How does grazing intensity influence the diversity of plants and insects in a species-rich upland grassland on basalt soils?

International audience; The effect of stocking rate on the species richness, abundance and functional diversity of vascular plants, butterflies and grasshoppers was examined in a semi-natural upland pasture in central France. Over a 5-year period, 3.6-ha plots were continuously grazed by Charolais heifers at 1.4 (High stocking rate), 1.0 (Intermediate) and 0.6 (Low) livestock units (LU) ha(-1). To evaluate botanical diversity, percentage cover of all plant species was estimated in late July in ten fixed 1 m x 1 m quadrats per plot. Butterflies were counted on three occasions between late June and early August along three fixed 50-m-long transects per plot using the 'Pollard walk', with grasshoppers being recorded on two occasions along the same transects. Diversity patterns of the three taxa were affected by stocking rate. For plants, species abundance changed more than species number. Abundance of forbs became higher under the Low compared with the High stocking rate. Stress-tolerant grasses were also more abundant in plots grazed at the Low stocking rate, while an opposite trend was observed for competitive grasses. Butterflies consistently responded to alterations in vegetation composition, especially to the dynamics of nectar plants. The species richness of grasshoppers also increased in plots at the Low stocking rate. The Low and Intermediate stocking rates were suitable for providing a high diversity of the three taxonomic groups. The results suggested that at least butterfly diversity would peak in vegetation taller than that of vascular plants.