Species diversity patterns derived from species-area models

Although area, species abundances, spatial distribution, and species richness have been central components of community ecology, their interrelationships are not completely understood. To describe these interrelationships, we study and test three patterns regarding species richness using species-area models. The first one is the widely accepted generalization that states that the number of species monotonically increases with sampling area. The second pattern predicts the decrease in species richness with the increase of species dominance in a given area. The third one predicts that spatial aggregation of individuals within species results in lower species richness in communities. These three generalizations were investigated by modeling and simulations. First, a random-placement species-area model was used to evaluate the effects of relative species abundances on species richness in a sampling area. Then, a nonrandom species-area model was derived which explicitly encompasses the spatial distributions of species; it served to evaluate the effects of heterogeneity in spatial distributions on species richness. Species-area models were numerically evaluated using parameters estimated from a tropical rain forest community, and simulations were conducted to support the numerical solutions. The three patterns regarding species diversity were consistently supported by the results. A discussion ensues, describing how the three patterns can be used to interpret and predict species diversity, and how they are supported by other diversity hypotheses. The three generalizations suggest that, if we want to understand species diversity, we should go and look for mechanisms that influence the abundances and spatial distributions of species. If a mechanism can make the species abundances more even, or their spatial distributions more regular, this factor likely contributes to species coexistence.