Community Structure: A Neutral Model Analysis

The relation of community structure to community function is the subject of much ecological theorizing. Three approaches to the problem, based on cybernetics, control theory, and niche theory, can be identified. They concur in giving biological interactions a major role in the determination of community structure in general, and species diversity in particular. To examine this body of theory, I use a "neutral" model for community development, a model which eliminates completely the biological interactions in question. Comparison of the patterns of community structure resulting from this model with the structure of natural communities provides an estimate of the effect of the biotic interactions. This can then be compared to the theoretically predicted effects. The neutral model used was originally developed in population genetics. In the context in which it is used here, it describes the stochastic development of a set of noninteracting populations, which colonize a community, persist temporarily, and eventually become extinct. The model predicts the form of the distribution of relative abundances and of species area curves, and sample values of species diversity. Comparisons of the model's predictions with actual community structure are made across what are taken to be gradients in the importance of biological interaction (successional vs. climax communities, high variation vs. low variation environments, the temperate zone vs. the tropics), using data on birds, fish, trees and insects. The results clearly contradict some of the theories of community structure. In those situations where biological interactions are predicted to generate the greatest increase in species diversity, the neutral model analysis shows that diversity is in fact significantly lower than would be expected in the absence of such interactions. Some implications of these findings for community theory are discussed.