Photograph by Paul Souders, Corbis
Published July 17, 2014
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.
This looks a particularly sketchy bit of work to me ie like why have polar bears lived on ice and eaten seals and not caribou and geese ?
I think it's highly probable that Polar Bears could experience a divergence in evolution. Some may chose to live on land, and will become more adapted as such. Others, however, may choose to reap the bounty of the sea, and evolve into some kind of whale-like creature. Only time will tell what evolution has in store for planet Earth....
Hope Mother Nature gives them the power to survive. The climatic change had disrupted the environment. We need to contribute something for better future.
The big question is, since all other bear species are land predators, why does the Polar Bear hunt on the sea ice in the first place? They must have been strictly a land animal at some time in their history. What forced that change?
This whole thing literally makes me sick to my stomach... such a magnificient animal, so specialized... have been around for so long, thru so much... And this is their demise???? Humans essentially????? And what a slow, painful way... starvation? I despise the human race... mostly...
It's a question of space, actually. And other requirements of life. Evolution has created polar bears, penguins, birds and little bugs, for example, so that there would be food chains and different parts of nature including species with different needs - enough food for everyone.
If all the glacier melts, the sea level will come up and cover the lower land areas. Then all the species have to live closer to each other, including humans. We all know that the most influential people in this world love (and have) money nowadays. With money they can cut down rainforests and build always better (read: bigger) shoppingmalls, for example. When they can do that, they can also easily torture species to death by sacrifising their ecosystems. We are accepting that cruelty and stupidity, because we don't really think about the future. Most of the people seem to think that the humans deserve the best parts of the cake because we are able to take them. It may sound hypocritical to say that we should live equally with the nature. Hasn't it always been 'a law of nature' that the strongest ones survive?
Yes and no. Evolution has actually "thought" about this system very carefully. Everything has its own place. Even if the nature's conditions change and species fight against each other, they live in harmony most of the time. Almost every extinction has a purpose - when species extinct, new ones can be born. Natural selection makes life continue though the environment changes.
Then there comes the human with its intelligence. I truly honore scientists, they have come up with great innovations, but why is the power used by the ones, who are hardly able to see one feet forward? I don't know if the human has been created somewhere or not, but even without believing to some upper power, we should channel our pride to helping the environment instead ruining it. That's the way we help ourselves the best.
@Dalton Clay a very rapid mass extinction is already happening
@Gerard Van der Leun You talk like a stalker.
@Cher Renke I don't think their extinction is inevitable. While conditions are deplorable for many, of the 19 recognized polar bear subpopulations, eight are declining, three are stable, one is even increasing.
@Cher Renke I dunno, there are plenty of cross-bred polar/brown bears in the wild, which suggests, as do recent genetic studies, that polar bears have not been speciated for that long, and are a quick-response adaptation.
@Marisa Cirino they won't go extinct - they'll adapt back to brown bears.
The Yellowstone River's oil spill was the first in U.S. frozen water in two-plus decades.
Welcome to Nagoro, Japan. Human population: 37. Doll population: 350. When villagers die or move away, a woman makes a life-size doll and places it in a spot that was meaningful to that person.
As an ancient drought took hold, a water temple saw more offerings from desperate Maya, archaeologists report.
The Future of Food
How do we feed nine billion people by 2050, and how do we do so sustainably?
We've made our magazine's best stories about the future of food available in a free iPad app.