YELLOWSTONE NATIONAL PARK, Wyoming – Yellowstone's wolves are back, helping revive parts of the ecosystem that changed drastically when this top-of-the-food-chain predator was killed off nearly a century ago. But Yellowstone is still not 100% back to normal – and it may never be.

“You put the predator back, that’s great, but conditions have changed so much in the intervening decades that putting the predator back is not enough to restore the ecosystem,” said Tom Hobbs, a Colorado State University ecology professor. “There’s not a quick fix for mistakes like exterminating apex predators.”

It's a sign of both the promise – and the limitations – of a multi-decade wildlife recovery effort. The reintroduction of the wolf nearly 25 years ago to the country's first national park has brought change: Overpopulated elk herds have thinned, allowing some willow and aspen groves to return and thereby creating better habitat for songbirds and beavers.

But even as this ecosystem shows signs of recovery, a complete restoration is nowhere to be found.

"In some places, I don't expect a full recovery of the ecosystem," said Bill Ripple, a distinguished professor of ecology at Oregon State University, who started working in Yellowstone in 1997. "It’s going to be a mixed bag for the longer term now in coming decades."

Yellowstone's vanishing wolves

The park radically changed after humans exterminated the gray wolf from Yellowstone in the mid-1920s due to predator control efforts. Elk herds ballooned over the next 70 years, overgrazing vast tracts of land and trees such as willow and aspen. Fewer trees sent the songbird population into decline. Beavers lost their food source and the lumber to build their dams. The lack of those dams caused streams to erode, making them deeper and not as wide and further degrading the conditions willow need to grow.

Today, nearly 25 years after wolves were reintroduced into the park, the top predators have helped parts of the ecosystem bounce back. They've significantly reduced elk herds, opening the door for willow, aspen, beaver and songbird populations to recover. But the wolves haven't been a silver bullet for the ecosystem as a whole.

“This idea that wolves have caused rapid and widespread restoration of the ecosystem is just bunk,” Hobbs said. “It’s just absolutely a fairytale.”

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Yellowstone's partial recovery has set off a heated debate in academia over how much bringing back an apex predator, such as the wolf, can help restore a devastated ecosystem. It's one with consequences stretching from the U.S. to India and Africa, where naturalists have pinned their hopes on keeping fragile ecosystems as intact as possible by avoiding the elimination of lions, tigers, sharks and other top predators.

“Maintaining intact ecosystems may be easier than fixing them after you’ve lost some of the parts,” Hobbs said.

Fewer elk, more songbirds

Most ecologists agree that Yellowstone has rebounded some. When Doug Smith, Yellowstone National Park’s wolf biologist, first arrived in 1994 shortly before wolves were reintroduced, some willow and aspen trees only came up to his knees. “Now I can’t see through it,” he said. “It’s like a forest.”

But the trees aren’t coming back in every corner of the park: In many spots willow groves haven't returned. Because willows need beaver to keep the streams from eroding and beavers need the willows to build their dams, it’s rather hard for both to come back simultaneously and in large numbers, said Hobbs, whose team has been conducting a long-term willow growth study in the park for 17 years.

The decrease in elk hasn’t allowed willows to recover because the streams changed significantly when wolves were absent.

“It doesn't really matter very much whether they're being browsed or not. They don't have adequate habitat to thrive,” Hobbs said. “The conditions that changed while wolves were absent created conditions that made it very difficult to restore willows.”

Grizzly population rebound

It’s not all about the wolves, even if they get the most attention. Over the past several decades, the number of other carnivores like the grizzly bear and mountain lion have also climbed, multiplying the impact of the top predators on the ecosystem.

“As a scientist, the challenge is to figure out how much ecological change since wolf reintroduction is attributable to wolves and how much of that change is due to other forces," said Dan MacNulty, an associate professor at Utah State University who studies the ecology of wolves and elk in the park.

How large the wolf's impact on the Yellowstone ecosystem is difficult to tease out in part because of nature's complexity and capacity for frequent change, he said. But money also plays a large role: It is difficult to adequately monitor all the potential drivers of change when funding for long-term research is so limited, he said.

“One of the grand challenges in ecology is to understand the consequences of predator removal and restoration in large-scale systems like Yellowstone. But the resources aren’t there. That really limits our power to know what’s going on,” he said. “A key reason why there’s so much scientific disagreement is that we haven’t been able to take all the necessary measurements over a long enough time and over a large enough number of organisms to come up with a more definitive answer.”

Despite all the disagreement, most ecologists say removing predators today would be a mistake.

"The way ecosystems put themselves back together after such a problem is still something that scientists are trying to understand," Ripple said. "The lesson is let’s not let things get as bad as they did with 70 years without wolves."

But there’s an even broader question that needs to be addressed: Can we restore apex predators and coexist with them?

“There’s not many places in the rest of the United States where this is happening,” Smith said. “There are lessons here that we can do this on human-dominated landscapes in other places, but I don’t know because it might involve more wolves, cougars and bears, and right there you have a problem because people have trouble living with those three carnivores.”