Restoring Yellowstone's aspen with wolves

Wolves (Canis lupus) were reintroduced to Yellowstone National Park in 1995-1996. We present data on a recent trophic cascade involving wolves, elk (Cervus elaphus), and aspen (Populus tremuloides) in Yellowstone's northern winter range that documents the first significant growth of aspen in over half a century. Results indicate reduced browsing and increased heights of young aspen during the last 4-5 years, particularly at high predation risk sites (riparian areas with downed logs). In contrast, young aspen in upland settings generally showed continued suppression with only a slight decrease in browsing levels and only a slight increase in height. Our findings are consistent with the combined effects of a behaviorally-mediated and density-mediated trophic cascade. Results provide an improved perspective for understanding trophic dynamics and spatially variable plant community growth patterns in this recovering ecosystem. (C) 2007 Elsevier Ltd. All rights reserved.