| Summary: | Apex predators regulate ecosystems through top-down processes. In the last century many predator populations crashed due to anthropogenic impacts, but recently some have recovered and are re-colonizing old areas, as well as expanding to new suitable habitats. Rapid loss or return of apex predators can destabilize ecosystems and cause consequences for their prey species as well as for livestock. In the Baltic Sea area, the white-tailed eagle (Haliaeetus albicilla) was virtually absent due to persecution and pollutants during the mid-20th century. The rapid growth of the eagle population from the 1980s onward initiated by extensive conservation efforts has not only brought the species back from the brink of extinction, but also caused growing predation pressure on its prey species, in particular the common eider (Somateria mollissima), which is the major prey of the eagles in the Åland islands and especially in the outer archipelago. In Lapland in northern Finland, where husbandry of semi-domestic reindeer is a traditional livelihood, concerns are rising that the expanding white-tailed eagle population poses a threat to reindeer calves. As top predators, the white-tailed eagles are sensitive to bioaccumulating hazardous substances. Despite the decreased levels of pollutants, the Baltic Sea is still the world’s most heavily polluted sea, while mercury levels in the major water bodies in Lapland, two artificial water reservoirs, are higher than in the Baltic. For successful conservation of both the white-tailed eagle and its prey species research is needed regarding the diet of the eagles and the mechanisms shaping the diet. This system also provides an opportunity to study the predator-prey dynamics following the rapid return of an apex predator to a system from where it has been virtually absent. In this thesis I cover (i) the prey use and nesting habitat choice of white-tailed eagles nesting in the Finnish coast and Lapland, (ii) the connections between the prey use and the nesting habitat, (iii) the use of ...
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