Moore, M. P.; Brasket, A. R.; Mokriski, M. J.; Alvarez, K.; Steinmeier, T.

Oxygen declines by ~10% for each 1000m gained above sea level. Although the impact of low oxygen on endotherms has received considerable attention, we know far less about whether or not hypoxia can set the elevational limits of terrestrial ectotherms. Here, we addressed this question using Nearctic dragonflies. First, we experimentally tested if dragonflies are vulnerable to the low oxygen found at high elevations. Our results show that even resting adults struggle with the PO2 that occurs at the highest elevations where dragonflies are found in Colorado, USA. Second, we used a comparative approach to test if the dragonfly species that can inhabit higher elevations are also more tolerant of low oxygen. Our analysis shows that a species upper elevational limit is precisely predicted by its vulnerability to low oxygen. Collectively, these results provide some of the first evidence that low oxygen can be an important constraint on the elevational maxima of terrestrial ectotherms.