Drosophila on Oranges: Colonization, Competition, and Coexistence

To investigate how two ecologically very similar species can coexist, I studied two species pairs of fruit flies in a California orange grove. These species are members of the guild of “cosmopolitan” Drosophila species. This guild is found almost worldwide, and it shows consistent dynamic patterns. Seasonal changes in the species composition in southern California were consistent over two years and were very similar to those previously recorded in Australia. However, the mechanisms maintaining the guild are poorly understood. In particular, the members of two species pairs, D. melanogaster/D. simulans and D. immigrans/D. hydei, have been classified as ecologically very similar. I investigated the extent to which two factors might be important and maintaining the integrity of this guild. First, Shorrocks and his coworkers have suggested that the spatial distribution of larvae among identical resource items can promote coexistence; and second, the state of decay of an orange might provide an additional resource dimension promoting coexistence. I carried out four fields experiments to record the sequence in which Drosophila colonized decaying oranges. I showed that, within individual oranges, larval competition was acting to reduce adult emergence, but that the spatial distribution of the species among oranges was not consistent with the Shorrocks model. However, the ecologically similar pairs were different in their pattern of colonization: D. simulans adults arrived earlier than D. melanogaster adults and D. immigrans earlier than D. hydei. These differences in adult arrival time were reflected in differences in successful oviposition, and are consistent with the hypothesis that the different species specialize on different parts of the process of decay.