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YELLOWSTONE NATIONAL PARK - At dusk, a crowd of late summer tourists scrambled to the top of a roadside hill in Hayden Valley eager to catch a glimpse of two wolves in the area.

While binoculars and expensive spotting scopes peered far across the valley, a lone brown beaver slipped into the nearby Yellowstone River and downstream, undetected by the hillside throng.

The moment might have been a metaphor.

The dramatic return of the wolf to Yellowstone grabbed worldwide attention, but the quiet resurgence of the beaver at the same time - particularly on the Northern Range - has barely been noticed.

Over the past decade, the number of beaver colonies counted in Yellowstone has grown from 49 to about 85. In the northern reaches of the park, the number has jumped from zero in 1996 to nine last year.

Secretive and generally nocturnal, beavers wield a mighty influence over their environment.

Rich habitat

They build dams, changing the flow of rivers and streams and creating rich habitat for a wide array of plants, birds, insects and other mammals.

"Nature thrives on diversity and beavers are a huge generator of diversity," said Doug Smith, leader of Yellowstone's wolf project who also conducts biannual beaver surveys.

Scientists agree the beavers' return has coincided with the return of the wolf and the willow, a favorite food and building material.

But, like many stories in Yellowstone, it's more complex than it seems.

Depending on to whom you talk, the beavers' return is related to wolves, climate, water flows, a nearby reintroduced population or some combination of those factors.

Whatever the case, it's another sign that at least a few mysteries remain in Yellowstone's complicated ecosystem.

"None of these stories are black and white," said Tom Hobbs, a professor at Colorado State University who has conducted studies in Yellowstone for years.

Beavers, the largest rodent in North America, have had a long-running presence in Yellowstone.

Records dating back to the late 1800s indicate that the population declined in the park and elsewhere as the fur trade grew. The population in Yellowstone bounced back a bit in the 1920s as beavers received protection from killing and aspen trees were abundant.

By the 1950s, the population dipped again after beavers ate too far into the aspen supply.

In the meantime, wolves had been wiped out in Yellowstone in the early part of the century, and the elk population flourished.

Without a top predator for 70 years, elk didn't have to move around as much, and spent much of their time eating willows and shrubs in low-lying river areas. Over the years, many theorized that elk were eating the landscape toward disaster, barely allowing sprouts to pop up before being devoured.

After wolves were reintroduced to Yellowstone in 1995 and 1996, biologists noticed that elk, wary of the predator in their midst, spent more time moving around and less time munching at one spot.

In turn, willows began to grow taller and thicker. And beavers had a chance to eat, thrive and build new colonies.

Granted there are still more studies needed, but Smith said it's hard to ignore the influence of Yellowstone's most recently returned predator.

"The missing ingredient for a century has been wolves," Smith said.

Using an estimate of six beavers per colony - which may or may not be the case in Yellowstone - there are today perhaps 500 or so beavers in the park.

Many are congregated in the vast Yellowstone River delta area southeast of Yellowstone Lake.

But the most significant increases in the beaver population on the Northern Range have corresponded with high wolf density and willows growing tall for the first time in years.

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Roy Renkin, a vegetation expert at Yellowstone, shades his opinion of the beavers' return more toward the influence of climate in recent years.

He doesn't discount the influence of wolves, but says willows may be benefiting from warming climate conditions around Yellowstone that are extending the growing season.

Renkin said one estimate shows the growing season starts 10 to 13 days earlier and ends 14 to 15 days later in a change that has coincided with the reintroduction of wolves.

The hypothesis is that as the growing season increases, plants are able to get sugars sooner and grow and then have energy left over to produce defensive chemicals that create an unpalatable taste for hungry elk.

"This is a stimulating idea," Renkin said. "It gets away from the direct wolf effect and goes more to climate."

There's some historical data indicating that the beaver population may have fluctuated significantly in Yellowstone over hundreds of years with ties to changes in climate and riparian habitat, Renkin said.

Both Smith and Renkin say the beavers' resurgence was also probably intensified by the reintroduction of beavers into the Gallatin National Forest not long before wolves were returned. Some of those beavers migrated downstream from the forest into Yellowstone.

Hobbs, the Colorado State professor, said beaver may also be responding to local conditions at particular streams and rivers in Yellowstone.

Although how much they're eaten by elk is a key factor for growing willows, so is the amount of water that's flowing nearby.

"Willows like to have their toes wet," Hobbs said. "Where water availability is low, then the willows may not be able to get big enough for beavers to come back."

Beavers, one of nature's great engineers, could be helping themselves, too. The more they slow the water down, the more water is available for willows, which means more food and lumber for beavers to survive.

"So much of life out there depends on water," Smith said. "And they're the ones that create still water."

Like everything else, beavers in Yellowstone are probably on some kind of cycle, Smith said, who cautioned against making judgments about whether the beavers' resurgence is a good or bad thing.

It just is, he said. And it's an intriguing chapter to watch as it unfolds.

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