Temporary non-crop habitats within arable fields: The effects of field defects on carabid beetle assemblages

Abstract

Landscape heterogeneity and higher complexity generally increase biodiversity in agroecosystems. Carabid beetles represent abundant and important predators of pests and weed seeds in temperate agroecosystems and are affected by landscape structure. Several studies have described the impact of permanent non-crop habitats such as woodlots, hedgerows, and grassy margins on carabid assemblages. However, temporal non-crop habitat islands within arable fields have been rarely investigated. The aim of this study was to investigate spatial distribution of carabid beetles within oilseed rape fields having temporary non-crop habitats (field defects). Field defects are areas where sown plants poorly develop due to sowing failures or extreme local conditions (soil humidity, missing nutrients). In twenty oilseed rape fields, we studied carabid assemblages collected with pitfall traps in three habitat types (field interiors, field defects, and boundaries between them) and in two sampling periods (spring and summer). Both activity-density and species richness were lower in field defects than in boundaries and field interiors during both sampling periods, indicating that field defects were not a preferred habitat for carabids. Activity-density and species richness significantly increased from spring to summer in all habitat types. Species composition of carabid assemblages significantly differed between field defects and field interiors or boundaries. Field defects were characterised by impoverished carabid assemblages and the presence of few indicator species. Interestingly, field defects with well-developed plant cover hosted carabid assemblages with species richness comparable to field interiors, indicating that re-sowing of field defects can support carabid populations within arable fields. However, the consequences of re-sowing on other arthropod taxa, e.g., insects requiring habitats with bare ground, and on populations of rare weeds need to be evaluated. The lack of effects of field defect size on carabid assemblages indicated that carabid beetles react to even very small patches with unsuitable conditions (e.g., very low humidity, high temperature or food scarcity).