The Insects of British Trees: Community Equilibration in Ecological Time

The most widely accepted model of insect-plant interaction derives from the correlation between insect species richness and the relative number of recorded Quaternary fossil remains of British tree species (Southwood, 1961).The implications of this relationship are: 1) that community formation and the equilibration of insect species richness is a very slow process, taking at least 2 million years, 2) that introduced plant species will necessarily have relatively few associated insect species, 3) that insect communities are basically different from vertebrate communities in that saturation of species does not occur (Whittaker, 1969).I present a model mutually exclusive to Southwood's, indicating that insects of British trees follow a species area relationship.The implications of this new finding are: 1) insect community formation and equilibration of species richness occurs at least within 300 years, 2) the insect diversity of introduced plants becomes a function mainly of the range of distribution of the plant within ecological time, 3) insect communities do not differ from vertebrate communities in terms of species saturation rate.The saturation occurs within ecological time for both groups.Plant/insect interactions are apposite to general ecological thought because they provide the only uncontested example of n on -equilibrium community biology; the number of insect species associated with tree species apparently can increase continually and without limit over long periods of geological time.This is suggested by Southwood (1961), who shows the number of insect species recorded from British trees to be positively and linearly correlated with the total number of Quaternary fossil records of the tree taxon.This implies community formation and any resultant equilibration of species number for these insects to be a very slow process, taking longer than the Quaternary or between 15,000 and 2 million years -depending upon "one of the most popular topics in all biogeography .... whether the whole fauna and flora of the British Isles immigrated in post-glacial times, or whether some fraction survived from an earlier time" (Deevey, 1949).Extremely protracted community development is logically necessary given the fact that there is no asymptote in Southwood's correlation and that native trees are on the same line as species introduced only 300 years ago.The Oaks provide a good example.Quercus petraea and Q. robur are native species that together have the richest associated insect fauna of any tree taxon in Great Britain (284 species), whereas Quercus ilex, introduced into Britain about 400 years ago has only 2 associated insect species.Southwood's correlation predicts that after another 14,600 to 2 million years in Britain Q. ilex will have many more species of insects associated with it, however by that time Q. petraea and Q. robur will also have acquired many more.But unless the relative abundance or distributions of these two taxa change, the difference between them in number 1 I thank Tom Ray for counting the 60 thousand-odd dots in Perring and Walters for this study.