Coexistence of Australian Rainforest Diptera Breeding in Fallen Fruit

(1) Twenty-eight species of Diptera, including five species of Drosophila were reared from fallen fruit collected in subtropical rainforest sites in eastern Australia. The situation was similar to that modelled by Atkinson & Shorrocks (1981). They showed that in discrete and ephemeral breeding sites, coexistence was facilitated if the competitors were aggregated. The aim here was to determine whether aggregation could explain the persistence of this diverse community of flies. (2) There was little niche differentiation among the flies, many of the species sharing the same kind of fruit. Though there was less overlap over individual fruit items, the dipteran species were not negatively associated as they would be if the low overlap was caused by systematic niche separation. (3) Low overlap over fruit items would be expected if the fly populations were kept at a low level by density-independent mortality and this might also explain the persistence of a diverse community. There was, however, widespread intraspecific competition, which would not be expected if density-independent mortality was important. (4) If this guild of flies was maintained by aggregation over discrete and ephemeral breeding sites we would expect low overlap of the species over fruit items, random associations between the species and evidence of intraspecific competition. These were all observed in this guild. Adult flies emerging from the fruits also showed sufficient aggregation to account for their coexistence, as did eggs laid in the wild on artificial breeding sites. The occurrence together of these phenomena suggests strongly that the model of Atkinson & Shorrocks (1981) best explains the coexistence of so many species of Diptera.