Intra-population variation in the effects of sea ice reduction on an Arctic breeding bird

The Arctic is warming four times faster than any other region on Earth, leading to a dramatic reduction in sea ice. Even though sea ice plays a key role in the ecology of many Arctic species, few studies have assessed the consequences of its disappearance on the dynamics of Arctic wildlife populations. Moreover, the potential intra-population variations in such effects remain largely overlooked. Here, using a 40-year time series, we assessed how sea ice changes in a High Arctic fjord affected the population dynamics of common eiders Somateria mollissima via changes in their fine-scale breeding distribution and how these effects varied among breeding sites. More specifically, some islands within the fjord used to be connected by landfast ice to the shore most of the spring and thus to be accessible to one of the main eider predators, the Arctic fox Vulpes lagopus. Following the disappearance of spring sea ice in the fjord, these islands recently became disconnected much earlier in the season and thus inaccessible to foxes. We tested the prediction that these islands now represent favorable nesting grounds for common eiders and that the breeding eider populations on these islands increased following the sea ice retreat. Our results support our prediction and the role played by fox predation in mediating the sea ice effects. Even though the overall eider population in the fjord has slightly declined in the last decades, the recent sea ice reduction has led to a rapid colonization of newly available breeding habitats and to an increasing number of breeding eiders there. Inter-annual changes in sea ice did not significantly affect the number of eiders breeding on the islands that were historically isolated from fox predation. Ignoring such intra-population variation between breeding sites in predation risk masks the effects of sea ice reduction on eider dynamics. Our study illustrates the complex and fine-scale effects of sea ice disappearance on Arctic wildlife and the potential importance of predation in mediating these effects.