Polar bears in a warming world

Melting ice and other threats have put the world's largest bear in peril

02-22-2010 // NW Staff

Polar bear cub and mom

 

The polar bear is heading toward an uncertain future in a rapidly changing Arctic world. Because of global warming, the ice that the bears depend on to supply food and shelter literally is melting under their paws.

Andrew Derocher, a polar bear expert at the University of Alberta, compares global warming’s impact on polar bears to the effects of clear-cutting on forest species. “That’s essentially what you’re doing when you take away the sea ice,” he says. “If you have an erosion of the bears’ habitat, you’ll see the populations erode and retreat northwards.”

Strange Days for the Polar Bear
In recent decades, scientists have found that temperatures are rising many times faster in the Arctic than anywhere else on Earth, reducing sea ice and affecting everything from river habitat to animal behavior. Already, polar bear biologists have recorded strange happenings across the species’ range, which spans five countries—Russia, Canada, Greenland, Norway and the United States:

  • For several years, Derocher and biologists Ian Stirling and Nick Lunn from the Canadian Wildlife Service have been documenting that polar bears at the southern edge of their range in Canada’s James and Hudson Bays are becoming thinner. Between 1980 and 2004, the average weight of a mature female dropped from 650 to 507 pounds. The animals’ reproductive rate and body condition also are declining.
  • In 2004, researchers from the U.S. Minerals Management Service discovered four drowned polar bears in the Beaufort Sea, something they rarely see because the animals are such strong swimmers and routinely travel long distances between ice floes. Researchers think the drownings happened because the polar ice cap had retreated a record 160 miles north of the Alaska coast, forcing the bears to swim extremely long distances to find solid ice.
  • Last year, polar bear expert Steven Amstrup, a U.S. Geological Survey researcher, published a shocking study on polar bears “stalking, killing and eating other polar bears” in the southern Beaufort Sea. Although the bears have been known to kill others’ cubs, competition for scarcer food may have pushed them toward cannibalism.

Toxins in habitat

Long-lasting pollutants that have worked their way into the Arctic are threatening the bears. Chemicals called PCBs that are flowing from southern climes into Arctic rivers are transported from species to species as animals prey on one another; the chemicals become more concentrated in each species along the food chain. Polar bears are at the far end of this chain—they eat animals with the highest concentrations of toxic chemicals in their bodies—so it is no surprise that scientists have reported high levels of toxics in the bears, levels that seem to be affecting the animals’ health and breeding. Derocher has detailed an unusual condition in which female bears display some male sexual characteristics. In other species, such gender-bending effects have been associated with exposure to human-generated chemicals, especially during pregnancy.

The master threat: global warming
 The specter of ongoing climate change, however, beats all these other threats. The polar bear’s entire life cycle is tied to seasonal changes in Arctic sea ice; the animal routinely travels 3,000 miles a year, moving from one chunk of floating ice to land to another chunk of floating ice. In one study, half of all Alaskan polar bear dens in which births took place were discovered on ice packs several hundred miles offshore. When females leave their dens in spring with their young, their lives become a sort of ice dance, moving farther offshore from May through August and back south in October. The relationship between ice and polar bear health and survival, biologists say, cannot be overstated.

Is what’s bad for polar bears bad for seals?
Of course! The link between survival and ice is as important for seals as it is for the bears that feed on them. Polar bears generally emerge in spring from their long rest in winter dens (they do not really hibernate during winter, but they are very inactive and lose weight) ravenous for newborn ringed seals. And in this matter of polar bear grocery shopping, global warming is on the bear’s side. Ringed seals give birth and raise pups in snow caves; if they lose a few weeks cave time because of earlier melting, their survival rates are likely to drop. When unseasonable April rainstorms melted seal dens in recent years, for example, bears preyed on the pups with greater ease, raising the specter of seal population crashes.

Polar prognosis
The bears' future is uncertain, and definitely not promising. “Ice-dependent seals, walrus and polar bears—I don’t know what they’re going to do,” says Craig George, a biologist with the North Slope Borough Department of Wildlife Management in Alaska.

 

Adapted from a National Wildlife magazine story by Daniel Glick, December/January 2007

 

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