Interspecific Competition between Drosophila melanogaster and Drosophila simulans: Effects of Adult Density, Species Frequency, Light, and Dietary Phosphorus-32 on Fecundity

Fecundity (number of eggs per surviving female) of Drosophila simulans st and D. melanogaster Or-R-C was measured over consecutive 12-h periods for 7 days post-eclosion in replicated experiments where adult density (6 levels), species frequency (6 levels), and P32 tissue content (2 levels) were varied factorially. Phosphorus-32 tissue content was introduced as a factor so that eggs of the two species could be distinguished in mixed-species populations. A similar experiment was done for D. simulans st and D. melanogaster yw. Least-squares analyses of variance indicated that: (1) D. melanogaster Or-R-C had highest fecundity, followed by D. melanogaster yw, then D. simulans st. (2) Competing D. melanogaster strains influenced fecundity of D. simulans st-fecundity being higher when yw was the competitor. (3) Fecundity changed with female age, but each strain displayed a unique pattern. (4) Increasing density reduced fecundity for all strains; similarly for st and yw, but more severely for Or-R-C. (5) Effects of species frequency were significant only for D. simulans st, its fecundity decreasing as its frequency increased. (6) P32 lowered fecundity. (7) Light (i.e., day vs. night) had a significant effect on fecundity for all strains. Effects of density and of its own frequency on the fecundity of D. simulans st apparently were due to intraspecific interactions affecting mating-an increasing proportion of females remaining unmated with increase in density or frequency. Reduced fecundity with increasing density in the D. melanogaster strains and effects on the proportions of eggs laid in the day and night in all three strains were explainable in terms of differing disruption at oviposition due to activity differences among the strains. Different patterns of relationship of fecundity with species frequency at the different densities for the two D. melanogaster strains were not simply interpretable, either in terms of inter-or intraspecific interactions.