These polar bears are getting fatter as sea ice melts. What's going on?
The finding offers a small window of hope for a polar bear population vulnerable to the effects of climate change.
Polar bears are often the poster children for the impacts of climate change because of how much they rely on the ice for survival. Less ice forces these powerful predators to swim further to find food or spend more time on land living off fat reserves. Many polar bear populations are at risk of starvation.
The effects ripple throughout their ecosystem. Without sea ice, algae can’t grow, zooplankton can’t nibble on algae and fish are left without enough plankton to eat. This impact goes all the way up the food chain to the ringed seals that polar bears love to hunt.
But researchers in Svalbard, Norway, found something unexpected about their polar bear population. When sea ice levels decreased around the archipelago, the bears got fatter, they report in a new study in the journal Scientific Reports.
Less ice, fatter bears
Bears in the Barents Sea around Svalbard have been studied through annual monitoring each spring since 1987. For scientists, having long-term data like this, with regular measurements, is a gold, mine because it allows them to spot patterns and trends. In this case, they noticed that the population seemed stable, despite this region losing ice twice as quickly as other polar bear habitats.
To find out what might be going on, researchers looked at the body size and chest circumference of 770 adult polar bears captured during this monitoring between 1992 and 2019. Leaner polar bears, with less fat stores to see them through hard times, can be an early warning sign of a struggling population, so body condition can indicate how a population is faring.
Based on other populations, the scientists expected to see the bears get skinnier as sea ice decreased. “I've seen myself—being in Svalbard—that the sea ice has disappeared very, very fast,” says the paper’s lead author Jon Aars, senior scientist at the Norwegian Polar Institute. “It would be natural to expect that [the sea ice loss] would have a negative effect on the bears, including on their body condition.”
Their new findings revealed the opposite. The bears’ body condition decreased between 1995 and 2000 before improving again, even though the region was rapidly losing sea ice after 2000. “I was a bit surprised when we found that it had actually increased instead of decreasing,” says Aars. “It's good news that they have coped so well, despite nowhere else in the Arctic having sea ice disappear at this rate.”
This doesn’t mean the bears aren’t affected by climate change. They have been forced to spend more time on land hunting less energy-rich foods, such as seabird eggs, and swim further between hunting and mating grounds. They have also lost important denning areas. “The good news is that they're still in good health,” Aars says.
Why Svalbard’s bears are thriving
One explanation for their success could be that while bears in many other populations don’t hunt on land—the energy expenditure isn’t worth the calories they gain—Svalbard’s bears are eating more land animals than they used to in the past.
These polar bears “have alternatives that they do not always have in other areas,” says Aars. For example, they catch reindeer, which have boomed after recovering from excessive hunting by humans. Reindeers provide a food supply during summer when polar bears are typically fasting.
Another possibility could be that the reduced sea ice could be forcing ringed seals into denser groups around the remaining patches of ice, making them easier to hunt.
The future for Svalbard’s bears
Whatever the reason, this news unfortunately isn’t as positive as it sounds. “I think it's a small window of hope,” says Alice Godden, senior research associate at the University of East Anglia in England who wasn’t involved in the study. She believes that the long-term outlook for the bears likely isn’t good. “Food availability is going to really be the driver whether they survive or not,” she says, adding that how much food they have is determined by how quickly temperatures rise as a result of carbon emissions.
The study’s authors warn that, at a certain point, the ecosystem will pass a tipping point and might experience severe, irreversible changes. “It will be harder to be a polar bear in Svalbard in the future,” says Aars.
The results were particularly unexpected because research has shown that the survival of other bears, particularly in the Western Hudson Bay and southern Beaufort Sea in the Canadian Arctic, declines when there is less sea ice. But comparing struggling bears to thriving ones isn’t helpful, experts say. “Each subpopulation of bears is going to be quite different,” says Godden. “You need to contextualize everything within their local habitat.”
She recently discovered that genetic changes might be helping southern Greenland’s bears adapt to a rapidly warming climate. “We saw changes in gene expression linked to fat metabolism, thermal tolerance and some links to ageing,” she says. She wonders whether DNA changes in the Svalbard bears could explain why they are also flourishing.
Although these polar bears are currently doing well, that won’t continue if the ice disappears completely, says Aars: “You don't have polar bears anywhere where you don't have sea ice for part of the year.”
