Hot deserts are found in North America (e.g. Mojave), South Asia, South & Central America, Africa and Australia. They are further subdivided into Hot Dry desert, and Semi-arid desert.
Hot Dry deserts are characterized by:
- Generally warm seasons throughout the year
- Very little or no rainfall during winter
- Extreme variation in temperature between night (-18ºC) and day (+49ºC). Heat escapes rapidly at night due to the lack of moisture in the air
- Unpredictable rainfall (intense short bursts, long rainless periods or low rainfall)
- Coarse soils with good drainage, with no subsurface water
Semi-arid deserts are characterized by:
- Moderately long and dry summers
- Cool, rainless winters
- Summer temperatures average 27ºC
- Average annual rainfall is 2-4 cm.
- Soils ranging from fine-textured sand to loose rock pieces or gravel
- Low salt concentration
- No sub-surface water
Animals that live in these harsh environments have developed mechanisms over time to survive in the desert.
For example, the Critically Endangered African Addax is one of the few large mammals that manage to survive here because of its ability to obtain all its moisture from the plants that make up its food supply, its specially adapted large spread-out hooves and its nomadic nature.
Other animals, such as the Mouse-Tailed bat of the Sahara region, gorge on insects during the hot season, storing up fat to tide them through the cool season when food is scarce. They go into a deep sleep at this time, gradually using up the fat supply. The camel is the most commonly known mammal in this group, storing fat in its hump(s), metabolizing it into water as needed. It also has large divided hoofs that enable the camel to walk across the sand without sinking in too deeply.
Some animals simply avoid the scorching daylight temperatures by sleeping in burrows, such as the Spadefoot toad (it stays underground for nine months – an extreme example). Many spiders, lizards and snakes take advantage of burrows.
Others remain relatively immobile during the hottest part of the day, seeking shelter in the shade of the plants or rocks. Examples include meerkats, jack rabbits, insects, lizards, and snakes.
Many adapt by coming out to feed only at night, and these are referred to as nocturnal animals. The Fennec Fox has characteristic huge ears which scatter the heat, but it is more active at night, staying in a burrow during the hottest part of the day.
Additional adaptations include the absence of sweat glands (to prevent loss of moisture), the presence of salt glands (allowing salt secretion without water loss) and the concentration of urine during hibernation (gerbils).
Animals such as the Desert Lark and the Fennec Fox have developed plumage or fur that perfectly match the colour of the desert soil, thus reflecting sunlight.
Reptiles are really quite remarkable in their adaptations to a desert life. They have a waterproof skin, lay hard-shelled eggs, regulate theirinternal body temperature according to the external temperature surrounding it, and produce uric acid instead of urine.
Birds have the ability simply to fly elsewhere to seek out water supplies or to hover high above in the cooler atmosphere. Birds, particularly in Australia, have the ability to synchronize their breeding period to any change in rainfall, whenever that may occur.
Birds such as the North American Wren take advantage of the prickly leaves of the cholla cactus or the sharp leaves of the yucca to build its large domed nest. These nests not only provide a place in which to lay their eggs, but also serve as shelterduring bad weather as well as a roosting place overnight.
The Australian female thorny devil lizard lays 3 to 10 eggs in November or December, the season when there is likely to be some rainfall. The young are a replica of their parent, complete with spines over its entire body, including its tail, enabling it to ward off potential predators.
Desert plants are remarkable in their ability to capture moisture when it is available and to store it during droughts. The two main adaptations are their ability to collect and store water, and features to reduce water loss.
Most desert plants are low shrubs, either evergreen or deciduous, short woody trees, with small leaves and very often with spines, thorns or aromatic oils.
Many have extensive root systems extending far away from the trunk in order to maximize the collection of any rainfall. These are known as xerophytes. The green Saguaro Cactus, found in the Sonoran Desert, takes advantage of this particular adaptation, but in addition photosynthesis (the production of carbohydrates using light and chlorophyll) takes place only in the top layer of the stem and not in its leaves.
The Prickly Pear Cactus in the Chihuahuan desert stores water in its spongy tissue, and its nasty thorns ward off animal predators.
Leaves are reduced to spines and not only protect the plant from being eaten but also shade the plant’s surface from losing water vapour (transpiration).
Some plants such as yuccas, agaves, etc. open their pores (stomata) only at night when water evaporation is less. Other plants have silvery or glossy leaves in order to reflect more radiant energy; they often have a bad odour or taste to ward off potential predators.
Some plants have a very short life cycle occurring only during periods of rain. During droughts, the seed has a waterproof coating and can lay dormant for years, if necessary, until rainfall occurs again.
Effects of Global Warming on Hot Deserts
On one hand, deserts are very vulnerable because of:
- Declining rainfall of 5 to 15% (Australia’s Great Victoria desert, the Atacama and the Colorado Great Basin)
- Paradoxically areas such as the Gobi will experience a 10-15% increase in rainfall
- An average of an increase of 3ºC in air temperature annually
- Diversion of large rivers that cross deserts to provide water to nearby cities and farms
- Construction of roads, settlement expansion and other development in nearby areas
- Desert plants are a potential source of new medicines and have not yet been explored in depth. Global warming may well destroy this untapped possible source of anti-cancer and anti-malarial compounds
On the other hand, adjacent areas are also vulnerable because of:
- Increase in winds that will pick up sand grains off the dunes and deposit them in vegetated areas, thus turning these now arable areas into desert. The continent of Africa is increasingly being affected
- Damming of the large rivers that cross deserts cause downstream water supplies to dry up, having a serious impact on flood plains and river ecology
- Rising water tables underneath the irrigated soils are prone to contamination due to increase in salt content as seawater seeps into underground water