Researchers from the University of Toronto have established a direct link between the population decline in polar bears living in Western Hudson Bay and shrinking sea ice caused by climate change.
The researchers developed a model, published in the journal Science , that finds population decline is the result of the bears not getting enough energy due to a lack of food caused by shorter hunting seasons on dwindling sea ice.
"A loss of sea ice means bears spend less time hunting seals and more time fasting on land," says Louise Archer, a postdoctoral researcher at U of T Scarborough who is the lead author of the study.
"This negatively affects the bears' energy balance, leading to reduced reproduction, cub survival and, ultimately, population decline."
The "bio-energetic" model developed by the researchers tracks the amount of energy the bears are getting from hunting seals and the amount of energy they need in order to grow and reproduce. What's unique about the model is that it follows the full lifecycle of individual polar bears - from cub to adulthood - and compares it to four decades of monitoring data from the Western Hudson Bay polar bear population between 1979 and 2021.
During this period, the region's polar bear population has declined by nearly 50 per cent. The monitoring data shows the average size of polar bears is also in decline. For example, the body mass of adult females has dropped by 39 kilograms and one-year-old cubs by 26 kilograms over a 37-year period.
The researchers' model provides a close match to the monitoring data, providing an accurate assessment of what will continue to happen to the polar bear population amid ongoing sea ice loss.
"Our model goes one step further than saying there's a correlation between declining sea ice and population decline," says Péter Molnár, an associate professor in U of T Scarborough's department of biological sciences who is a study co-author.
"It provides a mechanism that shows what happens when there is less ice, less feeding time and less energy overall," he says. "When we run the numbers, we get a near one-to-one match to what we're seeing in real life."
The researchers, who include co-authors from Environment and Climate Change Canada, noted that cubs face the brunt of climate-induced challenges.
Archer says shorter hunting periods result in mothers producing less milk. That, in turn, jeopardizes cub survival since cubs face reduced survival rates during their first fasting period if they fail to gain enough weight.
Mothers are also having fewer cubs. Monitoring data shows cub litter sizes have dropped 11 per cent compared to nearly 40 years ago and mothers are keeping their cubs longer because they aren't strong enough to live on their own.
"It's pretty simple - the survival of cubs directly impacts the survival of the population," says Archer, whose research is funded through a Mitacs Elevate postdoctoral fellowship and the non-profit organization Polar Bears International .
Western Hudson Bay has long been considered a bellwether for polar bear populations globally and, as the Arctic warms at a rate four times faster than the global average, the researchers warn of similar declines in other polar bear populations.
"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, who is an expert on how global warming impacts large mammals.
"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."
The researchers received support from the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.
