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Since reintroduction, wolves have been creating ecological change in Yellowstone National Park, including helping to increase beaver populations and bringing back willows and other vegetation.

Ever since the reintroduction of wolves to Yellowstone National Park in 1995, there has been controversy and debate about the benefits and ecosystem services that the predators have actually provided; whether it was a good idea to bring the wolves back to their ancestral home or not.

In 2014, I had written an article for the Good Nature Travel blog titled Video: Did the Reintroduction of Wolves Truly Change Yellowstone? In it, I mentioned that Arthur Middleton, at that time a postdoctoral fellow at the Yale School of Forestry and Environmental Studies, was challenging the accepted way that we thought about the wolves’ role in Yellowstone National Park. He had argued that wolves have made less of an impact on the park than expected, and that decades without wolves had damaged the parks’ willows far too much for the wolves to ever undo—even at very high levels of predation.

Fortunately, we now know, he was wrong.

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In Yellowstone, a principal staple of the moose diet is the leaves and twigs of the willow, followed by other woody browse species, such as gooseberry and buffaloberry.

Willows by the river

Aspens, cottonwoods, willows and other deciduous shrubs and trees play a pivotal role in the functioning of the natural ecosystem of the Northern Range, the hub of wildlife in Yellowstone National Park. Although they thrive in wetlands, willows can grow near seeps, springs and in any area where water is plentiful. Willows provide critical habitat for numerous species of native animals and plants.

Some wildlife, such as beavers and moose, are dependent on such riparian vegetation for forage. Beavers also use willows for lodge and dam building materials. And in addition to providing important habitat, food and shelter for a multitude of animals, woody riparian vegetation—such as willows—help stabilize stream banks, maintain nutrient cycles and regulate water temperatures.

Ever since the mid-1990s when gray wolves were reintroduced to Yellowstone National Park, there’s been disagreement among scientists and others over the degree to which willows may have recovered from decades of suppression by elk, followed by the restoration of wolves and subsequent reductions in elk numbers. Now, a new study led by Oregon State University puts the issue to rest.

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An estimated 80 percent of the bird diversity in the Rocky Mountains can be found in riparian zones, and several species are found exclusively in these regions.

According to that study, published in the science journal Ecosphere in May 2020, wolf predation on elk is a major reason for an increase in the height of willows in northern Yellowstone. The reduction of elk browsing over the last two decades in northern Yellowstone has allowed willows to grow taller in many places, despite a warming and drying climate. The scientists also state that willows aren’t recovering in some areas due to continued browsing by increased numbers of bison.

The new study compared data from three time periods: 1988–1993, when elk densities were high and most willows were very short; 2001–2004, when willows may have begun to recover; and 2016–2018. The researchers confirmed that willows have indeed increased in height and cover in response to a reduction in browsing by elk.

Passive restoration by predator return

Elk numbers in northern Yellowstone have declined from a high of nearly 20,000 in 1995—the year wolves were restored to the park—to 4,149 counted over two days in March 2019 by biologists with the Montana Department of Fish, Wildlife and Parks; the U.S. Forest Service; and the U.S. Geological Survey.


Following wolf restoration in the 1990s, elk numbers decreased; and researchers reported willows growing taller with the reductions in elk browsing, evidence of a shift toward willow recovery.

The Oregon State University researchers found a strong contrast between sites along streams compared to wet meadows. Willows in meadow sites did not increase in height, but willows in stream sites increased significantly, exceeding six feet—a height accessible to elk—in the summers of 2001–2004 and in the spring of 2016.

They also found a significant change in willow thickets at least six-and-a half feet in height along streams, with thickets occupying about 80 percent of willow patches in some sites, but as little as 22 percent in others. Tall willow thickets are an important habitat feature and an indicator of willow recovery.

Thus, passive restoration through the return of predators has begun to reverse the loss of willows, something the active culling of elk in the past was unable to accomplish.

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Beavers cut willows to store as a winter food source and to build dams. The structures have key feedbacks to willows: they raise the water level, giving willow roots easier access; and they increase flooding, a disturbance required for the plants’ reproduction.

Wolves by themselves

In fairness, wolves didn’t do this all by themselves. Other predators and hunters also affected Yellowstone’s elk; but, in the final analysis, this would not have happened without the wolves.

Too, the researchers point out, this doesn’t mean that willow habitats have been restored to the extent that existed in the early days of the park when beavers created large wetland expanses. That may eventually happen as beavers return, but it could take a long time to develop.

It is, however, a start.

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As long as there is negligible harm to the environment, couldn’t we bring back the wildlife we have lost due to our actions without the animals having to now prove their worth?

Stories by the wayside

But just as I did with the title of my 2014 article, I have another question for you: Do you think that when we return an animal that we have previously extirpated to its ancestral home, that animal must repay us by providing our current world with a benefit? Couldn’t we just try to restore wildlife to certain areas because we caused their absence in the first place?

Seven years ago, in that 2014 article, I had quoted Yale University’s Arthur Middleton: “When we tell the wolf story, we get the Yellowstone story wrong.”

I’m happy to say that it turns out that we didn’t.

Here’s to finding your true places and natural habitats,