Consequences To Polar Bears Due To Ice Retraction

Effects of Global Warming on Polar Bear Populations
(Click on Figure to Enlarge)
Original Diagram

The figure at the right shows a diagram of the information provided on this page. From the previous tabs, the story about how and why global warming is affecting the Arctic sea ice has been discussed. Now it is time to look more specifically at how the loss of sea ice is directly affecting the polar bear populations. This loss is causing a reduction of food resources, an increase in mortality events, a decrease in offspring survival rates, and the loss of natural habitat. (Click on diagram to enlarge for better clarity).

Feeding:

As the sea ice retracts earlier in the spring and continues to be less prominent in the summer months, a reduction in the amount of seals that polar bears have access to during the spring and summer is created. In 1998, the amount of summer ice in the Chukchi Sea covered 25% less area than during the minimum for the previous 35 years (3). As a result, polar bears must seek out other prey options in the absence of seals. That coupled with the loss of ice during the spring and summer months forces polar bears to seek out prey in terrestrial habitats (5). One of the main, coastal marine animals polar bears choose to eat are walruses. Unfortunately just as the seals begin to leave due to loss of ice, so will the walruses. The polar bears must then make a prey shift in order to interact with their new prey species. Thus polar bears shift to eating whatever food sources they can find, which are now mainly found on land. This leads to an increase in scavenging during the spring and summer months (5). Carcasses of whales are the primary food source for scavenging polar bears (2). Ultimately the retraction of ice in the summer months leads to the polar bears having to phenologically shift the amount of time they are able to fast (4). The bears must develop the ability to increase the amount of time they can go without food during a longer summer. A longer, fasting period directly relates to a reduction in adult polar bear body mass and in the likely success of offspring (2). Both of which cause a decrease in overall polar bear population and the amount of cubs born each year (3).

Reproduction:

As polar bears take longer fasting periods, due to earlier ice loss, a decrease in adult body mass has been detected (5). If the body mass of an adult female bear decreases below a ‘‘critical body mass’’ of 189 kg in the autumn, she will be unable to reproduce successfully (3). Through the late 1980's and the 1990's, a steady decline in body conditions of both males and females was documented (2). During the same period, fecundity decreased. Up until 1992 an annual reduction in average body mass of 4.7 kg for pregnant females was recorded (2). Assuming a continued loss of body mass at the same rate, they predicted that most females would be below the ‘‘critical body mass’’ for successful reproduction by 2014 (3). With the loss of ice, females are bearing fewer cubs and polar bear mating practices have changed. The ice is their primary mating ground. A decrease in ice means less mating and ultimately less offspring. If offspring are born, their survival rate has shown to be decreasing over the past twenty years (3).  This is due to very low body mass and decreased food resources to feed on when emerging from the dens. In addition, there has been increased predation from Arctic wolves looking for food themselves (3).

Mortality Events:

The retraction of ice has caused polar bears to die in different ways other than by starvation. One way is by cannibalism. With seal populations migrating away, polar bears are having to shift to different sources of prey. When they are unable to find scavenger prey, they have been shown to eat one another when in extreme starvation states. Over the past ten years six cases of cannibalism, within a population of polar bears near the Beaufort Sea have been documented (3). With shorter feeding periods in the spring and longer summers, there has been a reduction in available nutritional food sources. This forces polar bears to compromise their normal feeding habits in order to stay alive. A second mortality event, that has increased in the past ten years, are incidences of polar bear drownings. In 2004, there were four observed drownings in the Beaufort Sea (3). The bears were found between the coast and the offshore ice pack. In the recent late-ice years, the number of drownings have been increasing directly with an increasing distance of ice packs from the coast (11). Polar bears are designed for swimming in Arctic waters but are not able to swim for great distances. As the ice retracts farther away from the coast, the bears will try and swim out to the ice, in order to find food. The greater swimming distance requires more energy. The hungry bear runs out of energy and drowns before food can be obtained. A final mortality event is a result of an increase in the number of interactions between humans and polar bears. With earlier spring break-up and later fall freeze-up of the ice, there is a longer boating season in the Arctic (11). The hunting of polar bears from boats has since increased. However, this practice has been mitigated by quotas, for how many bears can be shot per year, for safety reasons. In the 1990's, the quota was three bears per year and currently it is ten (11). Besides hunting, the presence of increased boat traffic in the Arctic has produced more incidences of self-defense killings against polar bears (3).

Overall Habitat Loss:

Ultimately, the loss of ice results in a nearly, complete loss of habitat for the polar bear. This is especially detrimental to females, who rely on the sea ice for their maternity dens. It is important for females to reach the denning area in autumn, either by swimming or by walking on the ice. In the autumn, there is a large inter-annual variation in ice extent and distance, between the ice edge and the den sites (2). Observations have shown that the number of maternity dens at the island of Hopen at Svalbard, the southernmost of the denning islands in this archipelago, varied between 0 and 35 during 1994–2001 (3). The number of dens on Hopen was strongly, negatively correlated to the date of sea ice arrival, at the island, the previous autumn. In the fall of 1999, when the ice did not arrive until close to Christmas, the bears were not able to reach the island at all and there were no dens there (3). This information shows that a loss of ice created great distress for the females. This decrease in sea ice has caused the number of maternity dens found on ice packs to decrease over the years, from 67%, in 1985-1994, to 37%, in 1998-2004 (3). The denning distribution has changed in response to many conditions. Reductions in stable old ice, increases in unconsolidated ice, and the lengthening of the melt season have all reduced the availability and quality of pack ice denning habitat (11). At the same time, a decline in the physical stature and survival of cubs in the area has been observed (3). The females are starting to modify their behavior, from making dens on ice to making them on the coastal land. As a result, they have to adjust to different resources in order to feed their cubs once the family emerges from the den. They must also adapt to using new materials to build their dens with. Both of these environmental factors will initially increase the mortality rate among polar bears until they are firmly adapted to their new terrestrial habitat during the fall and winter months (3).