Niche separation of grizzly (Ursus arctos) and American black bears (Ursus americanus) in Yellowstone National Park

Three manuscripts constitute the following dissertation. Chapter one studied how sympatric grizzly bears and black bears in Yellowstone National Park relied on different food items to meet their energetic requirements. In particular, cutthroat trout (Oncorhynchus clarki), whitebark pine nuts (Pinus albicaulis), and elk (Cervus elaphus ) are three high quality food resources important to bears in the park and are declining. Male grizzly bears had the highest estimated dietary meat intake with female grizzly bears having intermediate intake and male black bears the lowest intake. Use of cutthroat trout was minimal, whereas elk constituted the majority of ungulates consumed. In years of good whitebark pine nut production all bears used these nuts almost exclusively. Black bears were more herbivorous than the socially dominant grizzly bear.;The second chapter evaluated whether grizzly bears and black bears were strictly nocturnal, diurnal, or crepuscular in the absence of switching caused by human influence. Although both species have shown flexibility in their activity profile in other parts of their range, black bears tend to be diurnal while grizzly bears are more often crepuscular. In Yellowstone National Park, grizzly bears were seasonally nocturnal, whereas black bears were strictly diurnal and did not temporally avoid the more socially dominant grizzly bear.;The final chapter evaluated the separation of grizzly bears and black bears spatially in their use of habitat types and areas close to humans (e.g., trails, developments, and roads). Non-forested habitat types were selected by male grizzly bears over forested habitat types, whereas black bears selected for forested habitat types. However, female grizzly bears selected both forested and non-forested habitat types in proportion to availability. Seasonal variation in the availability of preferred foods was reflected in seasonal differences in habitat selection. Despite individual variation, male grizzly bears avoided human use areas more than the more subordinate female grizzly bears and male black bears.;The analysis of dietary, spatial, and temporal separation of grizzly bears and black bears provides a more complete understanding of the niche separation between the two species. The more socially dominant male grizzly bear has higher energetic demands that requires higher dietary meat intake and therefore use of corresponding selected habitats. The more subordinate female grizzly bear and male black bear can better meet their energetic demands on lower quality food items (e.g., herbaceous material and insects). The analysis of all three variables, activity, diet, and habitat use, will hopefully help managers predict where both species and sex of bears will be depending on the season.