A newly discovered subpopulation of polar bears in remote southeastern Greenland have found a way to eke out a living under warm conditions similar to those forecasted for much of the Arctic later this century, scientists reported on June 16 in Science.
Warming temperatures are causing sea ice, which polar bears depend upon for survival, to diminish. However, the hardy Southeast Greenland bears use freshwater ice from coastal glaciers as platforms from which to hunt seals year-round. These glaciers may create previously unrecognized “climate refugia” for polar bears, the researchers wrote.
“Our study lays out the evidence for a previously undocumented and highly isolated subpopulation of polar bears on the southeast coast of Greenland surviving in a special way,” Kristin L. Laidre, a marine biologist at the University of Washington in Seattle, said in an email. “These bears can perhaps tell us a bit more about the future for the species.”
Although the glacier ice allows the bears to inhabit fjords that are sea-ice-free for more than eight months of the year, this habitat is unusual for most of the Arctic. “Glacier ice may help small numbers of polar bears survive for longer periods under climate warming, and may be important to the species persistence (meaning preventing extinction), but it is not available for the vast majority of polar bears,” Laidre acknowledged.
She and her collaborators identified the unique group while surveying polar bears along Greenland’s east coast, which is about 1,800 miles long. The team drew upon more than three decades of data from captured polar bears and samples collected from subsistence hunters. The researchers tracked the movements of bears wearing GPS collars as well as analyzing DNA and observing the demographics of polar bears in the region.
To their surprise, the researchers realized that bears in Southeast Greenland did not interact with bears along the northern part of the coastline. They found that these southerly bears were genetically distinct from the 19 other previously recognized polar bear subpopulations. “They are the most genetically isolated polar bears in the world,” noted Laidre.
The researchers estimated that this subpopulation has been largely isolated for hundreds of years. One explanation is that the landscape is difficult to navigate, with steep fjords separating mountains and narrow glaciers. Meanwhile, the bears are penned in by the Greenland Ice Sheet to the west and the open water of the Denmark Strait to the east. The bears may also be cut off from their northern neighbors by the fast-flowing East Greenland Coastal Current. The rugged landscape and barricading terrain might have made it more difficult for the Southeast Greenland bears to find mates, resulting in fewer cubs born than other subpopulations.
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Sea ice frozen to the shore, known as fast ice, typically persists from February to late May in this part of Greenland. This means that the bears are left without sea ice for more than 250 days of the year—over 100 days longer than they can go without feeding.
Elsewhere in the Arctic, polar bears move to land or head northward when sea ice recedes during the warmer months. But the freshwater ice flowing into the sea from the Greenland Ice Sheet, known as glacial mélange, allowed the Southeast Greenland bears to stay put and capture prey throughout the year.
Some bears remained in the same fjord for years. On 11 occasions, the researchers observed polar bears carried southwards on drift ice caught in the East Greenland Coastal Current. All the bears swam ashore and trekked back to their home fjord within a month or two.
The researchers calculated that there are several hundred polar bears in Southeast Greenland. However, it’s unclear whether their numbers are growing, decreasing, or holding steady. “This is important to know and requires further monitoring,” Laidre said.
Conserving this subpopulation is vital to preserve the genetic diversity of the species and understand how polar bears will be affected by climate change, she and her colleagues concluded. While glacial mélanges aren’t common, they can be found in other parts of Greenland and the Norwegian archipelago of Svalbard and are inhabited by ring seals, the main prey of polar bears. It’s possible that such conditions may give bears beyond the Southeast Greenland subpopulation a buffer against dwindling sea ice.
However, “loss of Arctic sea ice is still the primary threat to all polar bears,” Laidre said. “This study does not change that.” Even the freshwater ice havens may be transformed as the Arctic heats up and the Greenland Ice Sheet melts.
“Climate action is the single most important thing for the future of polar bears,” Laidre said.
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Robert Newton, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory who studies Arctic sea ice, praised the wealth of information that Laidre and her team analyzed to discover the new subpopulation.
“The article itself is very hopeful in the sense that it does appear that the polar bears can survive without sea ice as long as they have some alternative platform from which to hunt,” he said, noting that it can take hundreds of years for glaciers to retreat. “It is quite possible that polar bears as a species will survive the loss of sea ice in the Arctic even if most of the populations are forced into extinction or forced onto land where they’ll merge back into the brown bear populations.”
An important question for future studies is how well organisms further down the food chain, including seals, fish, crustaceans, and algae, will adapt in the face of shrinking sea ice, Newton added. Still, the findings indicate that glacial mélanges could offer a lifeline for polar bears until conditions stabilize.
“If we eventually do get greenhouse gasses and global warming under control and we’re able to return at some point to the historical temperatures at the surface of the Arctic, all of the modeling we’ve done indicates that the sea ice will return in short order,” he said. “If we can establish refugia and protect the animals there, we really are conserving something for the future.”
