The complexities of climate on the feeding ecology of the polar bear

There may be no more prevalent icon of climate change in the Arctic than the polar bear. The story is simple: a warming climate is causing reductions in the volume, extent and seasonal duration of Arctic sea ice. Polar bears use the sea ice surface for hunting seals, and reductions in sea ice result in lost hunting opportunities. Long-term studies have demonstrated that polar bear body condition, body size, reproduction, and survival is influenced by the extent and seasonal duration of sea ice. However, despite lost hunting opportunities as the underlying cause, no study has yet compared polar bear predation with climatic factors.

Seal kills were collected during research flights, easily spotted by the contrast of red blood on the white snow and ice. Samples collected from kill sites provided information on species, age-class and sex of prey.

Using observations of seals killed by polar bears in the spring between 1985-2011, we compared the influence of fine and large-scale factors on the likelihood of a predation event in our new paper "Multi-temporal factors influence predation for polar bears in a changing climate." Spring in the Arctic (generally defined as from maximum sea ice extent in late March to breakup in June) is a crucial feeding season. It has been estimated that polar bears may acquire as much as two-thirds of their energetic intake for the year during the spring. In our study, we found that while climatic factors were influential, the top predation model also included prey attributes. The reproductive activities and natality of the primary prey, ringed seals, accounted for much of the variation in the frequency of seal kills. Both fine and large-scale factors played a role, with local conditions influencing prey vulnerability as much as large-scale climatic settings.

After determining a top predation model, we wanted to know if it correlated with whether polar bears in our study area were fasting. Polar bears are unique from other bear species, in that they can enter a fasting state at anytime, lowering their metabolic rate, and using fat reserves for energy when prey are unavailable. Our top predation model was a significant predictor of the proportion of polar bears in a fasting state, suggesting the model was reflective of whether the population was likely to be feeding. We examined two periods in particular: 1985-86 and 2005-06. Despite the climatic conditions of both periods being nearly identical, we found that polar bears killed 50% less prey biomass in 2005-06 than in 1985-86. This correlated with an increase in the proportion of fasting polar bears, from 10% of the population in 1985-86 to 25% in 2005-06. Certainly the springs of 2005 and 2006 were much harder on polar bears of the Beaufort Sea than in the mid-1980s, and the difference between the years was likely related to the dynamics of the ringed seal population, for which further study is needed.

Recently, polar bears of the Southern Beaufort Sea subpopulation (overlapping with our study area) have come under the spotlight, due to a new study that found that their abundance may of declined by as much as 40% in the last decade. The decline was driven by poor survival between 2004-2006. However, the study found that metrics of sea ice were insufficient in explaining the trend. Given the importance of spring feeding to polar bears, the poor hunting conditions observed in our study may have contributed to the decline.

It is very likely that there is more to the polar bear - climate change story than just the dynamics of the sea ice. Consideration of the relationship between polar bears and their prey may open additional avenues to understanding the underlying drivers of how climate affects polar bears.

Nicholas W. Pilfold

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