A grey wolf on the hunt running alongside an elk in Yellowstone national park, Wyoming. Photograph: Barrett Hedges/NG/Getty Images
Grey wolves have returned to Yellowstone National Park, raising hopes that the area's ecosystem has bounced back. But the reintroduction of these mighty predators isn't enough to enable a full recovery, scientists say.
In the early 1900s, wolves largely vanished from Yellowstone. In their absence, elk — which were no longer hunted by wolves — ate more willows by streams. Without enough food or material to build their dams, beavers also declined. That made the willows' situation even worse, because willows depend on beaver dams to raise water tables.
After wolves came back in 1995, elk numbers dropped. People argued that by holding the elk in check, the new wolves allowed Yellowstone to undergo a "dramatic recovery," a research team writes in Proceedings of the Royal Society B. But willows must be at least 2 meters tall to escape the munching mouths of elk and provide enough food and dam material for beavers, the authors say.
To find out if the ecosystem passed this test, the team measured willows at four sites in northern Yellowstone from 2001 to 2010. Some plots within each site were fenced off to prevent elk from eating the plants. The researchers also built simulated beaver dams at some plots.
Even after 10 years, willows in the fenced plots without dams didn't reach the 2-meter mark, the authors report. The fenced willows only grew more than 2 meters tall if they also had simulated beaver dams nearby.
The study suggests that "effects of wolves alone were insufficient to restore riparian ecosystems," the team writes. And fixing the problem doesn't sound easy, since "the absence of beaver opposes the return of tall willows and the absence of tall willows opposes the return of beaver."