Testing the reliability and ecological implications of ramping rates in the measurement of Critical Thermal maximum

Abstract

Critical Thermal maximum (CTmax) is often used to characterize the upper thermal limits of organisms and represents a key trait for evaluating the fitness of ectotherms. The lack of standardization in CTmax assays has, however, introduced methodological problems in its measurement, which can lead to questionable estimates of species' upper thermal limits. Focusing on ants, which are model organisms for research on thermal ecology, we aim to obtain a reliable ramping rate that will yield the most rigorous measures of CTmax for the most species. After identifying three commonly used ramping rates (i.e., 0.2, 0.5 and 1.0°C min-1) in the literature, we experimentally determine their effects on the CTmax values of 27 species measured using dynamic assays. Next, we use static assays to evaluate the accuracy of these values in function of the time of exposure. Finally, we use field observations of species' foraging activities across a wide range of ground temperatures to identify the most biologically relevant CTmax values and to develop a standardized method. Our results demonstrate that the use of a 1°C min-1 ramping rate in dynamic assays yields the most reliable CTmax values for comparing ant species' upper thermal limits, which are further validated in static assays and field observations. We further illustrate how methodological biases in physiological trait measurements can affect subsequent analyses and conclusions on community comparisons between strata and habitats, and the detection of phylogenetic signal (Pagel's λ and Bloomberg's K). Overall, our study presents a methodological framework for identifying a reliable and standardized ramping rate to measure CTmax in ants, which can be applied to other ectotherms. Particular attention should be given to CTmax values obtained with less suitable ramping rates, and the potential biases they may introduce to trait-based research on global warming and habitat conversion, as well as inferences about phylogenetic conservatism.

Fig 1. Study diagram of testing implications of ramping rates in the measurement of Critical Thermal maximum.

(A) The first hypothesis (H1) examines if a positive relationship between the ramping rates and CT_max values exists for each ant species at the intraspecific level. (B) The second hypothesis (H2) examines the interspecific variations of exposure duration-based tolerance in function of the temperature treatments, we hypothesize that species assemblages show different exposure durations in their CT_{max (0.2)} values but presenting consistence in their CT_{max (0.5)} and CT_{max (1)} values. (C) Thermal performance framework of ectotherm on the basis of foraging behavior illustrate species activity in function of the temperature increase, FT_max recorded in the field presents critical and act as a thermal threshold for the organisms; the thermal performance curve is predicted based on the ant foraging activity in function of temperature. Through the comparison between CT_max and FT_max, the results can examine will the species stop at their CT_max if the environmental temperature reached their CT_max and provide a biologically relevant ranking of the CT_max values retrieved by different ramping rates (0.2, 0.5 and 1°Cmin^-1). (D) Examination of the effect of ramping rates used to measure CT_max values on the phylogenic signals using Pagel’s λ and Blomberg’s K. (E) Examination of the effects of the ramping rates used to measure CT_max values on community comparisons from different strata and habitats.

Fig 2. Results of dynamic assays.

Line plots of CT_max values measured in function of three ramping rates used for 27 ant species found in function of their vertical stratification (arboreal, ground and subterranean strata).

Fig 3. Results of static assays.

Mean exposure duration-based tolerance values (±SE) of 24 ant species for three temperatures based on the values retrieved in the CT_{max (0.2, 0.5, and 1.0)} treatments. Right y-axis refers to the duration tolerance the ants were maintaining their muscle control, and left y-axis refers to the log-transformed duration tolerance values.

Fig 4. Critical Thermal maximum vs. foraging temperature records.

Upper plot showing the difference between FT_max and CT_{max (0.2, 0.5, and 1.0)} and errors bars as standard deviation of the CT_max values. Lower plot shows the range and distribution frequency of all surface temperatures measured near baiting stations during the sampling period, independently of the presence of ants or not. Vertical lines indicate the FT_max values measured for each species in the field.

Fig 5. Critical Thermal maximum (CT_max) of 27 ant species in function of the phylogeny.

Color shading corresponds with the magnitude of thermal tolerance measured with different ramping rates, A. 0.2°C min^-1, B. 0.5°C min^-1, C. 1.0°C min^-1. Ant illustrations credited to Mr Runxi Wang with permission.