Global synthesis of aquatic insect heat tolerance reveals oxygen availability as a key driver of climate vulnerability

Accurately predicting species responses to climate change requires an understanding of the drivers of their thermal limits. Despite rapid warming of freshwater ecosystems worldwide, we still lack a global perspective on how upper thermal limits (UTLs) vary among aquatic insects, what constrains these limits, and how they contribute to species vulnerability. Here, we compiled a global dataset encompassing 423 aquatic insect species to test the effects of environmental conditions, organismal traits, acclimation history, and phylogenic relationships on patterns of heat tolerance. Maximum habitat temperatures were positively correlated with UTLs supporting the Climate Extremes Hypothesis, and insects relying exclusively on dissolved oxygen had the lowest UTLs supporting the Oxygen- and Capacity-Limited Thermal Tolerance hypothesis. Functional traits also explained substantial variation in UTLs; those that feed via scraping and shredding exhibited some of the lowest UTLs. Laboratory acclimation methods further influenced UTL estimates. Short-term exposure to higher acclimation temperatures increased UTLs, but longer exposure led to decreased heat tolerance. Finally, warming tolerance, i.e., the difference between UTL and the maximum habitat temperature) varied with breathing mode. Across latitude, warming tolerances were lowest for obligate dissolved oxygen-breathers but increased more rapidly in insects that can access terrestrial air. Collectively, these patterns indicate that oxygen is a key mechanism shaping thermal vulnerability in aquatic insects.