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 reindeer calves as prey, (iv) the association between nesting white-tailed eagles and the spatiotemporal population trends of the common eider, and (v) the consequences of prey use and nesting site choice on the mercury burden in white-tailed eagle nestlings in Lapland.
I found that the prey use of white-tailed eagles changed in time in the archipelago, and that it both in the archipelago and in the inland was influenced by the habitat in the nesting territories, which reflects the occurrence of prey species. In the inland, the white-tailed eagles preferred territories with higher proportions of lakes, peatbogs, and marshlands, which coincides with the higher occurrence of their preferred food source, fish. In the archipelago the main prey of the white-tailed eagles was common eider, which population in the Baltic has declined rapidly since a peak in 1997. The spatiotemporal changes in the core of the eider population distribution were influenced by the proximity of nesting white-tailed eagles. The eiders declined most in the outer archipelago and on unforested islands in the proximity of eagle nest. On the contrary, the population increased in the inner archipelago in areas with eagle nests. Finally, the prey use and nesting habitats influenced the eagles themselves. Elevated mercury burdens in white-tailed eagle nestlings in Lapland were linked with a diet on high trophic-level species and especially pike, the most important prey species in the area, as well as the proximity to a point source; the artificial reservoir of Porttipahta.
As flexible opportunists, the white-tailed eagles should not be food-deprived even though some prey species would decline. Likewise, the prey species should not be over-exploited, as the eagles use alternative prey species when a species decline. Population-level eagle-induced shifts has moved the core of the eider population towards safer nesting environments in the inner archipelago. However, the very rapid decrease of eiders calls for close monitoring and conservation efforts of the species.
As top predators, the white-tailed eagles are prone to accumulation of pollutants and are as such excellent sentinel species for environmental pollution. However, consequently they are also vulnerable and thus, even though the population today is viable, a continuum of the thorough monitoring of the white-tailed eagles is important.
The ecosystem resilience is today severely challenged by anthropogenic impacts. Returning apex predators can improve the states of the ecosystems by e.g. preventing over-grazing, controlling the growth of prey populations and restricting invasive mesopredators. Successful co-existence with humans requires a general acceptance of the predators which only can be gained through cooperation and involvement of stakeholders and inhabitants that are affected by the matter.