Limited plasticity in thermally tolerant ectotherm populations: evidence for a trade-off

Abstract

Many species face extinction risks owing to climate change, and there is an urgent need to identify which species' populations will be most vulnerable. Plasticity in heat tolerance, which includes acclimation or hardening, occurs when prior exposure to a warmer temperature changes an organism's upper thermal limit. The capacity for thermal acclimation could provide protection against warming, but prior work has found few generalizable patterns to explain variation in this trait. Here, we report the results of, to our knowledge, the first meta-analysis to examine within-species variation in thermal plasticity, using results from 20 studies (19 species) that quantified thermal acclimation capacities across 78 populations. We used meta-regression to evaluate two leading hypotheses. The climate variability hypothesis predicts that populations from more thermally variable habitats will have greater plasticity, while the trade-off hypothesis predicts that populations with the lowest heat tolerance will have the greatest plasticity. Our analysis indicates strong support for the trade-off hypothesis because populations with greater thermal tolerance had reduced plasticity. These results advance our understanding of variation in populations' susceptibility to climate change and imply that populations with the highest thermal tolerance may have limited phenotypic plasticity to adjust to ongoing climate warming.

Keywords: climate change; heat tolerance; local adaptation; phenotypic plasticity; thermal acclimation; trade-off hypothesis.

Figure 1.

Forest plot showing effect sizes (Hedges' g) for each included publication in the weighted analysis ordered in increasing mean effect size. Each point refers to a pairwise contrast between acclimation temperatures within a population. Error bars denote ± s.d. A positive value refers to an increase in upper thermal limits with acclimation at higher temperature whereas negative values refer to a decrease in thermal limits. The standardized magnitude of change in thermal limits is the measure of plasticity used in this study. (Online version in colour.)

Figure 2.

Graphs showing outputs from our meta-analytic regression model. Meta-analytic scatter plots show plasticity (standardized change in upper thermal limits) as a function of (a) standardized mean thermal limit of the lower acclimation temperature, (b) standardized difference in acclimation temperature and (c) standardized range in annual temperature. All variables were standardized so that the mean is zero. Solid line denotes model predictions, varying terrestrial ecosystems while holding other predictors at the mean. Dotted line shows 95% prediction intervals. (Online version in colour.)

Figure 3.

Plot showing the parameter estimates for each predictor in the model average in the weighted analysis. Error bars represent 95% confidence intervals.