Yellowstone National Park’s riverside cottonwoods stopped growing in the mid-1920s and 1930s. So did willows and aspen. Shoots sprang up and lingered for a while, but never matured. Park officials suspected elk were eating the new growth, but culling did not help.
Beginning in the late 1990s, however, things suddenly began to change. The elk moved away from the streams, and trees and willows began to grow. Researchers wondered why they’d gone.
They ruled out drought, flood, fire or climate change. Only one answer remained. Wolves.
"For 70 years, the elk congregated next to the rivers, eating the vegetation," said Oregon State University forest ecologist William Ripple, co-author of a study on the cottonwood recovery in the park. "They don’t do that anymore."
Nine years have passed since the United States Fish and Wildlife Service imported 15 grey wolves from Canada to colonise Yellowstone, wolfless since 1926, when hunters finished exterminating them as unwelcome pests and dangerous predators.
Today, the park has 250 to 300 wolves, too many to track them all with radio collars. They are no longer classified as an endangered species, but are now "threatened", and, if a dispute between the Fish and Wildlife Service and the state of Wyoming is resolved, they may soon be "delisted" altogether, allowing carefully-controlled hunting to resume.
But, for scientists, this triumph is only the beginning. Wolves, it turns out, constitute a "keystone species" that is reshaping an entire ecosystem in ways not foreseen when researchers began a crossed-fingers experiment in wildlife preservation.
Today, Yellowstone - America’s most famous stretch of wilderness - has become an ecologist’s bonanza. It appears to be evolving in reverse - returning to a time when flora and fauna were in a balance dictated by forces of nature, not by humans.
"For the first time in 70 years, the park has a complete suite of predators and prey," said Mr Ripple. "This is a grand experiment."
The cottonwood resurgence, reported late last year by Mr Ripple and Robert Beschta, a colleague at Oregon State University, documents what is described as a "cascade" of environmental changes.
Wolves stalk the elk, so elk leave the rivers, where they are vulnerable. The willows, cottonwoods and aspens grow, casting shade that cools the water to temperatures favoured by trout. Migratory birds return to roost in the new foliage.
But it does not stop there, said National Park Service wildlife biologist Douglas Smith, leader of the Yellowstone Wolf Project.
"In 1996, we had no beaver colonies, and now we have seven, because the beavers can eat the low-hanging willow branches," he said. And the beavers build dams, creating marshland that "brings back the otters, mink, muskrats and ducks".
Mr Smith said that it may take 20 to 30 years to measure the full effects of the wolves’ return. And with no ranching, farming or hunting allowed, the park is a perfect laboratory - a fully-protected piece of wild country covering 2.2 million acres.
Ten years ago, Yellowstone had 17,000 elk, the largest single population in the world. Weighing as much as 700 pounds apiece, they had no serious rivals. Grizzly bears, Yellowstone’s top predators, are capable of bringing down an adult elk, but they mainly prey on calves. Coyotes, though numerous, were much too small to attack elk.
"The first thing that happened was that the elk ignored the wolves," said Wildlife Conservation Society senior scientist Joel Berger. "The elk were treating 90-100lb wolves like they were 35lb coyotes. The elk were nave. They aren’t nave anymore."
The wolves flourished in what Smith described as "arguably the best wolf habitat on Earth", feasting on elk and multiplying rapidly both in Yellowstone and central Idaho.
Elk, not surprisingly, have suffered, both from weather and wolves, their numbers in the park shrinking to about 8,000 today. This, however, should not be cause for alarm, Mr Smith said, but instead regarded as another aspect of the park’s environmental transformation.
"Ravens, magpies, golden eagles, bears, bald eagles and coyotes feed off every elk kill," he explained, "and I’m not even mentioning smaller mammals or insects."
Wolves sit right below grizzlies on the Yellowstone food chain, not because of their individual prowess as hunters - they are no match for cougars or black bears - but because they hunt in packs and learn quickly.
And as the packs have grown, livestock has taken a hit. "I used to lose five or six head per year," said rancher Dave Nelson, who grazes about 1,000 cattle on land neighbouring Yellowstone. "Last year, I lost 21 head, and the year before that, I lost 15. I can’t tell you definitely it’s wolves, but I highly suspect it."
Mr Nelson, a past president of the Idaho Cattle Association, in recent years helped negotiate Idaho’s piece of a Fish and Wildlife Service- sponsored plan to drop wolves from the endangered-species list and give the states of Idaho, Montana and Wyoming the authority to allow hunting as long as the overall wolf population remains at 30 packs, with a pack defined as a breeding pair of wolves with cubs.
Federal authorities approved plans from Idaho and Montana two weeks ago, but refused to delist the wolf until Wyoming changes its management proposal to conform with its neighbours’. Despite the delay, the overall strategy has won cautious endorsement from both environmentalists and ranchers.
"We certainly would have liked to see more improvements, but the plan is acceptable," said Michael Scott, executive director of the Greater Yellowstone Coalition. "The states regulate, but federal authorities can intervene if wolf numbers plummet."
For Mr Nelson, selective hunting will keep the wolves honest - and out of the pasture. "They need to learn that they will pay a price if they come out of the woods to kill cattle," he said. "The way it is now, they’re half-domestic. They’re real brassy."
For now, elk remain the wolves’ meal of choice, but as the two populations reach equilibrium, researchers expect wolves to take on more difficult targets - moose first, and then bison. This last confrontation will test one of Mr Smith’s favourite hypotheses.
"Why do wolves hunt in packs? I think it’s for the bison," he suggests. "It takes three wolves to kill an elk, but I have seen ten wolves hanging off a 2,000lb bull bison.
"They killed it, but it took nine hours, and the bison killed one wolf, gored another and broke the leg of the alpha female. Bison just pound them."