The return of gray wolves has dramatically altered the landscape in portions of Yellowstone National Park, as new trees take root in areas where the predators have curbed the size of foraging elk herds, according to scientists in a new study.
Stands of aspen, willow and cottonwood are expanding in areas where for decades dense elk populations prevented new growth, said study author William Ripple from Oregon State University.
While other factors may play a role, from a changing climate to wildfires, more than a decade of research has confirmed earlier assertions that the return of Yellowstone's elk-hungry wolves has spurred new plant growth, he said.
The findings from Ripple and co-author Robert Beschta will be published in the scientific journal Biological Conservation. The study already has been released online.
Wolves are "apex predators, on top of the food web," Ripple said. "They're more than just charismatic animals that are nice to have around. We're finding that their function in nature is very important."
Wolves have spin-off benefits, too, the researchers said: As trees grow taller, the stands provide more habitat for yellow warblers and other songbirds and more food for beavers, which in turn construct ponds that attract fish, reptiles and amphibians.
The phenomenon has been described as a "landscape of fear" in which a predator's pursuit of prey has a cascading effect across the ecosystem.
Wolves were reintroduced to Yellowstone from Canada in 1995 and 1996 after being killed off early last century. About 100 now roam the park, thinning elk herd numbers as they feast on the big game animals year-round.
Other studies have indicated a single wolf can kill several elk or more each month during the winter.
Some scientists dispute the claim that wolves have sparked a restoration among Yellowstone's aspen.
In a 2010 study, U.S. Geological Survey scientist Matthew Kauffman agreed that foraging elk were the leading cause of the trees' decline in the park over more than a century. But Kauffman said the decline has continued since wolves returned, even in areas considered risky to elk because they are frequented by the predators.
That's because elk alter their behavior only slightly to avoid wolves, concluded Kauffman, who also heads the Wyoming Cooperative Fish and Wildlife Research Unit. Although elk numbers are down by about two-thirds in some areas of the park, herd numbers would have to drop even more for aspen to recover, Kauffman said.
Aaron Wirsing, a University of Washington biologist who studies predators' effects on ecosystems, said more than 15 years after wolves first came back to Yellowstone, the debate surrounding their impact continues to stir disagreement.
Others have said the work by Ripple and Beschta has not given enough credit to factors such as drought and stream levels. Wirsing said those other factors should not be ignored but added that the emerging consensus is wolves play a central role in the park's landscape.
"The weight of the evidence is certainly coming down in favor of wolves having a particularly profound impact on aspects of Yellowstone's ecosystem," he said.
Studies on other predators have found similar top-down effects on their surroundings. With fewer lions, researchers said increasing numbers of baboons in sub-Saharan Africa are pushing into settled areas where they raid farmers' crops more frequently and can spread intestinal parasites to humans. And fewer sharks along the East Coast has led to more rays, which in turn eat more scallops, wiping out some local fisheries.
One difference in those studies and the work in Yellowstone is that in most other cases, scientists have been left to study what was lost when a predator was gone. In the park, scientists have been able to track what happens after the predator came back.
"Being able to look back retrospectively gets us very close to being able to determine the impact of wolves," Wirsing said.
Ripple said the results of his work with Beschta suggest wolves also could have positive effects outside Yellowstone. More than 1,600 gray wolves now live in the Northern Rockies, and elk numbers have dropped as a result.
But given the backlash wolves have encountered among some ranchers and elk hunters, more research needs to be done to determine how many of the animals are needed to be an "ecologically effective" population, Ripple said.
"The question is, how many wolves does it take?" he said.