Wolves, cougars ambush smaller predators attracted by scraps

Wolves, cougars and other large predators are killing smaller carnivores, potentially controlling notoriously fecund species like coyotes.

Those were some of the findings of a University of Washington study published earlier this month in the journal Ecology Letters. The study synthesized data from 256 other studies looking at predation between carnivores, scavenging and resource availability.

The researchers found that small carnivores are generally drawn to the kill sites of larger predators, which should surprise no one. When the larger animals return, they often kill the small ones. The study, reflecting its regional interest in Washington, looked specifically at wolves, among other top predators.

“We initially thought maybe smaller carnivores are scavenging the wolf kills and benefiting,” senior author Laura Prugh, a wildlife ecologist and associate professor in the UW School of Environmental and Forest Sciences, said in a news release. “But then we realized that at these scavenging sites, they might be running into the wolves and getting killed. The scavenging, instead of providing a benefit, could actually be functioning as a trap that’s drawing in the smaller carnivores.”

The impetus for the study came from an observation Prugh made while working in Alaska’s Denali National Park. Her work there focused largely on how wolves (and other large predators) impact smaller predators called mesocarnivores. After finishing her postdoctoral work, she worked at the University of Alaska Fairbanks for nearly four years before heading south to UW.

“I’ve always been interested in how species interact with each other,” Prugh said. “Predator-prey interactions. How a change in one part of an ecosystem can affect other parts through these indirect pathways.”

During her time in Alaska, she noticed that smaller carnivores tended to scrounge wolf kills, but oddly, that didn’t seem to correlate to larger mesocarnviore populations.

“When we looked at a larger landscape scale, the smaller carnivores were a lot more abundant where the state had reduced the wolf population,” Prugh said in an interview.

She’d hoped to find other studies looking at both scavenging and mortality. She couldn’t, but the data from studies looking at mortality and data from studies looking at scavenging were consistent with her Alaskan observations.

The UW study also found that the diversity of carnivore species had a large overall impact on predation of smaller carnivores.

“When you had a system that just had two large carnivore species in it, the mortality rates were less than half than when you had three large carnivore species,” she said.

The findings could potentially have wildlife management implications. Smaller carnivores are, for the most part, equal-opportunity eaters. That’s means that they “tend to have widespread impacts on ecosystems.” Sage grouse near the Swanson Lakes, for instance, are competing with large coyote populations. Human-led efforts to kill the animals are mostly ineffective.

“If scavenging increases the risk of mortality of smaller carnivores, that might explain why it appears to be very hard for humans to replace the role of large carnivores in a landscape,” Prugh said in the release. “This link between scavenging and mortality might be one of the mechanisms that make large carnivores so effective in controlling smaller carnivores.”

If Prugh’s findings are correct, it might point toward one option for controlling populations of smaller carnivores.

“If we want to keep them at lower levels, that diversity becomes pretty important,” she said.

The study isn’t based on data from Washington, instead relying on a mishmash of other studies from other ecosystems.

That will change soon.

In 2016, UW, in conjunction with the Washington Department of Fish and Wildlife, embarked on an ambitious project hoping to document the various connections between humans, predators and prey, and how those relationships change with the return of wolves and other predators. Called the Washington State Predator-Prey Project, the large survey hopes to inform management of the “myriad species that comprise these systems.”

Since coming to UW, Prugh has tried to understand the tangled web of connection between predators, prey and – increasingly – humans. Since 2016, she’s worked on the Predator-Prey Study. Unlike in Denali National Park, neither of the two primary study areas – one in Central Washington and one in Eastern Washington – contains protected natural areas. Instead, it’s a mix of private and forest service land, all heavily used by people.

That’s given her, and others, a unique opportunity to see whether wolves and other animals are having the same ecosystem-wide impacts they’ve had in other “protected areas where they have mostly been studied so far,” she said.

“Washington has become such an exciting area for large predator research,” said Aaron Wirsing, an ecologist at UW, in a previous interview.

Wirsing was not involved in Prugh’s study, but he is working on the larger predator-prey study.

Wolves, famously and not quite accurately, have been cast as a great ecosystem balancer. A panacea against all sorts of excess. Most notably, biologists and ecologists noticed that heavily eroded streams in Yellowstone started to heal after wolves were reintroduced in 1995. What was going on? One theory, which went viral in a YouTube video titled “How Wolves Change Rivers” (41 million views and counting), argues that wolves reduced the elk population which, in turn, gave damaged streams, eroded by constant foraging from vociferous elk, a much-needed respite.

This linear, input equals output, hypothesis was attractive to wolf advocates and others looking for simple explanations to complex issues. Unfortunately, it’s not completely accurate. Instead, more nuanced research indicates the return of beavers may have played a larger role.

Which doesn’t mean wolves are without beneficial impacts, as indicated by Prugh’s recently published paper. Instead, their impacts must be viewed in the larger context.

The predator-prey project is still gathering data. Some findings may be published within a year, Prugh said. In the meantime, she hopes her paper spurs researchers in other regions to look more holistically at how carnivores interact.

“We’re not really getting a full understanding of how carnivore communities function by examining them separately,” she said.