How can we preserve and restore species richness of pollinating insects on agricultural land?

During recent decades, concern about the loss of biodiversity on agricultural land has increased, and semi-natural grasslands have been highlighted as critical habitats. Temperate European agricultural landscapes require distinct and appropriate management to prevent further impoverishment of the flora and fauna. This is especially urgent for pollinating insects that provide important ecosystem services. Our aim was to examine how species richness of three important groups of pollinating insects; solitary bees, butterflies and burnet moths are related to different farm characteristics, and if there are any differences between these three groups. A further aim was to test if red-listed species are related to any farm characteristics. Species richness of solitary bees, butterflies and burnets was measured on all semi-natural grasslands at 16 farms in a forest-dominated area of 50 km(2) in southern Sweden, using systematic transect walks in April to September 2003 (only butterflies and burnets) and 2005. Species richness of solitary bees and butterflies was intercorrelated, both before and after controlling for the area of semi-natural grassland. Species richness of solitary bees increased with the area of semi-natural grassland. After controlling for the effect of the area of semi-natural grassland species richness was strongly positively related with the density of the plant Knautia arvensis and negatively related with the proportion of grazed grassland. The results were similar for solitary bees and butterflies. The number of red-listed solitary bees was positively related to the proportion of meadows with late harvest (after mid-July) and decreased with increased farm isolation. The number of burnet species (all red-listed) was positively related to vegetation height, flower density and the proportion of meadows with late harvest on a farm. Areas with a high density of K. arvensis and with traditional hay-meadow with late harvest present, harbour most species. Promoting traditional hay-meadows, late extensive grazing and the herb K. arvensis, people managing agricultural biodiversity can encompass high species richness of pollinating insects and support red-listed species. Further, we suggest that the density of K. arvensis at a farm can be used as a biodiversity indicator, at least for pollinating insects.