Sensitivity to thermal extremes in Australian Drosophila implies similar impacts of climate change on the distribution of widespread and tropical species.

Climatic factors influence the distribution of ectotherms, raising the possibility that distributions of many species will shift rapidly under climate change and/or that species will become locally extinct. Recent studies have compared performance curves of species from different climate zones and suggested that tropical species may be more susceptible to climate change than those from temperate environments. However, in other comparisons involving responses to thermal extremes it has been suggested that mid-latitude populations are more susceptible. Using a group of 10 closely related Drosophila species with known tropical or widespread distribution, we undertake a detailed investigation of their growth performance curves and their tolerance to thermal extremes. Thermal sensitivity of life history traits (fecundity, developmental success, and developmental time) and adult heat resistance were similar in tropical and widespread species groups, while widespread species had higher adult cold tolerance under all acclimation regimes. Laboratory measurements of either population growth capacity or acute tolerance to heat and cold extremes were compared to daily air temperature under current (2002-2007) and future (2100) conditions to investigate if these traits could explain current distributions and, therefore, also forecast future effects of climate change. Life history traits examining the thermal sensitivity of population growth proved to be a poor predictor of current species distributions. In contrast, we validate that adult tolerance to thermal extremes provides a good correlate of current distributions. Thus, in their current distribution range, most of the examined species experience heat exposure close to, but rarely above, the functional heat resistance limit. Similarly, adult functional cold resistance proved a good predictor of species distribution in cooler climates. When using the species' functional tolerance limits under a global warming scenario, we find that both tropical and widespread Drosophila species will face a similar proportional reduction in distribution range under future warming.