Thermal tolerance and survival responses to scenarios of experimental climatic change: changing thermal variability reduces the heat and cold tolerance in a fly.

Climate change poses one of the greatest threats to biodiversity. Most analyses of the impacts have focused on changes in mean temperature, but increasing variance will also impact organisms and populations. We assessed the combined effects of the mean and the variance of temperature on thermal tolerances-i.e., critical thermal maxima, critical thermal minima, scope of thermal tolerance, and survival in Drosophila melanogaster. Our six experimental climatic scenarios were: constant mean with zero variance or constant variance or increasing variance; changing mean with zero variance or constant variance or increasing variance. Our key result was that environments with changing thermal variance reduce the scope of thermal tolerance and survival. Heat tolerance seems to be conserved, but cold tolerance decreases significantly with mean low as well as changing environmental temperatures. Flies acclimated to scenarios of changing variance-with either constant or changing mean temperatures-exhibited significantly lower survival rate. Our results imply that changing and constant variances would be just as important in future scenarios of climate change under greenhouse warming as increases in mean annual temperature. To develop more realistic predictions about the biological impacts of climate change, such interactions between the mean and variance of environmental temperature should be considered.