Strategies for Coping with Drought

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All life originated in the sea and all organisms that have left their ancestral home depend on an 'inner sea', high internal water content. This phylogenetic inheritage restricts life in many habitats, and obviously deserts are among the harshest in this respect. Even though deserts are not only water limited (they are also low in nutrients and energy resources), adaptations to cope with the spatiotemporal scarcity of water are predominant of most (if not all) true desert organisms.

All desert life forms, animals, plants, and microorganisms alike, employ one or more of three basic strategies to cope with the dearth of water: (1) drought evasion, a strategy of avoiding water stress temporarily in inactive states; (2) drought endurance, a suit of adaptations that reduce actual stress and enable being active during drought; and (3) drought resistance, a suit of adaptations evolved to avoid water stress altogether. Note that water and heat stresses are coupled, thus many of the adaptations mentioned below can be understood as strategies to cope with both.

1. Drought-evading organisms 'choose' to pass exceedingly dry periods in dormant stages. Predominant examples are short-lived (ephemeral) plants that survive the dry season or longer periods of drought in the dormant seed stage. Such annual plants are indeed very common in many deserts of the world and compose a large portion of the plant diversity in many areas (up to 80% of species richness). An equivalent for animals can be found in cryptobiosis of invertebrate eggs and larvae. Such aridopassivity can be found in fully developed organisms as well; examples are bulbous geophytes and desert animals that pass dry season belowground inactive (estivation). Choosing of less arid microsites is another way of avoiding drought. In animals, these are typically behavioral space choices (e.g., permanent habitation or temporary use of stress-protected microsites: below shrubs or stones, rock fissures, litter, below tree and shrub canopies, or even soaring in high air).

Likewise, many plants are restricted to favorable microsites (e.g., under tree and shrub canopies, runon microsites, algae growing under stones). Some organisms, mostly plants, are able to lose water almost completely and 'resurrect' once water becomes available again (poikilohydry: Selaginella species, algae, lichens, and moss species).

2. Drought endurance is a main strategy common among the dominant desert organisms worldwide. A suit of ecophysiological, morphological, and behavioral adaptations work together to reduce the most detrimental impacts of water stress.

Reducing water expenditure. Evergreen desert shrubs are capable of fine-tuned regulation of stomatal movement. Specialized photosynthetic pathways evolved in desert plants that minimize water loss and maximize carboxyla-tion. C4 and crassulacean acid metabolism (CAM) pathways are adaptations to hot temperatures, compared to the C3 pathway adapted to colder conditions. Animals of arid regions are able to regulate and restrict water loss by concentrating urine. Birds and reptiles excrete urinary waste as uric acid that can be concentrated and allow reabsorption of water in the urinary tract, a trait not available to mammals. Desert mammals and most other taxa excrete dry feces and reduce the urine flow rate. Water loss through surfaces is reduced in plants through an increase in thick lipid cuticulae, epidermal hair cover, sunken stomata, small surface/volume ratio (leafless plants with photosynthesizing stems - xenomorphic). Animals employ a variety of adaptations that reduce water loss: impermeable integuments (e.g., in arthropods), changes of lipid structure in the epidermis that create diffusion barriers to water vapor (some desert birds), denser hair or feather cover, and small surface-to-volume ratios (common in large mammals).

Prevention of overheating. High-temperature stress is closely connected to water stress as many of the ways of coping with higher temperatures involve expenditure of water, thereby exacerbating water stress. Examples are transpiration cooling in plants and evaporative cooling in animals (including humans; see below). Desert organisms typically have high heat tolerance and capability to function at high temperatures. The comparatively high-temperature optima and temperature compensation points ofphotosynthesis in plants and high body temperatures and high lethal temperatures in animals attest that. Among the most thermotolerant species are desert-dwelling ants that forage on extremely hot surfaces. A Saharan desert ant species (Cataglyphis bicolor) is noted to hold the record with a critical thermal maximum of 55 ± 1 ° C.

Apart from tolerating high temperatures, an array of mechanisms evolved to decrease or dissipate heat loads both in plant and animals. The formation of sheltering boundary layers, employment of insulating structures, and increase of reflection (white color, glossiness) are among these mechanisms. Behavioral space and temporal choices are a contribution to the prevention of overheating. Seeking of sheltered microhabitats and nocturnal activity of many (if not most) desert animals are obvious examples. The nocturnal CO2 uptake in CAM plants is an interesting analog to this.

3. Drought-resisting organisms employ adaptations that allow them to pass dry periods in an active state without experiencing physiological water stress. The succulence of many typical desert plants worldwide is a form of water storage that enables these plants to use water during dry periods. Examples for taxa that are rich in succulent species are the cacti (Cactaceae) and yuccas (Agavaeae) in the New World and some members of Euphorbiacea and Crassulaceae in the Old World. Succulent plants typically cannot become dormant and therefore require at least periodically predictable precipitation, a requirement that explains the general lack of succulent plants in extreme arid environments where prolonged droughts are common. Most succulent plants have fairly shallow root systems that react very quickly following larger rainfall events. An analog to plant succulence in animals can be found in desert snails that can store large amount of water. The ability of desert mammals (notably the camel) to store large amount of water in the blood is another analogous trait. The accumulation of fat tissue that can be metabolically transformed into water (see below) as a water storage mechanism is somewhat controversial and is more universally understood as being merely an energy source (e.g., fat reserves in desert reptile tails, body of rodents, and the famous camel's hump).

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