Wolves Shape Ecosystems By Preying On Beavers

A wolf skull with a beaver skull in its jaws. Beavers are immensly significant ecosystem engineers. By preying on dispersing beavers, wolves shape where their ponds are built and perhaps how many species an ecosystem can support. Voyaguers Wolf Project

Wolves' place in the environment is a hot topic right now. Colorado just narrowly voted to bring them back. The Trump administration decided gray wolves aren't endangered, even though scientists think they are, so people can hunt them. A new study finds wolves can change the environment in a manner not previously recognized, by preying on beavers without changing their behavior or overall numbers.

Beavers are vital ecosystem engineers. By building dams they slow the flow of rivers, cause pollutants to settle, diminish the effect of floods, but provide a greater flow in dry seasons. Animals and humans alike benefit from their presence, which is why they are being introduced to many places where they were once hunted out.

Wolves were not really thought to have a part in this, until a surprise observation. “In 2015, we documented a wolf killing a dispersing beaver in a newly-created pond," said Austin Homkes of the Voyageurs Wolf Project in a statement. "Within days of the wolf killing the beaver, the dam failed because there was no beaver left to maintain it.”

Dispersing beavers is the name given to juveniles leaving the parental lodge to establish a new colony. Although beavers build dams to protect their lodges from predators, it seems those in the process of establishing a new home are far from safe. Homkes' observation made zoologists wonder if there might be a period of vulnerability before the dam is properly established.

Homkes has now co-authored a paper in Science Advances exploring this possibility both by counting maintained and abandoned ponds, and by fitting wolves with tracking collars to monitor their beaver kills. Five years later, the pond in question has not been colonized by another beaver. Meanwhile, the authors found the wolves they tracked killed 4.5 beavers per wolf per year. In the process, wolves in Minnesota’s Greater Voyageurs ecosystem prevented the occupation of 88 ponds a year.

Given the vital role beavers play in the ecosystem, it might appear at first sight that the wolves' influence is destructive, but Homkes and co-authors think overall beaver numbers are determined by other factors. Instead, wolves change the distribution of dams, possibly facilitating a patchwork of forests and wetlands rather than larger aggregations of wet and dry areas. If so, they may be creating a richer and more diverse ecosystem than if the beavers were left to their own devices.

The idea that small numbers of apex predators can “rewild” areas, restoring them to near their natural state, caught on after a talk about the wolves of Yellowstone Park went viral. However, some scientists have questioned whether the effect in Yellowstone, and by implication elsewhere, has been greatly overstated. Homkes’ findings suggest the wolves influence many paths that may not be fully understood.

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