May 2008
The sea ice of the Arctic Ocean and the connected frozen seas is home to the largest and most predatory of the bear family. All bears alive today evolved 22 million years ago from a common ancestor — the Ursavus of Asia. Polar bears (Ursus maritimus) evolved from a group of brown bears (Ursus arctos) over 200,000 years ago, which became isolated from other brown bear populations by glaciers, possibly in Siberia. It is easy to imagine the evolutionary change in brown bears that inhabited a northern coast during a climatic cooling period, when food as tempting as unwary seal pups can be found offshore.
This polar bear cub, from the Beaufort Sea, Canada, is about 4 months old. Cubs, weighing about 10kg, were caught with their mother as part of a long-term population monitoring study.
Photograph: A. E. Derocher.
Adaptations and evolution
In a case of quantum evolution, polar bears evolved rapidly to exploit a vacant ecological niche as a specialized predator of seals. The rapid changes from a brown bear to a polar bear include these:
- white-yellow fur that helps them meld into their background as they sneak up on their prey
- claws that are catlike, an adaptation to grasp fleeing prey
- feet that are heavily furred to provide warmth
- smaller ears to avoid freezing in the frigid winters
- a narrower and more elongated skull, an adaptation perhaps to warm cold inhaled air, to aid the sense of smell, or to assist with the capture of prey trying to slip through a narrow opening to the safety of the sea below
- the ability to enter a fasting physiological state at any time of the year, unlike other bears, which enter this state only during winter torpor
- only four mammae (unlike brown bears with six) and a smaller litter size, an adaptation to the harsh environment for raising young
Living and dying on the sea ice is not conducive to creating a good fossil record, but polar bears existed in Europe during the last ice age, and fossil remains have been found in Germany, Sweden, Denmark, Norway, and England. Polar bears were present in Scandinavia as recently as 10,660 years ago. Unlike most large carnivores, the current distribution of polar bears is similar to that of the last few hundred years. No subspecies of polar bears are recognized.
Diet and feeding strategy
The lifestyle of polar bears is dramatically different from their brown bear ancestors. Despite their brief evolutionary separation, these two species exploit vastly different energy sources. Arctic brown bears are terrestrial, and most of their diet is vegetation augmented by animal protein. Polar bears, in contrast, are the most carnivorous of the bears, and two species of seals make up the bulk of their diet:
- The ringed seal (Phoca hispida), reaching over 60 kg and the most numerous seal in the Arctic, is their main prey.
- The larger bearded seal (Erignathus barbatus), which can top 400 kg, is also commonly taken.
Both of these seals rely on sea ice to reproduce and molt, and neither species is found where sea ice is absent. Polar bears are opportunistic and will exploit food sources from both marine and terrestrial sources. Seals, however, are what make the life history of the polar bear possible.
The fat-laden seals that polar bears eat allow them to grow larger than most brown bears. Being large is an advantage for staying warm in cold climates. More importantly, during the spring and early summer, polar bears gorge themselves on naïve newborn and recently weaned seal pups and deposit a thick fat layer that allows the bears to go through extended periods without food. Seals are an abundant food source that is converted into a portable energy store. Pregnant female polar bears in Hudson Bay can fast for up to 8 months and rear offspring (usually two) to about 10 kg before returning to the sea ice to feed on seals. Bears can double, triple, or quadruple their body mass during the spring. In contrast to brown bears, only pregnant female polar bears enter dens over winter; all other polar bears brave the harsh winter conditions trying to find a few seals to stem the loss of their precious fat stores.
Habitat
The sea ice is a dynamic habitat that undergoes huge annual variation in distribution and character. Polar bears follow the temporal shifts in habitats to access their prey. Annual sea ice, ice that forms and then melts within a single year, is the primary habitat of polar bears and is used various ways:
- as a platform to hunt
- as the habitat for mating
- for travel, migration, and connecting habitats
- as a summer refuge
- to den and produce young in some areas, primarily in Alaska
In response to the extreme variation in ice, female polar bears can cover huge areas in a single year, with many bears wandering over 200,000 km2 of sea ice. In contrast, brown bears use areas that are a tiny fraction of what polar bears use; a polar bear home range can be well over 1000 times that of a brown bear. In addition, the feeding strategies of polar bears are vastly different from their ancestors: Polar bears travel huge distances to exploit energy-rich foods, while Arctic brown bears move little and eke out an existence on an energy-poor diet.
Polar bears can be found at low densities right up to the North Pole, but their main habitat is the nearshore annual sea ice over the continental shelf, where biological productivity and their main prey are more abundant. The southernmost limit of the bears is in the subarctic waters of James Bay, in Canada, which is at about the same latitude as London, England. For management purposes, polar bears have been divided into 19 different subpopulations based on movement patterns of adult females wearing satellite radio collars.
Changing sea ice habitat
The main concern about climate change effects on polar bears is habitat loss and changes to sea ice habitat. The sea ice on which polar bears depend has undergone recent declines in area, duration of cover, and thickness as a result of climate warming. Observed changes to the sea ice and habitat that may affect polar bears are these:
- decline in maximum extent of sea ice in winter of about 1.5% per decade
- loss of multiyear ice (permanent polar pack ice), which is declining about 10% per decade
- increase in amount of open water
- shortening of the period of ice cover and lengthening of the open water period
- increase in the rate of ice drift
Studies suggest that the sea ice is changing faster that projected, and there are concerns that the loss of sea ice may have passed a tipping point that could accelerate future declines. Climate projections and sea ice models must be viewed with some caution, but the message is clear: Polar bear habitat is changing.
Climate induced changes to the sea ice may result in changes to the prey species available to polar bears. There are indications that harbour seal (Phoca vitulina) populations are increasing, and that harp seals (Pagophilus groenlandica) are expanding northward. These species are already prey items for some polar bear populations, but if the sea ice continues to change, we may see these species become more important to polar bears as long as the bears have a sea ice platform to hunt from. There is concern that if the ice conditions deteriorate too much, polar bears may be replaced by other top level predators, such as killer whales (Orcinus orca), which are largely excluded from ice covered seas.
Climate change effects on polar bears
The evidence for climate change affects on polar bears is not definitive. The definitive effects will come when subpopulations disappear. The status of the various subpopulations of polar bears varies widely: Some are in decline due to climate change effects, and others are not showing any indications of change. The effects of climate change can differ in space and time, but only two or three subpopulations are monitored adequately to be able to confirm long-term trends in abundance and thus provide some insight into what may befall the species over a broader area.
The most telling impacts of climate change on polar bears have been noted in western Hudson Bay, where declines in their body condition, reproduction, and survival have resulted in a 22% reduction in subpopulation size between 1987 and 2004. Earlier melting of the sea ice in Hudson Bay is the major driving force behind the population decline, but a continuing unsustainable harvest of seals has aggravated the situation. Earlier melting of sea ice has two consequences for polar bears: It shortens the feeding period at a time when recently weaned seal pups are available, and it lengthens the period the bears must fast with less stored fat. While polar bears are well adapted to extended fasts, there is a limit to how long they can survive without food. Females in poor condition give birth to small cubs that weigh less, and lighter cubs have lower survival rates. Over time, low survivorship to adulthood means the subpopulation will decline in number. There are data showing that polar bears in both the southern Beaufort Sea and southern Hudson Bay are also declining in condition, which is often a precursor to subpopulation declines.
Two young polar bears on the coast of Hudson Bay, Canada, are waiting for the sea ice to reform so they can return to their primary habitat and resume hunting seals.
Photograph: A. E. Derocher.
A warming climate is altering sea ice conditions and affecting polar bears in other ways. Sea ice in many areas shifts with wind and water currents, and polar bears often walk against the ice flow to remain in contact with their preferred habitats. Climate warming is reducing ice thickness and extent, which may result in greater ice drift. In effect, the polar bears are on a treadmill, and we are turning up the speed. More energy used for locomotion means there is less energy available for growth and reproduction. Like deforestation in terrestrial habitats, altered sea ice dynamics can increase habitat fragmentation, making movement across the landscape more difficult.
Some examples of the expected effects of changes in sea ice are the following:
- increased energetic costs of movement
- altered home range size and configuration
- altered subpopulation boundaries
- reduced access to den areas
- increased periods without access to prey
- altered prey species
- increased time spent swimming, which may chill small cubs and reduce their survival
Other events are more difficult to directly link to climate change but are consistent with predictions. Polar bears observed drowning off the coast of Alaska may have died due to the rapid northward retraction of the sea ice: More open water and greater distances between land and sea ice make it difficult for bears to find refuge. In the same area, killing and cannibalism observed among polar bears may be related to changes in sea ice conditions and lower availability of prey. Adult males appeared desperate enough to prey on other bears. Despite decades of research, such events were never recorded in the past in the Beaufort Sea, but it is consistent with a population under stress.
Changing sea ice conditions are affecting the bears’ hunting abilities. In the southern Beaufort Sea, bears were observed in 2005 through 2008 digging through solid ice trying to prey on seal pups. Normally, ringed seal pups are born under snow drifts, which the bears can excavate with relative ease, but clawing through ice up to 70 cm thick is inefficient and possibly an indication of low seal availability. Seals appear to be pupping under sea ice because of altered sea ice conditions and storm events that rafted thinner ice. The long-term consequences for polar bears are unknown, but a reduction in energy intake is likely to affect many aspects of the bear’s ecology.
Recently, a new study in Alaska revealed that polar bear dens on the pack ice declined from 62% between 1985 and 1994 to 37% between 1998 and 2004. This was probably a result of declines in the amount of stable old ice; increases in unconsolidated ice; lengthening of the ice-free period, which reduced the availability and quality of pack ice den habitat; and the long-term protection of denning females, which has resulted in bears with a fidelity to denning on land not being killed by hunters. As the ice continues to change, we can expect some den areas to be abandoned.
In some areas, the number of human-bear interactions is increasing. Nutritionally stressed bears that are spending more time on land are approaching settlements or hunting camps seeking food. As the sea ice continues to change and bears become increasingly stressed, further increases in interactions are expected.
The future
Will polar bears just adapt to a terrestrial life without the presence of sea ice? This notion has been naïvely proposed by some. Polar bears regularly attain body masses of over 300 kg for females and 500 kg for males. In contrast, brown bears living in the Arctic right next to polar bears rarely exceed 200 kg, reflecting the meager food resources of high latitude terrestrial environments. It is an odd view of evolution that would propose that a highly specialized species with over 200,000 years of evolution could respond in decades or, at best, centuries to the projected loss of their sea ice habitat. Regardless, the niche of a terrestrial Arctic bear is already filled by the brown bear, of which the grizzly is a subspecies.
Predicting the future is a precarious venture, but it is clear that the sea ice habitat of polar bears is changing rapidly. Highly specialized species are particularly vulnerable to the effects of habitat loss. In summary, the expected changes in polar bears related to climate warming include these:
- reduced access to prey species
- reduced body condition
- lower cub survival
- lower reproductive rates
- lower growth rates
- increased intraspecific aggression
- increased cannibalism
- lower adult survival
- altered movement rates
- shifting den areas
- shifting population boundaries
- increased bear-human interactions
- altered prey composition
- reduction in population size
Loss of sea ice is similar to deforestation of tropical rain forests: lose the habitat and, with few exceptions, you lose the species. Unlike other species, polar bears are unlikely to do well shifting their range further north because the polar basin is deep, cold, and unproductive. Losing the productive coastal habitats would be a serious loss, but the sea ice is more than just a platform, it is the habitat of polar bears and many of the species they rely upon. From phytoplankton to fish, the sea ice is an integral part of the Arctic marine ecosystem.
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