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 rolein the ecology of many Arctic species, few studies have assessed the conse-quences of its disappearance on the dynamics of Arctic wildlife populations.Moreover, the potential intra-population variations in such effects remainlargely overlooked. Here, using a 40-year time series, we assessed how sea icechanges in a High Arctic fjord affected the population dynamics of commoneiders Somateria mollissima via changes in their fine-scale breeding distribu-tion and how these effects varied among breeding sites. More specifically,some islands within the fjord used to be connected by landfast ice to the shoremost 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 inthe fjord, these islands recently became disconnected much earlier in the sea-son and thus inaccessible to foxes. We tested the prediction that these islandsnow represent favorable nesting grounds for common eiders and that thebreeding eider populations on these islands increased following the sea iceretreat. Our results support our prediction and the role played by fox predationin mediating the sea ice effects. Even though the overall eider population inthe fjord has slightly declined in the last decades, the recent sea ice reductionhas led to a rapid colonization of newly available breeding habitats and to anincreasing number of breeding eiders there. Inter-annual changes in sea icedid not significantly affect the number of eiders breeding on the islands thatwere historically isolated from fox predation. Ignoring such intra-populationvariation between breeding sites in predation risk masks the effects of sea icereduction on eider dynamics. Our study illustrates the complex and fine-scale effects of sea ice disappearance on Arctic wildlife and the potential importanceof predation in mediating these effects.