A general and dynamic species abundance model, embracing the lognormal andthe gamma models

One aspect of community ecology that has been given particular attention is the pattern of species abundances in a community. The species may have a w ide range of abundances; some are very common and others rare. When species abundance models are fitted to observations, the lognormal model and one o f the gamma models (e.g., the log-series model) are usually applied. The mo del that gives the best fit according to some goodness-of-fit test is then chosen. By applying a diffusion approximation for each species' dynamics wi th density regulation of the theta-logistic type, we here present a general species abundance model that embraces the two most widely applied species abundance models, the lognormal and the gamma. Our general model will, ther efore, provide a better fit than the two special cases, except when it corr esponds to one of them. In contrast to the classical models, ours is also d ynamic, making it possible to evaluate the fluctuations in species abundanc e over time through both biotic and abiotic factors. The model is fitted to several species abundance data sets and our results compared to previous a ttempts to fit a model, usually either the lognormal or the log-series.