Insects benefit from agri-environmental schemes aiming at grassland extensification

Abstract

One key target of agri-environmental schemes (AES) is maintaining biodiversity. However, if and how insect biomass and diversity benefit from grassland AES is not well known. Using Malaise trap samples from 72 grassland sites in four grassland dominated regions in Southern Germany, we investigated the effect of 1) local grassland AES (three levels: control, farm-wide, site-specific), and 2) an index of land use extensification (LEI, calculated for each site) on insect biomass and species richness of all insects, of five insect orders including all species (Coleoptera, Diptera, Hemiptera, Hymenoptera, Lepidoptera), and two pollinator groups (Syrphidae, Apiformes). Moreover, the effect of grassland AES in the surrounding landscape was quantified. Insect samples were subjected to metabarcoding to create DNA barcode sequences which were then compared with reference libraries (BOLD; NCBI) and additionally classified using the Ribosomal Database Project (RDP) classifier. Resulting taxonomies were combined into a consensus taxonomy table and the potential minimum number of species per sample was estimated. Statistical analyses were carried out using Generalized linear mixed models and Bayesian inference. Our results indicated that species richness of many insect groups benefited from AES. As predicted by the regression model, presence of site-specific grassland AES increased the species richness of all insects (∼8%), Hymenoptera (∼27%), Lepidoptera (∼35%) and Syrphidae (∼26%). While Apiformes benefited most, we found an average increase of species richness in the presence of farm-wide AES by ∼82% and even more with site-specific AES ∼120%. We found that an increasing LEI, and thus more extensive management, positively affected insect diversity in grassland habitats. No effect of local AES and LEI on insect biomass was found. Moreover, the amount of grassland AES in the landscape was less important for insect diversity. We conclude that many grassland insects (especially pollinators) benefit from extensive management. However, we showed that the studied AES variants comprise variable management intensities hampering general AES recommendations for promoting insects. AES programs should, therefore, specifically reward extensification measures that were shown to benefit insect diversity. The integration of the land use extensification index (LEI) as a tool in the AES framework seems a promising approach to preserve and promote of insect diversity.