There is no one-size-fits-all when it comes to understanding the impact climate change will have on the animal kingdom. In fact, some animals, such as the brown anole or certain mosquito species, are actually expected to thrive under a changing global climate. On the other hand, some species are more adversely affected by our changing world, and the polar bear happens to be one of the fairest of them all.
These Arctic dwellers need sea ice to survive. Ursus maritimus travel across ice patches in search of breathing holes or dens that indicate a seal – their primary food source – is nearby. When the ice breaks in early spring the bears become landlocked, unable to hunt along the ice. If they’re lucky, a dead whale carcass might float ashore and provide them with the fat and protein they need. If luck isn’t in their favor, they will fast on land as they wait for the ice to reform so they can hunt again. These changes were easier for the bears to adapt to during interglacial periods with low ice due to naturally occurring climate cycles, but in a world that is warming at twice the rate of the rest of the planet, the Arctic could be ice-free by 2040, making today’s bears particularly vulnerable to rapid sea-ice decline.
"I think this is likely one of the most probable explanations for how polar bears made it through previous warm interglacial periods," said co-author Ian Stirling in a statement. "But when we look at the situation now, ecologically, with respect to food sources, it's a very different picture. The potential of whale carcasses to bail bears out may still be important in a few areas but, quite simply, their overall availability is going to be substantially less than before humans invaded the Arctic."
Bears are opportunistic carnivores. Since the 1970s, observations of polar bears feeding on large bowhead and gray whale carcasses have been in the dozens. In 2017, more than 180 bears were seen scavenging on a dead bowhead.
New research published in Frontiers in Ecology and the Environment compiled years of field observations to find out just how important whale carcasses have been in helping bears survive when the Arctic hasn't had ice. They found that during ice-free summers, 1,000 polar bears need to eat around eight whales to survive. Meanwhile in springtime, when the bears require more food, the same number would need to eat 20 whales.
Long-term data collected in Russia’s Chukchi Sea suggests that enough whales die and float ashore each year to potentially meet this need, but this demand isn’t necessarily met in the other 18 subpopulations across the Arctic. On top of the unequal distribution of fatty whale carcasses, whale populations are getting smaller in general due to hunting, shipping, and industrial activities, which have also adversely affected bear populations.
The authors note that specific outcomes are difficult to predict, “largely because most projections are based on ecological data and population trends under present circumstances.”