Polar bears play an important role in the Arctic ecosystem: the food they leave behind feeds many other animals that enjoy the leftovers. According to a current estimate, Arctic polar bears leave behind around 7.6 million kilograms of their prey every year – mainly seals that they have transported from the sea to the ice surface. The polar bears therefore represent a crucial link between the material cycles in the sea and on land and provide the basis of life for numerous scavengers. However, this also means that if the polar bears dwindle, it will endanger many other Arctic species. From the researchers’ perspective, this underlines the urgency of protective measures.
Polar bears’ favorite food is seals – especially their nutritious blubber. The classic hunting strategy of the world’s largest land predator is to lie in wait on the Arctic sea ice at the seals’ ice-free breathing holes. Sooner or later a victim appears, is dragged onto the ice by the polar bear, killed and eaten. But when the polar bear has finished its meal, there is usually still plenty of meat left over for other animals. For example, arctic foxes are known to follow polar bears to feast on the remains of their prey.
No food waste
A team led by Holly Gamblin from the University of Manitoba in Canada has now investigated to what extent polar bears contribute to the diet of other Arctic species in this way. “Our results quantify for the first time the extent to which polar bears act as food providers for other species and highlight the interconnectedness of their ecosystem,” says Gamblin. According to the researchers’ estimates, each adult polar bear kills on average more than a ton of marine mammals per year and leaves around 30 percent of them behind as usable carrion. For all polar bears combined, these leftover food amounts to around 7.6 million kilograms per year – and are a vital food source for other species.
Animals that benefit from the incompletely consumed carcasses include at least eleven species of vertebrates, including arctic foxes, wolves, ravens, seagulls and snowy owls. Without the polar bears, they would have little chance of accessing the nutritious prey from the sea. “It is clear from our research that there is no other species that can adequately replace the polar bear’s hunt, in which it drags its prey from the water onto the sea ice, leaving behind extensive remains that other species can use,” says Gamblin. The polar bears and the seals they kill represent a unique link between the marine and terrestrial ecosystems of the Arctic.
Melting sea ice poses a danger
But climate change is making it increasingly difficult for polar bears to hunt seals. The sea ice continues to recede in summer and only slowly and incompletely regenerates in winter. As a result, the polar bear hunting season becomes shorter every year and at the same time often less successful. This is a problem for other species too: “Sea ice serves as a platform for many species to access carrion resources provided by polar bears, and ultimately the decline of sea ice will reduce access to this energy source,” says co-author Nicholas Pilfold of the San Diego Zoo Wildlife Alliance.
As polar bear populations shrink along with ice, the scavengers that depend on them will also find less food. “Our results suggest that the documented decline in polar bear populations in two subpopulations has already resulted in a loss of more than 300 tons of food resources to scavengers per year,” reports Pilfold. From the researchers’ perspective, the study underlines how important polar bears are for the entire Arctic ecosystem. Not only the polar bears themselves benefit from the protection of iconic predators, but also numerous other species that depend on them.
Source: Holly Gamblin (University of Manitoba, Canada) et al., Oikos, doi: 10.1002/oik.11628