25 years after returning to Yellowstone, wolves have helped stabilize the ecosystem

August 5, 2020
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Twenty-five years after gray wolves returned to Yellowstone National Park, the predators that some feared would wipe out elk have instead proved to be more of a stabilizing force. New research shows that by reducing populations and thinning out weak and sick animals, wolves are helping create more resilient elk herds.

For the past 12 years, elk numbers in the park’s largest herd have leveled off between about 6,000 and 8,000, instead of extreme boom-and-bust cycles due to climate fluctuations.

“Elk aren’t starving to death anymore,” says Chris Wilmers, a wildlife ecologist at the University of California, Santa Cruz.

During years with normal amounts of rain and snow, wolves primarily kill older cow elk, since they’re the easiest to hunt. But Wilmers led a recent study that showed during particularly dry years—when grass, shrubs, and wildflowers aren’t as lush—wolves switch to hunting bulls. These burly males already don’t eat much in the fall, focusing instead on making deafening bugles and smashing into each other while fighting over cows. All that effort burns calories, weakening them heading into winter. In dry years, they’re even more diminished.

As adaptable, intelligent predators, wolves have learned to recognize these conditions, and they would rather kill an undernourished 750-pound bull versus a 450-pound cow. So by targeting bulls during years of scarce food, they give the cows a chance to reproduce, thus keeping the population afloat. (Explore the Yellowstone most don’t see.)

But most importantly, the Yellowstone area’s wolves—which now number between 300 and 350—could help elk herds weather the perils of a more volatile climate, according to the study, published in the Journal of Animal Ecology. For instance, elk herds that maintain consistent numbers, rather than yo-yoing up and down, can better withstand more frequent droughts—one impact of climate change that is already occurring in the region.

“In a future that will be very unpredictable, we want a buffer” against mass die-offs, says Doug Smith, Yellowstone’s senior wildlife biologist, and wolves’ ability to keep elk herds balanced can play that role. Through hunting and management practices, “humans help stabilize elk populations, but they don’t do the same thing as wolves.”

The findings in Yellowstone are also relevant, he adds, as Coloradans vote in November whether to reintroduce wolves to their state, home to about 287,000 elk—the largest number in the U.S.

Finding a balance

For their study, Wilmers, Smith, and colleagues analyzed more than a thousand dead elk in Yellowstone—located in the states of Wyoming, Montana, and Idaho—over 20 years. The scientists spent about a month at the beginning and end of each winter tracking three wolf packs, locating every elk kill the wolves made; recording the dead animal’s age and sex; and removing a bone marrow sample, which determined the elk’s physical condition before death. The team then used satellite data to derive how much plant life was available for elk to eat each year, an amount dependent on snowmelt and rainfall.

Synthesizing all of that data revealed that wolves target bulls during years when vegetation is poor, offering a clearer understanding of how shifts in climate can change predators’ behaviors.

This is especially useful for managing and conserving wolves, which are still rebuilding their numbers after over a century of persecution. Before the 1900s, Yellowstone predators such as grizzly bears, black bears, wolves, and mountain lions thrived alongside robust populations of American bison, elk, mule deer, pronghorn, and bighorn sheep.

But a coordinated campaign by the federal government exterminated almost all those predators, and bison, from the area. The last known Yellowstone wolf pack was killed in 1926, and the canines were also wiped out in most of their historic range in much of the lower 48, hanging on in a few populations around the Great Lakes. (Read more about the history of Yellowstone National Park.)

Read More: https://www.nationalgeographic.com/animals/2020/07/yellowstone-wolves-reintroduction-helped-stabilize-ecosystem/

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