Plant–herbivore interactions affect the initial direction of community changes in an ecosystem manipulation experiment

Abstract

The lack of data on the combined effects of livestock species and grazing intensity makes it difficult to propose recommendations for the management of biodiversity and production in grassland ecosystems. We therefore divided a fertile grassland into 12 plots that were grazed by either cattle at a high or a low stocking density or by sheep at the same low stocking density. Grazing management had an immediate and direct impact on sward structure. We infer that dominant forbs ( Taraxacum officinale) and legumes ( Trifolium repens) were strongly disadvantaged by biomass accumulation in lightly grazed plots. Consequently, stocking density affected plant community composition right from the start of the second year of the survey. There was direct and indirect evidence that the selection made by sheep on forbs and legumes could be stronger than the selection made by cattle. The impact of this difference in selectivity took six years to manifest, when legume abundance became lowest and grass abundance peaked in sheep-grazed pastures. Conversely, neither species richness nor evenness of the plant community was modified over the first six years of the survey; it should therefore be continued in order to determine the equilibrium states that can be reached under the three management strategies.