Researchers: Willow decline is due to too few beavers

PROVO, Utah (AP) - A team of scientists believes willows have declined in Yellowstone National Park because there are too few beavers.

"We are ready to suggest that beavers are a keystone species to the riparian ecosystem of this park," said Rex Cates, a Brigham Young University chemical ecologist who led the research.

That means if Yellowstone wants to protect its remaining wet meadows, the beaver must play a role.

Cates is not yet advocating introducing more beavers to the park, but he said park officials should encourage more research on the findings, which were just published in The Wildlife Society Bulletin.

The decline of the willows is among the hottest ecological issues in the park.

"(Cates) will receive a lot of flak," said Don Despain, a former park ecologist now working for the biological division of the U.S. Geographical Survey in Bozeman, Mont.

Beavers are integral to the riparian zones because their dams increase the amount of water in the area, Cates said. Beaver ponds raise the water table, provide better sites for plant regeneration and help distribute nitrogen and phosphorus.

A persistent theory for the decline in willows, beavers and other animals and plants is that there are too many elk in the park.

Some scientists, including a contingent from Utah State University, argue that if park officials want to return Yellowstone to its natural state, they must drastically reduce the number of elk.

"The elk have destroyed the willows," said Charles Kay, a USU political science professor and critic of Yellowstone policy. "You have the worst overgrazed riparian areas in the country inside of Yellowstone National Park."

Cates said his findings show "it would be a drastic error to go in there and blow all those elk away."

Elk, whose numbers have more than quadrupled in the park since 1960, eat willows, and there seems to be little question that wintering elk in the northern part of the park have beaten down the region's once-dense stands of willows.

But healthy willows and browsers such as elk and deer have co-flourished peacefully for ages.

Cates and some other biologists believe something else is occurring in the environment to weaken the willows and make them more susceptible to elk.

Like most plants, willows have natural toxins that help ward off enemies.

When they sense they are being munched by a browser, willows will increase their production of tannins and phenolics, which are defensive chemicals that make themselves less palatable.

To test whether Yellowstone's willows are producing these chemicals adequately, Cates and his associates spent four years clipping shoots off of willow that have grown in fenced areas inaccessible to browsing animals. They compared those clipped shoots to similar willows growing in Rocky Mountain National Park in Colorado.

They found that the willows in Yellowstone did not increase their production of tannins and phenolics. In some cases, they actually decreased production of the chemicals.

The most obvious reason for the decline in the willows' health is a decrease in their suitable habitat, said Cates, noting that Yellowstone over the past century has become drier and warmer.

But "the real kicker," he said, is that there are significantly fewer beavers.

Despain said a big unknown is why the beavers' numbers have declined. One theory is that the elk herds, which burgeoned when wolves were exterminated and Indians stopped hunting in the park, consumed large amounts of willows and aspen, forcing the beavers out.