Abstract:Sea ice is declining over much of the Arctic. In Hudson Bay the ice melts completely each summer, and advances in break-up have resulted in longer ice-free seasons. Consequently, earlier break-up is implicated in declines in body condition, survival, and abundance of polar bears (Ursus maritimus Phipps, 1774) in the Western Hudson Bay (WH) subpopulation. We hypothesised that similar patterns would be evident in the neighbouring Southern Hudson Bay (SH) subpopulation. We examined trends 1980–2012 in break-up an… Show more
“…Although approach 3 reflected regional variability in sea-ice dynamics and polar bear ecology, it was strongly influenced by several well-studied subpopulations and did not reflect finer-scale variation. For example, within the divergent ecoregion, multiple estimates of N were available for the declining Southern Beaufort sea subpopulation [7], but not for the Chukchi sea subpopulation, which inhabits a more biologically productive region and has exhibited high recruitment despite sea-ice loss [9]. …”
Section: Resultsmentioning
confidence: 99%
“…Two have already experienced sea-ice-related demographic declines [7,8]. Others show signs of nutritional stress [9], have been reported as stable or productive [10] or have unknown status owing to deficient data [11]. Figure 1.The four polar bear ecoregions and 19 subpopulations.…”
Loss of Arctic sea ice owing to climate change is the primary threat to polar bears throughout their range. We evaluated the potential response of polar bears to sea-ice declines by (i) calculating generation length (GL) for the species, which determines the timeframe for conservation assessments; (ii) developing a standardized sea-ice metric representing important habitat; and (iii) using statistical models and computer simulation to project changes in the global population under three approaches relating polar bear abundance to sea ice. Mean GL was 11.5 years. Ice-covered days declined in all subpopulation areas during 1979–2014 (median −1.26 days year−1). The estimated probabilities that reductions in the mean global population size of polar bears will be greater than 30%, 50% and 80% over three generations (35–41 years) were 0.71 (range 0.20–0.95), 0.07 (range 0–0.35) and less than 0.01 (range 0–0.02), respectively. According to IUCN Red List reduction thresholds, which provide a common measure of extinction risk across taxa, these results are consistent with listing the species as vulnerable. Our findings support the potential for large declines in polar bear numbers owing to sea-ice loss, and highlight near-term uncertainty in statistical projections as well as the sensitivity of projections to different plausible assumptions.
“…Although approach 3 reflected regional variability in sea-ice dynamics and polar bear ecology, it was strongly influenced by several well-studied subpopulations and did not reflect finer-scale variation. For example, within the divergent ecoregion, multiple estimates of N were available for the declining Southern Beaufort sea subpopulation [7], but not for the Chukchi sea subpopulation, which inhabits a more biologically productive region and has exhibited high recruitment despite sea-ice loss [9]. …”
Section: Resultsmentioning
confidence: 99%
“…Two have already experienced sea-ice-related demographic declines [7,8]. Others show signs of nutritional stress [9], have been reported as stable or productive [10] or have unknown status owing to deficient data [11]. Figure 1.The four polar bear ecoregions and 19 subpopulations.…”
Loss of Arctic sea ice owing to climate change is the primary threat to polar bears throughout their range. We evaluated the potential response of polar bears to sea-ice declines by (i) calculating generation length (GL) for the species, which determines the timeframe for conservation assessments; (ii) developing a standardized sea-ice metric representing important habitat; and (iii) using statistical models and computer simulation to project changes in the global population under three approaches relating polar bear abundance to sea ice. Mean GL was 11.5 years. Ice-covered days declined in all subpopulation areas during 1979–2014 (median −1.26 days year−1). The estimated probabilities that reductions in the mean global population size of polar bears will be greater than 30%, 50% and 80% over three generations (35–41 years) were 0.71 (range 0.20–0.95), 0.07 (range 0–0.35) and less than 0.01 (range 0–0.02), respectively. According to IUCN Red List reduction thresholds, which provide a common measure of extinction risk across taxa, these results are consistent with listing the species as vulnerable. Our findings support the potential for large declines in polar bear numbers owing to sea-ice loss, and highlight near-term uncertainty in statistical projections as well as the sensitivity of projections to different plausible assumptions.
“…For example, declines in body condition were found in southern Beaufort Sea polar bears following years with reduced ice availability (Rode et al ., ). Polar bears in southern Hudson Bay have shown declines in body condition associated with a progressively longer ice‐free season (Obbard et al ., ). Past studies in western Hudson Bay have documented declines in body condition of adult males and females associated with earlier sea ice breakup, suggesting that a reduced foraging period affects nutritional condition (Stirling et al ., ).…”
Many species experience prolonged periods of fasting due to changes in habitat and food availability. Metrics that quantify energy reserves available during these periods allow for a better understanding of the interaction between environmental change and species survival. Body condition of polar bears has been assessed using morphometric and subjective indices, lipid content of adipose tissue, body composition models and, recently, bioelectrical impedance analysis (BIA). We assessed the utility of BIA and examined correlations among condition metrics for 134 free‐ranging polar bears on shore in western Hudson Bay in fall 2012–2013 and spring 2013–2014. We also examined long‐term inter‐annual and seasonal trends from 736 bears handled in 2004–2014. Total body fat, as estimated from BIA, was correlated with adipose tissue lipid content, energy density and fatness index, but not storage energy or skull width. Body condition was higher in adult and subadult females than males, consistent with energetic demands of gestation and lactation. Adult females had higher body fat in the fall than spring, and body fat decreased with increasing number of dependent offspring. Long‐term trends indicated a decline in body condition for all adult and subadult males and females. Although there were similar patterns among BIA and other established metrics, its limitations in the field suggest that BIA may not be the most efficient method of monitoring body composition in polar bears in comparison to other modeled metrics, such as energy density. Declines in polar bear body condition over time may be a reflection of contemporaneous changes in sea ice availability and population demography, and thus have implications for the long‐term conservation of this subpopulation.
“…The ecological niche of U. maritimus is the smallest in comparison with the other species and the only one entirely embedded within its estimated ‘M’ region, showing signals of realized niche reduction (Appendix S1: Table S.1.3; Figure S.1.3d); and also reaching along its distribution the limits of low temperature and precipitation conditions available on the Northern Hemisphere. Such niche divergence is most likely associated with its specialist habits, its notable adaptability, and biological features like hibernation and specialized diet, being the only land carnivore in the Arctic Circle that hunts seals and other sea mammals (Obbard et al, ), rendering it the bear species better adapted to a dry and cold climate. Liu et al () describe the evolution of U. maritimus as a rapid radiation and adaptation, in agreement with our results and with the findings of Rolland et al (), who indicate that species’ climatic niches can shift in a short time depending on their thermal regulation efficiency.…”
Aim
Assessing the relevance of niche evolution in the diversification patterns and geographical distribution of species driven by climate remains a challenge. We apply an integrative approach to evaluate the role of the environment on the phylogeography of bear species, incorporating fossil data to characterize the changes in the ecological niche through time. We evaluate our approach with the four extant species of bears within Ursus, the best represented taxon in the fossil record of the family Ursidae.
Location
Eurasia and North America.
Taxa
Asian black bear, Ursus thibetanus; American black bear, U. americanus; Brown bear, U. arctos; and Polar bear, U. maritimus.
Methods
We built a genetic and a geographical database from all published mitochondrial DNA sequences and of species occurrence records. We defined the most significant climatic variables based on each species ecological realm using correlation matrices, and characterized the ecological niches and existing environmental conditions with ellipsoid models. We inferred their current and Last Glacial Maximum (LGM) ecological niche modellings (ENMs) and compared the results with the fossil record. We estimated the times of divergence (d‐loop sequences) of lineages and applied a phyloclimatespace approach to discern the phylogeographical patterns along each species’ ecological space.
Results
Ecological niche modelling showed wider niches for U. thibetanus and U. americanus encompassing higher temperature and precipitation, while U. arctos and U. maritimus showed an opposite pattern. LGM models were consistent with the fossil record, predicting 55%–89% of the fossil occurrences (within their suitability areas). The phyloclimatespace revealed different degrees of environmental signal in the lineages’ phylogeographical patterns and ecological trajectories associated with LGM climatic conditions. Results indicated habitat tracking and ecological expansion since the LGM towards more extreme precipitation and temperature conditions for three species, except U. maritimus that showed ecological niche reduction.
Main Conclusions
Incorporating fossil information from the LGM improved our characterization and interpretation of ecological models, by enabling definition of the limits of the climatic conditions explored by the species in the past. Our approach also provided insights about the existing set of environmental conditions shaping the ecological niche divergence of Ursus bears. We were able to depict key features of the lineages’ evolutionary history, ecology and distribution, revealing the dynamics of niche occupation and the environmental signal on the phylogeographical patterns of Ursus.
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