Effects of insect herbivory on early plant succession: comparison of an English site and an American site

The effects of insect herbivores on plant community characteristics of early secondary succession were compared in an Old World site (southern Britain) and a New World site (Iowa, U.S.A.). Plant species richness, species diversity, cover abundance, foliage height diversity, and a height index, used as a measure of vegetational structure, were significantly greater at the New World site than at the Old World site. At the Old World site during the first year of succession on bare ground, exclusion of foliar herbivores by the use of non-persistent insecticides significantly increased plant species richness, diversity and the cover abundance of the vegetation. These differences were due to an increase in the annual species in insecticide-treated plots. In the second year the removal of herbivores significantly increased cover abundance due to the enhanced growth of perennial grasses. These results indicate that at the Old World site insect herbivores begin to slow the rate of succession as early as the second year because of their impact on perennial grasses. At the New World site, none of the community characteristics measured were altered by herbivore exclusion. This is due to the relatively small contribution of annuals to species richness and cover abundance during the first year and of perennial grasses to cover abundance during the second year. We suggest that insect herbivores will only affect secondary succession in old fields at the New World site when perennial grasses become abundant, normally three to four years after succession begins.