Limited scope for plasticity to increase upper thermal limits

Increases in average temperature and the frequency of extreme temperature events are likely to pose a major risk to species already close to their upper physiological thermal limits. The extent to which thermal phenotypic plasticity can buffer these changes and whether plasticity is constrained by basal tolerance levels, remains unknown. We examined the effect of developmental temperature under both constant and fluctuating thermal regimes (developmental acclimation), as well as short-term heat hardening on upper thermal limits (CTmax) in a tropical and temperate population of Drosophila melanogaster. We found evidence for thermal plasticity in response to both developmental acclimation and hardening treatments; CTmax increased at warmer developmental temperatures and with a prior heat hardening treatment. However, hardening and acclimation responses were small, improving CTmax by a maximum of 1.01° C. These results imply that overheating risk will only be minimally reduced by plasticit...