COMMUNITY ASSEMBLY IN A MODEL ECOSYSTEM

Environmental and biotic factors regulate species abundance and the co mposition of ecological communities. However, it is difficult to demon strate principles of community assembly in nature due to observational limitations and historical effects. Numerical simulation can be used to evaluate whether hypothesized mechanisms produce a pattern seen in natural communities. Most models of many-species communities assume si milar behavior by all individuals in a population. Here we describe an agent-based model of many-species communities in which interactions a mong individual agents are subjected to Darwinian selection (Holland's Echo model). To evaluate the response of the model to evolutionary me chanisms, we present a simple version of the model, and describe an ex periment which evaluated community assembly patterns with and without selection for genome-mediated interactions, at various levels of invas ion. Increased invasion rates decreased population size, but increased species richness and evenness. Interactions acted upon by natural sel ection resulted in greater population sizes and lower species richness and evenness than random interactions. Genotype abundances deviated f rom the expectation of equal abundances, and were affected by both the invasion rate and nature of inter-agent interactions. Species abundan ce patterns indicate that communities formed by random and selective i nteractions follow different assembly rules, and communities formed un der high invasion rates showed no coherent community-level pattern of genotype abundances. Thus, both invasion rate and type of interspecifi c interaction regulate the assembly of species into communities. (C) 1 997 Elsevier Science B.V.