Download Defining drivers of the trophic niche width in reef fish communities

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Remerciements
En cette fin de période de stage, je tiens tout particulièrement à remercier l’équipe de
l’IRD CoReUs du Laboratoire Arago de Banyuls-sur-mer, qui m’a accueillie et encadrée tout
au long de ces six mois : un grand merci à mon encadrant, Michel Kulbicki, pour sa grande
humanité, ses nombreuses corrections du brouillon de ce rapport mais aussi toutes ses
connaissances sur l’univers des poissons de récifs, qui pourtant n’a décidément pas fini de
nous cacher ses secrets. Merci également à René Galzin, de l’EPHE, d’avoir participé à la
mise en place de ce stage et de nous avoir rapporté du bout du monde de nouveaux
échantillons des Marquises, qui je l’espère dissiperont un peu les mystères isotopiques qui
entourent ces îles pour que nous puissions les étudier &.
Merci Valeriano, pour ton bel accent italien mais surtout pour ta présence, ton soutien
et ton aide précieuse pour l’utilisation de R et des tests statistiques. Un grand merci également
à tous ceux qui, de près ou de loin, se sont penchés sur mon travail pour y apporter leur
éclairage et leurs suggestions : David Mouillot et Sébastien Villéger, de l’UMR Ecosym à
Montpellier ainsi que Sarah Nahon, du CRIOBE (EPHE) à Perpignan. Les pistes que vous
nous avez suggérées ne tarderont pas, je l’espère, à être explorées !
Merci à tous ceux qui depuis plusieurs années ont agrémenté la base de données
isotopes de nombreuses nouvelles espèces et individus.
Par ailleurs, je tiens également à exprimer mes remerciements à l’équipe
administrative du Laboratoire Arago : merci à Philippe Lebaron, directeur de la station
marine, de m’avoir accueillie dans un cadre si idyllique. Merci au personnel de tous horizons
que j’ai pu croiser durant mon séjour, et qui ont rendu celui-ci agréable tout du long. Et bien
sûr, merci à tous les étudiants de passage, stagiaires et thésards de la station pour leur
gentillesse et leur bonne humeur &. Vous êtes nombreux et je vous exprime à tous ma
profonde reconnaissance. Merci notamment à Sandrine, le trio d’Amandines, Sara, Mathieu,
Antoine, Karine, Sabrina, Margot, Marlène, Cynthia, Elsa, Marine, Marion, Romain ainsi
qu’à tous les M1 qui ont mis le bazar et l’ambiance dans le foyer des étudiants &. Merci à
Maria-Jesus from Chile, à Sven from Ireland et à Tim from Germany de m’avoir permis
d’améliorer mon anglais et d’en savoir un peu plus sur leurs contrées respectives. Les soirées
plage et les difficiles nuit d’été passées à tenter de dormir malgré les concerts de la place
centrale resteront gravées dans ma mémoire, dans le bon sens du terme !
Merci finalement à mes professeurs de l’Agrocampus Ouest à Rennes, toujours
disposés à nous apporter leur aide et leur soutien, et à ma famille pour m’avoir encouragée
durant ces deux années de master à l’autre bout de la France. J’espère avoir été à la hauteur de
cet investissement.
Merci à tous !
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Résumé étendu
Les enjeux actuels de gestion de la biodiversité nécessitent une compréhension fine et
précise des processus écologiques qui gouvernent le fonctionnement des assemblages
complexes, tels que les communautés de poissons de récifs coralliens. Un suivi rigoureux et
des décisions efficaces s’appuient notamment sur la connaissance de la vulnérabilité de ces
systèmes, afin de prévoir leur capacité de résistance ou résilience suite à des contraintes ou
des perturbations (pêche, pollutions, etc.)
Dans le Pacifique Sud, la diversité des assemblages de poissons de récifs coralliens et
les bénéfices humains qui en découlent sont d’une importance considérable. Ces assemblages
sont également d’une très grande complexité, tant par leur composition taxonomique ou
fonctionnelle que par leur dynamique et leur fonctionnement. Leur étude nécessite donc
plusieurs approches complémentaires dont les caractéristiques de la niche trophique font
partie. Au cours de ce travail, les caractéristiques de la niche trophique d’une espèce donnée
seront résumées par la surface de niche trophique qu’elle occupe. Nous supposons que
certaines espèces adoptent un comportement « spécialiste » (faible diversité de proies, faible
variabilité intra-spécifique du niveau trophique) tandis que d’autres ont un comportement
plutôt opportuniste, donc « généraliste » (forte diversité des proies, possibilité de forte
variabilité de niveau trophique entre deux individus de la même espèce). L’hypothèse
principale de notre travail est que la combinaison de certaines caractéristiques intrinsèques à
l’espèce (classe alimentaire, taille, etc.) avec différents critères environnementaux (biotiques
ou abiotiques) vont favoriser la dominance, en termes de fréquence ou d’abondance,
d’espèces spécialistes, ou au contraire d’espèces généralistes. L'importance relative d'espèces
spécialistes ou généralistes peut influer sur les caractéristiques des communautés, telles que
résilience, résistance, stabilité.
L'étendue de la niche trophique de plusieurs espèces de poissons de récifs a été
mesurée via l’utilisation des ratios des isotopes stables 13C et 12C du carbone (!13C) et 14N et
15
N de l’azote (!15N). Ces isotopes sont issus du prélèvement de muscle blanc dorsal de
plusieurs milliers d’individus regroupant en moyenne 300 espèces, échantillonnées au niveau
de trois régions contrastées du Pacifique Sud : Nouvelle-Calédonie, Archipel des Gambier et
Atoll de Mururoa. Des prélèvements de sources de carbone (algues, plancton, matière
organique en suspension) ont également été effectués sur le site de Mururoa. Pour chaque
espèce, nous disposons également d’informations sur leurs traits de vie issus de la base de
données FishBase et des campagnes d’observations de l’IRD et de l’EPHE. Enfin, des
informations en termes de biomasse (g/m"), fréquence (présence/absence par site) et
abondance (poissons/m") des espèces sont disponibles via une base de données regroupant les
observations de transects UVC (Underwater Visual Census) effectués au niveau des régions
étudiées.
Nous avons défini une "niche isotopique" pour chaque espèce, basée sur la variabilité
intra-spécifique de !13C (représentative de la diversité de sources consommées) couplée à la
variabilité intra-spécifique de !15N (représentative de la diversité des niveaux trophiques
occupés). Cette surface de niche isotopique est calculée à l’aide de la méthodologie « Stable
Isotope Bayesian Ellipses in R » développée par Jackson et al. 2011 (SIBER - Package SIAR
in R Development Core Team 2012). Cette niche isotopique sera donc utilisée comme proxy
de la niche trophique, en tenant compte de certaines contraintes propres à l’utilisation des
isotopes stables afin d’éviter au maximum l’introduction de facteurs confondants.
Une première série d'analyses a examiné les variations de !13C et !15N en fonction de
la taille des poissons, de leur espèce, du groupe fonctionnel, du site ou de la région. Il y est
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notamment montré un effet de la taille qui se caractérise le plus souvent par une augmentation
du ratio isotopique de l’azote avec la taille individuelle. Toutefois, des tendances opposées
sont également observées de façon moins fréquente. De plus, ces relations entre taille et
signature isotopique
L’estimation de la niche isotopique des espèces communes à au moins deux des trois
régions étudiées suggère une forte variabilité de celle-ci, que l’on peut imputer à (1) la
variabilité inter-régionale de la signature isotopique des sources consommées, (2) à une
plasticité réelle du comportement alimentaire d’une même espèce d’une région à une autre. Le
rang des espèces le long du gradient spécialistes-généralistes reste cependant stable d’une
région à l’autre ; ce qui suggère un maintien du comportement alimentaire moyen des espèces
les unes par rapport aux autres entre deux régions contrastées.
Mis à part l'alimentation, les effets sur la niche isotopique des autres traits de vie tels
que la grégarité, le comportement nycthéméral, la mobilité ou encore le niveau dans la
colonne d’eau où évolue une espèce sont très variables et ont difficilement pu être reliés à des
facteurs environnementaux. De même, aucune relation significative n’a été mise en évidence
entre la surface de la niche isotopique et :
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La répartition géographique (indicatrice du degré d’endémisme) d’une espèce
L’indice de Singularité Fonctionnelle qui permet de classer les espèces en fonction
de l'originalité de leurs traits de vie
Comme précisé précédemment, seul le régime alimentaire indique une différenciation
en termes de largeur de niche trophique occupée, les espèces herbivores et omnivores
présentant en moyenne une niche trophique plus étendue, donc une alimentation relativement
plus diversifiée que les piscivores stricts, carnivores, planctonophages ou coralivores.
Toutefois, ce pattern n'est pas stable d’une région à l’autre, la surface de la niche isotopique
n'étant que faiblement liée aux traits de vie des espèces. Connaître le mode de vie précis d’une
espèce ne suffit donc pas à prévoir la niche trophique qu’elle occupera par rapport à une autre
espèce ayant le même comportement alimentaire. Seul ce dernier facteur sera donc retenu
pour analyser l'influence des facteurs environnementaux sur la niche isotopique.
En revanche, une relation significativement positive a été mise en évidence entre la
surface moyenne de niche isotopique et la fréquence, biomasse ou abondance des espèces
herbivores. Le phénomène inverse été observé chez quatre espèces coralivores. Cela suggère
que les interactions spécifiques au sein d’un groupe fonctionnel influencent le potentiel
d’occupation de la niche trophique, et qu’elles entraînent des réponses différentes en termes
de diversification du régime alimentaire selon le groupe fonctionnel considéré.
La seconde étape de ce travail se concentre sur l’atoll de Mururoa, où
l’échantillonnage est plus exhaustif, afin d’analyser l’influence d’un gradient
environnemental. Dans les écosystèmes récifaux, le degré d’exposition à l’océan est connu
pour être un facteur impactant la composition des assemblages ichtyologiques. L’atoll de
Mururoa a été subdivisé en quatre zones de superficies comparables selon un gradient
d'exposition à l’influence océanique. Les niches isotopiques des espèces disponibles sur au
moins deux zones de l’atoll ont alors été estimées à l’échelle de la zone. Les espèces étudiées
présentent une surface de niche isotopique très souvent significativement différente d’une
zone à l’autre mais sans qu’un pattern stable ait pu être mis en évidence, même en tenant
compte des traits de vie en particulier le régime alimentaire. Dans la zone lagonaire (zone
centrale de l’atoll, caractérisée par l’absence de récif mais la présence de pinacles) la surface
isotopique de la niche est cependant plus restreinte pour la majeure partie des espèces
étudiées, y suggérant une plus grande homogénéité des ressources. Cette seconde étape
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confirme la capacité d’une forte versatilité alimentaire de la plupart des espèces, le
comportement alimentaire étant contraint par les conditions locales.
La dernière étape a consisté à analyser la structuration de la niche isotopique globale,
c.à.d. celle occupée par l’ensemble des individus regroupés par groupes fonctionnels, et ce en
tenant compte de la zonation de l’atoll de Mururoa. Une méthodologie récemment dérivée des
métriques de Layman et al. 2007 a permis d’estimer la surface de niche isotopique occupée
par les individus d'un régime alimentaire tout en tenant compte de l’abondance ou de la
biomasse des différentes espèces qui le composent. Ceci a permis de mettre en évidence
certaines spécificités de groupes fonctionnels, notamment les herbivores, ces derniers
montrant une combinaison de plasticité au niveau de l'individu et de l'espèce qui leur confère
probablement une grande adaptabilité.
Ce travail, qui porte sur la plus importante base de données sur les isotopes stables de C et N
chez les poissons, suggère donc:
•
•
•
•
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une forte adaptabilité alimentaire, dans les limites de chaque comportement
trophique, qui devrait favoriser la résistance et la stabilité des communautés de
poissons de récif face aux perturbations
confirmation que les communautés très diversifiés sont probablement plus
résilientes
les stratégies opposées des herbivores et coralivores en termes de relation entre
largeur de niche trophique et abondance sont l'indication que les effets de ces
poissons sur l'équilibre algue-corail est sans doute bien plus complexe que ce qui
est actuellement décrit
l'absence de relation entre la largeur de la niche trophique et l'endémisme ou la
singularité fonctionnelle est un indice que le rôle écologique, en particulier en
termes de résistance et résilience, de telles espèces est probablement non lié à leurs
caractéristiques trophiques
l'absence de relation entre la largeur de la niche trophique et la plupart des traits de
vie est signe d'un grande flexibilité dans les relations trophiques au sein des
groupes fonctionnels, ce qui pourrait favoriser résilience et stabilité.
Les apports et les limites de l’utilisation des isotopes stables pour l’étude de la structure
fonctionnelle des communautés de poissons seront discutées. Enfin, quelques perspectives
seront alors présentées.
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Table of contents
Introduction & Review ................................................................................... ^!
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A) Reef Fish Communities in the South Pacific ........................................................
I. The ecological and trophic niche in ecology : specialists VS generalists ..........
II. Trophic specialization in reef fishes : a few examples from the literature .......
III. Biodiversity and community structure of reef fish across scales ....................
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B) On the use of stable isotopes to explore the trophic niche in ecology .................. C!
I. Principle & fields of application ........................................................................ C!
II. “A niche for isotopic ecology” (Newsome et al. 2007) .................................... C!
III. Limitations and constrains of the use of C and N stable isotopes in ecology .. B!
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In a Nutshell (1) .......................................................................................................................b!
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Materials and Methods ................................................................................... T!
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A) Three complementary databases ............................................................................ T!
I. Stable carbon and nitrogen isotopes sampling ................................................... T!
II. Field observations data : Underwater Visual Censuses (UVC) ........................ S!
III. Specific information : Biological and Behavioral Traits .............................. ^M!
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B) The population and community niche metrics ....................................................... ^^!
I. The SIBER ellipses methodology ....................................................................... ^^!
II. The Layman metrics modified by Cucherousset & Villéger (submitted) ......... ^O!
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In a Nutshell (2) .......................................................................................................................^L!
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Results ............................................................................................................... ^N!
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Part 1 : Preliminary analysis ....................................................................................... ^N!
(1) The differences between sources in a given Region ........................................... ^N!
(2) Effect of individual size on the isotopic signature ….......................................... ^N!
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Part 2 : Effect of inherent characteristics on the isotopic niche width ........................ ^B!
(1) The influence of biological and behavioral traits .............................................. ^B!
(2) The influence of the Geographical Range and Functional Distinctiveness ....... ^B!
(3) The influence of the frequency, biomass or density ............................................ ^T!
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Part 3 : Effect of environmental gradient in the isotopic niche width ........................ OM!
(1) The evolution of the isotopic niche width with habitat variations :
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feeding plasticity ?.................................................................................................... OM!
(2) The community scale : insights from the methodology of modified
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Layman’s metric .......................................................................................................... OM!
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Discussion ......................................................................................................... OO!
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Specialization in reef fish communities ...................................................................... OO!
Effect of individual size on isotopic signatures ............................................... OO!
Specialist to generalist gradient : is a typology at hand ? ............................... ON!
Effect of the biology of species on isotopic niche width ................................ ON!
Effect of endemism and functional distinctiveness on isotopic niche width .. OC!
Effect of frequency, biomass and density on isotopic niche width ................. OB!
Effect of an oceanic gradient ........................................................................... Ob!
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Stability versus Versatility .......................................................................................... OT!
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Comments & Critics of the methods : which perspectives ? ....................................... OS!
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Conclusion ........................................................................................................ LM!
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Literature Cited .................................................................................................. LO!
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Table of appendices
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!EE0-5.S*=*J*Geographical repartition of coral reef .................................................................... 8%
%
!EE0-5.S*==*J Major application of stable isotopes in ecology.................................................... * 9%
%
!EE0-5.S*===*J*Sampling design for isotopic analysis ................................................................. * :%
%
!EE0-5.S*=Q*J*Underwater Visual Censuses in Mururoa ............................................................ * 88%
%
!EE0-5.S*Q*J*R routine for the estimation of Functional Distinctiveness Index......................... * 89%
%
!EE0-5.S*Q=*J*Origin of the Layman’s derived metrics............................................................... 8:%
%
!EE0-5.S*Q==*J*Isotopic signatures of diet sources in New-Caledonia ........................................ *8;%
%
!EE0-5.S*Q===*J*Details about effect of size on isotopic signature .............................................. * 8<%
15
%
!EE0-5.S*=T*J*Example of the relationship between size and ! N for six species ..................... *8=%
%
!EE0-5.S*T*J*Main information about SIBER ellipses by specie within a region ......................* >?%
%
!EE0-5.S*T=*J*Table of significancy of the biological factors effect on isotopic niche width .... >9%
%
!EE0-5.S*T==*J*Residuals of the model (1) and (2) performed on Appendix XI .......................... >:%
%
!EE0-5.S*T===*J*Plots of isotopic signatures in zones of Mururoa ............................................... >@%
2
!EE0-5.S*T=Q*J*Table of density (individuals/m ) for each diet (regardless to specie)
%
within zones of Mururoa ........................................................................................................... *>;%
%
!EE0-5.S*TQ*J*Layman’s derived metrics graphical outputs ...................................................... *><%
%
!EE0-5.S*TQ=*J*Divergences between FishBase and isotopic data .............................................. 9=%
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List of figures
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Introduction &
Review
!
INTRODUCTION & REVIEW I
!
Explaining the origin and the future of biodiversity patterns around the world is at
present a major challenge. Many systems have been facing diversity loss during the last
decades. Global warming and anthropogenic disturbances are causing ecological
disequilibrium which consequences have to be evaluated in order to improve resource
management. Even though they occupy a very small proportion of the ocean surface (less than
0,1% of the total area of world oceans) and are restricted to subtropical regions (See
Appendix I), coral reef systems shelter nearly 25% of the total fish biodiversity. Thus they are
one of the most complex and diversified systems in the world. Because of the distribution of
these species along strong environmental gradients (Parravicini et al. 2013) these fish
assemblages show a great potential for ecological studies and understanding how
communities are structured. The major traits of the ecology of reef fish communities will be
briefly reviewed along with some of the major hypotheses on the processes underlying the
organization of these communities in order to set the outline of this report.
A) Reef Fish Communities in the South Pacific
I. The ecological and trophic niche in ecology : specialists VS generalists
The study of niche width represents a key point in ecology, however it covers different
concepts and there is no general consensus. The first ideas about ecological niche were
developed mainly by Grinnell (1917), Elton (1927) and Gause (1936) (See Chase & Leibold,
2003 for a review). For a better understanding of our framework, the major different
approaches used to define the niche of a given species are briefly described hereafter.
Hutchinson (1957) defined the ecological niche as an abstract hypervolume, i.e a ndimensional set of points in a space which axes represent varied environmental and/or biotic
variables. The environmental variables are subdivided in two categories that are the bionomic
axes (which represents the pool of resources used by a species) and the scenopoetic axes (the
set of habitat, physical and biogeochemical variables where the species could live)
(Hutchinson, 1978). Afterwards more precise distinctions were proposed :
!!!!"#$!%&'(($))'*(!+,!-#$!.)-/('*(!('0#$!!"
An ecological niche may be defined in
terms of the subset of the n-dimensional
space taken by a species from its
environment, but also in terms of the impact
of that species. Grinnell (1917) describes
the ecological niche as « the response of
species to a given set of variables » while
Elton (1927) is more specifically focused
on « the impact of species in the
environment », for instance in terms of
foraging strategies. Thus the trophic niche
is usually linked to the Eltonian approach of
the ecological niche.
!!!!"#$!12(3*4$(-*)!+,!-#$!&$*)'5$3!('0#$!!"
The whole range of environmental
constrains (both biotic and abiotic) rarely
enable species to be as efficient as they
theoretically could. Indeed, the biotic
relationships with other species and the
environmental variability of both available
resources and conditions may « shape » the
volume of the occupied niche of a species
(which represents the realized niche), even
though the inherent characteristics of
individuals (in particular phenotypic
features) would provide them a larger range
of
possibilities
(designed
as
the
fundamental niche).
Afterwards more precise distinctions need to be given accordingly to the objectives of
the study (Devictor et al. 2010). As an example, the study of foraging behavior is related to
eltonian realized niche. However the width of the ecological niche is concretely difficult to
measure since this notion is strongly dependent upon the considered spatial, temporal or
taxonomic scale. Devictor et al. (2010) defined four major types of factors that influence the
width of the ecological niche :
!
!"
•
•
•
•
Spatial and temporal variability of environmental conditions (for instance resource
availability or temperature range that optimize metabolic pathways, etc.)
Species interactions (predation, competition, increasing usually with species richness)
Local genetic speciation (that provides different patterns of performance in a given
habitat) and phenotypic plasticity of individuals (i.e. their potential of adaptability)
The considered level of organization (individual < population < community)
Our perception of a niche is also dependant on the methodology and sampling design.
Currently, the most common theory states that, due to trade-off constrains, a species
should either be adaptable (i.e with a large fundamental niche ; so it can fill in a diversified
set of narrower realized niches with respect to the environmental instability) but with low
performance, or specialized (i.e with a small range of usable environmental variables, for
instance a very specific kind of foraging resource) but with a high performance within the
range it belongs to. The degree of specialization of species in a given community or
ecosystem will shape its functioning (Devictor et al. 2008). Indeed the respective ecological
functions of these species will be different and provide different features against perturbations
or constrains (Burkepile & Hay, 2008). A system with a high degree of heterogeneity (in
terms of topography or substrat for instance, but also possibly a high species richness) could
either shelter generalist species that highly overlap or specialist species with a high degree of
resource partitioning, which in each case finally confers stability to the whole system as it
ends up into a certain dynamical equilibrium linked to interspecific competition. One may
propose the same and opposite hypotheses in an homogeneous habitat (for example in a very
small and/or isolated island when the amount of reef corals diversity is low because of larvae
dispersion constrains) with either specialized species which may be very competitive and
exclude generalists, or oppositely generalists which may be less competitive but more adapted
to changes in this homogeneous habitat.
However specialized species are more expected to be found in very temporally stable
and predictable environment, where the needed subset of environmental resource will be
available at all times ; while a changing and constrained habitat would be more adapted for
generalist species which could face the changes and therefore be more resilient in case of
major perturbation (Munday, 2004). Resilience describes the ability for a system to keep its
structure and functioning around a particular set of mutually reinforcing processes, known as
a regime (Notion introduced for the first time by Holling, 1973).
Thus, a stable ecosystem where most of species are specialists is likely to be less
resilient than a system dominated by generalists ; as species will be more sensitive to habitat
or resources variation (Pratchett et al. 2012). As a consequence perturbations in stable
environments are likely to generate larger risks than in less stable environments supporting a
similar but more generalized diversity. In the case of very diversified and threatened systems
like coral reef communities, the study of specialization is thus essential to evaluate the
vulnerability and the risks these communities are facing. Defining the degree of specialization
of these fish could enable better management measures to protect them and more accurate
predictions regarding biodiversity erosion.
!!"
INTRODUCTION & REVIEW I
!
II. Trophic specialization in reef fishes : a few examples from the literature
Several hypotheses have been proposed to explain the high diversity of coral reef
fishes (see review of Hixon, 2011). One hypothesis states that high intra- and interspecific
competition prevents species from becoming so dominant as to exclude others species.
Diversity is maintained thanks to resource or spatial partitioning. Therefore habitat or trophic
specializations are expected to be high when competition is important, i.e. when species
richness and abundance are high relatively to the available resources and shelters. Literature
confirms some of these statements, for instance on the Holocentridae family in the Atlantic
(Gladfelter & Johnson, 1983), Pomacentridae family in the Great Barrier Reef (Berumen &
Pratchett, 2008) but a second hypothesis could be proposed. A generalist behavior could also
enable a high diversity as the competitive exclusion would be less important. For instance in
the Toliara lagoon (Madagascar), Dascyllus aruanus is a highly habitat-specialized species
but within this population there is a partitioning of the resource both spatially and with fish
size. As a result the population of D. aruanus can be considered as foraging generalist even if
at the individual level these fish are specialists (Frédérich et al. 2010). Similarly, a study
conducted on two others damselfish species showed no distinction between their trophic niche
but only between their spatial niche (Dromard et al. 2013). Slatyer et al. (2013) in a review
suggested that endemism should be associated with a small niche, as the distribution of the
species is often restricted to quite specific habitats while widespread species use more
common resources (Hanski, 1993). However at present the life-history of endemic species has
not been much studied in reef fish communities.
III. Biodiversity and community structure of reef fish across scales
The dynamics of biodiversity and the community structure in reef fish strongly
depends on the spatial, temporal or even taxonomic scale of the study. Most of the existing
literature focuses on local drivers of biodiversity but regional drivers also need to be
accounted for (Tuya et al. 2011). According to Pandolfi (2002) for instance, determining the
temporal patterns of communities dynamic is essential to determine the time range in which
anthropic disturbance could modify the functioning of these systems. Besides both biotic and
abiotic drivers could shape the community structure in a reef system and they are expected to
be strongly linked, even though the biotic interaction factors may apply their influence mainly
at low geographical scales. Finally, the pool of species that we find at a given location
depends on the constraints applied by that we could call “ecological filters”. Biogeographical
factors, as reef area and reef isolation are important drivers of reef fish communities structure
(Mellin et al. 2010). For example, the islands theory developed by MacArthur and Wilson
according to which a large and not isolated area should shelter a higher biodiversity has been
showed to apply to coral reef (Galzin et al. 1994 ; McNeil et al., 2009 ; Huntington & Lirman,
2012). Species richness, density and biomass may change according to regional drivers like
sea surface temperature or nutrient input (Mellin et al. 2010b) whereas locally, drivers like
habitat complexity (i.e the topography of the reef), coral or algal covering, or even wave
exposure, distance to the ocean, reef type (Grimaud & Kulbicki, 1998 ; Fulton et al. 2005) as
well as biological features like predation (Hixon & Beets, 1993), competition, recruitement
and immigration influence the specific composition of communities (Cornell & Karlson, 2000
; Bellwood & Hughes, 2001 ; Bellwood et al. 2002).
Stability, resilience, resistance are a matter of scale, reef fish assemblages being more
stable at large scale than they are at a local scale (Pandolfi, 2002 ; Tuya et al. 2011 ;
Huntington & Lirman, 2012). A comparison of three major reef systems (Great Barrier Reef
of Australia, French Polynesia and Caribbean) shows that the structures of Labridae fish
communities are similar despite of very different geological and geographical histories, and a
!
!"
very low species overlap between two systems like Caribbean and Polynesia (Bellwood et al.
2002). This emphasizes the need to take functional groups in account when studying
community dynamics across different locations (Sale & Guy, 1992), so that species turnover
does not obscure our understanding of the influence of regional or local drivers.
A functional group is usually defined as a group of individuals or species that share a
comparable role or impact on their ecosystem, due to their intrinsic phenotypic or behavioral
features (i.e grazing, filtering, chasing, etc). The functional role is often linked to trophic
requirements, for example within reef fish herbivores there are several feeding behaviors
(grazing, scrapping, browsing and so on) which all target the same resource but end up in
different actions on the resource dynamics. Some functions may be central to the ecological
stability of a system and therefore their disappearance could induce major changes, the
extreme case being the transition between a stable and productive ecosystem toward a
degraded one. Yet all functional groups are not represented equally. In particular many rare
functions are filled by rare species (rare because of their endemism and/or their low
abundance) (Mouillot et al., 2013). Therefore some unique functions are characterized by a
low redundancy1 which increases their vulnerability and their disappearance may reduce the
potential of resilience of the system (Hoey & Bellwood, 2009 ; Mouillot et al. 2013).
Moreover, increasing species richness usually favors the redundancy of functional groups that
are already well represented (Halpern & Floeter, 2008 ; Guillemot et al. 2011). Even though
the same functional groups are found on most reefs, redundancy of each functional groups is
often unstable from one geographical area to another, attributing to each system unique
characteristics (Bellwood et al. 2004). Thus, in order to preserve these systems, it is essential
to understand their vulnerability. This notion of vulnerability seems quite elusive as it also
depends on actual existing threats and other ecological or biological characteristics, but it has
often been linked to the functional redundancy, considering that a system represented by
functional groups with low redundancy will be more vulnerable than a system in which
redundancy is high (McCann, 2000 ; Bellwood et al. 2004).
In conclusion, it is important to better understand functions in a community as they
insure the stability and the dynamic of the whole community. However, species redundancy
in functional groups may not be the only parameter that matters. As already mentioned, the
degree of specialization of each species in a given functional group within a habitat is also
important, as a given functional group is likely to be more vulnerable if only composed by
specialist species. However this approach has not been much used in ecological studies of reef
fish communities. Our knowledge on trophic specialization remains very patchy, with most
studies focusing on a single species or genus within narrow spatial limits. Patterns of reef fish
specialization have never been measured at a large scale or for a large pool of species within
different functional groups, and generally researchers focus on spatial specialization more
than on trophic specialization. The latter could be a particularly informative approach.
Foraging behavior in reef fish is mainly measured by two methods:
- Study of behavior in the field with direct observations
- Study of stomach contents
However, these methods rapidly reach their limitations when large scale studies are
needed. For instance, stomach content does only represent a snapshot of the diet of a given
individual at a given time, but does not provide a temporal integrated measurement. Similarly
stomach contents and field observations cannot provide information about what is really
assimilated by fishes. Thus these methods enable to observe some of the impact fish may have
on their environment, but they yield part of the information on the true width of their trophic
niche. This is where stable isotopes may constitute as a useful tool.
1
!!"
: Redundancy : In this case, the number of species within a functional group
INTRODUCTION & REVIEW I
!
B) On the use of stable isotopes to explore the trophic niche in ecology
I. Principle & fields of application
By their long-term stability in natural environment and the development of recent
effective technologic tools such as mass spectrometry, specific non-radiogenic isotopes have
become well used tools in biology (Post, 2002 ; Wolf et al. 2009). Stable isotopes are used in
many fields. In marine biology and ecology, they may be classified in three main approaches
presented in Appendix II. In ecology, the most common used isotopes are the ratios 12C/13C
and 14N/15N. These two elements are predominant in metabolic pathways, biogeochemistry
and biomass flows across trophic webs, but even though these ratios are relatively similar
among biological compartments, slight differences are detectable as a result of fractionation
processes (the proportion of heavy isotope conserved from source to product) and source
characteristics (Fry, 2006). This phenomenon is called “enrichment” and leads to each species
or individual to have a quite unique isotopic signature with enables to reconstruct some of its
history (Fry, 2006). Nitrogen isotope ratio !15N enables the assessment of a species trophic
level thanks to enrichment processes (DeNiro & Epstein, 1981). Indeed each trophic level
shows an increase of the !15N value which is a result of bioaccumulation of heavy isotopes
15
N from diet whereas 14N are easier to eliminate than 15N by metabolism and excretion. This
tracer enables to determine the position of a species relatively to the others in a trophic web
and is thus described as a predictor of the bionomic axis of a trophic niche (Hutchinson,
1978). Oppositely, carbon isotope ratios !13C are rather stable through trophic levels and
therefore enable to trace back the origin of the carbon sources in a given environment
(DeNiro & Epstein, 1978). It is thus described as a predictor of the range of scenopoetic axes
of a trophic niche (Hutchinson, 1978).
II. “A niche for isotopic ecology” (Newsome et al. 2007)
Newsome et al. (2007) provided a well documented review of the perspectives opened
by stable isotopes analysis in the field of trophic niche ecology. In order to rank species or
individuals within a species along a specialist / generalist gradient, they stated that plotting
!13C versus !15N values is somewhat analog to looking at two axes within the n-dimensional
niche described by Hutchinson. Indeed the isotopic signature is both influenced by what the
individuals consume and assimilate (bionomic axis given by !13C and !15N) and the habitat
and range of resources they use (scenopoetic axis given by !13C). Thus a different diet
behavior between two individuals of a given species will lead to a different isotopic signature
for each of them, and finally a higher variability in the !13C/!15N biplot than two individuals
with a similar diet. The !-space given by the combination of !13C and !15N in a biplot thus
constitutes an isotopic niche that could represent an accurate description of the trophic niche,
as both resources range and inter-individual variability are observable at the same time. In
addition, one, two or more species may be represented together in the same !-space in order
to compare their degree of specialization. This approach, first described by Bearhop et al.
(2004) and developed by Layman et al. (2007) and Jackson et al. (2011) with the use of
different statistical metrics, has often been applied to the study of a set of two or three species
in a narrow habitat but has never been tested at large scale on a wide pool of species.
Our objective is therefore to take advantage of these tools to bring some new insights
in the field of specialist to generalist species gradients within reef fish communities and test
for the potential influence of several factors (See “In a nutshell (1)” below). However, there
are a number of constrains to this approach which we will need to take into account.
!
!"
III. Limitations and constrains of the use of C and N stable isotopes in ecology
The estimation of isotopic niche requires to take into account many biological and technical
constrains (Table I). All of these contrains will be taken in account when possible.
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• Choose the most representative tissue : the white muscle
integrates patterns of feeding behavior on several weeks.
• Standardization could be done to make comparisons easier.
• Temporal contrasts have been avoided (i.e sampling is done in
a narrow time scale, if possible within a month).
• Spatial variability of isotopic baseline need to be assessed.
However feeding sources were sampled for one region only,
which limits inter regional comparisons.
• Sampling sources was limited because of the high fish species
diversity and the need for a large spectrum of sources. This is
why comparing locations with different isotope ratios for the
sources has to be conducted with much caution.
• Population VS Individual specialization is not easy to
distinguish but should not strongly interfere with conclusions as
the global pattern should be conserved. (See also “In a
nutshell (2)”)
• Coefficient of fractionation may differ according to species, life
stage, sex, sea surface temperature, etc. However it is very
difficult to assess. We chose to use the mean value currently
accepted in literature : 3,4 ‰ for nitrogen and 0,1 ‰ for
carbon.
• Same conditions as previous paragraph for assimilation rate,
but this aspect could not be taken into account here. However
this should not alter the generalist-to-specialist pattern if it does
exist.
Choice and justification
• Samples were analyzed in different laboratories. A test on 100
samples indicated no significant differences amongst labs.
• Lipid content may influence isotopic signatures, but
preliminary tests found no significant difference. Consequently
no delipidation was performed.
* Related to either the issue, the chosen solutions or both.
!!"
Post et al. 2002
Boecklen et al. 2011
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• Visualize intraspecific variability : we will account for the
individual size by sampling the wider size range as possible.
Sex effect could not be tested as it could not be determined in
all cases.
References *
Bearhop et al. 2004
Boecklen et al. 2011
Matthews & Mazumder, 2004
Newsome et al. 2007
Technical cautions
Choice and justification
Boecklen et al. 2011
Newsome et al. 2007
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Biological cautions
References *
INTRODUCTION & REVIEW I
!
6:!;!:<=2>?@@!ABC!
!
This review brings up several questions :
! Are the usual ecological statements about specialists / generalists dominance true
for reef fish communities? Do some environmental factors favor specialization ?
! Do some functional groups contains more specialized species than others ? Is the
trophic niche width conditioned by certain biological traits (Diet, Schooling, etc) ?
! Is the proportion of trophic specialist (or generalist) species an important
component of the structure of a given community ? What could we conclude in
terms of redundancy and potential resilience ?
! How accurate is stable isotope analysis to study community functioning and
stability at a large geographic and taxonomic scale ?
Our hypothesis :
! A specialized behavior is an inherent characteristic and might therefore be
maintained wherever the species lives.
! Species may however present a potential feeding plasticity across environmental
gradients.
"#!$%&&'$#!()*)+,#!(#-.##+!$-)(/*/-0!12!3#4$)-/*/-05!
! A singular combination of biological and behavioral traits will favor a specialized
feeding behavior: the more original a species happens to be, the more specialized it
is. In the same way, a high degree of endemism should favor trophic specialization.
! When competition is expected to be high, most of species will be either specialist
due to resource-partitioning or generalist with a high resource overlap.
! In a given functional group, relationship between species may interact with
specialization / generalism.
67&*/,)-/'+$!8'4!&'-#+-/)*!'8!4#$/*/#+,#!9!
Our framework :
The use of an important and large set of data will be used. This will enable to combine
the available knowledge about biology of hundreds of reef fish species and their stable
isotopes signatures, in a large geographic area of the South Pacific. Data on frequency,
biomass and density will also be available for most of these species.
! First we will attempt to classify species from specialist to generalist and see if this
classification remains stable across regions.
! Then for each region we will test for the influence of some biological traits and
dominance for each species on the isotopic niche width. A sheltered-to-exposed
gradient effect on niche width will also be tested in an atoll of French Polynesia.
! Finally a change in organizational scale (from species to functional group, given by
diet of species) will be performed in order to analyse the relationship amongst
species within a community.
! At last, the use of isotopes to assess for the specialist - generalist structure among
reef fishes communities will be discussed and some perspectives will be presented.
!
!"
Material &
Methods
!
MATERIALS AND METHODS II
!
A) Three complementary databases
This work uses three databases which are the result of a large sampling effort in several
contrasted regions of the South Pacific Ocean :
(i) The isotopes database contains the carbon and nitrogen stable isotopic ratios of an
important number of species sampled mainly in South of New-Caledonia and French
Polynesia (Gambier Islands and the atoll of Mururoa).
(ii) The field observations on reef fish (Underwater Visual Censuses) are compiled in
another database, which report biomass, frequency and abundance estimates.
(iii) The third database contains the observed characteristics of the sampled individuals,
and the life-history traits of each species derived from Kulbicki et al. (2011).
I. Stable carbon and nitrogen isotopes sampling
This study took place in three contrasted regions covering a large geographical area of
the South of Pacific Ocean, from New-Caledonia to French Polynesia as presented in Fig.1.
These three regions are distinct mainly by their geomorphology and the different types of reef
they present.
o South of New-Caledonia (NC) is a continental region that supports a large panel of
contrasted coastal environments. There is an important gradient in terrestrial influence
from the coast to the barrier reef. NC also shows the most diversified fish assemblages,
due to its history and its proximity to the Coral Triangle center of biodiversity (Kulbicki
et al., 2011 ; Parravicini et al. 2013).
o Gambier Islands (GB) is an archipelago with four major islands surrounded by a barrier
reef, at the extreme south-east of French Polynesia.
o Mururoa (MU) is an atoll of the Tuamotu archipelago, 400 km west from the Gambier
Islands. As in any atoll terrestrial influence is very limited. The reef belt surrounding the
lagoon supports a thin emerged sandy land and several motus. Along the East of the atoll,
the exposition to ocean is higher due to the presence of a large pass.
The atoll of Mururoa, where half of the sampling was performed, could be subdivided
into four different areas along an environmental gradients as described in Figure 1 (c). The
number of species and individuals captured for each region and area are summarized in Table
2. The detailed list of the families, genera and species sampled, as well as their diet are
presented in Appendix III. All the samples used for our analyses were collected at the same
season for all areas in a region, in order to reduce the temporal variability of isotopic
signature and specific composition of the assemblages. However, the period of sampling
could vary between two regions (Table 2), besides the isotopic baseline (main sources
isotopic signatures) is presently unavailable for two of the three regions, therefore no direct
comparison between regions will be presented. Fishes were stunned with rotenone then
picked up with scoop nets. Although it contains carbon, any potential effect of rotenone on
the carbon isotope signatures of fish can be considered as negligible since the residues are
rapidly cleared from fish organism (Gingerich et al. 1986). The fish were fork-length
measured (TL in mm) and placed on ice immediately after their removal from water. For each
individual, a fillet of white muscle tissue taken from the dorsal region was dissected for
isotopic analyses.
Isotopic signatures were accessed after the samples were freeze-dried, and stable
isotopes values were converted in ! notation as !13C or !15N = [(Rsample/Rstandard) - 1] x 1000,
where R is 13C/12C or 15N/14N. The standard reference was Pee Dee Belemnite carbonate for
CO2 and atmospheric nitrogen for N2.
"
"
!
!"
"
"
"
"
NEW-CALEDONIA
(Island)
"
"
GAMBIER
(Archipelago)
"
"
"
"
"
"
"
"
MURUROA
"
(Atoll)
"
"
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II. Field observations data : Underwater Visual Censuses (UVC)
The species composition of the sampled sites are available in an ecological database
containing the frequency, biomass and abundance for most of the sampled species of MU and
GB (such data are not directly available for NC, as UVC data collected there were not
simultaneous to isotope sampling). These data were collected by underwater transects
performed by IRD and EPHE. The sampling methods were different, IRD data was collected
with distance sampling (Labrosse et al. 2002), EPHE data were collected using fixed width
transects (Planes et al. 2005). UVC samples are non destructive, repeatable and represent a
large proportion of the reef fish communities (Kulbicki, 1990).
For the region of Mururoa, the counting strategy is stratified according to the same
subdivision as presented in Figure 1. The corresponding details (mainly the location of
transects and the number of species collected) are presented in Appendix IV.
!" !
MATERIALS AND METHODS II
!
III. Specific informations : Biological and Behavioral Traits
Biological traits are useful to classify species into functional groups (or functional
entities). Traits are also important to understand some of the differences in isotopic ratios
observed amongst and even within species. The traits used in this work were extracted from
Kulbicki et al. (2011), with a large part of the information obtained from FishBase1. The traits
retained for our analyses are indicated in Table 3. A species could not be listed in more than
one category by trait. On the basis of all these traits, an index of singularity was estimated for
each species. This index is derived from the Evolutional Distinctiveness (ED) described by
Isaac et al. (2007) and enables to classify species by a degree of « Functional singularity ».
From now this index will be designed as Functional Distinctiveness (FD).
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The Functional Distinctiveness value for a species will be higher when a species
presents a singular combination of traits relatively to the other species. FD value have been
estimated for each species within each region separately. As FDs are dependent upon the
!
1
: www.fishbase.org
!"#
species pool considered, we retained all the species available from checklists within each
region (Kulbicki et al. 2011). The R script used to derive Functional Distinctiveness from
Evolutional Distinctiveness is presented in Appendix V.
B) The population and community niche metrics
I. The SIBER ellipses methodology
As initially defined by Hutchinson (1957) and more recently by Newsome et al.
(2007), ecological niche could be approximated as a surface or volume between two or more
axes representing environmental and biotic variables. Stable isotope analysis provides
quantitative information on both bionomic (!15N) and scenopoetic factors (!13C). Thus the
core isotopic niche area could be! revealed by fitting standard ellipses to the isotopic data in
the bi-dimensional plot of !13C/!15N, as described in Jackson et al. (2011). The standard
ellipse area of a set of bivariate data is calculated from the variance and covariance of x and y
data and is expected to be less sensitive to sample size than former methods, which enable
robust estimations of the isotopic niche provided we have a minimum of ten samples for one
species. The steps providing the final mean value of the isotopic niche width of a species in a
given region are described in Figure 2 :
$%&''"&(")*+*+&,"-.",/&%01+"+123&/"-.","4&/1""
Biplot of !13C/!15N for Species A
(filled circles) and Species B
(Opened circles)
Step 1 : Standardization between 0 and 100.
Each value is transformed as :
Xstd = 100 # ((Xi - Xmin) / (Xmax - Xmin))
as proposed by Cucherousset & Villéger (2008).
Step 2 : Fitting of a standard ellipse on each
species, using the covariance table of the
bivariate data x and y to define the shape and
size of the ellipse :
The resultant standard ellipse area (SEA) is given by the formula "ab, a being the semi-major
axis of the ellipse and b the semi-minor axis, obtained via the eigenvalues of the matrix. SEA
can be corrected in order to account for small sample size : SEAc = SEA x [(n-1)/(n-2)].
Step 3 : The covariance matrix could also be estimated by a bayesian approach, which
enable an open number of estimates of the ellipse area (SEAB) and thus the constitution of a
confidence interval for SEA in order to perform statistical analyses.
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MATERIALS AND METHODS II
!
For our study, 500 simulations were performed for each species within a region or a
zone of Mururoa, using a Monte-Carlo Markov-Chain with the following priors as proposed
by Jackson et al. (2011) :
The priors : µx ~ dnorm(0, !2 = 103)
µy ~ dnorm(0, !2 = 103)
! ~ wishart-1(! = 2, V = )
The Likelihood : Yi ~ MVN([µx , µy] , !)
)
See Jackson et al. (2011) for detailed methodology on calculation of SIBER metric,
and Jackson et al. (2012) for an example of the use of these standard and bayesian ellipses in
a biological case of invasion ecology. As indicated in the mentioned steps, sampling locations
were treated separately to provide insight into the feeding ecology of species across our study
sites and avoid the issues of the missing isotopic baseline.
II. The Layman metrics modified by Cucherousset & Villéger (submitted)
Layman metrics, based on the elaboration of Convex Hulls in the bi-dimensional
" C/" N plot, were developed with the purpose to describe with precision the isotopic niche
of a species or assemblage of several species (Layman et al. 2007, Hoeinghaus & Zeug 2008,
Layman & Post 2008). These metrics are characterized by a set of seven indices ("13C Range
CR, "15N Range NR, Total area of Convex Hull TA, Mean distance to centroid CD, Mean
nearest neighbor distance NND and Standard deviation of nearest neighbor distance SDNND)
that, beyond the simple description of the area of a niche, also enable to describe the
relationship between individuals of a species or between several species of a group (a group
being defined, for us, by diet as it represent a functional group), in order to analyze their
functional structure and the potential effect of external factors (for example the habitat).
However, Cucherousset & Villéger proposed to weight the information brought by these
metrics with the abundance, biomass or other representative data of the importance of each
« individual » of the group (actual individual or a given species). Thus they derived the seven
Layman metrics into a set of six new metrics as described in Table 4 and Appendix VI.
13
15
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The trophic niche of a species may be represented by its isotopic niche width :
•
• Mean isotopic signature of !13C and !15N
Mean value of the isotopic niche core, given by SIBER ellipses
Assumptions : There is a gradient from specialist to generalist species :
Narrow niche
#!
"!
$!
Large niche
A = Specialist species with low degree of individual specialization
B = Generalist species with low degree of individual specialization
C = Generalist species with high degree of individual specialization
!"#$%&'!'(%)$*%)"#$+&#,!-$%&$./%0#*$12$:
(Hypothetized factors with statistical test that were used to test them)
1345463785$987:4;<$
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Individuals size : . Simple linear regression
. Model-based recursive partitioning (See
Zeileis et al. 2008)
Sheltered-to-exposed gradient
in the atoll of Mururoa :
Specific size
Diet
Home-Range
Schooling
Activity
Level
General linear model
runned on !13C, !15N
and SEAB.
Geographical Range
Functional Distinctiveness
Biomass/Frequency/Density
Large niche,
high or low
functional
redundancy
!"# !
Effect of zone tested on !13C, !15N
and SEAB with ANOVA for species
taken individually.
!"#$%!&
Narrow niche,
high or low
functional
redundancy
Results
!
RESULTS III
!
Part 1 : Preliminary analyses
(1) The differences between sources in a given Region
The isotopic signatures of some foraging sources are available but only in Mururoa
(Turf algae, Particulate Organic Matter POM and Zooplankton). Other sources have been
studied in New-Caledonia in 1996 (See Carassou et al. 2008 and Appendix VII) but have not
been recently re-sampled for the current study. No information on the isotopic values of
sources is available for the Gambier Islands either. The differences between zones of Mururoa
for each foraging sources were tested with a one-sided ANOVA. No significant differences
were found for POM across zones within Mururoa (p = 0.19 for !13C and p = 0.57 for !15N),
whereas Zooplankton was significantly different for !13C (p = 0.0007) and Turf was also
significantly different for !15N (p = 0.034). The Tukey HSD post-hoc test shows that these
differences are due to the values of the Ocean zone, that are higher than in the other zones for
!13C (i.e closer to zero) and lower for !15N (see also Appendix XIII for graphical
representation). As the number of species and individuals in New-Caledonia and Gambier is
quite low to work at a local scale, the estimation of the isotopic niche width by the SIBER
method will be performed at a regional scale, i.e by pooling the individuals sampled at
different locations even though the isotopic baseline remains unknow for each one of them.
This fact implies a hypothesis whereby either the isotopic baseline does not change
substantially across zones, or if it changes, the ecological pattern given by the isotopic niche
core study is still stable from one zone to another and will therefore be observable even when
pooling species from different locations. In Mururoa, the particular signature of Ocean zone
will be taken into account when studying environmental gradient effects on isotopic niche.
(2) Effect of individual size on the isotopic signature
The values of !13C or !15N are known to vary with the size of the individuals. As the
magnitude of these variations may influence our main hypotheses, they were evaluated for
each species within each region. For most species, a significant effect of the individual size
exists, at times combined with a significant interaction with the region (see Appendix VIII for
details).
• The dominant pattern was an absence of relationship between size and !13C but a
significant positive slope for !15N with size in a given species ;
• However, for 13 species among the 83 with a significant relationship between size and
at least one of the two isotopes values, the slope between size and !15N was negative.
These 13 species belong to various diets ;
• Twenty four species showed no significant relationship between size and !15N ;
• The slope between size and !13C was positive for 37 species, not significant for 33 and
negative for 19 species.
The relationship between !13C or !15N and size for a species may change with the
region. The slope of the regression may either increase or decrease or even reverse, from
positive to negative for instance, between two regions for the same species, indicating that the
relationship is not spatially stable. This could be verified for 33 species and show that 15
species kept a similar slope, 4 species presented a significant change in slope but no slope
reversal and 10 species reversed their slope across regions for at least one of the two isotopes
(Appendix VIII for details). When looking at the characteristics of these species, no general
common biological trait could explain the observed patterns. However the effect of the diet
the species belong to has been tested in order to work at a larger functional scale (Figure 3).
Indeed diet is expected to be an important factor to discriminate the ontogenetic size effect.
!
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Model-based recursive portioning trees (Zeileis et al., 2008) compare the level of
importance of each diet in predicting the relationship between size and !13C or !15N. Three
steps are followed for each recursive cycle during in developing the tree. A parametric model
is first fitted to the data, then parametric instability is tested with generalized M-"uctuation
tests (Zeileis and Hornik, 2007). If instability is detected, the data is split into two child data
sets, or nodes. The steps are repeated until all data sets are stable. This results in a tree
diagram with multiple terminal nodes that hold distinct sets of parametrically stable data, each
of which are represented by different models. When included as a factor of portioning in a
model-based recursive portioning (Figure 3), it may be noticed that if no pattern were
detected for !13C, diet influenced the relationship between size and !15N. In both Mururoa and
New-Caledonia, the pattern of portioning is equivalent, low trophic level diets (HM, HS, OM)
presenting a negative relationship between individual size and !15N while high trophic level
diets (FC, IM, IS, PK) display a positive relationship between size and !15N. Each circle
containing a p-value indicates that Diet influences the regression slope between size and !13C
or !15N, leading to consequently different regressions shape, which are summarized below.
For each node the number of individuals implicated in the analysis is given.
Specific examples are given on Appendix IX for one species per diet class. The chosen
species are among the most represented in our database and in the field. Both examples for
HM and HD show a decrease of the values of !15N with size. For instance, N. lituratus shows
a high decrease of !15N values for the smallest size range, then the relationship is more stable
if not increasing. Oppositely, A. nigrofuscus shows a stable decrease of !15N values when the
size becomes more important. The four last species presented in Appendix IX have an
!"# !
RESULTS III
!
increasing !15N signature when they grow. However these examples must not be taken as a
generality since there is a high variability across species even within the same diet.
Part 2 : Effect of inherent characteristics on the isotopic niche width
(Regional scale)
(1) The influence of biological and behavioral traits
As the level of sources may be significantly different amongst regions it is difficult to
make direct comparisons. Therefore, the three regions will be mainly compared on a relative
scale. The distribution of the mean isotopic signature for each species and for each diet class
(Figure 4) follows a similar pattern from one region to another. Piscivore and planktivore
species show the highest !15N values followed by corallivore and consumers of mobile
invertebrates. The diet with the lowest trophic levels, represented by omnivores and
herbivores present the lowest values of !15N.
The characteristics of SIBER ellipse for each species within a region are given in
Appendix X. The following analysis is performed with general linear model (family = gamma
with link function = inverse, as it enables the best fit and better residuals, See also Appendix
XII). Each biological and behavioral traits had significant effects on isotopic niche width
(Appendix XI), diet being the most important for both New-Caledonia and Mururoa (such a
model could not be performed in Gambier Islands as the number of species with more than
ten individuals was too low to accurately represent each biological trait). Before examining
the effect of the traits in details, the relative gradient from generalist to specialists species for
each region was compared. To realize a typology of species according to their degree of
isotopic specialization, species were ranked within a region from the largest isotopic niche
width (i.e the highest SEAB mean value) to the narrowest isotopic niche score (i.e the smallest
SEAB mean value). A graphical comparison of the ellipse areas for species in common across
two regions is given on Figure 5. In most instances the order of each species was kept
amongst regions within a trophic group. Thus, G. chilospilus presents a larger isotopic niche
than G. eurostus in both regions of Mururoa and Gambier Islands for instance. The same
pattern is observed for other diets except for “IM” as the rank from large to small isotopic
niche core vary between Mururoa and Gambier Islands.
As said before, all Biological and Behavioral Traits (Diet, Home-Range, Size of
species, Level, Schooling, Activity) had a significant effect on the mean SEAB value (p <
0.001). The details are presented in Appendix XI. A post-hoc test on the outputs of the
general linear model presented in Appendix XI was planed (instead of taking each factor
individually like a one-sided ANOVA and then using Tukey HSD), using the glht function of
the “multcomp” R package. However, a technical problem was encountered which could not
be fixed, which is the reason a ANOVA was used instead (Table 5). In Mururoa, the species
from Diet class (HM) have a mean SEAB value that is higher than the other functional groups.
The effect of the other traits is unstable as it could change across diet. Moreover, both of
general patterns and patterns of a given diet could evolve between Mururoa and NewCaledonia : the performed post-hoc test did not rank modalities in the same order.
(2) The influence of the Geographical Range and Functional Distinctiveness
No significant relationship between the Geographical Range of a given species and its
mean SEAB value could be found either in New-Caledonia, Gambier Islands or Mururoa
(Figure 6). Thus a narrower trophic niche is not linked to the degree of endemism, at least
when using isotopic variability as a proxy. In the same way, no relationship was found
between the mean SEAB value of species and their degree of Functional Distinctiveness
(Figure 7). Thus even a species that presents a singular combination of biological or
!
!"#
behavioral traits will not necessarily present a narrower isotopic niche width when using
isotopic variability as a proxy.
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RESULTS III
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* Modalities that need to be considered cautiously because only represented by one species.
(3) The influence of the frequency, biomass or density
Using the Underwater Visual Census database, the frequency (number of transects
where species is present), biomass (g/m2) and density (individuals/m2) of each species were
calculated for each zone of the regions of Mururoa and Gambier Islands. The relationship
between frequency or the rank in biomass/density of each species, and their mean SEAB value
is given in Figure 8. A weak but significantly positive relationship exists between each of
these measures and the width of the isotopic niche. When looking at the effect of the Diet (i.e
does the slope of the regression change according to the functional group) reveals that the
global pattern is mainly linked to herbivores (HM) for all three measures Frequency
(ANCOVA : p = 0.0107 for herbivores), Biomass (ANCOVA : p = 0.0139 for herbivores) or
Density (ANCOVA : p = 0.0403 for herbivores). For other diets, the regression is never
significant.
!
!"#
#
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#
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!"# !
RESULTS III
!
The case of coralivore : A previous SIBER analysis, run with a minimum of eight
individuals by species instead of ten, enabled the inclusion of two supplementary species from
Chaetodon genus, which are all coralivore to some extent. Even if the minimum number
individual is lower than recommended, it is interesting to report that the isotopic niche width
of these species decreased with increasing frequency (R2 = 0.92), biomass (R2 = 0.97) or
density (R2 = 0.87). Thus these four coralivores species present an opposite pattern relatively
to the strict herbivores.
The ecological metrics and the community functioning appears to be a better factor for
shaping the isotopic trophic niche than biological traits (except for diet) at least for herbivores
and coralivores. The next step will be to test the effect of a sheltered-to-exposed
environmental gradient on the local isotopic niche width of species, in order to see if
ecological features may influence the diversity of their diet in addition to the competition.
Part 3 : Effect of environmental gradient in the isotopic niche width #
(1) The evolution of the isotopic niche width with habitat variations : feeding plasticity ?
The mean SEAB values per diet and zone are presented in Appendix XIII. The zone
effect on the isotopic signatures !13C and !15N are summarized in Table 6 ; as well as the zone
effect on the isotopic niche width given by the SIBER ellipses for species taken individually.
A post-hoc test (glht, R package multicomp) performed on the entire pool of species shows
that the Lagoon zone (L) is separate from the others as it presents higher global !15N value
and lower !13C value (i.e more negative). The values of the available sources were different in
the Ocean zone. This suggests that the change of the global isotopic signature of species
between the zones of Mururoa are not - at least not only - the results of the variability of
sampled sources. For each species, a zone effect on the isotopic niche width was detected, the
only exception being Myripristis berndti. Testing together for the effect of zone on SIBER
ellipses and on the isotopic values for !13C and !15N allows to discriminate which species just
change their niche width from those which modify both their isotopic niche width and their
diet range. Most species present no significant difference in isotopic signature across zones.
However for several species, the change in the isotopic niche width is also accompanied by a
shift in the consumed sources, but without consistent pattern.
(2) The community scale : insights from the modified Layman’s metrics
The community scale analysis was performed by pooling species in functional groups
(i.e their diet class). Species were added to a functional group when both the isotopic
signature and the density metric were available (See Appendix XIV for the global density for
each diet). For each zone of the atoll of Mururoa, the six modified Layman’s metrics were
estimated using the script by the Ecosym laboratory (IRD, Montpellier) for all functional
groups. Graphical outputs of the script, which give a visual representation of the structure of
the community and the total area of the biplot occupied, are presented in Appendix XV. The
larger this area is, the wider the subsets of both bionomic and scenopoetic axes, where the
community could evolve, may be. The Trophic Divergence (TDiv), Trophic Eveness (TEven)
and Trophic Uniqueness (TUni) bring a representation of the relationship between species in a
given community. The major findings, when pooling all of these results for each functional
groups to compare zones of Mururoa (Figure 9) are :
- The Sheltered and the Lagoon zones present the narrowest Trophic Richness (< 0.15)
for all functional groups except IM in Lagoon zone (0.25). The general pattern for TRich is
quite maintained across zones for differents diet.
!
!"#
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- Trophic Divergence (TDiv) is stable across functional groups and zones (around
0.8). The low values for IM in Lagoon zone and PK in Ocean zone are due to the presence of
a very abundant species that influence the index (Amblycirrhitus bimacula for IM and
Dascyllus flavicaudus for PK). TDiv being relatively high (> 0.5 whatever the considered
zone), we may suppose the feeding behavior or/and diet of species to be quite singular for
each species within functional groups.
- The global pattern of Trophic Diversity (TDis) generally increases along the
gradient from sheltered to oceanic zone, except for the sessile invertebrates carnivores (IS) for
which the values decrease. For instance omnivore individuals (OM) present a consistently
increasing TDis values when going from Sheltered zone to Ocean. Piscivores (FC),
Planktivores (PK) and Herbivores (H) present a similar pattern with higher TDis in Lagoon
zone. Mobile invertebrates consumers (IM), oppositely, present lower TDis value in Lagoon
zone. Low values correspond to a community that presents a lower variability in the various
isotopic signatures (and by extension the trophic behavior) in species of this assemblage. This
!"# !
RESULTS III
!
means that a small TDiv corresponds to communities that occupy a narrow isotopic niche, or
present a small diversity between species or both.
- The Trophic Eveness (TEven) increases across the environmental Sheltered-toOcean gradient (except for the low value observed for Planktivores in Ocean zone). The
similarity between values increases as the oceanic influence increases. For instance, all values
for all diets cover the range from 0.1 to 0.9 in Sheltered zone while around 0.7 to 0.9 near to
the Ocean.
- Finally, the Trophic Uniqueness (TUni) gives a representation of the « trophic
singularity » of each species in a given functional group and zone. A similar pattern is
observed for Piscivores (FC), Herbivores (H) and Planktivores (PK) with TUni being higher
close to the ocean (Lagoon and Ocean zones). The three other diets (IM, IS and OM) display
more stable values across zones (except for Omnivores that show a slightly increasing value
when getting closer to the Ocean, but more slowly than H, PK and FC). IS and OM present
quite high values (around 0.6 to 0.7) whereas IM present the lowest values of all (around 0.2
to 0.4).
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!!"
Discussion &
Conclusion
!
DISCUSSION & CONCLUSION IV
!
! SPECIALIZATION IN REEF FISH COMMUNITIES
Effect of individual size on isotopic signatures
Results relative to the effect of size on !13C and !15N values showed that in more than
half of the studied species a significant relationship existed between size and the value of at
least one of the two isotope ratios. This ontogenic pattern has been reported for many fish
species (Galván et al. 2010) and could be explained by several factors. First, the modification
of isotopic signatures could be due to variations in individual metabolism, for example from
juvenile to sexually mature stage when gametogenesis requires more energy investment
(Kolasinski et al. 2009). Maturation could also enable a better assimilation of the ingested
items and thus modify the isotopic fractionation rate (Sweetings et al, 2007 (a) and (b)). The
isotopic signature can be modified by a change in foraging location (e.g. shift of habitat
between juveniles and adults) if the isotopic signature of the targeted source is different from
one foraging location to another (Gladfelter & Johnson 1983, Brischoux et al. 2011). Finally,
there may be a diet shift between early and late life stages (Cocheret de la Morinière et al.
2003, Graham et al. 2007, Frédérich et al. 2010). This diet shift could either avoid intraspecific competition between individuals of different size, or be only due to the fact that
larger individuals possess a better capture ability or could ingest prey from higher trophic
levels (Coates 1980, Luczkovich et al. 1995, Cocheret de la Morinière et al. 2003, Graham et
al. 2007). In our study, diet shift is the most probable explanation for species which amplitude
between the smallest and the largest individuals is important, based upon a mean fractionation
coefficient of 3,4‰ for !15N (Post et al. 2002). A diet shift may have important consequences
on trophic niche width as depending upon the proportion of juveniles and adults in a
population, the global isotopic niche width could change (size-linked intra-population
variability).
Moreover, in the literature the slope between size and !15N is positive whilst for !13C
there is usually no relationship (Galván et al. 2010 ; Greenwood et al. 2010). Cases when the
isotopic value of !15N decreases with size are not frequent in the literature. Our study is by far
the most comprehensive, covering almost 150 species, whereas so far the largest study
available looked at around ten species (Cocheret de la Morinière, 2003; Pasquaud et al. 2008),
and a we recorded a negative slope between size and !15N in more than ten cases, three of
which with a sufficient amplitude to induce a gradual change in trophic level (Acanthurus
nigrofuscus, Naso lituratus and Naso unicornis).These three herbivores contradict suggestions
of Cocheret de la Morinière et al. (2003) who found no significant relationship between size
and isotopic signatures for herbivores. Indeed for a given species, the comparison of our
results with the literature (e.g. Carassou et al. 2008, Frédérich et al. 2010, Greenwood et al.
2010) does not systematically lead to similar conclusions. Similarly our results show some
variability across regions. This suggests that some environmental, biotic or abiotic constrains
may influence the relationship of isotopic ratios with fish size (Reid et al. 2013, Nagelkerken
et al. 2008). By contrast, when scaling up to the next degree of organization, i.e. functional
groups, patterns are stronger as the relationship between size and isotopic ratios is conserved
between New-Caledonia and Mururoa despite major environmental differences. This shows a
convergence with the biodiversity patterns mentioned in the Review of this report : functional
groups show a stronger stability at large scale than species. However, within a given
functional group, the variability of the responses of the species is important. This suggests
that fish are very versatile / adaptable : beyond their biological traits (and by extension their
“evolutionary background”) there is a potential of adaptability that enables them to fit in
various habitat constrains, otherwise a conserved pattern across regions for each species
would have been the rule.
!
!"#
Specialist to generalist gradient : is a typology at hand ?
It was noticed that the rank of species along a generalist-to-specialist gradient remains
stable within a functional group across regions, the only exception being mobile invertebrates
consumers species. Despite a number limitations due to sampling and the absence of
confirmation from stomach content analyses and despite the difficulty to check for all the
underlying hypotheses (i.e fractionation rates by species, exact signatures of sources, etc) the
present results still constitute a strong argument to support the use of stable isotopes
variability as a proxy of trophic niche width. Up to now, specialization in coral reef fish has
been analyzed in a narrow range of species which are phylogenetically close, and the
specialization was often considered in its spatial aspect. Such a trophic specialist-to-generalist
gradient had never been analyzed, to our knowledge, at the scale of the present study which
shows that there is on one hand some constancy in the classification of species along this
specialist-generalist gradient but that the trophic niche width is highly variable across regions,
locations and even within species as discussed in the following paragraphs.
Effect of the biology of species on isotopic niche width
All the tested biological or behavioral traits have a significant effect on the mean
isotopic niche width of a species, implying that the degree of specialization is at least in part
linked to the evolutive traits of species (See Futuyma & Moreno, 1988 for a Review and
Litsios et al. 2012 for an example in the Pomacentridae family). In both New-Caledonia and
the atoll of Mururoa the most influential traits were diet and mean species size. This is
relevant insofar as the diet of species does not lead to the same opportunities in terms of prey
diversity. For instance, strict coralivore fish often present feeding preferences or space
partitioning in order to avoid habitat interspecific competition or aggressiveness (BouchonNavarro et al. 1985 ; Cox, 1994 ; Zekeria et al. 2002). Coral feeders have a narrower niche
width than piscivore or herbivore species, which are usually far more mobile and able to feed
on a wider range of prey (e.g for herbivores Roughgarden 1974). This is actually observed in
the atoll of Mururoa where sessile invertebrate feeders (most of which are coralivore) present
the narrowest isotopic niche width. In the same way, omnivore fish are expected to present
the widest isotopic niche, which was however not the case in Mururoa in contrast to NewCaledonia where this expectation was confirmed. In Mururoa the largest isotopic niche were
presented by the two herbivore diets (strict herbivore HM and mixed herbivore-detritivore
HD) followed by omnivore species. The simplest explanation is that omnivore species still
present the most eclectic diet, but in the atoll of Mururoa, herbivore species present a high
degree of inter-individual variability that results in a wide individual specialization (Green &
Bellwood, 2009 ; Araujo et al. 2011 ; Elmqvist et al. 2013), and thus a larger global specific
niche than omnivores. The absence of such patterns in New-Caledonia could be linked to
statistical power, as isotopic niche width could be estimated for only two herbivore and two
omnivore species in this region, whereas six to sixteen species of each diet were available in
Mururoa.
Behavioral traits like schooling or diurnal activity, for instance, are often linked to
trophic requirements, and therefore interact with diet. For example herbivore reef fish are
always diurnal species (the reason is uncertain but could be linked to either predation
avoidance or nutrient content), many carnivore species forage mostly at night because
invertebrates come out of their shelter at that time (Nagelkerken et al. 2000). In the same way,
large schools are more likely to shelter species of low trophic level (Foster, 1985 ; Welsh &
Bellwood, 2012) as they protect individuals from predators or enable them to reach their
resource more easily in case of direct competition. Oppositely predators are more often
!"# !
DISCUSSION & CONCLUSION IV
!
solitary species. Furthermore, behavioral traits are expected to have an influence on the
trophic niche width within a given functional group. For instance Lawson et al. (1999)
showed that an herbivore species forming large schools had a broader foraging influence than
another herbivore which does not aggregate. This very likely results into a broader diet range.
However, within our study the effect of a biological or behavioral trait could change across
regions (New-Caledonia vs the atoll of Mururoa). For example, when considering the
interaction between diurnal activity and piscivore species (FC), a larger isotopic niche was
observed for nocturnal species in Mururoa while the exact opposite was observed in NewCaledonia. Such interactions were frequent (observed for almost all biological traits across
regions) and show the versatility or adaptability of fish to food availability. However, there
was no predictable pattern, with the exception of the Level factor : the isotopic niche width
being almost always wider for species that live on the bottom than for species living higher in
the water column. This may be explained by either a higher prey diversity and availability, or
by a more generalist trophic behavior of these species in order to avoid competition over
ressource.
This absence of predictability in the effect of biological traits on trophic niche width
has to be associated with similar results found for : (1) individual size ; (2) within a species
across locations and; (3) the species effect within functional groups (the two latter points are
discussed further). All these results converge towards a high versatility or adaptability of
these reef fish to food resources. Very little literature is presently available on this topic.
Futuyma & Moreno (1988) suggested that the trophic niche width and specialization were the
result of a highly complex interaction between inherent (biological traits) and local constrains
that balance each other, and consequently it is very difficult to disentangle the respective
effect of each force. Our results strongly suggest that there is an interaction between food
availability and trophic niche width. Before investigating the consequences of this
adaptability/versatility of reef fish diet, we wish to examine two other species features:
endemism and functional distinctiveness.
Effect of endemism and functional distinctiveness on isotopic niche width
Narrow geographical ranges (endemism) and functional singularity may confer more
vulnerability to species. Endemism is expected to be linked to a narrow potential of habitat
adaptability and thus a relatively high degree of trophic specialization (Munday, 2004 ;
Slatyer et al. 2013). The combination of a narrow geographical range and low abundance
implies a double jeopardy (Hobbs el al. 2010). Double jeopardy combined with specialization
would be even more critical to species vulnerability. However we found no significant effect
of geographical range upon mean isotopic niche width of species, and neither did Hobbs et al.
(2010) with ecological specialization of angelfishes in the Indian Ocean. Endemism may
therefore not be linked to a better utilization of trophic resources or to the presence of specific
trophic resources. However the level of endemism was low in our data set. Data from
Marquesas Islands, which are characterized by a high level of endemism, are currently being
analyzed and initial results suggest also that trophic niche width is not related to endemism.
In the same way, there was no significant relationship between singularity of a species
and the mean value of its isotopic niche width, even inside a functional group, opposite to
expectations. Indeed the rarest species have been shown to often fill key roles in reef
ecosystems by supporting rare functional features ; and rare species often have a narrow
ecological niche (Mouillot et al. 2013). In the present study niche width could not be
associated to species singularity neither in New Caledonia nor Mururoa or the Gambier
archipelago. Thus these “singular” species are neither linked to a very high generalist
behavior nor oppositely to a very high specialization that could enable their persistence. As it
was hypothesized for degree of endemism, maybe singular species have developed some
!
!"#
evolutionary advantages in terms of spatial competition, or a better resistance towards
predation for instance. Hobbs et al. (2010) did not find any relationship between abundance
and ecological niche width while they hypothesized that the least abundant species would be
the most singular. Endemism and functional singularity yield therefore a similar signal than
the other analyses (size, location, functional groups), the trophic niche width of reef fishes is
highly variable and not linked to specific biological features. This merges with the
considerations of Pavoine et al. (2005) on the lack of predictability of species singularity.
Effect of frequency, biomass and density on isotopic niche width
Data from the atoll of Mururoa enabled to analyze the effects of species frequency,
biomass and density on trophic niche width. A significantly positive relationship with the
mean isotopic niche width was found. This suggests that the most common species, in terms
of occurrence or abundance, have a more eclectic feeding behavior than the species that are
less common. A high frequency could be the cause or consequence of a high degree of
opportunist feeding behavior, that enable the species to survive in a wide variety of locations
despite of varying prey availability or inter- (but more likely intraspecific) competition
(Semmens et al. 2009). Many studies have reported a positive relationship between abundance
and individual specialization that could lead to a larger global isotopic niche (Bean et al. 2002
; Araujo et al. 2011). A high biomass may originate either from a high density, or from a low
density but large individuals. Large individuals may have access to wider prey spectra and
therefore display larger mean values of isotopic niche width. When looking at the effect of the
species size class (Table 5), however, the largest species do not actually have necessarily a
larger niche width. So the relationship between biomass and mean isotopic niche width is
probably linked to the density effect.
When looking at the effect of the diet (i.e does the slope of the regression change
according to the functional group) it appears that the global pattern is mainly linked to
herbivores for all three measures frequency, biomass or density. This is in opposition to
Hobbs et al. (2010) who found no such relationship for Centropyge angelfishes which are
herbivore. However they took into account the habitat niche also which could explain this
dilution. The positive regression between abundance and isotopic niche width leads to quite
important considerations in terms of resilience, less abundant species being likely to become
more vulnerable to resource depletion. Oppositely, when considering the four corallivorous
species Chaetodon ephippium, C. flavirostris, C. lunulatus and C. trifascialis, the less
frequent or abundant (in terms of frequency, biomass and density) had the largest mean
isotopic niche width. This could be linked to a process of trophic niche-partitioning amongst
coralivore species. A frequent or abundant species may get more specialized in order to avoid
interspecific competition (Cox, 1994 ; Zekeria et al. 2002 ; Pratchett & Berumen, 2008) or has
a better access to its favorite resource (Berumen & Pratchett, 2008). Thus each species
focuses on a narrower range of items. However, as two of these four species were only
represented by eight individuals instead of the recommended minimum of ten (Jackson et al.
2011 ; Syväranta et al. 2013), this result may not be considered as plainly robust despite the
high level of significance. The opposition between herbivore and coralivore species, in terms
of the response of specialization to increasing competition may be considered to support our
previous results showing the diet versatility of species. Herbivore species may react to intraspecific competition, presenting a higher degree of individual specialization resulting into a
wide global isotopic niche which may overlap (or not; Bolnick et al. 2010) with other species
; whereas coralivores are more specialized in a common pool of resources which suggest dietpartitioning amongst other coralivore species. Balance between these two forces leads to
different responses as described by Bolnick et al. (2010) and thus to different vulnerability to
!"# !
DISCUSSION & CONCLUSION IV
!
resource depletion. For the other diets, the absence of any trend between frequency /
abundance and the mean isotopic niche width may reflect an adaptability to maintain a similar
diet for a wide range of resource conditions. Omnivore species could feed upon a wide range
of prey and are less likely to be constrained by competition. Planktivore species may not be
strongly selective and feed on the entire range of plankton organisms with low or no intra or
interspecific interactions, as the resource availability is high (however, intraspecific diet
variability has been shown to increase with density for Dascyllus aruanus, Frédérich et al.
2010). Carnivores and piscivores have usually wide home ranges which allow them to find
their favorite prey items, and therefore decrease their need to adapt to local competition
pressure.
Effect of an oceanic gradient
At the species scale
As the effect of distance from the ocean on community structure is well documented
(e.g. Grimaud & Kulbicki, 1998) we wanted to test if isotopic niche width could explain some
of this variation. In particular the effect of wave exposure, turbidity and currents condition
resource availability and habitat features thus influencing the functional structure of the
associated ecosystems (Grimaud & Kulbicki, 1998 ; Friedlander et al. 2003). For example
resource regularity is expected to be higher in sheltered than in exposed zones for herbivores
and planktivores which may induce wider isotopic niches. In Mururoa the mean isotopic niche
width of a given species varied across zones while the isotopic signatures of !13C and !15N
generally remained stable. This suggests a variation in prey selection or availability but
without a shift of sources. These variations could not be linked to any consistent pattern
within a functional or taxonomic group, in opposition to the patterns observed within
functional groups at the regional scale. This suggests that factors driving the isotopic niche
width at a local scale cannot be predicted from biological or ecological traits alone. In the
absence of local data on environmental variables such as nutrient levels, chlorophyll a,
topography of the substratum, and so on, it is not possible to know the influence of the local
environment on trophic niche width for a given species. However, shifting from the species
level to the entire available species pool within a functional group may bring stability, as
observed for the effect of individual size.
At the community scale : support from Layman’s modified metrics
In Mururoa, the environmental gradient from sheltered-to-exposed zones had a more
stable effect on the isotopic space at the functional group level. The narrower isotopic space
(TRic) in Sheltered and Lagoon zones for almost all diets, as the high mean Trophic Evenness
and Trophic Uniqueness values in the Lagoon zone suggest that species may share a similar
range of prey, but with a need for partitioning (1) because of low prey availability in the
Lagoon zone, the complexity of habitat being low or (2) because of high species richness in
the Sheltered zone, leading to competition. An example from planktivores may illustrate our
point : - According to UVC data the density of planktivores is higher in the Sheltered and
Lagoon zones (Given in Appendix XIV), which could be linked to the proximity to coral
communities that provide feeding sources. This converges with literature (Friedlander et al.
2003).
- Trophic Richness is weak in the Lagoon and Ocean zones, suggesting that fish probably
feed upon a similar range of zooplankton organisms. TRich is intermediate in the Sheltered
zone and high in South-West zone suggesting a trophic diversification.
- Plankton availability is thus supposedly higher in the Sheltered and South-West zones,
maybe because of a higher rate of nutrient input from the atoll ring.
!
!"#
- Trophic Eveness is maximum in the South-West and Lagoon zones, Trophic Uniqueness is
maximum in the Lagoon zone.
These results suggest that in the Lagoon zone density is high but prey availability is
low, resulting in a high degree of partitioning. In the Ocean zone there is a low density but
also a low prey availability, with a balance between these two constraints. Oppositely in the
Sheltered zone density is high and so is prey availability with a potential balance also between
these two constraints. In the South-West a low density goes with a high prey availability
which may explain the high Trophic Evenness and Trophic Richness : each species may feed
on its favorite prey as they are diversified with little competition expected to occur. However
this remains hypothetic as it is difficult to describe such an ecological pattern for all diets.
There are however some converging features, e.g.:
•
•
Trophic Richness is maximum for herbivores (with exception of mobile invertebrates
consumers) across all four zones, while Trophic Evenness and Uniqueness remain
relatively low, supporting that herbivores show both an intra-specific and interspecific diversity in their diet. This pleads for a globally generalist behavior of
herbivore species.
Oppositely, sessile invertebrates feeders (most of which are corallivores) present a low
Trophic Richness and a high Trophic Uniqueness, suggesting a high behavioral
specialization in conjunction with strict resource partitioning
The community approach is complementary to the study of the relationship at the
species level between abundance and isotopic niche width, as it accounts for species interrelationships with potential hypotheses on competition and resource. However, this approach
remains exploratory because (1) the species for which both abundance and isotopic data were
available may not represent the typical structure of a community in a given habitat ; (2) UVC
data are an instantaneous representation whereas isotopes integrate variations over several
months in diet ; (3) this methodological framework still needs to be fully validated. However,
at this stage the interest of such an approach is undeniable as the combination of isotopic
niche values and abundance data enables a precise representation of the trophic structure and
functioning of a community (Cucherousset & Villéger, submitted). Mouillot et al. (2013b)
suggested a similar tool to analyze the functional structure of communities while accounting
for phenological, morphological or any other measurable trait, susceptible to fill a potential
role in resilience of the system as expected for the degree of diet specialization.
! Stability versus Versatility
So far we found that the isotopic niche width may be mainly predicted from species
diet and abundance. There is also an effect of individual size and within a functional group
species tend to keep the same rank. However, in our opinion two major considerations
highlight this work : (a) as already often described in the literature, within a functional group
the species composition may change across space and time. However, the isotopic niche
width or the size-based response of these species may change as well, in other words species
may present a high degree of diet versatility which amplitude was unexpected ; in particular it
is observed for almost all studied levels. (b) all functional groups do not respond in a similar
manner, suggesting different answers to changes in environmental conditions and therefore
resilience (Nyström, 2006). For example coralivores may be much more vulnerable and less
resilient than herbivores (Wilson et al. 2008). Their susceptibility to habitat loss has been
demonstrated many times (Bouchon-Navarro et al. 1985 ; Pratchett et al. 2012). Oppositely
herbivore species may be well adapted in preventing shifts to macro-algae dominance in reef
ecosystems, as they display a high degree of diet versatility/adaptability both at the individual
and the species levels which corresponds with their generalist diet behavior (Roughgarden,
!"# !
DISCUSSION & CONCLUSION IV
!
1974) with these individual and species levels acting in a complementary way (Elmqvist et al.
2003). This versatility/adaptability of species may also be essential in the resilience potential
of a community. We found out that species could be way more adaptable than previously
supposed, as their isotopic niche width depends on both deterministic factors like traits or
abundance, but also stochastic factors as environmental constraints and/or prey availability.
Moreover, for some species, isotopic signatures showed a significant divergence compared to
the mean trophic levels given by Fishbase, which are based upon stomach contents or feeding
behavior (See Appendix XV). The trophic versatility of reef fishes is not an unknown pattern,
in particular Curtis-Quick et al. (2012) showed very similar processes on reef fish of the Great
Barrier in Australia, with at times a complete shift in diet according to environmental features.
Similarly Albouy et al. (2011) could not use ecomorphological traits to predict the level of
diet-partitioning or diet overlap, and Bellwood et al. (2006) stated that a specialized
morphology was not systematically linked to a specialist diet. Yet, the broad range of species
and geographical area we used for the current study highlights that this trophic
versatility/adaptability occurs at all scales. Besides looking at which guilds are the most
resilient one may ask to which degree of perturbation may a guild be resilient ? What are the
specific potential of adaptability and to what extent species may resist to factors like habitat
and environmental changes or variations in resource availability before they collapse ?
The trophic versatility of reef fishes may be considered in the light of the theory of the
diversity response, that states that the resilience of a guild does not depend only on its specific
redundancy and overall niche adaptability, but also on the diversity of responses within this
guild (Nyström et al. 2006 ; Mori et al. 2013). Versatility may act as a security to insure a
better resilience of coral reef fish assemblages despite habitat loss and other threats. This may
also explain in part the origin of the high biodiversity among coral reef fish : the instability of
recruitment (Hixon, 2011) combined with versatility could enable the coexistence of both
specialist and generalist species, preventing any competitive exclusion (even though there still
are ubiquitous species found over a wide range of conditions). Many other types of niche
specialization, generalization and many biological traits, besides trophic niche, could
contribute to the versatility or the diversity of responses inside a reef ecosystem (Curtis-Quick
et al. 2012). These may interact with the versatility / adaptability patterns we highlighted in
this work. However high resilience may not necessarily means a high ability for recovery.
These two notions have to be cautiously distinguished as even a pool of generalist and/or
adaptable species, like herbivores, may not easily recover after a phase shift (Elmqvist et al.
2003). For instance Bellwood et al. (2006b) showed that after a major collapse, the recovery
to the previous coral-dominated system occurred thanks to a single species, thus pointing to
the importance of keystone species. Therefore, even though the future of reef fish depends on
the health of coral reefs, the removal of species by overharvesting or pollution (Chabanet et
al. 2010) for instance may have unpredictable impact on the potential of resilience (Folke et
al. 2004 ; Bellwood et al. 2012 for an example with parrotfish)
! COMMENTS & CRITICS OF THE METHODS : WHICH PERSPECTIVES ?
The use of stable isotopes for a large scale study is at present uncommon. Most studies
that illustrate the use of SIBER ellipses focus on a range of two or three species that are
directly related (one being for example a direct competitor of the other - Jackson et al. 2012 ;
Guzzo et al. 2013 ; Hayden et al. 2013) in a very restricted geographical area. The use of such
methods for large scale considerations has much potential but brings also several important
issues. The Review section of this report indicated many technical and methodological
constrains which need to be considered when analyzing large scale patterns.
!
!"#
First, the absence of measures on potential sources could lead to some criticisms as
differences in isotope ratios may in part originate from differences in the sources and
therefore interfere in the analysis of the rank of species on a specialist-generalist gradient
(Bearhop et al. 2004 ; Araujo et al. 2011), particularly when patterns were slightly
pronounced. Solutions could be the analysis of a third isotope in order to obtain a
tridimentional niche and add an axis to the typology of generalist-to-specialist. Another
solution could have been to analyze the isotopic signature of blood, that integrate the diet
range at a narrower time scale (Boecklen et al. 2011). This could bring information on
temporal variability or inversely stability of diet range. However blood sampling on reef fish,
which are often very small would be a major challenge. The lack of sources is also
problematic when comparing contrasted zones or regions. A determination of a robust
isotopic baseline at each sampled location could enable a better standardization across areas
or regions which would allow direct comparisons, something which could not be done in the
present work. A precise knowledge about sources would also enable to get a very tight
estimation of the niche via SIAR mixing models for example, that enable to estimate the
relative proportion of each type of prey in the whole diet range of a species. However this is
probably out of reach in reef systems as sources are too numerous and may not be isotopically
sufficiently distinct. This could be a perspective for a smaller-scaled study in coral reef fishes
though, for instance to deepen some of our results. Another issue with SIBER ellipses
analysis is the difficulty to estimate overlap between a large set of species. Overlap can easily
be estimated between two species but this quickly becomes very complex when having more
than ten species as we did. The Layman’s derived metrics may bring some information related
to overlap but will not enable a precise analysis of competition.
There are numerous other sources of concern in this large scale analysis. The major
ones are ; (1) the difficulty to get samples for the same species over a wide array of
environmental conditions which is bound to the very high beta diversity of reef fish
assemblages ; (2) the need to sample a range of sizes for most species, as isotope ratios are
size dependant ; (3) get sufficient replicates in order to reduce variance ; (4) collect more
environmental information in order to analyze the effects of environmental factors such as
coral cover, habitat complexity, depth or fishing pressure. However the complexity and
diversity of reef fish assemblages and of their habitat makes such concerns very difficult to
take into account. The present study, despite its shortcomings, is by far the largest study to
date on the isotope ratios of fish and it is precisely this sample size which enabled us to
discover how versatile and adaptable reef fish may be regarding their diet.
CONCLUSION
This study proceeds to decline the use of the isotopic niche width at a large scale and
over a large set of species, which to our knowledge has never been done before. However,
such a large scale approach is confronted to the difficulty to account for all the very precise
considerations that stable isotopes biology usually requires.
Despite the shortcomings that were discussed, this work brings confirmations of
previous findings as well as some new insights and suggests some new venues that were
before scarcely studied and documented. The most interesting result may be the combination
of (1) the versatility of species found at nearly all studied levels : the effect of size on the
signature of !13C and !15N but also the effect of biological traits/environmental gradient on
the mean isotopic niche width inside a functional group, with (2) the stability observed when
considering the functional group instead of the species. The latter phenomenon was known
when considering reef fish species distribution across biogeographical regions in the world,
!"# !
DISCUSSION & CONCLUSION IV
!
but finding the same pattern with regards of trophic specialization leads to several interesting
conclusions in term of resilience and functional groups.
•
•
•
•
•
A high trophic versatility/adaptability may favor the resistance and stability of reef
fish assemblages to perturbations.
This confirms that assemblages with a higher diversity will probably be more resilient
The opposite strategies of coralivores and herbivores in the relationship between
abundance and trophic niche width suggest that the effects of these fish on coral-algae
phase shifts may be far more complex than presently thought.
The lack of relationship between trophic niche width with endemism and singularity
suggests that the ecological role, in particular resilience or resistance, of such species
is probably not related to their trophic characteristics.
The lack of relationships between trophic niche width and most life-history traits
suggest a higher flexibility in the trophic relationships amongst functional groups,
which may further favor resilience and stability.
This work also highlights several perspectives to explore :
Investigate the effects of environmental factor on the isotopic niche width by focusing on
subsets of the databases.
Complete the data on sources in order to improve our understanding of how the
generelist-to-specialist ranking may change across locations and regions. If the ranking of
species across this gradient, remains stable this would constitute a good index to make
comparative analyses of ecological processes across locations or regions.
Add information from other elements such as H, S, or P to distinguish different types of
specialization.
trophic overlap amongst species, amongst functional groups or even within species across
locations could be very informative on competition / exclusion processes. This overlap
can be easily be assessed using the SIBER ellipses, but in highly complex assemblages
the number of combinations rapidly becomes very important.
•
•
•
•
This study has shown both the potential of very large scale C and N isotope ratios studies and
their limitations. It is likely that technological progress may be decisive in this field with
smaller and smaller samples needed and more and more elements being available for analysis.
Ecology experts should not be worried, the journey is just beginning !
!
!
!"#
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!"#
Appendices
!
Appendix I : Geographical repartition of coral reef
Figure I-1: Map of geographical distribution of the major coral reefs around the world. Note
that the whole Indo-Pacific area concentrates the most important proportion of these reefs.
!
1
!
2
Appendix II : Major applications of stable isotopes in ecology
Table II-1 : Three major fields of application of stable isotope analysis in ecology, with a few
examples :
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Appendix III : Sampling design for isotopic analyses
Table III-1 : List of species captured within each region for isotopic analysis and their diet
(main characteristic) (Only species with a minimum of five individuals were included) :
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Appendix IV : Underwater Visual Census in Mururoa
Location of Underwater Visual Censuses transects in Mururoa :
Figure IV-1 : UVC Transects performed by EPHE (Ecole Pratique des Hautes Etudes)
Figure IV-2 : UVC Transects performed by IRD (Institut de Recherche pour le
Développement)
!
11
!
12
Appendix V : R routine for the estimation of Functional
Distinctiveness Index
R script and functions used to estimate Functional Distinctiveness Index (FD) from
Evolutional Distinveness (ED) of Isaac et al. (2007) :
# Required libraries :
library(MASS)
library(ape)
library(cluster)
library(picante)
library(ecodist)
library(vegan)
library(ade4)
# Upload the Biological & Behavioral Traits database :
THV <- read.csv2("THV_Region.csv", head = T, sep=";", dec=",")
# Upload the needed function « Phylo2phylog » :
phylo2phylog <function(phy, ...) {
if(!require(ade4)) {stop("This function requires the ade4 package")}
newick2phylog(write.tree(phy),...)
}
##### Example for Gambier islands #####
# Calcul of the interspecies distances :
dist.fish <- daisy(GB_THV, metric = "gower", stand = TRUE)
# Construction of the cluster on the species dissimilarity :
FD <- hclust(dist.fish, method="average")
# Transformation of the cluster in a phylogenetic format :
fish.func.tree=as.phylo(FD)
plot(fish.func.tree)
# Application of the ED function performed by Isaac et al. (2007) to obtain FD :
fish.func.phylog <- phylo2phylog(fish.func.tree)
fish.func.FD <- originality(fish.func.phylog, method=6) # 6 is equivalent to the ED function.
dotchart.phylog(fish.func.phylog, originality(fish.func.phylog, 6),
scaling=FALSE, yjoining=0, cleaves=0, ceti=0.5, csub=0.7, cdot=0.5) #
# Storage of the results :
write.csv(fish.func.ED)
##### End of script #####
!
13
Appendix VI : Origin of the Layman’s derived metric
Table VI-I : Litterature used by Cucherousset & Villéger* to construct their Layman’s
derived metrics indexes :
* for review only : the article has not been accepted yet, so the interpretation of these indexes
has to be taken with caution.
!
14
Literature cited in Appendix VI : !
!
Cornwell W.K., Schwilk D.W. & Ackerly D.D. (2006) A trait-based test for habitat filtering :
convex hull volume. Ecology, 87, 1465-1471.
Garnier E., Cortez J., Billès G., Navas M.-L., Roumet C., Debussche M., Laurent G.,
Blanchard A., Aubry D. & Bellmann A. (2004) Plant functional markers capture ecosystem
properties during secondary succession. Ecology, 85, 2630-2637.
Laliberté E. & Legendre P. (2010) A distance-based framework for measuring functional
diversity from multiple traits. Ecology, 91, 299-305.
Layman C.A., Arrington D.A., Montaña C.G. & Post D.M. (2007a) Can stable isotope ratios
provide for for community-wide measures of trophic structure ? Ecology, 88, 42-48.
Mouillot D., Graham N.A.J., Villéger S., Mason N.W.H. & Bellwood D.R. (2013) A
functional approach reveals community responses to disturbances. Trends in Ecology &
Evolution, 28, 167-177.
Villéger S., Grenouillet G. & Brosse S. (2013) Decomposing functional !-diversity reveals
that low functional !-diversity is driven by low functional turnover in European fish
assemblages. Global Ecology and Biogeography, in press.
Villéger S., Mason N.W.H. & Mouillot D. (2008) New multidimensional functional diversity
indices for a multifaceted framework in functional ecology. Ecology, 89, 2290-2301.
Villéger S., Novack-Gottshall P.M. & Mouillot D. (2011) The multidimensionality of the
niche reveals functional diversity changes in benthic marine biotas across geological time.
Ecology Letters, 14, 561-568.
!
15
Appendix VII : Isotopic signatures of diet sources in New-Caledonia,
from Carassou et al. 2008 :
Table VII-1: Extract from Table 3 of Carassou et al. (2008) publication (Realized on a subset
of the dataset used in the current work, data of New-Caledonia 1996).
!
16
Appendix VIII : Details about effect of size on isotopic signature
Table VIII-1: Detailed Table of Results of the Size or the Size x Region effect on the isotopic
signature of !13C and !15N for each species.
Only species for which significant linear regressions between size and isotopic signature
(p < 0.05) were found (for at least one isotope) are presented. For this analysis, a minimum of
five individuals per Region was required. The boxed species are those for which the
amplitude of the isotopic shift reach 3,4 ‰ for !15N in the sampled range of individual size, in
the region that are underlined. The question marks indicate a relationship between size and
isotopic signature which may be best fit by a non lienar relationship, therefore the linear
relationship has to be considered cautiously.
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!15N!
Appendix IX : Example of relationship between size and !15N for
six species (one per diet)
Size (mm)!
Figure IX-1: Example of the relationship between size and !15N for six species, one per diet,
in the region of Mururoa. The model for each species is represented in dotted lines with pvalue and adjusted R-squared.
!
19
Appendix X : Main information about SIBER ellipses by species
within a region
Table X-1: Main information about isotopic niche width based upon SIBER ellipses for each
species represented by at least ten individuals in each of the three regions of Mururoa, NewCaledonia and Gambier Islands. The Functional Distinctiveness Index (FD) is indicated for
each species also.
GAMBIER ISLANDS
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./.01/21./.&
8.04676/358&
85/03431367&
.5704//5/73&
.2102368756&
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.7048812381&
6103858258&
157042828.5&
4420/167884&
438085.78.1&
8/30.66.687&
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3/03411./85&
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11.02712824&
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20116723481&
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NEW-CALEDONIA
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MURUROA
-./01/-(
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+$$#%*+,$'+(
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)
)
Appendix XI : Table of significancy of the biological factors effect
on isotopic niche width
Table XI-1: significance of the biological factors, tested on (a) !13C and !15N, (b) SEAB
(i.e isotopic niche width) :
Despite the experimental design was unbalanced, the interactions between biological traits
(Table 5) have been tested even though they should be taken cautiously, as they strongly
improve the model (See residuals in Appendix XII). However the effect of each trait will be
tested within diet with ANOVA and Tukey post-hoc tests when possible (See Table 5 in Results
for details) in order to account for functional group without statistical bias.
Factor effect
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23
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Appendix XII : Residuals of the model (1) and (2) performed on
Appendix XI
Residuals of model (1) on !13C
Residuals of model (1) on !15N
Residuals of model (2) on !13C
Residuals of model (1) on ellipses
Appendix XI
Tableau des SIBER par specie et région,
avec data SEA.B, eccentricity, etc.
Residuals of model (2) on !15N
!
Residuals of model (2) on ellipses
24
Appendix XIII : Plots of isotopic signatures in zones of Mururoa
(a) Sheltered zone
(b) South-West zone
(c) Lagoon zone
(d) Ocean zone
Figure XIII-I : Plots of !15N against !13C signatures of fish species and different diets in
Mururoa for the (a) Sheltered, (b) South-West, (c) Lagoon and (d) Ocean zones. The grey
points represent the mean value per species, the mean value for each diet are indicated on the
graph in black points with errors bars.
!
25
Appendix XIV : Table of density (individuals/m2) for each diet
(regardless to species) within zones of Mururoa :
Table XIV-1 : Density (individuals/m2) for each diet within zones of Mururoa
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26
!
Appendix XV : Layman’s derived metrics graphical outputs
Legend of following figures : a. Mean isotopic position, b. Trophic Richness, c. Trophic Divergence,
d. Trophic Dispersion, e. Trophic Eveness, f. Trophic Uniqueness.
I. Piscivores (FC)
Figure XV-I (a) : Layman’s derived metrics values for FC in Sheltered zone of Mururoa
-
Figure XV-I (b) : Layman’s derived metrics values for FC in South-West zone of Mururoa
!
27
Figure XV-I (c) : Layman’s derived metrics values for FC in Lagoon zone of Mururoa
-
Figure XV-I (d) : Layman’s derived metrics values for FC in Ocean zone of Mururoa
!
28
Appendix XV : Layman’s derived metrics graphical outputs
II. Herbivores (HD + HM = H)
Figure XV-II (a) : Layman’s derived metrics values for H in Sheltered zone of Mururoa
-
Figure XV-II (b) : Layman’s derived metrics values for H in South-West zone of Mururoa
!
29
Figure XV-II (c) : Layman’s derived metrics values for H in Lagoon zone of Mururoa
-
Figure XV-II (d) : Layman’s derived metrics values for H in Ocean zone of Mururoa
!
30
Appendix XV : Layman’s derived metrics graphical outputs
III. Mobile invertebrate consumers (IM)
Figure XV-III (a) : Layman’s derived metrics values for IM in Sheltered zone of Mururoa
-
Figure XV-III (b) : Layman’s derived metrics values for IM in South-West zone of Mururoa
!
31
Figure XV-III (c) : Layman’s derived metrics values for IM in Lagoon zone of Mururoa
-
Figure XV-III (d) : Layman’s derived metrics values for IM in Ocean zone of Mururoa
!
32
Appendix XV : Layman’s derived metrics graphical outputs
IV. Sessile invertebrate consumers (IS)
Figure XV-IV (a) : Layman’s derived metrics values for IS in Sheltered zone of Mururoa
-
Figure XV-IV (b) : Layman’s derived metrics values for IS in South-West zone of Mururoa
!
33
Figure XV-IV (c) : Layman’s derived metrics values for IS in Ocean zone of Mururoa
(No sessile invertebrate consumer (IS) species were available for Lagoon zone in our data).
!
34
Appendix XV : Layman’s derived metrics graphical outputs
V. Omnivores (OM)
Figure XV-V (a) : Layman’s derived metrics values for OM in Sheltered zone of Mururoa
-
Figure XV-V (b) : Layman’s derived metrics values for OM in South-West zone of Mururoa
!
35
Figure XV-V (c) : Layman’s derived metrics values for OM in Lagoon zone of Mururoa
-
Figure XV-V (d) : Layman’s derived metrics values for OM in Ocean zone of Mururoa
!
36
Appendix XV : Layman’s derived metrics graphical outputs
VI. Planktivores (PK)
Figure XV-VI (a) : Layman’s derived metrics values for PK in Sheltered zone of Mururoa
-
Figure XV-VI (b) : Layman’s derived metrics values for PK in South-West zone of Mururoa
!
37
Figure XV-VI (c) : Layman’s derived metrics values for IS in Lagoon zone of Mururoa
-
Figure XV-VI (d) : Layman’s derived metrics values for IS in Ocean zone of Mururoa
!
38
Appendix XVI : Divergences between FishBase and isotopic data
Table XVI-I: Examples of divergences between isotopic mean value (!15N) and the Trophic
Level of a species as provided by Fishbase :
SPECIES
Trophic Level
(Fishbase)
Mean !15N
Suggestion
Acanthurus blochii
Apogon doryssa
Apogon exostigma
Bothus pantherinus
Lutjanus kasmira
P. johnstonianus
Pomacentrus adelus
Sargocentron rubrum
S. spiniferum
Sargocentron tiere
Chl. frontalis
Chl. microrhinos
Chl. sordidus
Scarus altipinnis
Scarus globiceps
Scarus niger
Scarus psittacus
Scarus schlegeli
2.27
3.24
3.45
3.96
3.48
2.26
2.26
3.48
3.48
3.58
2.59
2.59
2.52
2.59
2.52
2.52
2.52
2.52
8.88
9.81
6.91
6.66
11.36
8.95
9.05
11.15
10.45
10.60
5.14
5.82
5.77
5.01
5.96
5.84
5.22
5.58
Could eat small invertebrates ?
Could eat fish larvae
Could be planktivore
May not be piscivore
Mainly piscivore ?
Could be omnivore
Could be omnivore
Could be piscivores
Could be piscivores
Could be piscivores
May be herbivores
May be herbivores
May be herbivores
May be herbivores
May be herbivores
May be herbivores
May be herbivores
May be herbivores
Other examples could be given. These species are those for which a high divergence was
found with our data.
!
39
Diplôme : Master de l’Institut Supérieur des Sciences Agronomiques,
Agroalimentaires, Horticoles et du Paysage
Spécialité : Approche Ecosystémique de l’Halieutique
Spécialisation / option : Enseignant référent : Etienne RIVOT
Auteur(s) : Chrystelle DELORD
Organisme d'accueil : IRD CoReUs 2
Date de naissance* : 08 avril 1991
Adresse : Laboratoire Arago
Nb pages :
36
Annexe(s) : 16 (39 pp.)
Année de soutenance : 2013
Avenue du Fontaulé
66650 BANYULS-SUR-MER
Maître de stage : Michel KULBICKI
Titre français : Analyse des facteurs influençant la niche trophique dans les communautés de poissons
récifaux : apport des isotopes stables de C et N.
Titre anglais : Defining drivers of the trophic niche width in reef fish communities : support from C & N
stable isotopes.
Résumé (1600 caractères maximum) : Si la composition spécifique des communautés de poissons de récifs a été
fréquemment mise en relation avec les caractéristiques du milieu, il en est autrement de la composition fonctionnelle. A
l’aide des isotopes stables du carbone et de l’azote, considérés comme représentatifs du régime alimentaire la largeur
de la niche trophique de plusieurs espèces représentatives des groupes fonctionnels majeurs des poissons de récifs de
Nouvelle-Calédonie et Polynésie Française a été analysée. La taille des individus influe sur les compositions
isotopiques mais sans pattern régional. Aucune relation stable n’a pu être mise en évidence entre la largeur de la niche
trophique des espèces et leurs traits de vie (excepté l’alimentation), ni avec le degré d’endémisme ou la singularité
fonctionnelle. En revanche une relation significative existe entre la taille de niche trophique d’espèces herbivores ou
coralivores et leur fréquence/biomasse/abondance, suggérant un effet de la compétition. Aucune tendance stable n’a
été mise en évidence quant à l’effet d'un gradient d'influence océanique sur la surface de la niche trophique. Le
regroupement des observations par groupe fonctionnel, par contre, engendre davantage de stabilité quel que soit le
facteur considéré. Les résultats suggèrent que les poissons de récifs coralliens présentent une versatilité trophique
inattendue, leur conférerant un fort potentiel d’adaptabilité et donc de résilience face aux perturbations avec cependant
des variations suivant les groupes fonctionnels considérés.
Abstract (1600 caractères maximum) : While the species composition of reef fish communities has frequently
been linked to habitat characteristics, the functional structure of these communities still remains poorly known. With the
support of carbon and nitrogen stable isotopes, which are considered to be representative of diet, we attempt to
understand how the trophic niche width evolves for a representative number of species from the major functional groups
of reef fish communities from New-Caledonia and French Polynesia. Individual size influenced the isotopic composition,
but no regional pattern could be demonstrated. No stable relationship could be found between the trophic niche width
and the life-history traits of species (except for diet), nor with endemism or functional distinctiveness. However, a
significant relationship exists between niche trophic width of herbivores
or corallivore species and their
frequency/biomass/abundance, suggesting an effect of competition. No major trend was highlighted when considering a
gradient of oceanic influence on trophic niche width. Grouping data according to functional groups generated more
stable trophic niche widths, for anyof the analyzed factors . These results suggest that reef fish species are
unexpectedly versatile in respect of their trophic niche, giving them a high potential of adaptability and thus enhance
their resilience against perturbations. This potential may however vary from one functional group to another.
Mots-clés : isotopes stables, poissons de récifs, spécialisation, niche trophique, pacifique sud
Key words : stable isotopes, reef fishes, specialization, trophic niche, south pacific