Three decades of declining natural foods alters bottom-up pressures on American black bears

Abstract

Wildlife management entails the manipulation of bottom-up and top-down forces that affect populations. For bear management in North America, both food availability (bottom-up) and hunting pressure (top-down) can 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 (2015–2019) availability of 18 fruits and nuts representing the main foods of American black bears (Ursus americanus) in north-central Minnesota to what was available three decades earlier (1980s) on the same study area. Timber harvesting was routine in the 1980s but less intensive in recent decades, prompting us to hypothesize that forests matured and consequently produced less fruit for bears due to increased shade. 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 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 these groups, availability of the fruiting species changed little, but the probability of forest stands producing any fruit at all in the recent time period (~40%) was nearly half what it was in the 1980s (~80%). At the landscape scale, we estimated a ~70% decline in fruit biomass. Our hypothesis that this decline was due to reduced timber harvesting and thus denser canopy was only partially correct, as we observed the same decline in fruit production within forests of the same type and canopy closure, and also along edges of stands with high light penetration. Decline in fruit availability was due more to a complete lack of production in some stands of the same type and age rather than to general forest maturation. We explored three alternate hypotheses to explain this decline: (1) change in weather (which could affect fruiting directly, or cause issues with pollination), (2) increase in invasive earthworms, and (3) increased deer browsing. None of these were completely consistent with our findings, although it appeared from the extreme spatial variability in fruit production that on-the-ground factors were more of a driving force than weather. The magnitude and complexity of this change in bottom-up forces is a serious issue for bear management.