Abstract
In this study we tested the ability of a mechanistic model (Niche Mapper™) to accurately model adult, non-denning polar bear (Ursus maritimus) energetics while fasting during the ice-free season in the western Hudson Bay. The model uses a steady state heat balance approach, which calculates the metabolic rate that will allow an animal to maintain its core temperature in its particular microclimate conditions. Predicted weight loss for a 120 day fast typical of the 1990s was comparable to empirical studies of the population, and the model was able to reach a heat balance at the target metabolic rate for the entire fast, supporting use of the model to explore the impacts of climate change on polar bears. Niche Mapper predicted that all but the poorest condition bears would survive a 120 day fast under current climate conditions. When the fast extended to 180 days, Niche Mapper predicted mortality of up to 18% for males. Our results illustrate how environmental conditions, variation in animal properties, and thermoregulation processes may impact survival during extended fasts because polar bears were predicted to require additional energetic expenditure for thermoregulation during a 180 day fast. A uniform 3°C temperature increase reduced male mortality during a 180 day fast from 18% to 15%. Niche Mapper explicitly links an animal’s energetics to environmental conditions and thus can be a valuable tool to help inform predictions of climate-related population changes. Since Niche Mapper is a generic model, it can make energetic predictions for other species threatened by climate change.
Highlights
Ecological effects of recent climate warming on plants and animals have been documented for nearly every major taxonomic group around the globe from the equator to the poles [1]
Niche Mapper Overview Niche Mapper is a mechanistic model that has been previously described in detail and tested with a wide range of animal species including birds and mammals ranging from mice (Mus spp.) to oryx (Oryx leucoryx) through comparisons of observed to predicted metabolic rates and observed distributions across the landscape to predicted distributions based on energetic requirements (e.g., [27,28,29,30,31,32,33])
Radiant fur temperatures calculated by Niche Mapper correspond with those measured by Best [48] across a range of temperatures and wind speeds, differing from measured temperatures by,7.5% (Fig. 3)
Summary
Ecological effects of recent climate warming on plants and animals have been documented for nearly every major taxonomic group around the globe from the equator to the poles [1]. There has been an accelerating declining trend in Arctic sea ice extent in all months since 1979, a trend that is expected to continue in the future, and which has already impacted polar bear habitat quality [3,7,8]. Declines in sea ice extent and other alterations to the polar bear’s sea ice habitat are predicted to have a multitude of effects on the bears [9,10,11]. Population, body size and birth rate declines, presumably as a result of sea ice changes, have already been documented in some polar bear populations [9,12,13]
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