Study: Wolves Sustain Wildlife by Sharing Prey

Here's a bit of irony. The much-maligned wolf, one of the fiercest predators on the planet, has become an important provider of food for other animals in the few years since it was returned to Yellowstone National Park.

It's not that the wolf is all that magnanimous. It's just that the 14 wolf packs that now call Yellowstone home have found themselves in a land of plenty.

The National Park Service began reintroducing Northern Rocky Mountain wolves, a subspecies of the gray wolf (Canis lupus), to Yellowstone in 1995, much to the consternation of nearby ranchers who feared the predators would feed on their cattle, and the concerns of citizens who worried about the impact on other wildlife in the park.

These carnivores have indeed become major players in the park's ecological system, but it turns out that there are lots of other critters in the park that depend on the scraps left over from a wolf feast for their own survival.

Easy Come, Easy Go

The wolves don't really care all that much about the other animals, says animal ecologist Christopher C. Wilmers of the University of California, Berkeley. It's just that even a pack of wolves has trouble eating an entire elk, so after a kill, there's plenty of food left over.

Everything from coyotes to magpies invite themselves in for lunch, and the wolves "are sort of drunk on food," Wilmers says. "They move off rather than drunkenly fend off other scavenger species."

That's especially true during the hard months of winter, when food is scarce for most animals but plentiful for wolves. They know it's not going to be hard to get their next meal.

"In winter, the Yellowstone elk condition is highly correlated with snow depths," says Wilmers, whose research is part of the Yellowstone Wolf Project, an ambitious park service program aimed at determining the true impact of the return of the wolves. "As snow depths increase, the elks have a harder time moving around, and they have a harder time gaining access to the grass underneath. So they become increasingly vulnerable to predation by wolves."

Since the wolves know that, they "will just abandon the carcass and let other carnivores come and feed on it."

In the summer months those self-invited dinner guests may even include a grizzly or two, and nobody wants to argue with those characters.

Human Kills Feed Birds

Wilmers tracks wolves to their kill sites through the help of radio collars, and he found 202 wolf kills over a two-year period. He watches the feeding frenzy from afar with a powerful telescope. Usually he's so far away the wolves don't even know he's there. And when he happens to have a close encounter, the wolves just slip away, always eager to avoid humans.

In addition to the wolves, Wilmers has recently been studying the impact of elk remains left behind by hunters, and that turns out to be a very different story. His research, published in the current issue of the journal Ecology Letters, reveals that human hunters feed the few, wolves feed many.

Hunting season falls during a six-week period in early winter, and hunters take about 1,000 elk each season just outside the park, according to the state of Montana. Hunters usually gut the animals before trying to haul them out of the woods, "so you have gut piles literally just dotting the landscape," Wilmers says.

That's a huge amount of food, but it's concentrated in a relatively small area, and it's available for only a few weeks. Wilmers calls that a "pulse" resource because it is limited in both time and space.

"There's so much food that it's just saturating the local scavenger community and so the animals that are best able to utilize it are the ones that can get there from farther and farther away, like ravens and bald eagles," he says.

So it's an important, though temporal, resource for some speedy scavengers. Wolf kills, by contrast, occur throughout the region and during the entire year.

Whether the hunters or the wolves are more important to the survival of other species depends on your point of view, he adds.

"If the elk are killed by wolves, then that food is going to be more evenly distributed among the scavengers," Wilmers says. "If they are getting killed by hunters, then that food will go almost exclusively to bald eagles and ravens.

"So it just depends on what your value judgments are. If you want to feed more species, then you go with wolves. If you want to feed bald eagles and ravens then you go with hunters."

Unexpected Bonus

It's a curious footnote to the troubled history of Yellowstone wolves. They were all over the place when Yellowstone was established in 1872, but under the banner of predator control the wolves were gradually killed off, including at least 136 killed by park employees.

By 1970, they were gone entirely from Yellowstone, and survived only in two areas of the lower 48 states, upper Minnesota and on the Isle Royale in Michigan.

In 1987, the U.S. Fish and Wildlife Service proposed reintroducing wolves on an experimental basis to Yellowstone, although that clearly posed a danger to other animals in the park. The University of Wyoming predicted that wolves would reduce the elk population by 15 to 25 percent, for example. Moose and deer were expected to suffer similar declines. Whether they have had this effect remains to be determined.

But no one knew in those days that other animals would benefit from the reintroduction of the wolf. There are many days and nights when the snow covers the region so completely that it's hard for any animal to find a decent meal.

Except, it turns out, for the wolves. And those that dine on whatever the wolves leave behind.

The elk, of course, probably aren't a bit fond of this turn of events.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.