The polar bear, Ursus maritimus, is a large marine mammal and apex predator found across the circumpolar Arctic region. This keystone species relies almost entirely on the ocean environment, specifically the annual sea ice, for its survival. The International Union for Conservation of Nature currently lists the polar bear as Vulnerable. This status reflects a suspected future population reduction due to threats across its range.
The Primary Threat: Loss of Sea Ice
The Arctic sea ice acts as the polar bear’s primary habitat, serving as a platform for hunting, mating, and travel. Polar bears are uniquely adapted to catch their main prey, ringed and bearded seals, by waiting near breathing holes or ice edges. The seasonal loss of this ice significantly shortens the period when bears can effectively hunt, forcing them into extended fasting periods on land.
This reduction in hunting time leads directly to a decline in the bears’ body condition and overall health. Loss of access to sea ice means they must subsist longer on stored fat reserves. Polar bears cannot enter a state of “walking hibernation” to conserve energy during these ice-free months, meaning their metabolic rate remains high.
Increased open water forces bears to swim much longer distances to find stable ice or reach land. The energetic cost of swimming is approximately three to four times greater than walking, rapidly depleting their limited fat stores. This higher energy output combined with reduced food intake creates a severe energy deficit.
The prolonged fasting periods have a pronounced effect on reproduction and cub survival. Female bears require sufficient fat reserves to sustain themselves and produce milk during the denning period. Insufficient stored energy leads to lower milk production, resulting in reduced cub body mass and lower survival rates for young bears. Mothers are having fewer cubs over their lifetime, and cub survival is compromised if they cannot gain enough weight during the shorter hunting season.
Exposure to Environmental Contaminants
Polar bears occupy the highest trophic level in the Arctic marine food web, making them highly susceptible to chemical contamination through biomagnification. Contaminants from industrial and agricultural sources far to the south are transported by air and ocean currents, eventually accumulating in the Arctic. These chemicals are then concentrated at each successive level of the food chain, reaching peak levels in the apex predator.
The bears’ diet, which is rich in seal blubber, concentrates fat-soluble substances like Persistent Organic Pollutants (POPs) in their own thick layer of blubber. Key POPs found in polar bears include polychlorinated biphenyls (PCBs) and chlordanes, which are highly persistent compounds. Heavy metals such as mercury, a potent neurotoxin, also accumulate in their systems.
These high concentrations of contaminants weaken the bears’ physiological systems. POPs are known endocrine disruptors, meaning they interfere with the hormone system that regulates growth and reproduction. High levels of chlordanes have been linked to smaller testicle size in male bears and reduced ovary size in females, potentially compromising fertility.
Chemical exposure also suppresses the immune system, making contaminated bears more vulnerable to diseases and parasites. These toxins are passed from mother to offspring, both in utero and during lactation, leading to higher concentrations in cubs. This early-life exposure can result in developmental issues, including weak bones and altered lipid metabolism.
Increased Human-Bear Interactions
As sea ice retreats earlier and freezes later, polar bears are forced to spend more time foraging on land, bringing them into closer contact with Arctic communities and industrial sites. This increased overlap leads to a rise in human-bear conflict as hungry bears are attracted to human settlements for food sources like garbage or stored provisions. Conflict-related mortality is a significant factor, as bears are killed in defensive actions against humans or property.
Resource extraction activities, particularly oil and gas exploration, introduce additional risks to the species. The construction of physical infrastructure like roads and pipelines can fragment critical habitat and displace bears from denning areas. Industrial noise from machinery and low-flying aircraft can also cause stress or den abandonment, especially for pregnant females.
The greatest risk from industrial activity is the potential for an oil spill, which would have catastrophic consequences. Polar bears are extremely vulnerable to oil contamination because the oil destroys the insulating properties of their fur, leading quickly to hypothermia. Bears exposed to oil also ingest lethal amounts while grooming or eating contaminated prey, which can result in kidney failure, digestive disorders, and brain damage.
Subsistence hunting by Indigenous communities is managed through a strict quota system in countries like Canada, Greenland, and the United States. These quotas ensure the sustainability of the population while respecting the cultural and nutritional needs of local communities. All human-caused mortalities, including bears killed in defense of life and property, are tracked and counted against these regional harvest limits.