POLAR BEARS AND SEA ICE
Declines in polar bear survival during the period 2001–2005 were associated with longer annual ice-free periods over the continental shelf. Breeding probabilities also declined, but did not exhibit the same relationship to sea ice conditions as survival. We hypothesize that declining sea ice affects polar bear vital rates primarily via increased nutritional stress. In years with longer ice-free periods, polar bears have less time in summer and autumn to hunt over the continental shelf. Instead, they spend more time on multiyear ice over less-productive Arctic basin waters (Pomeroy 1997), or on land (Schliebe et al. 2008). Reduced foraging opportunities associated with longer ice-free periods, whether spent on land or over deep waters, likely cause polar bears to enter the winter in poorer nutritional condition.
Additional evidence suggests that polar bears in the southern Beaufort Sea are under increasing nutritional stress. From 1982 to 2006, body size and body condition for most sex and age classes were positively correlated with the availability of sea ice habitat, and exhibited a statistically significant decline during this period. Cub litter mass and the number of yearlings per female also declined following years with lower availability of sea ice habitat (Rode et al. 2009). Using serum biomarkers, Cherry et al. (2008) found that a higher proportion of polar bears were fasting in the springs of 2005–2006 (21·4% and 29·3%), compared to 1985–1986 (9·6% and 10·5%). The year 1985 had one of the lowest numbers of ice-free days on record, and 1986 was similar to 2001–2002, so this comparison is particularly relevant to our findings. Finally, the longer ice-free periods in 2004 and 2005 were associated with an unusual number of reports of inefficient foraging behaviours by polar bears (Stirling et al. 2008), observations of cannibalism (Amstrup et al. 2006) and observations of polar bears that had apparently starved to death (Regehr et al. 2006). Historically, such observations were rare or non-existent.
Polar bears depend on sea ice for movement and reproduction, as well as for hunting. In 2004, abrupt retreat of sea ice from the coast, combined with stormy weather, resulted in drownings in the southern Beaufort Sea (Monnett & Gleason 2006). Extensive open water and increased ice roughness, caused by the action of winter storms on thinner ice, may reduce foraging success (Stirling et al. 2008), increase the energetic costs of locomotion (Derocher, Lunn & Stirling 2004) and increase the risk of injury or death for cubs. Less stable sea ice also has apparently led to more females denning on land (Fischbach et al. 2007). Finally, the increasing seasonal retreat of sea ice may require polar bears to travel farther between multiyear pack ice, where most animals spend the summer, and the onshore denning areas or coastal hunting areas that they use at other times of the year (Bergen et al. 2007).
Climatic warming is likely also to have indirect ecological effects on Arctic marine mammals (Tynan & Demaster 1997; Derocher et al. 2004; Laidre et al. 2008). Polar bears are susceptible to changes in the abundance and age structure of seal populations (Stirling 2002). In particular, mortality rates of ringed seal pups, the most important component of the polar bear’s diet, may increase in years when the sea ice breaks up early. Pup mortality also may increase when warmer temperatures lead to rains early in the breeding season, which can melt the under-snow lairs that pups need for shelter (Smith & Harwood 2001; Stirling & Smith 2004). In some regions, climatic warming may temporarily increase the availability of alternate prey species for polar bears, such as recent increases in harp seals on the sea ice in Davis Strait (Iverson, Stirling & Lang 2006) and walruses in coastal haul-outs in the Chukchi Sea (N. Ovsyanikov, unpublished data). As top predators, polar bears can be expected to integrate ecological changes at lower trophic levels, which have been documented in northern Hudson Bay (Gaston et al. 2003) and may be occurring in other parts of the Arctic. Simultaneous with ecological changes, polar bears face increasing potential for conflicts with humans in a warming Arctic, as industrial activity expands (Arctic Climate Impact Assessment 2005), longer ice-free periods force polar bears to spend more time on land (Schliebe et al. 2008) and nutritional stress encourages polar bears to seek anthropogenic food sources (Regehr et al. 2007b).
Our time-varying survival estimates for 2001–2003 were similar to estimates for adult females in the southern Beaufort Sea from 1981 to 1992 (0·969; Amstrup & Durner 1995) and similar to, or higher than, estimates for adult females in other populations (0·940–0·997; Table 2 in Aars et al. 2006). Our survival estimates for 2004 and 2005 were lower than have been previously reported for polar bears. In an earlier, single-state analysis of capture–recapture data from the southern Beaufort Sea, Regehr et al. (2006) also found that survival may have declined from 2001 to 2005. Although Regehr et al. (2006) found weak support for a relationship between survival and the covariate ice(t), comparison of the two analyses is complicated by different model structures and data sets. We may have been more successful in detecting sea ice effects because the multistate models included different reproductive stages for adult females and because, unlike Regehr et al. (2006), we evaluated models with different recapture probabilities for females and males.
Our conclusions are strengthened by the use of multimodel inference and model averaging, and by agreement between models with parametric dependence on the environmental covariate ice(t) and models that allowed parameters to vary freely over time. Multimodel inference is particularly important for estimating statistical relationships from short time series of data in a variable environment. By permitting recapture probabilities to vary by sex, reproductive stage, tagging method and region of capture, we accounted for sources of heterogeneity often present in capture–recapture studies. Nonetheless, some individual heterogeneity may have resulted from the movement of polar bears with respect to the sampling area. For example, polar bears with small home ranges centred in the core of the sampling area may have been more likely to be captured than those with home ranges that were either large or centred near the edge of the sampling area. Our analysis of radiotelemetry data collected from 1985 to 2006 suggests emigration patterns in the southern Beaufort Sea are random, making it unlikely that survival estimates in the current study were biased due to Markovian dependence in temporary emigration (Kendall et al. 1997; Kendall & Nichols 2002; Schaub et al. 2004). Nonetheless, sea ice loss and increased variability in annual sea ice extent have the potential to affect polar bear distribution and movements, including the possible breakdown of historic population boundaries (Derocher et al. 2004). Although statistical tests for within-study changes in emigration were not significant, our ability to evaluate the type of emigration that occurred 2001–2006, and its potential effects on parameter estimates, was limited by the small sample size of radiotelemetry data. The lower-point estimate of the proportion of radiocollared polar bears inside the sampling area in 2005 and 2006, compared to 2002–2004, suggests caution in interpreting the magnitude of estimated declines in apparent survival.
IMPLICATIONS FOR CONSERVATION
The apparent dependence of polar bear vital rates on sea ice is relevant to evaluations of conservation status for this and other species. Moore & Huntington (2008) classify Arctic marine mammals into ice-obligate species (polar bear, walrus, bearded and ringed seals) and ice-associated species (beluga and bowhead whales; narwhal; harp, hooded, ribbon and spotted seals). Our results generalize most readily to ice-obligate marine mammals and to subarctic ice seals (see the analysis of sensitivity to climate change in Laidre et al. 2008). However, even species that depend directly on sea ice as a platform for foraging and other aspects of their life history may exhibit different responses to sea ice loss. Walrus, for example, are generally limited to foraging in waters <100 m deep. Their demography will be most affected by the distribution of sea ice over these shallow waters, although some walrus may be buffered from the effects of sea ice loss by their ability to use terrestrial haul-outs between feeding excursions (Sheffield & Grebmeier 2009). Additional demographic studies are needed to understand the impacts of climate change on Arctic marine mammals.
We believe that the analyses reported here and in a companion manuscript (Hunter et al. 2007) provide a template for assessments of extinction risk for other species with similar types of data. The first step is to estimate vital rates, which determine the potential for population growth, and to evaluate the relationships between vital rates and environmental conditions. If environmental conditions are expected to change, both the environment-dependent vital rates and the forecasted range of environmental conditions can be incorporated into a demographic model to project future population status. For polar bears in the southern Beaufort Sea, Hunter et al. (2007) used matrix-based projection models (e.g. Caswell 2001) to combine the vital rates estimated here with sea ice forecasts. That analysis indicated that the southern Beaufort Sea population faces a high risk of extirpation within the 21st century if sea ice loss continues as projected.
Sea ice declines and the associated impacts on marine mammals are expected to vary across the Arctic (Laidre et al. 2008; Moore & Huntington 2008; Thiemann, Derocher & Stirling 2008). Units based on taxonomy, genetic distinction, ecology and distribution are common in evaluations of conservation status (Green 2005), and can be used to extrapolate from well-studied populations to larger portions of a species’ range. Amstrup, Marcot & Douglas (2008) used regional differences in sea ice dynamics and ecology to identify four ‘ecoregions’ for polar bears. The Divergent Ice Ecoregion, characterized by the formation and subsequent melting or advection of annual sea ice, includes the southern Beaufort, Chukchi, Laptev, Kara and Barents seas (Fig. 1). It is reasonable to expect that the relationships between sea ice loss and polar bears in the southern Beaufort Sea also apply to other portions of the Divergent Ice Ecoregion, where sea ice loss has been greater (Meier, Stroeve & Fetterer 2007) but data on polar bears are not available. Sea ice declines throughout the Divergent Ice Ecoregion are projected to be long term and severe (Amstrup et al. 2008). Because this region includes c. 7500 polar bears, one-third of the current world population (Aars et al. 2006), our findings in the southern Beaufort Sea were considered relevant to the extinction risk facing a large portion of the world’s polar bears. This contributed to the listing, in May 2008, of polar bears as a threatened species under the US Endangered Species Act.