Coexistence of ecologically similar colonising species: Intra- and interspecific competition in Drosophila aldrichi and D. buzzatii

Two Drosophila species, D. aldrichi and D. buzzatii, colonised Opuntia cactus in Australia approx- imately 60 years ago. Only here are they sympatric. In laboratory population cages, D. buzzatii eliminated D. aldrichi in only a few generations. Thus analyses were done to identify competitive and life-history differences that may affect interactions between these species and their coexistence in nature. The two species differed in their responses with respect to preadult viability, size and developmental time, under intra- and interspecific larval competition, on simulated cactus rots in the laboratory. Increased intra- and interspecific density had similar effects on the larger of the two species, D. buzzatii. However, D. aldrichi showed greater decline in size and viability and greater increase in developmental time as a response to increased larval numbers of D. buzzatii, as compared with increased intraspecific density. Age of first reproduction and lifetime fecundity also favoured D. buzzatii under laboratory conditions. That D. buzzatii varied relatively less in developmental time suggested that it is better adapted to the smaller cactus necroses of O. stricta, present in the southern part of the cactus distribution where D. aldrichi is found rarely. Conversely, higher viability under crowded conditions suggested that D. aldrichi may be better adapted to poor environments such as older rots. © 1991 CSIRO.