RELATIVE ABUNDANCE DISTRIBUTIONS IN PLANT-COMMUNITIES - EFFECTS OF SPECIES RICHNESS AND OF SPATIAL SCALE

Relative abundance distributions (RADs) are an important feature of co mmunity structure, but little is known of the factors affecting which type of RAD is observed in a particular community. We examined the inf luences of species richness and of spatial scale on the RAD of plant c ommunities. The effect of species richness was examined by analysing s imulated communities generated under the Broken stick model, the Seque ntial breakage model, and a randomized version of Niche pre-emption mo del. In all cases, when there were few species in the community the da ta only occasionally gave the best fit to the model that was used to g enerate it. With 40-65 species, a best fit was obtained for the correc t model in about 75 % of cases, almost irrespective of the model. Effe cts of spatial scale were examined in data from four dune slacks and f rom two semi-arid grasslands, by analysing biomass values at a range o f sample sizes. The model that best fitted the whole sample differed b etween the four slacks and between the slacks and the semi-arid grassl ands. The change in which model of RAD fitted best, as sample size was reduced, varied between sites and between habitat types. At the small est sample sizes, the Zipf-Mandelbrot model often fitted, and in the s lack sites the Broken stick also, though neither fitted (in the vegeta tion examined) at larger spatial scales. It is concluded the RAD is af fected by species richness and by spatial scale, in ways that currentl y do not enable simple prediction. RADs can theoretically give informa tion on the processes such as resource partitioning, immigration and c ompetition that have structured the community, but they are a blunt to ol for this purpose.