Polar bear energetic model reveals drivers of polar bear population decline
Summary author: Walter Beckwith
Polar bears in Western Hudson Bay have seen their population nearly halved over the last several decades, largely due to dwindling sea ice and limited hunting opportunities, according to the findings of a novel bioenergetic model using data spanning more than 40 years. The findings reveal the relationship between bears’ individual energy needs and environmental limitations in driving population trends, highlighting energy as the central limiting factor behind the decline of a key Arctic apex predator. The Arctic is warming faster than any other region on Earth, leading to significant sea ice loss, ecosystem transformations, and heightened threats to ice-dependent species like polar bears (Ursus maritimus). These animals rely on sea ice to hunt seals, their primary food source, but as ice melts during warmer months, they are forced onto land or into less productive waters, relying on stored energy reserves due to the lack of adequate food sources. Food deprivation caused by changes in seasonal sea ice has been linked to declines in polar bear populations. However, conservation efforts are limited by a lack of data for most polar bear subpopulations and a framework to understand how sea ice loss affects the animals throughout their lives. To investigate the relationship between declining sea ice and polar bear populations, Louise Archer and colleagues compiled population monitoring and capture data collected from polar bears in Western Hudson Bay, Canada, over the last 42 years and developed an individual-based bioenergetic model. The model, grounded in physiological principles, integrates energy acquisition and expenditure – such as feeding, body maintenance, movement, growth, and reproduction – into a unified energy budget spanning an individual bear’s life cycle. The findings show that sea ice loss and resultant feeding limitations were the primary drivers of a ~50% population decline since the mid-1990s, demonstrating how individual energetic constraints shape population-level outcomes. What’s more, Archer et al. note that this framework, although developed for polar bears, is adaptable to other species facing constraints on foraging or energy use due to environmental or human-driven changes, offering broad utility in addressing global change impacts and informing conservation and policy decisions.
Journal
Science
Article Title
Energetic constraints drive the decline of a sentinel polar bear population
Article Publication Date
31-Jan-2025
Polar bear population decline the direct result of extended ‘energy deficit’ due to lack of food
Study finds polar bears are struggling to get enough to eat in face of dwindling sea ice due to climate change
University of Toronto
image:
Researcher Louise Archer.
view moreCredit: Handcraft Creative
U of T Scarborough researchers have directly linked population decline in polar bears living in Western Hudson Bay to shrinking sea ice caused by climate change.
The researchers developed a model that finds population decline is the result of the bears not getting enough energy, and that’s 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 U of T Scarborough postdoc and 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 currently 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 polar bear population in this region has declined by nearly 50 per cent. The monitoring data shows the average size of polar bears is also in decline. The body mass of adult females has dropped by 39kg (86lbs) and one-year-old cubs by 26kg (47lbs) over a 37-year period.
The researchers’ model provides a close match to the monitoring data, meaning it provides an accurate assessment of what is happening and will continue to happen to the polar bear population if it keeps experiencing sea ice loss and a greater amount of time in energy deficit.
“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 the Department of Biological Sciences at U of T Scarborough and co-author of the study.
“It provides a mechanism that shows what happens when there is less ice, less feeding time and less energy overall. When we run the numbers, we get a near one-to-one match to what we’re seeing in real life.”
Polar bear mom and cubs particularly vulnerable
The researchers, which include co-authors from Environment and Climate Change Canada, noted that cubs face the brunt of these climate-induced challenges.
Archer says that shorter hunting periods result in mothers producing less milk, which jeopardizes cub survival. The 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 almost 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.
Broader applications for the model
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 study, which is published in the journal Science, received funding from the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.
Journal
Science
Article Title
Energetic constraints drive the decline of a sentinel polar bear population
Article Publication Date
30-Jan-2025
