Abstract
Climate change is predicted to expand the ice-free season in western Hudson Bay and when it grows to 180 days, 28–48% of adult male polar bears are projected to starve unless nutritional deficits can be offset by foods consumed on land. We updated a dynamic energy budget model developed by Molnar et al. to allow influx of additional energy from novel terrestrial foods (lesser snow geese, eggs, caribou) that polar bears currently consume as part of a mixed diet while on land. We calculated the units of each prey, alone and in combination, needed to alleviate these lethal energy deficits under conditions of resting or limited movement (2 km d-1) prior to starvation. We further considered the total energy available from each sex and age class of each animal prey over the period they would overlap land-bound polar bears and calculated the maximum number of starving adult males that could be sustained on each food during the ice-free season. Our results suggest that the net energy from land-based food, after subtracting costs of limited movement to obtain it, could eliminate all projected nutritional deficits of starving adult male polar bears and likely other demographic groups as well. The hunting tactics employed, success rates as well as behavior and abundance of each prey will determine the realized energetic values for individual polar bears. Although climate change may cause a phenological mismatch between polar bears and their historical ice-based prey, it may simultaneously yield a new match with certain land-based foods. If polar bears can transition their foraging behavior to effectively exploit these resources, predictions for starvation-related mortality may be overestimated for western Hudson Bay. We also discuss potential complications with stable-carbon isotope studies to evaluate utilization of land-based foods by polar bears including metabolic effects of capture-related stress and consuming a mixed diet.
Highlights
Climate change is causing the sea ice in arctic regions to melt earlier in spring (e.g., [1,2]), leading to a trophic mismatch between polar bears and their primary spring prey, the pups of ringed seals (Phoca hispida) [3]
Molnár et al [8] used a mechanistic approach to predict polar bear survival that involved establishing a relationship between physical measures and body composition to determine how energy stores are incrementally depleted as polar bears spend longer periods on land during the ice-free season
Molnár et al [8] used the model to predict the proportions of adult males that would starve to death as the ice-free season expands to 180 days, a scenario predicted as ice conditions worsen in response to climate change
Summary
Climate change is causing the sea ice in arctic regions to melt earlier in spring (e.g., [1,2]), leading to a trophic mismatch between polar bears and their primary spring prey, the pups of ringed seals (Phoca hispida) [3]. In western Hudson Bay, polar bears have historically relied on the energy from hunting these seal pups to sustain them through the ice-free period on land until the ice reforms in fall [5,6]. Molnár et al [8] used a mechanistic approach to predict polar bear survival that involved establishing a relationship between physical measures (size and structure) and body composition to determine how energy stores are incrementally depleted as polar bears spend longer periods on land during the ice-free season. The model was parameterized with measurements of captured polar bears in western Hudson Bay and daily maintenance costs that are based on past patterns of average daily weight loss experienced by the bears until they returned to the ice [9,10]. The model takes into account somatic maintenance costs and the effects of limited movements (2 km d-1) but does not allow for energy influx into the system from consuming additional food on land
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