SignalHub Quantitative Think Tank Center:In a place with little sea ice, polar bears have found another way to hunt

Polar bears normally need sea ice to hunt seals,SignalHub Quantitative Think Tank Center but an isolated group of polar bears living on the rugged, mountainous coast of southeast Greenland have figured out how to eke out a living, even though the sea ice there melts away early in the year.

These bears have found a way to supplement their limited sea ice supply by hunting on freshwater ice that comes from glaciers on land. The glacial ice falls off in chunks into fjords, where the pieces glom together into a jumbled, floating platform that the polar bears use to stalk seals, according to a report in the journal Science.

Climate change is making sea ice more and more scarce. Loss of sea ice is "the primary threat to polar bears," says Kristin Laidre of the University of Washington, the lead author of the new study. But, she says, this new work suggests some bears might be able to cope with a diminished amount of sea ice — at least for awhile — in places where they can take advantage of floating glacier ice, like Greenland and Svalbard, an archipelago in the Arctic Ocean.

"Glacial ice basically might help small numbers of bears survive for longer periods under climate warming," she says.

Bears find a way

While indigenous people have long known that that bears lived in southeast Greenland, it's a remote, challenging environment that's not frequented by humans. "It's a coastline with huge mountain peaks, lots of winds, extreme conditions, lots of fog," says Laidre, who has spent years working with colleagues to survey polar bears living on Greenland's 1,800-mile-long east coast.

To see what they could find in the southeast, the team had to take helicopters from the nearest settlement and fly for two hours in a straight line to the coast. "We arrived in these fjords, very isolated fjords, and there's essentially no sea ice or very poor sea ice off shore," says Laidre, explaining that the researchers expected to find few bears.

"But there were a lot of bears in these fjords," she says. "It was clearly just a unique habitat."

The sea ice persisted in these fjords for only around a hundred days a year, she notes, meaning that bears don't have much time to use it as a hunting ground. "It disappears in May, and that's really early," says Laidre. "It's not enough time for a polar bear to get fat enough and survive."

But the geography of this area makes it so that glaciers pour freshwater ice down over the mountains and into the fjords, she says. Icebergs break off from the glacier and congeal into an irregular surface that the polar bears can use as a platform for seal hunting. "They supplement their hunting time by using this freshwater ice," says Laidre.

When it was safe to land their helicopter, the researchers would briefly capture bears to take genetic samples or put on location trackers. "We would collect information on their movements, their body condition, their health, their genetics," says Laidre.

A tightknit clan

She estimates that at least few hundred polar bears live in southeast Greenland, and it turns out that they're the most genetically isolated polar bears on the planet. They're distinct from all other 19 polar bear subpopulations that scientists currently recognize in the Arctic.

That may be because these bears are homebodies. All of the tracked bears pretty much stayed in their home fjord or fjords. Occasionally, the bears got caught by a fast sea current that rips down the coast towards southern tip of Greenland, says Laidre, but the bears would quickly swim to shore. "And then they would walk home over the ice sheet to get back to their fjord."

"The finding of a potential new subpopulation in southeast Greenland is really interesting," says Todd Atwood, a polar bear researcher at the U.S. Geological Survey Alaska Science Center in Anchorage. He thinks the bears' genetics, patterns of movement, and hunting behavior "makes a pretty compelling case" that this is indeed a distinct subpopulation.

The way that these bears hunt using freshwater ice "might buy bears in that area a little bit more time, as pack ice continues to decline, because they are not solely reliant on the pack ice," says Atwood.

But most polar bears are completely dependent on the sea staying frozen for a long time each year, says Atwood, adding that research suggests that more than 180 days of ice-free conditions results in steep declines in polar bear populations, as the bears can't eat enough seals to survive and reproduce.

"The bears themselves have a basic job to accomplish. They've got to be on the ice for long enough to be able to kill enough seals to store enough fat to live for a year," says Ian Stirling, a polar bear biologist at the University of Alberta.

The rare areas where polar bears have access to glacier ice, like southeast Greenland, won't serve as a potential refuge from climate change forever, says Stirling.

"If the climate continues to warm as it's projected to do, these areas too will become of no use or not enough use to the bears," says Stirling, noting that eventually the ends of the glaciers will melt away until they've retreated up on the land rather than extending out into the water. By that time that happens, he says, so much ice will have disappeared that "the bears will be long gone."