Analyzing patterns of species diversity as departures from random expectations

Insight into the factors that influence patterns of species diversity can be obtained by comparing real patterns to their counterparts derived under a model of randomness or a null model. For example, intraspecific aggregation is very common among a wide variety of protozoan, plant, and animal taxa. Despite this, ecologists do not yet fully appreciate the potential effect of intraspecific aggregation on patterns of species diversity. Intraspecific aggregation is predicted to limit alpha-diversity (mean species richness of a set of ecological communities) to a level less than that expected based on random distributions of individuals. Conversely, intraspecific aggregation should enhance beta-diversity (differences in species composition among communities). These effects on alphaand beta-diversity should be related to the mean amount of intraspecific aggregation within a species assemblage (group of taxonomically similar species distributed among a set of communities). I tested these predictions by applying additive diversity partitioning and a randomization routine to 28 arthropod assemblages. Intraspecific aggregation was very common. It significantly limited alpha-diversity to less than that expected under the random model. At the same time, intraspecific aggregation enhanced beta-diversity. These effects were greatest in assemblages where intraspecific aggregation was greatest. The next step for ecologists is to identify those factors that cause conspecifics to aggregate and thus also influence patterns of species diversity.