Temperature dependence of metabolic rate in tropical and temperate aquatic insects: Support for the Climate Variability Hypothesis in mayflies but not stoneflies

A fundamental gap in climate change vulnerability research is an understanding of the relative thermal sensitivity of ectotherms. Aquatic insects are vital to stream ecosystem function and biodiversity but insufficiently studied with respect to their thermal physiology. With global temperatures rising at an unprecedented rate, it is imperative that we know how aquatic insects respond to increasing temperature and whether these responses vary among taxa, latitudes, and elevations. We evaluated the thermal sensitivity of standard metabolic rate in stream‐dwelling baetid mayflies and perlid stoneflies across a ~2,000 m elevation gradient in the temperate Rocky Mountains in Colorado, USA, and the tropical Andes in Napo, Ecuador. We used temperature‐controlled water baths and microrespirometry to estimate changes in oxygen consumption. Tropical mayflies generally exhibited greater thermal sensitivity in metabolism compared to temperate mayflies; tropical mayfly metabolic rates increased more rapidly with temperature and the insects more frequently exhibited behavioral signs of thermal stress. By contrast, temperate and tropical stoneflies did not clearly differ. Varied responses to temperature among baetid mayflies and perlid stoneflies may reflect differences in evolutionary history or ecological roles as herbivores and predators, respectively. Our results show that there is physiological variation across elevations and species and that low‐elevation tropical mayflies may be especially imperiled by climate warming. Given such variation among species, broad generalizations about the vulnerability of tropical ectotherms should be made more cautiously. Schematic of expectations for change in standard metabolic rate (SMR; O2 consumption) and performance in response to temperature in an aquatic ectotherm. The thermal performance curve (TPC; green line) is a functional performance trait where higher performance is better. TOPT, where performance is highest, reflects the animal's preferred temperature. The oxygen consumption curve (dashed line) is a special case of TPC. T‐MRPEAK represents the temperature at which SMR is so high that it equals maximum metabolic rate. Predictions for variation in metabolic rate, based on the Climate Variability Hypothesis, are shown on the right. Our predictions were met in mayflies, but not stoneflies.