Polar bear survival

With a circumpolar distribution, the sea ice over the continental shelves is the primary habitat for polar bears and is used as the platform for traveling, hunting, mating and, in some areas, for denning (see Figure 14.2, Plate 22). Annual ice that forms during autumn and winter is the preferred habitat of the bears because the thicker multiyear ice found at higher latitudes has low primary productivity, and thus few seals. The key to the success of polar bears lies in exploiting the fat of other marine mammals and turning it into a portable food store for periods when food is unavailable (see Figure 14.3). When a seal is killed, the bears strip the fat from the carcass and then deposit it with little modification directly onto their own stores (Stirling and McEwan, 1975, Iverson et al, 2006). The bears are able to consume up to 20 per cent of their body mass in a single meal and for a

Figure 14.2 Circumpolar distribution of polar bears showing high and low density (see Plate 22 for color version)

Source: A. E. Derocher and Norwegian Polar Institute.

Figure 14.3 Partially consumed adult ringed seal killed by an adult male polar bear in the Beaufort Sea, Canada

Source: Photograph by A. E. Derocher.

500 kg (1100 lb) adult male this is a huge amount of energy. A bellyful of seal fat converted to a bear's own fat stores will help in the summer when seals are scarce or unavailable.

While superbly adapted to the highly variable conditions in the Arctic, polar bears live close to the limit of what is biologically possible. Pregnant females can successfully fast for over eight months while over-wintering in dens and rearing cubs, which weigh only 600 grams (1.5 lbs) at birth (Ramsay and Stirling, 1988). The tiny cubs are kept warm in snow dens and are fed rich milk containing up to 50 per cent fat. Success at rearing offspring, however, is tied to how much fat a mother bear is able to store and this, in turn, is linked to the dynamics of their sea ice habitat. If the sea ice melts earlier than normal, the bears lose the opportunity to add to their fat stores and are also forced to begin their fasting earlier. Delayed sea ice formation prolongs the fast and further depletes fat stores. If the fasting period is too long, mothers with cubs stop nursing and with little fat of their own, cubs can soon weaken and succumb to the harsh environment.

In addition, change in the sea ice can affect a bear's ability to hunt. Seal populations are dependent on specific ice conditions and if they are unavailable, the seals leave for other areas or fail to reproduce. Take away or alter the sea ice too much and the bears are pushed beyond their limits and populations decline through reduced recruitment.

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