Competition and Herbivory in Establishing Grassland Communities: Implications for Plant Biomass, Species Diversity and Soil Microbial Activity

The two main biotic factors affecting grassland plant species are herbivory and competition. We investigated the significance of both these factors in establishing grassland communities through manipulation experiments conducted in both winter and summer in glasshouse conditions. Manipulations consisted of addition of above-ground and below-ground herbivores, reduction of herbivory through use of an insecticide, and varying of competitive pressure by removing either dicotyledonous or monocotyledonous seedlings as they appeared. In the winter experiment, the total biomass of dicotyledonous plants was reduced by both above-ground and below-ground herbivory, while monocotyledonous plant biomass was unaffected. However, in the summer experiment, only those treatments in which competition was manipulated had any detectable effect. This suggests that the relative importance of competition and herbivory can vary considerably. The above-ground herbivory treatment sometimes stimulated and sometimes reduced plant species diversity, and sometimes also induced significantly higher dicotyledonous species diversity than the root herbivory treatment. Those treatments in which competitive pressure was manipulated had no detectable effects on diversity. There were many strong effects detected when plant species were considered individually, with different species responding to different treatments, and with all treatments affecting at least some species. Different species therefore respond quite differently to competition and herbivory. None of the treatments affected soil microbial activity or active soil microbial biomass in the winter experiment, although active biomass was significantly positively correlated with dicotyledonous plant biomass. In the summer experiment, soil activity was positively affected by monocotyledonous plants, while active biomass was the same in all treatments except for the treatment with all plants removed, where biomass was less. None of the soil biotic variables showed any relationship with plant species richness. Our study shows that competition and herbivory can both exert important influences at both the community and ecosystem levels of resolution.