Polar Bears: Struggling in the Warming Arctic
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Circumpolar distribution of polar bears (pink shading).
The polar bear is one of the world’s largest carnivores. It inhabits the ice-covered waters of the circumpolar Arctic, particularly annual ice over the continental shelf where biological productivity is highest.
For most of the year, polar bears hunt seals from sea ice. [1] Bears optimize foraging for seals by staking out seal breathing holes and haul-out sites. Rarely do they capture seals in open water.
During ice-free periods, female polar bears swim to shore from the southern edge of the pack ice and give birth on land, in dens. Recession of the Arctic ice cap as a result of warmer Arctic temperatures have extended the distance between denning and feeding sites. This increases the vulnerability of mothers and cubs.
Polar bears, when on land, mostly scavenge along the shore for food other than seals. A longer ice-free period means more time on land and intensified competition for this food. Because females are walking and swimming longer distances and spending more time on land, their body weights are decreasing. If this trend continues, the average weight of females during the next 20 to 30 years in western Hudson Bay will drop below 190 kg. Females weighing less than 190 kg seldom bear cubs. [2]
A longer time on land also increases encounters between polar bears and humans. Such encounters usually mean more bad news for the bears. If they damage property or threaten human safety, as they often do when scavenging for food, they are likely to be shot. Furthermore, repeated sightings of polar bears inflate estimates of their population sizes and have justified higher quotas for hunting by aboriginal people. [2]
Despite widespread interest about the plight of polar bears, information about their numbers is meager. Estimates of population size in any region are based on one or at most two scientifically conducted censuses.
Standard census methods such as “capture, mark, release, and recapture” may involve tracking bears from helicopters, shooting them with darts containing sedatives, tagging or collaring them, and then following them. These procedures may unduly stress the bears, change their behavior, or even compromise their survival. A novel, less invasive approach for tracking polar bears uses dogs to locate polar bear scat (feces), characterizes the age and food source of the scat, and identifies individual bears by DNA in the scat.[3]
Yearly estimates of body mass (mean with error bars indicating the variation in values around the mean) for polar bears in western Hudson Bay. Dashed blue line highlights the overall linear trend over time. (After Stirling and Parkinson 2006.)
[1] Derocher, A. E., N. J. Lunn, and I. Stirling (2004) Polar bears in a warming climate. Integrative and Comparative Biology 44:163-176.
[2] Stirling, I. and C. L. Parkinson (2006) Possible effects of climate warming on selected populations of polar bears (Ursus maritimus) in the Canadian Arctic. Arctic 59:261-275.
[3] Revkin, A. C. (2007) A team of 2, following the scent of polar bears. The New York Times, New York, June 5, 2007.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
