Apparent competition structures ecological assemblages

Competition is a major force in structuring ecological communities(1). It acts directly(2) or indirectly, in which case it maybe mediated by shared natural enemies and is known as 'apparent competition'(3-6). The effects of apparent competition on species coexistence are well known theoretically(7,8) but have not previously been demonstrated empirically in controlled multigenerational experiments. Here we report on the population dynamic consequences of apparent competition in a laboratory insect system with two host species and a common parasitoid attacking them. We find that whereas the two separate, single host-single parasitoid interactions are persistent, the three-species system with the parasitoid attacking both hosts species (which are not allowed to compete directly) is unstable, and that one of the host species is eliminated from the interaction owing to the effects of apparent competition.