Interspecific competition in Drosophila I. Reversal of competitive superiority due to varying concentration of ethanol

The experiments reported here were designed to investigate the outcome of interspecific competition between Drosophila melanogaster and D. simulans in environments varying in one factor only. The factor chosen was ethanol concentration in the medium; the strains used were D. melanogaster SM5 (Series I), D. melanogasterw, b , (Series II) and D. simulans v (both Series).
The results of competition over a number of generations, the longest experiment lasting 100 days, showed clear differences in the relative competitive abilities of the two species between the different environments. With no ethanol, D. simulans won in all replicates; with 896 ethanol, D. melanogaster won in all replicates; at intermediate (496) concentration, the winning species was not consistent. Experiments in Series II showed very similar results to their counterparts in Series I despite distinct genetic differences between the strains of D. melanogaster used.
These results tie in with published work, on (i) the relative susceptibility of single–species cultures of D. melanogaster and D. simulans to high ethanol concentrations, and (ii) distributional patterns observed in some field populations exposed to alcohols in their environment.