Complementarity of grasslands and cereal fields ensures carabid regional diversity in French farmlands
Résumé
Carabids (ground beetles) are important beneficial insects that contribute to biological control of pests and weeds in crop production. They can be found in different agricultural landscape habitats. However, little has been studied about the composition and similarity of carabid assemblages between two major land covers types: cropland and grassland. In a context of declining grassland areas in farmlands, highlighting complementarities of these two land covers is crucial to encourage land use planning that favors biodiversity. The objective of this study was to evaluate the relative and shared contribution of cereal crops and grasslands to carabid diversity in farmlands. Carabids were sampled in 104 pairs of cereal fields and grasslands in 3 agricultural regions of southeastern France. Carabid diversity was further analyzed with multiple regression models, including different local parameters. The correlations between assemblage similarities and geographical distance of sampled cereal fields and grasslands were also studied. Overall, 115 different species were found, 82 in cereal fields and 95 in grasslands. Per site species number and activity-density were higher in the two study regions with higher grassland cover. Evenness of carabid assemblages were higher in grasslands. Cropland and grassland remained strongly distinct, but assemblages showed higher similarity up to 4 km distance from each other. The study illustrates the major interest in preserving grassland within farmland for land use planners and/or policy makers. We found that grasslands and croplands complement each other. Though they host distinct communities, they provide continuous and different resources to the species throughout the year.