Habitat-specific population trajectories in boreal waterbirds: alarming trends and bioindicators for wetlands

Identification of conservation priorities is a fundamental issue for successful nature conservation. This can be done by looking at population trends of species in different habitats. Boreal zone is the key breeding area for many Eurasian waterbirds. Wetlands of the boreal zone can be roughly classified into two different groups: nutrient-poor oligotrophic and nutrient-rich eutrophic water ecosystems. Earlier work has shown that eutrophic lake-specialist species have declined compared with generalists and species preferring oligotrophic lakes. However, it is not known if species that live in both habitats show habitat-specific population trends. We used monitoring data of breeding waterbirds in Finland from 1986 to 2013 to evaluate habitat-specific population trends in five generalist duck species, which mostly overwinter in the coastal waters of West Europe. Population trend of tufted duck was highly significantly more positive in oligotrophic wetlands compared with populations in eutrophic wetlands, and two other species had similar but weaker effects. In addition, we established bird population state indicators for oligotrophic and eutrophic water ecosystems in Finland. The indicators revealed that the populations in eutrophic wetlands have on average halved since early 1990s. The populations in oligotrophic wetlands have remained stable, except a moderate decline since the late 2000s. Our findings support the idea that the waterbird populations of the eutrophic wetlands are in serious trouble. This is likely due to over-eutrophication of these ecosystems. Conservation and management actions should be urgently taken to halt the loss of biodiversity in these globally threatened freshwater ecosystems.