A comprehensive understanding of how species/functional group interactions determine population dynamics, community composition and their effect on ecosystem functioning is important. This paper presents data from a mesocosm experiment, based on the simplex design, to examine the effect of interactions between earthworm functional groups, food supply and initial overall biomass on community structure. All communities containing anéciques moved towards domination by anéciques. The survival of anéciques remained constant irrespective of initial conditions, with no effect of initial community structure, food supply or initial biomass. The proportional biomass of epigées increased when they were placed in communities dominated by anéciques. Initial overall biomass had a significant effect on the survival of endogées, with increased survival at low biomass. Juvenile production was significantly increased in communities that contained a higher initial abundance of epigées. The anéciques had significantly increased production of juveniles at lower levels of initial biomass. Overall, earthworm functional group diversity had an idiosyncratic effect on earthworm assemblage structure.