Effects of grazing intensity on pollinator abundance and diversity, and on pollination services

Abstract

1. Pollinating insects provide important ecosystem services and are influenced by the intensity of grazing. Based on the Intermediate Disturbance Hypothesis (IDH), pollinator diversity is expected to peak at intermediate grazing intensities. However, this hump-shaped relationship is rarely found. 2. The effect of grazing intensity was tested on flower cover, on the abundance and richness of bees, hoverflies and bee flies, and on pollination services to early-flowering bee-pollinated Asphodelus ramosus L. For that, we used data on 11 plant–pollinator phryganic communities from Lesvos Island (Greece) widely differing in grazing intensities. 3. Flower abundance and richness showed hump-shaped relationships with grazing intensity. Grazing affected the abundance and richness of bees and hoverflies directly and also indirectly, through changes in the flower community. Grazing influenced directly the richness but not the abundance of bee flies. Overall, pollinator abundance and richness showed hump-shaped relationships with grazing intensity, but variations in strength (hoverfly abundance) and direction (bee community) of the effect appeared along the season. Early in the season, grazing increased bee abundance but decreased richness, resulting in increased pollen limitation in A. ramosus. 4. The effects of grazing on pollinators vary with the intensity of the disturbance, generally supporting the IDH, and the timing of land-use activities may influence pollination services. Management strategies should include moderate grazing levels to preserve overall diversity in this area, however, the conservation of particular early bee or bee-pollinated species may benefit from reduced grazing in early spring.