Were you to poll some polar bears, the apex predators of the Arctic, they likely wouldn’t be very happy about the retreating ice brought on by climate change. Earlier, some scientists had speculated that the bears would adapt, transitioning from diets rich in fatty marine life to terrestrial foods like eggs, small mammals and vegetation. Now, new research by the U.S. Geological Survey finds that not only will they likely not thrive in such conditions, but their survival may be at stake.
“Although some polar bears may eat terrestrial foods, there is no evidence the behavior is widespread,” said Dr. Karyn Rode, lead author of the study and scientist with the USGS. “In the regions where terrestrial feeding by polar bears has been documented, polar bear body condition and survival rates have declined.”
To glimpse into the future of land-based polar bears, one need only to look to the current grizzly bears inhabiting the arctic region. Besides occurring in low densities, Arctic grizzly bears are among the smallest grizzly bears on Earth – and that’s with the way things are now, with polar bears spending most of their time on the ice sheets. Should they continue to retreat, polar bears may find themselves competing with grizzlies for already-limited terrestrial food sources.
“The smaller size and low population density of grizzly bears in the Arctic provides a clear indication of the nutritional limitations of onshore habitats for supporting large bodied polar bears in meaningful numbers,” said Rode. “Grizzly bears and polar bears are likely to increasingly interact and potentially compete for terrestrial resources.”
Scientists have drawn attention to instances of polar bears eating bird eggs, which would seemingly be a positive sign of adaptation. Bird eggs, after all, are calorically dense and plentiful. However, in Rode’s work, she and her colleagues observed few instances of such behavior – less than 30 bears have been observed consuming bird eggs from any one population, which typically range from 900 to 2000 individual birds.
It seems odd to find such a large predator (polar bears are among the largest bears on Earth) in such a cold, desolate environment. Polar bears owe their survival to their marine life diet, which is incredibly rich in fat – polar bears are thought to get more of their calories from fat than any other predatory species on Earth. By contrast, most prey species in the Arctic are lean, high-protein meals, which provide an inefficient source of the calories and nutrients polar bears need to maintain body mass and function. Though polar bears have been observed munching on vegetation, their bodies aren’t designed to digest it in the quantities needed to have a meaningful caloric impact.
“The reports of terrestrial feeding by polar bears provide important insights into the ecology of bears on land,” said Rode. “In this paper, we tried to put those observations into a broader context. Focused research will help us determine whether terrestrial foods could contribute to polar bear nutrition despite the physiological and nutritional limitations and the low availability of most terrestrial food resources. However, the evidence thus far suggests that increased consumption of terrestrial foods by polar bears is unlikely to offset declines in body condition and survival resulting from sea ice loss.”
How the bears will ultimately adapt is still a mystery. It’s possible, should the sea ice continue to retreat, that more available land will bring with it more available prey species, perhaps offsetting some of the losses from a marine life diet.