Warming undermines emergence success in a threatened alpine stonefly: A multi‐trait perspective on vulnerability to climate change

Vulnerability to warming is often assessed using short‐term metrics such as the critical thermal maximum (CTMAX), which represents an organism's ability to survive extreme heat. However, the long‐term effects of sub‐lethal warming are an essential link to fitness in the wild, and these effects are not adequately captured by metrics like CTMAX. The meltwater stonefly, Lednia tumana, is endemic to high‐elevation streams of Glacier National Park, MT, USA, and has long been considered acutely vulnerable to climate‐change‐associated stream warming. As a result, in 2019, it was listed as Threatened under the U.S. Endangered Species Act. This presumed vulnerability to warming was challenged by a recent study showing that nymphs can withstand short‐term exposure to temperatures as high as ~27°C. But whether they also tolerate exposure to chronic, long‐term warming remained unclear. By measuring fitness‐related traits at several ecologically relevant temperatures over several weeks, we show that L. tumana cannot complete its life‐cycle at temperatures only a few degrees above what some populations currently experience. The temperature at which growth rate was maximized appears to have a detrimental impact on other key traits (survival, emergence success and wing development), thus extending our understanding of L. tumana's vulnerability to climate change. Our results call into question the use of CTMAX as a sole metric of thermal sensitivity for a species, while highlighting the power and complexity of multi‐trait approaches to assessing vulnerability. Read the free Plain Language Summary for this article on the Journal blog. Resumen La vulnerabilidad al calentamiento se evalúa a menudo mediante medidas a corto plazo, como la temperatura crítica máxima (TCMAX), que representa la capacidad de un organismo para sobrevivir al calor extremo. Sin embargo, los efectos a largo plazo del calentamiento subletal son una parte esencial de la eficacia biológica en la naturaleza, y estos efectos no se capturan adecuadamente mediante medidas como TCMAX. La mosca de la roca del agua de deshielo (meltwater stonefly), Lednia tumana, es endémica de los arroyos de elevada altitud del Parque Nacional de los Glaciares, MT, EE. UU., y desde hace tiempo se considera extremadamente vulnerable al calentamiento de los arroyos asociado al cambio climático. Como resultado, en 2019, fue catalogada como Amenazada bajo la Ley para la Protección de Especies en Peligro de Extinción de EE. UU. Esta