8/19/2012 1:07:16 PM

Forest Fires in Denning Area

Just about anyone born in the last half-century knows who Smokey the Bear is. And many people are able to repeat his famous mantra: "Only YOU can prevent forest fires!" But I think even Smokey himself would be surprised to know that his distant cousin, the polar bear, is now dealing with the impacts of forest fires in the Canadian subarctic.

When most of us think about a polar bear den, we imagine a white snow bank with two little cubs peering out of a hole. But in the southern portion of the polar bear's range, the bears make use of earthen dens dug into peat moss. Female bears use these dens because there often isn't enough snow by mid-December to dig a snow den in which to give birth to their cubs. 

Evan Richardson at den site

Polar bear dig these earthen dens into frozen peat, typically under large spruce trees along the edges of lakes and streams. They can be used over several decades by any number of female bears. As a result, the dens remain as persistent landscape features unless they are disturbed.

In 1999, several large fires moved through the polar bear denning area in Churchill, Manitoba, and one of those fires burned down the Canadian Wildlife Service Polar Bear Research Camp. I happened to be on the helicopter the day this was discovered (although I was not yet working with polar bears).

As luck would have it, two years later, in 2001, I initiated a master's project with Dr. Ian Stirling at the University of Alberta to look at the impacts of these forest fires on polar bear denning habitat. We examined a number of burned and unburned dens. What we found was that forest fires significantly alter polar bear denning habitat by removing vegetation, primarily trees, which in turn weakened the stability of den sites and led to the collapse of dens.

Evan in fire-damaged den

In addition, the removal of the vegetation around the den sites resulted in a loss of permafrost, which is an important habitat feature that helps stabilize dens in addition to keeping them cool in the summer. The microhabitat inside these dens is important for pregnant female polar bears because it lets them escape the summer heat and harassment from biting insects, allowing them to save previous energy that will be required to nurture their cubs.

Although fires are not all that common in the Churchill denning area, one of the issues is that the trees and vegetation associated with den sites take a long time to recover from fire and it is not known under current climate change scenarios if the permafrost will ever recover. The other important factor we investigated in our study was the ignition sources for these fires, which are almost exclusively lightning strikes. You wouldn't think there would be a lot of lightning in the subarctic, but our study found that there was an average of 6,000 lightning strikes a year in our study area, a number that is predicted to increase with climate change. Warmer summer weather and drier conditions could further contribute to the risk of forest fires in the Churchill denning area in the future.

So why is this all so important?

Forest fire in July 2012

Maternity den sites are an essential part of polar bear reproduction as they provide a safe, warm place for cubs to be born. Newborn polar cubs are nearly helpless. Their eyes are closed, they are covered by only a thin layer of fur, and weigh about 600-800 grams (about the size of newborn Labrador puppy). The inside temperature of the peat dens in Churchill remains relatively warm in winter, around 0 degrees Celsius when it is -35 centigrade outside. After crawling into over a hundred of these dens for my master's I can definitely say that they are cozy.

Maternity dens are essential for the survival of newborn polar bear cubs and a lack of suitable denning habitat could influence female reproductive success. In July of this year several fires were burning in the Wildlife Management Area near Churchill, possiblly leading to further habitat loss in what used to be one of the largest polar bear denning areas in the world. Although the long-term impacts of forest fire activity on polar bear denning habitat in Churchill remain to be seen, the cumulative impacts of climate change have already resulted in a significant decline in the number of polar bears in this part of the world. If this population of bears is going to have a chance in the long run the most important thing you need to know is that "Only YOU can help prevent climate change!"

Forest fires in denning area, July 2012

Top photos, copyright Evan Richardson; bottom, copyright Daniel J. Cox/Natural Exposures.

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