Differential Effects of Above- and Below-Ground Insect Herbivory during Early Plant Succession

Abstract

The effects of aboveand below-ground insect herbivory on a natural plant community colonising bare ground were determined by manipulative field experiments. A foliar insecticide (Dimethoate-40) and a soil insecticide (Dursban 5G) were applied separately and in combination in a factorial experimental design over a two-year period. Characteristics of the developing vegetation were assessed in insecticidetreated and control plots. The insecticides had no independent effect on plant growth. Plant species richness and diversity were increased by the application of soil insecticide and, by the second season, depressed by foliar insecticide. Vegetation frequency and particularly cover abundance were enhanced by both compounds, with the soil insecticide having a greater effect in the second year. Vegetation height, assessed by a weighted mean height index, was only increased by the application of soil insecticide in the first year. Three major life-history groupings (annual and perennial herbs and perennial grasses) responded differently to herbivory. Annual herbs were promoted by foliar and soil insecticides in the first season, but only by the latter in the second. Within this general pattern, common species varied in response to the two compounds. Perennial grasses were strongly promoted by foliar insecticide and although few perennial herbs were present these increased in number of species and cover when below-ground herbivory was reduced. The different responses of life-history groupings and individual species to the two types of herbivory have a considerable effect on the pattern of early succession. The role of herbivory in relation to competition and seedling mortality in the developing plant community are discussed. The latter is relevant to the creation of species-rich communities of potential conservation value.