Desert Characteristics and Adaptations

Aspect 1 - Climate

Climate is one of the most important aspects that determines the desert biome. The hot desert biome (HDB) is characterised by high temperatures and low levels of precipitation all year round. The weather is very stable and skies are generally clear.

Precipitation in the HDB biome is usually between 0 and 250mm. Some receive less than 100mm. When rainfall does occur it can be unpredictable and often happens as convectional downpours. Evaporation is rapid and it may not have time to penetrate the soil.

The HDB often occurs where there are high pressure belts. At the equator the sun is directly overhead and there are high levels of evaporation. This creates a low pressure zone where warm saturated air rises and high levels of precipitation take place. This air moves back toward the poles cools and sinks creating a high pressure zone of dry descending air. This contributes to the lack of cloud cover.

High pressure zones happen at about 30'C N and 30'S of the equator. These are known as the horse latitudes. Trade winds move from these high pressure areas to low pressure at the equator. The cooler air is able to absorb moisture which reduces the level of precipitation. The Sahara desert is located where there is a high pressure belt and trade winds.

The HDB usually has a high diurnal temperature range, this is the difference between day and night temperatures. High pressure creates a lack of cloud cover and direct sunlight creates high temperatures. Daytime temperatures can be over 45'C. The lack of cloud cover at night means that heat escapes rapidly and temperatures can fall by as much as 30'C within 1 hour of sunset. Low temperatures at night time can cause fog or dew.

Cold ocean currents are also an important factor in creating a desert climate. Moisture laden winds move across the ocean. Cold ocean currents cool the prevailing winds, triggering rainfall over the sea. By the time the winds reach the land they are dry winds that absorb moisture, creating desert like conditions. The canary current is a cold ocean current beside the Sahara Desert.

The rain shadow effect can also be a factor in creating a desert climate. Precipitation takes place on the windward side of the mountains as warm air is forced upwards. On the other side (leeward side) cool dry air descends. The Gobi desert has been created in this way.

Continentality can also help to create the desert biome. In The Great Australian Desert, the winds have moved across land and they cannot absorb enough moisture or the precipitation has already taken place.

/

Aspect 2 - Soils

The zonal so or most common soil type in the HDB is arid soils. This aspect links heavily with climate and vegetation. This is usually infertile and is particularly influenced by the hot daytime temperatures and lack of rainfall.

The parent material is usually formed from mechanical weathering like exfoliation. The high diurnal temperature range allows mechanical weathering to take place. The texture of the soil tends to be coarse and gravely because exfoliation leaves behind angular pieces of scree.

In addition fine particles are blown away by winds and leave just coarser grained particles behind. This process is known as deflation.

Material can be washed from mountainous areas into desert areas by torrential convectional rain and flash flooding. Some low lying areas have developed deeper Aridisols as a result. Heavy rainfall can result in flash flooding and gully erosion, when heavy rainfall erodes small channels in the soil surface.

Aridisols can become fertile and produce plant life. They require water to do this. In the Sahara deserts this can happen in oases. Irrigation has been successful in some parts of California and Arizona.

Aridisols lack humus which is required to make soil fertile. Humus comes from the process of humification. This process involves the decay of plant litter into a jelly like substance that is rich in nutrients. The coarse texture and sparse or non-existent vegetation prevents the development of humus.

Microorganisms usually break down plant litter into humus. In the desert there is a lack of both microorganism and plant litter to create humus. Some humus can be found around the roots of plants in Aridisols but it is not widely distributed throughout the soil. This gives the soil a pale grey colour.

Chemical weathering is important to the development of soil horizons or layers. The low level of precipitation and high temperatures in hot deserts prevents chemical weathering. This means the soil can be deep but does not have clear layers of top soil, sub soil, and bedrock. The lack of organic matter also contributes to this.

Evaporation can exceed precipitation in hot deserts. This means that groundwater is drawn up through the soil in a process known as capillary action. When the groundwater evaporates it leaves behind minerals like salt. Salt pans can begin to develop on the surface. This process is known as salinisation.

In the Kalahari desert, limited rainfall from January to March creates shallow lakes where the water cannot pass through the salt pans.

Calcification also occurs in Aridisols. Nutrients that have been leached through the soil are drawn back up through capillary action. Similarly to salinisation it causes calcium to be deposited as a hard pan on the surface of the soil. A hard pan is impermeable and it can prevent the growth of plants.

The PH of the soil becomes alkaline if calcification takes place. This further reduces the productivity of the soil. The cement like layer of calcium is also known as a caliche.

Aspect 3 - Flora (Vegetation)

Another aspect is vegetation. It is sparse as a result of the lack of water, high evaporation, lack of humus, and coarse shallow soil. However, some flora have adapted to these conditions.

Phreatophytes have developed long root systems to overcome the characteristics of Aridisols and the lack of precipitation.

Long tap roots can move through the impermeable soil and reach ground water. The mesquite bush is a famous example of a plant with a tap root system. The roots can reach up to 50m. The mesquite is found in the Sonoran Desert of Mexico and Arizona.

Radial roots are another adaptation in order to access water. These are shallow roots that spread outwards over a large area to collect as much water as possible during periods of heavy rainfall. Shallow roots also avoid the need to move through calcite hardpans.

The Acacia tree actually combines both tap and radial roots to maximise the amount of water collected. The Saguaro cactus has a single tap root and two sets of radial roots. It can be found in the Sonoran Desert.

Plants in the desert have also adapted by being widely spaced. This allows radial roots to gather water and nutrients from a wide area. The creosote bush tends to be widely spaced.

Ephemerals are a group of plants that have adapted by having a very short growth cycle. These plants take advantage of short unpredictable periods of rainfall by completing their entire life cycle in a couple of weeks. Seeds remain dormant for long periods until rains arrive when the plant germinates, develops, and matures rapidly. The desert poppy is a good example of an ephemeral.

Succulents adapt to the climactic conditions by storing water. They are also known as xerophytes. They store water in their leaves, stems, and roots. Waxy leaves also prevent moisture loss through evaporation. Removal of moisture by dry desert wind is also referred to as desiccation.

The Saguaro Cactus from the Arizona desert is a good example of a succulent. The vertical grooves of the Saguaro allows it to expand and store water for long periods. The grooves also channel water toward the roots of the plant. A fully grown Saguaro can store up to 5 tonnes of water.

Like many cacti, the prickly pear has spines or thorns instead of leaves. Leaves loose moisture through transpiration, the smaller they are the less moisture is lost. Thorns also reduce the surface area of the plant and therefore it does not overheat as much.

Photosynthesis is carried out by chlorophyll in the stem. While the thorns also create still air around the plant. This reduces the impact of dry desert winds that hit the plants. Thorns can also protect plants birds and animals that bite succulents to get at the moisture.

Trichomes are plants with dense hairs. These hairs prevent moisture from moving over the surface of desert plants. The more air movement there is the more evaporation rates increase.

Nocturnal plants only bloom at night to avoid the high temperatures and evaporation rates. Cereus flowers remain closed during the day and then emerge at night. The desert dandelion is another example.

The Joshua tree is salt tolerant. This means that it can survive in soil that has been impacted by salinisation. It has branches that can grow up to 15m thick and it can lie dormant for long periods.

Aspect 4 - Fauna (Animals)

Fauna have made various adaptations to the extreme temperatures and lack of food or water. In particular diet and physical adaptations to their body parts.

Diet Adaptations

Some animals have adapted their diet in order to survive on very little food. Insects scorpions, and spiders are the most numerous desert animals. Scorpions and spiders are often venomous to kill prey with a single sting and therefore minimal effort.

Scorpions are able to slow down their metabolism and hibernate until there is prey nearby. They are then able to come out of hibernation rapidly. Scorpions are able to survive an entire year without eating.

The Dorcas Gazelle of North Africa can get most of their water from their diet. They can also convert their urine into uric acid pellets, this stops them from losing water. The Fennex Fox gets most of the moisture it requires from the blood of their prey.

Physical Adaptations

Some animals have physical adaptations to the climate conditions. Many animals have a light colour fur to camouflage themselves in the desert and to reflect sunlight keeping them cooler.

The kangaroo rat is a good example of this. This animal has small front feet to handle food and sharp claws to dug burrows. Digging underground is essential for many desert animals to escape peak daytime temperatures. Long back feet allow the kangaroo rat to leap away from predators while its tail can flick sand at them.

Camels are one of the larger animals found in the HDB. They have adapted to sandstorms by developing long eyelashes to prevent sand getting into their eyes. They can also tighten their nostrils to filter sand when they breathe.

To adapt to the heat camels have thick fur which protects them from sunlight. Their wide padded feet act like shoes allowing them to walk on unstable and hot sand.

Camels have also adapted to the lack of water and food in the HDB. They are able to store fat in their hump which acts as a reserve when there is a shortage. This can also be converted into water during droughts. Camels can consume over 100L of water at a time and store it. Furthermore, they can stop themselves from sweating to retain moisture and even turn urine into a solid to avoid water loss.

Temperature adaptations

Snakes move by sidewinding to ensure that only two points of their body are touching the hot sand at a time. The Mojave Desert Sidewinder is found in the south-western USA and the Namib Desert viper both use this movement. It also helps them to keep traction on the windy sands.

The thorny devil from the Australian Outback has adapted to the lack of water by developing a hard skin that can absorb water instantly.

Many animals have adapted to the temperature by burrowing underground or only emerging at night. The Cape ground squirrel lives in the driest areas of Southern Africa. It is a burrowing rodent that can also use its tail as a parasol.

The Fennec Fox is nocturnal. It sleeps in a den during the day and hunts at night time to avoid heat of the day. It has long large ears with many blood vessels that allow excess heat to leave its body. This is a common temperature adaptation among many animals in deserts. Jackrabbits also have large ears to lose heat.

The cactus pygmy owl is found in the deserts of Arizona. It carves out hollows in cacti to live in during the day and avoid the high temperatures.