Climate and competition combine to set elevational distributions of tropical rainforest Drosophila

Species turnover with elevation is a widespread phenomenon and provides valuable information on why and how ecological communities might reorganize as the climate warms. Tropical mountains typically have pronounced thermal gradients and intense species interactions, providing a testing ground for investigating the relationship between thermal tolerances and biotic interactions as the proximate factors influencing species distributions. We investigated temperature and interspecific competition as causes of species turnover and abundance changes of the nine most abundant species of Drosophila along elevational gradients in the Australian Wet Tropics. Thermal performance curves revealed that species distributions were better explained by their performance at extreme temperatures, rather than their thermal optima. Upper thermal limits varied less among species than lower thermal limits. Nonetheless, these small differences were associated with differences in centred elevation of distribution, consistent with environmental sorting as a driver of community composition at low-elevation sites. In contrast, community composition at cool, high elevations was driven by temperature-dependent interspecific competition rather than tolerance to low temperatures. These results run counter to common assumptions about the role of abiotic and biotic factors in structuring communities along thermal gradients, and indicate that tropical insects may be highly vulnerable to future warming. Our study illustrates the importance of experimental, quantitative tests across biological levels (i.e., individuals to populations) and temporal scales (i.e., within-generation to multi-generation) for characterizing effects of climate on a guild of closely-interacting species.