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Yellowstone's Top Dog

Kathleen M. Wong

By killing elk, wolves are slowly resuscitating a stagnant ecosystem. Photo: Buff & Gerald Corsi, Manzanita Collection

It’s winter in the Lamar Valley of Yellowstone National Park, and the Druid Peak wolf pack is on the run. Boiling over a small rise, they head straight for a band of grazing elk. Tails streaming, tongues flapping, the wolves split up. One chases a big-racked bull, another a spring calf. Then an old cow stumbles in the heavy snow, and the entire pack swivels in her direction. A wolf launches itself onto her haunches, another leaps for her nose, and together they drag her to the frozen ground. A final bite to the neck, and she lays still.

Killing is usually viewed as an act of destruction. Yet by killing elk, wolves are slowly resuscitating the stagnant ecosystem of Yellowstone National Park. The elk, say biologists, were suffocating the Lamar Valley with their abundance. So the wolf’s return to Yellowstone after an 80-year absence has had the cleansing effect of fire on an overgrown forest.

“Wolves are to Yellowstone what water is to the Everglades. As a keystone species, they affect just about everything you can name,” says Doug Smith, leader of the Yellowstone Wolf Project.

Less than a decade after 31 wild gray wolves were imported to the park from Canada, their numbers have swelled to more than 148 animals in 14 packs. They have since spilled over the park’s northern border and into a 1,530-square-kilometer area of lower-elevation valleys known as the northern range. And the cumulative appetite of all those wolves, biologists say, has spread their influence across the landscape like ripples in a pond.

The park’s many scavengers have benefited directly from wolf largesse. Ecologist Christopher Wilmers of the University of California, Berkeley, and colleagues have found that wolf kills supply food to a whole constellation of animal scavengers much more effectively than do human hunters. Over three winters, the scientists observed 202 wolf kills and tracked which scavenger species fed from the carcasses. They also counted the scavengers that visited 28 elk gut piles left by winter hunters just north of the park.

The scientists report in the journal Ecology Letters that more than 12 local scavenger species, including black and grizzly bear, red fox, crow, Clark’s nutcracker, and coyote dined off wolf kills. By comparison, only four species—bald and golden eagles, magpies, and ravens—all of which were also common at wolf kills, showed up at hunter gut piles. It’s no coincidence, Wilmers says, that the scavengers visiting hunter kills are all birds capable of flying long distances. They’re after carrion the locals are too full to eat.

Wilmers also found that the timing of wolf kills makes a big difference to local wildlife. Because there are so many hunters, they provide far more meat in just a few weeks than wolves do over an entire winter. But with hunter kills, Wilmers says, “It’s boom or bust. Most just ends up rotting because there’s too much meat for local birds and coyotes to deal with. With wolves, which kill all year round, there’s a more steady food supply.”

In fact, wolves are most generous with their food just when other animals need it most. Food is usually scarcest for wildlife at the close of a long winter. Elk, rabbits, and other grazers must paw through ice to munch frozen grasses or strip bark from aspens. Fish stay out of sight beneath thick translucent lake ice. Coyote, bald eagle, and raven go hungry.

Meanwhile, wolves are at the top of their game. Running easily across the surface of the snowdrifts, a pack can soon bring down a weakened elk. The excellent hunting, says Wilmers, can make defending kill leftovers more trouble than it’s worth. A 120-pound wolf may eat nearly 20 pounds of meat at a sitting. “They get so full they can hardly move; they’re drunk with food. So if others are coming in there to bother them, it’s easier for them to just move away.”

According to park ornithologist Terry McEnaney, it’s too early to tell whether wolf kills are boosting bird populations. The regular presence of winter carrion, he says, may be drawing more bald eagles and ravens into the park. But these birds leave come spring thaw, scattering far and wide to build their nests.

Even the most powerful predator in the park now demands its share of wolf bounty. In spring, grizzlies emerge ravenous from months of hibernation. If they happen upon a wolf kill, they will commandeer the meat for themselves, driving off an entire pack. In fall, grizzlies rely on white bark pine nuts to add the final pounds to their rolls of hibernation fat. But occasionally, as in 2002, the pine produces no seeds at all. When that happened, the grizzlies turned to wolf kills instead.

Pronghorn, too, have benefited from the presence of wolves. Coyote once took a terrible toll on pronghorn fawns. Now wolves, which don’t usually usually attack pronghorn, do kill competing coyote, hunting them down at carcasses and chasing them from dens. Coyote density has dropped to half of what it was before wolves returned, giving pronghorn a chance to mature and mate.

Having to be on the alert for wolves has readjusted the attitudes of local elk. Elk once lingered to eat in the shallow brushy streams, and browsed without fear in the aspen forests.

Occasionally they fell victim to cougars and grizzlies, but few of these solitary predators live in the park. And coyote generally tackled only young animals, so the elk ambled through the Lamar almost without fear.

Then overnight, they found themselves prime targets of the park’s newest residents. The presence of wolves, Smith says, “forced the elk to think differently. Now they’re not going places where they can’t see very well, because a wolf will be hiding from view.”

The elk’s new attention to self defense has begun to alter the landscape. In a recent issue of the journal Forest Ecology and Management, William Ripple of Oregon State University and colleagues compare measurements of cottonwood trees and other vegetation in the Lamar Valley before and after wolves. “The plants are growing tallest in areas with the highest predation risk,” Ripple says.

Willows were among the first plants to respond to the changes in the elk’s eating habits, says Ripple. “Before wolves, willow were less than a meter tall in most places. Today we can walk through streams on the northern range and they’ll be way over our heads; you can’t even see a person behind them.”

The return of willow may indirectly be helping beaver and other animals that rely on native trees. Beaver strip the bark from young willow shoots to eat, and build their dams from aspen poles. In 1996, the year of the wolves’ return, surveys turned up only a single beaver colony in the entire northern range. Now, as willow sprout again along streambeds, eight colonies are building dams in the area. In doing so, they create dams rich in aquatic plants, fish, and insects. These creatures, in turn, support populations of mink, otter, and even moose.

Ripple admits it’s tough to be sure whether local trees are springing back because of the wolves or some other environmental factor. However, his theory is bolstered by the fact that the growth in the water-loving plant occurred despite five dry years.

Ripple has found yet another effect of the wolves. For years, white-barked aspens with the quivering green and golden foliage have become an increasingly rare sight in the West. “They’re just dying out. There’s very little regrowth of young trees into mature trees.” Many theories have attempted to explain the decline, including the idea that grazers have been nibbling the aspens into oblivion. If so, elk populations would have hampered the aspen’s peculiar habit of regenerating by sprouting new shoots as well as setting seed.

The return of wolves to Yellowstone, Ripple realized, would be the perfect place to test the idea. So he and graduate student Eric Larsen took core samples from aspen trunks growing in wolf territory in the northern end of the park. By counting tree rings, they discovered that all the aspen in the area essentially stopped regenerating in the late 1920s and 1930s—immediately after wolves were eradicated from the park.

This finding may help explain why aspen forests are declining throughout the West. Wolves once ranged from Canada to Mexico, keeping aspen eaters such as deer, elk, and moose in check. Now, with wolves retracing their ancient tracks, these shimmery-leaved icons of the frontier may enjoy a resurgence.

Park neighbors watch the wolves’ return with a mixture of admiration and dread. Ranchers have lost a few livestock to the resourceful predators, while hunters complain that elk have become harder to locate. But, in reality, elk numbers have only dropped by a few thousand since wolves reclaimed their place in the park, and the remaining elk herds are much healthier. Wolves take the old and the sick, while hunters shoot animals in the prime of their reproductive lives.

“Before, elk were superabundant. And with the slight reduction, many more animals and plants benefit,” Smith says. “Things are slowly changing, and I’m convinced it’s for the better.”