Scientists discover a completely unexpected way wolves alter their ecosystems
This is a different kind of ecosystem engineering.
As fearsome apex predators at the top of the food chain, wolves linger in the cultural imagination as a symbol of true, dangerous wilderness.
But aside from playing on our imagination, scientists have long been believed wolves to be capable of powerfully reshaping their ecosystems through indirect interactions known as trophic cascades. But a new study published Friday in Science Advances reveals that this is not the only way they assert their influence on their environment.
And it has to do with one of wolves' innate characteristics — the ability to inspire fear.
Over the course of 2015 through 2019, scientists in the Voyageurs Wolf Project used GPS collars to track 32 gray wolves in the southern boreal ecosystem of northern Minnesota.
They examined the ways in which wolves affect their ecosystem by killing a humble, yet powerful ecosystem influencer — the beaver. Beavers dramatically "engineer" their landscapes by building hundreds, or even thousands, of ponds and dams that trap trees and vegetation in streams.
A gray wolf with a GPS collar.
"Ecosystem engineers are basically any species of animal that disproportionately impacts the ecosystem through their physical actions," Thomas Gable, lead author on the study and project lead for the Voyageurs Wolf Project, tells Inverse.
"A beaver is considered an ecosystem engineer because they build dams, and by building these dams, it impacts this huge part of the landscape," he explains.
Going into the study, Gable and his colleagues wanted to discover what happens when gray wolves prey on those dam-creating beavers. What they found significantly diverges from older research looking at the two species' influence.
Past studies often focused on gray wolves preying on elk in Yellowstone — work which has given rise to a scientifically-contested idea known as 'trophic cascades,' Gable explains.
A beaver swims in a pond.
The idea, Gable says, goes somewhat like this: "By instilling fear into their prey, [the wolves] changed the way that their prey were moving around the landscape. And by changing the way the elk were moving around the landscape, it prevented the elk from browsing as heavily on vegetation in certain. And, as a result, the vegetation started to grow better and recover. And that allowed a bunch of other species to come back into the ecosystem in higher numbers."
But Gable's study challenges the concept of trophic cascades, inviting a new way of thinking about how gray wolves influence the ecosystems around them.
"In our area, we actually don’t think wolves are having an impact on beavers via trophic cascades," he says. "Instead, we just think wolves are directly killing beavers, and when they directly kill a beaver, the beaver that’s created a pond basically drains it and doesn’t exist anymore because there’s no beaver left."
Essentially, their research suggests wolves may not directly impact the number of beaver ponds, but may instead alter the geographic distribution — the location — of these ponds.
A beaver nibbling on a stick.
"There’s not a lot of evidence to suggest that wolves are actually reducing beaver population size," Gable says.
The study relies on a key concept called "compensatory mortality," which helps to explain how populations change.
"The general premise is because some animals in a population die, it allows others in that population to continue or to live. And that’s what we think is happening with the beaver ponds," Gable says.
The study holds implications for understanding the affect of gray wolf populations on beavers living together in any boreal forest in the Northern Hemisphere, not just Minnesota, where this study took place.
"The premise is pretty straightforward. Dispersing beavers create new ponds. Wolves kill dispersing beavers that are creating ponds, and when they do it, the pond drains, and then there’s no pond there," Gable says.
"So we suspect that this is likely happening in a variety of different ecosystems."
An aerial survey of beaver dams and ponds.
Political implications — The findings have wider societal implications, too. The conservation of wolves is a heavily politicized issue in the United States.
In an order that goes into effect on January 4, 2021, the Trump administration effectively strips gray wolves of their protections under the Endangered Species Act. When it was announced, the measure prompted an outcry from wolf conservation groups, who are challenging the order in court.
"Because a lot of the discussion around the politics or management of wolves really comes down to societal values and not, what wolves do as a species?" Gable says. The main motivators, he says, seem to be "what does society want?"
"Do we want more wolves? Do we want less wolves? Do we hunt them? Do we not want wolves to be hunted?" He asks.
Whether this study justifies different protections for gray wolves will ultimately depend on the cultural beliefs of the person reading it, he says.
"All our work is doing is simply trying to understand the role that wolves play in ecosystems as a top predator. Whether or not people think that’s a good thing — that wolves are altering where beavers build ponds — will depend on the individual," Gable says.
Abstract: Gray wolves are a premier example of how predators can transform ecosystems through trophic cascades. How-ever, whether wolves change ecosystems as drastically as previously suggested has been increasingly questioned. We demonstrate how wolves alter wetland creation and recolonization by killing dispersing beavers. Beavers are ecosystem engineers that generate most wetland creation throughout boreal ecosystems. By studying beaver pond creation and recolonization patterns coupled with wolf predation on beavers, we determined that 84% of newly created and recolonized beaver ponds remained occupied until the fall, whereas 0% of newly created and recolonized ponds remained active after a wolf killed the dispersing beaver that colonized that pond. By affecting where and when beavers engineer ecosystems, wolves alter all of the ecological processes (e.g., water storage, nutrient cycling, and forest succession) that occur due to beaver-created impoundments. Our study demonstrates how predators have an outsized effect on ecosystems when they kill ecosystem engineers
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