Density, frequency and interspecific competition: Fertility ofDrosophila simulans andDrosophila melanogaster.

For one stock ofD. simulans and two stocks ofD. melanogaster the percentage of females that produced viable progeny (i.e. percent fertility) was measured at two female ages (50 and 72 hrs), for all combinations of six densities (20, 40, 80, 160, 320, 640 individuals) and three species frequencies (20, 80, 100%). Significant differences were recorded between species and betweenD. melanogaster stocks. Increasing density significantly reduced fertility forD. simulans but not for either of theD. melanogaster stocks. Although species frequency had no effect, the associated stock ofD. melanogaster influencedD. simulans fertility to a high degree-theOr-R-C stock reducingD. simulans fertility because of physical interference, this interference being particularly evident at low densities.