Joshua S. Halofsky
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Can wolves help restore aspen?
Joshua S. Halofsky, Washington State Department of Natural Resources, Olympia, WA, William J. Ripple, Robert L. Beschta, Jeff P. Hollenbeck, and Cristina Eisenberg, Oregon State University, Corvallis, OR
Trophic cascades theory states indirect influences of apex predators, such as gray wolves (Canis lupus), can be transmitted beyond their immediate prey base to lower trophic levels. Implied in this theory is that such predators may be needed to maintain biodiversity. The extirpation of the gray wolves from Yellowstone National Park by 1930 and their reintroduction in 1995-96 has facilitated the study of a terrestrial trophic cascade involving carnivore absence and subsequent presence at broad geographic and long temporal scales, appropriate for studying the responses of organisms such as elk (Cervus elaphus) and aspen (Populus tremuloides). This “natural experiment” has been the impetus for trophic cascade research in the northern Rockies, and for our Trophic Cascades Program (www.cof.orst.edu/cascades). This presentation will review our work documenting changes in aspen stand dynamics before wolf extirpation, during wolf absence, and following wolf reintroduction. We explore interactions between herbivory and fire in burned and unburned areas in predator absence and presence, and examine how wolves may not only influence aspen stand dynamics by directly reducing elk densities, but also through changes in elk behavior following wolf reintroduction.
While factors such as fire, climate, and natural stand succession can influence aspen, the correlative results from our work support the hypothesis of a trophic cascade between wolves, elk, and aspen. Whether the aspen responses we have observed thusfar following wolf reintroduction are the first indicators of a changing ecosystem due to renewed wolf presence, or an exception to a broader decline in aspen numbers, is still an unknown. However, given the general support for trophic cascade theory, and the changes reported in the Greater Yellowstone Ecosystem, we believe that predators such as wolves can play a role in the stand dynamics of woody deciduous species such as aspen.