Resource use decisions by consumers are a function of a dynamic spectrum of costs and rewards encountered while pursuing, handling, and consuming prey over space and time. Identifying spatial patterns of resource use for species can provide insight into ecological associations and inform management and conservation across ecological and political boundaries. The consumption of Pacific salmon (Oncorhynchus spp.) by the highly mobile grizzly bear (Ursus arctos horribilis) provides an opportunity to examine the outcome of foraging in complex geographies where these apparent trade-offs between reward and cost are prominent. High-meat (especially salmon) diets correspond with increased viability, reproductive success, and density for grizzly populations. In British Columbia (BC), watersheds that provide salmon ‘rewards’ are separated by costly complex topographies and human-caused fragmentation, which may limit access and subsequent consumption. We used hair samples and stable isotope analysis from female and male grizzly bears (n = 248, n= 340, respectively) collected from 1995 to 2014 across the 692,000 km2 of grizzly bear habitat in BC to estimate the contribution of salmon to annual diets. We examined how consumption varied by 1) region, 2) sex, 3) resource availability, and 4) natural and human-caused fragmentation.
Results/Conclusions
Estimates of salmon consumption revealed the geographic range and variability of grizzly bear diet on a western continental scale. Across BC, the distribution of salmon consumption was bimodal. Coastal bears consumed a high proportion of salmon (median = 0.63 annual diet for males and 0.57 for females), whereas in interior areas median salmon consumption rates were four times higher for males than for females (medians = 0.2 and 0.05, respectively). In the interior, where the Fraser River and associated watersheds flow far inland (>1000 km) towards more densely human-fragmented habitats, females relied on contributions from plants (median = 0.58) and terrestrial meat (median = 0.31) more than salmon, whereas males consumed more salmon than terrestrial meat (median = 0.08), with large contributions of plants (median = 0.63). To safeguard the bear-salmon predator-prey association that spans ocean, coastal, and interior regions, conservation planners and practitioners can consider managing across ecological and jurisdictional boundaries. More broadly, our landscape-scale approach highlights the importance of visualizing spatial patterns of dietary niche variation within populations to characterize ecological associations and inform management.