Does heat tolerance actually predict geographic thermal limits of animal species?

Abstract

To understand species' climatic vulnerability, our measures of species thermal tolerance should predict their geographic thermal limits. Yet, this assumption is ungranted. We tested if animals' heat tolerance restrict the warmest temperatures they can live at (Tmax), distinguishing among species differently challenged by their thermal environment. For that, we compiled 2350 measurements of species heat tolerance indexes and corresponding Tmax, measured at different microhabitats. We show that reptiles, a flagship for climatic vulnerability studies, are particularly unbounded by their heat tolerance. Contrarily, tolerance restricted marine fish geography in a non-linear fashion which contrasts with terrestrial taxa. Behavioral tolerance indexes, widely used to predict vulnerability, predicted Tmax inconsistently across Tmax indexes, or were inversely related to it. Animals heat tolerance restricts geographic limits more strongly for more thermally challenged species. In turn, factors uncoupling heat tolerance and Tmax (plasticity, thermoregulation, adaptation) should be more important for less thermally challenged species. Significance StatementTo identify climatic vulnerability, heat tolerance indices need to predict species' thermal limits to geographic distribution. Yet, we show that heat tolerance predicts geographic limits quite heterogeneously, depending on taxa, the type of measure of heat tolerance and how challenged are species at their hottest known location. Particularly, reptiles, a flagship of vulnerability studies, were less bounded by tolerance than taxa regarded as more capable to evade high temperatures, due to their capacity to evaporate water, find refuge, or migrate (Ex. amphibians, arthropods, birds and mammals). Measures of species behavioral heat tolerance may still need to improve. Factors thought to uncouple thermal tolerance and geographic limits should be stronger for less thermally challenged species.