Are Yellowstone National Park's Grizzlies Changing Denning Habits Due to Climate Change?

Are grizzlies in Yellowstone altering their denning habits because of climate change? NPS photo.

Are grizzlies in Yellowstone National Park altering their long winter's slumber due to a changing climate? Federal wildlife biologists think so, and want to take a closer look into this possibility.

"We've detected a slightly later period of denning in fall in male bears," Chuck Schwartz of the U.S. Geological Survey, who heads the Interagency Grizzly Bear Study Team, told the Great Falls Tribune. "We think that's related to climate warming."

There's been quite a bit of research into a warming climate in Yellowstone and what might result from that. Indeed, years of study show that the park is losing its snow cover earlier than historical records indicate it should.

Winters historically have been quick to arrive and long to depart in Yellowstone. From November through April, a steady series of storms typically leaves the landscape deep in snow and cloaked in frigid temperatures. These winters and their snows are a great equalizer in prey and predator relationships and help cull the sick and the old from the park’s elk and bison herds. Deep snows can favor either the predator, such as wolves pursuing elk or moose that flounder, or the prey, such as when the predators flounder in their pursuit. Hard winters can also lead to the deaths of ungulates weakened by disease, old age, or unable to reach nourishment under deep snow cover.

Unfortunately, there’s mounting evidence that Yellowstone’s winters are becoming milder and drier. During the past half-century snow depths across the park have steadily declined, particularly in the spring months, and temperatures have been on the rise. Driving these changes is the shifting El Nino Southern Oscillation away from the La Nina phase that produces colder, snowier winters, to the warmer, drier El Nino phase.

Now, in light of these trends, it's actually a good thing that Yellowstone's wolf recovery program has been so successful. If there weren't wolves in the park, that could have left some hungrier-than-usual scavengers -- coyotes, ravens, wolverines if any remain in the park, and yes, even grizzly bears -- come spring thaw. That's because less snow can mean less winter-kill and makes elk and bison more mobile sooner than normal because they're not struggling with deep snows.

Fortunately, when the federal government launched Yellowstone's wolf recovery program back in the 1990s it perhaps unwittingly provided a measure of climate-change adaptation for the park's wild kingdom. Why? Because thanks to the thriving wolf populations, even if winters become milder and less snowy there likely will be a relatively negligible fall-off in late-winter, early spring carrion that the above-mentioned scavengers rely on to get a big boost of protein when they most need it.

Studies snow that without wolves in Yellowstone, late-winter carrion would be expected to drop off by nearly a third in March and two-thirds in April with a waning snowpack. However, as long as healthy populations of wolves reside in the park the amount of carrion in March would only be expected to drop by 4 percent under climate-change scenarios, and by just 11 percent in April.

In other words, while snow cover might be easing up sooner than what once was considered normal, and so elk and bison are more mobile, so are wolves. And they're killing plenty of these and other ungulates and leaving plenty of carrion for the scavengers.

Now, as for the upcoming $30,000 study into how the changing weather patterns are affecting grizzlies in the park, Mr. Schwartz says biologists will studying denning schedules -- when bears turn in for the winter and arise in spring -- to see if there's a definite correlation with temperatures and snow melt.

Why the interest? Who cares if bears turn in a little later or rise a little earlier? Well, it could to lead to more conflicts with humans, and those often turn out bad for the bears. In fall, grizzlies are working hard to top off their caloric banks to make it through the winter. This often results in human conflicts as some bears head down to the lowlands in search of food and encounter humans.

Too, climate change is thought to be behind a major mountain pine attack in the Rockies that is wiping out lodgepole pines and making inroads on white bark pines, which grizzlies rely on for a protein-rich diet before turning in. If these white bark pines are wiped out -- and the Natural Resources Defense Council has petitioned the U.S. Fish and Wildlife Service to list them as an endangered species -- that could take one food source away from the grizzlies and add to their search down in the low country for food.

Now, according to the Great Falls newspaper, "The USGS discovered the trend of male bears entering dens later in the fall when it was asked to look at when and where bears den as part of a winter-use plan study. The USGS has not detected an early emergence by bears in the spring."

It will be interesting to see what this research turns up.