Deserts are places of extremes. Besides being dry, they experience intense sunshine, and a greater daily temperature range than any other land habitat.
Rain — when it comes — typically falls in brief but torrential downpours, while strong winds pick up sand and grit, carrying it almost horizontally through the air. Although no 2 deserts are identical, true desert is usually defined as having less than 15 cm (6 in) of rainfall a year.
Semi-desert has more rainfall — up to 40 cm (16 in) a year — which typically falls during a relatively short spring or wet season followed by months of drought.
Most of the world’s true desert is found in 2 belts, one straddling each of the tropics. Here, zones of high atmospheric pressure persist for months at a time, preventing low-pressure air from bringing in rain. Desert also forms where mountains block rain-bearing winds, and where cold, coastal currents chill the air so that it carries very little moisture inland. In true desert, the amount of rain is so meagre, and so unpredictable, that very few plants can survive.
The ones that do — such as cacti and other succulents – are highly effective at collecting and conserving what little water nature provides: they have large networks of shallow roots, which drain the surrounding ground so thoroughly that, often nothing else can grow nearby. For animals, this arid environment creates some interesting effects.
With so few plants. there is very little soil. which severely limits invertebrate life. Most small animals, such as insects, are found either on the plants themselves or in the debris that accumulates immediately beneath them. Larger animals, such as reptiles and rodents, venture away from these pockets of greenery, but even they have to be careful to avoid the worst ot the daytime heat. Lack of vegetation means that most of the ground is exposed.
Bare ground absorbs warmth very quickly when the sun rises, and re-radiates it once the sun has set. The dry air accentuates this effect, allowing daytime surface temperatures to soar to over 70°C (158°F). As a result, most animals living in true desert are active after dark. During the day, they hide away, leaving little sign of themselves apart from their tracks.
Compared with true desert semi-desert is more widespread and it is also much more biologically productive. It is found in every continent, including some regions far outside the tropics. The modest out nevertheless reliable rainfall that semi-desert receives has a dramatic effect on the landscape and the types of animals that it can support Plants often grow in profusion, creating tangled thickets of vegetation that provide plenty of cover.
There are woody species, which store above ground in their stems and leaves. Most of these desert species are well protected from plant-eaters for example have extremely sharp spines, while spurges exude a poisonous milk sap when they are damaged — but, for animals that can overcome these defenses, they are an important source of food.
Semi-desert also has plants known as desert ephemerals, which spring up rapidly after rain, flower, set seed, and then die. This short life cycle produces extra fresh food for animals, and adds to the stock of seeds scattered over the desert floor. While some semi-deserts are warm or hot throughout the year, others are surprisingly cold in Winter.
In the deserts of Central Asia and in the northern parts of Americas Great Basin — the desert region between the Rocky Mountains and the coastal ranges further west — temperatures can tail to -30°C (-22°F). In these areas, animals need protection against winter cold as well as against summer heat: small animals, such as insects usually become dormant in winter, and many burrowing mammals hibernate until the spring.
LIFE IN DESERT
In a habitat where moisture is scarce, obtaining and conserving water are every
animal’s top priorities. Desert animals practice a tight “water economy”, which
means collecting water wherever they can, and minimizing water loss wherever
However, being economical with water is not in itself enough to
guarantee survival: desert species have had to evolve various other adaptations
to enable them to cope with a wide range of temperatures and the ever-present
threat of food shortage. As a result, these animals are able to live in some of the
driest places on earth.
Most deserts have a scattering of oases, where animals gather to drink. Some species need to drink daily, which restricts how far they can roam from an oasis. Others can survive on their on-board reserves for days or even weeks. depending on the temperature.
A remarkable feature of desert life is that some animals can manage without drinking at all. Instead, they get all their water from their food. Some extract it from the moisture contained in food: but most use the food to manufacture metabolic water, which is created by chemical reactions when the energy in food is released.
Seed-eating rodents are expert at this: although their food looks dry, they are able to metabolize all the water they need, For drinkers and non-drinkers alike, water has to be eked out to make sure that it lasts. Compared with animals from other habitats, desert species lose very little moisture in their urine and droppings, and only a small amount is released from their skin and in their breath.
Desert species are also good at withstanding dehydration. The dromedary, or one-humped camel, can lose nearly one half of its body water and survive. For humans, losing just a fifth can be fatal.
To enable them to cope with erratic food supplies, many animals keep their own food supplies. Some do this by hiding food away, The North American kangaroo rat, for example constructs underground granaries that contain up to 5 kg (11 lb) of seeds.
But for predators, and for animals that browse on shurbs, creating such larders is not possible, their food is difficult to collect and to transport, and even if it could be hoarded it would be unlikely to remain usable for more than just a few days.
The answer is to store food inside the body. The classic example of this is the camel, which stores surplus food. In the form of fat, in its hump. Several other species, such as the Gila monster and fat-tailed dunnart, store food in their tails.
COPING WITH HEAT AND COLD
In desert, the temperature rarely stays steady for more than a few hours, and it can reach extremes of both heat and cold very quickly. Humans lose excess heat by sweating, but at very high temperatures this cooling system can use as much as 1 lite (85 fl. oz) of water an hour — far more than any desert animal could afford.
Desert animals tackle the heat problem in 2 ways: by reducing the heat they absorb, and by increasing the heat they give out. Light-colored skin or fur reflects some of the sun’s rays minimizing heat absorption; but a much more effective method — used by many desert animals — is to avoid the most intense heat by being nocturnal, spending the day sheltering underground. Burrows do not have to be very deep to make a difference: while the desert surface may be too hot to touch, the ground just a few centimeters below it will be relatively cool.
Getting rid of excess heat is more difficult, particularly when an animal’s body temperature is dangerously high. Lizards and snakes are often described as “cold-blooded”, but this actually means that their body temperature rises and falls with that of their surroundings. Although they thrive in warm habitats, and can survive with a body temperature of up to 44° C (111°F), they often have to sit out the hottest part of the day in shade.
Some desert birds cool down by panting, which involves fluttering the flap of skin over their throats. Desert kangaroos and wallabies lick their front legs,covering them with saliva. As the saliva evaporates, the animals blood cools down. In high-altitude desert region, such as the Gobi Desert of Central Asia and the Great Basin Desert of North America, winter can be extremely cold.
Animals have various ways of coping with this Most reptiles hibernate, while birds often fly to warmer climates. Mammals keep warm by growing thick fur, or by sheltering underground.
Desert animals often have highly variable breeding seasons. Instead of reproducing at a fixed time of year, many produce young when there is the best chance of finding food. Female Kangaroos for example, give birth extremely regularly when food is plentiful. but when food is scarce they stop breeding altogether. This flexible system is an efficient way of using resources because it prevents parents having to tend hungry youngsters when they; are hungry themselves.
Some desert species carry irregular breeding to extremes. Desert wildlife includes a number of animals that. paradoxically, live or breed in water and for those species reproducing is a highly unpredictable and time-sensitive business.
Such animals include burrowing frogs and toads, and also freshwater shrimps that live in temporary pools. For months or even years at a time, they are an invisible part of desert wildlife with the amphibian species lying hidden underground, and the shrimps present only as eggs in dried-up ground.
But immediately after a heavy storm, the frogs and toads dig their Way to the surface and the shrimp eggs hatch. Once active, these animals immediately set about finding mates because they have to complete their life cycles before the pools dry up again.
Desert sand makes life difficult for animals on the move. Large animals sink into it, while small ones struggle to climb up and down slopes of shifting grains. To combat the problem, some animals such as golden and marsupial moles, move through the sand rather than above it.
Others, such as camels and geckos have extra large feet which help to spread their body weight over the surface of the sand and so increase stability. Side-winding snakes have a different solution: they throw themselves forwards in a succession of sideways jumps, leaving a characteristic pattern of J-shaped tracks. In addition to saving energy this method helps to minimize contact with hot ground.
Some insects and lizards have learned to tolerate hot ground by alternating the feet that are in contact with the ground at any one time. Having long legs also helps as they hold the animals body away from the sand’s surface, where the heat is fiercest.