Independent and combined effects of daytime heat stress and nighttime recovery determine thermal performance

Abstract

Organisms often experience adverse high temperatures during the daytime, but they may also recover or repair themselves during nighttime when temperatures are more moderate. Thermal effects of daily fluctuating temperatures may thus be divided into two opposite processes (i.e., negative effects of daytime heat stress and positive effects of nighttime recovery). Despite recent progress on the consequences of increased daily temperature variability, the independent and combined effects of daytime and nighttime temperatures on organism performance remain unclear. By independently manipulating daily maximum and minimum temperatures, we tested how changes in daytime heat stress and nighttime recovery affect development, survival and heat tolerance of the lady beetle species Propylea japonica. Thermal effects on development and survival differed between daytime and nighttime. Daytime high temperatures had negative effects whereas nighttime mild temperatures had positive effects. The extent of daytime heat stress and nighttime recovery also affected development and critical thermal maximum, which indicates that there were both independent and combined effects of daytime and nighttime temperatures on thermal performances. Our findings provide insight into the thermal effect of day-to-night temperature variability and have important implications for predicting the impacts of diel asymmetric warming under climate change.