How Wolves Are Driving Down Mountain Lion Populations

Science
پنجشنبه 2 بهمن 1399 ساعت 14:41
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Smith sonian mag
The Lava Mountain Wolf Pack, the most populous of its kind in the American West, moved into carnivore biologist Mark Elbroch's study site in Wyoming's Teton Range in 2014. Various wolf packs took up residence in the nearly 900-square-mile area over the years, and this pack settled atop a massive, rocky cliffside. At the base of the cliffs lived a mountain lion named F47 and her three kittens.
Over the course of three months, the wolves killed off F47's kittens—one each month. Three other kittens in the study site also fell prey to the pack. Elbroch, the director of Panthera's Puma Program, didn’t see wolves attack the young cats with his own eyes, but he saw the aftermath: Bloodied bits of dismembered kittens strewn across the ground. "It was hard for us to watch," he says.
In the mid-1900s, scientists started trying to understand how North America's carnivores, like wolves and mountain lions, interact. Previous studies suggested that in places where mountain lions and wolves compete, wolves usually come out on top by stealing the lions' kills or changing where the cats hunt. But wildlife biologists weren't sure just how detrimental wolves could be to mountain lion populations. A study published in November in the journal Proceedings of the Royal Society B provided the first evidence that when the two species overlap, wolves have a greater effect on mountain lion populations than hunting by humans and the availability of prey. And the ways scientists say that wolves impact populations? By starving out adults and killing kittens.
In 1995, the U.S. Fish and Wildlife Service and biologists from Yellowstone National Park reintroduced gray wolves to the park after they were killed off from the region in the 1920s. The reintroduction created an invaluable opportunity for biologists to study how wolves shape their environment. Nearby, in the Grand Tetons, scientists established a study site in 2000 to monitor what ecological changes stemmed from the wolves’ return, and various teams from conservation organizations, universities and state and federal agencies used the locale to untangle how wolves interact with other species.
In an earlier study, Elbroch and his team reported that over the course of 15 years, the mountain lion population in the Greater Yellowstone Ecosystem in Wyoming dropped by 48 percent . They considered three main theories for the decline: human hunting of cats, a lack of prey or the presence of wolves.
To figure out what exactly cut the mountain lion population nearly in half, Elbroch and his team analyzed data gathered on 147 mountain lions living in the study site. During those years they tracked down cats and kittens and equipped them with telemetry or GPS collars that reported the predators’ locations. Not only did the collars provide Elbroch with an understanding of how the cats moved through the landscape, but they also helped the team locate mountain lions after they died—which allowed the researchers to determine how a cat met its demise. They then plugged that data, along with data on elk and wolves, into a population model to reveal patterns in the relationships between the three species.
The model was able to determine the strongest drivers of the declining mountain lion population during the study period, as well as forecast how the cats would fare in the future. Elbroch found the growing population of wolves was the culprit behind the massive decline.
Elbroch suspects that the drastic drop was largely due to wolves affecting the cats’ access to prey, namely elk. In a wolf-less study site, elk herds resided in the comfort of the mountains; when the wolves moved in, the herds started congregating in large groups in the open grasslands to protect themselves from a pack attack. Since mountain lions stealthily stalk and ambush their prey under the cover of brush, they couldn't reach the elk in the grasslands, and starvation became a more common cause of death among the cats.
The kittens didn’t fare well, either. The model showed that over the 16 years, roughly a third of kittens at the site survived until they were six months old, and only around a quarter ever made it to their first birthday. "This is the lowest survival ever reported for kittens anywhere," Elbroch says. For the youngsters, the predominant cause of death was from brutal wolf attacks that could wipe out entire generations of kittens, like the Lava Mountain Pack did in 2014.
The most recent study’s findings revealed that the wolves' impact on mountain lion abundance is so strong that the yearly average mortality associated with human hunting is comparable to the effects of just 20 wolves. That finding is remarkable given that more than 90 wolves were once recorded living in the study site, which would have placed an enormous amount of pressure on the cats.
Elbroch was shocked at how wolf-related deaths completely eclipsed those from hunters. "The narrative for mountain lions for the last 50, 60 years has been that human hunting is the primary driver of population dynamics for mountain lions, and that is absolutely true—except where they overlap with wolves,” he says. “Now we're going to have to rewrite the whole narrative that rolls up the bigger impact on population dynamics for mountain lions, which has huge ramifications for their management."
While mountain lion populations have expanded across most of the American West, wolves are still trying to recolonize parts of their historic range. Since wolves affect mountain lion abundance so greatly, the study suggests that in places where wolves and mountain lions overlap, officials should implement more restrictive hunting quotas on cats to avoid a crash in their population.
Korinna Domingo, the founder and director of the Cougar Conservancy, who was not involved in this study, suggests that wildlife managers should pause mountain lion hunting or reduce the quota in areas where wolves are moving back into the ecosystem. Doing so will allow the cats to deal with one major threat at a time instead of trying to evade both hunters and wolves simultaneously. "That needs to be a very serious consideration," she says.
But such scientific recommendations aren't always implemented in practice because wildlife management decisions don’t just depend on the science, they also rest on public input. The major comebacks of wolves and mountain in recent decades have been met with backlash, Elbroch says. Hunters argue that carnivores prey on elk and deer, reducing the number that are around to hunt, and ranchers worry that carnivores will prey on their livestock. When wildlife biologists come up with plans to manage carnivores, they try to balance hunters' and ranchers' desires to kill them with what the science says.
"I can build a population model and really look at what's the best way to actually manage populations based [for] mountain lions, or wolves or deer populations," says Mark Hurley, a wildlife research manager for Idaho, who was not involved in the study. "But when it comes right down to it, sometimes that really doesn't matter because it's people's values [at play]."
Oftentimes, the demands of stakeholders outweigh what the science says, according to Elbroch. Agencies are managing for the constituents that want fewer—or no—carnivores on the landscape, he says.
Given that wolves are recolonizing their historic ranges across the American West and moving into mountain lion territory, states that do not already have wolf populations will soon be faced with the challenge of managing them. They'll also have to reevaluate how they manage mountain lions when the wolves take up residence.
"Wolves are now in Washington, Oregon, California, Idaho, Wyoming, Montana and moving outward still,” says Elbroch. “And these are all states that have aggressive mountain lion management. It will be fascinating to see how they decide to alter their management of mountain lions as wolves move into the system."
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