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Polar Bears Threatened With Loss Of Food Sources

November 24, 2010

A continuation of current climate trends could eliminate the primary food source of polar bears amid fierce competition with grizzly bears as the two species are forced into a shared habitat, said evolutionary biologists at the University of California, Los Angeles.

The scientists used 3-D computer modeling to compare the skull and jaw strength of polar bears and brown bears, and found that polar bears are less suited to processing the tough omnivorous or herbivorous diets of their grizzly cousins.

The findings add to growing signs of threats to polar bears as a result of climate change that scientists attribute to man-made greenhouse gas emissions.

“This is one additional piece of evidence that things look pretty bleak for the polar bear, if current trends continue,” said Graham Slater, the study’s lead author, in an interview with Reuters.

Polar bears are already are losing habitat amid rising Arctic temperatures that are depleting the sea ice they depend on to hunt for seals. As the ice continues to diminish, the bears will be forced to seek additional food sources.

“To people who say polar bears can just change their diet, we are saying … they will have to, but it probably will not be sufficient for them, especially if they are co-existing with grizzly bears,” said Blair Van Valkenburgh, the study’s senior author.

The grizzlies have begun moving north in Canada as their natural habitat grows milder.  Simultaneously, polar bears are moving farther south due to melting ice, putting the two species on a collision course towards land best suited to the grizzly.

“These two species are already starting to come into contact,” Slater added.

The polar bear is expected to lose in the growing competition for the plant life that makes up the bulk of the grizzly’s diet because they lack the skull strength and tooth size needed for chewing plants, grass, tree bark and berries, the scientists said.

“Polar bears would not be able to break up the food as well in their mouths and would not digest it as well,” Van Valkenburgh told Reuters.

Seal blubber is softer and requires less chewing, putting less strain on the skull by comparison, he added.

Grizzly bears are a subspecies of brown bears, from which the polar bears evolved relatively recently.

And while the two are closely related, the split between them occurring roughly half a million years ago in response to the swift onset of glacial climates during the ice ages.

Polar bears are a “marvelous example of rapid adaptation to an extreme environment,” Slater told Reuters.

“The fact that we can lose them equally as rapidly as a result of human-mediated climate change is rather striking.”

The study, funded by the National Science Foundation, was published November 5, 2010 in the journal of the Public Library of Science, and can be viewed at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0013870.

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