Contrasting resource-dependent responses of hoverfly richness and density to landscape structure

Hoverflies (Diptera: Syrphidae) in agroecosystems have gained much attention recently because the larvae of some species are efficient control agents of crop aphids, and adult hoverflies provide pollination services to wild flowers and flowering crops. We assessed the density and species richness of hoverflies in 32 calcareous grasslands, which constitute a semi-natural habitat for adult hoverflies, by means of six transect walks from April to September 2004. Our results show that local habitat factors and landscape factors influenced hoverfly communities, and that their effects on hoverfly richness and density were quite contrary. Hoverfly species richness was affected by factors related to resource heterogeneity such as species richness of flowering plants, area of grassland habitat, and landscape diversity, which all imply the availability of diverse micro- and macrohabitats for adults and larvae. Hoverfly density, in contrast, depended on factors related to resource quantity, Such as the amount of pollen and nectar resources for adults and the amount of larval macrohabitats in the surrounding matrix. Therefore, both adult and larval habitat requirements have to be considered when analysing hoverfly communities in agricultural landscapes. Species guilds responded to specific land-use types Such as annual crops and woodland at different spatial scales, indicating variation in species' mobility and in the degree of spillover effects among neighbouring landscape elements. (C) 2008 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.