Constraints of tolerance: why are desiccation-tolerant organisms so small or rare?

Drying to equilibrium with the air kills nearly all animals and flowering plants, including livestock and crops. This makes drought a key ecological problem for terrestrial life and a major cause of human famine. However, the ability to tolerate complete desiccation is widespread in organisms that are either < 5 mm long or found mainly where desiccation-sensitive organisms are scarce. This suggests that there is a trade-off between desiccation tolerance and growth. Recent molecular and biochemical research shows that organisms tolerate desiccation through a set of mechanisms, including sugars that replace water and form glasses, proteins that stabilize macromolecules and membranes, and anti-oxidants that counter damage by reactive oxygen species. These protections are often induced by drying, and some of the genes involved may be homologous in microbes, plants and animals. Understanding how mechanisms of desiccation tolerance may constrain growth might show how to undo the constraint in some economically important macroorganisms and elucidate the much-studied but elusive relationship between tolerance of stress and productivity.