Wolf reintroduction reshapes Yellowstone ecology

Herbivores’ fear of predators influences vegetation growth, ecologists state

The 1995 reintroduction of wolves in the northern range of Yellowstone National Park has led to increased growth of willow and cottonwood in the park by causing fear responses in elk and other ungulates, according to William J. Ripple and Robert L. Beschta of Oregon State University in Corvallis. Ripple and Beschta, writing in the August 2004 issue of BioScience, argue that fear of predation when wolves are present changes grazing patterns, prompting grazers to avoid sites that deny them easy escape and to browse less thoroughly. The “terrain fear factor” consequently allows woody plants and trees to grow taller and thicker when wolves are present. This in turn has allowed beaver colonies to expand.

Ripple and Beschta base their conclusions on theory about feeding relationships between species, browsers’ risk of predation since the reintroduction of wolves into the park in 1995, and empirical research. In the view of Ripple and Beschta, extirpation of wolves in the early 20th century was “most likely the overriding cause of the precipitous decline and cessation in the recruitment of aspen, cottonwood and willow across the nothern range.”

The new theory thus brings a significant new factor into long-running, intense debates over the proper management of Yellowstone. Ripple and Beschta conclude that, for Yellowstone, “restoration goals should focus on the recovery of natural processes.” The identified fear factor might also be important in other regions, although the evidence is less well developed. Journalists may obtain copies of the article by contacting Donna Royston, AIBS communications representative.