Interacting effects of the abiotic and biotic environment on fitness of rainforest Drosophila

Interactions within and between species have significant effects on fitness, which are likely to vary across species ranges. However, empirical tests of this are rare, particularly under naturally varying field conditions. We transplanted 19 656 flies of two Australian tropical rainforest fly species ( Drosophila birchii and D. bunnanda ) along an elevation gradient in 972 vials at different intraspecific and interspecific densities, to test for effects of abiotic and biotic environmental variation on fitness. We recorded the number of male and female offspring of each species produced after one generation in the field. Productivity (number of offspring per female) of both species declined rapidly with increasing intraspecific and interspecific density, and with elevation. The effect of density was much greater at the warm, low elevation site than at sites higher up the gradient. Surprisingly, increasing interspecific, but not intraspecific, density was also associated with the production of offspring with a strongly male-biased sex ratio in both D. birchii and D. bunnanda . By contrast, in vials where only one of the species was present, the mean sex ratio was equal or slightly female-biased. Comparison of productivity of mixed and single-species vials suggests that higher mortality of female larvae in interspecific competition can partially, but not completely, explain the observed sex ratio difference. There were also differences between the species in the effect of interspecific competition on sex ratio across the elevation gradient, with effects weakest at the site where each is most locally abundant (low for D. bunnanda and high for D. birchii ). These results suggest that biotic interactions, both within and between species, are a critical factor shaping species distributions and their potential responses to environmental change.