U of T Scarborough researchers have established a direct connection between the declining population of polar bears in Western Hudson Bay and the loss of sea ice attributable to climate change.
U of T Scarborough researchers have established a direct connection between the declining population of polar bears in Western Hudson Bay and the loss of sea ice attributable to climate change.
The team created a model demonstrating that the decrease in the polar bear population is linked to insufficient energy intake, primarily due to shorter hunting seasons arising from the reduction of sea ice.
“With less sea ice, bears have fewer opportunities to hunt seals and end up fasting longer on land,” explains Louise Archer, a postdoctoral researcher at U of T Scarborough and the study’s lead author.
“This situation adversely affects their energy levels, resulting in lower reproduction rates, decreased survival of cubs, and leading to a decline in the overall population.”
The bio-energetic model designed by the researchers tracks the energy intake of polar bears from hunting seals compared to the energy they require for growth and reproduction. What sets this model apart is its consideration of the entire life cycle of individual polar bears—from cubs to adults—while utilizing data collected over four decades from the Western Hudson Bay polar bear population, spanning 1979 to 2021.
Over these years, the polar bear population in this area has decreased by almost 50%. Monitoring indicates that the average size of the bears has shrunk as well, with adult females losing 39 kg (86 lbs) and one-year-old cubs dropping by 26 kg (47 lbs) over a 37-year span.
The researchers’ model closely aligns with the monitoring data, offering an accurate evaluation of the current status and the future of the polar bear population if the loss of sea ice and energy deficiencies persist.
“Our model goes beyond merely showing a correlation between diminishing sea ice and population decrease,” states Péter Molnár, an associate professor in the Department of Biological Sciences at U of T Scarborough and a co-author of the study.
“It outlines the processes at play—less ice leads to reduced feeding opportunities and overall lower energy availability. Our calculations almost perfectly reflect the real-life trends we observe.”
Polar bear mothers and their cubs are especially at risk
The research team, which includes co-authors from Environment and Climate Change Canada, highlighted that cubs are particularly vulnerable to these climate-related issues.
Archer notes that shorter hunting seasons lead to decreased milk production among mothers, putting cub survival at risk. If cubs do not gain sufficient weight, their chances of survival diminish during their first fasting phase.
Additionally, mothers are giving birth to fewer cubs. The data reveals that the size of cub litters has decreased by 11% in comparison to figures from nearly 40 years ago, with mothers now keeping their cubs for extended periods because the cubs aren’t robust enough to fend for themselves.
“The survival of cubs is crucial for the population’s survival,” explains Archer, whose research is supported by a Mitacs Elevate postdoctoral fellowship and the non-profit organization Polar Bears International.
Wider implications for the model
Western Hudson Bay has long been viewed as an indicator for polar bear populations worldwide. As the Arctic experiences warming at a rate four times that of the global average, the researchers caution that other polar bear populations may face similar declines.
“This population is one of the southernmost for polar bears, and extensive monitoring has provided high-quality data,” says Molnár, an authority on the impacts of global warming on large mammals.
“It is reasonable to anticipate that what is occurring with polar bears in this area will also be seen in other regions, based on the anticipated losses of sea ice. This model effectively describes their potential future.”
The study has been published in the journal Science and has received financial backing from the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.
