An Arctic predator-prey system in flux: climate change impacts on coastal space use by polar bears and ringed seals.

Charmain D Hamilton,Rolf A Ims,Kit M Kovacs,Christian Lydersen,Jon Aars,Elizabeth Derryberry

doi:10.1111/1365-2656.12685

Charmain D Hamilton, Rolf A Ims + Show 4 more

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https://doi.org/10.1111/1365-2656.12685

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Abstract

Climate change is impacting different species at different rates, leading to alterations in biological interactions with ramifications for wider ecosystem functioning. Understanding these alterations can help improve predictive capacity and inform management efforts designed to mitigate against negative impacts. We investigated how the movement and space use patterns of polar bears (Ursus maritimus) in coastal areas in Svalbard, Norway, have been altered by a sudden decline in sea ice that occurred in 2006. We also investigated whether the spatial overlap between polar bears and their traditionally most important prey, ringed seals (Pusa hispida), has been affected by the sea-ice decline, as polar bears are dependent on a sea-ice platform for hunting seals. We attached biotelemetry devices to ringed seals (n=60, both sexes) and polar bears (n=67, all females) before (2002-2004) and after (2010-2013) a sudden decline in sea ice in Svalbard. We used linear mixed-effects models to evaluate the association of these species to environmental features and an approach based on Time Spent in Area to investigate changes in spatial overlap between the two species. Following the sea-ice reduction, polar bears spent the same amount of time close to tidal glacier fronts in the spring but less time in these areas during the summer and autumn. However, ringed seals did not alter their association with glacier fronts during summer, leading to a major decrease in spatial overlap values between these species in Svalbard's coastal areas. Polar bears now move greater distances daily and spend more time close to ground-nesting bird colonies, where bear predation can have substantial local effects. Our results indicate that sea-ice declines have impacted the degree of spatial overlap and hence the strength of the predator-prey relationship between polar bears and ringed seals, with consequences for the wider Arctic marine and terrestrial ecosystems. Shifts in ecological interactions are likely to become more widespread in many ecosystems as both predators and prey respond to changing environmental conditions induced by global warming, highlighting the importance of multi-species studies.

Highlights

Climate change is expected to have large consequences for the structure and function of ecosystems (IPCC 2014)
We investigated whether the spatial overlap between polar bears and their traditionally most important prey, ringed seals (Pusa hispida), has been affected by the sea-ice decline, as polar bears are dependent on a sea-ice platform for hunting seals
If ice conditions for seal hunting have deteriorated near tidal glacial fronts, polar bears may have decreased the amount of time spent in these areas, in which case polar bears could be expected to increase their use of terrestrial resources, such as bird colonies. To assess these alternative hypotheses, the present study investigated the habitat and space use patterns of polar bears that remain in coastal areas in Svalbard during the spring, summer and autumn, focusing on monthly home range size, the distance travelled per day and the association with environmental covariates such as tidal glacier fronts and bird colonies to determine whether the space use patterns of polar bears were affected by the sea-ice collapse

Summary

Introduction

Climate change is expected to have large consequences for the structure and function of ecosystems (IPCC 2014). Arctic endemic marine mammals are dependent on sea ice and these species are sensitive to changes in this habitat (Kovacs, Lydersen, Overland, & Moore, 2011; Laidre, Stern et al 2015). This is because of both the habitat loss issue and the sensitivity these species have to climate change due to their generally high trophic position(s) (see Doney et al, 2012; Gilman, Urban, Tewksbury, Gilchrist, & Holt, 2010). Sea-ice declines represent losses of shelter from inclement weather, protection from open-water predators (i.e. killer whales [Orcinus orca]) and many forms of human disturbance, foraging habitats, platforms for birthing, nursing, resting and moulting in the case of ice-associated seals and in a loss of hunting habitat and transport platforms for polar bears (Ursus maritimus; see Kovacs et al, 2011; Laidre, Stern et al 2015: Stirling & Derocher, 2012; for more details)

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