Vital rate responses to temperature lead to butterfly population declines under future warming scenarios

Due to their dependence on environmental temperatures, ectothermic animals are likely to be particularly sensitive to global climate change. Accurate prediction of ectotherm population responses to climate change requires a mechanistic understanding of effects of increased temperatures on survival and reproduction. Yet, despite organismal development through distinct life stages that may differ in sensitivity to temperature, most studies measure effects on a single life stage. Using a combination of greenhouse and field experiments, including a large restoration experiment, we measured the effects of temperature increases on all annual life stages in a multivoltine butterfly. We then used the vital rate – temperature relationships to develop a population model parameterized with (1) increased temperatures relative to observed field temperatures and (2) downscaled global climate model data. Our goal was to model population dynamics under future climate change in order to understand what temperature variables have the strongest effects on vital rates, which life stages is population growth most sensitive to under increased temperatures, and how projected warming will affect future growth rates. We found that maximum temperature over each life stage time period was the most important temperature variable explaining changes in vital rates with temperature. Population growth rates dropped below one (shrank) at temperature increases of 1.5 °C. Our population model projected that annual growth rates will go from growing to shrinking during the 2060s at latest under the RCP 8.5 scenario. Our findings demonstrate the need for conservation strategies for ectotherms that target specific vulnerable life stages and consider differing effects of climate variables.