Greater Yellowstone’s elk having fewer calves


Photo by Onwuma

Photo by Onwuma



Wolves have caused elk in the Greater Yellowstone Ecosystem to change their behavior and foraging habits so much so that herds are having fewer calves, mainly due to changes in their nutrition, according to Montana State University researchers.
During winter, nearly all elk in the Greater Yellowstone region are losing weight, said Scott Creel, ecology professor at MSU and lead author on a study published in the Proceedings of the National Academy of Sciences this week.

“Essentially, they are slowly starving,” Creel said. “Despite grazing and browsing during the winter, elk suffer a net loss of weight. If winter continued, they would all die, because dormant plants provide limited protein and energy, and snow makes it more difficult to graze efficiently.”

With the presence of wolves, elk browse more – eating woody shrubs or low tree branches in forested areas where they are safer – as opposed to grazing on grass in open meadows where they are more visible, and therefore more vulnerable to wolves.

Browsing provides good-quality food, but the change in foraging habits results in elk taking in 27 percent less food than their counterparts that live without wolves.

“Elk regularly hunted by wolves are essentially starving faster than those not hunted by wolves,” said Creel, who wrote the paper with his former doctoral students John Winnie, Jr., and David Christianson.

The decline in the Greater Yellowstone’s elk population since the reintroduction of wolves in 1995 has been greater than originally predicted.

In the three winters prior to wolf reintroduction, elk on Yellowstone’s northern range numbered roughly between 17,000 and 19,000. In the three winters prior to 2008, annual elk counts declined to between 6,279 and 6,738.

Obviously, wolves kill elk, and direct predation is responsible for much of the decline in elk numbers, but not all of it. Creel said the decline is also due to low calving rates, or a decline in the birth rate.

Researchers found that elk facing high levels of predation risk had substantially decreased progesterone levels prior to the annual birth pulse. Progesterone is necessary to maintain pregnancy.

But that raised another question: What was responsible for the decreased progesterone?

There were two competing theories: One suggested elk suffered from chronic stress due to the wolves’ presence. In mammals, stress causes the release of cortisol, a hormone that helps free up energy to fight or flee. But too much cortisol from chronic stress can shut down the immune and reproductive systems.

The other theory was that the elk weren’t getting enough to eat because they were always on the run from the wolves and spending more time in the forest, where food is sparse compared to grassy meadows. For wintering elk that are already on the edge of starvation, anything compromising nutrition could also cause the reproductive system to shut down.

The MSU researchers did chemical analysis of 1,200 fecal samples collected over 4 years, as well as urine samples for the study. They did not find the elevated levels of cortisol that would support the chronic stress theory.

However, they did find that those elk living in the presence of wolves had lower levels of progesterone, a hormone necessary to maintain pregnancy, than those elk that didn’t live with wolves.

“The elk are trading reproduction for longevity,” Creel said. “Elk are potentially long-lived, and many prior studies have shown that, in species like this, natural selection favors individuals who do not compromise their own survival for the sake of a single reproductive opportunity.”

If predators commonly affect the reproduction of their prey, it will change the thinking about predator-prey dynamics, and might change how wildlife managers plan for the reintroduction of predators, Creel said.

“This research shows that the total effect of a predator on prey numbers can be larger than one would determine simply by looking at the number that are killed,” he said.

Creel and his current doctoral student Paul Schuette are seeing if the theory holds up with other prey-predator populations, with a study of lions, spotted hyenas and a diverse array of prey animals on a Maasai Community Conservation Area in the South Rift of Kenya.

The study of Montana elk ruled out weather and grizzly bears as the cause of poor calf production.

“It is true that grizzlies prey on elk calves, and grizzly numbers have increased in the region,” Creel said. “However, the increase in total grizzly numbers has mainly been due to geographical expansion, rather than increases in the number of bears in places where they were already well-established at the time of wolf reintroduction.”

The research was funded by the National Science Foundation.

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