The oxygen and pressure physiology of upper elevational range limits in insects

Abstract Climate warming is widely expected to cause montane insects to shift upslope, and there is good evidence that some populations are doing so. Nevertheless, patterns are mixed—with evidence supporting upslope, downslope or even no movement of both lower and upper range limits. The diversity of responses likely reflects the diversity of physiological and ecological starting points, which differ enormously among species. Here, we assess how potential upslope movement is affected by lower levels of oxygen (hypoxia) and lower pressure (hypobaria) at higher elevations. We leverage a rich body of physiological literature to evaluate three oxygen‐ and pressure‐related hypotheses. First, high‐elevation hypoxia depresses metabolic rates and associated levels of performance, and this is particularly problematic for flying insects as air becomes less dense. We evaluate this idea by mapping published values of critical (partial pressure of oxygen that starts to depress performance) onto distributions of across elevations. Second, high‐elevation hypoxia and hypobaria elevate rates of water loss; we evaluate this idea using the literature on factors influencing patterns of insect gas exchange and water loss. And third, high‐elevation hypoxia depresses upper critical thermal limits; we summarize recent discussions about whether and how much oxygen influences critical thermal maxima (CT MAX ) in the context of potential heat stress at high elevations. Collectively, the evidence suggests that hypoxia may slow or prevent upslope movement in many species. Although many authors have acknowledged this possibility, it is time now to direct more experimental work at the problem. We suggest that the highest priorities should be work on (i) adult flight capacity, as flying insects have very high metabolic rates for their size, especially at higher elevations, and often thoracic temperatures elevated far above air temperature; (ii) chronic effects of hypobaric hypoxia on long‐duration life stages, which often includes in‐stream or overwintering stages; and (iii) comparisons of vulnerabilities of aquatic versus terrestrial species. Read the free Plain Language Summary for this article on the Journal blog.