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.

Polar bears (Ursus maritimus) and ringed seals (Pusa hispida) have a strong predator–prey relationship and are facing climate-associated Arctic habitat loss and harmful dietary exposure to total mercury (THg) and other pollutants. However, little is known about whether both species inhabiting the same area exhibit similar temporal patterns in Hg concentration, niche dynamics, and body fat indices. We used THg, δ13C, and δ15N values of western Hudson Bay polar bear hair (2004–2016) and ringed seal muscle samples (2003–2015) to investigate temporal trends of these variables and multidimensional niche metrics, as well as body fat indices for both species. We found a decline in THg concentration (by 3.8% per year) and δ13C (by 1.5‰) in ringed seals suggesting a change in feeding habits and carbon source use over time, whereas no significant changes occurred in polar bears. In contrast, the polar bear 3-dimensional niche size decreased by nearly half with no change in ringed seal niche size. The δ13C spacing between both species increased by approximately 1.5× suggesting different responses to annual changes in sympagic-pelagic carbon source production. Ringed seal body fat index was higher in years of earlier sea ice breakup with no change occurring in polar bears. These findings indicate that both species are responding differently to a changing environment suggesting a possible weakening of their predator–prey relationship in western Hudson Bay.

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The contribution of local (e.g., competition) and regional (e.g., dispersal) processes in the structure of communities remains an unresolved issue. In general, a tendency to assume local processes to be deterministic and regional to be stochastic dominates, although it is challenged. Fortunately, it can be cast as a testable proposition: if correct, the degree of determinism in the final community structure might indicate which process is more prominent in the control of community structure. However, recent findings have also suggested that stochastic patterns can arise from local processes and that dispersal can homogenize communities, which would make them appear deterministic irrespective of the mechanism involved. To evaluate these compet� ing expectations we conducted an experiment where the initial communities had the same composition and species abundances. We hypothesized that if local processes dominate, then arrays of communities will show divergence of community structures whether connected by dispersal or not (i.e., being fully isolated). Alternatively, if regional processes dominate, the dispersal connected communities should converge while isolated ones should not. We found, however, that both groups of experimental communities showed similar patterns of change � a decline in similarity and a tendency to diverge. This suggests that biological interactions, demographic stochasticity, or both, exert noticeable control over community structure such that they reduce simi� larity among replicate communities and diversify their final states. We speculate that these mechanisms enhance potential for species additions, particularly in conjunction with factors such as dispersal and the size of the regional species pool.