On the complexity of a simple environment:competition, resource partitioning and facilitation in a two-species drosphila system

An experimental system for studying ecological interactions between Drosophila species is described and the results of an extensive series of long-term experiments involving D. hydeiand D. melanogasterare presented. These reveal a variety of types of interaction and a variety of mechanisms producing stable coexistence. Some of the experimental results serve to confirm the predictions of competition theory: in one environmental régime D. hydeiand D. melanogaster coexist, despite interspecific competition, because of resource partitioning in the form of a difference in larval depth distributions. Other results urge the development of new bodies of theory: in a different environmental regime, D. hydei and D. melanogaster exhibit a previously unrecognized (+ , —) kind of interaction for which I propose the name contramensalism. This kind of interaction is of interest because it can have a balancing effect and can produce stable coexistence of closely related species in a way that is quite distinct from classical resource partitioning.