Effects of spatial and temporal heterogeneity of agricultural landscapes on carabid beetles assemblages (Coleoptera : Carabidae)

Previous work has demonstrated that landscapes which contain significant areas of semi-natural habitats support higher biodiversity and better ecosystem services. However, in agricultural landscapes the effects of the cropped area itself are often disregarded, despite the fact that the cultivated mosaic can be highly heterogeneous in space and time. In this study, we investigated the relative effects of the spatial and temporal heterogeneity of the crop mosaic on carabid beetles assemblages in agricultural landscapes of Brittany, western France. Spatial heterogeneity is characterized by the crop composition and configuration of the sampling year, while temporal heterogeneity induced by crop sequences is defined by the rate of crop rotation and the changes in crop composition over the last five years. We assume that croplands spatially heterogeneous should provide resources for more different species, and benefit to species that use more than one crop field type. On the other hand, crop's temporal heterogeneity should benefit species adapted to open and dynamic environments, which could increase carabid beetle abundances in sampled fields. Within the European FarmLand project (www.farmland-biodiversity.org), carabids were sampled using pitfall traps in 40 winter wheat fields, distributed along gradients in crop composition, crop configuration, and crop dynamic. The results show that both spatial and temporal heterogeneity have an influence on the structure and composition of carabid beetles assemblages, affecting essentially the open ground specialists adapted to dynamic environments. Changes in crop diversity over the last five years increase carabid abundances but decrease the evenness of the community; and crop diversity of the sampling year has a positive impact on the diversity of the community. However, these two levels of heterogeneity seems to affect the species differently according to their ecological traits. Species with high dispersal abilities are affected by spatial heterogeneity, while less mobile species are only influenced by crop dynamic.