Weaving A New Web: Wolves Change An Ecosystem
by Jim Robbins

In a field of snow brilliant in the winter sun, biologist Robert Crabtree bends over a dead, frozen coyote. Running a gloved hand through the thick gray and brown fur, he inspects it for signs of trauma. "We're looking for evidence that would fit the modus operandi of a wolf kill," says Crabtree, sounding more like a coroner than a biologist. "Has it been around wolves around the estimated time of death? Are there severe bites to the chest, broken ribs, internal bleeding, and possible torn ligaments and muscles around shoulders and hips. Because wolves pull a coyote apart. Mountain lions, on the other hand, are skull crunchers."

Crabtree, who runs a private, nonprofit research institute called Yellowstone Ecosystem Studies, is inspecting a lot of wolf-killed coyotes in Yellowstone these days. Ever since Canadian wolves were brought to the national park to re-establish a population in 1995 and 1996, the coyote population has been going through a dramatic restructuring. Since 1989, the Bozeman, Montana-based Crabtree and his wife Jennifer Sheldon, an expert on canids, have followed 179 radio-collared coyotes in the park. Until the wolves came back it was one of the densest and most stable coyote populations in the country because of the lack of human impacts.

But not anymore. In two years, 50 percent of the pre-wolf population of coyotes has been killed. And the re-appearance of the wolf has shaken the coyote social structure to its foundation. "They're being forced to shift their territories, and to give up their territories. If they don't, they get killed," declares Crabtree.

Thirteen coyote packs with a total of 80 individuals lived in the remote Lamar Valley, one of Crabtree's study areas, before the wolf returned. Now there are nine packs with only 36 individuals, an abrupt change for a population that has been stable for more than half a century. Crabtree believes wolves could eventually kill two-thirds of the coyote population.

It's not all bad news for coyotes. Those that survive, usually on the edge of wolf habitat, are flourishing. One pack, called the Amethyst pack, has ten members, the largest Crabtree has seen in the park. Most packs averaged six coyotes before the wolf. Crabtree says it’s because the wolves have made so much more protein available, in the form of dead elk. "If a coyote gets in there to a carcass, and doesn't get killed, it’s got a bonanza."

Crabtree and Sheldon have also observed that coyotes have changed the places where they spend their time, moving from open meadows to steep terrain. "Carcasses in the open don't attract coyotes much, and if they do, they're very nervous," says Crabtree. "That's because when a coyote gets chased on the flats it’s often killed. In the hills and steep terrain they feel more secure and they can get away. A lot of times a coyote will lead a wolf downhill, and as the wolf comes after it, it turns around and runs uphill. Wolves are bigger and can't stop and the coyote gets a huge lead."

And even though coyotes, at about 30 pounds, are a third the size of the average wolf, they are not pushovers. Only when a pack of wolves outnumbers coyotes do they attack; when there are more coyotes, they will attack wolves. Crabtree is not aware of any coyotes killing adult wolves, but thinks they might have killed some pups.

Biologists watched one scene unfold in which four coyotes attacked a wolf pup. A female wolf chased the coyotes off, but eventually they turned on her and pinned her down. She escaped, and swam across the Lamar River, where she was attacked on the other side by another pack of coyotes. Amazingly, she survived.

But coyotes are only part of the changing ecological story in Yellowstone. From elk to grizzly bears to rodents to raptors, the presence of the wolves is reshuffling the ecological deck in the park, altering relationships between species, having myriad unanticipated secondary and tertiary effects, and increasing species richness. "It's exciting," says Crabtree, who has completed eight years of a planned 15-year study of the effects of wolves on the Yellowstone Ecosystem. "If you want to understand an ecosystem you have to study long term," he explains. "What do two-or three-year studies have to do with a 30-year generation time of grizzlies, 300-year-old forests, or 10,000-year-old streams?"

The study is all the more interesting because it started several years before the first 33 wolves were reintroduced to Yellowstone in 1995 and 1996. The baseline of information Crabtree established shows precisely how dramatic the changes have been. "This is one of the great ecological experiments of this century, and the opportunity of a lifetime for an ecologist to answer the question of how large predators affect a system," Crabtree proclaims.

Besides coyotes, the species most affected by the wolves is their primary prey: elk. Roughly 20,000 elk graze the Northern Range, the largest expanse of ungulate habitat in the park. This is also where most of the wolves live. During 1996 the wolves made 142 known kills, 124 of which were elk. (In contrast, wolves killed only two bison .) On average, a pack of wolves killed an elk every one to five days, according to Doug Smith, wolf project leader for Yellowstone National Park. Further, Smith adds, as the snow and cold build in January, wolves do indeed cull the old and the sick, turning to adult bulls and cows that have been weakened by winter. The average age of a cow elk killed by wolves is 14 years--an old elk, says Smith--and most of the elk killed were females, either very young or old. The wolves kill calves in the early part of the winter.

Contrary to the wolf’s image as a highly efficient predator, studies in Yellowstone show that wolves are unsuccessful far more often they are successful. "For every hundred elk they chase, they kill two or three," Smith says. Elk that stand their ground against wolves have a much better chance of surviving the attack than those that run. It's too soon to tell whether wolves will significantly reduce the number of elk in Yellowstone. If the number of elk is reduced, most experts believe it probably won't be by much, because elk increase their reproductive rate when the herd numbers decrease. But the wolves may indirectly benefit some riparian plant species in Yellowstone, such as willows and aspen, which have diminished due to overgrazing, by forcing the elk out of the river bottoms to higher ground so they can be more aware of predators. Although the data are still inconclusive, Crabtree’s observations suggest that the elk have changed their behavior, and have been forced to become more vigilant. This could result in more elk dying over the winter, he says, because animals have to spend more time watching for wolves, and spend less time foraging.

One of the major elements of change in the ecosystem brought by the wolves is the new-found abundance of protein in the form of red meat. Grizzly bears, mountain lions, and coyotes don't kill many adult elk. Until the wolves arrived back, most elk were available only in the spring, after winter die-off. Now elk meat is available all year long. Once wolves have made a kill and fill their bellies, they become "meat drunk" and disappear to sleep it off. Other species that have been waiting move in. "A lot of other predators and scavengers have a seat at the wolf kill table," says John Varley, Chief Scientist for the park.

The year-round abundance of protein has increased the number of scavengers. The number of ravens sighted on carcasses has gone from four per carcass before the wolves, to eight per carcass now. The number of eagles has gone from one for every four carcasses to five for every four carcasses. "I'm suspicious that ravens and eagles are following wolves around," says Crabtree.

"When the sun comes up on a kill it can be stunning. You can see a grizzly bear, four or five ravens, coyotes, a fox, bald eagles, and golden eagles on the carcass. All at once. It's amazing to see how fast 900 pounds of meat goes," he says. "All that's left is a puff of fur." The wolves also take an occasional moose, deer, pronghorn antelope, and, surprisingly, killed one mountain goat that had left the mountains and wandered into the wrong place.

Other changes are taking place in the ecosystem, according to Crabtree, although they have not yet been quantified. He believes wolves have been an indirect boon to predators such as foxes, hawks, owls, eagles, badgers, and pine martens. Until the wolves came along, coyotes were eating 75 percent of the total number of microtine rodents, or voles, a third of the ground squirrels, and a quarter of the pocket gophers. With far fewer coyotes round, many more rodents are available to other predators. "Wolves are causing an explosion in species richness," said Crabtree. "They are a great way to increase biodiversity." Crabtree doesn't mean that there are more species around, but that there is a better balance of all species.

Unlike the portrayal of wolves in the film Never Cry Wolf, Crabtree says a typical wolf diet is only ten percent rodents, and far less for the wolves in Yellowstone, where so many elk are available. "It takes 6,000 mice to equal one elk," he said. "Imagine how much energy it would take to catch that many mice." In fact, there is so much meat available in Yellowstone that it is redefining the average size of wolf packs in North America.

"It creates delayed dispersal," says Crabtree. "There's room for young wolves to break out and establish new packs. But because food is so plentiful and there's less competition, they're delaying their dispersal and packs as large as 15 have formed. They could likely get bigger." Eight is the average pack size in North America.

The population of the threatened Yellowstone grizzly bear also appears to be getting a boost from the wolves. Cubs are born in the den in winter during the mother's hibernation. The number of cubs is directly dependent on the mother's nutritional status, which has been enhanced by the newly available elk carcasses. "I found eight wolf kills this October," Crabtree says. All of those kills had grizzlies consuming part of the elk. I've been there eight autumns and I usually see no grizzlies, or maybe one, in October."

Early one recent morning as a molten sun eased over a ridge and cast an orange glow, Crabtree stood along a park road, peering into a spotting scope, surveying a sprawling slice of Yellowstone National Park. A graduate student holds a small antenna, and a steady stream of beeps coming from a small receiver indicates that a radio-collared coyote is around. After a few minutes Crabtree spots the animal, loping warily through a grassy meadow.

A great deal of Crabtree's work in the park involves analyzing data collected remotely. But the real drama of the park's wolf activity has unfolded in the meadows along the park's main highway in the Lamar Valley, and researchers have seen behaviors that have rarely, if ever, been seen in other wolf studies. For example, Crabtree recently saw an elk running, with a wolf clamped on either side of the animal's neck.

Crabtree stresses that the study of wolves in Yellowstone has only begun. The density of the wolf populations and the fact that Yellowstone is a protected environment, he says, means that Yellowstone studies will make a major contribution to the body of scientific information on wolves. But perhaps the single most important feature of the Yellowstone wolves, says Crabtree, is the fact that they are going about their business out in the open. "Wolves evolved in the open, out on the great plains," he said. "The habitat they have left is in areas they had to retreat to--forest areas, suboptimal and protected. Yellowstone possesses the habitat they evolved to prefer. It's opened a whole new chapter in our understanding of carnivores because we can directly observe the events and the outcome of events. You see a full expression of those evolutionary traits. We get a picture of the real wolf."

Jim Robbins is a freelance writer in Helena, Montana, who writes regularly for the New York Times, Audubon, and other publications.

(ZooGoer 27(3) 1998. Copyright 1998 Friends of the National Zoo. All rights reserved.)

If you have a comment about Smithsonian Zoogoer magazine, please email it to us.

Page Controls