Trophic Relations of Brown and Black Bears in Several Western North American Ecosystems

Abstract

We examined the historical and current diets of brown bears (Ursus arctos) and black bears (U. americanus) in the Greater Yellowstone Ecosystem, Glacier National Park and immediately adjacent areas of national forests, Cabinet-Yaak mountains of northwestern Montana and northern Idaho, Blackfeet and Flathead Indian reservations east and south of Glacier National Park, the Kenai Peninsula of Alaska, and the southwestern states of Colorado, New Mexico, and Arizona. Dietary estimates are an essential first step in understanding variation in productivity and density of current populations and, therefore, predicting success of reintroduced populations. Hair or bone samples from 14 federal or state agencies, museums, and universities were examined via stable isotope analyses to quantify the importance of animal and plant resources to sympatric brown and black bears. Stable isotope analyses have numerous advantages over fecal analyses or direct observation because diets of (1) individuals and thereby specific age and sex classes within a population can be compared, and (2) long-dead bears can be compared to living bears to evaluate historical changes in ecosystems. Meat content of current brown bear diets, which varied extensively between individuals and age and sex classes in all populations, averaged 51 ± 19% (x ± SD) for subadult and adult males and females in the Greater Yellowstone Ecosystem and 11 ± 14% in Glacier National Park and Cabinet-Yaak mountains. Within these ecosystems, adult male brown bears were more carnivorous than any other age or sex class. Brown bears that used easily obtained, abundant meat sources had dietary meat contents generally ≥70%. The meat:plant ratios in the diets of historical Greater Yellowstone Ecosystem bears (1,000 YBP) and Glacier National Park bears (1908-18) were similar to ratios in current diets. Sympatric black bears in the Wyoming, Montana, and Idaho ecosystems had dietary meat:plant ratios that were not different from all brown bear age and sex classes, except adult males. Alaskan black bears made extensive use of salmon (Oncorhynchus spp.) when brown bears were rare (53 ± 28% of the diet), but did not use salmon when sympatric with abundant brown bears.