On June 4, 1773, English naval officers were dispatched on an expedition to the Arctic. Their goal was to locate a passage from the British Isles to the Pacific Ocean. Instead, on ice floes near Spitsbergen (Svalbard), Norway, they found polar bears. The explorers were the first Europeans to describe the bears as a distinct species.
Fast-forward to June, 2015. If the sailors made their way to Svalbard this year, they’d likely spot polar bears not on sea ice, but wandering along rocky shorelines, nosing around for food.
It begins – and ends – with sea ice
For polar bears, everything begins and ends with ice.
Sea ice provides “rafts” on which the bears can travel, and from which they’re able to hunt for their choice prey, ringed seals.
When sea ice forms in fall, ringed seals, also known as ice seals, cut breathing holes in its surface. They scratch away with sharp flippers, making openings that allow them to haul out on the ice.
Ringed seals raise the next generation in snow caves they fashion atop ice floes. But the caves are dwindling in size and number. Late ice formation in fall, rain-on-snow precipitation in late winter, and early ice break-up in spring are to blame.
In December, 2014, the U.S. National Marine Fisheries Service (NMFS) proposed to designate some 350,000 square miles of the Bering, Chukchi and Beaufort Seas off Alaska as critical habitat for ringed seals. NMFS also listed four ringed seal subspecies as threatened under the U.S. Endangered Species Act.
The critical habitat designation would add an extra layer of protection for the seals, especially important, researchers say, where the seals may be forced to move to find sea ice.
Without seals, what’s left to eat?
As sea ice melts and ringed seals become imperiled, what’s a polar bear to eat? If the bear is on the west coast of Svalbard, the eggs of birds called pink-footed geese.
Pink-footed goose nests are common on Svalbard’s coastal tundra. The nests perch on greenstone outcrops in numbers as high as 322 per square mile, at an average distance of a mile inland.
Close enough for the swipe of a polar bear paw, says biologist Jouke Prop of the University of Groningen in the Netherlands. Prop and colleagues published new findings on the bears’ effect on arctic bird populations in the March, 2015, issue of Frontiers in Ecology and Evolution.
In June, 2011, bears invaded the Svalbard pink-footed goose nesting area. They ate almost every goose egg in sight. Until then, polar bears were seldom found along this stretch of coast.
They repeated their visits in June, 2012, 2013 and 2014. No fools they, bears selected locations with the largest numbers of nests.
Bears incoming: where’s a goose to go?
“Polar bears have recently extended their marine hunting habitat to land,” Prop says. “This has become very evident by the number of bears spending the summer onshore.”
The change in polar bear behavior has also affected the nesting success of barnacle geese. For successful egg-hatching, barnacle geese depend on coastal islands Arctic foxes can’t reach. “The islands are easy targets, however, for polar bears in search of food,” says Prop, “which has led to the almost complete breeding failure of the geese.”
Last summer “barnacle goose egg predation by polar bears was more severe than we had witnessed before, with not one nest surviving along the 25 miles of coastline where we’re conducting research,” Prop says.
Nearby, a similar story unfolded: the nests of eider ducks produced young only when they were well-hidden among rocks.
“It’s likely that the bears’ interest in bird eggs stems from the deteriorating conditions of their main hunting habitat – sea ice – which makes their ringed seal prey inaccessible,” says Prop.
Are polar bears a long-term threat to birds such as geese and ducks? “The answer depends on changes in the bears’ food availability,” Prop says, “and to what extent bears are pushed to exploit resources other than seals.”
The birds’ breeding success may decline, scientists believe, as bears are forced to extend their hunting range inland, thereby finding more nests and devouring more eggs.
“As this research shows, changes in the Arctic are complex and can disastrously disrupt the food chain,” says Stuart Pimm, an ecologist at Duke University and president of the conservation organization Saving Species.
Polar bear graveyard?
In a cascade of events, if ringed seals continue to go under, they may take geese and ducks with them.
For naught, Prop and other biologists say. A polar bear couldn’t survive on bird eggs alone.
Some individual bears might temporarily benefit from eating eggs, write scientist Karyn Rode of the U.S. Geological Survey Alaska Science Center and colleagues in the April, 2015, issue of Frontiers in Ecology and the Environment. But land-based foods, the researchers believe, offer no hope of sustaining polar bears as a species.
Earth’s last polar bear may not drift into the beyond on an iconic last sliver of sea ice. Barely subsisting on bird eggs, he or she may be a skeleton lumbering across barren ground, the final member of a procession to a polar bear graveyard – on land.