Influence of resource subsidies on black bear foraging ecology across the Colorado landscape

Background/Question/Methods

As human-modified landscapes now predominate globally, the role of these novel habitats in modifying animal behavior is increasingly important. Human-derived foods have become a widely available and predictable resource. Foraging on such subsidies can alter interspecies relationships, and understanding how resource subsidies are utilized has important ecological and conservation implications.

As one of the more rapidly expanding wildland-urban interfaces, the Colorado Front Range presents black bears (Ursus americanus) with variable levels of human use and habitat quality. Despite extensive research into bear foraging ecology, little consensus exists about the relative influence of native and human food subsidies; some suggest bears become habituated to human foods, while others indicate foraging shifts to subsidies only when native foods are poor.

We aimed to explore this unresolved question by analyzing the effects of landscape factors on bear diet across Colorado. We estimated assimilated diet using stable isotope analysis of harvested black bear tissues to determine (i) how the consumption of food subsidies varied across the state (ii) the relative importance of road density and vegetative productivity on diet and (iii) if use of food subsidies increased risk of conflict.

Results/Conclusions

Preliminary results on hunter-harvested bears (n = 296) indicate that eastern CO bears foraged on human food subsidies (> 20% of diet) more than western bears (< 10% of diet). Regional trends were consistent across summer and fall, though all bear populations shifted towards more native foods in the fall. The most influential covariates on δ¹³C enrichment (as a tracer of human food subsidies) were age-sex class and road density. Adult females were the most enriched in δ¹³C, and road density was positively related to δ¹³C signature and thus, bear reliance on food subsidies, independent of vegetative productivity. In addition, the odds of being a nuisance bear increased by 60% for each unit increase in δ¹³C, indicating that foraging on subsidies increased risk.

Our study highlights the substantial use of food subsidies on the landscape by black bears. While demographic differences play a role in the foraging ecology of bears, availability of subsidies coincident with varying levels of human use appears to be the principle driver in predicting diet of this behaviorally plastic species. Thus, food subsidies may have the potential to shift the ecological role of black bears as they preferentially utilize this novel and calorically-rich resource.