​Killing Wolves Leads to More Livestock Deaths

The counterintuitive reason that lethal control doesn’t work.

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Dec 4 2014, 10:30am

This gray wolf knows how handsome it is. Image: Brooks Tracy, U.S. Fish and Wildlife Service

The reintroduction of gray wolves to the United States has been a touchy issue from the start, and only seems to attract more controversy as packs slowly recover their original territory. Conservationists and animal rights groups want to protect wolves from hunters, and farmers and ranchers want to protect their livestock from depredations. It's a stalemate that has inspired heated debates about the fate of the carnivores across the Northwest.

But today, a study published today in PLOS One suggests that the whole argument may be a moot point, because killing wolves doesn't decrease wolf attacks on livestock—it increases them.

The study's authors, ecologist Rob Wielgus and analyst Kaylie Peebles, reached this finding by collating data on wolf and livestock mortality in Idaho, Montana, and Wyoming from 1987 to 2012. "Predator control and sport hunting are often used to reduce predator populations and livestock depredations," the authors wrote in the abstract, "but the efficacy of lethal control has rarely been tested."

Wielgus and Peebles found that killing one wolf increased the odds of predatory attacks on sheep by four percent and cattle by five to six percent the following year. Killing 20 wolves led to double the number of livestock deaths, revealing a strong positive correlation between wolf culls and subsequent livestock predation.

Proportion of wolves killed the previous year versus the number of cattle depredated the following year. Image: Wielgus/Peebles/PLOS One

"I had no idea what the results were going to be, positive or negative," said Weilgus in a Washington State University Statement. "I said, 'Let's take a look at it and see what happened.' I was surprised that there was a big effect."

The authors suggest the reason for the counterintuitive correlation may have to do with pack cohesion. When a patriarch or matriarch dies, in wolves and in humans, the family dynamics are thrown off. In wolves, this means that younger wolves will form breeding pairs, forcing them to settle down in one place when they have pups. That precludes long trips hunting deer and elk, so the new parents prey on livestock instead.

Interestingly enough, wolves aren't the only predators that produce these results. Wielgus and Peebles contributed to a similar survey on cougar culls and livestock attacks, published in PLOS One in November 2013. That paper found that humans killing cougars could result in as much as a 240 percent increase in livestock predation from the big cats.

Colorado cougar. Image: Justin Shoemaker

Much like wolves, the social world of cougars is disrupted after so-called "cougar harvests," thrusting young, immature males into roles normally occupied by adults. Naturally, the less disciplined adolescents head over to the nearest place to get an easy meal: ranches and farms.

A similar blowback effect was also found between bear culls and livestock predation, so it may well be a wider consequence among several predatory species. As the authors concluded in the 2013 cougar study, "[w]idespread indiscriminate hunting does not appear to be an effective preventative and remedial method for reducing predator complaints and livestock depredations."

In theory, these findings should finally unite conservation and livestock interests against the practice of non-lethal control of predators. In practice, it will probably be messier. Regardless, Wielgus is confident that gray wolves will continue to rebound in the Northwest.

"The only way you're going to completely eliminate livestock depredations is to get rid of all the wolves," he said in the WSU statement, "and society has told us that that's not going to happen."