Theories and Models of Species Abundance

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Abstract
Three theories of explanation for the observed patterns of species abundance in samples from animal and plant communities are reviewed. These are the niche-preemption hypothesis associated with the log-series distribution, the theory based on the central-limit theorem to explain the truncated lognormal distribution, and the explanations associated with a recently published model of community dynamics. The relative performance of the log-series, lognormal, and dynamics models in predicting the species abundance in 222 samples from a wide range of communities is examined. The log-series provides the best prediction of only 4% of the samples, because there were either too few rare species or, more usually, the common species were too abundant. Only 28% of samples, those with a mode in the species-abundance distribution, may be described by a truncated lognormal, but even in these cases the lognormal parameters cannot reasonably be used to predict the abundance of species in the whole community. The species abundance in 94% of the samples may be explained by one of the predictions of the dynamics model. For most samples, the model predicts a concave dominance-diversity curve with a large number of rare species, but indicates that there may be a smaller proportion of rare species in communities where population growth is less restricted by competition or mortality. It is concluded that the theories associated with the log-series and lognormal models, which have been criticized previously for a lack of ecological realism, provide unsatisfactory explanations for species abundance because they are conceptually unsound and the models on which they are based rarely provide a satisfactory description of samples. This conclusion undermines the use of the diversity index .alpha. associated with these models. The dynamics model predicts the species-abundance pattern of most samples with greater accuracy and provides an explanation of species abundance based on recognized and testable ecological principles.