Have Wolves Returned Yellowstone to its Natural State?

Yellowstone National Park looks different than it did 30 years ago. That much everyone can agree upon. How different? And who or what is responsible for that change? The answer is where scientific opinion seems to diverge.

Most everyone has heard the story of the term “trophic cascade.” Most of us have seen the video. It has 45 million views on YouTube, in addition to frequently making the rounds on Facebook.

In case you haven’t seen it, the story goes like this: Wolves were returned to Yellowstone in 1995, decreasing the number of elk and making sure the elk didn’t just hang out in stream bottoms depleting vegetation. The lack of grazing in riparian areas led to more aspen, more willow, more birds, more beavers, more songbirds, more reptiles, more amphibians, and an ecosystem shift. Even rivers changed in response to the wolves.

That’s the story we all have heard. But it’s not the full story and may not be entirely right, according to some scientists who have studied the effects of the reintroduction of wolves to Yellowstone National Park.

“Trying to determine if our current ecosystem is in a restored state is a fool’s errand,” said Dan Stahler, leader of the Yellowstone Wolf Project, the Yellowstone Cougar Project, and the Elk Research and Monitoring Program for the park. “Change is happening, and it’s our job [as scientists] to monitor and understand these changes.”

Yellowstone became America’s first national park in the 1870s, and the park has changed dramatically since then. For years after the park was created, hunters and trappers were responsible for significant amounts of poaching. Bison in Yellowstone were reduced to a couple dozen. Ranching and farming operations existed in the Lamar Valley. Predator populations were largely decimated by the 1920s; wolves were entirely extirpated. Bears were fed at park garbage dumps and from the windows of cars. Beavers were largely extirpated, too.

But the park has worked to restore natural processes, with the most significant example being the reintroduction of wolves in 1995. Like the video said, wolves changed where elk and other species gather, predominantly removing them from overgrazing riparian areas. And since then, the park has changed significantly—even if not as dramatically as the video suggests. Yellowstone's Northern Range has changed from an elk-dominated landscape to a bison-dominated landscape. Beavers have returned to many, but not all, streams.

“What’s not in doubt in my mind is whether a trophic cascade has happened or not,” said Dan MacNulty, a professor of ecology at Utah State University. “The question boils down to how strong is that cascade? And No. 2, to what extent have wolves contributed to that cascade?”

A 20-year study released in February by ecologists at Colorado State University found that the removal of apex predators from Yellowstone, including wolves and smaller populations of cougars and bears, creates lasting changes that can’t be easily undone by reintroducing those predators. The paper, published in the journal Ecological Monographs, challenged the idea that the reintroduction of wolves restored Yellowstone to the state it was in prior to their removal.

Instead, the authors state, the park has transformed to an “alternative stable state”—basically grasslands instead of willow growing along small streams—that is resilient to the changes brought on by wolves.

“What I value about the study is it provides a really strong test of the strength of the trophic cascade that has occurred over the last 20 years,” MacNulty said.

Like the decline of wolves, the reintroduction of wolves didn’t occur in a vacuum. Beavers were also reintroduced to the Absaroka-Beartooth Wilderness in 1986 and have slowly worked their way south. Bison have come to be the dominant species in Lamar Valley, a place where they weren’t present in large numbers until the mid-2000s. Bear populations, resulting from increased protections under the Endangered Species Act, have also increased significantly.

Over the past 30 years, scientists have repeatedly published studies detailing the impact of wolves on Yellowstone ecosystems. “The simple message is that the changes that occur when apex predators are absent from food webs for a long time are not quickly reversed when you put the predators back,” said Tom Hobbs, a professor of natural resource ecology at Colorado State and coauthor of the study.

The Colorado State research involved a long-term experiment to test how various factors, including browsing and hydrology, impacted the recovery of willows in riparian areas. The researchers created simulated beaver dams and found that willows fared much better in the simulated dams than in natural areas and in riparian zones near those dams.

David Cooper, senior research scientist emeritus at Colorado State and another coauthor of the study, said the research shows that the natural relationship between beavers and willows is in a degraded state that will likely not be restored to for a long time. “Once the elk were super abundant, they outcompeted the beaver for food,” he said. “Willows are so beat down now that you know a small number of herbivores can maintain this degraded state, and beavers just can't get back into these systems to restore them.”

Because willows haven’t returned in many areas of the park, Cooper continued, beavers haven’t either. “Beavers need willows,” he said. “Willows need beavers, because beavers create these habitats where willows reproduce, and there hasn't really been very much willow reproduction for probably 75 years.”

There are more beavers on the landscape in Yellowstone’s Northern Range, but they have a different kind of habitat than they used to, and aren’t changing the streams as much as they once did.

“The habitat for beaver has degraded because willows are not present, and it's going to take a long time to put that back,” Hobbs said. “We can't rule out in the fullness of time, say the next 50 years … that willows will become taller. But there are a lot of other trajectories that could occur as well. All we can say for sure is that those changes in the willow communities that are needed to support beavers have not occurred in the 30 years since wolves were reintroduced and the other apex predators, cougars and grizzly bears, were restored naturally.”

Hobbs added that it’s likely these ecological shifts will continue to occur.

“We have a changing climate. We have invasive species. We have dynamics within animal and plant communities,” he said. “So, I think we should expect that that change to some extent is going to be the norm into the future.”

Where is the evidence for the changes that have been so widely praised? All across the Northern Range, said Luke Painter, an ecologist at Oregon State University, but it depends on where you look. For example, Blacktail Deer Creek and Crystal Creek have seen significantly more willow and aspen growth than the park’s famous Lamar Valley.

Painter has been studying the impacts of the return of wolves on the Northern Range for decades. His research group at Oregon State helped provide the analyses that supports the trophic cascade story. “We’ve been saying this over and over,” he said. “There are qualifiers. We’ve been careful to say it's a beginning process, and it’s not happening everywhere.”

Painter says the biggest change in Yellowstone has been the decrease of the elk population. Before wolves were reintroduced, around 80 percent of elk in the Northern Range wintered inside the park. Now, he said, 80 percent winter outside the park.

And the return of bison, which were basically absent from Lamar Valley summers until around 1985 and now are the dominant species in the valley year-round, has changed grazing patterns considerably. “It's a mixed bag, but it's ecologically very different than what we had before, which was pretty much grazing lawns everywhere,” Painter said. “Now we have grazing lawns in some places and not others.”

The trophic cascade has limitations in part because Yellowstone National Park has arbitrary borders that don’t allow natural species migration, especially for bison. Humans are also not permitted to hunt in the park.

“Livingston [Montana] is where [bison] want to live, but they’re not allowed to go to the Lower Yellowstone, which would be their core habitat,” Painter said. “A lot of ecologically important changes have happened, even though it is not all restored to this past condition. The future may never look like the past, with climate change and the restrictions imposed by human development.”

The changes that have occurred over the past 30 years are important and positive for the park, said Stahler, who recently took over leadership of the Yellowstone Wolf Project. He contends that strong research is needed to understand how the land is changing.

“It’s important for the public to understand that these ecosystems have this really complex relationship with the food web,” Stahler said. “And it’s an important part of our legacy to restore these natural processes. The restoration of an apex predator is an important step to restoring our natural ecosystems but what those ecosystems turn into, nature will decide—depending on how much we tinker. But putting those ecosystems back together is the most important part.”

Between a changing climate, increased visitation and invasive species, Yellowstone will always be in a state of flux, Stahler said. The park doesn’t look like it did in the 1870s, but it also doesn’t look like it did in the 1970s. It will likely look different in the 2070s.

“This idea that in Yellowstone we’re trying to get back to the place it used to be is a false idea,” Stahler said. “There’s been too many changes that have occurred in our history. What we can do is try to put the important pieces back together. Our management approach is not to manage for a specific ecological state, but to manage for ecological processes. Some might read the headline of this paper and say it didn’t fix things or heal things, but that's not the right question to ask. We’re not trying to create a landscape that looks exactly like it did in the past—that’s not possible. But we can put the pieces back in place and try to let nature take place.”

It makes sense that the YouTube video doesn’t have everything exactly right—in fact, it calls elk “deer.”

Hobbs said it’s important to realize that Yellowstone has issues in need of more attention.

“The problem with that video and the problem with the widespread belief that everything is wonderful in Yellowstone as a result of the reintroduction of wolves is that it deflects attention from real problems: invasive species, climate change, chronic underfunding,” Hobbs said. “These are real problems that Yellowstone has, and the idea that wolves and other carnivores have worked magic … deflects attention from those problems that are very, very real.”

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