In Thawing Arctic, Fragile Food Web at Risk of Unraveling (Part III)

In the Arctic, ringed seals and other ice-associated pinnipeds aren't merely the polar bear's prey. They're its raison d’être.

Fossil and DNA records suggest that the white bears began diverging from brown bears around 200,000 years ago. "Some brown bear populations figured out that all these little sausages were available out there on the ice," said biologist Brendan Kelly of the University of Alaska, "and with their powerful noses, the bears could easily smell out the seals."

Brown bears, especially the North American grizzly subspecies, are famously omnivorous. Their food web ranges from roots and berries to salmon and deer. Diet largely determines their size. The Kodiak subspecies is the largest — they rival polar bears in size — because Kodiaks consume massive amounts of southwestern Alaska’s protein- and fat-rich salmon.

Polar bears, by contrast, subsist almost entirely on Arctic ice seals, chiefly ringed and bearded seals. No other food comes close to providing the amounts of fat the bear needs to survive the Arctic's extreme cold.

"To a polar bear, seals are giant fat pills swimming around out there," says Steven Amstrup, a former U.S. Geological Survey wildlife biologist who is now the chief scientist for Polar Bears International. Amstrup has been studying the Alaskan population for more than 30 years. The U.S. Department of the Interior relied heavily on his research when it conferred threatened status on the bear in May 2008.

Amstrup has predicted that two-thirds of the world's polar bears could disappear by 2050 if greenhouse gas emissions continue to climb at their current rate.

If polar bears evolved from brown bears, and brown bears thrive in a land-based food web, it's natural to wonder whether polar bears could adapt by expanding their diet. Researchers have, over the years, recorded a number of instances of gastronomic experimentation by these innately curious creatures.

They have been seen feeding on white whales, narwhals, walrus, little auks, Brent geese, thick-billed murres, and ptarmigan. Biologists in Svalbard, the Arctic archipelago north of Norway, have reported polar bears stalking and killing reindeer. During late autumn, when the bears of Canada's Hudson Bay gather near the water's edge in Churchill, Manitoba, to await freeze-up, they've been observed eating berries, grass, moss, lichen, and marine algae.

Canadian researchers recently reported that in the springtime the Hudson Bay bears are increasingly raiding eggs and chicks from the nests of snow geese and thick-billed murre.

The Rise of the Polar Bear-Grizzly Hybrid

The polar bear's food web may be expanding, but experts like Amstrup see the bear's behavior as an expression of desperation, the equivalent of a polar explorer eating his shoes. Fat is the key. Even if skinnier, less insulated polar bears were to survive, reproductive rates would plummet. Female polar bears only bear cubs when their bodies have sufficient fat stores; when the fat's not there, the bear's body reabsorbs the embryo.

Looking for the polar bear to survive by expanding its food web, Amstrup concluded, was a fool’s gambit. "We just don’t see any evidence that suggests there's any prey on land that's abundant enough to support polar bears in anything like their current population," he says.

Could polar bears adapt through interbreeding? Reports of polar bear-grizzly hybrids obtained from hunters in the Canadian Arctic have raised questions about a possible increase in interspecies breeding driven by climate change.

A recent article by Kelly in Nature highlighted confirmed reports of two "grolar bears." One was a second-generation hybrid, which indicates that the cross-species bears can survive and reproduce. "The rapid disappearance of the Arctic ice cap is removing the barrier that’s kept a number of species isolated from each other for at least 10,000 years," Kelly told me. Pinnipeds, he believes, are especially strong candidates for hybridization, because many species have a similar number of chromosomes.

"By melting the seasonal ice cap," he said, "we're speeding up evolution."