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Outsized effect of predation: Wolves alter wetland creation and recolonization by killing ecosystem engineers, Thomas D. Gable, Sean M. Johnson-Bice, Austin T. Homkes, Steve K. Windels, Joseph K. Bump, Science Advances 2020 Nov
By Ben Goldfarb
The alpha male of the Cranberry Bay wolf pack, dubbed V083 by researchers, is a canine with a singular specialty: killing beavers. V083 roams Voyageurs National Park in northern Minnesota, and in the spring and summer, he and his packmates prey heavily on busy rodents, ambushing them along foraging trails and waterways. This year alone, V083 devoured 36 beavers, the equivalent of seven colonies.
Such kills have an outsize impact, according to a new study. By influencing where beavers live and build dams, the wolves shape Voyageurs’s vast wetlands—an ecological chain reaction that alters the contours of the land itself. “Looking at it over time,” says Tom Gable, a biologist at the Voyageurs Wolf Project and lead author of the study published today in Science Advances, “you start to see how interconnected wolves are to wetland creation.”
The research will likely add fuel to a years long scientific debate over the role that wolves and other predators play in shaping ecosystems. In Yellowstone National Park, years of fieldwork suggested wolves reintroduced there in 1995 thinned herds of elk, in turn reducing grazing on stream-side plants and helping stabilize eroding creek banks. But subsequent studies have suggested the story is more complicated, and that wolves aren’t the sole agents of change.
Now, the Voyageurs Wolf Project, a long-term investigation of Canis lupus’s predatory tactics, sheds more light on the issue. Between 2015 and 2019, biologists strapped satellite collars to 32 wolves and monitored their movements; when a wolf lingered in one location, it was usually because it had made a kill. Gable and his colleagues then hiked to the presumed kill sites, picking through blood-smeared fur and bone to figure out what the wolves had feasted on.
In many cases, the victims were beavers—ecosystem engineers that transform their surroundings by building dams and creating ponds. They’re especially prolific in Voyageurs, where their ponds cover 13% of the park’s total area.
Although the tracked wolves ate plenty of beavers—some packs killed 40% of the rodents in their territories each year—they didn’t have a major impact on long-term beaver numbers, the researchers found. But the wolves did influence where their prey lived. In particular, Gable and his colleagues learned that wolves frequently ate “dispersing” beavers—nomads that had left their home lodges to colonize new areas. These beavers were especially vulnerable because they had to repeatedly venture onto land to harvest sticks for their nascent dams. After wolves killed these colonists, the researchers found their partial ponds remained unoccupied for the rest of the year. So the predators prevented forests from fully transitioning to ponds and wetlands—forestalling dramatic environmental change.
Compared with Yellowstone’s complex and contested wolf dynamic, Voyageurs offers a clear example of what scientists call a trophic cascade: When wolves eat beavers, beavers can’t construct ponds. “There’s been a lot of interest in trying to understand how large carnivores are connected to riparian ecosystems and wetlands,” Gable says. “Our work has presented this simple mechanism that you could explain to a 5-year-old.”
Still, the scale of the Voyageurs cascade isn’t clear. Every year, the region’s wolves alter about 88 beaver ponds—hardly an overwhelming transformation on such a vast landscape. “Ponds are coming and going in various places over time, but the numbers suggest it’s just a small part of what’s going on in the landscape out there,” says Robert Beschta, a hydrologist at Oregon State University, Corvallis, who has studied the effects of wolf predation in Yellowstone.
Gable notes wolf impacts add up with time. Over 10 years, he estimates, Voyageurs’s wolf packs may affect one pond for every 2.1 square kilometers of land. And the phenomenon may not be isolated to northern Minnesota. “Given the fact that wolves and beavers co-occur across a substantial portion of the Northern Hemisphere,” Gable says, “this mechanism is likely occurring everywhere wolves are preying on beavers.”