Variations in environmental temperature across geographic gradients are expected to produce physiological differentiation and local adaptation of ectotherms depending on their thermal tolerances and physiological sensitivities. Numerous studies have focused on the relationship between upper thermal tolerance limits and geographical gradient, which is important for our understanding of species response to climate change. However, current studies are strongly skewed to global-scale projects even though smaller scale studies can convey equally significant findings. Here, we focus on the thermal tolerance variations of closely located populations along a 500-km latitudinal gradient in the Republic of Korea. We investigated variations in the critical thermal maximum (CTmax) of larvae of Rana uenoi and Bufo gargarizans. We exposed tadpoles under increased temperatures and recorded CTmax as the point when the righting response was lost. Then we performed regression analyses to explore the relationship between the geographical gradient and CTmax using linear mixed-effect models method in the R package lme4.
Results/Conclusions
Latitude was not significant for CTmax in B. gargarizans, whereas CTmax generally increased with latitudes in R. uenoi (R2 = 0.08, P < 0.05). Furthermore, average CTmax of B. gargarizans (38.04 °C ± 0.6) was much higher than that of R .uenoi (35.84 °C ± 2.27). No relationship between CTmax and geographical gradient and higher values of CTmax might be critical for the larger distribution range of B. gargarizans, because these features of CTmax may enable this species to adapt both dry and moist environments. The positive relationship between CTmax and latitude may suggest a high degree of sensitivity to an environmental gradient in R. uenoi. This pattern is opposite to the general pattern, which is the negative relationship between CTmax and latitude. This opposite pattern of thermal response in R. uenoi may be explained by the counter-gradient variation model, suggesting an evolutionary trade-off acting on thermal physiology. Finally, we suggest the necessity of individual species research for practical and reliable assessments on species response to climate change as they exhibited physiological differentiation.