The Fractal Model: a new model to describe the species accumulation process and relative abundance distribution (RAD)

Quantifying a sampled collection of taxa is a widespread problem in ecology . A number of diversity indices have been proposed and used in numerous wor ks in spite of a lack of statistical characteristics and tests of compariso n. These Relative Abundance Distributions (RAD) can also be described using graphic representations exhibiting both the number of taxa and the quantit ative distribution of the individuals which constitute these taxa. Fitting these RADs to deterministic or stochastic models remains problematic for di fferent reasons like scale dependence or complexity of the numerical proced ures. In our study, we introduce a new model to describe RAD and ecological succession. This model is a Fractal Model based on the assumption that dur ing an ecological succession/accumulation process, at each step of the succ ession, K new species appear which are k times more abundant with K = k(d) (where d is a fractal dimension). We use a Monte-Carlo procedure with a Kol mogorov-Smirnov distance to fit this model and to estimate parameters. Thro ugh the study of entomological data from a Mediterranean Man And Biosphere reserve, we demonstrate the qualities of the new Fractal Model. Thus, the F ractal Model was rejected for none of the 13 RADs tested. In addition, the parameters (K, k and d) are independent of the Hill family diversity indice s and of three evenness indices and are able to provide additional informat ion concerning both the diversity of the sampled ecosystem and development of this biodiversily during the species accumulation process in this ecosys tem.