The ecology of fear: scientists say wolves are helping restore Yellowstone's ecosystem.

For four years, they toiled in parallel wilderness laboratories a thousand miles from home, both quietly minding their own business, which is plant science. Bill Ripple and Bob Beschta were working in separate obscurity, attempting to explain the decline and rise of three key tree species in Yellowstone National Park's northern range.

The two Oregon State University scientists scarcely noticed the distant mournful howls of the gray wolf. They paid scant attention to the caravans of wolf watchers who traversed Yellowstone's remote Lamar Valley with their binoculars and video cameras. Ripple and Beschta concede they had little more than a passing interest in what park naturalist James Halfpenny calls "the greatest ecology experiment of the 20th century."

"Too busy," recalls Beschta, a professor emeritus in forest engineering whose forte is stream hydrology and riparian areas. One moment, they are immersed in the obscure study of aspen, willow and cottonwood; the next, they are suddenly, unwittingly joined at the hip boot on the national ecological stage for their landmark assertions about ... wolves.

After spending four years connecting dozens of dots, they published a study crediting Canis lupus with unilaterally beginning a dramatic restoration of the ailing Lamar River valley. And the implications, like the wolves themselves, have spread beyond the borders of Yellowstone to other regions of the Pacific Northwest, where splinter packs are eventually expected to take up residence. "To me, it's an incredible trip Bill and I have embarked on," Beschta says. "But it was not by intent. We were not going over there to show that wolves are doing incredible things."

Their unlikely journey began in the summer of 2000, when Ripple and OSU graduate student Eric Larsen first ventured to Yellowstone. They hoped to explain a 70-year gap in aspen recruitment in the northern range, a mountainous region outside the park's famed caldera and geyser basins. It wasn't long before they connected their first dots. In what Ripple and Larsen affectionately call their "'aha' moment," they noted that most aspens were either younger than 10 or older than 80.

The trees had stopped growing about the time Yellowstone's wolves were eradicated in the 1920s. Ripple and Larsen suspected that wolf predation was impacting elk, which in turn was impacting aspen growth, but they weren't sure how--and they were apprehensive about going public without more thorough knowledge.

Given the intense emotions surrounding the wolf, especially in nearby ranching and hunting communities that view the nomadic predator as little more than a frothing killer of elk, cattle and sheep, they knew their claims would elicit skepticism. "I thought, 'If this is wrong, boy is there going to be egg on my face,'" Ripple says. "But I felt strongly about it intellectually and intuitively, so we went ahead and published [in 2000]. But we put it in at the end and discussed some other potential causes."

The next summer, though, Ripple literally stumbled on what appeared to be similar phenomena with cottonwood and willow along Soda Butte Creek, his thoroughfare into the northern range's high country. He e-mailed Beschta, who had been struck by the lack of vegetation along the Lamar during his first visit in 1997. Still, Beschta had been skeptical when Larsen had attempted to correlate the park's population of some 120 to 160 wolves with aspen recruitment during a thesis defense.

"I could never figure out how wolves could eat that many elk to make a difference," he recalls. Beschta didn't get on board until he confirmed a similar age gap in riparian cottonwood and willow growth in the Lamar Valley. The broad swath of grassland bisecting the northern range, nicknamed the "Serengeti of Yellowstone" for its prolific wildlife, was conspicuously absent of middle-aged trees.

Beschta found cottonwoods dating to the time of Lewis and Clark, others growing since the Civil War and still others born in the Roaring '20s. Yet, "something shut it off past the 1920s." Beschta crunched data back in Corvallis and compared his figures to Ripple and Larsen's. "It was like, 'Holy smokes, they're right!'" Beschta says.

Yet the connecting of dots still had only just begun. Wolves had been reintroduced into the park and central Idaho in the winter of 1995 to 1996, and in 2001 both Ripple and Beschta noticed young willows and cottonwoods were beginning to reach heights unseen for many decades.

Ripple and Beschta began touting a concept called "trophic cascade," where a top predator has a profound domino effect on an entire ecosystem. In Yellowstone, they believe, the wolf's impact starts with its favorite cuisine, the elk. They believe the wolf's return has restored a long-lost "ecology of fear" in the ungulate, which instinctively recognizes that it can no longer browse in the open or as casually on aspen, willow and cottonwood. The key is not simply that elk numbers are lower. Populations were similar in the 1960s, when elk were removed or killed by the Park Service, but the saplings were still mowed down. The difference is that the elk won't stand in one place as long with wolves nearby.

Willow and cottonwood are returning in northern Yellowstone, and a ripple effect has begun. Beaver are recolonizing due to the availability of trees. New dams and lodges have re-created wetlands, which in turn are restoring habitat for native trout and songbirds. Berry bushes are also reappearing, offering hope anew for Yellowstone's island of endangered grizzly bears. The predator's future has been clouded by beetle decimation of whitebark pine and the cones it needs for hibernation sustenance.

"This ecological chain reaction is what's so amazing," Ripple says. "This is a great experiment that I feel honored to be a part of."

But skepticism abounds. Some park biologists say the return of willow, cottonwood and, to a lesser extent, aspen reflects a natural cycle. Another supposition is that recent flooding has reseeded the area with cottonwood and willow saplings. Duncan T. Patten, a biologist at Montana State University in nearby Bozeman, suggests that unusually mild recent winters have reduced snow cover and provided more grasses, the elk's food of choice.

Robert Crabtree, chief scientist at the Yellowstone Ecological Research Center in Bozeman, told a Portland, Oregon, newspaper that ecosystems are simply too complex for a single explanation as basic as ecology of fear. "Everybody's in a rush to grab the thunder and attention of wolves," Crabtree says. "Certainly, wolves are a factor, but the whole story is not being told. You can't ignore other explanations for one pet theory."

Ripple and Beschta agree that other factors play a role, but they say logic and evidence is overwhelming. They're unconvinced that climate, elk hunting or flooding were decisive.

The two scientists are now more attentive to wolves, which are seen more frequently in Yellowstone than anywhere in the world. Those distant mournful howls have entirely different meaning. "It's fun to see them at play and hunting and all aspects of wolf societies," Ripple says.

Beschta adds, "Ranchers have real concerns, and so do hunters at some level, but everything's pretty much that wolves are bad, bad, bad. Hopefully, if we've done anything at all, we've broadened the discussion on wolves to include some of these very, very important ecological components." CONTACT: Oregon State Leopold Project, www.cof.orst. edu/wolves.