A large population of polar bears in eastern Alaska and western Canada has decreased by 40 percent since the beginning of the new millennium, according to a study by the U.S. Geological Survey and Environment Canada.

Researchers tagged and released polar bears in the Beaufort Sea from 2001 to 2010 and found that the population decreased from 1,600 bears in 2004 to 900 in 2010.

They also found that just two of the 80 polar bear cubs they tracked from 2003 to 2007 survived. Typically, about half of the cubs will make it to adulthood.

"For reasons that are not clear, survival of adults and cubs began to improve in 2007 and abundance was comparatively stable from 2008 to 2010 with approximately 900 bears in 2010," the researchers wrote in their paper, which was published in the current issue of Ecological Applications. "However, survival of subadult bears declined throughout the entire period."

Polar bears, which are dependent on sea ice to survive, have long been a symbol of the effects of global warming, and scientists have followed them closely.

But while the study finds this particular population in decline, other polar bear populations are stable. One in northern Canada's M'Clintock Channel is even growing.

However, of the 19 known polar bear populations scattered across the Arctic, little is known about most of them. Nine are so remote that they've hardly been studied at all.

Location, location, location

So why are some populations stable — or even thriving — while the Beaufort Sea bears are dropping in number?

It all has to do with location.

"If you're in the high Arctic, there's a greater possibility of population stability," David Koons, a Utah State University professor who studies animal populations, told National Geographic. "There is more ice pack and prey availability."

polar bear on iceThe most southern polar bear populations, such as the Beaufort Sea bears, are experiencing the effects of global warming first as their range heats up faster, making it more susceptible to melting sea ice.

Polar bears spend much of their lives on sea ice where they catch seals, their primary prey, which are also dependent on ice. But since the late 1970s, global warming has caused ice to retreat by 12 percent per decade, according to NASA.

Even "when that ice is there, it's really jumbled up [due to freezing and refreezing]," said study leader Jeff Bromaghin. "The seals may be there, but [the polar bears] can't get to them. We saw several cases of bears clawing through really thick chunks of ice to get to seals."

In 2007, the U.S. Geological Survey estimated that the global polar bear population will shrink to a third of its current size by 2050 due to habitat loss and diminished access to prey, but Bromaghin says that estimate might be too conservative.

"Actually, observed loss of sea ice in the Arctic has been greater than earlier climate models. We're losing ice faster than forecasted."

How are polar bears coping?

With less sea ice to hunt from, the Beaufort Sea polar bears are seeking other food sources. Researchers have observed them eating caribou, whale carcasses and snow goose eggs.

In 2004, the study authors confirmed three instances of cannibalism among the bears.

"They're starting to use land when food sources are limited. They'll eat whatever they can catch, but it's not enough to sustain them in the long run," Bromaghin said. "Every scrap of evidence suggests that polar bears are linked to sea ice. There's no evidence they can live on land."

The Beaufort Sea polar bears aren't the only ones eating eggs. So are the bears that live along the western Hudson Bay in Canada, a population that's been stable for the last 10 years, following a decline of 15 percent to 22 percent from 1987 to 2004.

“The early 2000s were very bad years for our bears and everyone agrees on that,” said Kelsey Eliasson, a polar bear guide and author of "Polar Bears of Churchill." "However, since 2004, there have been some good years and some bad years and in the past five years, I would say conditions are actually, for the most part, positive for these bears."

One reason for the bears' steady numbers may be that they've found another food source. With sea ice melting earlier, the bears come on shore earlier when the area's 75,000 pairs of snow geese are breeding, meaning there are plenty of birds and eggs for hungry bears.

polar bear walking along Hudson BayEliasson lives in Churchill, Manitoba, which is known as the "Polar Bear Capital of the World."

The small town located on the Hudson Bay is a popular tourist destination for those looking to catch a glimpse of polar bears when they come ashore in the fall. And locals say there's been no shortage of bears.

The animals often wonder into town in search of food, which is why Manitoba Conservation established the Polar Bear Alert program. During bear season, four natural resources officers patrol the area and monitor a 24-hour bear hotline.

"The Polar Bear Alert Program handled a total of 52 bears this season, which is higher than the past two years," said Brett Wlock, a natural resources officer who has worked in Churchill for five years. "In comparison, last season we handled 38 polar bears and the previous season, 24."

Although some polar bear populations are stable or growing, conservationists say we should look to the shrinking number of Beaufort Sea bears as an indication of what's to come.

"Climate change is not some future threat," Sarah Uhlemann, senior attorney for the Center for Biological Diversity, told The Los Angeles Times. "Global warming is happening now and killing polar bears now."

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Polar bear on ice (inset top): Juan-Vidal Díaz/flickr; polar bear by Hudson Bay (inset below): Paul J. Richards/Getty Images