Agri-environmental schemes affect the trophic niche size and diet of common carabid species in agricultural landscapes: Numeric and trophic response of carabids

1. Carabids are abundant in agroecosystems and provide biocontrol services by feeding on weed seeds and pests. We compare the effects of two fundamentally different agri-environmental schemes (AES) on the numeric and trophic responses of four common carabid species: Anchomenus dorsalis, Poecilus cupreus, Pseudoophonus rufipes and Pterostichus melanarius. 2. In six cereal fields under organic farming and six sown flowering fields, 10 290 individuals of the four species were sampled to quantify their numeric response. The activity density of all species was higher in organically managed cereal fields compared to flowering fields. 3. We analysed naturally occurring stable isotope ratios of carbon and nitrogen in 138 carabid individuals to quantify isotopic niches and the diet contribution of soil- versus plant-based resources. Resources from the soil ('brown') energy channel were more important than resources from the plant ('green') energy channel in the diet of all species, particularly in P. melanarius. 4. Isotopic niches were larger for P. rufipes and P. melanarius in flowering fields than in organically managed cereal fields. Moreover, differences between AES types were observed in the diet contribution derived from both energy channels: A. dorsalis and P. cupreus incorporated higher proportions of plant-based resources in organically managed cereal fields compared to flowering fields. 5. Similar numeric, but different trophic responses suggest that AES type may alter arthropod food webs beyond simple changes in carabid densities. Such 'hidden' modifications in the resource utilisation of generalist predators deserve more attention to adequately evaluate the ecological consequences of AES.