Regional biodiversity in an agricultural landscape: the contribution of seminatural habitat islands

An important goal of ecological compensation measures in agricultural areas is the conservation and enhancement of regional species diversity. However, some current European agri-environment schemes seem to be rather ineffective. A likely explanation is the lack of source populations in intensely cultivated landscapes. Remnants of natural and seminatural habitats can contribute to regional biodiversity in various ways: as essential habitats for specialised species, as stepping stones, and as temporary habitats for hibernation, larval development, or preovipository feeding. The overall percentage of arthropod species, for which seminatural habitats are an essential prerequisite for living in an agricultural landscape, was assessed with a 5 km long transect of 18 standardised trapping stations. The transect extended from an isolated area of wetland through intensely managed crop fields and grassland to an isolated semiarid meadow bordered by mixed forest. For more than 1000 arthropod species the spatial and temporal distribution of a one year's catch along the transect was interpreted with regard to their affinity to seminatural habitats. Experts were asked to judge questionable cases of apparent ubiquist species. All in all, more than 63% of all animal species (except for soil and water fauna) living in the agriculturally managed areas of the Limpach valley seem to depend on the presence of seminatural habitats. We conclude that remnant islands of natural or seminatural habitats provide the most important source populations for agri-environment schemes in order to enhance biodiversity in an otherwise depleted agricultural landscape.