What Biome Does a Polar Bear Live In?

Polar bears live in the Arctic tundra biome, but calling it “tundra” only tells part of the story. Their true habitat is the sea ice that forms over the Arctic Ocean and its surrounding seas, a frozen platform that functions as hunting ground, travel corridor, and dining table. Ecologists sometimes classify this environment as the marine Arctic or Arctic marine biome to distinguish it from the treeless land we typically picture when we hear “tundra.” The distinction matters because a polar bear without sea ice is a polar bear in trouble.

Why Sea Ice Is the Biome That Matters

When biome maps label the top of the globe, they usually show “Arctic tundra” stretching across northern Canada, Alaska, Scandinavia, and Russia. That label applies to the land, but polar bears spend most of their active hunting lives offshore on frozen ocean. Sea ice is not simply a surface to walk on. It hosts a complex community of algae growing on its underside, invertebrates that feed on those algae, fish that feed on the invertebrates, and seals that haul out on the ice to rest and give birth. Polar bears sit at the top of that chain. They forage “nearly exclusively” on sea ice, making it the substrate that defines their ecological existence.¹

The Arctic itself is formally defined as the area north of the 10 °C isotherm for July, a boundary that roughly follows the treeline.² Within that boundary, winter temperatures can plunge to around −40 °C or colder, daylight vanishes for weeks, and blizzards are routine. Polar bears evolved to thrive in exactly these conditions. But rather than enduring the barren land surface, they follow the ice edge, migrating with it as it expands in autumn and retreats in spring.

How Polar Bears Choose Their Ice

Not all sea ice is equal. The Arctic produces several distinct ice types depending on age, thickness, and how it moves. Research tracking bears in the Canadian High Arctic and along southeastern Baffin Island found that during spring and summer, bears in the Archipelago region favored landfast ice (ice anchored to the shore), while bears along Baffin Island preferred moving pack ice made up of thick first-year floes. Both ice types likely represent areas where most seal pupping occurs in spring.³ In winter, bears from both regions shifted to first-year ice where new ice was forming, and in autumn they moved onto multiyear ice that had survived the summer melt.³

This seasonal switching is strategic. First-year ice in winter tends to be thinner and easier for seals to maintain breathing holes in, which makes hunting productive. By autumn, when the summer melt has reduced thinner ice, the older, thicker multiyear ice is what remains, and bears move onto it because it is what is available. The ice is not just a biome background; it is an actively chosen, season-specific habitat.

When Polar Bears Come Ashore

Despite their dependence on marine ice, polar bears also use land, especially for denning. Pregnant females dig maternity dens in deep snowdrifts on land, typically from late October through March. In western Hudson Bay, researchers examined over 100 den sites and found that bears denned almost exclusively in peat banks along the edges of creeks, rivers, and lakes, adjacent to open lichen-covered tundra.⁴ The terrain provides both the deep snow accumulation needed to insulate a den and the topographic relief that traps windblown snow into drifts.

On Russia’s Wrangel Island, modeling of snow deposition patterns showed that about 70% of known maternal dens were located within 100 meters of modeled snowdrift sites, confirming that the physical landscape channels snow into predictable locations and bears find them reliably.⁵ So while the Arctic tundra is not where bears do most of their feeding, it provides a critical nursery habitat. Cubs are born in these dens and do not emerge until spring, when their mother leads them back to the sea ice to begin hunting.

In populations like the Western Hudson Bay group, GPS collar data from adult females showed that distance to the denning area was the most important factor in habitat selection across all seasons, suggesting a strong fidelity to these terrestrial refuge areas.⁶ Even when on the ice, these bears stay loosely tethered to familiar onshore denning zones.

The Food Chain Under the Ice

A biome is more than temperature and terrain; it is a web of organisms that sustain each other. The Arctic marine biome starts with ice algae, single-celled organisms that grow on the undersurface of sea ice during the long polar spring. These algae are a surprisingly durable energy source. A large survey across Arctic shelves detected ice algal carbon signatures in 96% of the organisms investigated, spanning invertebrates, fish, seabirds, and marine mammals, collected year-round from January through December.⁷ That finding reflects the fact that ice algal carbon sinks to the seafloor and gets retained in the sediment community, making it available to bottom-dwelling organisms long after the ice itself melts. The entire benthic food web acts as a pantry stocked during the ice-covered months.

For polar bears, the link is more direct. They hunt ringed seals and bearded seals, which depend on the same ice ecosystem. Research in the Canadian Arctic found that bears mainly kill newborn ringed seal pups in their subnivean birth lairs on landfast ice, while bearded seals are an important prey species along southeastern Baffin Island.⁸ Everything connects back to ice: algae feed the food web, seals breed on the ice, and bears hunt on it.

Polar bears also play a role as ecosystem providers. An average polar bear kills roughly 1,000 kilograms of marine mammal biomass each year, and because bears preferentially eat blubber and abandon the rest of the carcass, around 30% of that biomass is left as carrion. Across the species’ entire range, this adds up to an estimated 7.6 million kilograms of carrion per year, feeding at least eleven vertebrate scavenger species.⁹ Arctic foxes, gulls, and ravens are among the animals that depend on polar bear leftovers.

How Polar Bears Became an Arctic Species

Polar bears did not always exist as a distinct species. Genomic analyses estimate that polar bears diverged from brown bears somewhere between roughly 340,000 and 600,000 years ago, during the middle Pleistocene.¹⁰ That timeframe gave them hundreds of thousands of years to evolve the traits needed for life on sea ice. One striking adaptation involves their cardiovascular system. Of the top sixteen genes under the strongest positive selection on the polar bear lineage, nine are associated with heart and vascular function. A gene called APOB, which encodes a key component of LDL cholesterol particles, shows especially strong selection. Mutations in this gene may explain how polar bears tolerate lifelong high levels of LDL cholesterol from a diet rich in seal blubber, something that would cause severe heart disease in humans.¹¹

Their body also adapted mechanically. Polar bear paw pads have papillae (small raised bumps) that are about 1.5 times taller and create roughly 1.3 times more true surface area than those of brown bears or black bears. Friction experiments with 3D-printed model surfaces showed that these features increase grip on snow by a factor of 1.3 to 1.5.¹² This is a direct physical adaptation to the ice-and-snow biome: better traction means more efficient travel and more successful ambush hunts on slippery surfaces.

A Biome in Decline

The defining feature of the polar bear’s biome is disappearing. Across all nineteen defined polar bear management regions, the sea ice season is getting shorter. Spring ice retreat is arriving three to nine days earlier per decade, autumn ice advance is coming three to nine days later per decade, and the total number of ice-covered days is dropping by seven to nineteen days per decade, depending on the region.¹³ The Barents Sea and central Arctic Basin are losing ice fastest. Average sea ice concentration from June through October is declining by one to nine percent per decade across all regions.¹³

When the ice melts early, bears are forced ashore before they have built up enough fat reserves, and they must wait on land until freeze-up returns in autumn. That time ashore is essentially a fast. A study tracking twenty polar bears on land in Manitoba from August to September found that 19 of 20 bears lost body mass at rates of 0.4 to 1.7 kilograms per day. Despite eating berries, vegetation, birds, bones, and even garbage, the terrestrial food did little to offset the energy deficit.¹⁴ Some bears adopted a hibernation-like strategy of minimal activity, while others made exhausting open-water swims of 54 to 175 kilometers, but neither approach prevented weight loss.

This is not just a polar bear problem. The skull and teeth of polar bears are specialized for a soft, lipid-rich diet of seal blubber and flesh, distinctly different from the broader molar dentition of grizzly bears. Dietary analysis in Alaska found that polar bears maintained a conserved absence of hard-object feeding for roughly a thousand years, until the 21st century. Some modern Alaskan bears now show signs of consuming harder foods, including more complete carcass use and terrestrial items, despite being poorly suited for it.¹⁵ A specialist forced into generalist habits is a warning sign.

Glacial Ice as an Unexpected Alternative

In Southeast Greenland, researchers documented a previously unknown subpopulation of polar bears that hunts year-round using freshwater glacial mélange, the mix of icebergs and brash ice calving off marine-terminating glaciers, instead of the annual sea ice that most polar bears rely on.¹⁶ This glacial platform provides a hunting surface even when seasonal sea ice has melted, suggesting that marine-terminating glaciers could serve as limited climate refugia. The finding is fascinating but sobering: only a handful of Arctic fjords produce this kind of persistent ice, so the refuge is tiny compared to the continental-scale sea ice habitat that most bears need.

Pollution at the Top of the Food Chain

Living at the apex of a marine food web means polar bears accumulate pollutants that have traveled thousands of kilometers on ocean and air currents. Persistent organic pollutants like brominated and chlorinated compounds enter the Arctic ecosystem at low concentrations, get absorbed by small organisms, concentrate in seals, and then concentrate further in the bears that eat those seals. Research on East Greenland polar bears found increasingly complex mixtures of bioaccumulated and in some cases biomagnified chlorinated and brominated contaminants, a contributing stress factor on top of the energy challenges from habitat loss.¹⁷ The biome may look pristine, but its remoteness does not protect it from industrial chemicals.

Different Ice, Different Microbes

The biome even shapes the invisible community inside a polar bear’s gut. A comparison of gut microbiomes between two subpopulations, one in the Southern Beaufort Sea and one in East Greenland, found substantial differences linked to their different sea ice environments and foraging habits. Bears in the Southern Beaufort Sea, where ice loss has been more severe and diets have shifted, carried a distinct microbial community compared to East Greenland bears living with more stable ice conditions.¹⁸ The shifting foraging habits driven by sea ice loss may be altering the bears’ gut bacteria, with potential downstream effects on nutrition and physiology. When the biome changes, everything changes, right down to the microbes inside the predator at its peak.

What Indigenous Communities Have Observed

Scientific field studies of polar bear habitat are relatively recent, but Indigenous peoples in the Arctic have been observing these bears for millennia. Interviews with Cree elders and hunters in the Northern Eeyou Marine Region of Québec revealed detailed knowledge of polar bear distribution, terrestrial habitat use, denning behavior, and foraging patterns. Participants were unanimous in noting a warming climate and a prolonged ice-free season in recent years.¹⁹ This kind of long-term observational knowledge fills gaps that satellite tracking and short-duration field studies cannot, particularly regarding how bears used the landscape before modern monitoring began. Indigenous communities often report changes in bear behavior and distribution years before they show up in scientific datasets, making traditional ecological knowledge an important complement to remote sensing and collar data.