The polar bear, Ursus maritimus, is the largest terrestrial carnivore, yet it is classified as a marine mammal due to its fundamental dependence on the ocean and the sea ice that covers it. These animals inhabit the circumpolar Arctic region. Today, the global polar bear population is highly vulnerable, a status driven almost entirely by the rapid changes occurring in their frozen habitat.
The Critical Role of Sea Ice in Survival
Sea ice represents a dynamic, functional habitat that sustains the polar bear’s entire life cycle. The species relies on this frozen surface as a traveling corridor, enabling them to move vast distances to find mates and access feeding areas. The ice is also where they construct maternity dens, providing a sheltered location for the birth and nursing of cubs before emerging in the spring. The most fundamental function of the sea ice is its role as the primary hunting ground for the bears’ high-fat prey, seals. Polar bears employ a still-hunting technique, waiting patiently near seal breathing holes or stalking seals resting on the surface.
Primary Threat: Habitat Fragmentation and Loss
The primary mechanism by which polar bears are becoming endangered is the destruction and instability of their sea ice habitat caused by rising global temperatures. Climate change is causing the sea ice to break up earlier in the spring and freeze up later in the autumn, drastically shortening the time bears have to hunt and build up fat reserves. In some regions, the ice-free period has lengthened by up to 41 days per decade, forcing bears to spend longer periods fasting on land.
This loss of stable, continuous ice results in habitat fragmentation, where large expanses of ice are broken into smaller, isolated floes. Polar bears are compelled to undertake longer, more energetically costly swims in open water to reach remaining ice or land. These extended swims are particularly dangerous for young cubs, who may not have the stamina to survive the journey, further contributing to population decline.
Biological Impacts on Health and Reproduction
The direct consequence of a shorter hunting season and longer fasting period is severe nutritional stress, which manifests in reduced body mass and overall fitness. Polar bears accumulate the majority of their annual energy reserves during the spring and early summer when seal pups are readily available. A single successful hunt can provide weeks of sustenance.
Females are particularly susceptible to this decline in body condition, as they require substantial fat reserves to sustain pregnancy and lactation. Studies have shown that extended fasting on land is linked to mothers producing milk with significantly lower energy content or stopping lactation altogether. This poor maternal condition directly impacts reproductive success, leading to smaller average litter sizes and lower survival rates for cubs-of-the-year. In the Western Hudson Bay subpopulation, a decline in cub survival and reduced body size in adults has been directly linked to the earlier breakup of sea ice.
Secondary Stressors and Current Conservation Status
While the loss of sea ice is the overwhelming threat, polar bears also face secondary stressors that compound their vulnerability. As bears are forced onto land for longer periods, they increasingly venture into human settlements, leading to heightened human-bear conflict and subsequent management removals. This increased interaction also puts them at risk from industrial development, such as oil and gas exploration, which can lead to habitat disturbance and oil spills.
Another concern is the bioaccumulation of environmental pollutants, such as PCBs and heavy metals, which are transported to the Arctic and enter the marine food web. Since polar bears are apex predators, these toxins concentrate in their fat reserves, potentially compromising their immune and reproductive systems. The global population of the species is currently listed as “Vulnerable” on the IUCN Red List based on the projection that the population will decline by more than 30% over the next three generations due to sea ice loss. The United States also lists the polar bear as “Threatened” under the Endangered Species Act, recognizing the severe peril posed by the ongoing climate-driven changes to their Arctic home.