Oxygen availability becomes a limiting factor for organisms living:

1. Air is a mixture whose density falls with altitude. 2. Fewer air molecules in a given volume means fewer oxygen molecules too. 3. Oxygen availability can be an issue for organisms living at very high elevations, where there are fewer molecules of oxygen in the air. 4. At low elevations the proportion of oxygen is the same and the density is higher, so supply is not limiting. 5. In water, by contrast, oxygen limitation comes from decomposition rather than altitude. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 35 'Ecology and the Biosphere', sections 35.1-35.5_