American black bears: Strategies for living in a fragmented, agricultural landscape

Abstract

The American black bear (Ursus americanus) is a forest-dwelling species, but also an ecological opportunist. Few studies have investigated ecological and landscape requirements for this species in highly-fragmented habitats. Northwestern Minnesota is particularly well suited for such a study because it marks the historical western edge of black bears in eastern U.S.. This area is a patchwork of forest (<20% coverage) and agricultural lands, with contiguous forests eastward and agriculture westward. My dissertation, spurred by this intriguing increase and expansion of bears at the edge of their range, focuses broadly on two interrelated fundamental ecological questions: (1) how do bears respond to fragmentation of forested habitat, and (2) how is the edge of a bear's geographic range delimited? Results from stable isotope analysis illustrated how a small portion of the landscape used for corn and sunflower production (1-4% annually) was a major portion of some bears' diets. We found the degree of crop consumption varied with natural forage availability, demographics, size and health, space use patterns and landscape fragmentation surrounding the individual. Captive bear food preference trials revealed that male bears were much more apt to try novel, high-calorie foods, but females learned to do so after more exposure. My analysis utilizing advanced biologger technology of bears' heart rates revealed that bears were not stressed when foraging in cropfields or in small patches of forest but when crossing open fields without foods, their heart rates were typically faster than expected for their rate of travel, indicating a stress response. I found bears in this area have the largest home ranges ever recorded for the species, so I used short-term (weekly) home ranges to estimate how landscape, habitat type, caloric availability and demographics affected the amount of area a bear used at different times of year. I used results from this analysis to produce regional maps of bear habitat quality under varying natural and anthropogenic food conditions, showing the probable geographic limit of this range. It appears that for bears to expand much farther west they would need to cross a large expanse of unsuitable habitat or slacken habitat requirements.