Water Uptake in Deserts Animals

Vertebrates are able to obtain water from three sources: (1) free water, (2) moisture contained in food, and (3) metabolic water formed during the process of cellular respiration. Some are able to receive water from all three sources, while others are able to exploit only one or two methods. Highly mobile animals tend to be restricted to the use of open water sources that are often sparse and far between. Typical examples are desert birds that fly in regular intervals to the few bodies of water available. To mention are the desert-adapted orders of sand grouse (Pteroclidiformes) and some doves (Columbiformes) that tend to visit standing water in large flocks at dawn and/or dusk. The former are even known to transport water soaked in their specialized belly feathers to their flightless chicks. Many desert animals are able to use available water opportunistically by drinking large quantities in short time. This ability is proverbial in the camel that can take up to 30% of its body weight in a few minutes. Camels and other desert mammals have resistant blood cells that can withstand osmotic imbalance. Animals living in more mesic environments (including humans) would destroy their red blood cell at such high water content in their blood. Much of the free available water has high salinity, and so it is not a surprise that many desert animals show high salt tolerance, for instance by employing salt-excreting glands. Other animals, mostly the ones that are restricted in their mobility (e.g., mammals, reptiles, and insects), rely on water obtained from their food. Carnivorous and insectivorous animals typically receive enough water from their prey. Herbivores do so as well, as long as the moisture content of the consumed plant material is relatively high (>15% of fresh weight: fresh shoots and leaves, fruits, and berries). The ultimate desert-adapted method however is the extraction of metabolic water. Especially seed-eating (granivorous) animals are able to metabolically oxidize fat, carbohydrate, or protein. Rodents and some groups of desert birds (e.g., larks, Old World and New World sparrows) are able to convert these energy sources into water: 1 g of fat produces 1.1 g of water, 1 g of protein produces 0.4 g of water, and 1 g of carbohydrates produces 0.6 g of water. Schmidt-Nielson has shown that kangaroo rats (genus Dipodomys) are able to obtain 90% of their water balance from metabolic water derived from consumed seeds. The remaining 10% is obtained from moisture stored in seeds. The use of already stored body fat as source of water is controversial. It has been argued that metabolizing fat and other storage sources into water requires increased ventilation and therefore increases water loss by transpiration from lung tissue. At the most, no net gain of water will be the result. According to this, the camel's hump might function simply as a fat energy storage facility, one that is situated in one place in order to reduce isolation and allow dissipation of heat.

In areas with high humidity, animals are able to receive water from dew. Such direct uptake as the main source of water is probably restricted to arthropods and some mollusks (snails). There is some evidence that rodents can utilize condensation by water enrichment of stored food (Figure 11).

Plants (and microorganisms)

Plants, with few exceptions, depend on water uptake by their roots from the soil. Due to low soil matrix water potentials and high salinity in arid regions, such soil water is often not readily available. One way for desert plants to overcome this restriction physiologically is to osmoregu-late the plant cell water potentials to overcome the low potentials of desert soils, a mechanism that also aids them in extracting water from saline solutions. Indeed, some of the lowest water potentials have been measured in desert shrubs (—8 to — 16MPa (mesic plants rarely go below —2 to — 3 MPa)) and salt-tolerant (halophytes)

Desert Rodents

Figure 11 Desert sand rat (Psammomys obesus). As the scientific name implies, this day-active desert rodent can store large amounts of body fat as reserves during unproductive seasons. Like other desert rodents, it obtains all of its needed water through its plant diet. Negev Desert, Mitzpe Ramon, Israel, May 2003. Photograph by C. Holzapfel.

Figure 11 Desert sand rat (Psammomys obesus). As the scientific name implies, this day-active desert rodent can store large amounts of body fat as reserves during unproductive seasons. Like other desert rodents, it obtains all of its needed water through its plant diet. Negev Desert, Mitzpe Ramon, Israel, May 2003. Photograph by C. Holzapfel.

desert perennials (as low as —9MPa). In general, many desert plants tend be deep rooted and are therefore able to exploit water reserves that tend to be available in the deeper soil layers. Due to the need of desert plants to forage extensively for water, root-to-shoot ratio of desert plants is typically high and rooting depths are larger than in other ecosystems. In extreme cases, as in phreatopytes, rooting depth can exceed 50 m. This was found for mes-quite trees (genus Prosopis) that are practically independent from local precipitation and are able to maintain very high transpiration rates for prolonged periods. In contrast and as mentioned before, many succulent plants that store water in their tissues tend to be shallowly rooted and are able to intercept even light summer rains that do not cause a deeper recharge of soils and would otherwise be lost to evaporation. Annual plants and most grasses also benefit from being shallowly rooted. In general, many desert plants can react quickly to available water by deploying fast-growing 'water roots' from special dormant root meristems. Shallow rooting plants show temporally intensive water exploitation patterns while plants with deeper root systems are characterized by spatially extensive water exploitation patterns.

Some deep-rooted perennial plants exhibit hydraulic redistribution from deeper soils to shallow soils. Water is absorbed from the soil at greater depth during the day and moves via the transpiration stream upward into shallower roots and the aboveground parts of the plant. At night when the air is more humid and plant stomata are closed, plants become often fully hydrated and water may be exuded from the root into the dry shallow soil. This pattern, described as hydraulic lift, may have nutritional benefits for the perennial plant itself, as it enables it to utilize the nutrients from what would have otherwise been dry soil. Released water - on the other hand -might become available for competing plants. Hydraulic lift has been described in almost all of the dominant shrubs of the arid Western US (e.g., Artemisia tridentata, Larrea tridentata, Ambrosia dumosa) and might be prevalent all over the world's arid zones.

Plants of saline habitats, halophytes, must be able to acquire water with high salt concentrations. They need to overcome the high osmotic pressure of saline solutions and need to avoid the potential toxicity of some ions (Na+, Cl-). In order to achieve such a high salt tolerance, halophytes employ strategies as osmoregulation, dilution of inner cell salt concentration by succulence, and use specialized salt-excreting glands.

Special water-rich habitats within deserts, for instance, permanent stream sides and springs, attract extrazonal plants that often possess only few aridity adaptations. Found in these oases are wetland plants and some salttolerant tree species that can be characterized as 'water spenders'. Good examples are palm trees (the date palm Phoenix dactylifera and the Californian palm Washingtonia filifera) and salt cedars (Tamarix species).

Direct uptake of condensed atmospheric water (dew and fog) and water vapor is generally possible only for some specialized poikilohydrous vascular plants, but is of much greater importance for microbiotic organisms such as lichens and cyanobacteria.

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