Download Assessing the likely effectiveness of multispecies overlap analysis

Contributed Paper
Assessing the likely effectiveness of multispecies
management for imperiled desert fishes with niche
overlap analysis
Brian G. Laub∗ ‡ and Phaedra Budy∗ †
∗
Department of Watershed Sciences, The Ecology Center, Utah State University, 5210 Old Main Hill, Logan, UT 84322, U.S.A.
†U.S. Geological Survey, Utah Cooperative Fish and Wildlife Research Unit, Utah State University, 5290 Old Main Hill, Logan, UT
84322, U.S.A.
Abstract: A critical decision in species conservation is whether to target individual species or a complex
of ecologically similar species. Management of multispecies complexes is likely to be most effective when
species share similar distributions, threats, and response to threats. We used niche overlap analysis to assess
ecological similarity of 3 sensitive desert fish species currently managed as an ecological complex. We measured
the amount of shared distribution of multiple habitat and life history parameters between each pair of species.
Habitat use and multiple life history parameters, including maximum body length, spawning temperature, and
longevity, differed significantly among the 3 species. The differences in habitat use and life history parameters
among the species suggest they are likely to respond differently to similar threats and that most management
actions will not benefit all 3 species equally. Habitat restoration, frequency of stream dewatering, non-native
species control, and management efforts in tributaries versus main stem rivers are all likely to impact each
of the species differently. Our results demonstrate that niche overlap analysis provides a powerful tool for
assessing the likely effectiveness of multispecies versus single-species conservation plans.
Keywords: Catostomus discobolus, Catostomus latipinnis, ecological similarity, Gila robusta, habitat selection, life history characteristics, niche overlap, threat response
Evaluación de la Posible Efectividad del Manejo Multi-Especie paraPeces de Desierto en Peligro Mediante el Análisis
de Traslape de Nichos
Resumen: Una decisión crı́tica en la conservación de especies es si uno se debe enfocar en especies individuales o en un complejo de especies similares ecológicamente. El manejo de complejos multi-especie suele ser
más efectivo cuando las especies comparten amenazas, respuestas a las amenazas y distribuciones similares.
Usamos el análisis de traslape de nichos para evaluar la similitud ecológica de tres especies sensibles de peces
de desierto que se manejan actualmente como un complejo ecológico. Medimos la cantidad de distribución
compartida del hábitat múltiple y los parámetros de historia de vida entre cada par de especies. El uso de
hábitat y muchos parámetros de historia de vida, incluyendo la longitud máxima del cuerpo, la temperatura
de desove y la longevidad, difirieron significativamente entre las tres especies. Las diferencias en el uso de
hábitat y los parámetros de historia de vida entre las especies sugiere que probablemente respondan de manera
diferente a amenazas similares y que la mayorı́a de las acciones de manejo no beneficiarán equitativamente
a las tres especies. Tanto la restauración del hábitat como la frecuencia con la que se le retira agua a los
arroyos, el control de especies no-nativas y los esfuerzos de manejo en los cuerpos de agua tributarios contra
los esfuerzos en los rı́os principales, tienen la probabilidad de impactar de distintas formas a cada una de las
especies. Nuestros resultados demuestran que el análisis de traslape de nichos proporciona una herramienta
‡email [email protected]
Paper submitted July 24, 2014; revised manuscript accepted November 1, 2014.
1
Conservation Biology, Volume 00, No. 0, 1–11
C 2015 Society for Conservation Biology
DOI: 10.1111/cobi.12457
Effectiveness of Multispecies Management
2
poderosa para evaluar la posible efectividad de los planes de conservación multi-especie en contraste con los
de especie individual.
Palabras Clave: caracterı́sticas de historia de vida, respuesta a la amenaza, selección de hábitat, similitud
ecológica, traslape de nichos, Catostomus discobolus, Catostomus latipinnis, Gila robusta
Introduction
Efforts to conserve threatened and endangered species
are often hampered by limited knowledge of species life
history and habitat use and by limited resource availability
for gathering such information (Tear et al. 1995; Miller
et al. 2002). Scientists and management agencies have
expressed interest in developing plans that focus on
ecosystems or landscapes and address multiple species
simultaneously, with the goal of more efficient resource
use (Franklin 1993; FAO 2003; Pikitch et al. 2004). In the
context of the U.S. Endangered Species Act (ESA), the U.S.
Fish and Wildlife Service recommends that multispecies
plans be developed for species that are taxonomically related and face similar threats or share a threatened ecosystem (USFWS 1990). Several approaches for addressing the
conservation needs of multiple species simultaneously
have been developed, including the umbrella species
concept, in which habitat conservation for one wideranging species is assumed to protect habitat for species
with narrower habitats (Roberge & Angelstam 2004), and
the focal or surrogate species concept, in which the most
sensitive species to a particular threat are used to manage
the threat for co-occurring species (Lambeck 1997).
However, research into life history trait variation
shows that closely related species or individuals within
species often have diverse responses to similar management actions or anthropogenic threats (Elmqvist
et al. 2003). In addition, multispecies management approaches and plans are often limited in their effectiveness
(Andelman & Fagan 2000; Lunquist et al. 2002; Roberge
& Angelstam 2004). Development of multispecies plans
may not be based on a thorough understanding of species
biology but rather on general notions about what is more
ecologically beneficial; this can preclude implementation
of specific recovery actions (Boersma et al. 2001; Clark
& Harvey 2002). Multispecies conservation plans may be
useful when resources are limited, but there is a need to
critically assess the likely effectiveness of planned multispecies management plans or conservation agreements.
Successful multispecies recovery plans under the ESA
usually involve species that face similar threats (Clark
& Harvey 2002). Research into the appropriateness of
using a surrogate species approach has also focused on
the need to determine whether individual species are
responding similarly to threatening processes (Lambeck
1997; Carignan & Villard 2002). Life history parameters
can be useful in this regard as gages of species’ sensitivity
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Volume 00, No. 0, 2015
to disturbance (Williams et al. 2010; Mims & Olden 2012;
Taylor et al. 2014). Complementarity in geographic distribution and habitat and resource use have also been emphasized as key criteria for the appropriateness of using
surrogate species (Block et al. 1987; Andelman & Fagan
2000; Hitt & Frissell 2004). Life history parameters and
habitat use have also been used to evaluate the similarity
of species’ functional roles within ecosystems (Brandl &
Bellwood 2014). Collectively, these studies suggest that
the likely effectiveness of multispecies management can
be critically assessed by examining the threats to species
in question, their response to those threats, their distributional overlap, their habitat requirements, and the
similarity of their life history strategies.
Niche overlap modeling could potentially provide a
useful tool for quantitatively assessing ecological similarity of species and hence the likely effectiveness of
joint conservation plans. The ecological niche of a given
species can be determined by assessing species use of a
particular resource across the range of resource values
and is typically represented by a bell-shaped curve (May
& MacArthur 1972). Niche overlap between 2 species is
calculated as the area of overlap of the curves. Niche overlap modeling is often viewed as an estimate of the shared
resource use of co-occurring species (Colwell & Futuyma
1971) and thus can be used to quantify the ecological
similarity of species (Schoener 1970; Broennimann et al.
2012). In addition, niche overlap modeling highlights life
history differences between species and resources (e.g.,
habitat) for which species have substantially different
use patterns, information that is useful in evaluating how
species may respond differently to management actions.
Freshwater biodiversity is severely threatened worldwide, and development of effective and efficient conservation plans for managing many aquatic organisms are
urgently needed (Dudgeon et al. 2006). We evaluated
the likely effectiveness of a multispecies conservation
agreement involving 3 sensitive native fish species widely
distributed throughout the Colorado River Basin in North
America. We collected data from the literature on the
habitat and life history requirements of the 3 species and
used niche overlap modeling to quantify their ecological
similarity. Our objectives were to explore the potential
value of niche overlap modeling as a tool for gaging the
likely effectiveness of multispecies conservation plans
and identify key habitat and life history differences between the 3 species that could lead to different responses
to threats and conservation actions.
Laub & Budy
3
Methods
at maturity, diet breadth, longevity, age at maturity, and
spawning initiation temperature (Olden et al. 2006). All
life history traits were continuous variables.
The Three Species Conservation Agreement (TSCA)
We focused on a conservation agreement signed in 2005
(the TSCA) by 6 states (Arizona, Colorado, Nevada, New
Mexico, Utah, and Wyoming) to coordinate management
of 3 native fish species widely distributed throughout
the Colorado River Basin (UDNR 2006). The 3 species
are the bluehead sucker (Catostomus discobolus), flannelmouth sucker (Catostomus latipinnis), and roundtail
chub (Gila robusta). Populations of bluehead sucker
from the Bear and Snake River Basins in northern Utah
and southern Idaho were also included in the TSCA.
These populations have recently been proposed for separate species status (Unmack et al. 2014) but were included in our study. Currently, all 3 species are listed
as sensitive throughout their range, and roundtail chub
populations in the lower Colorado River Basin are under
consideration for ESA listing. The 3 species currently
occupy about half of their historic range (Bezzerides
& Bestgen 2002), and one of the primary goals of the
TSCA is to prevent listing of the species. Major threats to
their populations include habitat loss, alteration of natural
flow regimes, competition and predation from non-native
species, hybridization with non-native species, and population fragmentation due to dams and diversions (UDNR
2006).
STUDY APPROACH
To quantify ecological similarity of the 3 species, we calculated the niche overlap among the species of a number
of habitat and life history traits (Table 1). Niche overlap
was calculated using the methods of Geange et al. (2011),
which provides a universal scale of niche overlap values
(0–1) for many different types of data, including categorical and continuous data. The main steps in calculating
niche overlap were: obtain data on the distribution of
values for each species for each parameter of interest,
model the data distribution as a probability density function, and estimate the niche overlap value between any
2 species as the area of intersection of the 2 probability
density curves.
We used 3 parameters to gage the similarity of habitat
use among the 3 species: flow selectivity, depth selectivity, and substrate selectivity. All habitat parameters were
classified into discrete classes (< 0.21, 0.21–0.4, 0.41–
0.6, 0.61–0.8, and > 0.8 m/s for flow; 0–20, 21–40, 41–
60, 61–80, 81–100, and >100 cm for depth; silt, sand,
fine gravel, coarse gravel, small cobble, large cobble,
boulder, and bedrock for substrate); therefore, habitat
data were categorical. Life history traits were selected
from a suite of traits used previously to categorize life
history strategies of native Colorado River Basin fish and
included length of larvae, maximum body length, length
DATA COLLECTION
We surveyed the literature for books, theses, agency
reports, and peer-reviewed articles that presented data
on habitat selection or life history traits for any of the
species throughout their range (references in Supporting
Information). Selection is the use of a habitat component
relative to its availability (Rosenfeld 2003); therefore, selectivity data were collected only from references that
either reported habitat selectivity directly or reported
habitat availability and use. We calculated a niche overlap value for each reference that reported data for at
least 2 species, rather than compiling all selectivity data
into a single analysis because different papers often used
different techniques for assessing habitat availability and
use.
Data on life history parameters were collected from
each river tributary or lake studied in surveyed references. Data for individual fish were not available because
this would have required tracking individual fish throughout their lives. For example, estimating length at maturity for an individual fish would require measurement of
length when that fish matured, but individual maturation
dates are rarely known. Thus, we collected either the
minimum (age at maturity, length at maturity, spawning
initiation temperature, and length of larvae at hatch) or
maximum (maximum body length, longevity) values reported for a species in each river tributary or lake studied
in each reference. Data on diet breadth were also collected on a per-water-body basis. In each water body, all
types of diet items consumed across all individuals were
combined into one estimate of diet breadth. Because
life history data were not collected for individual fish,
we assessed the range of variability across populations
for each species. All data on each life history parameter
were compiled into a single data set and used to model
a probability distribution for each species and calculate
niche overlap (see Fig. 1 for summary).
DATA ANALYSES
The probability density of habitat selectivity data was
modeled using a binomial distribution function because
habitat variables were categorical (Geange et al. 2011).
All life history traits were continuous variables, and most
life history probability density functions were modeled
using a kernel density approximation (Silverman 1986).
We used kernel density estimation rather than normal
curves because data were not consistently normally distributed and kernel density estimates provided greater
flexibility in modeling probability density functions to
observed data distributions. However, we fit a normal
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Effectiveness of Multispecies Management
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Table 1. Ecological and management importance of habitat and life history traits used in analyzing ecological similarity of bluehead sucker,
flannelmouth sucker, and roundtail chub.
Parameter
Habitat selectivity
Depth
Ecological importance
Relevance to management
shows whether species select deep versus
shallow habitats
Flow
shows whether species select fast versus slow
flowing habitats
Substrate
shows whether species select fine versus
coarse substrate
Life history traits
Length of larvae
Max. body length
Length at maturity
Diet breadth
Longevity
Age at maturity
Spawning temperature
determinant of the larval period, during which
fish have particular habitat and food
requirements and are susceptible to
predation by small fish
indicator of fish size, which determines
metabolism rates, susceptibility to
gape-limited predators, and potential for
completing life cycle at small sizes
related to length of larval and juvenile period
and thus susceptibility of larval and juvenile
stages to mortality; indicates potential for
completing the life cycle at small sizes
indicates degree of omnivory versus
specialization on particular food resources
indicates importance of consistent
reproduction events to maintaining the
population—species with greater longevity
have more opportunities to reproduce
with longevity indicates timespan for potential
reproduction; similar to length at maturity;
indicates potential to mature early from the
susceptible larval or juvenile phase
determines timing of spawning
distribution to the mean and standard deviation of age at
maturity because < 6 data points were available for each
species. Values for maximum body length, diet breadth,
longevity, length at maturity, and age at maturity were
log-transformed to prevent density estimation for values
outside possible ranges of each parameter (e.g., maximum body length cannot be ࣘ 0; Geange et al. 2011).
We used the density function in the base package
of R version 3.0.2 (R Foundation for Statistical Computing, Vienna) with default parameters to derive kernel density estimates. The default method for estimating the bandwidth in this package is Silverman’s (1986)
equation:
h = 0.9σ n− 5 ,
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(1)
indicates how conservation or restoration of
particular habitat types, such as pools vs. riffles
will likely impact each species
indicates how conservation or restoration of
particular habitat types, such as pools versus riffles
will likely impact each species
indicates how conservation or restoration of
particular habitat types will likely impact each
species and how fine sediments may impact
species uniquely
species with smaller larvae may be more susceptible
to mortality during the larval period, such as
changes to larval habitat and presence of
non-native predators
smaller fish may be more susceptible to predation
over their lifespan, but may be able to survive and
reproduce in smaller streams
smaller lengths at maturity indicate less susceptibility
to predation during larval and juvenile stages and
potential to survive and reproduce in small
tributary streams
species with wider diet breadth may be less
susceptible to alterations in particular food sources
or stream trophic state, particularly shifts between
dominance by algal versus detrital production
species with greater longevity are less susceptible to
yearly fluctuations in environmental conditions
such as drought
species with lower age at maturity may be less
susceptible to mortality sources during the larval
and juvenile stages and may be able to complete
the life cycle at smaller sizes
susceptibility of species with different initiation
temperatures to temperature regime changes;
timing of spawning may also determine
susceptibility of larvae and juveniles to dewatering
in spawning tributaries and competition with
other species
where n is sample size and σ is minimum (standard
deviation, interquartile range)/1.34. Visual inspection of
estimated density curves plotted on histograms revealed
no severe over or under-fitting. However, because the
choice of bandwidth can influence the calculated niche
overlap value (Mouillot et al. 2005), we also investigated the effect of the bandwidth selection on calculated
niche overlap values by performing sensitivity analysis
of the bandwidth parameter for all niche overlap values calculated using kernel density estimates (Supporting
Information).
To gage whether a given niche overlap value could
be classified as evidence for significant similarity or a
significant difference between species, we tested the
null hypothesis that the given niche overlap value is no
For all papers
Laub & Budy
5
Within each paper
Results
Identify how many individual populations are sampled
(one for each lake or stream)
For each population
For each species
Identify papers that reported length data
approximately 0.62, and we used this value to determine
ecological significance of niche overlap values.
Identify maximum length value reported
Collect all values reported
and determine data distribution
Calculate niche overlap between pairs of species
using data distributions
Figure 1. Flowchart of method used to collect and
analyze maximum body length data from the
literature.
different from a value expected by chance given the data
(Geange et al. 2011). The test was performed for parameters that were modeled by kernel density estimates by
generating 1000 permutations of species labels for all data
points used to calculate a given niche overlap value (i.e.,
permutation testing). Permutation testing was necessary
because a single value of niche overlap was generated for
each habitat and life history trait as opposed to a sample of
niche overlap values that could be used in statistical tests
(e.g., ANOVA). For age at maturity, which was modeled
with a normal distribution, we drew 10 random values for
each species based on the mean and standard deviation
for that species, shuffled species labels of the randomly
drawn values, and then modeled the 10 values for each
species with a kernel density estimation, This procedure
was repeated 1000 times to define a null distribution of
niche overlap values. We considered a given niche overlap value indicated a statistically significant difference
between species if the value fell below the 5th percentile
of the null distribution.
To gage ecological significance of niche overlap values,
we used a result from niche theory that suggests that
under normal environmental variation, the ratio of the
difference between means to the standard deviation of
species distributions along an environmental gradient is
limited to about one due to constraints on species packing (i.e., maximum niche overlap) (May & MacArthur
1972). This limit corresponds to a niche overlap value of
Habitat Selection
Sufficient data for calculating habitat selectivity overlap
between at least one pair of species was obtained from
4 sources (Table 2). Most papers only reported average
values for habitat use and availability, which meant there
was insufficient data to perform permutation tests for
most estimates of habitat selection overlap. Thus, we
focused primarily on the San Rafael River in southeastern Utah because raw data for permutation testing was
available for this site (Bottcher 2009). On the San Rafael
River, the mean niche overlap value for habitat selection between bluehead and flannelmouth sucker was
0.61. For bluehead sucker and roundtail chub it was
0.45, and for flannelmouth sucker and roundtail chub
it was 0.69. Roundtail chub tended to select slower velocity water than the sucker species, and the difference
was statistically significant relative to bluehead sucker
(Table 2). Flannelmouth sucker demonstrated high overlap with roundtail chub in terms of depth selection and
high overlap with bluehead sucker in terms of flow selection. Estimated habitat selectivity overlap also tended
to be high in other river systems, particularly for young
fish, but spawning habitat selectivity overlap between
bluehead sucker and flannelmouth sucker was relatively
low (Table 2).
Life History Parameters
Mean niche overlap values for all life history parameters were 0.55 between bluehead sucker and flannelmouth sucker, 0.59 between bluehead sucker and roundtail chub, and 0.48 between flannelmouth sucker and
roundtail chub. We observed no significant differences
in length of larvae between the species. However, significant differences in maximum body length and length
at maturity were observed between flannelmouth sucker
and both bluehead sucker and roundtail chub; flannelmouth sucker obtained larger body sizes more frequently
and had larger size at maturity than the other 2 species
(Table 3; Figs. 2a & 2b).
Flannelmouth sucker and roundtail chub also differed
significantly in terms of diet breadth. Roundtail chub had
the most variable diet breadth between river systems;
it was the only species that consumed vertebrate prey.
Flannelmouth sucker had a bimodal distribution, with
many instances of very narrow diet breadth and many
instances of relatively high diet breadth (Fig. 2c).
Roundtail chub tended to have lower longevity than
both sucker species, and the difference was significant for the comparison between roundtail chub and
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Table 2. Estimates of habitat selectivity niche overlap (NO) calculated from literature sources that reported data for at least 2 of the species
examined.
Species comparisona
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v FMS
BHS v RTC
BHS v RTC
BHS v RTC
FMS v RTC
FMS v RTC
FMS v RTC
Life stage
Habitat parameter
NO
p
Citation
Larvae
Juveniles
Adults
Adults
Adults
Spawning adults
Spawning adults
Spawning adults
Adults
Adults
Adults
Adults
Adults
Adults
Adults
Adults
Adults
Substrate
Substrate
Substrate
Primary channel meso habitatsb
Secondary channel meso habitats
Spawning depth
Spawning velocity
Spawning substrate
Depth
Velocity
Substrate
Depth
Velocity
Substrate
Depth
Velocity
Substrate
0.90
0.76
0.73
0.73
0.64
0.33
0.36
0.68
0.52
0.82
0.47
0.50
0.26
0.61
0.82
0.42
0.82
NA
Gido & Propst 1999
Gido & Propst 1999
Gido & Propst 1999
Paroz et al. 2007
Paroz et al. 2007
Otis 1994
Otis 1994
Otis 1994
Bottcher 2009c
Bottcher 2009
Bottcher 2009
Bottcher 2009
Bottcher 2009
Bottcher 2009
Bottcher 2009
Bottcher 2009
Bottcher 2009
0.27
0.80
0.10
0.39
0.02
0.58
0.81
0.14
0.74
a Abbreviations: BHS, bluehead sucker; FMS,
b Meso habitats are macro-scale geomorphic
flannelmouth sucker; RTC, roundtail chub.
features such as pools, riffles, and runs.
c Niche overlap values from Bottcher (2009) were tested for statistical significance.
Table 3. Estimates of niche overlap (NO) for selected life history parameters of bluehead sucker (BHS), flannelmouth sucker (FMS), and roundtail
chub (RTC).
Na
Parameter
Length of larvae
Max body length
Length at maturity
Diet breadth
Longevity
Age at maturityb
Spawning temp
BHS v Fms
BHS v RTC
FMS v RTC
BHS
FMS
RTC
NO
p
NO
p
NO
p
11
71
24
8
9
2
20
10
66
25
10
15
6
14
7
52
18
9
8
3
11
0.79
0.43
0.57
0.71
0.70
0.001
0.66
0.82
<0.01
<0.01
0.53
0.18
<0.01
0.08
0.63
0.91
0.74
0.83
0.51
0.001
0.5
0.12
0.88
0.17
0.65
0.20
<0.01
0.01
0.55
0.43
0.41
0.55
0.31
0.66
0.44
0.11
<0.01
<0.01
0.05
<0.01
0.26
<0.01
a Number
b Age
of data points for each species for the given life history parameter.
at maturity niche overlap calculated using a normal distribution rather than a kernel density estimate.
flannelmouth sucker (Fig. 2e). Bluehead sucker had significantly lower age at maturity relative to both flannelmouth sucker and roundtail chub (Fig. 2f).
Flannelmouth sucker differed significantly from roundtail chub in terms of spawning initiation temperature, and
density plots indicated that all 3 of the species had unique
distributions along this parameter (Fig. 2d).
Composite Niche Overlap
The overall niche overlap across all habitat and life history parameters was 0.57 between bluehead sucker and
flannelmouth sucker, 0.55 between bluhead sucker and
roundtail chub, and 0.54 between flannelmouth sucker
and roundtail chub. All 3 composite niche overlap values
and all niche overlap values for individual parameters that
were statistically significantly different were <0.62, the
theoretical upper limit for niche overlap based on species
packing theory.
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Discussion
Our results suggest that bluehead sucker, flannelmouth
sucker, and roundtail chub are sufficiently distinct in multiple ecological aspects as to question the usefulness of
a multispecies approach. We found high niche overlap
in habitat selection among the 3 species in several rivers
in the Colorado River Basin, particularly for young fish.
This was unexpected given the general perception that
the species select unique habitats (e.g., Bezzerides &
Bestgen 2002). A likely explanation for this observation is that many of the rivers where habitat selection has been estimated, particularly in the San Rafael
River, are severely degraded, such that little habitat exists (Rosenfeld 2003; Bottcher 2009). The sensitivity of
niche overlap to environmental conditions highlights the
importance of sampling habitat selection across the full
range of occupied habitat. In particular, investigation of
free-flowing rivers with intact habitat diversity where
Laub & Budy
7
(a)
(b)
2.5
1.5
Density
2.0
Density
2.0
BHS
FMS
RTC
1.5
1.0
1.0
0.5
0.5
0.0
0.0
3
4
5
6
7
8
3
Ln(Max Body Length) (mm)
(c)
4
5
6
7
Ln(Length At Maturity) (mm)
(d)
3.0
0.4
2.5
Density
Density
0.3
2.0
1.5
1.0
0.2
0.1
0.5
0.0
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
Ln(Diet Breadth)
10
15
20
25
Spawning Temperature (deg C)
(e)
(f)
2.5
5
2.0
4
Density
Density
5
1.5
1.0
0.5
3
2
1
0.0
0
1
2
3
4
Ln(Longevity) (Yr)
5
−1
0
1
2
Ln(Age At Maturity)
the 3 species coexist would help clarify natural niche
overlap. Understanding how niche overlap values differ
under relatively pristine and degraded conditions would
help in predicting how species habitat use and interactions are likely to change under fluctuating environmental conditions, information that would be unavailable if
the species were managed as a single ecological unit.
There were, however, some important differences in
habitat selection among the species in the San Rafael
River. Roundtail chub showed low overlap with the
sucker species for flow and depth variables. This difference reflects a greater selectivity for pools by roundtail chub. The presence of pools depends on floods
that scour sediment and is frequently associated with
large logs and wood accumulations (Lisle 1979; Fausch
& Northcote 1992; Abbe & Montgomery 1996). Many
rivers within the Colorado Basin have been affected by
dams and diversions which have severely reduced spring
3
Figure 2. Species density
distributions for selected life history
parameters: (a) maximum body
length, (b) length at maturity,
(c) diet breadth, (d) spawning
temperature, and (e) longevity
(BHS, bluehead sucker; FMS,
flannelmouth sucker; RTC,
roundtail chub). These parameters
were estimated using kernel density
models. Density distributions for
age at maturity (f) were modeled
using normal distributions.
floods and associated scouring processes (Stanford 1994).
In addition, native riparian vegetation has been degraded
and cottonwood recruitment is very low in many systems (Stromberg et al. 2007). These changes have likely
affected pools to a greater extent than riffles and runs
and thus may have affected roundtail chub more severely
than the other 2 species. More research is necessary to
confirm our conclusions because habitat selectivity determinations are not always consistent among river systems
(Thomas & Bovee 1993).
Ecological differences among the 3 species were also
reflected in life history traits. Flannelmouth sucker were
larger than the other 2 species, likely because flannelmouth sucker were more common in large main stem
rivers, where large fish are more likely to be found,
as opposed to smaller tributaries at higher elevations,
where large-bodied fish are absent and roundtail chub
and bluhead sucker were more common (e.g., Mueller &
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8
Wydoski 2004; Sweet & Hubert 2010; Laske et al. 2011).
Spawning temperatures were unique for each species.
Roundtail chub spawned at the warmest temperatures,
and flannelmouth sucker spawned at the coldest. Roundtail chub tended to have higher age at maturity than
the sucker species but lower longevity, suggesting that
roundtail chub adults have the shortest time as reproductive individuals and thus may require successful recruitment classes more frequently than the 2 sucker species.
However, data on age at maturity and longevity were
rarely both available for the same population; further investigation into length of reproductive period for the 3
species in different populations is needed.
Our results regarding overlap in life history traits are
likely robust to sample size and the specific parameters
used to model the data distributions. First, sensitivity
analysis indicated that most estimates of statistical significance for individual parameters did not change when
the bandwidth was varied by a factor of 2 (Supporting
Information). Second, in our literature survey, we collected data across a large proportion of the range of each
species from a variety of references and therefore likely
captured nearly the full range of variability in life history
traits known for each species. Increasing the sample size
for each life history trait would likely refine the shape of
the distribution for each species but have a small effect on
the range of the distribution. Thus, additional data is likely
to decrease estimated niche overlap, further highlighting
the differences among species.
Differences in habitat and life history traits could potentially be due to spatial separation of the species, but
this is unlikely. Each of the species is widely distributed
throughout the Colorado Plateau, and life history data
spanned a large proportion of this range. Habitat selection data was limited, but the main river for which
data were available (San Rafael River) is home to all
3 species. Therefore, the relatively low niche overlap
we observed likely reflects general ecological differences
among the species. The species’ niche overlaps may have
been higher if data were used only where they occur in
isolation. Species are expected to differ in their resource
use based on niche partitioning theory (May & MacArthur
1972), and the niche overlaps we found likely reflect the
realized niche of each species rather than the fundamental niche (Hutchinson 1957). Indeed, species shift and
narrow their trophic niche space in the presence of nonnative fish species (Walsworth et al. 2013), suggesting
that niche overlaps will be lower under co-occurrence
than in isolation. More exploration of the effect of competition in shaping the niche overlap between species
would be useful. Nonetheless, the significant differences
among the species across a broad distributional range,
suggests they are likely to respond differently to management actions where they co-occur.
Identifying the specific parameters where species differ can provide guidance as to what management actions
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Volume 00, No. 0, 2015
Effectiveness of Multispecies Management
are likely to benefit multiple species versus those that
are likely to benefit one species over another (Table 4).
For example, one of the main goals of the TSCA is to
enhance and maintain habitat for each species, but due
to differences in habitat selection, efforts to restore pools
are predicted to benefit roundtail chub to a greater degree
than the suckers. Water acquisition and recommendations for natural flow regimes may benefit all 3 species
because natural flow regimes are the driving force behind physical processes that sustain habitats and ecosystem processes to which native fish are adapted (Poff
et al. 1997). However, understanding how species are
likely to respond to alteration of ecosystem processes
will still be critical for determining management impacts
(Carroll et al. 2001; Quinn & Collie 2005). Furthermore,
in river systems, recovery of natural flow regimes is infeasible in many locations and conservation of threatened
species is likely to require a combination of managed
flows and active management activities, such as nonnative species control and habitat restoration (Propst
et al. 2008). Where site-specific management activities
are undertaken, understanding how each species is likely
to respond individually will be beneficial.
In scenarios similar to the one presented here, where a
group of species share similar management status, distribution, or phylogeny but differ in their likely responses
to conservation actions, a hybrid management approach
may be useful. In such an approach, species-specific conservation objectives are developed under the broad scope
of a conservation agreement or ecosystem-based management plan. The plan then focuses on objectives related to
recovery and conservation of ecosystem processes, such
as natural disturbance regimes and provision of connectivity between habitat patches to ensure metacommunity
persistence. Species-specific plans would predict how
species are likely to respond to ecosystem-based management and would guide development of more site-specific
management actions such as non-native species control
or habitat restoration. In the case we examined, the conservation agreement could focus on recovery of natural
flow regimes and identification of fish passage barriers
that prevent migration between populations. Speciesspecific plans could identify priority areas for habitat
improvement or removal of hybridization threats that
would most benefit each species. A similar strategy has
been proposed for other desert fishes, with emphasis on
the conservation benefits of single-species approaches
and the opportunities for managing communities provided by a basin-scale approach (Rinne & Stefferud 1999).
Niche overlap analysis would support such an approach
by identifying ecological similarities between species and
likely response differences to management actions.
The method of niche overlap analysis we used successfully identified key habitat and life history trait differences among multiple species that are likely to influence how individual species respond uniquely to threats
Laub & Budy
9
Table 4. Potential response differences of bluehead sucker, flannelmouth sucker, and roundtail chub to actions proposed under the conservation
agreement used to manage the 3 species (Three Species Conservation Agreement) based on differences in habitat and life history traits.
Management action
proposed under
conservation agreement
Enhance and maintain
habitat for each species
Control threats posed by
non-native species that
compete with, prey
upon, or hybridize with
each species
Expand each species
through transplant
activities or
reintroductions
Water acquisition and flow
recommendations
Riparian vegetation
restoration
Potential response differences
Roundtail chub most responsive to pool habitat, suckers to
riffle habitat; due to larger size, flannelmouth sucker less
likely to respond to habitat restoration in small tributaries
Hybridization threats differ; flannelmouth sucker may be
less susceptible to predation due to larger body sizes;
roundtail chub may be most susceptible to competition
and predation during larval stages due to lower length at
hatching; competition between each species and
non-natives likely varies based on diet breadth of
non-natives versus the 3 species
Suitability of different river systems likely to differ for each
of the 3 species depending on habitat attributes, available
prey resources, temperature regimes that promote
spawning activities, and presence of non-native fish
Recovery of natural flow regimes likely to benefit all 3
species, but timing and duration of flood flows vs. low
water periods and recovery of flows in tributaries versus
main stems will likely impact species uniquely
Changes in detrital inputs, availability of large wood in
streams, and alteration of shading patterns likely to
impact food availability, habitat availability, and
temperature patterns, all of which are likely to lead to
response differences between species
and management actions (e.g., Mims & Olden 2012).
By aggregating multiple data sources into a universal
scale of species differences (Geange et al. 2011), the
method provides an efficient way to use available data.
The method not only quantifies the degree of overlap for
many different parameters and data types, but also provides, through density estimation plots, a visual demonstration of how species are using resources uniquely.
Niche overlap analysis is grounded in well-established
ecological theory developed explicitly for the purpose
of identifying the unique distributions and roles of organisms in ecosystems (Vandermeer 1972) and has been
used previously in diverse ecosystem types to estimate
temporal shifts in spatial resource use (Broennimann
et al. 2012), assess different functional roles of species
(Brandl & Bellwood 2014), and explore potential drivers
of biodiversity and coexistence (Schoener 1970; Mason
et al. 2008). The method has also been used to examine
response diversity of species to management changes
in diverse ecosystems, including bees and reef fishes
(Williams et al. 2010; Taylor et al. 2014). Therefore, the
application of this method is likely to be useful in many situations beyond desert fish assemblages where managers
face a choice between developing single or multispecies
conservation plans.
Acknowledgments
Support for this work was provided by the Bureau of
Land Management and the U.S. Geological Survey, Utah
Habitat and life history traits
used to inform response
differences
Habitat selectivity, max. body
length
Max. body length, length of
larvae, diet breadth
Habitat selectivity diet breadth
spawning temperature
Max. body length spawning
temperature
Diet breadth, habitat
selectivity, spawning
temperature
Cooperative Fish and Wildlife Research Unit (in-kind).
J. Bottcher and T. Walsworth generously provided data
on the 3 species in the San Rafael River. S. Bonar and
3 anonymous reviewers provided insightful comments
that improved the manuscript. Any use of trade, firm, or
product names is for descriptive purposes only and does
not imply endorsement by the U.S. Government.
Supporting Information
The list of references used in the literature survey
(Appendix S1) and the methods and results of sensitivity analysis of the kernel density bandwidth parameter
(Appendix S2) are available online. The authors are solely
responsible for the content and functionality of these
materials. Queries (other than absence of the material)
should be directed to the corresponding author.
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