Study Shows Mountain Lions Have Unexpected Predator

“The big take home of this paper is that wolves have the strongest effect on the survival and abundance of mountain lions in the system,” Dr. Mark Elbroch told MeatEater. Elbroch is the lead author of the study and the puma program director for Panthera, a wild cat conservation group.

“It’s dramatic. Wolves are a power on the landscape. We saw cats respond, but we thought it was anecdotal. The strength of the [wolves’] effects surprised every one of us,” Elbroch said.

Mountain lion populations decreased 48% in the southern Greater Yellowstone Ecosystem between 2000 and 2015. Biologists have tried for the last several years to identify the primary driver in this decline. A 2018 study by Elbroch and his colleagues posited that the drop was likely caused by three primary factors: regulated human hunting, wolves killing lion kittens, and increased starvation.

This latest paper, which Elbroch and his co-authors published late last year in the peer-reviewed academic journal Proceedings of the Royal Society B, indicates that wolves drive both kitten mortality and starvation, and their impact eclipses human hunting by a factor of four. In concrete terms, the study estimates that just 20 wolves can account for the effects of all human predation in the study area.

Digging Deeper How have wolves so dramatically reduced mountain lion populations? The fact that wolves kill lion kittens grabbed headlines when the study was published last year, but Elbroch told us that, based on the newest research, the effect of wolf packs on elk herds is likely a bigger driver of the decline.

Only about five wolves occupied the 888-square-mile study area when researchers began tracking mountain lion populations in northwest Wyoming in the year 2000. The wolf population peaked at 91 individuals, and researchers noticed a dramatic shift in the location of the elk herds.

“When wolves dropped into the system and started to increase, the elk started coming out of the woods and hanging out in the open. More and more of them started moving onto the National Elk Refuge in dramatic numbers,” Elbroch said.

Elbroch and his colleagues noticed a 70% shift in the location of the elk population. Rather than living in the mountains where lions could more easily stalk and kill them, the elk began congregating in the open where weaker elk could find safety from wolf packs.

“There was a huge reduction in the number of elk available to mountain lions, which is what we think was driving a lot of this starvation,” Elbroch said. “The wolves were blocking elk from mountain lions in two ways. One, by moving them into places where mountain lions are uncomfortable, like the wide open. And two, by literally chasing them off their kills.”

While there’s evidence to suggest elk avoid areas of the landscape where mountain lions might be present, wolves have forced them to change their behavior to a greater extent than cats have. “In our system, as wolves became a power on the landscape, elk seemed to be avoiding wolves more than trying to avoid cats,” Elbroch said.

Starvation drove most of the lion decline, but kitten predation was still a major problem. Over the course of the study, only about a third of kittens survived until they were six months old, and only around a quarter ever made it to their first birthday. Wolf predation was the predominant cause of death.

"This is the lowest survival ever reported for kittens anywhere," Elbroch told Smithsonian Magazine.

The wolves didn’t seem to be interested in the young lions as food. Elbroch said researchers would find kitten parts strewn on the ground after a wolf kill, which he interpreted as the wolves treating the cats as competition. Elbroch’s team observed one instance in which a mountain lion killed a wolf pup, but Elbroch said that that behavior is rare even outside their study area.

A “Solid” Study It’s worth noting again that Elbroch’s study was funded in part by Panthera, a wild cat advocacy organization. The wildlife research institute Craighead Beringia South contributed funds to the project in the beginning, but Panthera took over at the end to “wrap it up,” Elbroch said.

We ran the study past Jim Heffelfinger, a wildlife research scientist at the University of Arizona and a frequent guest on the MeatEater Podcast. Heffelfinger called the paper “interesting and solid from a scientific standpoint” and appreciated that the “Discussion” portion of the study remained reasonable without undue speculation or advocacy. He noted that researchers couldn’t tease the effect of the declining elk herd apart from the effect of the wolves, which he described as “a weakness they could not control or overcome.”

Elbroch admitted that wolves and elk are so closely correlated that separating their effect on lions is extremely difficult. The paper also acknowledges that cats likely starved both from wolves blocking access to elk and from decreasing elk numbers. Elk managers and hunters in the study area have reduced the herds by about 30% since the study began, and Heffelfinger argued that such a reduction in prey should not be underestimated.

Elbroch nonetheless insisted that the number of elk is less important than the location of the elk. “We know that wolves are playing a role in limiting elk, but I think the number of elk is more driven by human hunting than any carnivore in the system,” Elbroch acknowledged. “But the wolves did something completely different: they changed where the elk were on the landscape at different times of year.”

Elbroch and his team were able to draw their conclusions using a relatively new kind of analysis called an “integrated population model.” Where previous models forced researchers to work with only a single kind of data, this model allowed them to input a variety of information types, including kitten counts by researchers at lion dens, abundance estimates for different animals, and GPS and VHF collar data.

“With the traditional survival analyses of 10 years ago, you had to have one kind of data that was consistently collected because the models wouldn’t allow you to pull in different kinds of data with different rules into a single analysis,” he said. Their integrated population model allowed them to “pull it all together in a single, more elegant execution of the analysis.”

What Now? If Elbroch’s findings are proven by further research, the paper represents a shift in how we understand mountain lion mortality.

“Everyone in the West says human harvest has the greatest effect on mountain lions, and I think overall, that’s true, in most systems. But most systems don’t have wolves,” Elbroch said.

Elbroch emphasized that the mountain lion population isn’t on the verge of collapse. “We know that historically cats and wolves have lived together. I have no expectation that wolves will drive them to the dust and there will be no more mountain lions in the mountains.”

He does, however, expect a new normal. “We’ll see a new pattern where mountain lions are never going to be as abundant as they were when there were no wolves. That’s just the reality. That’s not good or bad. This is just the way it plays out,” he said.

He and his colleagues have not called for an end to mountain lion hunting, but the paper does recommend that wildlife biologists take proactive steps to protect lions. It’s unclear what that might look like when translated to hunting regulation. Elbroch acknowledged (somewhat wryly) that state agencies must account for a wide variety of factors, and that the abundance of one specific species isn’t always at the top of the priority list.

Many Western states may be forced to make decisions whether they will work to reduce the number of wolves, set stricter limits on mountain lion hunting, or continue on the current trajectory. Elk feedlots in Wyoming likely “exacerbated” the problem, Elbroch said, but most regions with wolf populations should expect some reduction in the number of lions.

Hunters may also have to adjust to this new normal, Heffelfinger pointed out. Wolves will alter the argument that human hunters alone must manage prey numbers. Now, it looks like humans may not be the primary force managing second-level predators, either. It might be true, as mountain lion hunters have claimed, that there are more mountain lions in the modern era than ever before in the relatively recent history of North America.

“The 95% of the public that doesn't hunt might see it as selfish that we don't want wolves restored because that means fewer cougar and elk tags,” Heffelfinger said. “We hunters have to tread lightly here. Cougars and wolves are not funding the North American Model of Wildlife Conservation, but they deserve to be restored. I think hunters may have to concede some tags in the long run to make room for wolves [and] to maintain the current high support from the nonhunting majority.”

Whatever happens at the policy level, Elbroch’s paper provides a window into what the landscape looked like before European contact. “We don’t know the way North America was before European settlement. We just have stories. I think it’s fair to conjecture that wolves limited cats across North America, especially in these open terrains,” he said. “Wolves very likely put bookends on the number of mountain lions that could live in a place. For me, that’s fascinating ecology to ponder.”

Feature image via John Hafner.