Researchers say wolves likely aren’t the cause of a substantial drop in Yellowstone elk numbers, just the beneficiaries of an enduring drought that has choked off forage and left the ungulates weak during the winter months.

Biologists have documented a decline of about 50 percent – from roughly 20,000 animals to about 10,000 – in Yellowstone’s elk population since federal officials started reintroducing wolves to the park in 1995.

In more than 13 years of data collection, park biologists have documented roughly 2,000 wolf kills.

Twice a year, once in early winter and once in late winter, Yellowstone researchers spend 30 days looking for wolf kills with ground crews and airplanes. Scientists note the location of the kill, the pack that killed the elk, and the elk’s age and sex. Biologists also measure the fat content of the dead elk’s bone marrow.

The marrow lets researchers gauge the fitness of each animal. Bone marrow with high fat content shows that the elk ate well during the summer. Bone marrow with low fat content likely means the elk didn’t find enough high-quality forage, and therefore entered the winter months in poorer shape.

Typically, in years with heavy snow, wolves usually kill more elk because the ungulates get bogged down and can’t forage as effectively. And in years with light snow, wolves kill fewer elk because the elk can run faster and have more access to forage, said Doug Smith, Yellowstone wolf project leader.

But according to Smith, light snow is only a short-term advantage for elk. In the long term, less snow could mean less forage for animals during the summer. Then elk enter the next year’s winter weaker, and therefore more susceptible to wolf predation.

The first few years of the study, between 1995 and 1997, with average or above-average snowfall, wolf kills occurred as expected.

However, as the study progressed and drought began to dominate the Yellowstone ecosystem, elk kills began to increase.

“In the first few years, all elk – cows, calves and bulls – were in good shape early in the winter,” Smith said. “But with less snow and less rain since ’96-’97, we saw a decline in the condition of elk entering winter. More and more cows and bulls are entering winter already in poor shape.”



Easier targets

This general decline in elk body condition corresponded with an increase in the number of wolf kills, even in years with light snow, when elk normally have the advantage. According to Smith, this increase in wolf kills is likely due to drought because less high-quality forage is available to elk during the summer to carry them through the winter months.

A look at the bone marrow shows that wolves are either selecting out the weak elk and only the weak elk, or all elk are in poor shape, Smith said.

The drought is good news for wolves because they can kill more elk, but Smith says the predators likely aren’t having a huge effect on the elk population as a whole. Some of the elk in poor condition would likely die of starvation anyway.

“Before 1995, we had wetter winters and summers, and the range could support more elk,” Smith said. “The ability of the land to support elk has declined. There are too many elk for the range.”   

“You get more and more elk in poor shape, and the wolves kill those elk,” he said. “Without wolves, the elk population in Yellowstone would have declined anyway. Would it have declined as much? Probably not.”

Beyond the drought’s effects on the elk population as a whole, Smith said the lack of rain and snow probably has allowed wolves to start killing bull elk early in the year.

Because bulls rut, competing with other males for breeding rights with females, they spend more time in the late fall jostling with competitors than eating.

“Elk biologists tell me they might not eat for two or three weeks,” Smith said. “The rut wears them down even more.”

Smith thinks this period of quasi-fasting during the rut could place some bulls, already in poor shape from the drought, in greater danger of being eaten by wolves, though he says the idea is just speculation at this point.

By the time March rolls around, Smith said, even in years with good precipitation and, consequently, good forage, all of the elk are in poor shape.



Range ‘worst I’ve ever seen’

This winter could see the same pronounced effect on predation because of drought.

“The range in Yellowstone going into this winter is the worst I’ve ever seen,” Smith said. “What we’re waiting to see is how that plays out in terms of wolf predation.”

Other predators like grizzly bears and mountain lions have also had an effect on elk populations. Grizzly bears, Smith said, are “huge calf predators,” and mountain lions have reached their carrying capacity in the park. Further, state wildlife managers are managing elk herds at lower levels compared with years past.

If the past decade or so had normal precipitation, Smith said, wolves “would have killed elk that probably would have lived,” he said. “[The drought has caused] a constriction of elk carrying capacity that wolves have been able to take advantage of.”

While it’s hard to predict what will happen in a given year, Smith said 13 years of data on thousands of elk kills give a more complete picture.  

“We’re learning that no winter is alike, and early winter and late winter is always different,” he said. “If you took out any three-year period, your conclusions about wolves in Yellowstone would be completely different than if you looked at that entire 13-year data set.”

Smith said he hopes to further test his theory about the relationships among drought, wolves and elk by looking at satellite photos of forage conditions in Yellowstone during the past decade.