Annual Temperature Range Drives Thermal Breadth of Freshwater Insects Across Multiple Spatial Scales

Thermal breadth (T_br) is a critical trait influencing organisms' response to climate change. Three hypotheses seek to explain interspecific and interpopulation variation in T_br, each relying on a different temperature driver: Climate Variability Hypothesis (CVH)–annual temperature range; Thermal Constraints Hypothesis (TCH)–mean annual temperature; and Diel Narrowing Hypothesis (DNH)–mean diel temperature range. We test these hypotheses by measuring the T_br of freshwater insects (Ephemeroptera, Plecoptera and Trichoptera) along elevational gradients in the Americas and Australia, contrasting warm and cool adapted species from low climate variability tropical regions and high climate variability temperate regions. We find strong and consistent support for the CVH, limited support for the TCH (in two of four regions) and no support for the DNH. We conclude that T_br of freshwater insects is better explained by annual temperature variability, rather than high temperatures or diel temperature variability.