Not all deserts are hot and sandy. A desert is defined by its lack of water rather than its temperature. This does not mean that there is no water but that rainfalls, and other inputs of water, are irregular and infrequent. Deserts cover one-third of the Earth's land surface (Figure 2.1). This includes semi-arid areas (annual precipitation of less than 600 millimetres), arid areas (less than 200 millimetres) and hyper-arid areas (less than 25 millimetres). Deserts form in the leeward side of mountain ranges, in inland areas which are remote from oceans and where dry stable air masses form, resisting convective currents which would bring rain. Most of the world's hot deserts (subtropical deserts such as the Australian and Arabian deserts and the Sahara Desert) are located
figure 2.1 The world's major deserts. (1) Great Basin; (2)Mojave/Sonoran; (3) Baja California; (4) Chihuahuan; (5) Peruvian; (6) Atacama (Chile); (7) Patagonian; (8) Sahara; (9) Namib; (10) Kalahari; (11) Arabian; (12) Turkestan; (13) Iranian; (14) Thar; (15) Gobi; (16) Australian. Drawing by Ken Miller.
in the area between 25° to 35° north and south of the equator, with dry air tending to be trapped between major wind belts and storm systems. Cool coastal deserts (such as the Namib Desert, the Peruvian-Chilean deserts and the desert of Baja California in Mexico) form in subtropical areas where there are cold sea currents from polar regions. The moist, cold air formed by contact with the cold ocean current moves inland as a thin layer beneath the hot and dry tropical air. This does not produce rainfall, but results in condensation at night that the organisms of these deserts use as a source of moisture. Temperate or cold deserts form in the rain shadow of mountain ranges or in areas which are at a great distance from oceans. The Gobi Desert of central Asia is an example, whose elevation and distance from the coast results in extremely dry and cold conditions. The polar deserts of Antarctica are rather different from other deserts since there is no shortage of water in Antarctica - it is thought to hold 90 per cent of the world's store of freshwater, with 99 per cent of the surface of Antarctica being covered by ice. Water as ice or snow is, however, not available to organisms and the small areas of ice-free land in Antarctica are largely dry. The Dry Valleys of Victoria Land (also called the Ross Desert) are the most extensive areas of ice-free land in Antarctica. They are extremely dry and parts of them are thought not to have received any rainfall, or other precipitation, for at least the last two million years. Deserts result from low rainfall but they are also places of extreme temperatures and of rapid changes in temperature. In the Sahara Desert, air temperatures can regularly reach in excess of 50 °C and ground temperatures can be as high as 70 °C, while in the Gobi Desert air temperatures are as low as —40°C at night.
When it does rain in a desert, the rain can be very heavy. This causes floods, but the water rapidly drains away or sinks into the ground. Water is thus only briefly available to desert organisms, unless they can access areas where water accumulates and persists for longer periods of time. Desert organisms have various mechanisms for coping with infrequent and unpredictable access to water. If there is no moisture available at all, however, there is no life. As well as limited access to water, desert organisms have the problem of hanging on to the water they have. This is partly because the air is very dry, but high temperatures and wind may also result in high rates of water loss from their bodies. They either have to survive the water loss or limit it so that they can retain sufficient water to last until it becomes available again.
The main stresses faced by desert organisms are thus infrequent and unpredictable access to water, high rates of water loss, extreme temperatures and large temperature ranges. Other problems may include precarious environments due to periodic floods, high winds and unstable ground, exposure to solar radiation and limited access to food or nutrients.
Organisms show two broad responses to the extreme conditions that they face in deserts. They have adaptations which enable them to function in the face of low water availability and high temperatures or they escape, retreat from or avoid the harsh conditions, restricting their activity or growth to periods that are more favourable. This corresponds to the capacity adaptations (adaptations which allow the organism to operate under extreme conditions) and resistance adaptations (adaptations which allow the organism to avoid or survive the stress until the conditions favourable for its growth return) that were described in Chapter 1. Many organisms show a mixture of these two responses. Which response is favoured by the organism depends on the particular demands of its habitat, its evolutionary history and factors such as its size and degree of mobility.
Capacity adaptation with respect to low water availability involves the ability to access the sources of water which are available, the ability to restrict water loss from the organism and the ability to store water. The response to high temperatures involves the organism's ability to avoid gaining heat and the ability to lose heat. Large organisms (like a camel) have a greater ability to store water than do small organisms (like a bacterium) and they have less of a problem with water loss since they have a smaller surface area in relation to their volume. Problems of gaining heat are also diminished, for a similar reason, but, conversely, they are less able to lose heat. The suite of adaptations which the organism has evolved also depends on the complexity of the organism. Mammals have a more sophisticated suite of adaptations than do bacteria. Large animals can move greater distances than small animals, while plants and microorganisms may have no or little ability to move. Animals can have behavioural adaptations in response to the harsh conditions - for example, migrating to find water sources - while plants and microorganisms have no, or a restricted, ability to respond in this way.
Resistance adaptation involves escaping from or avoiding the harsh conditions. This operates on various time scales, ranging from avoiding activity during the hottest part of the day to being active only at night through to lying dormant for months or years until it rains and conditions favourable for growth return. Cryptobiosis is the ultimate resistance adaptation, with the organism capable of surviving in an ametabolic dormant state for many years. Again, the kind of adaptations which the organism has evolved depends on its evolutionary history. Resistance adaptations which involve behavioural responses, such as burrowing into the sand, are found in animals. Plants and microorganisms have a greater tendency to dormancy, particularly in the formation of seeds and spores which lie dormant until the return of favourable conditions.
Mammals are the most complex of organisms and have evolved sophisticated behavioural and physiological responses to the demands of their environments. Their size and complexity does, however, impose some restrictions on the sorts of adaptations they have been able to develop and, for example, they are not capable of cryptobiosis. The camel is perhaps the most famous of desert mammals so let us look at why it is such a successful desert inhabitant.
There are two species of camels: the dromedary (one-humped or Arabian) camel and the Bactrian (two-humped) camel. Dromedaries are associated with hot, dry, flat deserts (subtropical deserts such as the Sahara Desert and Arabian Desert) while Bactrian camels live in mountainous, rocky regions (cold temperate deserts, such as the Gobi Desert). The two species coexist in Turkey, Afghanistan and Turkmenistan where they interbreed, indicating that they are closely related. The following discussion is focussed on the dromedary, which will be referred to simply as the camel. Dromedaries are entirely domesticated and so aspects of their biology are determined by their relationship to humans.
Camels are herbivores, browsing and grazing on desert vegetation. The desert vegetation is sparse because of the low rainfall. Annual plants, which provide lush grazing, appear only after rainfall and thus do not provide a reliable food source. Camels rely on hardy perennials which provide a more permanent food source that is adapted to the low rainfall. The main features of this food for the camel are that it is sparse, tough and salty, with small, curled leaves which may be reduced to thorns. Camels browse over a large area, often covering distances of 50 kilometres per day in their search for food. They browse sparsely, taking a bite from one plant and then a bite from another. This avoids overbrowsing the vegetation and allows it to recover; camels' use of their food source is thus sustainable. They eat a variety of plants and can eat plants which other animals cannot. Their ability to go for long periods without water means that they can feed on areas remote from water sources. The leathery lips and tongue of the camel allow it to contend with the tough vegetation and it can even eat the thorns themselves. The camel's physiology allows it to cope with the salt intake from its food and from drinking salty water; indeed, they need the salt and, in the absence of natural sources of salt, need to be given a daily ration of as much as 45-60 grams.
During the cooler months of the year, camels can rely entirely on their food plants for their intake of water, which on average consist of 50-60 per cent water during the cool season. Even when temperatures reach 30-35 °C, camels can go for as long as 15 days without drinking. It is only when temperatures exceed 40 °C that they need to drink at short and regular intervals. When they do drink, camels have a tremendous capacity to take in water. A camel which is dehydrated, after several days without water under hot conditions, may drink up to 200 litres of water over several hours and as much as 130 litres (about a bath full) in a few minutes during its first drinking session. This water is rapidly transferred to the camel's bloodstream, where it is available to rehydrate the tissues. This rapid dilution of the blood by an influx of water would be fatal for most animals since the resulting osmotic stress would cause the red blood cells to rupture. The camel's red blood cells are, however, unaffected. The ability to drink a large quantity of water in a short time also means that it can spend a minimum amount of time in overgrazed areas at water sources. In dehydrating conditions, the water content of the milk of lactating camels is actually increased, to ensure the survival of the young.
Camels can survive severe dehydration resulting in the loss of 20-25 per cent of their body weight (30 per cent of total body water). In contrast, humans die if they lose water equivalent to about 12 per cent of body weight. Most of this water is lost from the camel's gut and the spaces between the cells. Relatively little water is lost from the cells of the camel during dehydration. A dehydrated camel develops a hollow in its side behind the ribs. Nomads can judge to within 10 litres how much water a camel needs to drink from the shape of the hollow. No other mammal can survive without water for weeks on end and still remain active, a feat which makes camels invaluable to humans in arid areas. As well as surviving high levels of dehydration, camels have a number of mechanisms for conserving water within their bodies. The large body size of the camel means that the loss of water by evaporation through the skin is less than it is for smaller animals. Camels reduce their reliance on sweating to cool down at high temperatures. They produce a concentrated urine (high in salts and urea but low in water), relatively dry faeces and breathe slowly, reducing water loss in the breath. The requirement for a high salt intake is driven by the production of a concentrated urine. The nasal passages have large cavities which act to moisten dry inspired air and to recover water from expired gases.
There is no evidence that camels can store water anywhere in their bodies. Pliny the Elder (c. AD 23-79) was the first to suggest that camels store water in their stomach. However, there is no anatomical or physiological evidence that supports the suggestion that camels can store water in their stomach, or any other part of their body. Water is not stored in the hump. The hump consists mainly of fat and acts as a food store. The metabolism of fat does produce water, but this is exceeded by the amount of water lost in the gases expired during the respiration necessary to gain the oxygen involved in fat metabolism. Fat is an excellent insulator and if it was evenly distributed under the surface of the skin the camel would be unable to lose heat efficiently through its surface. Concentrating fat reserves in the hump (and around the kidneys) allows heat to be lost unimpeded over the remainder of the skin.
Most mammals control their internal temperatures within fairly tight limits. They lose heat by sweating, panting and by exposing areas of skin to the air. These mechanisms also result in water loss. Camels reduce the water loss by allowing their internal temperatures to fluctuate more than do other mammals. The difference between their internal temperature early in the morning and in the afternoon can be as much as 6 °C. By tolerating the increase in internal temperature, the camel conserves the water which would be lost through sweating and other mechanisms involved in losing heat. It is estimated that camels save as much as 5 litres of water per day by allowing their internal temperature to fluctuate in this way. Camels reduce heat gain by having a light-coloured skin and a thin woolly coat which shields the body from the sun. They face the sun, thus reducing the area exposed to heat radiation and the fat in their hump insulates the more exposed back. Their tall body shape and long legs allow heat to be lost from the hairless, poorly insulated lower half of the body and raises the body above the ground to where temperatures are cooler. The mechanisms which enable camels to survive in the desert are summarised in Figure 2.2.
Other large grazing mammals of subtropical deserts include antelopes, gazelles, zebu cattle and wild asses, and, in Australia, kangaroos
conduction, convection and radiation from the animal
reflected radiation heat generated metabolically water generated metabolically faeces and food and drink evaporation faeces and reflected radiation heat generated metabolically water generated metabolically food and drink evaporation
figure 2.2 The major routes for the gain and loss of heat and water in an animal such as a camel.
and wallabies. These share some of the adaptations to the desert environment shown by camels, although none show the same degree of resistance. Mammalian desert carnivores include foxes, jackals, hyenas, coyotes, small cats, badgers, skunks, ferrets, some Australian carnivorous marsupials and the dingo. Only foxes are found in extremely arid areas. These carnivores obtain most, if not all, of their water requirements from consuming the bodies of their prey.
Small desert mammals can avoid the worst stresses of heat and desiccation by retreating during the day, or during the hottest part of the day, into burrows or rock crevices. Most of these are rodents, no bigger than the size of a rat. Not only is the temperature within the burrow lower than that outside during the day, but the humidity is two to five times higher, due in part to the breath of the animal. By avoiding the heat of the day, desert rodents escape the temperature regulation problems posed by exposure to the sun. They do not sweat since their small body size, and hence relatively large surface area, means they cannot survive the loss of water. A rodent may, however, find itself trapped outside during the day - due, perhaps, to being driven from its burrow by a predator. Kangaroo-rats have an emergency temperature regulation system which enables them to survive in such situations. They produce copious saliva which wets the fur of the chin and the throat, producing cooling by evaporation. Normally, water loss is confined to the breath and this loss is reduced by cooling the air as it leaves the nose, so that it carries less water. Other water conservation measures include the production of a concentrated urine and dry faeces. Many desert rodents rely entirely on their food as a source of water. Egyptian jerboas and American kangaroo-rats can live indefinitely on a dry diet of plant seeds. Metabolic water, produced as a product of the oxidation of food, can be a major source of water for animals in arid areas.
Desert birds can travel large distances to obtain water. Their water supply problems become more acute during the breeding season when they must also satisfy the needs of their nestlings. The African sand grouse often nests 40 kilometres away from standing water. The male bird ferries water for his chicks by soaking his belly feathers. These have a unique structure which enables them to act like a sponge, soaking up and holding the water. The American 'Road Runner' bird produces a liquid from its stomach which trickles down the back of its throat and is drunk by its nestlings. The feathers of birds not only enable them to fly but are also very good insulation, protecting the bird from the heat of the sun. If they need to lose heat, they flutter their throats, creating a cooling flow of air through the moist inside of the mouth. Ostriches can raise their sparsely distributed dorsal feathers, allowing a cooling flow of air over a large area of their skin surface.
Desert reptiles, such as snakes, lizards, geckos and tortoises, like small mammals, avoid the hottest part of the day by burrowing into the ground or by sheltering in rock crevices. They have to perform a delicate balancing act apropos their internal temperature. The temperature needs to be high enough to allow the animal to be active, but, if it is too high, the animal will die. Reptiles have a very limited ability to generate their own heat, in contrast to mammals and birds, and rely on the heat of the sun to raise their temperature sufficiently for activity to occur. It is often cold at night in the desert and few reptiles are nocturnal. In the morning, they bask in the sun, absorbing the heat and raising their internal temperature to a level where they can become active. As the heat of the day increases, they retreat to their shelters. During the late afternoon and early evening, temperatures become suitable for activity again. Reptiles can extend their periods of activity by adaptations which enable them to gain or lose heat faster when they need to. Some Namib dune lizards, when they emerge in the morning, press their bodies against the warm sand to gain heat from the surface. As the temperature rises, they adopt a stilt-like posture which raises the body above the surface and periodically raise diagonally opposed limbs, allowing more heat loss.
Desert amphibians have particular problems with water availability, since they need to lay their eggs in water. They are consequently restricted to areas of deserts where a suitable body of water is at least periodically available. Desert ponds form after heavy rainfall and rapidly dry up. The rainfalls which supply sufficient water for amphibians to breed may be separated by long dry periods. One of the best known desert amphibians is the spadefoot toad of the Sonoran Desert of North America. The toads go into a dormant state during dry periods. They burrow nearly a metre into the ground to find relatively moist conditions where they can remain for as long as nine months. Some amphibians are surrounded by a mud cocoon while others produce a cocoon of dry dead skin which prevents rapid water loss. The animals also accumulate urea which raises the osmotic concentration of their tissues and results in a net flow of water into their bodies from the surrounding soil. They can also tolerate dehydration of the tissues and store water in the form of a dilute urine in the bladder. When it rains, the spadefoot toad emerges and breeds. The eggs develop rapidly and the pond becomes full of tadpoles. The pond may also become colonised by algae and fairy shrimps (small crustaceans). If fairy shrimps are present, two types of tadpoles are formed. Herbivorous tadpoles feed on the algae while larger carnivorous tadpoles feed not only on the shrimps but also on their herbivorous colleagues. The formation of these two types of tadpole is an insurance policy for the toad species. If it rains again, the pond will become muddy and the carnivorous tadpoles will not grow well since it is difficult for them to see their prey. The herbivorous tadpoles will continue to feed on the algae and may mature in large numbers. If it fails to rain, it is important that at least some individuals complete their development. The carnivorous and cannibalistic tadpoles develop quickly by consuming their siblings. They compete for the rapidly disappearing puddles of water and a few survive to breed after the next rainfall.
There are three broad groups of desert invertebrates: those permanently inhabiting the soil (dominated by nematodes, mites and spring-tails), those active on the surface for at least part of the time (centipedes, spiders, scorpions and a wide variety of insects) and those associated with temporary bodies of water (shrimps, crabs, nematodes and some insects). Other less prominent desert invertebrates include snails, woodlice, earthworms and millipedes.
Soil invertebrates feed on organic material that is buried by the action of wind and rain, on the microorganisms associated with its decomposition or on other soil invertebrates. Plant material is also buried by the activities of animals which are active on the surface, such as termites, ants and other insects. The highest densities of nematodes are found close to desert vegetation and in the top 10 centimetres of the soil. While the soil and the proximity of vegetation provides some protection against high temperatures and high rates of water loss, desiccation is likely to be a major challenge to their survival.
Arthropods active on the surface of the ground still spend a major part of their time beneath the surface where they are protected against the harsh environment and may only emerge to forage, collect water and perhaps find a mate. Ants and termites are major components of the invertebrate fauna of deserts, with their biomass exceeding that of all other invertebrates combined and even that of vertebrates in some places. A range of ant species is found in deserts, particularly in the arid regions of Australia. They live in nests beneath the ground. Termites are found in deserts throughout the world. Their nests extend both above and below the ground, but, in the hottest deserts, the nests are almost entirely underground. A foraging area is established around the nest. Most desert termites have little resistance to water loss and only forage when the humidity is high. Ants are more tolerant of desiccation but still retreat to the nest when temperatures are high. The architecture of the elaborately constructed termite mounds creates a flow of air which ventilates and cools the nest, and the orientation of the mound to the sun means that they tend to only absorb heat when air temperatures are low.
A wide variety of other groups of insects feed on desert vegetation -among the most prominent are the grasshoppers and locusts. Desert locusts occur in two forms which for a long time were thought to be separate species. Their life cycle allows them to take advantage of the flush of desert vegetation that occurs following rain. During dry periods, they are pale in colour, solitary and breed once per year. When it rains, the resulting flush of vegetation allows many more of the young to survive than do during dry periods. These become crowded together on the dwindling vegetation as it shrinks during subsequent dry periods. The crowding stimulates them to convert to the darker gregarious form and they can produce huge swarms numbering in the millions. Low pressure weather systems stimulate them to launch into the air where they are carried in a weather pattern which is likely to produce more rain and hence vegetation and food. These swarms have enormous destructive power and are a threat to human agriculture since they devour all vegetation in their path. Eventually, the numbers dwindle and they resume the solitary form.
Beetles from the family Tenebrionidae are often thought to be the insects that are best adapted to desert life, with their numbers increasing in areas adverse to other forms of life. They can live on dry foods without any water and some are active during the hottest part of the day. They are extremely tolerant of dehydration. The osmotic concentration of their body fluids (haemolymph) is maintained during episodes of dehydration and rehydration, although the haemolymph volume changes, thus maintaining the physiological integrity of their cells. Tenebrionids have a very waxy cuticle, which reduces water loss across their surface. Their dorsal wing cases (elytra) are fused and form a cavity covering their backs. The respiratory openings (spiracles) open into this cavity and hence are not in direct contact with dry air. This reduces the amount of water lost during respiration. They reabsorb water from the rectum and produce very dry faeces. Namib tenebrionid beetles collect moisture from the fog which originates from the cold coastal currents offshore from the Namib Desert. When fogs occur at night, they migrate to the crest of a sand dune, where condensation is greatest. They stand with their heads down and face into the wind. Moisture from the fog condenses on their backs and trickles down to their mouths (Figure 2.3).
Insects and other arthropods are such successful inhabitants of deserts, despite their small size and hence relatively large surface area, because of their ability to restrict water loss. They do this by having a waxy cuticle, which is impermeable to water, and a respiratory system involving openings in the cuticle and air tubes (tracheae and trache-oles), which supplies sufficient oxygen to the tissues while restricting water loss. Water loss via the faeces is restricted by reabsorbing water through the rectum. Water is gained by the ingestion of food and collecting condensation and some species can absorb water from the air, even at relative humidities as low as 81 per cent. Water within the cells is maintained, with the volume of water in the body cavity decreasing during dehydration while maintaining its osmotic concentration. The most extreme conditions may be avoided by burrowing, by dormancy during adverse periods and by daily and seasonal patterns of activity and reproduction. Temperature regulation may also involve body colouration and behaviours, postures and orientations which allow the animal to gain or lose heat.
The invertebrate inhabitants of temporary desert ponds have to survive periods of extreme desiccation. Such problems are also faced by those inhabiting temporary bodies of water in other environments and will be considered later in this chapter.
Almost all desert areas support some sort of vegetation, with dunes and bare rock supporting the least. Plants face similar problems to those of animals in desert areas, particularly extreme temperatures and low, irregular and unpredictable water supply. In some ways, their water supply problems are worse than those faced by animals since they cannot rely on their food as a source of water. Plants produce the sugars they use to fuel their metabolism via photosynthesis. They need carbon dioxide from the air and water for photosynthesis. The carbon dioxide enters the plant via pores (stomata) in the leaves. If the stomata are open to allow the entry of air, they also allow the loss of water. This process is called transpiration. Plants can compensate for the water lost through transpiration by taking up water through the roots. In a desert, the air is very dry, resulting potentially in very high rates of transpiration. This, together with low water availability, means that desert plants have problems maintaining their water content.
Desert plants have solved their water problems in a variety of ways. The vast majority of desert plant species are ephemeral. One of the most spectacular sights in nature is the growth and blooming of ephemeral desert plants after a rainfall. Ephemeral annual plants lie dormant as seeds which only germinate after rain. They then grow, flower and set seed within just a few weeks so that they can complete their life cycle while water is still available. This is, of course, a resistance strategy with the plant lying dormant, in the form of seed, until favourable conditions return. The seeds may be dry enough for their metabolism to cease and to become anhydrobiotic. Germination of seed after rainfall could be a very risky strategy since there may not be sufficient water for the plant to complete its development and it may wither and die before setting seed again. Some desert plants have solved this problem by having a chemical in the seed which inhibits germination. If there is sufficient rainfall, the chemical is washed away and the seed germinates, but, if there is only a little water available, germination is prevented. Annuals (which die after setting seed) make up the majority of ephemeral plants. Ephemeral perennials survive as an underground bulb or corm. Ephemeral annuals are favoured in deserts with the greatest variation in rainfall. In the most arid areas of North American deserts, such as Death Valley, where rainfall is very variable, ephemeral annuals make up 96 per cent of plants. Where there is less variability in rainfall, perennials predominate.
Perennial plants survive in desert environments by gathering water efficiently and by adaptations which allow them to retain that water. Temperate trees and shrubs have as much tissue below ground, in the form of roots, as there is above ground. Desert plants have from two to six times as much tissue below ground as above, allowing their roots to gather water over a wide area. This means that the plants are widely spaced and that desert vegetation is typically sparse. Some desert trees and shrubs can send out lateral roots as far as 75 metres to reach water sources. Transpiration is reduced by decreasing the surface area through which water can be lost. The cuticle of desert plants is waxy so that water loss is restricted to open stomata. Most of the plant tissue is below ground, leaves are small and reduced to spines or are lost altogether, with the stem taking over the role of photosynthesis. Some plants only grow leaves after rain and shed them again as the soil dries out. Other desert plants have remarkable abilities to store water after rainfall. Cacti, yuccas and euphorbias are called succulents because their thick, spongy leaves and stems can absorb and store large quantities of water. The stems of some cacti are pleated into grooves which enable them to expand as the plants take up water after rain. The mature saguaro cactus is able to store up to eight tons of water and can survive up to two years without rainfall. Succulents also have the ability to store the carbon dioxide which is produced by their metabolism at night and reuse it during the day for photosynthesis. This recycling of carbon dioxide reduces the need to allow air into the plant and thus reduces the loss of water through transpiration.
Many of the adaptations of desert plants have more to do with protecting the plant against excessive heat than with preventing water loss. The leaves of desert plants and the pads and stems of succulents are orientated with their thin edge facing the sun, reducing heat gain. Reduction of leaves to form spines protects the plant against heat gain by reflecting solar radiation. The spines also create a still layer of air which reduces the rate of heat gain from the surrounding air and protect the plant against grazing animals.
At first glance, many desert soils appear lifeless and sterile. In fact, they contain numerous microorganisms such as bacteria, algae, protozoa and fungi. There is probably no soil in the desert, or indeed anywhere on Earth, that is entirely sterile. The numbers of microorganisms are, of course, related to the availability of nutrients and of water. They are most numerous in the upper part of the soil and in association with the roots of plants. Large areas of deserts are covered by hardened surfaces called, variously, desert pavements, desert crusts and desert varnishes. These are inhabited by microorganisms, which are involved in their formation, and such soils are also referred to as cryptogamic crusts.
These crusts aid the survival of the microorganisms which help form them by modifying the habitat so that it is more stable and retains water better. Blue-green algae from the Atacama Desert of northern Chile produce gelatinous sheaths which trap sand grains and form cushion-like pellets which shade the small area of soil beneath and help them retain water.
Desert microorganisms can gather enough moisture from rainfall and/or condensation to support their growth, but numbers are drastically reduced when the soil dries out. Some can produce cysts, spores or mucilage which reduce the rate of water loss and enable them to survive anhydrobiotically. Normal growth forms, however, have little ability to prevent water loss. In spite of this, spore-forming bacteria make up a minority of the total number of bacterial species. In order to survive in dry soil, microorganisms must either be able to survive desiccation by anhydrobiosis or able to recolonise the area when water returns. We know much less about the abilities of microorganisms to survive dry conditions than we do about their survival at high temperatures (thermophiles) and high salt concentrations (halophiles).
Some microorganisms avoid harsh conditions by sheltering in favourable microhabitats, such as under a rock or pebble. Some even live within rocks (endolithic microorganisms), either in fissures or cracks (chasmoendolithic) or within the porous structure of the rock itself (cryptoendolithic). Photosynthetic bacteria (cyanobacteria) or algae form the basis of these microbial communities, with fungi, lichens and other bacteria associated with them. The rock must be transparent and the microbes close enough to its surface to allow light to reach them for photosynthesis, but the layer of overlying rock is sufficient to protect them against desiccation and other hazards.
temporary deserts and temporary waters Deserts are not the only places where organisms may have problems with desiccation. Have you cleaned out your gutters recently? If you have not done it for a while, you may well find things growing in them. I am writing this section while staying at Rothamsted Manor, which accommodates visiting scientists and students working at Rothamsted Experimental Station in Harpenden - the main agricultural research station in the UK. My room has a ladder leading onto the roof, providing a fire escape. The slate roof of the Manor is a temporary desert ecosystem. The tiles are covered with lichens and there is moss growing between them. Most of the Manor dates from the sixteenth century, although parts are from the twelfth century, and the lichens may well be several hundred years old. The mosses tend to grow in areas where water accumulates when it rains. Take a look around the outside of your house. Wherever water accumulates during rain, and where conditions are relatively undisturbed, you are likely to find mosses growing. Drier areas (like the surface of the roof tiles) may be colonised by lichens and cyanobacteria.
I took a scraping of the dry moss and, after soaking it in water overnight, examined it under a microscope. I could see several different kinds of organisms living in association with the moss. These included microscopic animals - rotifers, tardigrades and nematodes - as well as microorganisms such as protozoa, fungi and bacteria. There is a whole community of organisms living on the roof of the Manor. These observations repeat those of Antoni Van Leeuwenhoek, a seventeenth-century Dutch scientist, who was one of the first to observe living organisms using the recently invented microscope. His description of what he called 'animalcules' is given in Chapter 3.
The roof of the Manor is a temporary desert because, although it rains quite a lot during the year, the water rapidly drains away and the roof organisms have to survive periods of desiccation. There is little material to hold the water for long and the moss dries out. The roof community thus has to survive periods of extreme desiccation lasting several days or even weeks. They are also likely to be exposed to quite high temperatures. The air temperature here, while writing this, was 29 °C and the surface of the dark slate roof, which absorbs the heat of the sun, was likely to be much higher. Similar habitats provided by human activity include the tops and sides of walls and other structures, and 'container habitats' such as discarded tin cans, bottles, cooking pots and car tyres. Natural and artificial container habitats are particularly significant in the tropics where they often act as breeding grounds for mosquitoes which transmit diseases such as malaria and yellow fever.
There are many more natural situations where organisms are exposed to periods of desiccation, even though the rainfall of the general area they inhabit means it is far from being a desert - their microhabitat is a temporary desert. Some examples include the exposed surface of the soil and the edges of lakes, ponds and streams which desiccate when the water level falls. Small bodies of water may dry out completely. Organisms living on the surface of the aerial parts of plants (the bits above the ground, such as the bark of a tree or the canopy of a forest) and those living in plant tissue which dries out when the plant dies or sets seed are exposed to desiccation. Natural container habitats provided by plants include treeholes, the junctions between leaves and stems (for example, in bromeliads) and in flowers. Some of these periodically dry out.
Parasites living in the intestine of other animals need to infect new hosts to ensure their survival. They form eggs, larvae or cysts which pass out of the host when it defecates and initiate the free-living phase of their life cycle. Dung is also colonised by a variety of free-living organisms such as nematodes, earthworms, fly larvae, beetles, fungi and bacteria. Fly larvae must develop into adults before the dung desiccates or disintegrates and have very rapid rates of developments. Free-living nematodes and microorganisms and the free-living stages of parasites can tolerate the desiccation and remain within the dry dung until they are liberated by rainfall.
Although the area in which we live may seem far from being a desert, we can, if we look carefully, find many places which are temporary deserts where organisms will be exposed to desiccation and other extreme environmental stresses.
Temporary ponds and streams, which periodically dry up, are found in most parts of the world. These are inhabited by organisms that can survive the stresses involved in dry phases of varying durations, inter vals and intensities. Desert ponds and streams provide some of the most extreme of these types of habitat. These form only after rainfall. Winds and hot, dry air produce rapid rates of evaporation meaning that ponds can be very short lived, often lasting only one or two weeks. Rain-filled rock pools which form in shallow depressions in rocks are common features of tropical and subtropical regions. These contain water for varying lengths of time, from a day up to several weeks. The more short-lived the pools, the greater are the demands on the organisms that live there.
Organisms have various ways by which they can exploit temporary bodies of water. They may be transient inhabitants, colonising the pond when it fills with water and leaving it when it dries. Many insects only live as larvae within these waters and rapidly complete their development to emerge as adults which fly away to seek more permanent waters. As we have seen, desert amphibians burrow into the mud as the pond dries and lie dormant until the next rainfall. Although we think of frogs and toads as being associated with moist environments, they can in fact successfully survive in periodically dry areas. In the western deserts of Australia, the numbers of frogs emerging from their burrows after rain is so large that they can disrupt rail travel - the rail tracks become slippery as thousands of frogs are crushed beneath the wheels of the trains.
Other animals also escape the threat of desiccation by burrowing. Some freshwater crayfish can burrow to depths of a metre or more to reach ground water. They remain active in their burrows by building chimneys which supply oxygen, but these can be plugged with mud to reduce water loss during dry weather. The water at the bottom of these crayfish burrows is used as a refuge for a variety of small invertebrates and a whole community of animals inhabits them. Other crustaceans burrow to avoid adverse conditions, as do some pond snails.
The African lungfish can survive when its river habitat dries. As the water level falls, it burrows into the mud of the river and secretes a cocoon of mucus (Figure 2.4). The cocoon is waterproofed by a layer of lipid and the fish lies folded on itself with its head next to a small
opening at the top of the cocoon. The fish's oxygen uptake reduces to 10 per cent of normal, its heartbeat slows, its tissues partly dehydrate, urine production ceases and the fish switches from producing ammonia to urea, which is less toxic and can accumulate in the blood and tissues without adverse effects. Part of the cocoon extends into the mouth of the fish and acts as a respiratory tube. The lungfish can survive in this dormant state for at least six months. When water returns, the lungfish becomes active again in a matter of minutes. The fish cannot rely on its gills to supply it with oxygen and it must swim to the surface in order to reach the air before it drowns.
Most invertebrate animals and all microorganisms are too small to escape desiccation by burrowing very far and their relatively large surface area in relation to their volume means that they cannot prevent water from being lost from their bodies. They survive by becoming dormant and entering into anhydrobiosis. Small crustaceans are some of the most obvious inhabitants of the most transient of temporary ponds. These include fairy and brine shrimps (Anostraca), tadpole shrimps (Notostraca), clam shrimps (Conchostraca), water fleas (Cladocera), seed shrimps (Ostracoda) and copepods. Some species survive anhydrobiotically as a juvenile stage, with a few forming protective cysts. Most species produce resistant eggs which lie dormant in the dried mud of the pond and hatch en masse when it rains. Anhydrobiosis thus not only allows the animal to survive the period of desiccation but also allows its life cycle to become synchronised with periods of favourable conditions. Only a few days after rain, these ponds can be swarming with millions of tiny shrimps which have emerged from eggs lying dormant in the dried mud.
Other invertebrates of temporary ponds survive periods of desiccation in a state of anhydrobiosis. These include nematodes, rotifers, tar-digrades and some insect larvae. One of the most spectacular examples is the larva of a midge (a chironomid insect), Polypedilium vander-planki. This inhabits shallow, exposed, rain-filled rock pools in the African 'kopjes' (isolated hillsides) of Nigeria and Uganda. At the start of the rainy season, these ponds may fill with water and dry out several times. Even during the dry season, it may rain and the ponds briefly fill with water. The larvae are thus exposed to cycles of desiccation and rehydration. The larvae can survive anhydrobiotically and can lose 99 per cent of their body water. This is the largest animal known to survive anhydrobiosis.
Microorganisms, such as protozoa, bacteria, fungi and algae, are found in abundance in temporary waters. Little is known of their adaptations to these habitats, although they may produce cysts (protozoa), spores (bacteria and fungi) or modified cells with thickened walls, mucilage sheaths and an accumulation of oils (algae) which enable them to survive anhydrobiotically.
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