Wolf Reintroduction to Yellowstone National Park Helped Restore Aspen Trees — but There Is Still Much to Learn

Our story starts 100 years ago, in 1926, when the last wolf in Yellowstone National Park was shot and killed. For nearly 70 years, there were no wolves in the park. And the landscape drastically changed.

Without a crucial apex predator, elk populations spiked. In the 1940s, biologists at Yellowstone became concerned because the park’s massive elk herd was overgrazing the range. By the 1950s, at least one effect of this overgrazing was apparent. The park’s aspen trees were in decline.

However, when wolves were brought back to Yellowstone in 1995, the results were surprising.

Read More: Wolves Keep the U.S. Ecosystem in Check

Aspen Trees Dying in Yellowstone

An aspen stand is really a single organism, Luke Painter, an ecologist at Oregon State University who studies plant and animal interactions, explained to Discover. Painter is also the lead author on the 2025 study in Forest Ecology and Management about this subject.

Though they appear to be individual trees, aspens are actually connected by their roots. Aspens reproduce primarily by sending up shoots, often called suckers, and these suckers were being eaten by elk.

By the turn of the twenty-first century, the older trees were dying out and not being replaced by young trees.

Why Aspen Trees Are Important

This is not just a matter of losing pretty trees. Aspens play an important role in the park’s ecosystem. Plants grow in the understory of aspen stands, providing food for pollinators such as bumblebees, butterflies, and hummingbirds. Birds nest in the trees’ cavities.

Aspens are also an important source of food and building material for beavers. Beavers create wetlands and ponds that support even more habitat diversity. This is especially important now, thanks to rising heat and drought driven by climate change, said Painter.

So overgrazing the aspen shoots was a problem not just for the aspens, but for many other species as well. From the 1940s through 1968, park rangers tried thinning the herds by culling some of the elk, but that didn’t have much effect on the aspen. The solution that did work was unexpected.

The Reintroduction of Yellowstone Wolves

According to the National Park Service, the park began a hands-off approach to management during the 1960s. They wanted to let the ecosystem manage itself.

Of course, that couldn’t be done without wolves, because wolves are a natural part of Yellowstone’s ecosystem. So starting in 1995, experts brought grey wolves from Canada to Yellowstone’s Lamar Valley, reintroducing the species after an almost 70-year absence.

The wolves were not brought back to the park to save the aspens in particular, but simply because they belong there. (In addition, wolves were listed under the Endangered Species Act, which requires federal agencies to take steps to recover listed species.)

Still, biologists weren’t expecting the effect the wolves had. Painter’s research shows that when the wolves were reintroduced, the aspens (and willows and cottonwoods, too) began to recover.

“The idea that wolves would make that kind of difference in the aspen really was not something that was in the minds of people that were reintroducing the wolves,” Painter told Discover. “They thought the wolves might help a bit with controlling the elk population, but they really didn’t expect this kind of trophic cascades effect.” Yet that’s what happened.

The simple story is this: Wolves thinned the elk herds. Fewer elk munching on aspen shoots saved the aspen trees. Then beavers, wetlands, songbirds, and much else began to recover. This is called a trophic cascade — when a change in predator population has effects that cascade not only downward, but throughout an ecosystem.

A Lot Left to Learn

But as with all good stories, if you look closely, things are a bit more complex than they appear. For one thing, the aspens have not recovered in all parts of the park. In some places, bison are eating aspen shoots, along with young willows and cottonwoods. Wolves rarely eat bison, and with less competition from elk, the bison herds are growing.

“Bison are not as effective as elk at suppressing these plants,” Painter said, “but in some places they are having effects on the aspen recovery.”

Another complexity is that, though wolves are the primary reason for the aspens’ recovery thus far, they are not the only ones. For example, wolves have acted in combination with other predators, such as cougars and bears, to thin elk herds. And in some places, fallen trees could be providing some protection for the young shoots, Painter added.

These complexities were addressed in a 2026 correction paper and a 2026 response paper in Forest Ecology and Management.

This is a happy story, but not a simple one. Ecosystems are incredibly complex, and small changes can have big effects. When you leave an ecosystem alone — let it manage itself, as the Park Service has chosen to do — you can’t know what will happen, but you can learn from it. And Yellowstone has a lot yet to teach us.

Read More: Wolf Reduction Helped Caribou Calves Survive — but Only in Rugged Terrain

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