LB 24-257 - Life cycle complexity drives nonlinear, size-decoupled climate vulnerabilities across ontogeny

Background/Question/Methods:

A commonly used method to assess ectothermic species vulnerability to climate change relies on determining organismal physiological tolerances of environmental temperature change. However, these vulnerability assessments are often based on physiological tolerances from mature developmental stages. Recent work suggests early developmental stages might be more vulnerable, and that risks of organisms to climate change are being underestimated as a consequence. Here, using the painted lady butterfly (Vanessa cardui) as a model system, we quantified two common physiological traits used in vulnerability assessments, including the critical thermal maximum and thermal sensitivity of metabolic rate. We measured these traits across ontogeny from the third larval instar through all subsequent larval instars, pupation, and adult eclosion. Additionally, we measured individuals following developmental acclimation at 20 versus 30 °C to estimate the effects of thermal acclimation across ontogeny. We expected that vulnerability based on physiological traits would largely track developmental variation in body size. Specifically, we expected that, owing to often positive associations between body size and increased critical thermal maximum and lower thermal sensitivity of metabolic rate, later developmental stages characterized by larger body size would be the least vulnerable.

Results/Conclusions:

Contrary to our expectations, we found that the earliest developmental stage, the third larval instar, having the smallest body size of the stages we measured, exhibited the lowest vulnerability, i.e. they had the highest critical thermal maximum and the lowest thermal sensitivity of metabolic rate. Across ontogeny, we found vulnerabilities to be largely decoupled from developmental changes in body size. Results from the two developmental acclimation temperature treatments were generally similar, although some marginal differences in the rank order of vulnerability between the fourth and fifth larval instars were observed. At the intra-stage level, we found that the relationship between body size and physiology changed across ontogeny, with tradeoffs being evident early in development, and the typical positive association between body size and physiology (higher critical thermal maximum) being evident at later developmental stages. These results suggest that a simple relationship between earlier developmental stage and enhanced vulnerability to climate change cannot be assumed for all species. Such findings could be important for vulnerability assessments of other organisms with complex life cycles and for other long-distance migratory ectotherms, such as the imperiled Monarch butterfly.