ABSTRACT
The effects of climate and plant phenological changes on herbivorous
species are widely recognized, yet less research has focused on
omnivorous and carnivorous species, even though they also have
vegetative components to their diet. The historical focus on predators
regarding simple interactions between obligate carnivores and their prey
over-simplifies many species’ roles within ecological communities by
casting them purely as predators and minimizes other, equally important
roles within the community. We used a long-term, individual-based data
set on a polyphagous mammal, the brown bear (Ursus arctos ), to
estimate diet over 25 years identify long-term patterns and factors
contributing to annual variation in diet. We used carbon and nitrogen
stable isotope values measured in hair and Bayesian mixing models to
determine annual diet among three demographic bear classes, and then
used linear regression models to relate diet to indices of food
availability. We found that while diet varied among years and
demographic classes, variation in both carbon and nitrogen values were
explained by bilberry (Vaccinium myrtillus ) productivity.
Additionally, proportions of animal-derived foods decreased through
time, while proportions of bilberry increased, even as the moose
population in Sweden increased over this same period. While meat and
animal-derived foods are considered higher quality foods for bears, bear
diet did not respond to changing moose availability. Our results
highlight that even though vegetative diet components in predators are
typically considered less important to predator ecology, brown bears in
Sweden responded to changes in primary production, regardless of prey
availability. It will be crucial to put more emphasis on the vegetative
parts of diets as we predict how species and ecological communities
respond to climate change because predators serve many more functions
within their community besides predation alone.
Keywords: brown bear, carnivore, diet, omnivory, primary
productivity, stable isotopes, Ursus arctos , Vaccinium .