Species richness of insects and mites on trees: expanding Southwood

1. Data on the species richness of phytophagous insects and mites associate d with 25 tree genera occurring as natives in Germany were compiled and com pared to data for British trees published by Kennedy & Southwood (1984). Fo r tree genera occurring in Germany and Britain patterns of species richness and composition of phytophage faunas were similar. 2. Present abundance of trees, their distributional history during the Holo cene, morphological traits and taxonomic isolation were used to explain the variance of species richness and proportion of specialists across tree gen era occurring as natives in Germany. Tree genera were either used as indepe ndent data points or to calculate phylogenetically independent contrasts. F or the latter approach, a phylogeny for the tree genera was generated from published rbcL gene sequences. In general, the conclusions from the two typ es of analyses were similar. 3. The species richness of phytophages on German tree genera were positivel y related to present tree abundance, tree height and tree abundance derived from pollen samples. For phylogenetically independent contrasts the length of time a genus was present since the end of the last glaciation also beca me significant. 4. The proportions of specialists showed a negative relationship with prese nt abundance of trees, a positive relationship with taxonomic isolation as well as the length of time of genus was present since the end of the last g laciation. For phylogenetically independent contrasts only the latter two v ariables remained significant. 6. Overall the results support the species-area and the coevolutionary hypo theses.