Variation in food abundance and distribution influences animal foraging behavior, but response is contingent on the amount of resource heterogeneity detected, which is consistent with environmental ‘grain’ size. Large mammals presumably perceive their surroundings at broad spatial scales, but the importance of landscape-level food resource properties for habitat use is generally less understood. We evaluated the role of heterogeneity of Canada buffaloberry Shepherdiacanadensis, defined by fruit distribution and variability in patch quality (fruit density), in grizzly bear Ursus arctos habitat selection by comparing patch- and landscape-level foraging strategies (resource use). Our objectives were to: 1) identify the spatial scale at which grizzly bears select buffaloberry fruit resources; 2) determine whether patch- or landscape-level foraging strategies explain resource use; and 3) assess the importance of resource heterogeneity in structuring habitat selection. Buffaloberry patch and landscape variables were combined with GPS radio-telemetry data from eight collared grizzly bears in west-central Alberta, Canada, to fit resource selection functions (RSFs). We found that a spatial scale of 1887 m, corresponding to an average travel distance for bears over 5.5 h, was the most supported scale for buffaloberry use. Landscape-level foraging strategies generally had more support than those of the patch-level, with spatial heterogeneity of buffaloberry patches best explaining grizzly bear selection for fruit resources. Bears selected for areas with a wider distribution of buffaloberry fruit and greater variability in patch quality, thus providing both a higher probability of shrub encounter and greater contrast between resource patches. A negative interaction between distribution and variability, however, indicated a tradeoff where use of areas with a more widespread fruit distribution decreased when variability in resource quality was high. These results demonstrate the influence of food resource heterogeneity on animal habitat use and emphasize the value in considering spatial scale in studies of animal—resource interactions.
Environmental heterogeneity influences animal behavior (Wiens and Milne 1989, Crist et al. 1992, With 1994) with selection for food resources depending partly on how the properties of those resources vary in space (Heinrich 1979). The response of an animal to variation in food abundance and distribution is contingent on the amount of resource heterogeneity that it detects in its surroundings, which is consistent with its environmental ‘grain (Levins 1968). Optimal foraging theory proposes that animals will seek to acquire food resources at the lowest energetic cost, thereby maximizing efficiency and fitness (Charnov 1976). This assumes that animals have perfect knowledge of the heterogeneity of these resources (Rapport 1991); however, this information is typically incomplete (Pyke 1984) as it is constrained by their grain size. Generally, grain size increases with body size (With 1994, Ritchie 1998, Mech and Zollner 2002), suggesting that large mammals would perceive their environment at a relatively broad spatial scale beyond that of the local patch.
As experimental scales should be dictated by the organism and phenomenon of study (Wiens et al. 1986, Addicott et al. 1987), examination of selection for food resources (Nielsen et al. 2010) should therefore not only consider local supply, but also incorporate landscape-level resource estimates to better reflect the amount of environmental heterogeneity large mammals perceive. Grizzly (brown) bears Ursus arctos are habitat generalists with a diverse, omnivorous diet (Hamer et al. 1991, Mattson et al. 1991, McLellan and Hovey 1995, Munro et al. 2006) that enables them to adjust their foraging behavior based on annual and seasonal food availability (Jonkel and Cowan 1971, Bunnell and Tait 1981, Deacy et al. 2017). Resource abundance and distribution therefore affect habitat use (Jonkel and Cowan 1971, Nielsen et al. 2004a, 2010), and accounting for this heterogeneity by acknowledging their grain size could provide valuable insight into the landscape factors most relevant for habitat selection.
Resource selection functions (RSFs) are statistical tools for evaluating animal habitat selection that estimate relative probability of use given particular environmental site characteristics (Manly et al. 2002). RSF models that have been developed for grizzly bears previously (Nielsen et al. 2002, Moe et al. 2007, Goldstein et al. 2010, Peters et al. 2015) have mainly evaluated the effects of environmental variables, such as habitat cover type and elevation, measured at the local patch-level. Questions of spatial scale, which are essential for testing the influence of environmental heterogeneity, have been largely overlooked in the context of grizzly bear habitat selection, although these have been investigated for other animals such as ungulates (Boyce et al. 2003, Anderson et al. 2005). The few studies that have directly considered scale have focused on the extent of the landscape available for bear use (Nielsen et al. 2004a, Ciarniello et al. 2007), rather than the spatial scale at which properties of the resource units themselves were estimated. Grizzly bear habitat selection models also seldom incorporate food resource attributes as explanatory variables, despite food-probability models often explaining bear selection more effectively than those that are habitat-oriented (Nielsen et al. 2003). Buffaloberry occurrence in particular, along with that of a few other key food items, significantly predicts bear foraging activity (Nielsen et al. 2010). Spatial variation of food resources appears to be more influential for grizzly bear habitat use than temporal availability (Mangipane et al. 2018); however, the importance of food resource heterogeneity relative to other food properties has not been explored, although it has been demonstrated to strongly affect grizzly bear foraging behavior (Searle et al. 2006).
Canada buffaloberry Shepherdia canadensis (L.) Nutt. is one of the primary fruit resources for grizzly bears in the Canadian Rocky Mountains, where it comprises a major component of their summer and early fall diet (Munro et al. 2006). The fruit is especially crucial during hyperphagia, when bears increase their food consumption to build body fat reserves in preparation for winter denning (Nelson 1980). Grizzly bear selection for buffaloberry has been assessed using predictions of shrub occurrence (Nielsen et al. 2003, 2010), but given the dioecious habit of this species and that only female plants bear fruit, occurrence does not correspond t