Climate variability predicts thermal limits of aquatic insects across elevation and latitude

Janzen's extension of the climate variability hypothesis posits that increased seasonal variation at high latitudes should result in greater temperature overlap across elevations, and favor wider thermal breadths in temperate organisms compared to their tropical counterparts. We tested these predictions by measuring stream temperatures and thermal breadths (i.e. the difference between the critical thermal maximum and minimum) of 62 aquatic insect species from temperate (Colorado, USA) and tropical (Papallacta, Ecuador) streams spanning an elevation gradient of ca. 2000m. Temperate streams exhibited greater seasonal temperature variation and overlap across elevations than tropical streams, and as predicted, temperate aquatic insects exhibited broader thermal breadths than tropical insects. However, elevation had contrasting effects on patterns of thermal breadth. In temperate species, thermal breadth decreased with increasing elevation because CTMAX declined with elevation while CTMIN ...