Polar Bear Model Unveils Decline Drivers

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.