Reduced natural foods alter bottom-up pressures on American black bears

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Reduced natural foods alter bottom-up pressures on American black bears

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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.


University of Minnesota M.S. thesis. July 2018. Major: Natural Resources Science and Management. Advisor: James Forester. 1 computer file (PDF); iv, 57 pages.

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Rettler, Spencer. (2018). Reduced natural foods alter bottom-up pressures on American black bears. Retrieved from the University Digital Conservancy,

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