GRANTS PASS, Ore. — Scientists studying the broader effects of wolf reintroduction said a growing body of evidence suggests that killing off predators such as wolves and grizzly bears in the last century started a cascade of effects that threw ecosystems out of balance.

Researchers from Oregon State University found that a thriving wolf population not only changes where and how elk browse -- it even reverberates down to the number of willows that grow next to streams.

"We are just at the very infancy of understanding the importance of these apex predators sitting at the top of the food chain affecting entire ecosystems," said forest resources professor William J. Ripple, co-author of a new study.

The research, published in the Oct. 25 issue of the journal Forest Ecology and Management, comes as states begin to wrestle with a problem they haven't faced in nearly a century: how to deal with wolves.

After wolves were reintroduced to Yellowstone National Park in 1995 and 1996, researchers noticed they were most successful bringing down elk where the prey had to deal with a change in terrain, such as crossing a stream. Elk soon learned to avoid those areas.

"When you remove the wolves, the elk are able to browse unimpeded wherever they want, as long as they want," said co-author Robert L. Beschta. "Now that the wolves are back, the ecology of fear comes into play."

Comparing old photographs and other descriptions of the area with present conditions, Ripple and Beschta found streamside vegetation sharply declined in the mid-1920s, about the time the last wolves were killed.

Vegetation along streams prevents erosion, cools the water for fish, cycles nutrients through the food web, and provides habitat for birds and amphibians.

Ripple and Beschta established a study area in 2003 in the Gallatin National Forest where Tepee Creek runs into the upper Gallatin River. Elk use the area for winter range. No livestock graze there. Three wolves from Yellowstone arrived in 1996, and the pack grew to as many as 13.

The scientists charted the growth of willows along 1.8 miles of the Gallatin and 1.8 miles of Tepee Creek, and compared growth inside and outside two fenced-off areas.

Outside the fenced-off area near the river, considered a high-risk area for wolf attack, the amount of willows eaten dropped from 92 percent in 1998 to zero percent in 2002. There was little change around the fenced-off area in an open area away from the river identified as low-risk for wolf attacks.

Jim Peek, professor emeritus of wildlife biology at the University of Idaho, said it was too early to know whether the study's findings would hold up over time, but the observations were valid.

"It's important work, because it directs our attention toward things other than the fact that predators eat prey," Peek said. He said he doubted wolves would have as much impact on elk browsing in more open country, such as central Wyoming.

Duncan T. Patten, with Montana State University, agreed that wolves have changed elk behavior, but felt reduced elk numbers, mild winters that allowed elk to eat grass instead of willows, and changes in the river from beaver dams were more likely explanations.

Source: Associated Press