High Altitude Limits of Living Things

West, John B. High altitude limits of living things. High Alt Med Biol. 22:342-345, 2021.-The tolerance of animals to high altitude is generally limited by the low partial pressure of oxygen (PO₂) in the air. Plant growth at high altitude is also limited but by different mechanisms. This article is a brief survey of the limiting factors of all living things. By a curious coincidence, the highest point on earth, that is Mt. Everest at 8,848 m, appears to be right at the limit of human tolerance to hypoxia. The altitude of the highest permanent human habitation, that is a town, is 5,100 m. This altitude is partly determined by the hypoxia, but also by economic factors. For other terrestrial mammals, birds, and insects, the highest altitudes for permanent habitation apparently belong to field mice (Phyllotis xanthopygus rupestris) and jumping spiders (Euophrys omnisuperstes) at about 6,700 m. Birds have been known to fly as high as 11,000 m although how much they are elevated by atmospheric updrafts is not clear. The record for animals for survival in extreme hypoxia is arguably held by the primitive invertebrate, the tardigrade (Hypsibius dujardini). This has been shown to tolerate the hard vacuum of space where the PO₂ is essentially zero for many days. Less is known about the tolerance of plants to extreme altitude. However, vascular plants have been collected at >6,000 m in the Himalayas, and moss grows even higher. Lichens are very tolerant of severe hypoxia. There is evidence that global warming is increasing the highest altitudes at which plants can survive.