seeds, rice seeds (no rice plant
fragments were found), algae and
unidentifiable material. Each item
was dried at 60°C, until constant
weight. The importance of each food
item was expressed as a percentage of
total weight within the gut.
6.3.6. Stable isotope and nutrient
content analyses
Samples of submerged macrophytes,
rice plants and seeds seasonally
available in the lagoon and the
studied rice-field were randomly
collected for stable isotope and
nutrient content analyses. Leaves
samples of R. cirrhosa (RUP-L), P.
pectinatus (POT-L) and rice plants
(Rice), and R. cirrhosa seeds (RUPS), P. pectinatus seeds (POT-S), rice
seeds (Rice-S) and the muscle of A.
platyrhynchos and F. atra (n = 5
sample for each food item and each
consumer) previously dried to a
constant weight (60°C, 24 h) were
ground to fine powder for stable
isotope analyses (δ15N and δ13C‰)
and nutrient content (C:N molar
ratio). Analyses were carried out with
an EA-IRMS (Thermo Finnigan)
analyzer
in
continuous
flow
configuration. The average difference
in isotopic composition between the
sample and reference material (δsamplestandard, expressed as ‰) is determined
by the equation:
[(Rsample – Rstandard)/Rstandard] × 1000 =
δ sample-standard
where Rsample is 13C/12C or 15N/14N in
the sample, Rstandard is 13C/12C or
15
N/14N in the working reference
(CaCO3 from Vienna Pee Dee
belemnite [PDB] and atmospheric
nitrogen is the standard for δ13C and
δ15N measurements, respectively),
calibrated against an internal
standard (Atropine, IAEA and/or
UGS). The analytical precision of the
methods used was 0.15‰.
6.3.7. Statistical analyses
We used a one-way analyses of
variance (ANOVA) with two levels
(summer; autumn) to separately
investigate seasonal variations in A.
platyrhynchos and F. atra abundance.
Differences in biomass and shoot
density between treatments were
investigated separately for each
season (summer and autumn) using a
one way-ANOVA with two levels
(control; exclusion). These tests were
177
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
carried out using GMAV 5 software
(Underwood et al. 2002). Rice seeds
were present only in autumn and
differences in rice seed biomass and
density
between
control
and
exclusion cages were assessed with a
Mann-Whitney test for independent
samples (due to lack of normality and
homoscedasticity in the data). We
used the same test to analyze seasonal
differences in rice consumption from
tethers. These tests were carried out
using SPSS Statistics version 18
(SPSS, Inc. 2009).
Differences
in
stable
isotope
15
13
composition (δ N and δ C) and
nutrient contents (C:N molar ratio)
between food items were investigated
with a one-way ANOVA with 6
levels using GMAV 5 software
(Underwood et al. 2002). IsoSource
(Phillips & Gregg 2003) isotope
mixing models were used to identify
the contributions of each food source
to the diet of A. platyrhynchos and F.
atra. The input parameters for the
model were the isotopic values for
the consumers and the trophic
resources (measured in this study)
and the overall rate of fractionation.
The model was run using
fractionation values (3.2‰ for δ15N
and 1.45‰ for δ13C) from other
178
studies on waterfowl with similar
herbivorous diets (Hobson &
Bairlein 2003; Evans Ogden et al.
2004; Inger et al. 2006). Since no
differences in the isotopic signals
were found between seeds and plant
tissue, for A. platyrhynchos we ran the
model using the average of seed and
plant tissue in δ15N and δ13C values
of rice, R. cirrhosa and P. pectinatus,
and for F. atra the isotopic values of
each plant species’ tissues (Phillips &
Gregg 2003).
For all ANOVAs, assumptions of
normality and homogeneity of
variance were assessed with the
Kolmogorov-Smirnov and Cochran’s
C
tests,
respectively.
When
assumptions could not be met by
transformation, the significance level
was set at 0.01 to reduce the
possibility of a Type I error
(Underwood 1997). The Student
Newman-Keuls post-hoc test (Zar
1984) was then used to investigate
the presence of significant groupings.
All ANOVAs and post-hoc test were
carried out using GMAV 5 software
(Underwood et al. 2002).
Significant patterns of variation in
gut contents between A. platyrhynchos
and F. atra were identified with n-
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
MDS
ordination
(Bray-Curtis
similarity index) and the analyse of
similarity
(ANOSIM).
Finally,
percentage of similarity were given
using SIMPER analyses to identify
which dietary items primarily
accounted for observed differences in
the stomach contents of A.
platyrhynchos and F. atra, using
PRIMER-E
software
package
(Clarke & Warwick 2001).
6.4. RESULTS
6.4.1. Waterfowl populations
No significant differences between
seasons were detected, either in A.
platyrhynchos (F1 = -3.340, p = 0.117)
or F. atra abundances (F1 = 2.9, p =
0.140). However, the populations of
both species tended to increase from
summer to autumn, especially A.
platyrhynchos (Fig. 6.2A). The total
of individuals counted in the study
area in summer was 235 (84% A.
platyrhynchos; 16% F. atra) and in
autumn 492 individuals (86% A.
platyrhynchos;
14%
F.
atra).
Considering the total individuals
counted in the lagoon and the ricefield area, A. platyrhynchos showed
higher percentages in the rice field
(summer: 57%; autumn: 38%) while
Figure 6.2. A. Abundances (Mean ± SE) of
Anas platyrhynchos and Fulica atra in the
study area of the survey location (individuals
ha-1) and B. percentages of each species
present in the study area of the survey
location (RIC) and in the neighboring
lagoon (LAG) during the study period. NS:
Not significant results.
F. atra had higher percentages in the
lagoon (summer: 23%; autumn: 47%)
(Fig. 6.2B).
6.4.2. Exclusion cages and tethering
experiments
In summer, rice plant biomass was
significantly higher in exclusion cages
(206.01 ± 36.39 g DW m-2) than in
control cages (96.21 ± 10.32 g DW
m-2; F1 = 8.73, p = 0.014, Fig. 6.3A).
In autumn, significantly higher
values were recorded for control
179
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
cages (695.01 ± 108.18 g DW m-2)
than for exclusion cages (312.13 ±
31.37 g DW m-2; F1 = 13.85, p =
0.004, Fig. 6.3A). For rice shoots
density, summer values in control
and exclusion cages did not differ
significantly (136.26 ± 8.10 shoots m2
and 137.29 ± 7.22 shoots m-2
respectively; F1 = 0.01, p = 0.927),
whereas, significantly greater rice
shoots density was observed in
control compared to exclusion cages
(346.67 ± 28.26 shoots m-2 versus
170.53 ± 11.23 shoots m-2
respectively; F1 = 38.02, p < 0.001,
Fig. 6.3B) in autumn. Analyses of
exclusion cage data did not show
significant differences for seed
biomass (Z = - 1.922, p = 0.055, Fig.
6.3C), whereas seed density was
significantly higher in control than in
exclusion cages (Z = - 2.082, p =
0.037, Fig. 6.3D). In the tethering
experiments, rice plant consumption
in autumn was significantly higher
than in summer (Z = - 2.612, p =
0.009). Consumption recorded in
autumn was 0.067 ± 0.071 g WW d1
, with 0.37 ± 0.14 g DW of seeds
lost per tether and day.
Figure 6.3. Exclusion cages experiments: A. Rice biomass (g DW m-2); B. Rice density (shoots
m-2) and C. Rice seed biomass (g DW Seeds·m-2) and D. Rice seed density m-2. Mean ± SE.
Significant differences are: * p < 0.05, *** p < 0.001, NS: not significant.
180
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
Table 6.1. SIMPER (Clarke & Warwick 2001) analysis showing the contribution of food items
to dissimilarity (Jaccard Index) between Anas platyrhynchos and Fulica atra stomach contents.
Ruppia cirrhosa (RUP-L); R. cirrhosa seeds (RUP-S); Potamogeton pectinatus leaves (POT-L); P.
pectinatus seeds (POT-S); Rice plant (Rice); Rice seeds (Rice-S). The cut off for low
contributions: 90%.
A. platyrhynchos (Average similarity: 20.53)
Species
Av. Abund.
Av. Sim.
Sim. /SD
Cont. (%)
Cum. (%)
RUP-S
39.1
12.72
0.39
62.31
62.31
Rice-S
29.38
6.53
0.28
31.81
94.12
Av. Abund.
Av. Sim.
Sim. /SD
Cont. (%)
Cum. (%)
88.89
77.78
1.84
100
100
F. atra (Average similarity: 77.78)
Species
POT-L
A. platyrhynchos and F. atra (Average dissimilarity = 94.64)
A. platyrhynchos
F. atra
Av. Abund.
Av. Abund.
Av. Dis.
Dis. /SD
Cont. (%)
Cum. (%)
4.84
88.89
42.56
2.71
44.97
44.77
RUP-S
39.1
0
19.55
0.81
20.66
65.03
Rice-S
29.38
0
14.69
0.67
15.52
81.15
RUP-L
9.54
11.1
9.27
0.59
9.79
90.95
Species
POT-L
6.4.3. Gut contents
Gut contents of A. platyrhynchos
consisted of rice seeds (29%) and R.
cirrhosa seeds (39%) and to a lesser
extent: P. pectinatus seeds, leaf
fragments of R. cirrhosa and P.
pectinatus, algae and unidentifiable
material, together constituting the
remaining 32%. Fulica atra guts
contained only leaf fragments of P.
pectinatus (90%) and R. cirrhosa
(10%).
n-MDS ordination of the items
found in the gut contents showed
different
groupings
for
A.
platyrhynchos and F. atra samples.
ANOSIM results confirmed that
these
two
groupings
were
significantly different (Global R =
0.424; p = 0.002). The average
similarity among A. platyrhynchos was
only 21%, whereas for F. atra the
value was 78%. The average
dissimilarity between A. platyrhynchos
and F. atra was 95%, mostly due to
differences in presence of P.
pectinatus leaf fragments (45%)
(Table 6.1).
181
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
6.4.4. Stable isotope analyses
δ15N and δ13C signatures revealed
significant differences among food
items (One-way ANOVA, p < 0.001,
Table 6.2). The highest δ15N
signatures were found in P. pectinatus
vegetative tissues and seeds and R.
cirrhosa vegetative tissues, and the
lowest in R. cirrhosa seeds and in rice
fragments and seeds (Fig. 6.4A). For
δ13C signatures, rice fragments and
seeds showed the highest values,
whereas the lowest were found in R.
cirrhosa and P. pectinatus vegetative
tissues and their seeds (Fig. 6.4A).
Results from the IsoSource model
(Phillips & Gregg 2003) indicated
that the diet of A. platyrhynchos and
F. atra mostly constituted of rice
plants and P. pectinatus with a low
contribution of R. cirrhosa (A.
platyrhynchos: rice 62%; P. pectinatus
38%; R. cirrhosa 0%; and F. atra: rice
61%; P. pectinatus 36% and R.
cirrhosa 2% at the percentile 50%).
These results point to rice plants as
the main food for both consumers
with a non-negligible contribution of
P. pectinatus.
182
6.4.5. Nutrient contents in
waterfowl and food resources
C:N molar ratios were significantly
different between food items (Oneway ANOVA, p < 0.001, Table 2).
The highest C:N molar ratios were
found in P. pectinatus seeds and rice
plants and the lowest in rice seeds, R.
cirrhosa seeds and in leaves of R.
cirrhosa and P. pectinatus (Fig. 6.4B).
6.5. DISCUSSION
The results confirm that waterfowl
feed on rice fields during the growing
season in Mediterranean wetlands
such as the Ebro Delta, either on
seedlings or seeds according to their
availability. This can lead to a
significant reduction in rice plant
biomass, as we detected in summer.
Isotope and gut content analyses
confirm the importance of rice in
both species’ diets but also suggest
that waterfowl may undergo seasonal
dietary variations, mostly influenced
by changes in the availability of food
resources in the area, rather than by
their nutritional quality.
Anas platyrhynchos and F. atra
abundances recorded in our study
appear to increase from summer to
autumn,
following
patterns
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
previously reported (Martínez-Vilalta
1996; Marco-Méndez et al. 2015b).
Although both species were present
in the rice-field during the whole
study period, A. platyrhynchos was the
most abundant (summer: 84%;
autumn: 86%) while F. atra seemed
to be more present in the
neighboring lagoon (summer: 72%;
autumn: 86%). In this study, more
intense grazing activity in the ricefield was not related to population
increase, as commonly reported
(Perrow et al. 1997; Mitchell &
Perrow 1998), but results suggest
that
resident
populations
of
waterfowl use rice-field habitats
throughout the whole growing
season.
In summer, waterfowl abundance
seems to be lower but cage
experiments detected rice damage
through their grazing, leading to a
significant reduction in plan biomass.
In contrast, consumption was
undetectable by tethers, which we
think could be due to the low
number of waterfowl encountering
tethers. This possibly resulted from
their high mobility in the area and/or
the high abundance of other
resources in the neighboring lagoon
(Menéndez & Sánchez 1998;
Menéndez & Comín 2000). In fact,
we
know
from
the
other
simultaneous study (Marco-Méndez
et al. 2015b) that waterfowl were also
visiting the lagoon in summer and
grazing actively on R. cirrhosa,
probably influenced by the higher
abundance of its flowers. This
together with waterfowl abundance
data suggests that in summer both
species were probably feeding actively
in both habitats. Therefore, this
grazing on the rice-field seems
plausible
despite
tethering
experiments not detecting such
consumption.
In autumn, although waterfowl
abundance tends to increase, cage
experiments did not detect the effects
of waterfowl grazing on the rice-field
either in plant or seed biomass.
However, tethering experiments did
show grazing with an emphasis on
seed consumption. Here, the lower
biomass of rice plants and seeds
inside exclusion cages, versus control
cages, suggests that grazing effects
were possibly masked by a ‘shading
effect’. We hypothesize that the
netting mesh used (1.5 cm2) could
have
created
light-limiting
conditions during the time duration
of the experiment, thus reducing
183
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
growth and seed production in the
rice plants (Bello et al. 1995; Schou
et al. 1998). Alternatively, rice plants
may also have compensated for
waterfowl grazing by growing more
(Howe & Westley 1988; Prado et al.
2011), but this cannot be assessed
from the data in this study. The
grazing detected by the tethering
experiment appears to be consistent
Figure 6.4. A. δ15N and δ13C signatures and B. C:N ratios in the muscle of A. platyrhynchos and
F. atra and in the different food items. Mean ± SE.
184
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
with
the
greater
waterfowl
abundance, rice biomass and seeds in
autumn, while both ‘shading’ effects
and plant compensatory mechanisms
are likely to have interfered in cage
experiment results. Hence, despite
these possible ‘masking effects’ our
results suggest that waterfowl did
feed on rice-fields over the entire
study period, causing a notable
reduction in rice plant biomass in
summer and in rice seeds in autumn.
R. cirrhosa fragments were the most
important for F. atra, confirming
their reported dietary preferences
(Rodríguez-Villafañe et al. 2007;
Stafford et al. 2010). These results
are in concordance with the local
abundance of R. cirrhosa and P.
pectinatus in the Ebro Delta lagoons
(Prado et al. 2011), showing that
waterfowl were also feeding there at
the time of collection (autumnwinter). In particular, P. pectinatus
and R. cirrhosa seeds (commonly
available in autumn-winter; Green et
al. 2002; Rodríguez-Pérez & Green
2006) and flowers (in summer;
Marco-Méndez et al. 2015b) are
known to be subjected to high
seasonal herbivory by waterfowl, with
an important impact on plant
communities (Rodríguez-Pérez &
Green 2006; Marco-Méndez et al.
2015b). Regarding rice seeds, the fact
Gut content analyses showed
differences between waterfowl species
diets, identifying A. platyrhynchos as
primarily a granivorous species and to
the F. atra as primarily a herbivore
species. Among the range of food
items found in gut contents, rice
seeds and R. cirrhosa seeds were the
most important food resource for A.
platyrhynchos, while P. pectinatus and
Table 6.2. Differences in δ15N and δ13 C signatures and nutrient contents (C:N ratios) between
food items: Ruppia cirrhosa leaves (RUP-L); R. cirrhosa seeds (SD-S); Potamogeton pectinatus
leaves (POT-L); P. pectinatus seeds (POT-S); Rice plant (Rice); Rice seeds (Rice-S). Significant
differences are indicated: *** p < 0.001. Differences between groups investigated with the Student
Newman-Keuls post-hoc test (Zar 1984) are indicated. No transformation was carried out.
Source of
variation
δ15N
δ13C
df
MS
F
p
Food item
5
55.846
311.542
***
Residual
24
SNK
0.179
POT-S>RUP-L=POT-L=
=RUP-S=Rice-S=Rice
C:N
df
MS
F
p
5
197.245
1670.849
***
24
0.118
RUP-S=POT-L=RUP-L=
=POT-S>Rice-S=Rice
df
MS
5
232.737
F
p
17.62 ***
24
13.208
POT-S=Rice>Rice-S=
=RUP-S=RUP-L=POT-L
185
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
that individuals were collected during
the hunting season (autumn-winter)
means that individuals could obtain
them directly from the ripe plants or
from grain not collected during
harvest (Stafford et al. 2006). It is
also known that hunters intentionally
spill commercial rice grains (without
the husk) to attract waterfowl.
However, in our study we only
detected this type of rice (clearly
different from locally available rice
still in the husk) in one individual,
which was discarded for dietary
analyses. We think that the high
presence of seeds in guts of A.
platyrhynchos seems consistent with
the seed consumption detected by
tethers in autumn and with A.
platyrhynchos abundance, indicating
the rice fields are important feeding
habitats for this species (Stafford et
al. 2006, 2010). The importance of
submerged macrophytes in the diet
of F. atra also concurs with its
greater presence in the lagoon and
with previous studies (RodriguezVillafañe et al. 2007; Marco-Méndez
et al. 2015b). However, the lack of
rice plant tissue and seeds in its guts
contrasts strikingly with its consistent
presence in the rice field during the
whole growing season. This could be
186
due to the seasonal restrictions in the
collection of individuals for analysis
(some F. atra were hunted in
November) and that guts may reflect
a post-harvest scenario; as a
consequence of decreasing food
availability in the rice field (despite
residual uncollected seeds) F. atra
probably moved to the lagoon for
feeding.
Stable isotopes analyses and mixingmodel results showed that in the long
term both F. atra and A.
platyrhynchos seem to acquire most of
their dietary needs from rice plants
and P. pectinatus (seeds or plant
tissue according to their seasonal
availability). These results seem to
match with gut content analyses for
A. platyrhynchos, showing that
individuals were intensively feeding
in the short or long term in the rice
field during the growing season.
However, gut content analyses of F.
atra pointed to a diet based on
macrophytes, while mixing-model
results also showed an important
long-term contribution of rice plants
to their diet. This apparent
discrepancy between gut and stable
isotope analyses is likely to be related
to temporal changes in the diet. In
fact, while gut contents can only
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
reflect waterfowl diet at the moment
of
collection
(autumn-winter),
waterfowl muscle tissue can retain
information of previous feeding
locations for periods of several weeks
before individuals were collected;
from 12 to 28 days (Tieszen et al.
1983; Hobson and Clark 1992a).
According to this, stable isotope
analyses suggest that F. atra was
feeding in rice fields in the growing
season (at least at the end of the
season), while guts seem to be
reflecting a post-harvest scenario
where F. atra feed mainly in the
lagoon. In fact, changes in diet and
foraging
behavior
have
been
previously reported in both species
(Guillemain
&
Fritz
2002;
Guillemain et al. 2002; Dessborn et
al. 2011). These allow them to face
seasonal changes in abundance or
availability of food within the
wetland (Rodríguez-Pérez & Green
2006) and respond to the differential
nutritional requirements of wintering
(higher
consumption
of
carbohydrate-rich seeds) or breeding
seasons (increased protein-rich food;
Baldassarre & Bolen 1984; Hohman
et al. 1996). In addition, studies also
suggest that herbivores maximize the
consumption of food items with
higher nutritional, protein and
energy contents (Hughes 1980). In
accordance with this, high nutritional
contents could influence higher
consumption of macrophytes by F.
atra. Similarly, seed selectivity of
waterfowl may be related to other
factors not measured here, such as
high protein and/or energy contents
(Hughes 1980; Sedinger & Raveling
1984; Murphy & King 1987) or by
the ease with which they can be
digested
(Charalambidou
&
Santamaría 2002). Ultimately, our
results suggest that waterfowl feed
complementarily
on
rice-fields
during the growing season; this is
mostly influenced by seasonal
changes in the availability of food
resources in the area, rather than by
their nutritional quality.
Our results confirm that in
Mediterranean regions (where there
has been an important reduction of
natural wetlands) rice-fields act as a
complementary feeding habitat for
waterfowl during the growing season.
Hence, this use of such croplands by
waterfowl could help mitigate the
loss of natural habitats in other areas
progressively
dominated
by
agriculture.
For
this
reason,
increasing collaboration between
187
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
researchers, farmers and agronomists
is needed to better understand how
appropriate
field
and
crop
management can increase the
conservation value of farmland for
birds, without compromising its
economic viability. Future research
could include: the availability and
value of wildfowl foods other than
188
rice in the fields, importance of field
edges
and
water
delivery
infrastructure, effects of rice farming
on population dynamics and a clearer
analysis of how socio-economic
factors influence the ecology and
conservation of wetland birds using
rice-fields.
CHAPTER 6: Rice fields used as feeding habitats for waterfowl throughout the growing season
189
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
190
CHAPTER 7: General Discussion
7. General Discussion
7.1. Pros and cons of using
different methodological
approaches in the study of
herbivore preferences and
consumption rates across
different ecosystems
The main purpose of this thesis was
to identify what factors drive
herbivore preference and final
consumption rates across different
macrophyte
ecosystems.
Using
combined methods allowed a broader
vision of the macrophyte-herbivore
interactions taking place in each case
under study. However, each method
had some associated limitations that
must be taken into account in the
interpretation of results (Table 7.1).
Results obtained using each method
were compared in order to avoid
over- or under-estimations of
herbivory. For example, estimates
from tethering experiments and
feeding observations in the field may
be influenced by several factors acting
differently from one meadow to
another or between times for the
same species, causing high variability
in estimates (see Section 7.2). Given
the multiple factors that may
influence herbivores’ consumption
and feeding behavior in each case
under study, it is possible that
consumption estimates did not
necessarily reflect food preference. In
contrast, food choice experiments
allow testing herbivore preferences by
isolating them from field factors
(e.g., predation, competition…),
which may help to elucidate how
factors related to food palatability
influence food selectivity. Regarding
diet, gut content gives a short-time
period of dietary information that
can be limited by food availability at
the moment of individual collection
and food digestibility, while isotope
analyses
give
longer-term
information but restricted to the
turn-over rate of the tissue
investigated. Accordingly, from
combining methods we sometimes
found contradictory results such as
that the preferred food (identified by
food choice experiments) was not
always the most consumed in the
field (tethering experiments and
191
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
feeding observation) or the most
present in gut contents (short-term
diet), although it may contribute to
long-term diet patterns (isotopic
analyses). This may suggest that
herbivores do not always have the
chance to feed on their preferred
food in the field, since under natural
condition they may be influenced by
192
other factors acting differently in
each studied case. For these reasons
we think that combining methods
was the best way to approach and
understand herbivory patterns in
macrophyte ecosystems and to
identify the main driving factors
involved.
CHAPTER 7: General Discussion
Table 7.1. Suitability of the different methods employed
NEGATIVE
POSITIVE
TETHERING
•
•
Easy to perform by any researcher in any habitat: leaf area can be easily measured
‘in situ’ using graph paper and after collection also by image analysis.
•
When leaf area cannot be measured, working with wet weights can be less
accurate.
Possible artifact effect: tethers can influence feeding behavior and consumption
rates.
Possible artifact effect: tethers can alter plant status as a result of leaf
manipulation.
Higher temporal and spatial variability in the estimates due to the high variability
in the encounter tether-herbivore, especially in cases of species with high mobility
(e.g., fish and waterfowl) or with patchy foraging strategies
•
•
•
•
•
RECOMMENDATIONS
It is an accurate method to quantify consumption in a short time period.
•
•
•
•
Tethering experiments should be performed as simply and non-intrusively as
possible, integrating them discreetly into the habitat of study so as not to alter
feeding behavior.
Before deployment leaves should be carefully manipulated during measurement,
trying to keep intact the shoots or stolons of the macrophyte.
Tethering experiments should be performed during a relatively short time period
appropriate to the macrophyte species, to avoid leaf degradation over time as much
as possible.
Control replicates must be used to account for variation in wet weight during the
experimental period for further correction in consumption rate estimates (e.g.,
replicates protected against herbivores).
Growth rates must be monitored during the experimental period for further
correction of consumption rate estimates (e.g., punching when working with
surface area).
In order to avoid over- or under-estimation of macrophyte consumption associated
with the wide variability in the encounter tether-herbivores in cases of high
mobility or patchy foraging strategies, experiments should be highly replicated
both in space and time.
193
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
POSITIVE
FOOD CHOICE EXPERIMENTS
•
•
•
NEGATIVE
•
•
•
•
•
•
RECOMMENDATIONS
•
•
•
•
•
•
•
194
An accurate method to quantify consumption in a short time period.
Easy to perform by any researcher in any habitat: leaf area can be easily measured ‘in
situ’ using graph paper and after collection also by image analysis.
Inclusion cages allow isolating small herbivores (e.g., sea urchins) from other food
resources and to accurately test their preferences after some time without feeding.
When leaf area cannot be measured, working with wet weights can be less accurate.
Paired replicates or tethers can influence feeding behavior and consumption rates or
alter plant status as a result of leaf manipulation (artifact effect).
When using food choice tethering in the field: Higher temporal and spatial variability
in the estimates due to the high variability in the encounter tether-herbivore,
especially in cases of species with high mobility (e.g., fish and waterfowl) or patchy
foraging strategies. In these cases, herbivores could not be kept without feeding before
the experiment.
In cases of herbivore inclusion in experimental cages (e.g., sea urchins), stress after
handling, fasting and inclusion can affect survival and feeding behavior during the
experiment.
When using food choice-tethering in the field, tethers should be deployed in a nonintrusive way, integrating it discreetly into the study habitat not to alter feeding
behavior.
Before experiments, leaves should be carefully manipulated during measurement,
trying to keep macrophyte shoots or stolons intact .
Experiments should be performed during a relatively short period appropriate for the
macrophyte species, to avoid leaf degradation as much as possible.
Control replicates must be used to account for variation in wet weight during the
experimental period, for further correction in consumption rate estimates (e.g.,
replicates protected against herbivores).
Growth rates must be monitored during the experimental period for further correction
in consumption rate estimates (e.g., punching when working with surface area).
When using food choice tethering in the field with species with high mobility, in
order to avoid over- or under-estimation of macrophyte consumption associated with
the high variability or patchy foraging strategies, experiment should be highly
replicated both in space and time.
In cases of herbivore inclusion in experimental cages (e.g., sea urchins): after handling
and fasting sea urchin individuals should be observed carefully before using them in
the experiment, to avoid mortality.
Time duration of experiment should be appropriate for the species, not to alter
feeding behavior under inclusion condition and to avoid macrophyte degradation.
Cage size should be adequate for the number of individuals included (e.g., 3-4
individuals in 50 cm X 50 cm X 50cm cage) to reduce stress and mortality.
CHAPTER 7: General Discussion
EXCLUSION CAGE EXPERIMENTS
RECOMMENDATIONS
NEGATIVE
POSITIVE
•
•
•
•
•
•
•
•
•
An accurate method to quantify herbivory impact on macrophyte biomass, canopy
height or flowering in different ecosystems during long time periods.
Macrophytes are not manipulated before experiment.
Samples can be taken at different times during experiments to monitor natural
changes in macrophyte variables in the habitat and quantify differences due to
herbivory (control/open vs. exclusion/closed cages).
Usable with different macrophyte species and for both small (e.g., sea urchins) and
large herbivores (waterfowl).
Cages mesh can cause ‘shading effect’ (light reduction) and consequently reduce
biomass and canopy height (especially in long experimental periods), which can
interfere in grazing impact estimation.
In long-term experiments, periodic sample collection inside cages can affect plant
status and consequently trigger plant biomass and canopy height variation that can
interfere in final grazing impact estimation.
Cage mesh should be appropriate for the ecosystems and macrophytes under study
(e.g., turbidity, canopy height…).
Controls should be carried out to measure a possible ‘artifact effect’ of the mesh over
macrophyte variables, for further correction of grazing impact quantification.
In long-term experiments, where periodic sampling is required, cage size and sample
size should be properly designed and adapted to each case (e.g., macrophyte
distribution or canopy height, herbivore size…), reducing as much as possible the
number of destructive sample collections.
195
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
FEEDING OBSERVATIONS
RECOMMENDATIONS
NEGATIVE
POSITIVE
•
196
•
•
•
•
•
•
•
•
Allow feeding behavior to be observed in natural conditions without experimental
interference.
Provide complementary information about feeding habits to compare with tethering
and food choice results under experimental conditions.
Can provide not only information about feeding behavior but also about
abundances, movement patterns and habitat uses.
In underwater studies divers can interfere in feeding behavior.
Time consuming task: observation should be highly replicated in space and time
especially in the case of mobile species like fish or waterfowl.
Weather conditions can interfere in abundances and feeding behavior.
Divers should keep a distance from schools to reduce interference in feeding
behavior.
If possible, video recording is the best way to analyze feeding behavior more
precisely, especially in the case of mobile herbivores.
In order to reduce variability, censuses should be highly replicated and if possible
carried out by the same observer, at the same time of day and under good weather
conditions.
CHAPTER 7: General Discussion
RECOMMENDATIONS
NEGATIVE
POSITIVE
GUT CONTENT ANALYSES
•
•
•
•
•
•
•
•
•
Give information about food items consumed immediately before collection (short
term).
Complement experimental results and feeding observation.
There is no experimental or observational interference during feeding.
Destructive method: this methodology implies killing the animal.
Collection can be difficult if unobtainable by fishermen (fish) or hunters
(waterfowl).
Time consuming task: in dissection and identification of each food item.
Digestibility: different food items can have different digestibility affecting their
identification and dietary contribution estimates.
Short term information: gut content provides dietary information valid only at the
time of collection.
To have long-term information about the diet, a large number of individuals
should be collected through the time and space. This can be especially important
in high mobility species, whose diet can show higher variability according to
changes in food resources availability.
197
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
STABLE ISOTOPE ANALYSES (SIA) AND MIXING MODELS
POSITIVE
•
•
•
NEGATIVE
•
•
•
•
RECOMMENDATIONS
•
•
•
•
198
Stable isotope analyses in different tissues can provide short or long term information
about the diet or feeding habitat (e.g., liver, muscle or blood).
Some tissue samples can be collected from the field avoiding killing the animal (e.g.,
feathers).
Mixing models: Under appropriate conditions, it is possible to use the stable isotope
ratios in tissues to quantify the relative importance of different dietary items to a
consumer, based on the premise that consumers’ tissues will resemble the long-term
isotopic composition of the diet (Fry & Sherr 1974; Minagawa & Wada 1974).
The sensitivity of the mixing model is mostly affected by the different isotopic
signatures of the sources (Phillips & Gregg 2003), so when sources have similar
isotopic signatures (e.g., seeds and leaves of the same macrophyte species, or genus
such as Caulerpa) the model cannot discriminate their contribution to the diet.
Sometimes it is difficult to find a fractionation value appropriate for the herbivore and
diet under study, from the literature available.
First models were restricted by the number of sources (dietary categories) and
isotopes used in the study, such that feasible solutions could only be attained if the
number of sources were equal to or less than the number of isotopes plus one. Hence
dual isotope studies could only ever partition the contribution of three dietary sources
(Phillips & Gregg 2001).
More advanced methods have relaxed these restrictions, although the outputs of these
multi-source mixing models have a much lower resolution (Phillips & Gregg 2003).
Moore & Seemens (2008) developed a Bayesian-mixing model that estimates
probability distributions of source contributions to a mixture while explicitly
accounting for uncertainty associated with multiple sources, fractionation and isotope
signatures. This model also allows for optional prior data inputs into analyses.
In this thesis we combined both IsoSource (Phillips & Gregg 2003) and MixSir
(Moore & Seemens 2008) methods. Despite MixSir having been proven to be more
accurate given the contributions to a diet, our results were overall more coherent with
IsoSource than with MixSir. In some cases, contributions provided by the two models
were similar but in other cases results were quite different and those given by MixSir
were misleading. We decided that the criteria of the researcher should prevail in
interpreting the results and they were not included in the study.
Fractionation values selected to carry out analyses should not be generalized for all
studies (e.g., 3.4‰ for δ15N and 0.5‰ for δ13C) and must be suitable for the type of
diet and consumer under study: a waterfowl, a fish or a sea urchin would have
different diet assimilation, therefore whenever possible an adequate fractionation
value should be used for each case.
The tissue employed (e.g., muscle) would have an associated turnover rate to take into
account when mixing model results are interpreted. For example, muscle tissue may
reflect waterfowl diet/habitat several weeks before individuals were collected (from 12
to 28 days, the time of synthesis of the tissue employed).
CHAPTER 7: General Discussion
7.2. Grazing intensity
estimation across different
macrophyte ecosystems
Consumption rates were greatly
variable between seasons, macrophyte
and herbivore species (Fig. 7.1, Table
7.2). Surprisingly, despite being the
smallest herbivores, sea urchins
recorded the highest consumption
rates, Lytechinus variegatus (Lam.) on
decayed
leaves
of
Thalassia
testudinum Banks ex König and
Paracentrotus lividus (Lam.) above
Cymodocea nodosa (Ucria) Ascherson
in summer 2011. This confirms that
marine herbivores other than large
vertebrates can also exert an
important role in seagrass ecosystems
(see review by Heck & Valentine
2006) (Fig. 7.1). In addition, higher
herbivory in Mediterranean meadows
was not always mostly due to Sarpa
salpa (L.), which had a low
consumption of Posidonia oceanica
(L.) Delile and no detectable
consumption of C. nodosa at our
study sites. However, in the study
carried out in 2012 (Chapter 4), C.
nodosa was the macrophyte most
consumed by S. salpa, recording in
Figure 7.1. Percentage of the amount of macrophytes offered that were consumed per day in the
different times of study. Herbivore species responsible for consumption are indicated (U: sea
urchins P. lividus and L. variegatus, in purple); F: the fish S. salpa and W: waterfowl F. atra and
A. platyrhynchos). Lowest consumption rates cannot be visualized (e.g., ≤ 0.01 % see Table 7.1).
199
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
summer
consumption
rates
significantly higher than reported for
the other macrophytes during the
whole study (Table 7.2). Our results
suggest that herbivory on C. nodosa
(e.g., 2.82% of macrophyte removed
by P. lividus in Chapter 3 and 0.51%
by S. salpa in Chapter 4) can also be
important and should not be
underestimated. Despite the fact
lower
consumption
and
no
significant differences were detected,
results also show that consumption of
Caulerpa prolifera (Forsskål) JV
Lamouroux can exceed (in summer)
or at least equal (in autumn) P.
oceanica consumption detected in our
study (see Table 7.2).
After C. nodosa, the highest
consumption rates were recorded for
Potamogeton pectinatus L. In the
wetland this species was the
macrophyte most consumed by
waterfowl in all the seasons studied
(summer 2010, autumn 2010 and
winter 2011), followed by Ruppia
cirrhosa (Petagna) Grande and rice
plants with seeds (Oriza sativa L.).
Regarding the cage experiments,
results pointed to R. cirrhosa as the
most consumed macrophyte. From
the differences found between
200
control and exclusion cages, it was
estimated that the biomass and
canopy height of R. cirrhosa reduced
in
a
month
by
waterfowl
consumption was ca. 38% and a 36%
respectively. These results are within
the estimated range for similar
waterfowl species affecting other
macrophyte species such as Zostera
japonica Ascherson & Graebner or Z.
marina L. (Baldwin & Lovvorn
1994a; Madsen 1988; Portig et al.
1994) (see Table 7.3).
Overall, these results reflect high
variability in the herbivory intensity
detected
for
the
different
macrophytes and herbivores studied.
Furthermore, we realized that in
some cases, the high herbivory levels
quantified seemed to be reflecting a
particular situation associated with
habitat features and uncommonly
high herbivore abundances, while in
other cases, the high mobility of
herbivores seemed to have accounted
for the variability and inconsistency
in the rates recorded. In fact, thanks
to combining methodologies we were
able to conduct a more cautious
interpretation of our results that
identified the potential factors
involved in each study.
CHAPTER 7: General Discussion
Table 7.2. Comparison of macrophyte consumption rates found in this thesis by tethering
experiments. In order to compare studies we estimated % of initial amount of macrophytes
offered that was consumed per day
Chapter 2: Marco-Méndez et al. 2012; Chapter 3: Marco-Méndez et al. 2015a; Chapter 4: Marco-Méndez
et al. submitted; Chapter 5: Marco-Méndez et al. 2015b; Chapter 6: Marco-Méndez et al. in press.
Source
Chapter 2
Chapter 2
Chapter 3
Chapter 3
Chapter 3
Chapter 3
Chapter 4
Chapter 4
Chapter 4
Chapter 4
Chapter 4
Chapter 4
Chapter 4
Chapter 4
Chapter 5
Chapter 5
Chapter 6
Chapter 5
Chapter 5
Chapter 6
Chapter 6
Chapter 5
Chapter 5
Time of
study
Autumn
2009
Autumn
2009
Summer
2011
Summer
2011
Summer
2011
Summer
2011
Summer
2012
Summer
2012
Summer
2012
Summer
2012
Autumn
2012
Autumn
2012
Autumn
2012
Autumn
2012
Summer
2010
Summer
2010
Summer
2010
Autumn
2010
Autumn
2010
Autumn
2010
Autumn
2010
Winter
2011
Winter
2011
Concumption rates
(tethering experiment)
%
removed
per day
12.15 ± 1.3 mg DW shoot−1 d−1
3.02
0 mg DW shoot−1 d−1
0
C. nodosa
0.24 ± 0.086 cm2 shoot–1 d–1
2.82
19.1 ± 2.6 ind m−2
P. oceanica
0.105 ± 0.06 cm2 shoot–1 d–1
0.19
S. salpa
0.5 ± 0.1 ind m-2
C. nodosa
0 cm2 shoot–1 d–1
0
S. salpa
0.5 ± 0.1 ind m-2
P. oceanica
0.004 ± 0.004 cm2 shoot–1 d–1
0.01
S. salpa
0.3 ± 0.1 ind m-2
C. nodosa
12.75 ± 3.42 mg WW d-1
0.51
S. salpa
0.3 ± 0.1 ind m-2
P. oceanica
0.163 ± 0.115 mg WW d-1
0.01
S. salpa
0.3 ± 0.1 ind m-2
C. prolifera
0.53 ± 0.21 mg WW d-1
0.01
S. salpa
0.3 ± 0.1 ind m-2
C. cylindracea
0 mg WW d-1
0
S. salpa
0.04 ± 0.03 ind m-2
C. nodosa
0.054 ± 0.038 mg WW d-1
< 0.01
S. salpa
0.04 ± 0.03 ind m-2
P. oceanica
0.274 ± 0.190 mg WW d-1
0.01
S. salpa
0.04 ± 0.03 ind m-2
C. prolifera
0.21 ± 0.116 mg WW d-1
< 0.01
S. salpa
0.04 ± 0.03 ind m-2
C. cylindracea
0 mg WW d-1
0
R. cirrhosa
0.022 ± 0.0108 g WW d-1
0.66
P. pectinatus
0.047 ± 0.0157 g WW d-1
1.39
Rice plants
0 g WW d-1
0
R. cirrhosa
0.0175 ± 0.007 g WW d-1
0.52
P. pectinatus
0.048 ± 0.117 g WW d-1
1.25
Rice plants
0.0669 ± 0.07 g WW d-1
0.58
Rice seeds
0.013 ± 0.004 g WW d-1
-
R. cirrhosa
0.004 ± 0.004 g WW d-1
0.17
P. pectinatus
0.017 ± 0.07 g WW d-1
0.59
Herbivore species
Herbivore
abundance
L. variegatus
8.8 ± 1.7 ind m−2
L. variegatus
8.8 ± 1.7 ind m−2
P. lividus
19.1 ± 2.6 ind m−2
P. lividus
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
F. atra,
A. platyrhynchos
ha-1;
0.4 ± 0.2 ind
0.1 ± 0.02 ind ha-1
0.4 ± 0.2 ind ha-1;
0.1 ± 0.02 ind ha-1
0.16 ± 0.04 ind ha-1;
0.9 ± 0.3 ind ha-1
2.3 ± 0.2 ind ha-1;
0.4 ± 0.1 ind ha-1
2.3 ± 0.2 ind ha-1;
0.4 ± 0.1 ind ha-1
0.3 ± 0.1 ind ha-1;
1.9 ± 0.5 ind ha-1
0.3 ± 0.1 ind ha-1;
1.9 ± 0.5 ind ha-1
2.8 ±1.2 ind ha-1;
0.3 ± 0.04 ind ha-1
2.8 ±1.2 ind ha-1;
0.3 ± 0.04 ind ha-1
Macrophytes
species
T. testudinum
(Decayedl)
T. testudinum
(Green)
201
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Table 7.3. Comparison of waterfowl grazing effect on macrophytes found in this thesis by
exclusion cage experiments.
Source
Time of
study
Herbivore
species
Abundance
Macrophyte
species
Chapter 5
Summer
2010
F. atra,
A. platyrhynchos
0.4 ± 0.2 ind ha-1;
0.1 ± 0.02 ind ha-1
R. cirrhosa
Chapter 5
Summer
2010
F. atra,
A. platyrhynchos
0.4 ± 0.2 ind ha-1;
0.1 ± 0.02 ind ha-1
R. cirrhosa
(flowers)
Chapter 6
Summer
2010
F. atra,
A. platyrhynchos
0.16 ± 0.04 ind ha-1;
0.9 ± 0.3 ind ha-1
Rice plant
7.3 Main factors driving
herbivory across different
macrophyte ecosystems
7.3.1. Influence of herbivore
abundance and feeding behavior
In the eastern Gulf of Mexico, events
of intense herbivory in which L.
variegatus may remove up to 50 to
90% of the annual aboveground
primary production (APP) of T.
testudinum are not uncommon and
may be attributable to dramatic
increases in sea urchin density (Camp
et al. 1973; Greenway 1976;
Valentine & Heck 1991; Heck &
Valentine 1995). According to
Valentine & Heck (1991), L.
variegatus can regulate the biomass
and size of subtropical turtlegrass (T.
testudinum) meadows in the northern
Gulf of Mexico, particularly where
sea urchin densities exceed 20 ind
202
Grazing impact estimated
by exclusion cage experiment
Canopy height was reduced ∼38
% and ∼36.5 % in open cages vs.
exclusion cages
Flowers abundance was ∼8 times
higher inside exclusion cages;
1015.7 ± 269.8 flw m−2
Biomass was reduced ∼53 % in
open cages vs. exclusion cages
m−2 in winter and 40 ind m−2 during
summer and fall. Since densities of L.
variegatus during our study were
lower (8.8 ± 1.7 ind m−2), numbers
were not enough to ‘overgraze’ the
seagrass meadow (Fig. 7.2). In
addition, feeding observations noted
ca. 5 times more individuals feeding
on detrital material than on green
leaves, pointed to a detritivorous role
within seagrass beds.
In Mediterranean seagrass meadows,
P. oceanica has been reported to be
subjected to heavy herbivore pressure
in shallow meadows (Prado et al.
2007a, 2008a), mostly attributed to
the fish S. salpa (̴ 70% of total annual
losses) (Fig. 7.3) and to lesser extent
to P. lividus (the remaining 30%),
(Prado et al. 2007a). However, in
this thesis (Chapter 3), our tethering
results showed that herbivory on C.
nodosa can also be important and that
CHAPTER 7: General Discussion
Fig. 7.2. Lytechinus variegatus with green and decayed leaves of in a meadow of Thalassia
testudinum (Chapter 2). Photo: P. Prado
P. lividus was mainly responsible for
the consumption of both seagrasses
at our study site (96% of P. oceanica
and 100% of C. nodosa, see Fig. 7.1
and Table 7.2). This elevated
herbivory by P. lividus is explained by
the high abundance of sea urchins in
this mixed seagrass-rocky habitat (19
ind m-2) which differs notably from
commonly reported abundances in P.
oceanica (2–3 ind m-2; Boudouresque
& Verlaque 2001, 2013; Tomas et al.
2005b) and C. nodosa meadows (ca.
0.7 ind m-2; see Fernandez &
Boudouresque 1997). The high
availability of rocky substrate (ca.
51% of coverage) that provided
shelter from predation did likely
influence these densities (Ebling et
al. 1966; Boudouresque & Verlaque,
2001, 2013). Despite the higher
consumption of C. nodosa detected by
tethers, feeding observations showed
that most of the individuals were
hidden within rocky shelters and
passively caught drifting food items
(e.g., detached leaves of P. oceanica)
without venturing into the meadow
203
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
(Marco-Méndez, pers. observ.). The
observed behavior suggests that sea
urchins prioritize staying safe within
their shelter over searching for their
preferred food (Boudouresque &
Verlaque 2001, 2013), therefore
higher levels of urchin herbivory may
be due to the proximity of tethers to
these shelters, which made the
preferred food available to these
individuals.
The first study of the fish S. salpa,
carried out in summer 2011 (Chapter
3), suggested that the lower herbivory
rates compared to P. lividus (4% of P.
oceanica and no consumption of C.
nodosa; see Table 7.2) was due to low
fish abundance (0.49 ind m-2)
compared to P. lividus and to fish
mobility within its home range (ca. 1
ha according to Jadot et al. 2002),
which could have led to the low
number of fish-tether encounters,
underestimating consumption by
fish. In fact, in the second study
carried out in 2012 (Chapter 4),
enhanced fish abundance was
responsible for high herbivory in
summer, especially on C. nodosa, but
autumn fish migration caused lower
consumption rates of all macrophyte
species (in concordance with Ruitton
et al. 2006; Tomas et al. 2011b).
204
This apparent contradiction between
the
two
studies
concerning
consumption rates in the mixed
meadow reinforces the theory that
herbivory varies strongly both
spatially and temporally (Tomas et al.
2005a; Prado et al. 2008a). It is not
only influenced by temporal changes
in fish abundances (as fish
abundances were similar), but also by
fish home-range size (high mobility
could have affected the variability in
the
encounter
herbivore-tether
between experiments), habitat type,
and selection (meadows were
different in each study), or variability
in behavior of S. salpa individuals
(Jadot et al. 2002, 2006).
In the study carried out in the Ebro
Delta (Chapter 5), total waterfowl
abundance (F. atra and A.
platyrhynchos) in the lagoon increased
from summer to winter (from 0.51
ind ha-1 to 3.14 ind ha-1), F. atra
being the most abundant species in
the entire lagoon throughout the
study.
This
seasonal
pattern
concurred with those previously
reported for coots and ducks in other
Mediterranean
wetlands,
with
abundances peaking in OctoberNovember during the post-breeding
period and a mean density of 2.9 ind
CHAPTER 7: General Discussion
Figure 7.3. School of Sarpa salpa in a Posidonia oceanica meadow. Photo: LM. Ferrero-Vicente.
ha-1 (Rodriguez-Pérez & Green
2006). However, contrasting with
previous studies that suggested that
grazing activity in Central and
Northern Europe takes place in late
autumn and winter due to population
increase (Van Donk & Otte 1996;
Søndergaard et al. 1996; Froelich &
Lodge
2000;
Santamaría
&
Rodríguez-Gironés 2002), our study
suggests that major effects of
waterbirds
on
submerged
macrophytes
in
Mediterranean
lagoons were not driven by seasonal
variations in waterfowl abundance
(Fig. 7.4). In summer, although
waterfowl abundance was lower,
grazing effects were evident in R.
cirrhosa biomass, canopy height and
flowers, which underwent the most
intense herbivory in open cages
(flower abundance was ̴ 8 times
higher inside exclusion cages).
Similar results were found in the
neighboring rice field studied
(Chapter 6), where waterfowl
abundances tended to increase from
summer to autumn (Martínez-Vilalta
1996; Marco-Méndez et al. 2015b).
Although both waterfowl species
were present in the rice-field during
the whole study period, A.
platyrhynchos was the most abundant
(summer: 84%; autumn: 86%) while
F. atra seemed to be more present in
the neighboring lagoon (summer:
205
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 7.4. Anas platyrhynchos feeding on Ruppia cirrhosa around experimental exclusion cages in
the Ebro Delta (Chapter 5). Image captured by the time lapse video-recording camera used for
feeding behavior observation.
72.2%; autumn: 86.3%). However,
here as in the previous study, more
intense grazing activity in the ricefield was not related to population
increase, as commonly reported
(Perrow et al. 1997; Mitchell &
Perrow 1998), but results suggested
that
resident
populations
of
waterfowl use rice-field habitats
throughout the whole growing
season.
Overall, all these studies suggest that
herbivory across different ecosystems
did not follow a general pattern but is
highly variable in time and space
depending on a regional combination
of animal numbers, feeding behavior,
habitat structure and compositional
abundance of food resources.
206
7.3.2 Influence of food availability
and food preferences: The role of
epiphytes and flowers
Differences
in
the
temporal
availability of food items can lead to
profound dietary changes as often
reported (Boudouresque & Verlaque
2001; Wernberg et al. 2003). After
the study carried out in St Joseph
Bay, Florida (Chapter 2) it was
suggested that high fall abundances
of decayed leaves trapped within T.
testudinum meadows could benefit
green leaves by reducing grazing by
L. variegatus and regulating the
nature and magnitude of herbivore
impacts on seagrass beds. In fact,
previous studies had suggested that
the impact of herbivores on seagrass
productivity and the dynamics of the
CHAPTER 7: General Discussion
bed may be related to the relative
availability of green vs. decayed leaves
(e.g., Tubbs & Tubbs 1982, 1983).
In our study carried out in fall, when
the abundance of detrital leaves is
typically high and often exceeds the
biomass of green leaves (in this case
by ca. 25%), detrital epiphytized
leaves were indeed the most
consumed, and the preferred food
item of L. variegatus. This confirms
its detritivorous role within seagrass
beds. Our food-choice experiments
indicated that these differences in
consumption were strongly mediated
by epiphytes; detrital leaves were
largely preferred over green leaves but
only when epiphytes were present
(Montague et al. 1991; Greenway
1995). These results confirmed
previous evidence that decayed plant
material is an important dietary
resource for sea urchins (Lowe &
Lawrence 1976; Klumpp & Van der
Valk 1984). Moreover, they show
that this dietary choice can be largely
dependent on the presence of
epiphytes. This was supported by
isotopic
analyses
pointing
to
epiphytes as the most important
food, followed by green leaves and
decayed leaves. Given the importance
of detrital resources highlighted by
this experiment, part of the large
spatial and temporal variability in sea
urchin grazing observed to date may
Figure 7.5. Food choice experiment where paired epiphytized leaves of Cymodocea nodosa and
Posidonia oceanica were offered to Paracentrotus lividus individuals (Chapter 3).
207
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
in fact be related to differences in the
biomass of decayed leaves trapped
within meadows.
Regarding Mediterranean herbivores, past studies have indicated that
P. lividus (Boudouresque & Verlaque
2001, 2013; Tomas et al. 2005b) and
S. salpa (Verlaque 1981, 1985)
preferentially feed on the epiphytes
of P. oceanica leaves, so during this
thesis work we have pursued
clarification of their mediating role in
the consumption and food selectivity
of these herbivores in Mediterranean
meadows. In the first study, carried
out in a Mediterranean mixed habitat
in late summer (Chapter 3), we
found that C. nodosa and P. oceanica
displayed similar availability at the
study site and that leaf consumption
did not vary significantly between
seagrass species. However, food
choice experiments revealed a slight
preference of S. salpa for C. nodosa,
whereas this was strongly evident for
P. lividus, which in the presence of
epiphytes showed an ca. 8 times
higher consumption of C. nodosa
than P. oceanica (Fig. 7.5). In fact,
the epiphytic community structure
revealed
important
differences
between seagrass species that might
have further influenced herbivore
preferences
and
consumption.
Cymodocea nodosa displayed higher
Figure 7.6. School of Sarpa salpa in the mixed meadow of Posidonia oceanica, Cymodocea nodosa,
Caulerpa prolifera and Caulerpa cylindracea (Chapter 4).
208
CHAPTER 7: General Discussion
epiphytic coverage and mean number
of leaf epiphytic taxa. Therefore, this
study confirmed the role of epiphytes
in mediating seagrass consumption
and preferences (e.g., Greenway
1995; Marco-Mendez et al. 2012),
especially for P. lividus whose diet is
greatly supplemented by epiphytes
(Tomas et al. 2005b).
prolifera in the diet of S. salpa
(Chapter 3). Tethering experiments
showed that C. nodosa was the
macrophyte most consumed by the
fish, summer consumption rates
being significantly higher than
reported for P. oceanica, C. prolifera
and C. cylindracea, during the whole
study.
In the second study carried out in
another Mediterranean mixed habitat
(Chapter 4; Fig. 7.6), our results
confirmed the previously reported
importance of C. nodosa and C.
From both studies (Chapter 3 and 4),
we concluded that under summer
conditions, when S. salpa densities
were the highest, C. nodosa was the
‘most
consumed’
macrophyte,
Figure 7.7. Flowering of Ruppia cirrhosa in the Ebro Delta lagoon (Chapter 5).
209
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
probably influenced by the higher
nutritional quality of its leaves and
epiphytes as well as by temporal
differences
in
the
epiphytic
community composition (MarcoMéndez et al. 2015a). Differences
found in consumption rates between
studies (Chapter 3 and 4) were
attributed to natural variability in fish
abundance
and
distribution
associated with fish mobility within
their home-range, habitat type and
selection, or variability in behavior of
S. salpa individuals (Jadot et al. 2002,
2006; see Section 7.2.1). Despite C.
prolifera being a consistently selected
food by S. salpa, food choices also
indicated that the preference for C.
prolifera vs. seagrasses can be
dissipated in the presence of
epiphytes, which may have mainly
accounted for herbivory patterns in
the mixed meadow. IsoSource
mixing model (Phillips & Gregg
2003) results ultimately reflected the
preferences
and
consumption
patterns observed during the study
and supports the previously reported
importance of epiphytes (MarcoMéndez et al. 2012, 2015a) and
Caulerpa species in S. salpa herbivory
(Ruitton et al. 2006; Tomas et al.
2011b).
210
In Chapter 5 we found that, in
Mediterranean aquatic lagoons,
waterfowl grazing impact on
submerged
macrophytes
was
influenced by the seasonal changes in
food availability and flowering
events, rather than by waterfowl
abundances (Fig. 7.7). In fact, greater
effects were detected in R. cirrhosa
and in summer due to the higher
abundance of flowers (ca. 10 times
higher than P. pectinatus inside
exclusion cages) which appeared to
be a key factor controlling herbivory
pressure. In autumn, flowers
disappeared, and its lack of effect on
biomass and canopy height of both
R. cirrhosa and P. pectinatus was
attributed
to
the
enhanced
availability of other resources such as
rice seeds (due to the harvest season)
or the proliferation of floating
macroalgal mats. Later in winter, the
lower biomass and canopy height
recorded for both macrophytes could
have made them a less accessible
resource and therefore, harder to be
found by waterfowl, particularly due
to the greater water turbidity during
this period. We hypothesize that in
this season, the reported ability of
ducks and coots to switch to feeding
on invertebrates and seeds may help
CHAPTER 7: General Discussion
them persist within the lagoon area,
in spite of the scarcity of submerged
vegetation (Rodríguez-Pérez &
Green 2006).
over the entire study period, causing
a notable reduction in rice-plant
biomass in summer and in rice seeds
in autumn.
In the simultaneous study carried out
in a neighboring rice-field (Chapter
6), waterfowl abundances and
patterns of herbivory concurred with
those reported for the lagoon. In
summer,
waterfowl
abundance
tended to be lower but cage
experiments detected rice damage by
grazing, leading to a significant
reduction in plant biomass. In
autumn,
although
waterfowl
abundance tends to increase, cage
experiments did not detect the effects
of waterfowl grazing on the rice-field
in either plant or seed biomass.
However, tethering experiments did
show grazing with an emphasis on
rice seed consumption. Here, grazing
detected by the tethering experiment
appears to be consistent with the
greater waterfowl abundance, rice
biomass and seeds in autumn, while
both ‘shading’ effects and plant
compensatory mechanisms are likely
to have interfered in cage experiment
results. Hence, despite these possible
‘masking effects’ our results suggested
that waterfowl did feed on rice-fields
Food items found in gut content
(Fig. 7.8) and stable isotope analyses
were in concordance with the local
abundance of R. cirrhosa and P.
pectinatus in the Ebro Delta lagoon
(Prado et al. 2011), showing that
waterfowl were also feeding there at
the time of collection (autumnwinter). In addition, stable isotope
analyses confirmed the importance of
rice in both species’ diets, suggesting
that both were alternatively feeding
in rice fields in the growing season.
Combined dietary results indicate
that waterfowl may undergo seasonal
dietary variations, mostly influenced
by changes in the availability of food
resources in the area. This may allow
them to face seasonal changes in
abundance or availability of food
within the wetland (Rodríguez-Pérez
& Green 2006) and respond to the
different nutritional requirements of
wintering (higher consumption of
carbohydrate-rich seeds) or breeding
seasons (increased protein-rich food;
Baldassarre & Bolen 1984; Hohman
et al. 1996).
211
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 7.8. Gut content of Anas
platyrhynchos (A) with high presence of
Ruppia cirrhosa seeds (B) and rice seeds (C),
and guts of Fulica atra with fragments of R.
cirrhosa and Potamogeton pectinatus leaves
(D).
7.3.3. Influence of nutritional
content (C:N molar ratio) on food
selectivity
Plant nutritional quality (often
expressed as leaf nitrogen content)
has been shown to play a central role
in determining herbivore feeding
patterns (e.g., Onuf et al. 1977;
Slansky & Feeny 1977; Kraft &
212
Denno 1982; Coley 1983). Several
studies have investigated the
relationship between grazing rates
and nutrient concentrations in
seagrass systems, suggesting that
higher nutrient content is a factor
triggering
preference
and
consumption of primary producers by
herbivores (e.g., McGlathery 1995;
Heck et al. 2000; Prado et al. 2010b).
However, several authors have found
that increased nutrient content in
seagrass leaves does not necessarily
lead to higher consumption rates by
herbivores (for a review see Cebrián
2004). During this PhD work we
investigated the potential mediating
role of nutrient contents of different
macrophytes and their seeds in
herbivore
selectivity
and
consumption.
In the study carried out in St Joseph
Bay (Chapter 2), detrital leaves had
approximately half the nitrogen
content of green leaves alone and a
nearly 2-fold higher C:N ratio, but
consumption rates of both types of
leaves without epiphytes were not
significantly different. Similarly,
urchins consumed decayed leaves
with epiphytes to a greater extent
than green leaves with epiphytes,
even though the total nutrient
CHAPTER 7: General Discussion
content in leaves and epiphytes was
not significantly different. The lack
of
correlation
between
the
consumption rates and nutrient
content of the diet was also observed
in the food-choice experiments when
one type of leaf (either green or
decayed), with or without epiphytes,
was offered to the urchins. This
suggests that nutrient content itself
did not determine food selectivity of
the L. variegatus sea urchins in our
study.
In the first study carried out in a
Mediterranean
mixed
meadow
(Chapter 3), food choice experiments
revealed a slight preference of S. salpa
for C. nodosa, whereas it was strongly
evident for P. lividus. In the presence
of epiphytes, P. lividus showed a ca. 8
times higher consumption of C.
nodosa than P. oceanica, and values
also remained higher when epiphytes
were removed. Here, the lower C:N
values recorded in non-epiphytized
C. nodosa leaves suggested that this
preference could be related to the
higher nutritional quality of this
species. Furthermore, C:N ratios in
P. oceanica leaves (epiphytized and
non-epiphytized) were ca. 3 times
higher than in epiphytes. Similar
results were found for C. nodosa, for
which the leaves (epiphytized and
non-epiphytized) had C:N ratios
about twice higher than epiphytes.
These different nutritional contents
in the two seagrasses and their
epiphytes support the hypothesis that
the presence of epiphytes can
increase seagrass consumption rates
and mediate herbivore preferences
(Marco-Méndez et al. 2012) because
of their higher nutritional value, or
lower C:N ratios (e.g., Alcoverro et
al. 1997, 2000). Additionally, the
higher
nutritional
value
of
epiphytized leaves of C. nodosa
compared to epiphytized leaves of P.
oceanica may be involved in the
observed food preferences. This is
clearly due to the higher quality of
the leaves themselves plus the added
higher nutritional value of their
epiphytes. From this we can conclude
that different nutritional content in
leaves and epiphytes were likely
mediating herbivore preferences for
Mediterranean seagrass species.
In the second study carried out in a
Mediterranean meadow (Chapter 4),
we tested consumption by S. salpa
and its preferences among different
macrophyte species, finding similar
results. C. nodosa was the ‘most
consumed’ macrophyte, and from the
213
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
C:N results we also concluded it was
probably influenced by the higher
nutritional quality of its leaves and
epiphytes (lower C:N ratios) as well
as by temporal differences in the
epiphytic community composition
(Marco-Méndez et al. 2015a).
However, food choice, feeding
observations and gut content analyses
pointed to the alga C. prolifera as a
food consistently selected by S. salpa.
In fact, C:N ratios values were about
twice higher in non-epiphytized
leaves of both seagrasses than in C.
prolifera.
These
nutritional
differences were slightly reduced
when seagrasses were epiphytized.
Together with the significantly lower
C:N ratios and higher nutritional
content of epiphytes compared to C.
prolifera (%N was ca. 3 times higher
and %C ca. 4 times higher), such
differences could explain why the
preference for C. prolifera vs.
seagrasses loses strength in the
presence of epiphytes. This partly
accounts for the herbivory patterns in
the mixed meadow (i.e., tethering
results). It also confirms that
epiphytes and their higher nutritional
value (Alcoverro et al. 1997, 2000)
can mediate herbivore preferences
and consumption rates (Marco214
Méndez et al. 2012). Furthermore, it
seems that because of the higher
quality of its leaves (lower C:N ratio),
plus the increased nutritional value
resulting from the presence of
epiphytes, higher herbivory was
recorded for C. nodosa than P.
oceanica in the mixed meadow
(Marco-Méndez et al. 2015a). The
higher nutritional value among
Caulerpa species may also partly
explain S. salpa’s preference for C.
prolifera. From these results we can
also conclude that epiphyte and
macrophyte nutrient contents were
likely mediating S. salpa consumption
and preferences for C. nodosa and C.
prolifera in Mediterranean meadows
(Fig. 7.9) despite other factors
possibly being also involved.
As regards waterfowl herbivory in the
Mediterranean wetland (Chapters 5
& 6), nutrient content in food
resources did not by itself explain the
pattern of consumption observed
during the studies, since similar
higher nutritional contents were
found in the most important food
items: rice seeds, R. cirrhosa seeds, R.
cirrhosa and P. pectinatus, (Fig. 7.8).
Our results suggest that waterfowl
may undergo seasonal dietary
variations, mostly influenced by
CHAPTER 7: General Discussion
changes in the availability of food
resources in the area, rather than by
their nutritional quality. However we
did not rule out other features, not
measured here, that could have been
involved in waterfowl food selectivity
(e.g., protein and energy contents or
digestibility; Hughes 1980; Sedinger
& Raveling 1984; Charalambidou &
Santamaría 2002).
Overall, results suggest that in some
cases nutrient content is an
important factor driving food
selectivity and consumption in
macrophytes ecosystems. However,
higher nutrient content did not
always lead to higher consumption
rates because macrophyte-herbivore
interactions depend on the herbivore
and macrophyte species involved.
There is a lack of correlation in some
studies
between
macrophyte
consumption and the nutrient
content of the diet. Selectivity of L.
variegatus for decayed leaves, and
high presence of different seeds in
guts of A. platyrhynchos despite lower
nutritional content are clear instances
of this. Therefore, other macrophyte
features could also be mediating food
selectivity. Even when nutritional
content explained the main patterns
of consumption and preferences,
some results suggest that other
features must be involved. For
example, the consistent selectivity of
S. salpa and P. lividus for C. nodosa
over P. oceanica, even when epiphytes
were removed. In fact, the quality or
‘palatability’ of the food is also
determined by other macrophyte
features
such
as
secondary
metabolites or toughness or (Fritz &
Simms 1992), energy contents
(Hughes 1980) or varying levels of
structural carbohydrates (cellulose)
affecting food digestibility and
absorption (e.g., Klumpp & Nichols
1983). These food quality traits not
only vary greatly between plant
species, but can also differ
substantially between different parts
of the same plant (flowers and seeds
vs. leaves) and can critically mediate
plant-herbivore interactions (Raupp
& Denno 1983; Poore 1994; Orians
et al. 2002; Taylor et al. 2002).
Despite not measuring them during
this thesis work, we hypothesized
that they could have also interfered
to a greater or lesser extent in food
selectivity, depending on the species.
215
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 7.9. Sarpa salpa feeding on Caulerpa prolifera and Cymodocea nodosa in the mixed meadow
(Chapter 4). Image captured during feeding observations.
7.3.4. Potential influence of
macrophyte chemical and structural
defenses on food selectivity
The limited literature concerning
chemical deterrence in macrophytes
suggests that some species with
relatively slow growth rates, such as
P. oceanica (Hemminga & Duarte
2000), are chemically defended
whereas other ephemeral pioneer
species with a fast growth rate such
as Halophila minor (Zoll.) Hartog
(Vermaat et al. 1995) show no
deterrence (Paul et al. 1990).
According to this theory, fast
growing species such as C. nodosa
would be less chemically defended
and then more intensely grazed by
216
herbivores than slower growing
species like P. oceanica (Cebrián &
Duarte 1998). This seems coherent
with the different consumption rates
and selectivity found in our studies
(Chapter 3 and 4).
It has also been shown that such
chemical features can affect each
herbivore differently (Vergés et al.
2007a). Regarding sea urchins,
Vergés et al. found that P. lividus’s
preference for different P. oceanica
tissues was inversely related to the
concentration
of
phenolic
compounds in these tissues (Vergés
et al. 2007a) and that leaf structural
defenses are the primary factor in
determining the feeding choices of P.
CHAPTER 7: General Discussion
lividus, whereas nutrient content and
chemistry
are
of
secondary
importance (Vergés et al. 2007b). As
with P. oceanica, weaker chemical and
structural defenses in decayed T.
testudium leaves could well explain
why they were the preferred food of
the sea urchin L. variegatus despite
their lower nutritional content.
Therefore, patterns of herbivory
followed by P. lividus in our studies
are coherent with these previous
findings
e.g., preferences
for
epiphytized leaves that are less
chemically and structurally defended,
and for fast-growing seagrass species
(e.g., C. nodosa) which are presumed
to be less chemically defended.
Indeed, the different consumption
and macrophyte selectivity of sea
urchins observed during our studies
may respond to a combination of
differences in structural defenses,
epiphyte communities, nutritional
content and chemical deterrence
among macrophyte species.
Regarding fish grazing, Verges et al.
(2011) reported in contrast that S.
salpa preferred the most nutritious
but chemically defended younger
leaves of P. oceanica, indicating a full
adaptation to consuming this
macrophyte and a greater impact of
this herbivore on the plant. Although
phenolic compounds have been
shown to deter feeding and/or
decrease the assimilation efficiencies
of some herbivorous fish (Van
Alstyne & Paul 1990; Irelan & Horn
1991), many other studies have
shown no correlation between
grazing susceptibility and phenolic
content (Steinberg & Paul 1990;
Steinberg et al. 1991). Similar results
have been found with caulerpenyne, a
secondary metabolite synthesized by
species of the genus Caulerpa which
in some cases seems not to deter fish
grazing (Meyer & Paul 1992). This
indicates that toxicity is not intrinsic
to the compound, but a result of
metabolite-consumer
interactions
(Paul 1992). From this evidence, the
greater herbivory on C. nodosa vs P.
oceanica by S. salpa detected in our
studies must have been greatly
influenced
by
differences
in
nutritional content rather than by
differences in potentially deterrent
chemical compounds. The observed
preference of S. salpa for C. prolifera
vs. seagrasses (without epiphytes)
confirms that this fish could indeed
have evolved tolerance to some
secondary metabolites such as
caulerpenyne as previously reported
217
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Figure 7.10. Cymodocea nodosa, Caulerpa prolifera and Caulerpa cylindracea in the mixed meadow
(Chapter 4).
(Tomas et al. 2011b; Cornell &
Hawkins 2003), therefore chemical
deterrence was unlikely to be a factor
determining patterns of S. salpa
herbivory during the study (Fig.
7.10).
On examining waterfowl herbivory,
we detected a strong selectivity for
flowers and seeds that could not be
explained by nutritional content and
therefore hypothesized that other
unmeasured features could have been
involved
in
waterfowl
food
selectivity. The optimal defense
theory (ODT) predicts that tissues
that contribute most to plant fitness
(flowers, fruits, seeds etc.) or are
218
more prone to predation should be
better defended (McKey 1979;
Rhoades 1979). This contrasted with
the high consumption of flowers
detected in our study. It is however
coincident with Vergés et al. (2007b)
who detected that despite being the
preferred
food
of
urchins,
inflorescences
were
chemically
defended, had higher levels of
phenolics and lower nutrient and
calorie
content
than
leaves,
suggesting that structural defenses
were important factors driving their
preferences. We hypothesized that
high selectivity for flowers and
possibly seeds vs. macrophyte leaves
could be highly mediated by leaf
CHAPTER 7: General Discussion
structural defenses. However, other
features may explain herbivore
selectivity among different species of
flowers or seeds, such as high fat or
high crude protein content or the
ease with which they can be digested
(Hughes 1980; Sedinger & Raveling
1984; Charalambidou & Santamaría,
2002).
General
studies
confirm
the
defensive value of macrophyte
chemical composition and structural
compounds, but also that it cannot
be generalized. The chemical
defenses of given macrophyte species
function as a specific interaction
between such secondary metabolites
and the particular herbivore species
attacking the macrophyte, since
compounds that deter feeding in one
consumer may have no effect on
another.
7.4. Collected summary of the
main driving factors identified
across the different ecosystems
studied
During this thesis we found that
herbivory-macrophyte
interactions
can be influenced by several factors
acting at the same time, although
some of them were clearly identified
as main drivers (see Fig. 7.11; Table
7.4). From our studies we can see
that ecosystems may not respond
linearly to a top-down or a bottomup control, as herbivores and
macrophytes respond differently
between
species
and
their
interactions
are
much
more
complicated than we initially
theorized (see Introduction).
From all our studies we can affirm
that variations in food resource
availability, influenced by different
abiotic conditions (e.g., seasonal light
intensity and temperature), generally
have a strong influence on
macrophyte consumption across
different ecosystems. Examples are
availability of decayed leaves, higher
epiphytic biomass or seasonal
changes in epiphytic communities,
flower abundances; rice plant or seeds
availability. In the first study, the
high availability in the fall of highly
epiphytized decayed leaves of T.
testudinum, the preferred food of L.
variegatus, appeared as the main
driving factor of its grazing in the
seagrass
meadow.
However,
throughout
our
studies,
we
sometimes found that herbivores
showed a preference for a given
resource, that despite being available
219
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
in the habitat studied, was not the
most consumed (e.g., Chapter 4).
Apparently this is contradictory, but
it makes sense if we consider that
sometimes the preferred food cannot
be easily found under natural
conditions and/or that herbivore
feeding behavior can be influenced by
factors other than food preference. P.
lividus reveals a strong preference for
C. nodosa, likely mediated by its
higher nutritional quality, as well as
by epiphyte load and community
structure. However, P. lividus
normally inhabits rocky shelters
prioritizing for survival strategies
(Ebling et al. 1966; Boudouresque &
Verlaque 2001, 2013), which maybe
explains why urchins are scarce in
other meadows where there are no
Figure 7.11. Scheme summarizing different factors involved in macrophyte-herbivore
interactions across different ecosystems. Arrows indicate cascading effect from bottom-up (green)
or top-down (black) control. Main driving factors identified during this thesis are highlighted in
red boxes.
220
CHAPTER 7: General Discussion
Table 7.4. Summary of the main driving factors identified in this thesis and their effect
(Yes/No) on the different macrophytes-herbivore interactions studied.
Factor
Interaction
L. variegatus-
Herbivore Feeding Feeding
Food
Flowers/
Epiphytes
abundances behavior preference availability
Seeds
C:N
No
Yes
Yes
Yes
Yes
-
No
No
Yes
Yes
Yes
Yes
-
No
P. lividus-P. oceanica
Yes
Yes
Yes
Yes
Yes
-
Yes
P. lividus-C. nodosa
Yes
Yes
Yes
Yes
Yes
-
Yes
S. salpa-P. oceanica
Yes
Yes
Yes
Yes
Yes
-
Yes
S. salpa-C. nodosa
Yes
Yes
Yes
Yes
Yes
-
Yes
S. salpa-C. prolifera
Yes
Yes
Yes
Yes
Yes
-
Yes
S. salpa-C. cylindracea
No
Yes
Yes
Yes
No
-
Yes
Waterfowl-R. cirrhosa
No
Yes
-
Yes
No
Yes
No
Waterfowl-P. pectinatus
No
Yes
-
Yes
No
Yes
No
Waterfowl-Rice
No
Yes
-
Yes
No
Yes
No
T. testudinum (decayed leaves)
L. variegatusT. testudinum (green leaves)
rocky
shelters
(Fernandez &
Boudouresque
1997)
and
consequently
have
not
been
suggested as potential herbivores of
C. nodosa. In our case, the
combination of the availability of the
preferred food, C. nodosa, in a rocky
habitat favoring high abundances of
sea-urchins (due to the easy
availability of rocky shelters)
influenced the herbivory pattern.
Similar conclusions can be drawn
from herbivory patterns of S. salpa
across the different studies. Here,
even if all macrophyte species
availability did not differ between
seasons, the fish consumed high
amounts of C. nodosa in summer and
only showed a marked preference for
the alga C. prolifera vs. seagrasses in
the absence of epiphytes. In this case,
the observed herbivory patterns seem
to respond to a combination of
natural seasonal, temporal and spatial
variabilities in fish abundance, and to
distribution and feeding behavior
(Peirano et al. 2001; Jadot et al.
2002, 2006), differences in epiphyte
load and community structure and
different nutritional contents.
In Chapters 5 and 6, we found a clear
disconnection between seasonal
abundances in waterfowl and their
221
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
seasonal impact on the submerged
macrophytes of the lagoon and ricefields. In both studies, greater effects
were detected in summer, despite
waterfowl abundances being the
lowest then in both habitats studied.
The seasonal availability of flowers in
the lagoon explained major effects
detected on R. cirrhosa in summer. In
the rice field, greater impact was also
detected in summer on the newly
planted seedlings, despite important
consumption of rice seeds also being
detected in autumn. In this study,
dietary analyses were crucial to
understanding
that
waterfowl
switched their diet according to
222
seasonal variation in food availability
(Guillemain
&
Fritz
2002;
Guillemain et al. 2002; Dessborn et
al. 2011), which appears to be the
main factor driving waterfowl
herbivory in Mediterranean wetlands.
We can conclude that herbivore
abundances, feeding behavior and
preferences,
changes
in
food
availability, epiphyte biomass, flowers
and seeds and C:N molar ratios were
the main driving factors behind the
different
interactions
studied,
although they may trigger positive or
negative
effects
on
different
macrophytes.
CHAPTER 7: General Discussion
223
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Photo: L.M. Ferrero-Vicente
224
Conclusions / Conclusiones
CONCLUSIONS
1. Herbivory intensity measured by
consumption rates was highly
variable among seasons, macrophytes
and herbivores species, confirming
that herbivory intensity is highly
variable in space and time, depending
on the species involved and the
factors influencing their complex
interactions in each ecosystem.
2.
Lytechinus variegatus showed a
preference for consumption of
decayed leaves rather than green
leaves, related to their higher
epiphyte biomass/cover. Seasonal
availability of decayed leaves in fall
could reduce herbivore impact on
photosynthetically active green leaves
and substantially alter the magnitude
of top-down effects within Thalassia
testudinum meadows in the eastern
Gulf of Mexico.
3.
Paracentrotus lividus showed a
strong preference for Cymodocea
nodosa vs. Posidonia oceanica and was
primarily
responsible
for
the
consumption of both seagrasses in
the Mediterranean mixed meadow.
The higher nutritional quality of C.
nodosa and higher coverage and
number of epiphytes on its leaves
appears to explain the greater
herbivory on this species.
4. The pattern of herbivory by Sarpa
salpa in the different Mediterranean
meadows studied also showed high
spatial and temporal variability.
When densities of S. salpa
temporarily increased, C. nodosa was
the most consumed macrophyte,
probably influenced by the higher
nutritional quality of its leaves and
epiphytes as well as by temporal
differences
in
its
epiphytic
community composition.
225
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
5.
Caulerpa prolifera was the most
preferred food item of S. salpa
(according
to
food
choice
experiments, feeding observations
and gut content analyses), but
preference over seagrasses was
weakened in the presence of
epiphytes, which appear to account
for the herbivory patterns observed in
the mixed meadow.
6.
Herbivory
patterns
of
Paracentrotus lividus and Sarpa salpa
also suggest that final consumption
rates and dietary composition are not
only determined by food preferences,
but also by other factors that
influence herbivore behavior (e.g.,
predation risk and/or home-range
mobility).
7.
Seasonal changes in the
availability of food resources and
flowering
events
rather
than
waterfowl abundance were the main
factors driving waterfowl grazing
impact on submerged macrophytes in
Mediterranean wetlands.
226
8.
During the growing season in
Mediterranean regions, rice-fields act
as a complementary feeding habitat
for waterfowl, which could help
mitigate the loss of natural habitats
in
other
areas
progressively
dominated by agriculture.
9.
The combination of experimental
approaches (tethering and food
choice
experiments),
field
measurements
(macrophyte
availability and feeding observation),
along with dietary analyses using
methods that integrate temporal
variability into resource acquisition,
provide a better knowledge of the
multiple factors involved in complex
macrophyte-herbivore interactions.
Conclusions / Conclusiones
CONCLUSIONES
1.
Las tasas de consumo fueron muy
variables entre épocas, y especies de
macrófitos y herbívoros lo que
confirma que la intensidad de la
herbivoría es muy variable en el
espacio y el tiempo, y dependiente de
las especies involucradas y los
distintos factores que puedan influir
en las complejas interacciones de
cada ecosistema.
2.
Lytechinus variegatus manifestó
un consumo preferente de hojas en
descomposición frente a las hojas
verdes, relacionado con una mayor
biomasa de epífitos en las hojas en
descomposición.
La
alta
disponibilidad
de
hojas
en
decomposición en otoño podría
reducir el impacto sobre las hojas
verdes fotosintéticamente activas y
modificar
sustancialmente
la
magnitud de los efectos Top-down en
las praderas de Thalassia testudinum
del este del Golfo de México.
3.
Paracentrotus lividus mostró una
fuerte preferencia por Cymodocea
nodosa vs. Posidonia oceanica y fue el
principal responsable del consumo de
ambas especies en la pradera mixta
del Mediterráneo. La mayor calidad
nutricional de C. nodosa junto con la
mayor cobertura y número de taxones
de epífitos de sus hojas parece
explicar la mayor herbivoría sobre
esta especie.
4.
El patrón de herbivoría de Sarpa
salpa registrado en las diferentes
praderas mediterráneas estudiadas
mostró una alta variabilidad espacial
y temporal. Cuando las densidades de
S. salpa aumentaron temporalmente,
Cymodocea nodosa fue el macrófito
más consumido, probablemente
influenciado por la mayor calidad
nutricional de sus hojas y epífitos, así
como por las diferencias temporales
en la composición de la comunidad
epífita.
227
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
5.
Caulerpa prolifera fue el alimento
preferido de S. salpa (de acuerdo con
los experimentos de preferencia, las
observaciones del comportamiento
alimenticio en el campo y los
contenidos estomacales), aunque esta
preferencia por el alga frente a las
fanerógamas se disipaba en presencia
de epífitos, lo que parece explicar el
patrón de herbivoría observado en la
pradera mixta.
6.
Los patrones de herbivoría
observados para Paracentrotus lividus
y Sarpa salpa también sugieren que
las tasas de consumo final y la dieta
no sólo están determinadas por las
preferencias
alimenticias
sino
también por otros factores que
influyen en el comportamiento de los
herbívoros (por ejemplo, el riesgo de
depredación y/o rango de movilidad).
7.
Los cambios estacionales en la
disponibilidad de los recursos
alimenticios y los eventos de
floración fueron los principales
factores que impulsaron los efectos
del pastoreo de las aves acuáticas en
los macrófitos sumergidos en los
humedales mediterráneos y no la
abundancia de aves acuáticas.
228
8.
Durante la temporada del cultivo
de
arroz
en
las
regiones
mediterráneas, los arrozales actúan
como un hábitat de alimentación
complementario para las aves
acuáticas, lo que podría ayudar a
mitigar la pérdida de hábitats
naturales
en
otras
áreas
progresivamente dominadas por la
agricultura.
9.
La combinación de métodos
experimentales (e.g., tethering y
experimentos
de
preferencias),
mediciones y observaciones en el
campo (disponibilidad de macrófitos;
observación del comportamiento
alimenticio) y métodos de análisis de
la dieta que integran la variabilidad
temporal en la adquisición de
recursos, proporciona un mejor
conocimiento de los múltiples
factores que intervienen en las
complejas
interacciones
entre
macrófitos y herbívoros.
CONCLUSIONES
229
Factors Driving Herbivores Consumption and Feeding Preferences across Different Macrophytes Ecosystems
Illustration:
Author Unknown. Botanical Illustration, c.1885. Meyers
Konversations – Lexikon. Bibliographisches Institut Leipzig.
230
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