Oikos 116: 1431 1437, 2007
doi: 10.1111/j.2007.0030-1299.15833.x,
Copyright # Oikos 2007, ISSN 0030-1299
Subject Editor: Kenneth Schmidt, Accepted 24 April 2007
Individual foraging specialization: niche width versus niche
overlap
Brooke L. Sargeant
B. L. Sargeant (sargeant@fiu.edu), Dept of Biology, Georgetown Univ., Washington, DC 20057, USA. Present address: Dept of
Biological Sciences, Florida International Univ., 11200 SW 8th St., Miami, FL 33199, USA.
A recent review of individual specialization suggests it is
widespread and has important implications for conservation, ecology and evolution (Bolnick et al. 2003).
Dozens of studies have identified ‘‘individual foraging
specialization’’ in a variety of species, yet upon closer
examination these studies differ markedly in how they
define and quantify specialization. Given such diverse
approaches, characteristics of niches that provide
information about individual differences and quantify
individual foraging specialization need to be clearly
identified.
Individual foraging specialization has been defined
in many ways, although all definitions involve the use of
different dietary items or foraging tactics by individuals
within a single population. A direct corollary is that at
least some individuals must have narrower niches than
the population niche overall. Following Bolnick and
colleagues’ (2002) interpretation of Roughgarden’s
(1972) niche width concept, population-level niche
width (‘‘total niche width’’, TNW) can be subdivided
into a ‘‘within-individual component’’ (WIC) and a
‘‘between-individual component’’ (BIC). Thus, WIC/
TNW provides a measure of how much variation in
TNW can be explained by variation within or between
individuals. As WIC/TNW increases, most of the
variation in TNW results from variation within
individuals, but as it decreases, variation in TNW
results from variation between individuals. Thus, unless
all individuals have identical niches, at least some niches
will be narrower than that of the population and
individuals will differ from one another.
Individual differences clearly differentiate individual
foraging specialization from ecological specialization in
species or populations (Levins 1968, Fox and Morrow
1981, Futuyma and Moreno 1988) and predict
individual differences in niches. But further distinctions
about how specialization is defined and therefore who is
most specialized vary substantially between studies.
While it may initially seem that a strict definition
could be applied (e.g. a specialist uses 50% of resources
used by the population), biologically meaningful
patterns of niche use are likely to depend on what
resource and species are being considered. For example,
Futuyma and Moreno (1988) recommended against a
strict, rigid definition of resource specialization, viewing
specialization and generalization as extremes of a
continuum based on Hutchinson’s (1957) n-dimensional hypervolume of niche space. Therefore, similar to
traditional approaches to generalization and specialization, individual specialization should describe individual niche use only in terms relative to the population’s
niche use.
So what, exactly, is individual foraging specialization? Generally, two different conceptual approaches
have emerged, although both compare an individual’s
niche to the population’s niche. In the first, which I will
refer to as the niche width specialization concept,
individual foraging specialists are individuals with
narrower foraging niches than the population’s niche
(Roughgarden 1972, West 1986, Werner and Sherry
1987, Holbrook and Schmitt 1992, reviewed by
Bolnick et al. 2003). This should include definitions
that describe ‘‘the consistent use’’ of resources or tactics
in which alternative resources or tactics are used by the
population (Werner et al. 1981, Partridge and Green
1985, Schindler et al. 1997), as consistency should
result in a narrower niche by reducing richness or
evenness (below). In the second approach, which I will
refer to as the niche overlap concept, specialists have
niches that exhibit little overlap with the population
1431
niche (Bolnick et al. 2002, Estes et al. 2003, Svanbäck
and Persson 2004). A variant of this concept is that
specialists are those who use rare dietary items or tactics.
While the use of a rare resource is suggestive of low
overlap with population dietary values, the relative use
of the rare resource by the individual and by the
population will influence such overlap measures. For
the purposes of this discussion, the use of a ‘‘rare’’
resource will refer to the rarity of this resource in the
population’s realized niche, but does not imply the
actual availability of this resource. Some have referred
to such behaviors themselves as ‘‘foraging specializations’’, which does not reflect the process of ecological
specialization and may be more similar to terminology
regarding morphological or evolutionary ‘‘specializations’’ as opposed to ecological specialization (Robinson
and Wilson 1998, Ferry-Graham et al. 2002). These
behaviors are better termed ‘‘rare’’ or ‘‘novel’’, rather
than ‘‘specialized’’ or ‘‘specializations’’.
The use of multiple definitions in studies of
individual foraging specialization presents three main
problems. First, without unified approaches, broadscale comparative studies are hindered, especially
because different measures of individual foraging
specialization do not always correspond. Second, some
definitions conflict with the traditional view of ecological specialization as having reduced niche width. For
example, the use of a rare dietary resource or tactic or
having little dietary overlap with the population may
not reflect niche width. Third, this ambiguity has
blurred analysis of the functions and evolution of
individual foraging specialization. The inherently flexible nature of the generalization specialization continuum, however, does not preclude biologically
meaningful and consistent definitions and measurements. Perhaps a useful approach is to first consider
what characterizes a niche, and then determine how
these characteristics relate to the two concepts of
individual foraging specialization described above.
Additionally, hypothesized ultimate explanations for
individual foraging specialization will be discussed in
terms of their predicted relationships with niche
characteristics.
Measuring individual niches
At least four critical characteristics of a niche can be
identified with regard to different resource categories
(e.g. prey species): niche width, richness, evenness, and
niche overlap. For each characteristic, indices such as
those in Table 1 can be calculated for each individual
and index means can be used to describe the characteristic for the population. Niche width (sometimes
referred to as breadth) summarizes the diversity of an
individual’s niche, often in terms of the proportion of
their resources in various categories (Roughgarden 1979,
Schindler et al. 1997, Bolnick et al. 2002, Svanbäck and
Persson 2004). Individual niche width (WICsi, where s
designates Shannon index and i designates individual)
can be quantified using Roughgarden’s (1979) adaptation of the Shannon biodiversity index. Similarly, WICs/
TNWs is a categorical equivalent of WIC/TNW
described above (Bolnick et al. 2002). When measured
using the Shannon index, individual niche width is
influenced simultaneously by richness and evenness
(Magurran 2004), richness (Ri) being the number of
resource categories used and evenness (Ei) referring to
Table 1. Characteristics of individual niches for categorical resource data.
Niche characteristic
Index for individual
Incorporates
population niche?
Width
/
WICsi aj pij lnpij
No
Richness
/
Ri ji
No
Evenness
/
Overlap
Ei WICsi
lnji
1=ni
Wi(li)
/
; liPj
!nij
qj
pij
Notes
Roughgarden’s (1979:528) measure of
individual niche width for categorical
niches, from Bolnick et al. (2002)
No
Adapted from Pielou’s (1975:15) evenness
measure
Yes
Adapted from Petraitis’ W (1979) for
interspecific diet overlap by Bolnick et al.
(2002)
Individuals (i) have consumed a number of items (n) in several resource categories (j), such that: 1) ji is the number of resource
categories used by the individual i; 2) nij is the number of items in the jth resource category for individual i; 3) ni is the number of
items for individual i; 4) pij is the proportion of items in the jth resource category for individual i; and 5) qj is the proportion of items
in the jth resource category for the population (see Bolnick et al. 2002 for additional details).
1432
the relative use of each resource. Evenness can be
calculated using Pielou’s (1975) evenness index adapted
for individual niche width. Because niche width (as
measured using WICsi) is maximized (for a given
number of resource categories) when all categories are
used equally, a comparison of the observed width to the
maximized width provides an index of evenness (Pielou
1975). Richness and evenness correlate positively with
niche width, such that width increases with increasing
numbers of resources used and is maximized where all
resources are used equally. An individual with higher
richness and lower evenness than another individual may
have higher or lower niche width, depending on the
strength of the differences, although the Shannon index
is generally influenced more heavily by richness
(Magurran 2004). Individual niches may be evaluated
also in terms of deviation from population resource
values (Bolnick et al. 2002), which I refer to as niche
overlap (Wi). Bolnick and colleagues (2002) provide an
adaptation of Petraitis’ W, originally calculated for
interspecific diet overlap (Petraitis 1979), to calculate
intraspecific overlap. Unlike the other niche characteristics, the definition and calculation of niche overlap
incorporate the resource use of the population. The
lower the Wi value, the less an individual overlaps with
the population’s resource use.
Several further points detailing the definition of
individual specialization are worth considering. While
biologists typically measure realized niches when studying individual specialization in nature, consideration of
both realized and fundamental niches may be required
to understand the causes of individual specialization
(Bolnick et al. 2003). For example, individuals may
have different fundamental niches (resulting in different
realized niches) due to phenotypic differences. Alternatively, individuals may have identical fundamental
niches, but differences in ability to exploit that niche, as
in interference competition, could generate individual
differences. Long-term consistency is another important
component of niche use (Schindler et al. 1997, Bolnick
et al. 2002), and can be described in terms of niche
characteristics by comparing their values over time.
Additionally, depending on the niche that is being
measured, individual foraging specializations may be
defined in terms of dietary species or sizes, behavioral
tactics, and potentially any other foraging niche. As
such, they are not restricted solely to differences in prey
species. Further, ecological specialization is niche
specific; an individual could be a generalist in terms
of prey species but be more specialized in prey size, for
example. Similarly, the degree to which an individual is
an ecological specialist in terms of niche width may not
address morphological or evolutionary ‘‘specializations’’, which are not measured in terms of ecological
niches (Ferry-Graham et al. 2002). Finally, the calculations presented here refer to cases in which the niche
axis is categorical and would not be applicable to a
continuous niche axis (e.g. prey size) (but see Bolnick
et al. 2002 for an example of an index for continuous
niches). Additionally, several other indices may be used
to measure niche components, each with different
strengths and weaknesses (Levins 1968, Bolnick et al.
2002, Magurran 2004). The indices presented here
were selected due to their ease of calculation, common
usage, and ability to estimate desired niche properties,
but have some weaknesses. For example, WICsi is zero
for monophagous individuals and can artificially depress population WICs values.
Often the niche characteristics presented above are
positively correlated, but this does not have to be the
case, particularly when niche overlap is compared to
niche width or richness. For example, consider a
hypothetical population (Table 2) in which most
individuals (A D) utilize only one resource, thereby
having a niche width (WICsi) of 0. However, individual
E has begun to use two additional resources; this
individual has a calculated niche width of 1.10. Thus,
individual E uses two rare resources and has a wider
niche than the other members of the population. When
niche overlap (Wi) is calculated in terms of each
individual’s overlap with population diet proportions,
individual E exhibits the least niche overlap as a result
of its divergence from others. According to the niche
width concept of individual foraging specialization, E is
the least specialized in the population. Individual E is
the most specialized, on the other hand, when viewed in
terms of the niche overlap concept. Clearly, the niche
width concept and the niche overlap concept can
provide conflicting results regarding who is most
specialized.
Table 2. Niche width compared to niche overlap.
Individual
A
B
C
D
E
a
Prey type 1
Prey type 2
Prey type 3
Ri
Eai
WICsi
Wi
90
90
90
90
30
0
0
0
0
30
0
0
0
0
30
1
1
1
1
3
.
.
.
.
1
0
0
0
0
1.10
0.87
0.87
0.87
0.87
0.47
Evenness cannot be calculated for monophagous individuals.
1433
As the previous example illustrates, use of a rare
resource does not necessarily reflect niche width. A
similar exercise reveals that defining individual foraging
specialization in terms of the use of a rare resource may
not match the outcome of the niche overlap concept
(Table 3). Although individual A is the only individual
using prey type 1, according to the niche width concept,
B has the narrowest niche and would be the most
individually specialized forager. But because all individuals except B use prey type 3 and B has an unusually
high diet proportion of prey type 2, B shows the least
overlap with the population’s niche. In this case,
individual B is most specialized in terms of both the
niche width concept and the niche overlap concept,
even though only A uses prey type 1.
Admittedly, these examples have been constructed to
illustrate the potential conflicts among these concepts of
individual foraging specialization, and the frequency of
such conflicts in biological populations is unclear.
Despite numerous studies documenting individual
differences, relatively few explicitly measure niche use
in terms of the characteristics described here. Yet it is
not hard to imagine a scenario in which such conflicts
would occur. For example, consider a case of niche
expansion in response to high intraspecific competition
(Bolnick 2001 for an experimental example), where (at
least at the start of expansion) a few individuals could
use a wide range of poorer quality resources while the
majority of individuals use a small range of high quality
resources. Such niche width expansion incorporating
lower quality resources has been predicted to occur in
response to competition (Roughgarden 1972, Wilson
and Turelli 1986). Additionally, niche width has been
shown to be uncorrelated with niche overlap in wild
Indian Ocean bottlenose dolphins (Tursiops sp.)
(Sargeant 2005). Clearly, more studies are needed to
identify the frequency with which these patterns occur.
Historically, ecological specialization has generally
referred to niche width, with specialists having narrower
niches than either the available niche or the population
niche (Levins 1968, Fox and Morrow 1981, Futuyma
and Moreno 1988, Jaenike 1990, Bolnick et al. 2003).
Importantly, many studies do not clearly define
individual specialization or distinguish findings based
on niche width and niche overlap. In many cases, even
when individual specialization is explicitly defined using
overlap measures, ‘‘niche width’’, ‘‘niche breadth’’,
‘‘variation’’, ‘‘overlap’’, and ‘‘specialization’’ appear
interchangeably throughout the text. For example, in
some cases both individual calculations of niche width
and niche overlap have been considered to ‘‘estimate
individual specialization for each individual’’ (Bolnick
et al. 2002). The measures of population level incidence
of individual specialization used by Bolnick and
colleagues (2002) (including WICs/TNWs and W
(mean of the Wi values)), however, may have greater
agreement than similar measures for individuals. Indeed, they found high correlation between WICs/
TNWs and W using simulated data sets.
Given the historical framework of ecological specialization, ‘‘individual foraging specialization’’ should be
defined as the use of a narrower niche than the
population niche and should be measured on a
continuum of width. WICs/TNWs can be calculated
to assess how much of the population’s niche width is a
reflection of variation within or between individuals,
and WICsi/TNWs can be used to calculate a measure of
specialization for each individual. Because TNWs is
equal for all individuals in the same population, this is
the same as comparing WICs values directly when
examining a single population. Therefore, individuals
with lower niche widths are relatively more specialized
than individuals with higher niche widths within the
same population. While niche overlap should not be
equated with measures of specialization, it does provide
useful information regarding individual patterns of
niche use and has important implications when individual foraging specializations result from density- and
frequency-dependent fitness effects. Therefore, to understand the nature and evolution of individual foraging
specialization, niche overlap should also be measured.
For the rest of this paper, ‘‘individual foraging specialists’’ will refer to individuals with relatively narrow
niche widths compared to the population.
Functions of individual foraging
specialization
Discriminating between niche overlap and niche width
is more than just a semantic argument. Such clarification
Table 3. Use of rare resources compared to niche width and niche overlap.
Individual
A
B
C
D
E
a
Prey type 1
Prey type 2
Prey type 3
Ri
Eai
WICsi
Wi
30
0
0
0
0
30
90
45
45
45
30
0
45
45
45
3
1
2
2
2
1
.
1
1
1
1.10
0
0.69
0.69
0.69
0.72
0.57
0.91
0.91
0.91
Evenness cannot be calculated for monophagous individuals.
1434
should illuminate discussion of the functions of individual foraging specialization. Proximate explanations
for individual foraging specialization include differences
in genotype, phenotype, environment, individual learning, or social learning, or combinations of these factors
(Partridge and Green 1985). These address how
behaviors develop through genetic, environmental,
sensory, and/or hormonal pathways at a mechanistic
level. At the ultimate level, which addresses how a
behavior has evolved, tradeoffs related to phenotypic
differences, habitat heterogeneity, or other characteristics, and intraspecific competition have been proposed
to explain the function and evolution of individual
foraging specialization (Partridge and Green 1985).
Individuals may differ in phenotype, such that diets or
tactics provide different energy intake rates depending
on these phenotypes (Roughgarden 1974, Price 1987).
A second evolutionary explanation is that individual
resource use is limited by access to resources,
but individuals have different local environments (Holbrook and Schmitt 1992). When individuals are
restricted in their habitat use (by costs of travel,
territoriality, etc.), they experience tradeoffs between
using one habitat and another. In this case, patchy
habitats could drive population variation in resource use
by providing different prey types or physical substrates,
such that use of particular resources or tactics provides
higher foraging success. Phenotypic and habitat tradeoffs have been commonly discussed, but use of multiple
resources can result in tradeoffs with foraging efficiency
through additional mechanisms (reviewed by Bolnick
et al. 2003). Individuals in this scenario benefit by
having a low niche width that affords decreased handling
time, increased digestive efficiency, or other benefits, but
individuals differ from conspecifics due to differences in
ecology, phenotype, or learning (Werner et al. 1981).
Tradeoffs may be critical to individual specialization, as
models predict individuals should expand their niches to
match the population niche unless prevented by some
cost(s) (Taper and Case 1985, Ackermann and Doebeli
2004). Finally, frequency-dependent selection could
also be occurring, with the best strategy depending on
the strategies used by others when there is intense
intraspecific competition (McLaughlin et al. 1999,
Bolnick 2001, McLaughlin 2001, Estes et al. 2003,
Svanbäck and Persson 2004).
While all of these hypotheses predict individual
differences, they do not always predict who will be most
specialized in terms of niche width. For an individual,
hypotheses regarding tradeoffs generally predict decreasing niche width with increasing constraints or in
some cases increasing fitness (if reducing niche width
increases energy intake). Because niche overlap is
influenced by what the rest of the population is doing,
however, a theoretical prediction of the relationship
between tradeoffs experienced by an individual and
their niche overlap is unclear. Unlike ultimate hypotheses for the adaptive function of individual foraging
specialization based on tradeoffs, which offer no a priori
predictions about niche overlap, competition could
result in a negative relationship between fitness and
overlap because initially uncommon patterns of niche
use should provide decreased competition. Applying the
theory of ideal free distributions (Fretwell and Lucas
1970) to inter-individual niche overlap, individuals
may eventually have equal fitness at equilibrium if they
are flexible in their behavior (Partridge and Green
1985, Estes et al. 2003). At equilibrium, no correlation
would be expected between niche overlap and fitness.
Thus, density- and frequency-dependent fitness benefits
associated with use of different resources may favor
individual foraging specialization secondarily and generate predicted correlations between foraging success
and niche overlap, but will not generate predictions
about how specialization in terms of niche width will
relate to foraging success. Svanbäck and Bolnick (2005)
used optimal diet theory to provide a framework for
how, once individuals experience phenotypic tradeoffs,
high population density can promote inter-individual
diet variation.
In short, ultimate explanations for the evolution of
the individual differences underlying individual specialization may be divided into those that reduce intraindividual variation (reduce niche width) and those
that increase inter-individual variation (reduce niche
overlap). Tradeoffs or interactions with conspecifics
(e.g. social hierarchies) may cause individuals to
specialize by generating different optima for different
individuals and favor reduced niche width, while high
competition resulting in frequency dependence can
favor individuals with low niche overlap. Both
mechanisms may occur, and in particular the effects
of competition may depend heavily on the strength of
competition and the intensity of tradeoffs and differences in prey rankings (Svanbäck and Bolnick 2005).
The above examples are simplified cases, and multiple
factors may actually drive niche diversification. Nonetheless, discussions regarding the function and evolution of individual foraging specialization should be
explicit about how the factors driving niche diversification and population niche expansion are predicted
to influence particular niche characteristics. Clearly,
hypotheses regarding tradeoffs are relevant to niche
width, whereas those regarding competition are more
closely tied to niche overlap.
Conclusions
It is now evident that individual foraging specialization
is frequent and has important ecological, evolutionary,
and conservation consequences (Bolnick et al. 2003).
1435
Particularly, individual foraging specialization is relevant to recent attention to disruptive selection and
speciation (Bolnick 2004) and social learning and
animal culture (Werner and Sherry 1987, Rendell and
Whitehead 2001, Estes et al. 2003, Tinker 2004).
Given the wide applicability of individual foraging
specialization, it is vital that studies measure not only
differences among individuals, but also the degree of
specialization in populations and individuals in a
quantitative way (Bolnick et al. 2002). The niche
characteristics described here provide useful ways of
describing an individual’s niche, and niche width in
particular can be used to quantify the degree to which
an individual is specialized relative to the population
and can provide information about an individual’s prey
preferences and tradeoffs that may constrain their
resource use. Describing the use of a rare resource or
deviation from the population niche provides useful
information about individual differences and the origin
of individual foraging specialization because measures
of niche overlap can reveal how much the individual is
competing with the rest of the population for resources.
However, because niche overlap and rarity do not
necessarily correspond to individual niche width, they
should not be used to describe whether an individual is
itself a specialist. Use of multiple standardized and
quantitative methods within and among studies will
advance our understanding of the function and evolution of individual foraging specialization, providing
direction for future research.
Acknowledgements Rachel Barr, Dan Bolnick, Sasha Dall,
Larry Douglass, Janet Mann, Richard Svanbäck, Martha
Weiss and Aaron Wirsing provided helpful discussions and
comments on drafts of this manuscript.
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