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Polar bear population reaches its limits

Despite being listed on the on the endangered species list, polar bear populations in certain areas have reached their “carrying capacity” — the maximum environmentally sustainable population size.

A little-noticed report from earlier this year found that polar bear populations in the Davis Straits have flourished despite shrinking Arctic sea ice since the 1970s. In fact, the polar bears in Davis Strait, which lies between Greenland and Canada, now have a greater population density than any other seasonal-ice subpopulation and may have reached the outer limits of what the region can sustain.

More significantly, this is at least the second peer-reviewed report on polar bears in which researchers admit they cannot distinguish between a polar bear population that has reached the population limits of what its environment will support versus one being hurt by global warming.

Dr. Susan J. Crockford, a zoologist with more than 35 years of experience, found numerous signs that the polar bear population was reaching its limit.

“We have at least two reports in the peer-reviewed literature that state flat-out that the presumed negative effects of declining sea ice on a population’s size are indistinguishable from a population that is as large as it can get,” Crockford wrote in a blog post. 

The report found that despite the shrinking sea ice, the estimated number of bears in the region shot up to 2,158 polar bears in 2007 from only about 1,400 in 1993. The population density of the area comes out to be 5.1 bears per 1,000 square kilometers of sea ice habitat and is “greater than polar bear densities in other seasonal-ice subpopulations, which are approximately 3.5 bears/1,000 km2,” according to Crockford.

“Rather than being proven victims of Arctic sea ice in a ‘death spiral’ due to global warming, when they finally present the data, biologists have to admit that they cannot actually tell the difference between a polar bear population that is so large that it can no longer increase and one that is suffering a population decline because of reduced sea ice,” Crockford added.

The report found that the Davis Strait polar bears have low birth rates, average adult survival rates and high population densities. However, researchers were unsure whether low birth rates were due to global warming or because of the large population density.

A similar conclusion was reached by Andrew Derocher in 2005 referring to his study on polar bears in the Barents Sea between 1998 and 2002: “given that the population may be showing density-dependent responses, it is not possible to differentiate the climatic effects from population effects.”

Polar bears were the first species to be listed on the Endangered Species List because of global warming. They were classified as threatened under the law in May 2008.

“There are far more polar bears alive today than there were 40 years ago,” author Zac Unger told NPR in an interview about his new book, “Never Look a Polar Bear in The Eye.” “There are about 25,000 polar bears alive today worldwide. In 1973, there was a global hunting ban. So once hunting was dramatically reduced, the population exploded.”

According to the U.S. Fish and Wildlife Service, it’s estimated there are between 20,000 to 25,000 polar bears worldwide, living in Canada, Greenland, the northern Russian coast, islands of the Norwegian coast and the northwest Alaskan coast.

Polar bear hunting is banned in the U.S., and Alaska Natives can only hunt polar bears for tribal needs. There are also restrictions on the importation of polar bears and polar bear parts or products. The U.S. and other countries, such as the Russian Federation, support an international effort to protect the bears.

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