In a world where climate change is rapidly changing natural habitats, polar bears have become a symbol of the fight for survival. Summers are getting longer, temperatures on the planet are rising and there is less and less ice. As the Arctic's frozen blocks disappear, bears will spend more time on land, where it is difficult to find food, as their traditional hunting strategies that exploit sea ice are less effective. A study published today in Nature Communications presents a portrait of the challenges the 25,000 polar bears remaining on the planet face in avoiding starvation.
In late spring and early summer, polar bears use the sea ice as a hunting platform. They feed primarily on seals, which give birth and wean their young in the same season. As the ice melts, the Arctic giants will need to slow down as much as possible to conserve energy. A team of scientists led by wildlife biologist Anthony Pagano of the United States Geological Survey observed polar bears up close for three weeks over the summer. To accomplish this, they attached collars equipped with cameras and GPS to twenty bears in western Hudson Bay, Manitoba, Canada. In this area, the population has declined by about 30% since 1987 and the ice-free period has increased by three weeks since 1979, meaning bears have stayed on land for about 130 days in the last decade.
After analyzing the images, they found that the bears tried various tactics to feed themselves and maintain their energy reserves, including eating and resting at similar levels to grizzly bears when they enter hibernation. 70% of those active ate terrestrial foods such as berries, herbs and bird carcasses. Three bears dared to swim long distances to find food in the water, and although two of them found remains of belugas and seals, they were unable to eat while swimming or bring them to land.
Regardless of their diet or rest strategy, there was a consistent decline in body mass and 19 of the 20 participants lost an average of 1 kilogram per day. “A further increase in the length of summer on land will be associated with an increased risk of hunger,” says Pagano. “Neither strategy will allow polar bears to exist on land beyond a certain period of time. Even the bears that were foraging lost weight just as quickly as those that were lying down,” adds Charles Robbins, director of the Bear Center at Washington State University and co-author of the study.
It has been speculated that polar bears may adapt to ice-free seasons by searching for terrestrial food, but the study shows that it is not that easy to find and that it does not provide them with the nutrients or energy they need, so they end up suffering from famine. David Nogués Bravo, a macroecologist at the Center for Macroecology, Evolution and Climate at the University of Copenhagen, who did not take part in the study, explains that the lack of food “reduces the probability of maintaining generational alternation,” adding that “terrestrial foods Given “they had some energy advantage, but ultimately had to expend more energy to access those resources.”
A polar bear in western Hudson Bay.David McGeachy
The ice that covers the Arctic reaches its minimum in September each year and each year is smaller than the previous one. According to NASA data based on several of its satellites, the extent of the polar ice cap has declined by 12.6% each decade since 1980. A 2023 study, supported by observations from NASA and ESA satellites, predicts that between 2030 and 2050 the ice-free September will come. And if greenhouse gas emissions are not reduced, the Arctic region will be without ice for up to half a year by the year 2100.
The study's findings have important implications for polar bear conservation, but also invite us to examine the impact it has on the Arctic's marine and terrestrial ecosystems. According to Nogués, the presence of these animals, which hunt longer in land areas, has indirect effects on other species such as birds. “Bears are capable of eating dozens of eggs in a short period of time, reducing the likelihood of these bird populations surviving,” he adds.
The fact that polar bears are in serious danger of extinction is nothing new. What is surprising is the massive decline in young animals and the starvation of adults due to melting sea ice. “I have seen polar bears in the Arctic and it becomes grim to follow the path of this species to its probable functional extinction in the not too distant future,” says Nogués, who emphasizes that it is important to take into account “the biodiversity crisis and Climate crises are not two different crises, but two sides of the same coin.”
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