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

Spencer J. Rettler, Andrew N. Tri, Véronique St-Louis, James D. Forester, David L. Garshelis

Research output: Contribution to journalArticlepeer-review

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.

Original languageEnglish (US)
Article number119267
JournalForest Ecology and Management
Volume493
DOIs
StatePublished - Aug 1 2021

Bibliographical note

Funding Information:
This work was supported by the Wildlife Restoration (Pittman-Robertson) Program grant [W-68-D-15, 2015–2016 & W-71-R-7, 2017-2019]. Rettler received additional funding from the Department of Fisheries, Wildlife, and Conservation Biology at the University of Minnesota – Twin Cities and Forester was supported by the MN Agricultural Experiment Station [MIN-41-020].

Funding Information:
Thank you to Daniel Dewey, Chuck Fortier, Sean Konkolics, Michael McMahon, Andrew Michel, and Stefan Nelson for your invaluable assistance in the field. We would also like to thank the University of Minnesota ? Twin Cities Fisheries and Wildlife undergraduate club for their help entering data, the staff at the Marcell Experimental Forest for providing housing during field seasons, the U.S. Forest Service employees of the Chippewa National Forest for sharing spatial forestry data, and John Latimer and Dallas Hudson for sharing their meticulously collected plant phenology data. Thank you to the two anonymous reviewers for their comments in an earlier version of the manuscript. Finally, we would like to thank Karen Noyce and Pam Coy for their shared knowledge from the initial study that inspired this research project. This work was supported by the Wildlife Restoration (Pittman-Robertson) Program grant [W-68-D-15, 2015?2016 & W-71-R-7, 2017-2019]. Rettler received additional funding from the Department of Fisheries, Wildlife, and Conservation Biology at the University of Minnesota ? Twin Cities and Forester was supported by the MN Agricultural Experiment Station [MIN-41-020].

Publisher Copyright:
© 2021 Elsevier B.V.

Keywords

  • American black bears
  • Food availability
  • Forest composition
  • Fruit production
  • Timber harvest

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