Scripps Howard News Service

Agile, fast and fierce, cougars once hunted their prey throughout Yosemite National Park, but as human invasions and hunting began about 80 years ago, the predators steadily disappeared.

As a result, the entire ecology of the fabled valley was disrupted, researchers say they have discovered.

With fewer cougars to prey on them, Yosemite's mule deer multiplied and browsed on the tasty shoots of young black oak trees. The oaks disappeared, pines and firs replaced them, and even the wild evening primrose, known for its nutritious roots and its perfumed oil, grew rare among the altered plant growth in the valley's moist meadow soils.

A new study by Oregon State University scientists says the vanishing of the cougar had long-term effects on Yosemite that closely resemble the ecological impact of the disappearance of wolves in Yellowstone.

"The loss of top predators, whether it's wolves in Yellowstone or cougars in Yosemite, is having a severe and degrading impact on plant communities," says William Ripple of Oregon State's department of forest resources, the lead author of a report published online in the journal Biological Conservation.

The study by Ripple and his Oregon State colleague Robert Beschta is the first of its kind ever conducted in Yosemite. It is an effort to find evidence confirming a still-controversial theory about how the loss of a breed of predatory animal can affect an entire ecological system.

"There are interactions everywhere in nature," Ripple said Thursday. "In Yosemite, the predators like cougars are tightly linked to the herbivores, like deer. The herbivores browse on the young oak shoots and keep their numbers down so that other trees, like pines and firs, can invade. They, in turn, are linked to other plants -- all along the line."

But scientists at Yosemite, who -- like most Californians -- refer to cougars by their more common name of mountain lions, say they are not so sure that Ripple and Beschta haven't generalized too much from limited data.

"They've brought up a really complicated issue and an interesting theory, but it needs much more rigorous study," said Nikki Nicholas, the park's chief of resources management.

"A lot of it is up to speculation," added Steve Thompson, senior wildlife biologist at the park.

And Leslie S. Chow, a wildlife biologist and cougar specialist with the U.S. Geological Survey at Yosemite, said no one actually knows how many cougars -- or mountain lions -- now live in the park or ever did. A male cougar, he said, has a typical range of at least 100 square miles, and he estimates there may have been no more than 20 to 50 within the park in the 1920s, when hunters were hired to kill as many of them as possible.

Six years ago, a careful survey based on captures and DNA from the animals' scat indicated that about 25 were still in the park, he said.

"Those are the only hard numbers to be sure," Chow said.

The kind of slow chain reaction that Ripple and Beschta have studied, in which a change in one key plant or animal population can lead to major disturbances in the numbers of other species, is known as a "trophic cascade."

As one example of trophic cascade resulting from population changes in predators, the two scientists researched the effects of reintroducing wolves into Yellowstone National Park. They found that because the wolves are now preying on the Yellowstone elk, the smaller elk population is allowing riverside cottonwood trees and willows -- once nearing extinction -- to recover and bring back a variety of other plants along the many streams in the park.

The Oregon scientists say that although they have found at least preliminary confirming evidence for another kind of cascade in Yosemite, they agree that still more research is needed to be sure the "cascade" hypothesis is valid.

E-mail David Perlman at dperlman(at)sfchronicle.com.

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