Does killing wolves that attack sheep and cattle reduce livestock deaths over the long run?
That’s a hotly debated issue in Washington, where wolf packs are expanding into areas where sheep and cattle graze, creating conflicts that sometimes end with wolves being killed by the state Department of Fish and Wildlife to protect livestock.
In 2014, Washington State University researchers published a study that said killing problem wolves can actually increase livestock attacks the following year by disrupting the packs’ social structure. Killing the pack’s leaders resulted in more breeding pairs, more wolf pups and more livestock attacks in the future, the study said.
The research analyzed 25 years of data on wolf attacks on sheep and cattle in Idaho, Montana and Wyoming.
Now, a dueling study from the University of Washington offers a different conclusion.
Three UW researchers, who aren’t wildlife biologists but were intrigued by the earlier study, analyzed the same data with a different statistical approach, said Nabin Baral, one of the study’s authors.
Their work indicated that killing wolves that prey on livestock can lead to a short-term increase in attacks, particularly for sheep. But the year after the wolves were killed, livestock attacks went down.
The study was published last week in the journal PLOS ONE, which also published the earlier WSU research. Baral, whose main area of research is biofuels, said the study is part of the effort to better understand the effects of killing wolves to protect livestock.
But there are gaps in the data, which make it difficult to draw hard conclusions, he said. For instance, the researchers don’t know whether individual ranchers stepped up efforts to protect their animals following wolf attacks, which would influence livestock survival rates, Baral said.
Rob Wielgus, author of the WSU research, criticized the UW study and its conclusions Tuesday, saying the work is based on flawed statistical modeling.
“The authors are not wildlife biologists; they’ve never studied wolves,” said Wielgus, who contends the study was designed in a way that is “a biological impossibility.”
Wielgus’ research showed that killing wolves increased livestock attacks the next year unless the number of wolves killed exceeded the species’ reproductive growth rate of about 25 percent per year. Then, overall wolf populations fell and the number of livestock attacks dropped the following year.
Donny Martorello, the state Department of Fish and Wildlife’s wolf policy lead, said vigorous debate and rebuttal is not unusual in scientific literature.
“What you are seeing is an evolution of the science,” he said. “It’s a good dialog and it will better the science.”
The department’s practice is to use lethal means to control wolves attacking livestock when preventive measures have been tried and failed, Martorello said. The ongoing research will help the department learn about the longer-term effects of lethal control.
A study published last year in the Journal of Wildlife Management said removing all or part of a wolf pack increased the length of time between attacks on livestock. Wielgus said those study results align with his research, which indicates that high numbers of wolves must be killed before livestock attacks decrease.