A century ago, the wolves of Yellowstone National Park were still killed as varmints—and by the 1940s, they had almost vanished completely. Today, 18 years after their return, the predators are often hailed as saviors for restoring the ecological balance of one of America's wildest landscapes. But a new study says that Yellowstone's wolves probably can't turn back the ecological clock completely—at least not without help from a less glamorous animal: the beaver.
"It'd be nice to have a simple story to tell, that everything that changed when you removed wolves will change back when you bring them back in," says study author and ecologist Kristin Marshall, who carried out the research as a Ph.D. student at Colorado State University, Fort Collins. But "ecology is complicated. Our study shows it's more complicated than we thought."
The researchers focused on riparian, or streamside, habitats in the park's northern range, the 1400-square-kilometer area where many of Yellowstone's elk and bison spend the winter. Roughly 25 to 30 wolves now live in the northern range, thanks to a reintroduction program in the mid-1990s. But the area is still largely bereft of the beavers that once populated its small streams. That's in part because an elk population boom—a consequence of the extermination of wolves—deprived beavers of the willows they need for food and construction material.
If beavers need willows, willows also need beavers. Beaver dams help create mud flats where new willows can sprout; they also raise the water table, supplying more water to willow roots. When wolves vanished, the willows of the northern range faced a double whammy: too many elk, too few beaver. The result was a scarcity of the thick, lush willow patches needed for a healthy riparian zone.
To find out whether wolves could rescue the willows, Marshall and her colleagues charted willow growth at four sites in the northern range. At each site, the researchers fenced some plots to provide total protection from browsing elk and other animals. They also built dams—hauling in logs by helicopter—in streams near some plots to mimic the effect of beavers. Some plots were dammed and fenced; control plots were neither dammed nor fenced, making wild wolves their only possible protection from elk.
After 10 years, the fenced willows that weren't close to dams, though they'd suffered no browsing at all, on average were far shorter than 2 meters, the team reports online today in Proceedings of the Royal Society B: Biological Sciences. That's the threshold height that makes willows tall enough to reproduce despite voracious elk. Unfenced willows along dammed streams didn't make the threshold either. Only a combination of dams and fences provided the right conditions for the willows to grow to a self-sustaining height.
"Those plants that we protected completely from browsing—nothing touched them for 10 years, and they still didn't make it past that threshold, on average," Marshall says. "That really suggests that even if wolves were completely reducing the impact of elk … we still wouldn't have complete recovery for willows."
Marshall and her team also measured the height of 113 willows in the northern range that were not part of the experimental or control plots. They found that a plot of the trees' heights in 2010 strongly resembled a plot of willow heights in 1990, 5 years before wolves were reintroduced. The researchers argue that these results show that there aren't many more tall willows now than there were before wolves came back.
"This is a really well-executed and convincing study," says ecologist Scott Creel of Montana State University in Bozeman. "They make a pretty compelling argument that … you can't just send [the ecosystem] back the way it came."
The paper "demonstrates that it isn't just wolves and elk in that system -- it's much more complicated," adds University of Alberta ecologist Mark Boyce, who has studied various species at Yellowstone's. "There are many connections among various species … that influence the recovery."
Not everyone is convinced. Riparian ecologist Robert Beschta of Oregon State University, Corvallis, who also researches post-wolf recovery at Yellowstone but did not contribute to the new study, points out that the willows in the Marshall team's control plots—undammed, unfenced, but subject to wolf patrols—did grow slightly taller over the 10-year study period. "Recovery of heavily degraded ecosystems takes time," he says. "They set the bar very high by saying, 'This system hasn't recovered yet (with) wolves, and therefore the effect of wolves is maybe minimal or non-existent.' "
All agree, however, that beavers might help the willows and riparian zones make a comeback. But until willows are vigorous, beavers could starve. It's hard to see a way out of this "chicken-and-egg" problem, Marshall says. Perhaps if Yellowstone got a very wet year, encouraging willow growth, combined with a year that saw a low level of elk browsing, beavers could establish a foothold in the small streams of the northern range, as they have in other parts of the park. "It's feasible that it could happen on its own," she says. "It's just not likely in the next few years."