Calcareous grassland fragments as sources of bee pollinators for the surrounding agricultural landscape

Abstract

In Central Europe, agricultural intensification has led to increasing fragmentation and loss of semi-natural habitats. In turn, ecosystem services such as pollination are being lost when insect pollinators depend on semi-natural habitats. Calcareous grasslands are a highly threatened, biodiverse type of semi-natural habitat that may substantially support wild pollinators and pollination services to surrounding habitats. Here, we studied spillover of pollinators and pollination services from calcareous grassland fragments of different sizes into the surrounding landscape for solitary and social bees, as well as hoverflies. We selected eight grassland fragments of varying sizes (0.05–9.41 ha) surrounded by simplified agricultural landscapes near Göttingen, Germany. To quantify pollination spillover, we established Phacelia tanacetifolia (Boraginaceae) plots at distances ranging from 0 to 400 m from grassland edges alongside field roads. Our experiment revealed the importance of calcareous grassland fragments as sources of solitary bees, of which visitation to Phacelia plants decreased with increasing distance from fragments. Larger grassland fragments supported more than twice as many solitary bees as smaller ones. The limited foraging range of solitary bees appeared to be compensated by other groups, such as bumblebees and hoverflies, which were less affected by distance, suggesting a greater forage radius and/or independence from the grassland fragments as habitat. Seed production of Phacelia plants increased with overall pollinator visitation and solitary bee visits specifically, but did not decrease with distance. In conclusion, calcareous grasslands need to be conserved or restored as major bee habitats, which support spillover of pollination services into the adjacent agricultural landscape.