College of Forestry

Global Trophic Cascades Program

Wolves enhance biodiversity in Yellowstone, report says.


Date: Wednesday, October 29, 2003

RICHARD L. HILL - The Oregonian

Summary: Change in elk behavior leads to cottonwood growth, which benefits fish and birds, OSU researchers find

The return of wolves to Yellowstone National Park appears to be boosting biological diversity around streams and helping nearly extinct stands of cottonwood trees flourish again, say two Oregon researchers.

William J. Ripple and Robert L. Beschta of Oregon State University say the reintroduction of wolves to the park has helped change elk behavior, which in turn benefits the vegetation. The elk, while browsing on young cottonwoods and willows, now avoid high-risk areas such as streambeds.

"The willow and cottonwood are growing on sites that have poor visibility for the elk or have poor escape terrain," said Ripple, a professor of forestry and director of OSU's Environmental Remote Sensing Applications Laboratory. "If elk felt like they're going to be cornered by a wolf pack without good escape options, they may not go into that area and browse the plants." Ripple and Beschta reported their findings in the journal Forest Ecology and Management, and Beschta described his research in the journal Ecological Applications

Beschta, professor emeritus of forestry, said they found thousands of cottonwood seedlings in one part of the elks' winter range along the Lamar River. He said there should have been young trees, but none were there because long-term elk browsing had prevented seedlings from getting taller. They found a pattern of small seedlings along with large cottonwood trees about 70 years old, with little or no sizes in between.

Wolves were killed off in the Yellowstone area by the 1920s, allowing elk to browse anywhere without fear of the predators.

The research found that streamside cottonwoods and other vegetation has started to grow more abundant and taller after the reintroduction of the wolf to the park in 1995. About 200 wolves are now in the Yellowstone area.

In their report in Forest Ecology and Management, the scientists noted when they compared woody plant heights shown in photographs taken before 1998 with those shown in 2001-02 photos, they found an increase in the height of riparian woody plants for six of eight sites along Soda Butte Creek and the Lamar River in the northeastern part of the park.

Ripple and Beschta said their evidence illustrates how the beneficial effects of a top predator can ripple through an ecosystem.

"Once the shrubs start growing along the stream bank, they will decrease the erosion and add cooling shade over the stream, which fish like," Ripple said. "So there's better fish habitat and more birds."

They also found a wintering beaver colony had become established on Soda Butte Creek, a rare occurrence in the past few decades.

Ripple and Eric Larsen, a researcher formerly at OSU and now at the University of Wisconsin at Stevens Point, reported three years ago in the journal Biological Conservation that the decline of aspen groves in Yellowstone and other areas of the Rocky Mountains may be at least partly because of the absence of wolves.

Using historic documents, aerial photographs and tree-ring dating techniques, Ripple and Larsen determined that aspen growth in Yellowstone vanished almost as soon as the wolves had been exterminated 70 years ago.

Ripple said he plans to investigate whether aspens may be doing better in the park similar to the cottonwoods and willows.