Response of Yellowstone grizzly bears to changes in food resources: a synthesis

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Whitebark pine is a diminished resource for grizzly bears in the Greater Yellowstone Ecosystem (GYE), but bears whose habitat overlaps with whitebark pine continue to forage on pine seeds when available. Despite evidence that the current mountain pine beetle outbreak is waning and that the effects from blister rust are limited and manageable, the long-term future of whitebark pine remains uncertain during climate change. However, evidence from the analyses presented in this report suggests that whitebark pine decline has had no profound negative effects on grizzly bears at the individual or population level. The findings of analyses presented here indicate that the Yellowstone grizzly bear population has shown notable resilience in the face of decline of whitebark pine and natural stochasticity of other food resources within the GYE. Grizzly bears obtained sufficient alternative foods through diet shifts and have maintained body mass and percent body fat over time. Extensive demographic analyses did not show a decline in population but only a slowing of population growth since the 2000s, possibly indicating the population is near carrying capacity. Evidence from demographic analyses indicates that the change in population trajectory was more associated with grizzly bear density, primarily through reduced cub survival and reproductive transition, rather than whitebark pine decline. Finally the number of fall mortalities increased during 2000–2012, but effect sizes seem small and were not apparent in survival estimates of independent-aged bears for that period. Therefore, the increase in number of mortalities is unlikely to have affected population growth.

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ELECTRONIC FILE - Zoology: Mammals

Suggested citation:
Interagency Grizzly Bear Study Team. 2013. Response of Yellowstone grizzly bears to changes in food resources: a synthesis. Report to the Interagency Grizzly Bear Committee and Yellowstone Ecosystem Subcommittee. Interagency Grizzly Bear Study Team, U.S. Geological Survey, Northern Rocky Mountain Science Center, Bozeman, Montana, USA.

SWAP (2/19/2016) citation:
[IGBST] Interagency Grizzly Bear Study Team. 2013. Response of Yellowstone grizzly bears to changes in food resources: a synthesis. Final report to the Interagency Grizzly Bear Committee and Yellowstone Ecosystem Subcommittee. Bozeman (MT): US Geological Survey, Northern Rocky Mountain Science Center. 54 p. [accessed 2016 Feb 10]. http://www.nrmsc.usgs.gov/files/norock/IGBST/IGBST_FoodSynReport120213.pdf.