The Insects of British Trees: Community Equilibration in Ecological Time

Abstract

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 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 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 rate. The occurs within ecological for both groups. Plant/insect interactions are apposite to general ecological thought because they provide the only uncontested example of non-equilibrium community biology; the number of insect species associated with tree species apparently can increase continually and without limit over long periods of 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 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 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 1I thank Tom Ray for counting the 60 thousand-odd dots in Perring and Walters for this study. 2 Department of Biological Sciences, Florida State University, Tallahassee, Florida 32306. ANN. MISSOURI BOT. GARD. 61: 692-701. 1974. This content downloaded from 157.55.39.162 on Thu, 11 Aug 2016 05:03:32 UTC All use subject to http://about.jstor.org/terms 1974] STRONG INSECTS OF BRITISH TREES 693 of associated insect species will remain constant. The non-asymptotic nature of the correlation implies that the rate of addition of insect species to the fauna of a tree taxon is independent of the number of insects already in that fauna. Perhaps within another million years, saturation (MacArthur, 1965) or equilibrium (MacArthur & Wilson, 1963) will have occurred for the insect biotas of the oldest or most perenially abundant tree species, and continued existence in the flora will not add any more insect species to their faunae. However Southwood's correlation gives no indication that these faunae will ever saturate. The correlation supports Whittaker's contention that similar generalizations may not apply to insect and bird communities: Bird communities can thus be saturated, determinate in their evolution of diversity; but plant and insect communities may show indefinite, indeterminate evolutionary increase in diversity (Whittaker, 1969). However there is a problem with Southwood's correlation. It ignores the influence of present abundance of host taxa upon associated insect species richness. The general importance of present conditions in determining has been well established by the species-area relationship. For many taxocenes of birds, reptiles, phanerogams, and invertebrates the number of species in a biota is a function of the present area supporting the biota, when geographically close and similar areas are considered. (See Simberloff, 1972, for review.) This relationship, S = kAz (Preston, 1960) where S = number of species, = area of the region considered, k = a constant for the biota in question, and z = ratio of log species addition per log area addition, has been most often demonstrated to explain the difference in richness among the biotas of oceanic islands, but there is no reason that it should not be valid for on the mainland as well. Indeed, Janzen (1968) has suggested that host plants are islands in a sea of vegetation for phytophagous insects. Hence I reason that for the British insect/tree system the analogue of area, of A in the species-area relationship, should be the present range or breadth of distribution of a host tree taxon. The idea that cumulative ancient host tree abundance determines species richness of associated insect faunae is an extension of the popular notion that the antiquity of a biota determines its species richness (Wallace, 1878; Wimpenny, 1941; Dunbar, 1960; Fischer, 1960; Zenkevitch, 1961; Sanders, 1968). This geological theory of organic proposes that the number of species in a biota tends to grow steadily through long periods of time. It is obvious that theories and recent or ecological theories are competing or alternative explanations of species richness. That a biotas's antiquity determines its species richness is in conflict with the idea that ecological conditions during recent times control richness. However both may operate, one being a major factor and the other being minor, or both may be of relatively equal importance, or finally one of the two may be shown to be overwhelmingly more important in determining species richness. By subtracting the amount of variation in insect richness attributable to variation in present day range from the total amount of insect richness variation it is possible to establish the proportion contributed by cumulative ancient abundance of the host taxon. This content downloaded from 157.55.39.162 on Thu, 11 Aug 2016 05:03:32 UTC All use subject to http://about.jstor.org/terms 694 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 61 C,') L100 5 LUJ oak. willot * (U) birch *