A research group has just created a model that quantifies the link between sea ice and polar bears, providing an accurate assessment of what will happen to polar bear populations if sea ice continues to decline.
A group of researchers from the University of Toronto Scarborough have established a direct link between the reduction in sea ice caused by global warming and the decline in polar bear populations.
While this may seem like nothing new, the model developed by the scientists represents a real breakthrough, as it now makes it possible to quantify the link between sea ice and polar bears.
Researchers have developed a bioenergetic model that follows polar bears throughout their lives. Their results have been published in Science on January 30. The model takes into account energy input (calorie absorption) and energy output linked to hunting, reproduction, growth and movement. Essential data. “The amount of energy ingested versus the amount of energy used determines how fat an individual is, which in turn decides whether a bear can successfully reproduce, rear offspring, and even survive”, explains Dr. Louise Archer, a postdoctoral researcher at the University of Toronto Scarborough and lead author of the study, in a press release issued by Polar Bears International, which helped fund Archer’s research.
The model developed by Archer and colleagues follows the life cycle of an individual, from cub to adult bear. This comprehensive model was compared with monitoring data from the Western Hudson Bay polar bear population between 1979 and 2021.
And the results are clear: the Western Hudson Bay polar bear population has declined by almost 50% between 1979 and 2021. And it’s not just the population that has shrunk. The animals, too, have seen their mass melt away. Female polar bears lost 39 kilos over the same period, and one-year-old cubs 26 kilos.
“Our model goes one step further than saying there’s a correlation between declining sea ice and population decline”, notes Péter Molnár, Associate Professor in the Department of Biological Sciences at the University of Toronto at Scarborough and co-author of the study, in a press release issued by the university on January 31. “It provides a mechanism that shows what happens when there is less ice, less feeding time and less energy overall […] When we run the numbers, we get a near one-to-one match to what we’re seeing in real life.”
And as the model is based on energy intake and expenditure, it could be adapted to analyze other species.
Ice, seal and bear
Polar bears are extremely well adapted to the extreme environment of the Arctic. They are also highly specialized predators, whose diet is largely dependent on the seals they hunt on the sea ice. In the absence of sea ice, which is becoming increasingly scarce as a result of global warming, polar bears return to land, where they are forced to find whatever sustenance they can.
The problem is that the food they find is not enough to meet the polar bears’ high energy requirements. As a result, the bears find themselves fasting. If the sea ice forms quickly, the polar bears can return to their hunting grounds and make up the deficit caused by fasting. But if the sea ice forms late, the bears spend more time on land and their fasting is prolonged, resulting in shorter hunting periods.
This has an impact not only on the adult bears, but also on the cubs, who receive less milk and less calorie-rich milk from mothers who are already struggling to cover their own energy needs. As a result, the cubs’ survival is jeopardized, as they are unable to put on enough weight to cope with their first period of fasting.
One of the consequences is that mothers keep their offspring with them for longer, as they are not strong enough to survive on their own. This offspring also tends to diminish, as females produce fewer and fewer cubs. Researchers found that litter size had fallen by 11% in forty years. A direct threat to the health of a population, and even to its survival.
A model for all bears
For the researchers, the conclusions they have drawn from their model are not just confined to western Hudson Bay, but can be applied to all polar bears in the Arctic. “This is one of the southernmost populations of polar bears and it’s been monitored for a long time, so we have very good data to work with,” says Molnár. “There’s every reason to believe what is happening to polar bears in this region will also happen to polar bears in other regions, based on projected sea ice loss trajectories. This model basically describes their future.”
A bleak future for a species whose survival depends on an environment that is melting faster and faster.
Link to the study: Louise C. Archer et al, Energetic constraints drive the decline of a sentinel polar bear population. Science387, 516-521(2025). DOI:10.1126/science.adp3752
Mirjana Binggeli, Polar Journal AG
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