Polar bears live in 19 relatively discrete subpopulations around the circumpolar north and the effects of climate warming are becoming evident throughout their range (IUCN/SSC Polar Bear Specialist Group, 2006) (see Figure 14.4). Despite the sense that many people think of polar bears as a species living in the far reaches of the Arctic, the southernmost bears live at the same latitude as London, England. Other bears live near the North Pole, but the densities at high latitudes are thought to be very low and virtually nothing is known about this area.
Research has shown that the effects of climate change are apparent in polar bear subpopulations in a myriad of manners. Most often, the first symptom is a decline in the condition of the bears when they have difficulty obtaining sufficient prey. While the Arctic is a harsh and variable environment, the whole life history strategy of polar bears is tied to trading off reproduction to ensure survival. Females have the potential to reproduce many times over a 25-year lifespan. Thus, if hunting conditions are poor and body condition declines, the reproductive output of females drops. Most notable are smaller litter sizes, smaller cubs and cubs with lower survival rates. If sea ice conditions do not improve, the low recruitment rates to a subpopulation result in a slow decline. If the conditions are severe enough, the survival rates of sub-adults and adults are lower and a precipitous population decline can follow. When coupled with the slow maturation of the bears and the prolonged mother—offspring period (normally 2.5 years), polar bear populations are naturally slow to change in numbers, but once depressed in abundance, they are very slow to recover if the conditions allow.
To date, the effects of climate change are most evident in three subpopulations. The body mass of bears has declined in the Southern Beaufort Sea, Western Hudson Bay and Southern Hudson Bay subpopulations (Derocher and Stirling, 1995; Stirling et al, 1999; Regehr et al, 2006; Obbard et al, 2006) (see Figure 14.4). The first two subpopulations have declined by 17 per cent over 20 years and 22 per cent over the last decade, respectively (IUCN/SSC Polar Bear Specialist Group, 2006; Regehr et al, 2007). Changes in sea ice conditions are clearly implicated. While these rates of decline may seem low, for long-lived mammals with low reproductive potential, the rates are a serious concern for the persistence of the populations. Changes in sea ice conditions are clearly implicated but making these linkages is very difficult. Most other subpopulations lack sufficient monitoring
Note: SB=Southern Beaufort Sea, NB=Northern Beaufort Sea, VM=Viscount Melville, NW=Norwegian Bay, LS=Lancaster Sound, GB=Gulf of Boothia, MC=M'Clintock Channel, FB=Foxe Basin, WH=Western Hudson Bay, SH = Southern Hudson Bay, KB=Kane Basin, BB=Baffin Bay and DS=Davis Strait. QE=Queen Elizabeth but is now considered part of the Arctic Basin subpopulation.
Source: IUCN/SSC Polar Bear Specialist Group, 2006.
data to determine their trends, but symptoms of climate change are chronic and increasing in regularity. So far, the changes that have been observed can be typified as gradual declines punctuated by unusual events. Of concern is the possibility of climate warming creating a year or two of very poor conditions that could result in rapid population declines. The strategy of polar bears to store body fat to buffer poor years and periods without food can only be pushed so far.
Recent observations of polar bears drowning can be related to the sudden changes in sea ice patterns (Monnett and Gleason, 2006). As the sea ice has retreated from the Alaskan coast, polar bears are unable to maintain contact with the ice and with more open water; larger waves that form due to the increased fetch on the water make it difficult for the bears to regain contact with their primary habitat or, alternatively, land. The effects of this loss are unknown, but it appears to be a recent development. Polar bears are excellent swimmers, but, unlike seals and whales, they rely on walking for most of their movements and cannot remain permanently in water.
Further, as the sea ice cracks and drifts more on wind and current, the bears are effectively on a treadmill that humans are steadily increasing in speed (Mauritzen et al, 2003). The more energy that is used for locomotion, the less energy there is for growth and reproduction. Signs are showing up in several populations that polar bears are shrinking, not only in mass, but body length as well (Atkinson et al, 1996; Derocher, 2005).
Additional indicators of change come from recent observations of intraspecific killing and cannibalism (see Figure 14.5). In the Southern Beaufort Sea subpopulation, four adult females were killed and partially consumed by other bears, presumably by food stressed adult males (Amstrup et al, 2006; Stirling et al, in review). Cannibalism has been observed to occur in polar bears before, but the clear suggestion of an increase in occurrence is cause for concern. During the same period as the cannibalism observations, polar bears were attempting to prey on seals by digging through ice rather than through snow due to changes in the habitat (Stirling et al, in review).
Changes in prey consumed by polar bears are already underway in Hudson Bay, where the bears are consuming more harbor seals that are normally associated with temperate waters (Iverson et al, 2006). In other areas, the number of problem bears has increased as food stressed bears seek alternate food sources (Stirling and Parkinson, 2006). New data show that polar bears off the Alaska coast that once commonly denned on the sea ice are abandoning the drifting ice and are shifting to denning on land (A. Fischbach, personal communication). Even on land, polar bears may not be safe inside their dens. Warm weather, particularly rain in spring, can lead to den collapse and result in the death of both mother and cubs (Clarkson and Irish, 1991).
Other changes are likely to be more subtle, but they are appearing. Changes in the home range sizes and movement rates of polar bears have been documented (Parks et al, 2006). Other changes are possible and could include shifts in subpopulation boundaries and new diseases. These sorts of changes are difficult to monitor given the dispersed nature of polar bears. There is no single definitive study that can link climate change and polar bears; scientists have to rely on numerous smaller research studies and work to 'connect the dots'. The collective experience of polar bear researchers and northerners who live on the land are
Source: Photography by A. E. Derocher.
developing a clear picture of the situation and it does not look good for the Arctic ice bear.
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