Adult population density, fecundity and productivity in Drosophila melanogaster and Drosophila simulans.

Single-species cultures of D. melanogaster Oregon-R-C and D. simulans v were set up with 5, 50, 100, 200, 300 or 400 pairs of parents. These parents were discarded after 48 hours, and the numbers and wet weights of emerging progeny recorded twice daily. For each culture, the fitness components estimated were total number of progeny, total progeny biomass, average male and average female wet weights, mean developmental period, and sex ratio. D. melanogaster had higher progeny productivity and longer mean developmental period. For both species, as adult density increased, progeny number per culture increased to a maximum and then decreased, but the average number of progeny per female decreased rapidly from the lowest density. The cause of this decreased progeny number per female differed in the two species. For simulans, it was due to decreased fecundity per female, possibly a behavioural response to crowding. For melanogaster, the decreased progeny number per female was mainly due to reduced immature stage viability as a result of increased larval crowding. Reduction in fecundity per female was relatively small, as compared with simulans.