As Arctic sea ice melts earlier each year, polar bears in some parts of Norway and Greenland are abandoning ice floes for dry land and their favorite meal—seals—for seabird eggs. The shift in diet could sound a death knell for popular nesting grounds of barnacle geese, eider ducks, and glaucous gulls, researchers warn this month in Frontiers in Ecology and Evolution.
Calling the results the “cascading effects of climate change,” the team of European researchers found that over the course of the past 10 years, dramatic increases in summertime nest predation correlated with diminishing sea ice coverage in the Arctic Ocean. Their research combined NASA satellite imagery with on-the-ground observations at one seabird nesting site in northeast Greenland and four in the Svalbard archipelago of Norway. “[At our site] we had the first polar bear predation in 2004,” says Jouke Prop, an ornithologist from the University of Groningen in the Netherlands, who was a principal author and led observations at Nordenskiöldkysten, one of the Norwegian sites. “In most of the years since, predation has been really severe.” He has observed polar bears eating more than 200 eggs in 2 hours, and last year no chicks or eggs of any species—barnacle geese, eiders, and glaucous gulls—survived.
According to Prop, several dozen of Svalbard’s polar bears have learned when and where to return each year for the best meals. Although they compose a small percentage of the total bear population, their impact is catastrophic. Prop has calculated that a hungry bear can raid 50 nests in just an hour and a half, consuming enough for a 20-kilogram omelet—before sleeping for 12 to 16 hours and then awaking for another meal. Polar bear cubs also get in on the action, rolling eggs on the ground like balls before eating them.
As their hunting behavior shifts from ice to land, the polar bears “have progressively arrived earlier and earlier to have access to more eggs,” says biologist Børge Moe, another principal author of the study who works at the Norwegian Institute for Nature Research in Kongsfjorden, where seabird egg predation is just beginning to increase. At sites in southern Svalbard, bears are showing up at the nesting sites as early as mid-June. But as bears vie to be first on the scene, they will eventually arrive during the critical period when birds are just settling in, usually in early June, Prop predicts. And that may force birds to abandon their breeding colonies.
“This paper ties it all together and shows a very clear relationship between the disappearance of sea ice and increasing predation intensity on seabirds,” says Andrew Derocher, a polar bear specialist and Arctic ecologist at the University of Alberta in Edmonton, Canada. In the long run, he says, the seabirds may outlast the bears. “What we’re seeing is probably a very transient effect on these seabird populations,” he says. As sea ice disappears, polar bear populations will crash harder than the seabirds.
Meanwhile, the scientists are proving as adaptable as the bears. “We really had to get accustomed to this,” says Prop, a specialist in barnacle geese. When the bears started raiding the colonies he had been studying his whole life, he and other researchers were torn. Should they chase the bears off, as did another team he heard of in Canada? Or should they continue their observations, interfering with the colonies as little as possible? Eventually, they settled on the latter.
“Initially the bears were destroying our main object of study—lifetime reproduction of geese. It was a blow, but then we realized bears were becoming part of our study,” Prop says. He marvels at what has happened since. “Ten years ago I would never have believed the situation we are now in, and it’s impossible to make predictions. Three or four years more, and then we are at the really critical point to see how the birds will react.”