Insect pollinators profit from moderate diversification of intensively used grasslands

Many grasslands of Central Europe reached a high productivity due to intensification, but to the cost of plant diversity. This loss is further reflected in the decrease of insect pollinators like wild bees and hoverflies that depend on diverse plants as nectar and pollen resources. Schemes for in-field measurements to the benefit of biodiversity are rarely considered in such intensively managed grasslands, so we developed five seed mixes of increasing diversity to investigate their value to insect pollinators and monitored them over a three-year period under regular agricultural management with four to five cuts per year. The abundance of wild bees increased with the overall floral resource availability, while hoverflies showed an increased abundance with the presence of ribwort plantain. The amount of bumblebees doubled from the first to the following study years, suggesting a benefit of the mass-flowering clover to colony establishment for the following year. The benefit was mainly to generalists with good flight abilities. While there were no significant differences in pollinator species richness between the seed mixes, it was higher than on nearby permanent grasslands of the same management intensity, suggesting that floral resource availability is more important than the age of grassland sites. Overall, even a moderate increase in plant diversity in grasslands offers a great opportunity to support pollinator abundance due to the big share of land this habitat type covers in many regions. Moderate diversification can complement high-value habitats and diverse landscape elements in safeguarding farmland biodiversity.