Rapid deterioration of Aquatic Warbler Acrocephalus paludicola habitats at the western margin of the breeding range

Abstract

In the last German breeding area of the rapidly declining ''Pomeranian'' population of the Aquatic War- bler (Acrocephalus paludicola), the Lower Oder Valley National Park, we investigated changes in habitat suit- ability between 1993 and 2006 by combining monitoring results with repeated assessments of vegetation structure and composition, site conditions, and land use. Sites with recent Aquatic Warbler records showed shorter and sparser vegetation, a thinner litter layer, and a higher total plant species richness and cover of small and least competitive (CSR) species than abandoned or unoccupied sites. On a long-term study plot, during a period of late mowing and subsequent cessation of land use, vegetation height increased, the cover of CSR species decreased, and the site became abandoned by Aquatic Warblers. The probability of Aquatic Warbler occurrence was dependent on elevation and increased with the proportion of early mown or grazed area in the preceding year, with early use being most important on slightly higher elevated sites. This rapid deterioration of eutrophic habitats by delayed or discon- tinued land use is atypical for the majority of Aquatic Warbler breeding habitats. We conclude that both late or no land use and land use during the breeding season nega- tively affect the Pomeranian breeding sites and that a more sophisticated and flexible land management is urgently needed.