Interspecific competition in Drosophila: II. Competitive outcome in some 2–resource environments

Populations of Drosophila metanogaster and D. simulans were made to compete in a number of 2–resource environments. The resources used were all based on a standard Drosophila medium and differed from each other only in their concentrations of ethanol. Each experiment comprised three cages started at a species–frequency of 0.8 D. melanogaster and three at 0.2 D. melanogaster. This enabled screening for a wide range of equilibria. Reversal of competitive superiority was achieved (unintentionally) by raising the alcohol level of one of the resources. Thus while D. simulans won in an environment with media containing 0 and 896 ethanol, D. melanogaster won if the ethanol concentrations were 0 and 10%, or 0 and 12%. This parallels the reversal of dominance reported earlier for single–resource systems. However, in none of the 2–resource systems studied was there a stable equilibrium in species–frequency–in fact there was no significant frequency–dependence at all. An artificial selection experiment produced a stock of D. melanogaster ovipositing a higher proportion of eggs on the alcoholic resource. Thus, this stock was more divergent from D. simulans, in at least one aspect of resource use, than was the original stock of D. melanogaster. However, competition between stock D. simulans and selected D. melanogaster showed no frequency–dependence. Again, reversal of competitive dominance was unintentionally achieved. The implications of these results are discussed, and the need for a comprehensive study of resource–utilization in a pair of competing species is stressed.