Trophic cascades from wolves to grizzly bears in Yellowstone
Article first published online: 4 SEP 2013
© 2013 The Authors. Journal of Animal Ecology © 2013 British Ecological Society
Journal of Animal Ecology
Volume 83, Issue 1, pages 223–233, January 2014
How to Cite
Ripple, W. J., Beschta, R. L., Fortin, J. K., Robbins, C. T. (2014), Trophic cascades from wolves to grizzly bears in Yellowstone. Journal of Animal Ecology, 83: 223–233. doi: 10.1111/1365-2656.12123
- Issue published online: 18 DEC 2013
- Article first published online: 4 SEP 2013
- Accepted manuscript online: 29 JUL 2013 12:05AM EST
- Manuscript Accepted: 16 JUL 2013
- Manuscript Received: 11 APR 2013
- L.L. Stewart Faculty Scholars award
- Oregon State University Foundation
- United States Geological Survey (USGS)
- Biological Resources Division Natural Resources Preservation Program
- Interagency Grizzly Bear Study Team of the USGS Northern Rocky Mountain Science Center
- Washington State University Bear Center
- Bear Management Office of the Yellowstone Center for Resources in Yellowstone National Park
- berry-producing shrubs;
- endangered species;
- trophic interactions;
- We explored multiple linkages among grey wolves (Canis lupus), elk (Cervus elaphus), berry-producing shrubs and grizzly bears (Ursus arctos) in Yellowstone National Park.
- We hypothesized competition between elk and grizzly bears whereby, in the absence of wolves, increases in elk numbers would increase browsing on berry-producing shrubs and decrease fruit availability to grizzly bears. After wolves were reintroduced and with a reduced elk population, we hypothesized there would be an increase in the establishment of berry-producing shrubs, such as serviceberry (Amelanchier alnifolia), which is a major berry-producing plant. We also hypothesized that the percentage fruit in the grizzly bear diet would be greater after than before wolf reintroduction.
- We compared the frequency of fruit in grizzly bear scats to elk densities prior to wolf reintroduction during a time of increasing elk densities (1968–1987). For a period after wolf reintroduction, we calculated the percentage fruit in grizzly bear scat by month based on scats collected in 2007–2009 (n = 778 scats) and compared these results to scat data collected before wolf reintroduction. Additionally, we developed an age structure for serviceberry showing the origination year of stems in a northern range study area.
- We found that over a 19-year period, the percentage frequency of fruit in the grizzly diet (6231 scats) was inversely correlated (P < 0·001) with elk population size. The average percentage fruit in grizzly bear scats was higher after wolf reintroduction in July (0·3% vs. 5·9%) and August (7·8% vs. 14·6%) than before. All measured serviceberry stems accessible to ungulates originated since wolf reintroduction, while protected serviceberry growing in a nearby ungulate exclosure originated both before and after wolf reintroduction. Moreover, in recent years, browsing of serviceberry outside of the exclosure decreased while their heights increased.
- Overall, these results are consistent with a trophic cascade involving increased predation by wolves and other large carnivores on elk, a reduced and redistributed elk population, decreased herbivory and increased production of plant-based foods that may aid threatened grizzly bears.