A groundbreaking study from the University of Toronto Scarborough has revealed a direct link between shrinking sea ice caused by climate change and the alarming population decline of polar bears in Western Hudson Bay. The research provides a clear mechanism explaining how diminishing hunting opportunities on sea ice are leading to energy deficits, reduced reproduction, and ultimately, the collapse of this iconic Arctic species.
A Shrinking Habitat, A Shrinking Population
Polar bears rely on sea ice as a platform for hunting seals, their primary food source. However, as global temperatures rise, Arctic sea ice is melting at an unprecedented rate. In Western Hudson Bay—one of the most extensively studied polar bear populations—this loss of habitat has resulted in a nearly 50% population decline over the past four decades (1979–2021).
The study, published in the journal Science, attributes this decline to the bears’ inability to meet their energy needs due to shorter hunting seasons. “A loss of sea ice means bears spend less time hunting seals and more time fasting on land,” explains Louise Archer, a postdoctoral researcher at U of T Scarborough and lead author of the study. “This negatively affects the bears’ energy balance, leading to reduced reproduction, cub survival, and ultimately population decline.”
A Bio-Energetic Model Tracks Energy Deficits
To better understand the relationship between sea ice loss and polar bear survival, the researchers developed a “bio-energetic model” that tracks the energy dynamics of individual polar bears throughout their lifespans—from cubs to adults. This innovative approach compares modeled outcomes with four decades of real-world monitoring data, revealing a near-perfect match.
“What’s unique about our model is that it doesn’t just show a correlation between sea ice loss and population decline—it explains how these changes occur,” says Péter Molnár, an associate professor in the Department of Biological Sciences at U of T Scarborough and co-author of the study. “When we run the numbers, we get a one-to-one match to what we’re seeing in real life.”
The model shows that as polar bears spend more time fasting on land, they burn through their fat reserves faster, leaving insufficient energy for growth, reproduction, and cub rearing. Over time, this leads to smaller body sizes, fewer cubs, and lower survival rates.
Cubs Face the Brunt of the Crisis
Young polar bears are particularly vulnerable to the impacts of climate change. Monitoring data reveals that adult female body mass has dropped by 39 kg (86 lbs), while one-year-old cubs have lost 26 kg (47 lbs) over 37 years. Mothers struggling with nutritional stress produce less milk, jeopardizing cub survival during critical fasting periods. Additionally, litter sizes have decreased by 11%, and mothers are keeping their cubs longer because they aren’t strong enough to live independently.
“The survival of cubs directly impacts the survival of the population,” Archer emphasizes. “If cubs don’t survive, the population can’t sustain itself.”
A Warning for Polar Bears Worldwide
Western Hudson Bay is considered a bellwether for polar bear populations globally. As one of the southernmost groups of polar bears, this population has been closely monitored for decades, providing a wealth of data to inform predictions about other regions. With the Arctic warming at a rate four times faster than the global average, researchers warn that similar declines are likely occurring—or will soon occur—in other polar bear populations.
“This is not just a problem for Western Hudson Bay; it’s a glimpse into the future for polar bears everywhere,” Molnár says. “Based on projected sea ice loss trajectories, this model essentially describes their fate if we don’t take action.”
Broader Implications for Conservation and Climate Action
The study underscores the urgent need for global efforts to address greenhouse gas emissions. While protecting polar bears requires safeguarding their Arctic habitats, the root cause of their plight lies in human-induced greenhouse gas emissions driving Arctic warming. Without significant reductions in carbon emissions, the long-term survival of polar bears—and countless other species—is at risk.
“This research provides a powerful tool for understanding the impacts of climate change on wildlife,” Archer notes. “But it also serves as a call to action. We have the knowledge to predict these outcomes, but now we need the will to prevent them.”
A Wake-Up Call for Humanity
The plight of polar bears in Western Hudson Bay is more than an environmental tragedy—it’s a stark reminder of the interconnectedness of ecosystems and the cascading effects of climate change. As apex predators, polar bears play a crucial role in maintaining the balance of the Arctic ecosystem. Their decline signals broader ecological disruptions that could have far-reaching consequences.
By shedding light on the mechanisms driving polar bear population declines, this study reinforces the importance of addressing carbon emissions head-on. For polar bears—and for humanity—the time to act is now.
Source: Materials provided by University of Toronto. Original written by Don Campbell. Note: Content may be edited for style and length.
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