Configurational Landscape Heterogeneity: Crop-Fallow Boundaries Enhance the Taxonomic Diversity of Carabid Beetles and Spiders

Arthropod biodiversity in agricultural landscapes is favoured by high levels of compositional and configurational heterogeneity. In terms of composition, the high value of non-crop habitats is firmly established. In contrast, the causes underlying positive effects of configurational heterogeneity, i.e. patch size and boundary length, are less well understood. We here test the hypothesis that boundaries comprise valuable habitats in its own right by enhancing the taxonomic diversity of carabid beetles and spiders. We used pitfall traps placed along transects running from the interior of wheat fields across the habitat boundary to the interior of unmanaged grassland fallows. Taxonomic diversity was highest around the boundaries due to (1) spillover effects but also (2) species showing a distinct preference for edge habitat. For carabids, habitat boundaries harboured distinct species assemblages. Grassland fallows had, especially in spiders, positive effects on the taxonomic diversity of crop fields close to the boundary, while no negative effects of crop fields on adjacent fallows could be found. Overall, taxonomic diversity was higher in grassland fallows than in crop fields, though differences were surprisingly small. The high taxonomic diversity found for boundaries suggest that these may comprise valuable habitats in its own rights, which in turn may causally underpin positive effects of configurational heterogeneity on arthropod diversity in agroecosystems. Thus, increasing configurational heterogeneity, e.g. by promoting small field sizes and high edge density, seems an important tool for preserving farmland biodiversity.