Browsing by Subject "American black bears"

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    American black bears: Strategies for living in a fragmented, agricultural landscape
    (2014-08) Ditmer, Mark Allan
    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.
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    Reduced natural foods alter bottom-up pressures on American black bears
    (2018-07) Rettler, Spencer
    Wildlife management is often based on the manipulation of bottom-up and top-down forces. For bear management in North America, food availability (bottom-up) and hunting pressure (top-down) are the primary factors that limit population growth. Whereas seasonal and year-to-year variation in production of fruits and nuts (i.e., primary bear foods) has been widely recognized, long-term trends in natural food production have rarely been reported. Here we compared the current (2015–2017) availability of 18 fruits and nuts, which represent the main foods of American black bears (Ursus americanus) in north-central Minnesota, to what was available during the 1980s. The study area was the same in both time frames, constituting primarily National Forest, where timber harvesting was routine in the 1980s but much less so in recent decades. We hypothesized that forests matured over the 25-year period and consequently produced less fruit for bears. Within each of 12 forest types, we measured the abundance, productivity and biomass (kg/ha) of each fruit-bearing plant species using the same methodology as used in the 1980s. Bootstrapped independent two-sample t-tests identified species-specific changes in food availability between the two sampling periods; generalized linear mixed-effects models were used to quantify changes for groups of fruit-producing species (i.e., summer/fall foods and short/tall shrubs). For all groups of species, the probability of forest stands producing any fruit at all in the recent time period was nearly half (~40%) what it was in the 1980s (~80%). At the landscape scale, we estimated a ~70% decline in biomass availability due to a reduction in young forest types that produce the most fruit, and also a ubiquitous decline in fruit production across most forest types. Our hypothesis that this decline was due to reduced timber harvesting was only partially correct, as we observed the same decline in fruit production within mature forest of the same type and canopy closure, and also along edges of stands with high light penetration. Earlier springs, with potentially greater vulnerability of flowers to frost, and an ongoing invasion of invasive earthworms, which are known to radically affect the soil, may be additional causes for diminished fruit production. Bears will likely need to alter their foraging habits to compensate for lower natural foods as the landscape continues to change. This study demonstrates the complexity of forest management and its unintended effects on wildlife.