BILLINGS, Mont. (AP) — The return of wolves and cougars to Yellowstone National Park is helping restore a landscape that had been altered in their absence and allowing streams to return to a more natural state, according to a new study.

The widespread extermination of wolves and cougars early last century meant elk herds that the carnivores prey on were able to grow in size. The swollen herds ate away willow plants and other vegetation along the park's streams, causing erosion damage.

But in recent years, resurgent populations of wolves and cougars have restored the park's natural balance by knocking back elk numbers and changing the herds' behaviors, according to Robert Beschta and William Ripple of the Oregon State University College of Forestry.

Cougars returned in the 1980s and wolves were reintroduced about a decade later. In the years since, the park's elk population has dropped dramatically. As a result, willows are rebounding and streams are recovering, the researchers said.

The findings were included in a peer-reviewed study published in the journal Ecohydrology. The researchers cautioned that the healing is in the early stages and could take a long time to complete.

The idea that bringing back wolves and cougars would have cascading effects across Yellowstone's ecosystem has long been of interest to researchers.

Prior studies concentrated on the impacts to other animals and plants. Ripple and Beschta took that a step further to see what happens to the landscape itself.

"This is the first time that we studied how the reintroduction of wolves, along with other large carnivores, affect non-living landscape features outside of the food web," Ripple and Beschta said in a statement to The Associated Press.

John Winnie, an associate teaching professor at Montana State University, said he disagrees with the assertion by Beschta and Ripple that changes to elk behavior is what's driving the effects on the landscape. Winnie says the sheer number of elk plays a more important role.

But regardless of the mechanism, Winnie says he's in agreement that cougars and wolves can indirectly influence the park's streams.

"Simply having more vegetation growing along stream banks is going to change how rivers behave," Winnie said. "That's basic stream ecology."