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

Jennifer Kay Fortin

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, whitebark pine nuts, and elk 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 while elk constituting the majority of ungulates consumed. In years of good whitebark pine nut production all bears utilized the food item 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 human influence to switch. Although both species have shown flexibility in their activity profile in other areas black bears tend to be diurnal while grizzly bears are more often crepuscular. Although grizzly bears were seasonally nocturnal, 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 habitat use and areas close to humans (e.g., trails, developments, and roads). Non-forested habitat types were preferred by male grizzly bears over forested habitat types while black bears showed the opposite preference for forested habitat types. However, female grizzly bears showed no preference for either forested or non-forested habitat types. Seasonal variation in preferred food source availability 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 preferred 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.

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

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