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