Assimilated diet patterns of American black bears in the Sierra Nevada and western Great Basin, Nevada, USA

Abstract

Abstract: In western Nevada, USA, the American black bear (Ursus americanus) coexists with humans and increasing urban sprawl. Hotels, casinos, restaurants, and homeowners dispose large quantities of high-protein, calorie-rich foods, often in unsecured waste containers. We used 173 hair samples from black bears captured in western Nevada from 2003 to 2010 and conducted δ13C and δ15N analysis to examine anthropogenic food use. We developed a set of a priori models to examine the effect of biological (sex, age class, mass category [considering sex and age]), chronological (season, molt phase, and year), and spatial (urban–wildland classification [UW class]) factors potentially affecting use of anthropogenic foods and accompanying stable isotope signatures. Bears in above-average mass categories had enriched 13C and 15N signatures compared with bears of below-average mass. Wildland bears had depleted 13C compared with urban bears and appeared to use human foods less. Postmolt hair samples (representing late-spring–early summer diet) were depleted in both 13C and 15N relative to premolt hairs (late-summer–autumn diet), indicating changes in food availability. Male black bears had enriched 15N compared with females, indicating more meat in their diet. Our results indicated substantial 13C and 15N enrichment of black bear diets in Nevada, which was affected by biological, chronological, and spatial factors. Using mixing models of the 2 isotopes, we found both urban and wildland bears relied on natural and anthropogenic foods, with wildland bears using wild foods more often. There was only 3.8% difference in the median use of human foods between urban and wildland bears, but great variability for individual bears in each location category. Our results affirmed that, to effectively address human–bear conflicts, officials should emphasize exclusion of anthropogenic food attractants on a year-round basis and further try to understand factors affecting individual bear use of garbage.