Community Assembly in a Model Ecosystem

Environmental and biotic factors regulate species abundance and the composition of ecological communities. However, it is difficult to demonstrate 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 similar behavior by all individuals in a population. Here we describe an agent-based model of many-species communities in which interactions among individual agents are subjected to Darwinian selection (Holland's Echo model). To evaluate the response of the model to evolutionary mechanisms, we present a simple version of the model, and describe an experiment which evaluated community assembly patterns with and without selection for genome-mediated interactions, at various levels of invasion. Increased invasion rates decreased population size, but increased species richness and evenness. Interactions acted upon by natural selection resulted in greater population sizes and lower species richness and evenness than random interactions. Genotype abundances deviated from the expectation of equal abundances, and were affected by both the invasion rate and nature of interagent interactions. Species abundance patterns indicate that communities formed by random and selective interactions follow different assembly rules, and communities formed under high invasion rates showed no coherent community-level pattern of genotype abundances. Thus, both invasion rate and type of interspecific interaction regulate the assembly of species into communities.