YELLOWSTONE NATIONAL PARK, Wyo. -- Doug Smith is a tall man, well over 6 feet, but he still has to stretch if he wants to reach the top of the willows along Blacktail Deer Creek.

Nine years ago, when he first moved to this park, they barely reached his knees. Up the road a ways, beavers have built four colonies, including a beaver lodge on Soda Butte Creek, the first such structure in decades on the park's famous northern range.

Until the last couple years, there weren't enough willows around for them to eat and use in dam building.

Smith, the park's top wolf biologist, said he believes wolves have helped bring back the beaver. That's because they've made the elk more wary, forced them to spend less time in the flat creek bottoms where the willows grow and they are easier prey for wolves.

The condition of willow, aspen and cottonwood stands in the park's northern range has been a hot issue for a century.

They have been used as an indicator of range conditions and are seen as a valuable entity in their own right. Birds and amphibians rely on them. They're important for erosion control. Elk, moose and deer browse them. They're important to bugs and fish and everything that eats them.

Photographs from the 1890s show streambeds choked with willows and the reason for their decline has provoked intense debate, much of which centered around elk numbers. Until 1968, the National Park Service killed a whole bunch of elk, shooting them from helicopters, chasing them into traps and even operating a slaughterhouse for a time at the Lamar Ranger Station.

Willow thickets "are a critical community for maintaining biodiversity," said Duncan Patten, an ecologist who has studied the Northern Range for years, including a stint with a prestigious National Academy of Sciences panel of experts.

And lots of people, especially scientists, have noticed the tentative return of willows in some places.

"It's the hottest research topic in the park right now," Smith said. But not everybody is certain how big a role wolves are playing. As with most things in Yellowstone's complicated ecosystem, there's an awful lot going on and it's difficult to sort out cause and effect.

Wolves were reintroduced to Yellowstone in 1995 and 1996, a time when the northern range elk herd was at its highest ever, nearly 20,000 animals.

Willows started bouncing back the next year. But there were also record floods in 1996 and 1997, followed by a series of mild winters and dry summers, and a variety of snow conditions that left different foods available to elk at different times.

Plus, the number of moose, which eat a lot of willow, is down dramatically since the big fires of 1988.

Add in things like changing water tables and variations in spring rains and you see how complicated the situation can become.

"It's never just one thing," said Bob Crabtree, head of Yellowstone Ecological Research Center, an independent entity in Bozeman. "It's multiple factors."

Smith agrees that the research jury is still out, still pondering the full connection between wolves and willow, between beaver and weather. It will take years of work to come up with a final answer.

But wolves, he says, "are certainly a leading cause" of the willow resurgence. And Patten maintains that a hard winter or two could force hungry elk back into the willow stands, where they would graze heavily.

Until this point, most political and media attention on the impacts of wolves has focused on elk numbers and how many will be available for human hunters outside the park.

"That's a very valid question to ask," Smith said. But it's not the only question. "The frame of reference is important as well, if your definition of wildlife is deer and elk," he said. "But there's a very good argument out there that wolves are increasing biodiversity."

He and two other veteran biologists, Rolf Peterson of Michigan and Doug Houston of Colorado, published a peer-reviewed article in the April edition of the scholarly journal BioScience that examines the impacts of wolves.

The scientists wrote the article partly in response to the widespread public speculation about wolves.

"In time, the fear that wolves will kill all the elk will ... be put to rest," the article predicts. In 2002, the elk-to-wolf ratio in the park was 166, close to the 1993 prediction of 154. Small and mid-size carnivores like red foxes, fishers, wolverines, lynx and bobcats might benefit from wolves and the carcasses they make available, the article speculates, while coyotes, which have seen their numbers reduced by 90 percent in some areas, will continue to take a drubbing from their larger competitors.

Scavengers like ravens and eagles gorge on wolf kills and grizzly bears have learned to steal carcasses. One bear was seen fending off a pack of 24 wolves.

The small cougar population grew slightly during the 1990s. Nobody yet knows what the future size of the elk herd will be. Earlier scientific reports put the northern range's carrying capacity at 5,000 to 6,000 animals, but not everybody agrees with that. The latest count tallied about 10,000 elk.

The future herd size will depend partly on midwinter hunting seasons outside the park, where elk migrate. The late-hunt quota has been reduced by one third in recent years and may be reduced further.

Until a few years ago, critics assailed the Park Service for perceived overgrazing of the northern range by too many elk. Now the concern has switched.

"Media attention has abruptly switched from concern about too many elk for the northern range to concern about too few elk for human hunters outside the park," the paper says.