CHEYENNE, Wyo. — Wolves don’t appear to startle female elk that range east of Yellowstone National Park sufficiently or frequently enough during the winter to cause them to lose weight and reduce their ability to carry fetuses to birth in the springtime, according to a study released Tuesday.
Previous studies offered no consensus on whether Yellowstone’s reintroduced wolves have caused “non-consumptive” effects — impacts of predators on prey besides killing them — on the area’s elk.
Some biologists have theorized that Yellowstone’s wolves create an environment of fear sufficient to make elk less able to forage, get enough nutrition and reproduce.
But researcher Arthur Middleton and others who monitored wolves and elk in an area about 30 miles east of Yellowstone from January through March of 2008, 2009 and 2010 reached the opposite conclusion: wolves don’t harm elk with fright.
“There were clear responses of elk to wolves. Elk moved at higher rates. They displaced away from the encounter. They became more vigilant,” Middleton said.
Elk disturbed by approaching wolves, however, didn’t move very far — on a scale better measured in yards than miles — and typically didn’t retreat from their open winter ranges to hide in treed areas with lower-quality forage. The elk followed normal patterns of ruminating under tree cover during the day and spent the rest of the time in the open, Middleton said.
Elk took little evasive action until wolves were relatively close by, within a half-mile or so. Such close encounters happened only once every nine days, on average, for individual elk. That wasn’t enough to significantly reduce body fat or cause elk to miscarry their calves, Middleton said.
“Elk behavior responses to wolves were just too weak and too infrequent to accumulate over the course of winter in such a way as to reduce their nutrition and cause them to lose pregnancy,” he said.
Grizzly bears and poor summer forage conditions caused by several years of drought have a bigger effect on elk health, the researchers concluded in findings published in the journal Ecology Letters.
Middleton and his colleagues conducted their research as part of the Wyoming Cooperative Fish and Wildlife Research Unit, a U.S. Geological Survey program involving the University of Wyoming and Wyoming Game and Fish Department. Middleton got his doctorate in ecology from UW and is now with the Yale School of Forestry and Environmental Studies.
The study focused on the Clarks Fork elk herd and Sunlight, Hoodoo, Beartooth and Absaroka wolf packs. About two-thirds of the elk herd spends the whole year in the area northwest of Cody, but the other one-third migrates to summer ranges in Yellowstone.
Besides tracking and visually documenting the relative movements of elk and wolves, they tranquilized and examined the same elk in late summer and late winter. They compared how much fat the elk gained and lost each year and, using ultrasound, checked to see if the elk were pregnant in late winter.