Faunal assembly in chironomids (Diptera): generic association and spread

Faunal assembly in chironomids was analysed using published data in the fauna of three rivers in southern England and of Great Britain as a whole. Two questions, both relating to the generic composition of local faunal assemblages, were asked: (i) to what extent are faunas from different sites within a single geographical area associated in terms of the occurrence of genera (‘generic association’); (ü) given the total number of species observed, is the number of different genera represented (‘generic spread’) more than would be expected by chance. The latter question relates to the hypothesis that if interspecific competition is a strong organizing force of local faunal assemblages, species would be more likely to occur as widely spread out among different genera, thus decreasing overlap in resource use and hence the possibility of outright competition. Null models of species occurrence were constructed through Monte Carlo simulation to evaluate quantitatively these two questions. Generic association of faunal assemblages from three localities was statistically significant, though there was some variation among different subfamilies of Chironomidae. No evidence of generic diversification (more genera occurring in the fauna than expected by chance) was found. These results and the original questions were given critical considerations in the discussion.