ALPERT, P; Univ. of Massachusetts – Amherst: The constraints of tolerance: why are desiccation-tolerant organisms small and rare?

The ability to tolerate desiccation is common in terrestrial microbes, bryophytes, and lichens but rare in vascular plants. No desiccation-tolerant plant grows more than 3 m tall, and over 75% of the vascular species are associated with rock outcrops where desiccation-sensitive plants cannot survive. The size of desiccation-tolerant plants may be constrained by inability to re-establish upward movement of water in dried stems above 3 m. The abundance of desiccation-tolerant plants may be constrained by trade offs between tolerance and growth rate; maximum growth rate may be lower in more tolerant species of plants, causing them to be out-competed by less tolerant species. In animals, desiccation tolerance appears to be common in three phyla, very rare in arthropods, and otherwise absent. Tolerant animals are almost all microscopic, and none exceeds 5 cm. Like the tolerant vascular plants, the tolerant arthropods are restricted to habitats where water availability is at least periodically very low. What may constrain the maximum size of desiccation-tolerant animals is largely unknown, nor is it well known how growth rates compare in more and less tolerant species. However, tolerance in both plants and animals appears to rely on the synthesis of solutes that maintain the configuration of proteins and the structure of membranes in the absence of water. This requirement for tolerance may impose energetic costs and limits on metabolism that cause selection against tolerance except where it is essential for survival.