'Landscape of Fear' Not Impacting Yellowstone's Elk

As discussed below, thinking about Yellowstone wolf reintroductions continues to evolve.

NOTE: this article was originally published to Science.org on June 11, 2013. It was written byVirginia Morell.

Who's afraid of the big bad wolf?

Not elk in the Greater Yellowstone area of Wyoming and Montana. That's the contention of a new study that disputes the notion that Rocky Mountain gray wolves, which were reintroduced into the region in 1995, have turned the once peaceful area into a "landscape of fear." After the wolves' return, scientists noticed that the aspen trees and willows began to recover, while elk numbers declined. Researchers attributed the trees' new growth to the wolves, because the elk could no longer blithely feed; they had to be vigilant and on the move. That added stress, some suggested, could also cause female elk to have fewer successful pregnancies, which would account for the elk population's dropping numbers. But a new study published online today in Ecology Letters suggests that the elk aren't that stressed by the wolves. After tracking both species in the region for three winters, and recording elk behaviors as wolves approached, the scientists argue that elk haven't dramatically altered how or where they feed. Only when wolves approach an elk within 1 kilometer (which happens on average about once every 9 days), as in the photo above, do elk pay close attention. The scientists also collected data on female elks' body fat and pregnancy rates and compared these to 19 other populations in the northwest that aren't hunted by wolves—there was no noticeable difference. Still, wolves do affect the elks' numbers in one way: They eat them.

NOTE: below was taken from Science magazine's October 2024 issue, also written by Virginia Morell

In 1995, officials began to reintroduceseemingly triggering an ecological transformation. Elk had been overrunning thepark, because most of their predators hadbeen hunted out. But the few dozen returning wolves apparently had an outsize impact. They created a “landscape of fear,” scaring elk away from their favorite dining spots of aspen stands and willow thickets. Elk ate worse diets, had fewer offspring—and trees, rivers, grasses, even songbirds returned, in a glorious example of a revitalized ecosystem. Or so the story went.

Now, 29 years later, a detailed study challenges that tale. What eased the impact of elk wasn’t fear of wolves, but a guild of predators, including human hunters, which killed enough elk to let vegetation partly recover according to a new paper out on 23 October in Ecological Monographs. There is little evidence wolves frighten elk merely by their presence, the authors say. They even chal- lenge the received wisdom that the park’s ecosystem has been restored.

“It’s high-impact science … with an unusually excellent long-term data set,” says ecologist Tom Hobbs of Colorado State University. “The Yellowstone wolf-elk system should no longer be held up as evidence that wolves changed elk behaviors and saved the park.”

That’s the simplistic version of the hypothesis ecologists William Ripple and Robert Beschta at Oregon State University put forth in the early 2000s: The presence of wolves—even their scent—helps trees mature by instilling fear in elk that would otherwise blithely graze the sprouts to the ground. The supposed impacts of the Yellowstone wolves, which numbered 124 as of January, have been celebrated around the world and become a model for using predators to restore other overgrazed habitats, such as parts of Scotland and a national park in Mozambique.

Elaine Brice, a postdoc ecologist now at Cornell University, remembers learning the story in high school biology: “Predators had magical protective properties and could fix damaged environments,” she says. But she later learned some studies cast doubt on the landscape of fear hypothesis. She wondered: “If wolves do have an effect on elk and habitat restoration, is it due to reducing [elk] numbers or to creating fear?”

Brice and colleagues analyzed data collected in 113 plots inside aspen stands in northern Yellowstone from 1999 to 2019. They measured the trees’ height and tracked elk feeding in the stands. Wolves’ impact on the elk was weak, they report. Male elk—which face greater risk from wolves—avoided the densest stands. But females did not change their behavior except in winter, when both sexes stayed away from aspen stands, where snow forms deep drifts that slow their flight. Both sexes also avoided feeding at dawn and dusk when wolves were out.

Overall, “Wolves may have changed some elk behaviors, but not to the degree necessary to really affect aspen growth,” says co-author and ecologist Eric Larsen of the University of Wisconsin-Stevens Point, who collected the aspen field data.

It seemed more likely the trees got a respite simply because elk numbers dropped. In 1997, northern Yellowstone elk numbered nearly 20,000; a decade later the area held 8335 elk and 101 wolves. But Brice and her team say wolves alone didn’t thin the herds.

Instead, drawing on other studies, the scientists show elk numbers were brought down by multiple predators: grizzly and black bears, cougars, wolves, and human hunters. From 1995 to 2011, humans killed 16,700 elk and wolves killed 9100. The decline in elk had far more impact than the relatively small change in their behavior, Brice says. “There were fewer elk, so the trees had a chance.”

The scientists also conclude that in fact the northern park has only patchily recovered. Aspen in northern Yellowstone are still struggling to grow into mature forests.

“This study provides convincing evidence that wolf effects on Yellowstone’s aspen are likely weaker than earlier studies indicated,” says Laura Prugh, a wildlife ecologist at the University of Washington, Seattle. But, she says, “Weak as they are, they’re still effects. … Wolves [can’t] kill elk without also scaring them.”

Ripple, who advanced the original idea, thinks the new study may have missed some data. “Their one small plot per sampled aspen stand may not have been large enough” to capture predation risk for elk in the entire stand, he says. Beschta thinks the data show “predation risk did have an important role,” allowing young aspen, alder, and willows to grow taller.

Ripple adds that testing such hypotheses will always be difficult in a large ecosystem where events unfold more like a multiplayer drama than an experiment. Today, bison—also heavy grazers—roam where elk used to; drought has stunted some aspen; beaver are beginning to return. “This is not the end of the ‘landscape of fear’ hypothesis,” he insists. “The science is just in its infancy.”

Brice agrees. But she cautions that “you can’t just restore a predator and expect everything—plants, insects, rivers—to recover. It’s far more complex than that.”