Introduction
Foraging decisions are essential because they directly affect survival
through nutrient and energy acquisition (Sih, 2011). In most cases,
foragers choose from a variety of food options. Many factors affect
these choices including differences in availability and accessibility.
Understanding the factors that affect these decisions provides insight
into how animals budget their time in addition to the selective
pressures which shape feeding preferences (Ydenberg, Brown, & Stephens
2007).
One way researchers document these choices is through cafeteria trials,
in which foragers are offered many food items simultaneously (Morrison
& Hik, 2008; Woods 2009; Dostaler, Ouellet, Therrien, & Côté 2011).
Choices made during these trials presumably reflect choices made during
normal foraging in the wild. Cafeteria trials allow for direct tests of
the balances between energetic costs and benefits because they allow
researchers to control some of the variables that affect foraging
decisions such as search time and relative abundance of different food
types. It is also important to consider the number of options offered in
these experiments, as foraging preferences can vary with the diversity
of options (Wang, Wang, Liu, Huang, & Hodgkinson 2011). Moreover,
cafeteria trials conducted in the laboratory can control for several
environmental variables.
A wide variety of approaches can discriminate foraging preferences from
cafeteria trials (Elston, Illius, & Gordon 1996). Analysis of
preference using pairwise comparisons can be problematic because not
only does this oversimplify foraging decisions, but the probability of
observing a spurious preference for any food type increases with the
number of comparisons (Thomas & Taylor, 1990). Moreover, ranking
foraging preferences based on pairwise comparisons overlooks the
importance of the presence of alternative food types on foraging
decisions (Woods, 2009). Other problems plague some approaches. For
example, if a forager avoids a given resource, this can cause
mathematical or computational problems because of zeros in the data
(Elston, Illius, & Gordon 1996). In addition, zeros are a problem for
statistical approaches that require chi-square tests to determine
preferences (Neu, Byers, & Peek 1974), as these tests require most
expected frequencies to be greater than one (van Emden, 2008).
Resource consumption curves provide an excellent approach to measuring
foraging preferences (Krebs, 1999). Foragers are monitored over time to
determine how much resource is consumed in the absence of other
resources. The utility of the consumption curve is dependent on the time
scale of measurement of the amount consumed. Measuring consumption too
often may interrupt or negatively affect foraging behavior, introducing
a bias into the analysis. Also, this index assumes preferences are
additive and there is no interaction between different resource types.
The preferable approach to analyzing data from cafeteria trials is to
develop an index that incorporates preferences across many food types
simultaneously. The Shannon Diversity Index accomplishes this task.
Considering each food item, the index quantifies not only the number of
food types in the diet (analogous to species richness) but also the
distribution of food items within each food type (analogous to
evenness). Species diversity is useful if foragers avoid certain food
types, but evenness is preferable if foragers sample all food types
available, and provides the most information concerning feeding
preferences. Given a starting amount of resources, a null hypothesis of
equal preference can be calculated and compared to the observed value
with a simple hypothesis test. One shortcoming of the index is that is
only indicates if a preference exists but does not allow for
identification of which food type is preferred without further testing.
It does, however, allow comparisons between groups of individuals (i.e.,
different species, males versus females, etc.) and could be applied to
any amount of n food types. Its simplicity is also a benefit.
This study assesses the efficacy of using Shannon diversity indices to
test hypotheses concerning foraging preferences. When foragers include
more resources in their diets, or increase their diet breadth, this
should be reflected by higher values of the Shannon Diversity index
(Hs ). Nuanced differences in diets can be
determined by testing evenness, or the distribution of selected food
items into different resource categories. In the case of equal
preferences for all food types, values of Shannon Evenness (J’ )
should be unaffected by diet breadth. This interpretation of Evenness
provides an excellent null model, as it assumes the complete lack of
partial preferences (Stephens & Krebs 1986), and generates a single
value to compare to actual diets. Null models demonstrated how the
indices reacted to changes in diet breadth, and field preferences were
tested using data generated from a series of cafeteria trials on common
forest granivores.
Forest granivores provide an excellent model system to address questions
about foraging decisions. Seeds are a convenient resource for
representing the variety of food items occurring in natural environments
because they are ubiquitous, easily measured, and readily manipulated.
Moreover, studying seed consumption can lead to insights about how
granivores may affect forest composition through their foraging
decisions (Meiners & Stiles, 1997; Lobo, Green, & Millar 2013).
Foragers encounter many different seed types leading to species-specific
preferences. For example, deer mice demonstrate divergent seed
preferences: the woodland deer mouse (Peromyscus maniculatus
gracilis ), shows a strong preference for red maple (Acer rubrum )
over sugar maple (A. saccharum ) seeds, while the seed preferences
of the white-footed deer mouse (P. leucopus ) are less clear
(Cramer, 2014). In the Great Lakes region, two common and well-studied
seed predators are eastern chipmunks (Tamias striatus ) and least
chipmunks (Neotamias minimus ) (Hsia & Francl, 2009; Myers,
Lundrigan, Hoffman, Haraminac, & Seto 2009; Penner & Devenport, 2011;
Jenkins & Devenport, 2014).
Ecological differences between these species should lead to differential
foraging preferences. T. striatus is a habitat generalist
(Snyder, 1982) and is more common in the rodent community than N.
minimus , a habitat specialist that prefers mesic areas (Verts &
Carraway, 2001). There is also a distinctive body size difference:N. minimus is approximately half the mass of T. striatus .
In addition, there are behavioral differences between these chipmunks:T. striatus can be active year-round and are larder hoarders,
whereas N. minimus undergoes seasonal torpor in the winter months
and stores seeds in small dispersed caches.
Regardless of these differences, studies in other systems with multiple
species of chipmunk have demonstrated prevalent interspecific
competition. This may be ameliorated in Great Lakes forests by limiting
similarity, in which foraging choices between similar species is
mediated by seed size. Granivores differentially harvest seeds of
different sizes, and the size of seed selected for is positively
correlated with body size [Brown & Lieberman, 1973; Emmons, 1980).
Small seeds may not provide enough energy for larger granivores to
survive, but smaller seeds are more accessible to smaller granivores.
Thus, it is reasonable to expect that differential seed preferences
between these species are based on seed size.
Two sets of hypotheses were generated for the null model analysis and
the field-test using actual recorded diets. For the null model, we
predicted that the value of the Shannon index would increase as more
food types were added to the diet, but that Evenness would remain
constant regardless of diet breadth. For the chipmunk model system, we
predicted that each species would demonstrate partial preferences based
on seed size: T. striatus should select larger seeds and N.
minimus select small seeds.