Determinants of local species richness of diurnal Lepidoptera in boreal agricultural landscapes

The determinants of local species richness of diurnal Lepidoptera were studied in 68 agricultural landscapes in five geographic regions by sampling field margins, road verges, forest edges and patches of semi-natural grassland. Butterflies and day-active moths were counted and habitat quality was measured in 1191 separate 50 m transects. Generalized linear mixed models (GLMM) were built to explain patterns of species richness by first adjusting for the effects of geographic location and weather conditions during the butterfly counts. Habitat type and quality largely determined local species richness, whereas the explanatory power of the local habitat quantity and management variables was limited. Species richness was highest in patches of semi-natural grassland, second highest in forest edges and lowest in road and field margins surrounded by cultivated fields. Species richness increased with abundance of nectar flowers and margin width, and decreased with increasing soil moisture and within-patch openness. Landscape openness showed a unimodal response with a maximum species richness at intermediate values. Largely the same variables affected species richness of moths and butterflies, but moths appeared to be more sensitive to negative effects of windiness and intensive management and less dependent on abundant flowers and increasing margin width than butterflies. The results highlight the significance of even small patches of semi-natural grasslands and open, sunny forest edges for species richness in typical modern farmland with relatively intensive agriculture. The effectiveness of agri-environment schemes in promoting biodiversity could be increased by directing more efforts to maintenance and restoration of these habitats.