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

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.