A Top Dog Takes Over

Exterminated from Yellowstone National Park eight decades ago, gray wolves are back—and boosting the park’s biodiversity

  • Sharon Levy
  • Oct 01, 2003
FLEAS STILL HOP at the entrance to the den, dug into the side of a small hill on the rolling surface of Blacktail Deer Plateau in Yellowstone National Park. The Leopold Pack, with eight adolescent pups, left this safe haven within the last couple of weeks. Standing amid the litter of a recently active wolf den—dozens of scats, a scattering of sticks and elk bones marked with the fine scratches of sharp puppy teeth—one is struck by the way the site seems to disappear into the landscape. Walk a few yards away, look back, and if you didn’t know better, you’d never guess the wolves had been there.

Like its den, the gray wolf’s impact on Yellowstone, though profound, can be difficult to see with the untrained eye. Exterminated from the park in the 1920s, the species was reintroduced seven years ago. Since then, scientists have uncovered evidence that when the wolf vanished from the park, several other species were affected as well. Wolves, it turns out, may shape the destinies not just of their prey and other predators but also of plants and songbirds—a timely finding as officials consider decreasing protection for this imperiled species. "With the wolf back in place as the top carnivore, biodiversity is greater," says biologist Doug Smith, leader of the National Park Service’s Yellowstone Wolf Project. "The return of the wolf is the best thing to happen to Yellowstone in the past century."


POWERFUL PREDATORS: When wolves were eradicated from the park 80 years ago, Yellowstone’s elk numbers increased, and groves of aspen—browsed heavily by these ungulates—began to decline. With wolves back preying on elk, young aspen trees are once again thriving. Wolves also affect predators such as coyotes, whose populations have fallen by half since the top dog’s return. In the park’s northern range, a male wolf chases a coyote (above) that had tried to prevent it from excavating a den to reach a litter of coyote pups

Since the arrival of Euro-American settlers, the park has experienced a series of sometimes drastic changes. Wildlife was hunted intensively at first, then protected piecemeal according to the prevailing attitudes of the times. Elk, targeted by commercial hunters in the 1800s, were later protected, while park managers mandated destruction of predators such as wolves. Today elk are plentiful, and populations of wolves—as well as mountain lions and black and grizzly bears—are recovering.

One major change took place after the last of Yellowstone’s wolves were trapped and shot 80 years ago, when growth of new aspen trees came to a halt. Photographs taken in the 1890s show aspen groves standing tall on Blacktail Deer Plateau and elsewhere in the park, many of them gone today. Since Yellowstone’s establishment in 1872, 95 percent of its aspen forests have been lost. Though they never covered more than 4 percent of the park’s area (in its northern range), aspen groves support a greater variety and abundance of birds and understory plants than do surrounding Douglas fir and lodgepole pine forests.

The culprits behind the decline of Yellowstone’s aspen were elk, whose populations began to boom after the wolf disappeared. The hungry animals devoured new growth of willow as well as aspen. For many years, the park service controlled elk numbers by culling herds, but in 1968—in response to a public outcry to Congress—it made a controversial decision to minimize human meddling with the park’s wildlife, including elk. Without culling or natural predation, there was then nothing to limit the grazers but the carrying capacity of the land.

In a study of the trees’ growth rings, ecologists William Ripple of Oregon State University and Eric Larsen of the University of Wisconsin–Stevens Point found that aspen that have managed to grow to tree height—rather than being eaten by elk as sprouts—began their lives between 1700 and the 1920s. After that, aspen continued to germinate, but they were heavily browsed by elk and seldom managed to grow taller than six or seven feet. Ripple says their study supports the hypothesis that wolves, through their effects on elk, help aspen flourish. A recent National Academy of Sciences report agrees that heavy browsing by elk has been the driving force behind the park’s aspen decline, though it notes that climate change and fire suppression also have affected the trees’ growth.

Ripple and Larsen, who launched a long-term study of aspen, elk and wolves three years ago, are finding that even if wolves don’t change the number of elk, they may still protect aspen by changing the way the grazers behave. Aspen stands make good cover for wolves, and elk soon learn to steer clear of places where wolves spend time. They may browse around the edges of aspen forests in wolf territory, but they won’t go into the woods where they would be most vulnerable to attack. As a result, aspen in areas frequented by wolves are growing tall for the first time in decades.


DAMMING EVIDENCE: The loss of Yellowstone’s aspen (above) and willow particularly hurt the park’s resident beavers. Booming in the 1920s, when trees of all ages provided stem sizes the animals need, beaver populations began dropping by the 1950s. Though too many beavers may themselves damage aspen groves, the industrious rodents also help the trees by flooding the land and creating better aspen habitat.

Heavy browsing by elk during the wolf’s absence may have affected other species too. Before the predator’s reintroduction, beavers had vanished from the park’s northern range, although they flourished in the 1920s when aspen groves harbored trees of all ages, including the stem sizes preferred for dam-building. The animals themselves contributed to aspen decline—to some degree eating themselves out of house and home—but some ecologists believe the tree would have continued to regenerate had they not suffered from browsing by elk. Today, following their reintroduction on U.S. Forest Service land adjacent to Yellowstone, beavers are returning to the parts of the park frequented by wolves. In these areas, not only aspen but also willows are sending up tall shoots again.

Ripple believes that the wolf, the beaver and the aspen all have powerful effects on one another. "If any of these three are missing from the system," he says, "problems could occur, especially if large numbers of ungulates are present." He enumerates the links between the species: Wolves travel along streams and often prey on beaver; beaver need aspen for food and dam construction; wolves control elk browsing patterns, allowing aspen to grow; and beaver dams flood the land, creating more good habitat for aspen.

In Banff National Park, Canada, research underway by University of Alberta biologist Mark Hebblewhite bolsters recent findings from Yellowstone. Before wolves recolonized the park’s Bow Valley in 1986, he says, "elk and beaver were obviously competing for willow." Another Banff researcher, biologist Cliff Nietvelt, found that beavers select willow stems of a size that vanishes when elk populations are dense. In Banff, which is a much harsher habitat for elk than Yellowstone, the wolf’s return has reduced the elk population by 50 to 60 percent, allowing a dramatic rebirth of willow and aspen in wolf territories. Beaver dams built from these trees create ponds that are biodiversity hot spots. "The only time I’ve ever seen large numbers of boreal toads breeding in the park," says Hebblewhite, "was at a beaver pond about a mile from an active wolf den."

In Yellowstone, the wolf’s reintroduction promises a cascade of effects on other creatures as well. During the top dog’s long absence, the park’s coyote population boomed, for example, reaching some of the highest densities ever recorded in North America. Wolves often kill coyotes or usurp their territories, and since the wolf’s return, biologist Robert Crabtree of the Yellowstone Ecological Research Center estimates that coyote numbers have dropped by 50 percent. This is good news for coyote competitors such as red fox; sightings of these animals are already up in areas frequented by wolves. Crabtree and Smith expect that numbers of some other mid-sized predators, including wolverines and fishers (a rare species of weasel), will also increase.

Just south of Yellowstone, in Grand Teton National Park and nearby national forest lands surrounding Jackson Hole, Wyoming, moose—which devour willow—are the problem browsers. Once rare, moose numbers began expanding in the 1880s when humans wiped out both the wolf and grizzly bear. Today the animal’s population density is about five times higher inside the park than it is outside, where people are allowed to hunt moose. Though the grizzly population is now expanding and Grand Teton is home to two wolf packs that moved south from Yellowstone, these predators are not yet numerous enough to have a significant impact on moose.

In 1998, biologists Joel Berger of the Wildlife Conservation Society and Peter Stacey of the University of New Mexico studied moose effects on willow habitat and on the birds that live there. "The willows inside the park are heavily browsed, so they’re smaller and have many more dead stems than those outside the park," says Stacey. When he censused songbirds in willow inside and outside Grand Teton, he discovered a greater variety of species and higher abundance outside the park, where the willow habitat was healthier and less affected by moose. MacGillivray’s warbler and the gray catbird, two species that depend on dense willow thickets for nesting, are absent from the park, but found outside it.

"Nobody suspected that there would be a higher diversity of birds outside the park," says Stacey. "We think of parks as being places that are set up to preserve biodiversity. What we found is just the opposite, because the park has been missing some key ecosystem components."


HIDDEN CONNECTIONS: Wolves reintroduced to Yellowstone (above) and elsewhere may even benefit songbirds. In Canada’s Banff National Park, scientists discovered that the American redstart, a bird that requires mature willow to nest, thrives in areas where wolves have decreased elk density. Outside Grand Teton National Park, Wyoming, the predators have provided a boost to willow-nesting gray catbirds and MacGillivray’s warblers. Despite such growing evidence of the wolf’s beneficial impact on ecosystems, federal officials have proposed decreasing protection for this carnivore.

Similar discoveries come out of Banff. Hebblewhite and Nietvelt found that in areas where wolves have reduced elk density, thriving willow habitats host a diverse array of songbirds—including the American redstart, a species that needs mature willow to nest successfully. Near the town of Banff, where numerous elk have taken up residence on lawns and golf courses (safe from wolf predation), willows don’t grow above ankle height. The birds there are grassland species, and there is less diversity than in wolf territory. Redstarts do not nest there at all.

Hebblewhite believes that over time, the wolf will prove to have as powerful an influence on Yellowstone’s ecology as it has on Banff’s—and that the wolf will have the last word in a long-standing argument among ranchers, hunters, and wildlife scientists over the best way to manage grazing animals like elk and moose. "Restoring predators in the areas that you can is the easiest solution," he says. "It’s the best way of achieving some measure of ecological integrity."

Some argue that human hunters should control elk populations. But hunting cannot mimic the impact of large carnivores. "Predators select different prey than do human hunters," says NWF Senior Scientist Steve Torbit. People target healthy males, while top carnivores take old, young and weakened animals as well as females. "It’s the females that control population growth," says Torbit. "Even where elk are hunted, populations can continue to grow, and we are beginning to see a loss of healthy aspen groves outside of national parks in the West. Wolves are the missing component."

Research results emerging from both Yellowstone and Banff are particularly timely today. In April, the U.S. Fish and Wildlife Service, declaring its gray wolf recovery program a success, changed the predator’s status from endangered to threatened everywhere but in the Southwest. The agency also said it would soon begin the process of removing the wolf from the Endangered Species List altogether as well as abandon efforts to restore the animals in geographic areas such as New England. Once found throughout the United States, gray wolves today are thriving only in Alaska and the Northern Rockies and Great Lakes regions. The plan has drawn criticism from conservation organizations, including NWF.

Even fully protected by law, wolves remain vulnerable. In Yellowstone and central Idaho, young adults raised in reintroduced packs have begun to travel, looking for new places to live. Many of these pioneers will end up being killed or relocated when they wander into areas where people live. Despite growing evidence that wolves play a vital role in healthy ecosystems, resistance to the predators’ presence on many ranch lands and urban fringes remains intense. "It’s tough to find places that are wild enough to hold wolves," says Smith.

California writer Sharon Levy saw and heard wild wolves for the first time when she visited Yellowstone National Park to report this story.

NWF Priority

Restoring Wolves

For more than two decades, NWF has worked to restore wolves, both in the public mind and on lands where the predators once lived. Education efforts include producing feature-length films, a wolf newsletter and materials for schoolchildren. NWF also works with other groups trying to protect and reintroduce wolves, including the U.S. National Park Service, the U.S. Fish and Wildlife Service, the Nez Perce Tribe and the White Mountain Apache Tribe. In Maine, NWF’s Northeastern Natural Resource Center is helping state wildlife biologists investigate reports that wolves may have returned to that state from Canada. On the policy front, NWF is working to ensure that a proposal to take wolves off the federal list of endangered species in the Northern Rockies and Great Lakes does not lead to an abandonment of wolf recovery efforts in other parts of the country such as the Northeast and the Southern Rockies.

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