Trophic relations of brown and black bears in several western North American ecosystems

We examined the historical and current diets of brown bears (Ursus arctos) and black bears (U. americanus) in the Greater Yellowstone Ecosystem, Glaci er National Park and immediately adjacent areas of national forests, Cabine t-Yaak mountains of northwestern Montana and northern Idaho, Blackfeet and Flathead Indian reservations east and south of Glacier National Park, the K enai Peninsula of Alaska, and the southwestern states of Colorado, New Mexi co, and Arizona. Dietary estimates are an essential first step in understan ding variation in productivity and density of current populations and, ther efore, 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 diet s of (1) individuals and thereby specific age and sex classes within a popu lation can be compared, and (2) long-dead bears can be compared to living b ears 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) over bar +/- SD) f or subadult and adult males and females in the Greater Yellowstone Ecosyste m and 11 +/- 14% in Glacier National Park and Cabinet-Yaak mountains. Withi n these ecosystems, adult male brown bears were more carnivorous than any o ther age or sex class. Brown bears that used easily obtained, abundant meat sources had dietary meat contents generally greater than or equal to 70%. The meat:plant ratios in the diets of historical Greater Yellowstone Ecosys tem bears (1,000 YBP) and Glacier National Park bears (1908-18) were simila r to ratios in current diets. Sympatric black bears in the Wyoming, Montana , and Idaho ecosystems had dietary meat:plant ratios that were not differen t from all brown bear age and sex classes, except adult males. Alaskan blac k bears made extensive use of salmon (Oncorhynchus spp.) when brown bears w ere rare (53 +/- 28% of the diet), but did not use salmon when sympatric wi th abundant brown bears.