Neutral or non‐neutral communities: temporal dynamics provide the answer

Abstract

Neutral models for community dynamics are based on the assumption that temporal variation is only influenced by demographic stochasticity, ignoring the influence of fluctuations in the environment. Demographic and environmental stochasticity are known to have very different effects on the population dynamics of single species. Communities, however, are usually analyzed with emphasis on the shape of the species abundance distribution rather than on their dynamical characteristics. Accordingly, examination of the validity of the neutral model has typically been based on statistical analyses of the shape of the species abundance distribution or form of the species–area relationships. Here we illustrate that both demographic and environmental noise must be included to obtain realistic description of community dynamics. We do this by a simple approach comparing two different dynamic models both leading to Fisher's log series species abundance distribution: the neutral model with only demographic noise, and a limiting form of an approximate gamma model most strongly affected by environmental noise. While keeping Fisher's diversity parameter α constant, the turnover rates for observable species become much greater and average life‐times of species much smaller in the gamma model than in the neutral model. In practice, the neutral model tends to produce unrealistically small turnover rates for large populations. This illustrates that realistic assumptions about the stochastic components of the underlying population dynamics of single species are important for correctly characterizing dynamical changes in species composition of communities. This approach can provide a powerful tool for identifying the principal factors affecting the structure of communities.