In a single year, one polar bear can leave roughly 300 kilograms of prey for other animals to dine on. Altogether, the carnivores provide 7.6 million kilograms of carrion for scavengers throughout the Arctic, researchers estimate.
The findings, reported October 28 in Oikos, highlight the crucial role these apex predators play in feeding a vast array of species and hints at the way that food web might be shaken as climate change warms the Arctic, endangering polar bear populations.
Scientists have long known that polar bears typically eat the bladder of their prey — usually seals — then leave the rest. However, the biomass of those leftovers and their importance has been long overlooked, says Nicholas Pilfold, a scientist at the San Diego Zoo Wildlife Alliance who worked for 15 years in the Arctic.
Pilfold and colleagues dove into studies, observations and anecdotes of scavenging activity around carcasses dating as far back as the 1930s. The team also analyzed studies on how many calories seals can provide and how many seals polar bears consume each year.
During its yearly hunting peak, a polar bear kills roughly one seal every three to five days — usually ringed seals — which equates to about 1,000 kilograms of food every year. A bear eats a majority of that mass, leaving some 30 percent up for grabs. Considering there are an estimated 26,000 polar bears in the Arctic, all those leftovers add up to millions of kilograms of food for scavengers, including arctic foxes, gulls, ravens and other polar bears. Occasionally even snowy owls, wolves and grizzly bears will feast on the remains.
These species wouldn’t be able to access this kind of food if the polar bears didn’t leave it behind, says study coauthor Holly Gamblin, a wildlife biologist at the University of Manitoba in Winnipeg, Canada.
Pilfold has seen some of these interactions firsthand. “The foxes will follow in the tracks of where the polar bears have gone,” waiting for the bears to abandon the carrion. Birds would also fly overhead and wait their turn, he says. “There’s just this cacophony of sound of gulls that are all swirling around, and they’re all trying to get some of that seal.”
Jon Aars, a polar bear expert at the Norwegian Polar Institute in Tromsø, says the findings come as no surprise. The leftovers are probably a “rather important,” food source for other species, “particularly at a time of the year when alternative food is not so easy to get,” he says.
But the Arctic is warming, affecting polar bear populations and, in turn, their leftovers. “If we’re going to start to see declines in polar bears, we’re likely going to see declines in that carrion biomass,” Pilfold says.
For example, an estimated 323,000 kilograms of carrion per year has been lost in two regions where polar bear sub-populations have been declining, the team calculates. Melting ice might also make it harder for some scavengers to reach the bears’ leftovers.
It’s difficult to predict the effect that the decrease of polar bears’ carrion might have on other species, Aars says. But “it will have [an] impact one way or another,” depending on which species and which “part of the Arctic we look at.”