Plasticity in thermal tolerance has limited potential to buffer ectotherms from global warming

Abstract

Global warming is increasing the overheating risk for many organisms, though the potential for plasticity in thermal tolerance to mitigate this risk is largely unknown. In part, this shortcoming stems from a lack of knowledge about global and taxonomic patterns of variation in tolerance plasticity. To address this critical issue, we test leading hypotheses for broad-scale variation in ectotherm tolerance plasticity using a dataset that includes vertebrate and invertebrate taxa from terrestrial, freshwater and marine habitats. Contrary to expectation, plasticity in heat tolerance was unrelated to latitude or thermal seasonality. However, plasticity in cold tolerance is associated with thermal seasonality in some habitat types. In addition, aquatic taxa have approximately twice the plasticity of terrestrial taxa. Based on the observed patterns of variation in tolerance plasticity, we propose that limited potential for behavioural plasticity (i.e. behavioural thermoregulation) favours the evolution of greater plasticity in physiological traits, consistent with the 'Bogert effect'. Finally, we find that all ectotherms have relatively low acclimation in thermal tolerance and demonstrate that overheating risk will be minimally reduced by acclimation in even the most plastic groups. Our analysis indicates that behavioural and evolutionary mechanisms will be critical in allowing ectotherms to buffer themselves from extreme temperatures.

Keywords: acclimation; climate change; ectotherm; plasticity; thermal tolerance.

Figure 1.

Acclimation in thermal tolerance in relation to absolute latitude, seasonality and habitat. Seasonality is the standard deviation of weekly mean environmental temperatures (see §2). (a) CT_max ARR with respect to latitude. (b) CT_min ARR with respect to latitude. (c) CT_max ARR with respect to seasonality and (d) CT_min ARR with respect to seasonality. (Online version in colour.)

Figure 2.

Relationship between acclimation and inherent thermal tolerances. (a) CT_max ARR with respect to maximum CT_max and (b) CT_min ARR with respect to minimum CT_min. (Online version in colour.)

Figure 3.

Acclimation in (a) upper and (b) lower thermal tolerance by clade. (Online version in colour.)

Figure 4.

Predicted changes in thermal safety margin given changes in mean body temperature. Coloured lines are based on mean CT_max ARR values for each clade. Grey polygon bounds the central 95% of CT_max ARR measurements across all clades.