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Don't Blame the Big Bad Wolf
In the ten years since gray wolves were introduced to Yellowstone National Park, elk numbers have dropped by over 40 percent. But don't be too quick to blame the big, bad wolf, cautions a Michigan Tech scientist. Years of drought and pressure from the elk's primary predator, the human hunter, appear to have had a far greater impact on the region's elk population.
A statistical analysis of data from the Yellowstone region shows that drought conditions and hunting pressure alone would be expected to cut in half the number of elk from 1995 to 2005. In reality, the elk population fell 44 percent, from about 17,000 to 9,500 during the same time period.
"You don't need wolves in the picture at all to explain the population drop," said John Vucetich, an assistant professor in the School of Forest Resources and Environmental Science.
Vucetich built a number of computer models using 1961-95 data on the Yellowstone elk population, before wolves were introduced. The models incorporated three variables: the region's harvest rate, snowfall, and precipitation.
Using the best of these models, he applied it to population figures from 1995 to 2004. It predicted that, given the hunting pressure and the region's climate, the elk population would drop nearly 8 percent a year, on average. Over nine years, these variables alone would cut elk numbers approximately in half.
He's not surprised, however, that many would finger the wolf as the main culprit. "The belief certainly seems rooted in common sense," he said. "But our work shows that the timing of the elk decline is coincidental with wolf reintroduction and that other factors are to blame.
"Whether or not wolves had been introduced, you'd have seen fewer elk anyway."
"Wolf impacts on elk are one of the most controversial aspects of wolf restoration in all the West," said Douglas W. Smith, leader of the National Park Service's Yellowstone Wolf Project. "There are many opinions, but few have any data to back them up. This is one of the first definitive analyses about the impacts of wolves on elk and indicates that for the first ten years, wolves may not have had a dramatic impact on elk numbers-that elk were declining anyway."
"This viewpoint has been hotly contested by other researchers and by the states of Montana and Wyoming, which are responsible for elk management," Smith added. "Continued debate and research hopefully will create clarity and agreement for the benefit of elk, wolves, and humans."
In nature, the relationship between predators and prey is complex and not always transparent. Wolves certainly prey on elk, but they tend to take weak or vulnerable animals. "Most of the elk killed by wolves would have died even if the wolves had not killed them," Vucetich said, either from old age, disease, or the effects of the drought. Hunters, on the other hand, kill more randomly, so they can have a bigger impact on the survival of the herd by taking out animals of peak reproductive capacity.
Just because the decline in the elk population has been driven by harvest and drought does not mean that wolves will not affect elk numbers in the future. "They very well may," Vucetich said. "But it doesn't matter if wolves cause elk decline or not. They can coexist with elk, and they belong in the Yellowstone ecosystem.
"It does matter that much of the elk decline is caused by human harvest," he said. "We share elk with wolves. Humans take elk for recreational purposes in many locations throughout the West. Wolves are restricted to a tiny portion of their former range and take elk for their survival. Given these circumstances, we have to decide how much sharing is right."
Assistant Professor Michael P. Nelson, an environmental ethicist in the Department of Philosophy at the University of Idaho, predicted that Vucetich's work would not be readily accepted by some interest groups.
"One of the main motivators behind the effort to remove wolves from the endangered species list in the West is the assumption that they are largely responsible for noticeable elk herd reductions," Nelson said. "If that's true, then wolves are therefore linked to decreased hunter success rates, and therefore linked to decreased hunting tourism dollars. Vucetich's findings will be viewed as surprising because so many people are invested in the claim that wolves are responsible for elk herd reductions. Though they probably won't change their minds given this one study, it certainly does muddy the waters."
The study is partially funded by a grant from the National Science Foundation. Vucetich, Smith, and Daniel R. Stahler, of the National Park Service's Yellowstone Center for Resources, have coauthored a paper on the work, which was published in Oikos, an international journal of ecological research.
Vucetich studies wolves and moose on Isle Royale National Park. Along with Smith, Stahler works on the Yellowstone Wolf Project.