Abstract

1. Both top-down and bottom-up influences, such as grazing herbivores and edaphic factors, may maintain species-rich vegetation by preventing dominant plants from reducing diversity. However, the interaction between grazing and other processes maintaining diversity, particularly in ecosystems with multiple herbivores, is poorly understood. We manipulated access by small and large vertebrate herbivores on an edaphically heterogeneous site. We investigated whether: (i) grazing and soil properties interacted in their impact on vegetation, (ii) the effects of herbivores on different plant functional groups depended on soil properties, and (iii) small and large herbivores were functionally equivalent. 2. Treatments allowed mixed rabbit and livestock grazing, rabbit grazing only or no grazing and were replicated within three areas differing in vegetation, soil nutrient availability and pH. Soil properties, plant species composition, vegetation height and above- and below-ground biomass were measured after 6 years. 3. Bottom-up and top-down impacts were both important, with soil properties and grazing explaining 42.1% and 9.2% of the variability in species composition between plots, respectively. Grazing enhanced the impact of soil properties on the plant community by preventing dominance by Ulex europaeus and maintaining differences in species composition between the areas. 4. Grazing consistently increased species richness across vegetation types, but the responses of different plant functional groups depended on area. For instance, grazing removal caused graminoid abundance to increase in the area where grasses were the dominant functional group, but to decrease in other areas. 5. Small herbivores (rabbits) were only partially functionally equivalent to larger grazing herbivores (livestock), as rabbit grazing pressure did not increase in plots ungrazed by large herbivores. The results suggested a facilitative relationship between large and small herbivores, with large herbivores improving forage quality and increasing access to plots by rabbits. 6. Synthesis. (i) Grazing and soil properties interacted in their impact on the composition and diversity of vegetation, (ii) the effects of herbivores on particular plant functional groups depended on the characteristics of the area, and (iii) small and large herbivores were not functionally equivalent, with small vertebrate herbivores unable to replace the impact of larger herbivores when these were removed.