Die Bedeutung kleiner Feuchtgebiete für den Artenschutz: Synthese einer Populationsstudie an der Rohrammer Emberiza schoeniclus

摘要

Many species persist in fragmented populations as a consequence of past and ongoing habitat destruction. Despite this, the importance of remnant fragments varying in size for the conservation of species is often unclear. From 2002 to 2006, we studiedthe role of small and large wetland fragments (2-247 ha) within a 200 km2 area in northeastern Switzerland for the persistence of the Reed Bunting Emberiza schoeniclus, a characteristic species of wetlands. Results from both artificial nest experiments and real nests showed that nest success increased with vegetation cover around and above the nest, distance from the water-sided reed edge, and to a lesser extent also with distance from the land-sided reed edge, and the size and shape (expressed as the ratio between edge length and area of the patch) of the reed patch containing the nest. These findings suggest that nest success of the Reed Bunting fundamentally depends on the quality of the reed patches: large and old reed patches with dense vegetationare habitats of good quality and conducive to nest success, whereas young and small patches, such as one-year-old reed stripes ("rotational fallows"), are suboptimal habitats. Probability of nest predation was not related to whether or not a nest was visited by observers. Reproductive performance of the Reed Bunting in small and large wetland fragments did not basically differ, although small fragments were better reproductive grounds in one of four years. Recruitment probability, defined as probability that individuals born and ringed in our study area were re-sighted as breeders in a subsequent year, was not related to fragment size either. Based on demographic and population genetic analyses, the local populations in the different fragments were strongly connected, thus forming a patchy population (and not a metapopulation, despite the fragmented distribution). Small and large fragments turned out to be mostly sinks, and the annual growth rate of the entire population network was insufficient forself-maintenance. Both findings imply that the persistence of the local populations and of the entire patchy population depended on immigration, which was found to annually range from 43.8 to 61.4 %. We conclude from this study that small and large wetland fragments contributed equally to the population network of the Reed Bunting in northeastern Switzerland, even though they have to be considered as sinks from a demographic perspective. For the conservation of the Reed Bunting, future management of wetlands should aim at sparing large reed patches along the water's edge to provide adequate nest sites.