Researcher paints broad picture of wolf-prey interactions with Jackson Hole study

BILLINGS – Since wolves were reintroduced into Yellowstone National Park in 1995, Kristin Barker jokingly estimated there have been “12.75 bajillion studies” conducted to answer: How a recovering population of predators would affect their prey populations as well as other aspects of the ecosystem?

“So after several decades of good, concerted study, what have we learned?” she asked a crowd gathered at the Buffalo Bill Center of the West in Cody, Wyoming, on Thursday.

Barker is the research coordinator for the Beyond Yellowstone Program, a new group supporting local conservation efforts with best available science. Her presentation was based on her most recent research into wolf avoidance of humans using collared wolves and cow elk in the Jackson Hole area.

“We have learned that wolves decrease the population size of their prey … sometimes. After wolf reintroduction, some elk population sizes decreased, others remained stable and others increased,” Barker said.

“We have also learned that wolves change the behavior and distribution of their prey … sometimes,” she added. “Sometimes wolves show up and elk bounce into a drainage. Sometimes wolves show up and elk give them the side-eye and go back to eating.”

Variations

Likewise, Barker said some studies have shown the reintroduction of wolves to Yellowstone caused a trophic cascade, where the reduction in elk caused aspen regeneration leading to more songbirds and habitat for species like beavers. Other studies have debunked this idea. Why are the results conflicting?

There are variations in the landscapes studied, but that doesn’t explain the gaps, Barker said.

“One thing that could be going on is that prey should be responding to the actual risk of predation on the landscape,” she said. “But knowing what predation risk actually is really hard to measure and identify.”

Consequently, researchers often use substitutes. So an open, flat landscape might be considered a high-risk area for prey species, as would a place where wolves spend more time.

“But we know those proxies don’t always represent the actual risk of an animal dying on the landscape,” Barker said.

Since wildlife, both prey and predators, will alter their behavior around humans, this factor also has to be considered in research examining predation. For example, the human shield hypothesis argues that large predators avoid areas where there are humans. On the other hand, there’s the argument that humans might increase the risk of predation because their watered lawns or agricultural fields may concentrate prey species like elk and deer, providing predators an attractive source of food.

Jackson Hole

Barker’s study set out to try and answer some of these questions in the Jackson Hole area in winter. To do that, the researchers identified areas where collared wolves congregated and then investigated to see if they had made a kill. If so, the animal remains were analyzed through necropsy to figure out if the wolves were scavenging or had made the kill themselves. Out of more than 1,000 sites checked, 170 had wolf kills – mainly elk – that were analyzed in the three winters between 2019 and 2021. The majority of other places where wolves concentrated were bedding areas.

The region provided a good sample of varied conditions. Jackson Hole is close to people, roads and trails. It also has elk winter feedgrounds, attracting the animals. In the region’s forests, wolves are hunted, whereas nearby Grand Teton National Park does not allow wolf hunting.

“So basically what we were doing is we were figuring out how the probability of a wolf killing something in a certain area changed as a function of environmental influences and human influences,” Barker said. “And we tested whether different versions of those things matched what we saw in reality.”

Findings

The research showed wolves were more likely to make kills in wooded areas, places with shallow snow and in drainages and valley bottoms. Humans were found to increase and decrease the risk of predation, depending on the situation.

So there was a higher risk of predation near feedgrounds, snowmobile and ski trails, but mostly at night. Wolves tended to avoid roads.

“Something else we also found is that prey availability is a big driver of how wolves respond to humans,” Barker said. “Wolves prioritize acquiring prey over responding to humans.”

That means when wolves expand into areas with less to eat, they may be more willing to take risks near humans to acquire food. Wolves use of trails rather than roads in the study area may be due to the fact that trail use occurred mainly in the day, while traffic was flowing into the night on roadways.

So humans have an effect on wolf predation, but wolves are constantly considering the risks of eating versus a possible interaction with humans.

“So you can’t paint a broad stroke and say, ‘Humans will decrease or increase the risk of predation,’ ” Barker said.

Bold elk

Next, the study looked at whether collared cow elk were likely to travel, forage or rest in risky places at risky times and to also see if they behaved differently depending on how much risk they had previously experienced.

What the research showed is that elk didn’t always change their behavior when they encountered a wolf. They did, however, more consistently change their behavior when they were in risky places. Some elk that had faced more risk were less worried about hanging out in areas where wolves were more likely to make a kill. Were their personalities just more risk averse, or were they just more interested in eating than being killed? Possibly the elk were in a bigger group, which the study couldn’t measure because only some of the elk were collared.

Barker theorized that without accounting for an individual’s experience with risk, researchers might have difficulty detecting common behavioral responses. This could explain why different studies have come to opposite conclusions.

“So it makes sense that someone studying an elk population that’s just experiencing wolves right after reintroduction would find different behaviors than someone studying a population that’s had wolves around for generations,” Barker said.

Also, with different human influences on the landscape, researchers are going to see different impacts of wolves on their prey, she added.

Wrap up

“So overall we see differences based on food scarcity, based on different kinds of human influences, based on elk experiences,” she said. “So this very clear-cut relationship that has been put forward of wolves affecting elk, affecting vegetation could absolutely be true in some places and times, but it would be extraordinarily unlikely for it to play out in the exact same way across all of the different places and times in this broad, varied ecosystem that people are looking across.”

Barker then asked her audience if wolves were the “ecosystem angel that is tirelessly restoring natural processes?” Or is the wolf the “ecosystem devil who is just eating thousands and thousands of animals and killing things for fun?” To each question, Barker firmly asserted the predator is neither.

“Wolves are just another species that has co-evolved in concert with all of the other species that have come together to make up the ecosystem that we live in here today … just another animal trying to survive in a pretty harsh place.”