Re-established grasslands on farmland promote pollinators more than predators

Abstract

Agricultural intensification is among the major threats to farmland biodiversity and associated ecosystem services. Agri-environment schemes in Europe aim to counteract the continuous decrease of permanent grassland habitat and related decline of biodiversity in agricultural landscapes. We established new grasslands adjacent to permanent old grasslands to study effects on arthropod pollinators and predators. Over three years, we assessed the effects of newly established grasslands on activity density, species richness and assemblages of pollinators (syrphids, bumblebees, solitary bees) and ground-dwelling predatory arthropods (carabids, spiders, ants). Pollinator activity density and species richness of all three pollinator groups approximately doubled in newly established grasslands compared to old grasslands, indicating suitability of new grasslands as pollinator feeding habitat. Predator responses were less consistent. Species richness and activity density of carabids and ants did not increase in new grasslands, indicating slower adaptation to new habitats. Spider activity density increased in the newly established grasslands and in adjacent cereal fields, but no significant differences in species richness compared to old grasslands were observed. Bumblebees and solitary bees showed distinct distance decays in cereal fields. Syrphid activity density declined in cereal fields far away from the newly established grasslands, but not nearby. Carabids increased and ants decreased in species richness and activity density with increasing distance from old grasslands in cereal fields and newly established grasslands alike. In contrast, spider species richness and activity density decreased with increasing distance from old grasslands in cereal fields only, but there was no distance decay in newly established grasslands. While pollinators utilized the additional floral resources within the whole area of newly established grassland quickly, predators reacted much slower. Even after three years, there was no measurable response of carabid and ant activity density and only a minor response of spiders. This clearly shows that short-term measures are not sufficient to sustainably conserve farmland biodiversity. The conservation of permanent grassland and landscape heterogeneity should be prioritized. However, the restoration of new grassland should also be encouraged by agri-environment schemes as a supplement, to enhance functional biodiversity in agricultural landscapes.