The diet of black bears tracks the human footprint across a rapidly developing landscape

Abstract

Food subsidies have become a widely available and predictable resource in human-modified landscapes for many vertebrate species. Such resources can alter individual foraging behavior of animals, and induce population-wide changes. Yet, little consensus exists about the relative influence of the availabilities of native and human food subsidies to wildlife foraging throughout altered landscapes. We explored this unresolved question by analyzing the effects of landscape factors on American black bear (Ursus americanus) diet across the state of Colorado, USA. We estimated assimilated diet using stable isotope analysis of harvested black bear tissues to determine the contribution of human-derived foods to bear diets throughout Colorado, as well as how increasing reliance on human-derived food subsidies increases the risk of conflict. We found that bears (n=296) showed strong regional diet variability, but substantial use of human-derived food subsidies in eastern Colorado (>30% assimilated diet). The age-sex class of the bear and housing density of its harvest location were the most influential predictors of 13C enrichment (a tracer of human food subsidies). Furthermore, foraging on subsidies increased risk of conflict; the odds of being a nuisance bear increased by 60% for each ~1‰ increase in δ13C. Our study confirms the efficacy of δ13C as a proxy for human activity, and indicates that while demographic differences play a clear role in the foraging ecology of bears, availability of subsidies coincident with varying levels of human activity appears to be a major driver in predicting black bear diet throughout the western United States.