Intense solar radiation, lashing winds, and little moisture i.e. less than 10 inches (25 cm) of rainfall create some of the harshest living condition in the biosphere called hot desert. In hot desert ecosystem generally with cloudless skies, the sun quickly heats the desert by day, producing the highest air temperatures (recorded as 57.8º C in Death Valley, California) in the biosphere.
In contrast, the nights are very cold, as the temperature goes down tremendously due to loss of heat into the atmosphere through radiation. There is little water and temperatures vary widely, one may bake during the day and freeze at night.
High temperatures during daytime and persistent winds accelerate water evaporation and transpiration of water vapour from plants. High evapotranspiration and low rainfall is the chief characteristic of desert ecosystem, thus producing sparse perennial vegetation of widely spaced shrubs. The winters are quite cold, temperatures sometimes below freezing point, while heat during summer is intense and scorching. Scarcity of rain fall in deserts ecosystem can be due to high subtropical pressure (Sahara and Australian deserts), geographical position in the rain shadows (western North American deserts) and due to high altitude (Tibetan, Bolivian or Gobi deserts).
Deserts ecosystem cover about 14 percent of the earth’s land and occur mainly near 30º north and south latitude where global air currents create belts of descending dry air. Some desert ecosystem are also produced in the rain shadows of high mountain ranges, leeward slopes that face away from incoming storms and thereby receive little rainfall. Most deserts ecosystem receives some rain during the year and has at least a sparse cover of vegetation.
Annual net primary productivity of true deserts is less than 2000 kg per hectare. The dominant soils of the arid zone are light-textured and devoid of any significant structural development. These are prone to severe wind erosion. Desert ecosystems have very low water retention capacity – with high infiltration rate and low hydraulic conductivity. Water is hardly retained in deserts soil as it is not soaked into the earth, and rushes off in torrents. The moisture in the arid zones is insufficient to support living beings.
But despite such harsh living conditions, desert ecosystem exhibits a spectacular biological diversity. A large number of plant and animal species thrive in the deserts due to their morphological, anatomical, physiological and behavioural adaptations.
Desert Ecosystem – Plant Adaptations
Plants have evolved many adaptions for surviving the rigors of the desert. There are three life-forms of plants that are adapted to desert ecosystem: a) ephemeral annuals, b) succulents, and c) desert shrubs.
Ephemeral annuals are also called as ‘drought evaders’ or ‘drought escapers’. They germinate, grow, flower, and release seeds within the brief period (6-8 weeks) when water is available and temperatures are warm. The seeds remain dormant, resisting drought and heat, until the following spring. Seeds wait out adverse environmental conditions, sometimes for decades, and will germinate and grow only when specific requirements are met. With their small size and large shoots in relation to roots, they are well adapted to dry habitats. They escape dryness in both external and internal environments. Desert sunflower and desert marigold complete their life cycles during brief rainy seasons.
The succulent plants suffer from dryness in only external environment. Their succulent, fleshy stems, leaves and roots serve as water storage organs (water storage region is present in these organs) which accumulate large amount of water during brief rainy seasons. Opuntia, Aloe, Euphorbia, Yucca and Agave have mastered the art of enduring in the desert ecosystem by economizing in their expenditures of moisture.
They rely on their waxy coatings, spongy stem and/or leaf tissues, root structures and their night time stomata openings to carefully regulate their water use. At night the temperatures are lower and humidity higher than during the day, so less water is lost through transpiration. Such plants are sometimes called “drought endures”. In Opuntia spp. (Cactus), the stem modified into a thick, fleshy, green, life-like structure called phylloclade which manufacture food by photosynthesis and conserve water. Their leaves modified into spines which retard transpiration, promote dew formation at their tips, protect from insolation and from thirsty animals. The bulk of the tissue consists of large, round, pitted, parenchymatous water-storing cells. The cell sap is mucilaginous which helps in checking evaporation of water. The extensive shallow root systems are usually radial, allowing for the quick acquisition of large quantities of water during the rainy period.
The leaves are fleshy in Aloe spp. with marginal spines and a large water-storing tissue. The succulent Euphorbia spp. has succulent stem which store large quantities of water during rainy season. It contains toxic milky latex that irritates skin and eyes. The stipules become modified into spines. The toxic substances and spines prevent them from predator animals. The Century plant (Agave spp.) has saw-toothed leaves with waxy coatings that render them nearly waterproof and so prevents loss of water.
The leaves of these plants channel rain water to the plant’s base. It also contains toxic chemicals like oxalate crystals and irritating substances that can irritate the skin and mucous membranes and can cause digestive problems in their predator animals. The Joshua tree (Yucca spp.) is a very tough plant. The leaves are stiff and very pointed. The roots become fleshy to store water in Asparagus spp.
The shrub in desert ecosystem or non-succulent perennials suffers from dryness both in their internal as well as external environments. Their morphological and physiological features include rapid elongation and extensive root system, high osmotic pressure and endurance of desiccation, ability to reduce transpiration and reduction in size of leaf blade. Root system is very extensive i.e. more than 30 m long (Alfalfa spp.) to siphon deep groundwater supplies.
There is waxy coating and sunken type of stomata on leaves, which reduces loss of water during transpiration. Desert grasses have rolled and folded leaves so that the sunken stomata become hidden to minimize the rates of transpiration. In desert ecosystem, individual plants are scattered thinly with large bare areas in between. These spacing reduces competition for a scarce resource; otherwise intense competition for water might result in the death or stunting of all of the plants.
The grasses (bunchgrass) in desert ecosystem also grow in isolated tufts. During extremely hot and dry period, the parts of the plants that are above the land may wither and die, but the root systems remain alive. Desert mariposa and desert lily have bulbs that may remain dormant for several years until a deep soaking rain awakens them. The extensive bare ground in desert ecosystem is not necessarily free of plants. Mosses, algae, and lichens may be present which form a stabilizing crust on sands and soils.
Desert Ecosystem – Animal Adaptations
Animals of desert ecosystem are much more affected by extremes of temperature than desert plants because the biological processes of animal tissue function properly within a relatively narrow temperature range. Thus, most of the animals in desert ecosystem rely on their behavioural, physiological and structural adaptations to avoid the desert heat and dryness.
The drought evader animals adopt either a short annual life cycle that revolve around the scanty rains or undergo aestivation (e.g. ground squirrel). During aestivation, the breathing, heartbeat and other body activities slowdown, this in turn decreases the need of water. Many lay eggs that survive until the next rains when they hatch in the transient puddles. On the onset of rains, a variety of animal like grasshoppers, butterflies, bees, beetles, and spider’s and more may be seen in the desert ecosystem. Amphibians like spade foot toad dig burrow with the help of its spade-like feet and goes to sleep till the rains arrive. It can undergo aestivation for 8-10 months. The birds make nest and reproduce during the rainy season when there is abundant food.
The drought resistant animals are active and carry their normal function throughout the year. They circumvent aridity and heat through morphological and physiological adaptations or by modifying their feeding and activity patterns. They remain in cool, humid underground burrows during the day time and search for food only at night when temperatures are lower.
Some xerocole rodents of desert ecosystem, that are active in the day periodically seek burrows and passively lose heat through conduction by pressing their bodies against the burrow walls. The desert toad uses a survival strategy similar to that employed by succulent plants. It stores water in its urinary bladder. The reptiles and some insects are pre-adapted to the hot desert ecosystem. They excrete a dry metabolic waste product in the form of uric acid and guanine so that water loss is minimal. They have thick waterproof skin that also minimizes water loss.
Desert spiders, mites and insects secrete a waxy layer over their cuticles. Wax is impermeable to water thus prevents loss of water from their bodies. Mammals as a group are not well adapted to desert life because they excrete urea, which involves the loss of much water.
Most of the mammals of desert ecosystem, like kangaroo rat, the pocket mouse and the jerboa have adapted nocturnal habitat. They seal their burrows by day to keep their chamber moist, and can live throughout year without drinking water. They feed on dry seeds and dry plants even when succulent green plants are available. They remain in burrows during the day, and conserve water by excreting very concentrated urine and by hygroscopic water in their food. Thus, adaptation to the desert ecosystem by these rodents is as much behavioural as physiological. Other desert ecosystem mammals like mule, deer and elk avoid the extreme temperatures of the day by limiting activity hours to dawn and dusk. The wood rats survive in parts of the desert by eating dry food as well as succulent cacti or other plants that store water. Jackrabbits and kit fox have large ears that reduce the need of water evaporation to regulate the body temperature. Their ears release heat during their resting periods in a cool, shady place.
The camel in the desert ecosystem can go for long periods without water because their body tissues can tolerate elevation in body temperature and a degree of dehydration. However, it uses water for temperature regulation. The body temperature of camel drops to 33.8ºC over night and rising to 40.6ºC by day when the animal begins to sweat.
Opposed to popular belief, camels do not store water in their hump. Their hump stores fat which yields water after its metabolic oxidation. The kangaroo rat and jerboa have long legs, which help them in jumping and swift running as well as in lifting the body above the ground and thus reducing direct contact with the hot sand.
Desert Gerbils have hairy soles on their feet which allow them excellent traction on sand. The sand rat feeds on plants that have very salty sap which can be toxic in large quantities. Thus, rats simply retain the water and excrete urine that is about four times as salty as sea water. The desert birds utilize a salt gland to help in the maintenance of water balance. They occasionally drink water from dew or other sources.
Thus, these unique natural habitats (desert) with their incredibly diverse flora and fauna have been home to some of the world’s oldest civilizations. The desert ecosystem in California support about 1200 plant species, 200 species of vertebrate animals and numerous insects and other invertebrates. Therefore, the conception of a desert as an uninhabited wasteland is not correct. Besides, we should always remember that the desert is easily damaged and is very, very slow to recover. Thus, fragile beauty and unique heritage of world’s deserts deserve protection.
The general conception about desert, being uninhabited wasteland is not true. This ecosystem which covers 14 percent of earths land surface is actually reservoir of rich and diverse flora and fauna.