Study: Drought, Not Wolves, Impact Yellowstone Elk Reproductive Rates

With the Yellowstone elk population declining in recent years, a study from Wyoming-based researchers indicates that drought, and not the presence of wolves, is to blame for declining reproductive rates.

The decline of elk populations in the greater Yellowstone ecosystem is a well-documented phenomena. And many in the region blame the reintroduction of wolves to the Yellowstone area for the decline. But correlation certainly does not equal causation, and the study led by recent University of Wyoming Ph.D. graduate Arthur Middleton indicates that the presence of wolves isn’t strong enough to reduce the body condition and reproductive rates of female elk.

The findings were published in the scientific journal Ecology Letters. They’re based on multiple years of observation by Middleton and his colleagues at the Wyoming Cooperative Fish and Wildlife Research Unit — a U.S. Geological Survey program housed at the University of Wyoming in cooperation with the Wyoming Game and Fish Department. Middleton and colleagues used state-of-the-art GPS collars and firsthand observation to track the interactions of the Clarks Fork herd with wolves from the Sunlight, Hoodoo, Beartooth and Absaroka wolf packs in 2008, 2009 and 2010. The detailed movement data on both wolves and elk allowed the researchers to identify each time one of the collared elk encountered a collared wolf.

The study’s key finding is that even though elk varied widely in their encounters with wolves, those that encountered wolves frequently were not less fat — or any less likely to be pregnant — than those that rarely bumped into the predators. This finding differs from some previous studies that indicated wolves influence elk behavior strongly enough to contribute to regionwide declines in calf production.

“Elk respond to wolves, but less strongly and less frequently than we thought,” says Middleton who, for three years, closely followed the Clarks Fork elk herd west of Cody, along with the wolf packs that prey on it. “We found that wolves influence elk behavior, but the responses were subtle and — over the course of winter — did not reduce body fat or pregnancy. Our work indicates that the effect of wolves on elk populations is limited to direct predation and doesn’t include so-called harassment, stress and fear, which have been proposed as additional indirect effects on prey populations.”

The elk herd, one of several migratory herds in the greater Yellowstone area, spends summers in Yellowstone National Park and moves into the Sunlight Basin during winter. The researchers also recaptured GPS-collared elk at the end of winter and the end of summer each year to assess their annual fat dynamics and pregnancy rates using ultrasonography.

The researchers found that when wolves approached within 1 kilometer (a little over a half mile), elk increased their rates of movement, displacement and vigilance. However, the behaviors lasted only about 24 hours and didn’t significantly reduce elk foraging or force elk into poor habitats. And such encounters with wolves took place at a rate of only one in nine days on average for the migratory elk in the Clarks Fork herd — the maximum was once every four days.

“Our research was unique in that we tracked wolves while also monitoring the movements, foraging behavior, body fat and pregnancy of the elk they hunted,” says Middleton, who worked under Matt Kauffman, head of the Wyoming Cooperative Fish and Wildlife Research Unit under a dual appointment with the USGS and UW’s Department of Zoology and Physiology, where he is an assistant professor. “This approach, essentially connecting the dots from wolf movements all the way to elk behavior and nutrition, revealed that elk respond to wolves too weakly and too infrequently for those behaviors to carry nutritional costs.”

The study casts additional doubt on the idea that wolf reintroduction has caused what scientists call a “behaviorally mediated trophic cascade” in the greater Yellowstone ecosystem — prompting elk to alter their foraging behavior or avoid risky areas, thereby allowing aspen and willows to recover from overbrowsing. These new findings are consistent with studies by Kauffman and others showing little or no evidence for cascading effects caused by purported broad-scale shifts in elk habitat use or foraging behavior in response to wolves.

Both Middleton and Kauffman have pointed out in their studies that the wide-ranging hunting strategy of wolves, which differs from the tactics of a stalking predator, might be the reason that elk responses are too weak and inconsistent to alter their foraging patterns or nutritional gain.

“A key factor in the ability of predators to cause these sorts of cascading effects is the ways in which they hunt and kill their prey,” says Kauffman, who initiated the study in 2006 along with Game and Fish Department wildlife biologist Doug McWhirter and U.S. Fish and Wildlife Service wolf manager Mike Jimenez. “Wolves are coursing predators that chase down their prey, as opposed to stalking predators that lurk in concealed areas. We are learning that coursing predators are less likely to induce strong behavioral responses in their prey, and this new work suggests that the coursing hunting mode of wolves may constrain both their ability to influence prey condition and cause cascading ecological effects on plants.”

Kauffman adds that the concept of non-consumptive effects of predators on prey has been well tested in small, well-controlled environments — often involving insect predators and prey — but that researchers are still sorting out how it all works in the large landscapes occupied by species such as wolves and elk.

Calf production has been declining among migratory elk herds in the greater Yellowstone area, but wolves may not be the primary culprit, Middleton says. For the Clarks Fork herd at least, other recent research findings point to high rates of bear predation and reduced habitat quality due to drought — both on summer ranges largely inside Yellowstone — as being the more likely cause of declines in elk calf numbers.

“The recovery of large carnivores, particularly grizzly bears, has brought major increases in predation on newborn elk during early summer,” wrote Middleton, who added that the region has experienced severe drought and warmer temperatures in recent years. “These effects of drought and predation could largely explain both low pregnancy and declining calf production among elk of the Yellowstone region.”

Kauffman, a USGS scientist, is one of the co-authors of the report published today — along with a collaborative team of researchers from the Game and Fish Department, the Fish and Wildlife Service, the National Council for Air and Stream Improvement, Yellowstone National Park and UW.

To read the article summarizing the research in Ecology Letters, go to

Image by Jim Peaco, courtesy National Park Service.

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