Photograph by Paul Souders, Corbis

Read Caption

A polar bear saunters along the rocky shore of Canada's Sentry Island in the Hudson Bay, in the summer of 2013.

Photograph by Paul Souders, Corbis

As Sea Ice Shrinks, Can Polar Bears Survive on Land?

The predators can stay alive on goose eggs and caribou, scientist argues.

Here's one possible summer menu for polar bears being forced to stay on land due to a lack of reliable sea ice: 60 snow goose egg clutches, 53 goslings, 63 adult geese, 3 caribou calves, and 3 adult caribou. Garnish with berries. Bon appétit.

Linda J. Gormezano, an ecologist at the American Museum of Natural History in New York City, isn't a polar bear chef. But she has been figuring out what polar bears might have to eat to survive increasingly long ice-free seasons in the western part of Canada's Hudson Bay area (map).

Her calculations, presented this week at the North America Congress for Conservation Biology in Missoula, Montana, suggest that the predators may be able to survive for six months on land-based foods, and, because of climate change, they may have to.

Other scientists have warned that a lack of sea ice due to warming temperatures in the Arctic—and declining seal prey populations—could drive this population extinct by 2020. (Read about polar bears in trouble in National Geographic magazine.)

There are fewer than a thousand bears in the western Hudson Bay, out of a total global population of up to 25,000, but these bears are among the most visible.

They stay on the sea ice as long as it lasts, eating seals. When the sea ice breaks up, they come on land. During the land season, tourists travel to Churchill, Manitoba, to go out among the bears in special "tundra buggies." (Also see "Annual Polar Bear Migration Under Way: How It Works and How Climate Change Could Be Impacting It.")

The conventional wisdom has been that polar bears don't eat on land or do much at all except snooze and conserve their energy before the bay refreezes.

But as the climate warms, the sea ice doesn't last as long. Already the bears are showing up on land, on average, three weeks earlier than in the 1980s, according to Gormezano. And by the late 2060s, the western Hudson Bay could be ice free for up to six months.

Turf, Not Surf

In 2010 an analysis of the bears' energy needs by ecologist Peter Molnar—now at Princeton University—and his colleagues suggested that six months without food would kill 28 to 48 percent of adult male polar bears. At that rate, the population would likely crash.

But this analysis assumed that bears don't eat anything on land. Gormezano said this isn't true.

Her analysis of polar bear poop and observations of behavior suggest that the bears eat lots of land food, including lesser snow geese and their eggs, as well as caribou. So as part of her Ph.D. work under polar bear field biologist Robert F. Rockwell, she repeated Molnar's analysis but added varying levels of these foods. (Watch: Polar Bear Eats Seal in First Ever POV Video)

Bears that come ashore fat and sleek in May in the 2060s, she predicts, will be able to keep their energy reserves topped up with the goose eggs that become available a few weeks later and by the odd caribou calf. (See pictures: "80 Polar Bears Throng Village in Search of Whale.")

An adult male in bad condition would need the full menu of tundra foods listed above to make it through the ice-free months. Gormezano's analysis considered adult males, as did Molnar's, because the math is more straightforward without factoring in the demands of growth or reproduction.

But her observations show that typically females and cubs chase geese and gnaw on caribou.

Polar Bear Decline

An unanswered question is whether the energetic cost of chasing after a goose or a caribou is worth the calories the food supplies. If so, there are lots and lots of geese and, these days at least, plenty of caribou in the Hudson Bay area, Gormezano said.

"There is potential for this to prevent starvation under the scenarios that Molnar put out there," she said. "However, it does depend on how much energy they spend getting this food." (See more polar bear pictures.)

Molnar hasn't looked at the analysis in depth, but he's skeptical, given trends in this population of polar bears.

"The population has declined quite substantially between 1995 and 2005," he said.

"We know that their body conditions are getting worse. They are getting thinner. The real question is not, Can you put land-based feeding into the models or not? The real question is, Why aren't the bears eating enough to prevent declines?"

Follow Emma Marris on Twitter.