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 herb ivory and competition. We investigated the significance of both these factors in establishing grassland communities through manipulation exp eriments conducted in both winter and summer in glasshouse conditions. Manipulations consisted of addition of aboveground and below-ground h erbivores, reduction of herbivory through use of an insecticide, and v arying of competitive pressure by removing either dicotyledonous or mo nocotyledonous seedlings as they appeared. In the winter experiment, t he total biomass of dicotyledonous plants was reduced by both above-gr ound and belowground herbivory, while monocotyledonous plant biomass w as unaffected. However, in the summer experiment, only those treatment s in which competition was manipulated had any detectable effect. This suggests that the relative importance of competition and herbivory ca n vary considerably. The above-ground herbivory treatment sometimes st imulated and sometimes reduced plant species diversity, and sometimes also induced significantly higher dicotyledonous species diversity tha n 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 treatment s, and with all treatments affecting at least some species. Different species therefore respond quite differently to competition and herbivo ry. None of the treatments affected soil microbial activity or active soil microbial biomass in the winter experiment, although active bioma ss was significantly positively correlated with dicotyledonous plant b iomass. In the summer experiment, soil activity was positively affecte d by monocotyledonous plants, while active biomass was the same in all treatments except for the treatment with all plants removed, where bi omass was less. None of the soil biotic variables showed any relations hip with plant species richness. Our study shows that competition and herbivory can both exert important influences at both the community an d ecosystem levels of resolution.