Receding sea ice bringing polar bears closer to humans
More time on land presents many problems for mammals
In recent years, polar bears have become the lovable poster children for climate change in the Arctic. And as their preferred sea-ice habitat continues to diminish year after year, thanks to warming temperatures in the region, it seems there’s no end to the polar bear’s troubles. Now, a new study in PLOS One has brought to light one more problem for one more polar bear population. Bears in the Chukchi Sea region — the body of water between Russia and Alaska — are spending more time on land in the summers as the amount of summer sea-ice in the Chukchi Sea continues to shrink. And that could lead to myriad problems, not only for the bears, but also for the humans they may run into.
“For this particular population of polar bears, there’s been very little study of land use,” said Karyn Rode, a research wildlife biologist with the U.S. Geological Survey and lead author of the new paper. There are 19 polar bear subpopulations, and only a few have been studied for the landuse patterns, including bears in Canada’s Foxe Basin and Hudson Bay region. “So really, the only work (for the Chukchi Sea bears) is some un- published reports by Russian scientists who have been on the island in the summertime and observed polar bears there,” Rode said.
The researchers chose to examine the Chukchi Sea bears for two reasons, said Rode. First, these bears are one of just two subpopulations that the U.S. is charged with managing (the other being the Beaufort Sea bears). Second, the Chukchi Sea is recognized as one of the Arctic regions experiencing the most dramatic recent sea-ice declines.
It’s a problem that’s not just exclusive to bears, either.
The Chukchi Sea has garnered international attention the past few summers when hordes of walruses were forced to haul out on the Alaskan shore after their preferred seaice habitat melted away.
For this paper, the researchers examined data from two study periods: 1986 to 1995 and 2008 to 2013. In both periods, scientists radio-collared bears (103 of them in the first study period and 47 in the second) and observed their movements during the summer months. Only female bears were included in the study, largely thanks to a quirk in male polar bear biology: male bears’ necks are wider than their heads, meaning the males are frequently able to slip loose of their radio collars.
In the first study period, the researchers found that 20 per cent of all the bears spent more than seven days on land, while in the second period this proportion rose to 38.9 per cent. The bears also increased the average amount of time they spent on land by 30 days. Most of these bears spent their time on either Wrangel Island or nearby Herald Island off the Russian coast, although a few also summered on the Russian or Alaskan coasts, and they included denning bears, or bears having cubs.
The study also suggests that changes in sea ice are, in fact, what’s driving the bears’ behaviour. Data indicate that ice is retreating, or melting and shrinking northward, earlier in the season than it used to. In northern parts of the study region, the researchers found that ice retreated 20 to 40 days earlier between 2008 and 2013 than it did between 1986 and 1995. In more southern areas, it’s retreating up to 15 days earlier.
“I think the article is quite interesting to see how they use the sea-ice data and the temporal information they have to start looking at correlations over time,” said Vicki Sahanatien, a recent doctoral program graduate of the University of Alberta, who was not involved with this paper but whose research has focused on polar bears in Canada’s Foxe Basin.
One of the biggest concerns, though, is how much longer the bears are staying on land once they come off the ice. Thirty days is an impressive amount of time for the bears to extend their time on land, because many scientists believe bears don’t feed as well on land as they do on sea ice.
Polar bears use sea ice as their hunting grounds, perching on the ice to snatch up tasty seals as they surface. On land, bears are often forced to pursue smaller prey or scavenge carcasses of already dead animals, so scientists worry that their nutrition could suffer the more time they spend on land.