Photograph by Joel Sartore, Nat Geo Image Collection
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Pelts from four gray wolves shot by United States federal officials in Montana hang over a truck. A new study weakens the theory that killing predators can help protect livestock.

Photograph by Joel Sartore, Nat Geo Image Collection

The Case for Mass Slaughter of Predators Just Got Weaker

A new study found that there’s little evidence that lethal predator control does anything to help ranchers.

Wildlife officials in Washington State recently green-lit a controversial plan to kill a pack of wolves fingered as the culprits behind a spate of attacks on cows there. The way the state sees it, taking out the carnivores could help prevent more livestock losses.

The United States used this justification to kill thousands of coyotes, wolves, bears, and other predators last year. Other nations, including Canada and Finland, have also authorized predator hunts for this reason.

But these killings might not solve any problems after all. A new study published Thursday in the journal Frontiers in Ecology and the Environment found that there's little scientific evidence that killing predators actually accomplishes the goal of protecting livestock.

"We know anecdotes and perceptions don’t get us very far when we’re dealing with a problem like livestock predation," says Adrian Treves, a conservation biologist from the University of Wisconsin who co-authored the paper. "The science of predator control has been slow and not very advanced."

Hunting for Evidence

Treves and his colleagues reviewed previous research attempting to measure the effectiveness of various predator-control methods in North America and Europe. Some studies looked at whether killing predators meant fewer livestock deaths, while others examined the success of nonlethal deterrents, such as the use of guard dogs and flag-lined ropes or wires.

The scientists found that most of the research doesn’t hold up scientifically. Only two of the studies were deemed top notch because they took into consideration the possible effects of things like disease, weather and other elements that could influence livestock deaths. But neither study focused on the effectiveness of killing predators. Instead the papers concluded that certain nonlethal predator-control methods helped ward off future attacks on livestock.

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Wolves killed this Montana rancher's calf. In 2010 predators killed 220,000 cattle and calves in the United States.

As for the studies that did evaluate the effectiveness of lethal predator control, seven used shoddier science but were reliable enough to “draw an inference about,” Treves says.

But only two showed that the killings prevented livestock loss, while the majority found that killing predators either doesn’t help eliminate attacks on farm animals or actually makes things worse by increasing predation (meanwhile most of the papers falling into this category that examined nonlethal predator control methods found them successful).

For instance, one study, published in 2013 that focused on Washington State, concluded that hunting cougars only increased their attacks on livestock. That’s because research has shown that hunting older males—be they cougars, wolves, or black bears—tends to lead to more predation because those males keep out the youngsters, which are more aggressive.

"Lethal methods tend to be more risky for livestock and therefore for livestock owners than nonlethal methods," Treves says.

A Way Forward

Treves wants the findings to prompt governments and hunters to stop shooting, trapping, and using other methods to kill “problem” predators until better science becomes available. The way he sees it, this will aid ranchers as well as the public, whose tax dollars help fund the wildlife agencies making decisions about how to deal with predation. (Also see “This Government Program's Job Is to Kill Wildlife.”)

“People deserve to hear the options and understand the evidence, especially if our government claims to be science-based in our policies,” Treves says.

In the United States, employees with a government program called Wildlife Services kill thousands of predators a year as part of its mission to solve conflicts between humans and wildlife. Despite growing criticism in recent years, the program has always said the methods they use are legitimate.

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Under the U.S. government's Wildlife Services program, more than a million predators, like this gray wolf in Idaho, have been killed since 2000.

“Wildlife Services’ policies and decisions are based on the best available science,” Pamela Boehland, a spokeswoman for the program, wrote in an email. “Not all wildlife damage problems can be resolved using nonlethal techniques alone. Even with the use of single or combined nonlethal methods, livestock losses to predators often continue.”

But according to Treves, the new study shows there’s not enough science to support the killing of these animals. “Any government action that destroys wildlife should be scrutinized to a higher level,” he says.

Doug Smith, a senior wildlife biologist with Yellowstone National Park, who wasn’t involved in the study, says that the new data fill a gap in the research into the effectiveness of predator-control methods. “It’s about time that lethal and nonlethal control had a critical evaluation,” he says.

Smith points out that a move away from killing predators would require a culture shift among ranchers, who often jump to lethal predator control because it offers a quick and easy fix with short-term results. “People are instant gratification creatures,” he explains. “A lot of ranchers are very comfortable with that model.”

This story was produced by National Geographic’s Special Investigations Unit, which focuses on wildlife crime and is made possible by grants from the BAND Foundation and the Woodtiger Fund. Read more stories from the SIU on Wildlife Watch. Send tips, feedback, and story ideas to