Too many missed meals may be the larger cause of the decline of elk in the Greater Yellowstone Ecosystem - not wolf predation or the elk's fear of being eaten by wolves, according to a newly published study.
"What seems to be happening is all these subtle behavioral responses seem to be adding up over winter," said Scott Creel, an ecology professor at Montana State University and the lead author of a study that appears this week in the Proceedings of the National Academy of Sciences. "I was initially a little surprised. When I began looking at elk-wolf interactions, I thought predation would be the main cause."
The study builds on similar theories Creel and his associates have written about over the course of the seven-year study.
Blame to go around
Over the years, various explanations have been given for the elk population decline in the Greater Yellowstone area - extended drought that diminished forage, predation by wolves, too high a hunter harvest late in the winter and predation on elk calves by bears.
Wolf predation seemed the most likely cause, since the Northern Yellowstone elk herd stood at 17,000 to 19,000 before the wolves' reintroduction to Yellowstone National Park in 1995. This year, 7,109 elk were counted, an increase from the previous two years when 6,279 and 6,738 elk were counted.
The cow-to-calf ratio in the Northern Range this year was 22 calves per 100 cows, double what it was the year before. Wildlife managers would like to see 20 to 30 calves per 100 cows.
Although some calves are being lost to predators, relatively few were being killed by wolves in their first six months of life, according to Creel's radio-collar studies.
That pointed to a low birthrate as the cause.
One theory to explain the reduced number of calves was that stress from fear of predation was causing the cow elk's cortisol levels to increase to the point that they produced less progesterone, a hormone critical to carrying a pregnancy to term.
But high cortisol levels weren't detected in the 1,200 fecal and urine samples taken from elk in the Paradise Valley, Wall Creek Wildlife Management Area and the Gallatin Range over four winters. Elk in the Elkhorn Mountains, where wolves aren't present, were included in the study for comparison.
The elk living with wolves did, however, have lower progesterone levels compared with those not living with elk.
What the researchers found was that whereas the elk used to migrate down to large meadows to forage on grasses the entire winter, the presence of wolves had pushed the elk into the mountains. That means that rather than grazing, as they'd prefer, the elk are turning to browse for food - shoots, leaves and twigs on bushes and trees. Although nutritious and high in protein, the browse isn't enough to pack on pounds to keep impregnated elk healthy.
Elk without the pressure of wolves ate 27 percent more food, the study found.
"Elk prefer to graze if given the choice," Creel said. "But the elk in our study area weren't hanging out in the grassy patches when the wolves were around."
Creel's study ruled out drought as the cause, since the study took place during some mild winters that should have helped elk survive and improved their body condition in winter. And elk across the rest of the state are at historic high numbers.
Upon reflection, Creel said that the Yellowstone elk herd will never likely rebound to its historic highs as long as wolves are present in the ecosystem.
"My intuition is that we have now seen a jump from one equilibrium to another," he said. "It looks to have stabilized at about 6,000 to 7,000 elk.
"But you could certainly make an argument that this is cyclic, and this is the first trough in the cycle," he added. "Perhaps the elk numbers will go back up, and then we'll bounce back and forth."
Creel noted that elk and wolves in the Greater Yellowstone Ecosystem are "tightly coupled. ... It's a pretty hard thing to know whether they'll cycle or settle into a new equilibrium."
Creel will now move on to studies of predators and prey in East Africa to see if the relationship to reproduction occurs in other species, or if it is specific to elk and wolves.
John Winnie Jr. and David Christianson, Creel's former doctoral students, co-authored the study. Their work was funded by the National Science Foundation.
Contact Brett French at email@example.com or at 657-1387.