Divergence of thermal physiological traits in terrestrial breeding frogs along a tropical elevational gradient.

Rudolf von May,Craig Moritz,Ana Carolina Carnaval,Ammon Corl,Roy Santa‐Cruz,Alessandro Catenazzi

doi:10.1002/ece3.2929

Abstract

Critical thermal limits are thought to be correlated with the elevational distribution of species living in tropical montane regions, but with upper limits being relatively invariant compared to lower limits. To test this hypothesis, we examined the variation of thermal physiological traits in a group of terrestrial breeding frogs (Craugastoridae) distributed along a tropical elevational gradient. We measured the critical thermal maximum (CT max; n = 22 species) and critical thermal minimum (CT min; n = 14 species) of frogs captured between the Amazon floodplain (250 m asl) and the high Andes (3,800 m asl). After inferring a multilocus species tree, we conducted a phylogenetically informed test of whether body size, body mass, and elevation contributed to the observed variation in CT max and CT min along the gradient. We also tested whether CT max and CT min exhibit different rates of change given that critical thermal limits (and their plasticity) may have evolved differently in response to different temperature constraints along the gradient. Variation of critical thermal traits was significantly correlated with species’ elevational midpoint, their maximum and minimum elevations, as well as the maximum air temperature and the maximum operative temperature as measured across this gradient. Both thermal limits showed substantial variation, but CT min exhibited relatively faster rates of change than CT max, as observed in other taxa. Nonetheless, our findings call for caution in assuming inflexibility of upper thermal limits and underscore the value of collecting additional empirical data on species’ thermal physiology across elevational gradients.

Highlights

In a rapidly changing world, many species are faced with shrinking habitat and novel climatic conditions
The high variability observed in both critical thermal maximum (CTmax) and critical thermal minimum (CTmin) among closely related species (Figure 3 and Fig. S4) supports the idea that thermal traits in ectotherms can adjust through evolutionary time
Our tests of phylogenetic signal focusing on CTmax indicated that closely related species are less similar than expected under a Brownian motion model of evolution, supporting the idea that even upper thermal limits can change rapidly in this diverse amphibian clade

Summary

Introduction

In a rapidly changing world, many species are faced with shrinking habitat and novel climatic conditions. | 3258 predicting elevational range shifts in montane organisms, especially in the context of climate change, most predictions about future geographic ranges are based on correlative models that ignore species’ evolutionary history and eco-physiology (Colwell, Brehm, Cardelús, Gilman, & Longino, 2008; Laurance et al, 2011; VanDerWal et al, 2012). Empirical data on critical thermal limits of most tropical montane taxa remain unknown. Tropical lowland taxa, especially ectotherms, are thought to live near their thermal optimum, so increased temperatures due to changing climates would lead to decreased fitness (Colwell et al, 2008; Huey et al, 2009; Sunday et al, 2014). As with species living at higher elevations, empirical data on species’ critical thermal limits are not available for most tropical lowland taxa

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