Temporal variation in translocated Isle Royale wolf diet reflects
optimal foraging.
Adia R. Sovie1, Mark C. Romanski2,
Elizabeth K. Orning3, David G.
Marneweck,3,4, Rachel Nichols3, Seth
Moore5, Jerrold L. Belant1
1 Department of Fisheries and Wildlife, Michigan State
University, 480 Wilson Road, East Lansing, MI 48824, USA
2 National Park Service, 800 E Lakeshore Drive, Isle
Royale National Park, Houghton, MI 49931 USA
3 State University of New York, College of
Environmental Science and Forestry. 1 Forest Drive, Syracuse, NY 13210
4 Conservation Alpha, Johannesburg, South Africa
5Department of Biology and Environment, Grand Portage
Band of Lake Superior Chippewa, 27 Store Rd, Grand Portage, MN 55605 USA
Abstract
Wolves (Canis lupus ) can exert top-down pressure and shape
ecological communities through selective predation of ungulates and
beavers (Castor canadensis ). Considering their ability to shape
communities through predation, understanding wolf foraging decisions is
critical to predicting their ecosystem level effects. Specifically, if
wolves are optimal foragers, consumers that optimize tradeoffs between
cost and benefits of prey acquisition, changes in these factors may lead
to prey switching or negative-density dependent selection with potential
consequences for community stability. For wolves, factors affecting cost
and benefits include prey vulnerability, risk, reward, and availability
which can vary temporally. We described wolf diet in by frequency of
occurrence and percent biomass and characterized diet in relation to
optimal foraging using prey remains found in wolf scats on Isle Royale
National Park, Michigan, USA during May–October 2019–2020. We used
logistic regression to estimate prey consumption over time. We predicted
prey with temporal variation in cost (vulnerability and/or availability)
such as adult and calf moose (Alces alces ) and beaver to vary in
wolves’ diet. We analyzed 206 scats and identified 62% of remains as
beaver, 26% as and moose, and 12% as other (birds, smaller mammals,
and wolves). Adult moose were more likely to occur in wolf scat in May,
when moose are in poor condition following winter. Similarly, the
occurrence of moose calves peaked June–mid July following parturition
but before their vulnerability declined as they matured. In contrast,
beaver occurrence in wolf scat did not change over time, possibly
reflecting the importance of low handling cost prey items for recently
introduced lone or paired wolves. Our results demonstrate that wolf diet
is plastic and responsive to temporal changes in prey acquisition cost
as predicted by optimal foraging theory. Temporal fluctuation in diet
may influence wolves’ ecological role if prey respond to increased
predation risk by altering their foraging or breeding behavior.
Keywords: Optimal foraging, Isle Royale, beaver, moose, wolf, diet
Introduction
Predation can shape ecosystems through controlling prey population
abundance and influencing where prey forage, resulting in landscape
scale changes to vegetation and nutrient flows (Terborgh et al. ,
2001; Halpern et al. , 2006; Estes et al. , 2011).
Specifically, wolf (Canis lupus ) predation can produce strong
top-down structuring in many systems (Bump et al., 2009; Ripple
and Beschta, 2012). While wolf predation can influence prey activity
(Suraci et al. , 2016; Kohl et al 2018; Gaynor et al. ,
2019), it is less clear if wolves are optimal foragers or if prey
demography and behavior can drive wolf foraging decisions (Abrams, 1992;
Katz et al. , 2015). Under optimal foraging theory, consumers
optimize caloric intake by balancing the calories gained from a food
item and the caloric and safety costs of searching for and handling the
item (Pyke et al. , 1977). For wolves, the caloric benefits of
prey items are relatively stable while the costs associated with
acquisition [search time, subjugation difficulty, and risk of
injury] change temporally. If wolves are optimal foragers, foraging
patterns may change over time, resulting in prey switching with the
potential for cascading ecological effects (Garrott et al. , 2007;
Latham et al. , 2013; Basille et al. , 2013).
For wolves, the cost of prey acquisition includes search and handling
time as well as injury risk, factors that can change over weeks or
months (Griffiths, 1980; Mukherjee and Heithaus, 2013). Predators often
target abundant prey to reduce the caloric costs associated with search
time (Griffiths, 1980). The availability of prey can change temporally
as species migrate into or out of a feeding range, emerge from
hibernation or torpor, or following a birth pulse (Metz et al. ,
2012; Petroelje et al. , 2014). Wolves do not always
preferentially attack the most abundant prey, vulnerability and risk are
also factors driving wolf foraging decisions (Mech & Peterson 2003;
Tallian et al. , 2017; Hoy et al. , 2021). Larger animals
require greater effort and risk to subdue (Griffiths 1980; MacNultyet al. , 2009). In contrast, smaller animals or prey in poor
condition exhaust quicker, require less effort to subdue, and are less
likely to inflict injuries (MacNulty et al. , 2009; Krumm et
al. , 2010). Prey risk and vulnerability change temporally (Metz et al
2012). For example, ungulates are often nutritionally deficient after
winter (Huggard 1993; Kautz et al. , 2020) and young animals
develop the speed and endurance necessary to evade capture as they
mature (Severud et al. , 2019). If wolves are optimal foragers
these temporal shifts in prey demography and behavior may alter wolf
diet.
Recently reintroduced wolves on Isle Royale National Park (IRNP),
Michigan, USA provide a unique opportunity to investigate wolf diet
relative to optimal foraging theory. Wolves on IRNP prey on a limited
number of species that predictably vary in caloric value, availability,
and vulnerability, allowing us to evaluate hypotheses of optimal
foraging and prey switching. During the ice-free season (April–October)
wolves in IRNP historically preyed on moose and beavers but also
consumed snowshoe hare (Lepus americanus) , small mammals, and
birds (Thurber and Peterson, 1993). Wolves in IRNP consume 4-7kg/wolf of
biomass a day (Thurber and Peterson, 1993), and should select for large
prey to maximize caloric intake (Carbone et al. , 1999). Adult
moose (263 kg) are a high calorie prey item but require considerable
effort to locate and subdue (Sand et al. , 2005). Adult moose are
most vulnerable in May due to winter malnutrition but regain fat
reserves after green-up in June and July (Parker, 2003; Tischleret al. , 2019). In contrast, moose calves (45kg) are not as
calorically rewarding as adults but require less handling time and risk
(Hoy et al. , 2021). Calves are unavailable until parturition in
late May and are guarded by their mothers for the first few weeks of
their lives (Edwards, 1983; Stephens and Peterson, 1984). Moose calves
are most vulnerable to wolf attack in late June and into July, as they
spend less time with their mothers, but are capable of outrunning wolves
by September (Severud et al. , 2019). Beavers (12 kg) are a low
risk and abundant food source on IRNP during the ice-free season. In May
following ice-out and in September before ponds freeze, beavers are
vulnerable to wolf predation when they forage on land to repair lodges
or restore food caches (Gable et al. , 2017).
We estimated wolf diets from scats in IRNP in relation to optimal
foraging during the ice-free period 2019–2020. We hypothesized that
wolf diets would reflect an optimal foraging strategy that tracks
temporal shifts in prey acquisitions costs. Specifically, we predicted
that due to their high caloric value moose would be the primary
component of wolf diet by both frequency of occurrence and biomass in
scats. We also predicted the presence of moose in the wolf diet will be
highest in May and decline thereafter. Further, we predicted that moose
calf occurrence in scats would follow a nonlinear relationship, peaking
in late-June after parturition and declining in late summer as calves
grow and become less vulnerable to wolf predation. We predicted that
beaver occurrence in wolf diets would vary with peaks in May and
September. Finally, we predicted that other prey (e.g., snowshoe hares,
small mammals, and birds) would increase in occurrence in late summer as
moose become more difficult to kill and wolves increase their use of
alternative prey (Gable et al. , 2018).
Methods