Data collected from radio-collared polar bears have confirmed their close ties to the ice. For example, between May 1985 and April 2001, we obtained 34,034 high-quality satellite radio-locations of polar bears in the Chukchi and Beaufort Sea areas of Alaska and northwestern Canada. Some collars had duty cycles that allowed them to transmit more frequently than other collars. When duty cycles were standardized so that each bear contributed one relocation per week, only 975 (7%) of 14,622 weekly locations were on land (Amstrup et al. 2000; Amstrup unpubl. data). Most of those were bears occupying maternal dens for the winter. In the polar basin area, polar bears truly are pelagic organisms (Garner et al. 1994)!
Telemetry data also have proven that polar bears do not wander aimlessly on the ice, nor are they carried passively with the ocean currents as previously thought (Pedersen 1945). Rather, they occupy multiannual activity areas outside of which they seldom venture. Annual activity areas of female polar bears monitored by radiotelemetry for multiyear periods varied among years. Collared animals, however, seemed to use seasonally preferred or "core" regions every year despite variation in annual activity area boundaries (Amstrup et al. 2000). This suggests that activity areas of polar bears, when viewed over multiyear periods, might be called home ranges. All areas of the home range, however, will not be used each year. Sea-ice habitat quality varies temporally as well as geographically (Stirling and Smith 1975; Ferguson et al. 1997, 1998, 2000a, 2000b; DeMaster et al. 1980; Amstrup et al. 2000). In areas of volatile ice, a large multiannual home range of which only a portion is used in any one season or year is an important part of the polar bear life history strategy.
Linear movements and activity areas are very large compared to those of most terrestrial mammals, and they vary in different regions of the globe, presumably because of variation in patterns of productivity and other sea-ice characteristics. In the Beaufort Sea, where polar bears have been followed by radiotelemetry for 20 years (Amstrup et al. 2000), total annual movements, calculated as the sum of straight-line distances separating consecutive weekly relocations, averaged 3415 km and ranged up to 6200 km. Movement rates of >4 km/hr were sometimes sustained for long periods, and movements of >50 km/day were observed. Annual activity areas of 75 radio-collared female polar bears in the Beaufort Sea region averaged 149,000 km2. The smallest annual activity area was nearly 13,000 km2, whereas the largest was 597,000 km2 (Amstrup et al. 2000).
Whereas movements of polar bears in the Beaufort Sea are impressive in their magnitude, movements of bears in areas of more dynamic ice may be even greater. The mean activity area size for six bears followed by satellite telemetry in the Chukchi Sea was 244,463 km2 (Garner et al. 1990). The mean annual distance moved by those bears was 5542 km. The potential mobility of polar bears in regions of volatile ice was illustrated by a mean rate of northerly spring movement of 14.1 km/day at a time when ice was moving as much as 15.5 km/day in the opposite direction (Garner et al. 1990).
In contrast, Schweinsburg and Lee (1982) reported maximum activity areas of <23,000 km2 in the Canadian Arctic Archipelago. Ferguson et al. (1999) also reported very large-scale movements for polar bears in the volatile sea-ice conditions of Davis Strait and Baffin Bay, and much smaller movements for bears in the interior of the Canadian Arctic Archipelago. The sea-ice of the Chukchi and Beaufort Seas and Baffin Bay is more dynamic and unpredictable than the ice in much of the Canadian Arctic Archipelago. The mobility of polar bears appears to be directly related to that variability (Garner et al. 1990, 1994; Gloersen et al. 1992; Messier et al. 1992; Ferguson et al. 2001).
Seasonal movement patterns of polar bears serve to emphasize the role of sea-ice in their life cycle. In the Beaufort Sea, the largest monthly activity areas were in June�July and November�December. These also were the months of highest movement rate. This matches the patterns of ice ablation and formation observed in the area (Gloersen et al. 1992). Polar bears catch seals mainly by still-hunting (Stirling and LaTour 1978). The volatile summer and autumn ice must minimize predictability of seal hunting opportunity. That unpredictability could require longer movements and larger activity areas during seasons of freeze-up and break-up. From May through August, measured net monthly movements of polar bears in the Beaufort Sea were significantly to the north for all bears. In October bears moved back to the south (Stirling 1990; Amstrup et al. 2000). Those movements appeared to be correlated with general patterns of ice formation and ablation. Between May and August, the ice of the southern Beaufort Sea is degrading (Gloersen et al. 1992). October is usually the month of freeze-up in the southern Beaufort Sea and may be the first time in months when ice is available over the shallow water near-shore. Polar bears summering on the persistent pack ice quickly move into shallow-water areas as soon as new annual ice forms in autumn, and they disperse easterly and westerly as ice solidifies through winter.
In contrast to polar bears of the Beaufort Sea region, Messier et al. (1992) reported that peak movement rates of instrumented polar bears in Viscount Melville Sound within the Canadian High Arctic Archipelago occurred from May to July. Movements, although increasing after January, were less from October through March. Ferguson et al. (2001) reported high movement rates in spring and summer in the High Arctic, and Messier et al. (1992) reported increasing mobility from January through spring in the Canadian Arctic. Polar bears in the Beaufort Sea also demonstrate high summer movement rates apparently because of rapidly changing ice conditions. In the southern and northern Beaufort Sea areas, movement rates remained high in November and December and low in May. The lower level of winter movement among polar bears of Viscount Melville Sound may be a consequence of the year-round abundance of multiyear ice (Gloersen et al. 1992; Messier et al. 1992; Ferguson et al. 2001). The density of ringed seals is lower there than in most other areas of polar bear habitat from Alaska through to West Greenland (Stirling and �ritsland 1995), and seals that are present in Viscount Melville Sound tend to be more concentrated along tidal cracks and pressure ridges that parallel the island coastlines (Kingsley et al. 1985). By comparison, the annual ice that predominates in most of the southern Beaufort Sea is more dynamic, and allows a greater amount of sunlight into the water column to support primary productivity. This facilitates easier access to air for seals to breath, and supports higher densities and numbers of ringed seals and polar bears (Stirling et al. 1982; Kingsley et al. 1985; Stirling and �ritsland 1995).
Polar bears in the Beaufort Sea may spend more time in winter actively foraging, and those in the Viscount Melville Sound area may spend more time resting and conserving energy. Messier et al. (1992) reported that long periods of "sheltering" were common among bears wintering in Viscount Melville Sound, and attributed this behavior to the poor foraging conditions there. Another factor may be the greater predictability of the foraging conditions in the stable ice of the High Arctic. With less change in the character of the sea-ice after freeze-up, polar bears may be able to determine the profitable hunting areas in early winter. Predictable sea-ice conditions could help bears minimize midwinter searching for good hunting areas and maximize benefits of sheltering. The constantly changing sea-ice in places like the Beaufort Sea or Baffin Bay, however, may require major modifications of foraging strategy from month to month or even day to day during break-up, freeze-up, or periods of strong winds. Polar bears are adaptable enough to modify their foraging patterns for the extreme range of sea-ice scenarios (Ferguson et al. 2001).
FIGURE 27.8. The circumpolar range of polar bears is subdivided, according to observed movement patterns, into 20 hypothesized populations or stocks. 1, Western Hudson Bay; 2, southern Hudson Bay; 3, Foxe Basin; 4, Lancaster Sound; 5, Baffin Bay; 6, Norwegian Bay; 7, Kane Basin; 8, Queen Elizabeth Islands; 9, Davis Strait; 10, Gulf of Boothia; 11, M'Clintock Channel; 12, Viscount Melville Sound; 13, northern Beaufort Sea; 14, southern Beaufort Sea; 15, Chukchi Sea; 16, Laptev Sea; 17, Novaya Zemlya; 18, Svalbard; 19, East Greenland; 20, Arctic basin. Boundaries are constantly being adjusted as new data and ecological insights are obtained. SOURCE: Adapted from Lunn et al. (2003:23). Click image to enlarge.
Just as the labile nature of the sea-ice results in annual variability in the distribution of suitable habitat for polar bears, it also eliminates any benefit to polar bears of defending territories. The location of resources is less predictable than resources on which terrestrial predators depend. Seals tend to be distributed over very large areas at low densities (Bunnell and Hamilton 1983). Furthermore, their distribution, density, and productivity are extremely variable among years (DeMaster et al. 1980; Stirling et al. 1982; Stirling and �ritsland 1995). As radiotelemetry studies have shown, female polar bears show only general fidelity to seasonal feeding areas (Ferguson et al. 1997; Amstrup et al. 2000). Absence of strict fidelity, especially during breeding and denning seasons (Garner et al. 1994; Amstrup and Gardner 1994), essentially prohibits defendable territories. Males similarly must be free of the need to defend territories if they are to maximize their potential for finding mates each year (Ramsay and Stirling 1986b).
Although there may be limited spatial segregation among individual polar bears, telemetry studies have demonstrated spatial segregation among groups or stocks of polar bears in different regions (Schweinsburg and Lee 1982; Amstrup et al. 1986, 2000; Garner et al. 1990, 1994; Messier et al. 1992; Amstrup and Gardner 1994; Bethke et al. 1996; Ferguson et al. 1999). Patterns in spatial segregation suggested by telemetry data, survey and reconnaissance, marking and tagging studies, and traditional knowledge resulted in recognition of 19 partially discrete polar bear groups (Lunn et al. 2002:21�35). There is considerable overlap in areas occupied by members of these groups, and boundaries separating the groups have been adjusted as new data were collected. Nonetheless, these boundaries are thought to be ecologically meaningful, and the units they describe are managed as populations.
A 20th polar bear population may occur in the central polar basin (Table 27.1). It is unclear whether bears that occur in this region are simply visitors from populations nearer to islands and continental shorelines or whether there are animals that spend all of their time in these high-latitude regions far from any land. The frequency of recent observations deep in the polar basin, however, mandates recognition that a separate stock could occur there (Fig. 27.8).
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