With polar bears' icy homes melting due to climate change, scientists are worried about mass polar bear starvation, and while one new model suggests that the bears' diets may be flexible enough to cope, other scientists challenge that model's assumptions.
Polar bears currently depend on sea ice when foraging, especially when hunting seal pups, so many scientists predict that polar bears will starve by 2068, when "annual ice breakup is expected to separate the bears from their sea-ice hunting grounds for a consecutive 180 days each year," reports the American Museum of Natural History, "creating ice-free seasons that will last two months longer than those in the 1980s."
But those studies failed to account for dietary flexibility, say scientists at the AMNH, in a June paper published in PLoS-ONE. "Polar bears are opportunists," says AMNH ornithologist Robert Rockwell, that "have been documented consuming various types and combinations of land-based food since the earliest natural history records."
Dr. Rockwell and his graduate student, Linda Gormezano, focused on terrestrial food sources to determine if they can offer enough energy to sustain adult male polar bears during long summer months, when the bears can’t rely on ice-hunting to build up fat reserves.
Based on a mathematical model, they concluded that prey such as caribou, snow geese, and snow goose eggs can theoretically provide plenty of calories, well more than the bears burn in hunting them.
But Rockwell and Gormezano’s model shouldn’t be misinterpreted to suggest that polar bears, which were first listed as a threatened species under the Endangered Species Act in 2008, are now free from the risk of mass starvation and extinction.
Polar bear experts Steven Amstrup and Karyn Rode, among others, still consider polar bears to be very much in danger.
"Rockwell and Gormezano were explicit about the assumptions they made, but it’s important to realize that there were a lot of assumptions made," says Dr. Rode, an Alaska-based biologist with the United States Geological Survey (USGS).
While some polar bears have been observed eating terrestrial foods, including berries, bird eggs, and mammals, it remains uncommon, she says. "Nowhere has it been documented that terrestrial foods are becoming a more important component of polar bear diets," wrote Dr. Rode and her colleagues in their April 2015 study, "Can polar bears use terrestrial foods to offset lost ice-based hunting opportunities?" published in Frontiers of Ecology and Environment.
They observed fewer than 30 polar bears – within populations ranging from 900 to 2,500 individuals – consuming energy-dense terrestrial foods, such as bird eggs.
Rockwell and Gormezano suggest that "social learning and energetic need" could push terrestrial feeding to become widespread among polar bears, but that’s a big assumption, says Dr. Amstrup, the chief scientist with Polar Bear International and a coauthor on the April paper.
"A hungry polar bear isn’t going to just lay down and die," agrees Amstrup, but "there doesn’t appear to be enough food to support the entire polar bear population if sea ice completely melted."
Yes, he says, polar bear dietary habits can evolve, and they have to some extent, but Rockwell and Gormezano’s model assumes there is enough terrestrial prey for all polar bears to survive, which "simply isn’t reasonable," he says.
It’s not a question of their flexibility, he says, but whether available resources will allow them to adapt. Polar bears have to compete with smaller, omnivorous brown bears that already depend on the Arctic’s comparatively scarce terrestrial food sources. "The Arctic environment is poor in the eyes of a bear," says Amstrup. "It isn’t reasonable to think it could suddenly support the world’s largest bears."
The grizzly bears that share some of the polar bears’ habitat are the smallest of their kind and appear only in small densities, so they can survive even though "food resources are pretty poor, with low availability of these types of terrestrial foods available in the bears’ range," says Rode.
"Polar bears have a higher energetic requirement than the grizzly bears that use those resources," she adds.
"Evidence from captive studies with brown bears suggest that a higher fat content allows for a lower caloric diet," says Rode, but if polar bears were to transition from lipid-based seals to high-protein sources like bird eggs, they would need to consume a greater quantity of food in order to survive. That spells trouble for the bird populations.
Furthermore, Rockwell and Gormezano’s model was based on research in the Hudson Bay. "The amount of terrestrial prey in the Hudson Bay region is substantially higher than in any other area of polar bear habitation," says Amstrup. "We don’t know if many polar bears are taking advantage of these foods, and even if they were, it’s ... a small geographic region."
So what hope is there for the polar bears?
"Greenhouse gas mitigation is the answer," says Amstrup. "If we don’t stop the sea ice from melting, polar bears will go extinct."
Amstrup agrees with the consensus of most polar bear researchers that with no change in greenhouse gas emissions, two-thirds of the world’s polar bears will die by the middle of this century and the species could be extinct by the end of the century.
Polar Bears International, a nonprofit dedicated to the bears’ survival, suggests that everyday decisions, like bicycling instead of driving a car, or using energy-efficient appliances, could add up to a large cumulative effect.
Research into the bears’ adaptability, like the Rockwell and Gormezano study, can help scientists predict how many polar bears could be sustained by land-based foods, which may suggest other solutions to the problem. Rode is also working on mitigating other challenges polar bears face, such as modeling patterns of land use to help predict polar bear-human interaction, so it can be handled without further harm to the giant animals.
"There is little doubt that polar bears are very susceptible as global climate change continues to drastically alter the landscape of the northern polar regions," says Rockwell, "but we’re finding that they might be more resilient than is commonly thought." But while resilience could extend the bears’ ability to survive, it has yet to be confirmed whether terrestrial foods could save the species.
Amstrup, like most polar bear scientists, does not share Rockwell’s optimism. "While it’s tempting to think that polar bears could survive by switching to a terrestrial diet, [our] paper establishes in no uncertain terms that land-based foods do not offer any hope of polar bear salvation," he said in April. "If we don’t save the sea ice, polar bears will indeed be gone."
[Editor's note: This story has been revised to include a wider range of scientific opinions.]