Imagine a world where water is in such short supply, and so

unpredictable, that it determines the pattern of life. Not just across the landscape, or

through the cycle of a year, but across a decade, a century or a

millennium.

DesertCaina, Maslog, Rustia

Deserts are difficult places for people because of their dryness, their unpredictable ecology, and also because they are patchy environments — where food, water, and plants are often concentrated in small pockets.

True deserts are ecosystems with sparse or no vegetation due to low, infrequent and largely unpredictable rainfall and often high evaporation. Many deserts have high average temperatures during the day and cool nights due to no, or little, cloud.

What is a desert?

Land where evaporation exceeds rainfallDryness - a combination of low rainfall and high water loss from evaporation, surface run-off and seepage deep into the soil.

Low rainfall, high evaporation and a wide daily range in temperature (hot by day-cool by night due to low humidity)

Deserts occupy about 26 percent of the continental area

Occur in two distinct belts between 15 degrees and 35 degrees latitude in both the Northern and Southern Hemispheres – Tropic of Cancer and Tropic of Capricorn

Why are deserts dry?To understand why deserts are dry you need to consider the water cycle.

On earth, water follows a path called the water cycle. The sun’s energy heats water making it evaporate from the land and sea.

As the water vapor rises it cools and forms clouds. Wind may blow the clouds over land and as the clouds are blown upward over hills and mountains the water droplets cool and fall as rain.

Low humidity allows up to 90% of solar radiation to penetrate the atmosphere and heat the ground. At night, the desert yields the accumulated heat back to the atmosphere

Rain, when it falls, is often heavy and unable to soak into the dry earth, rushes off in torrents to basins below

Average Rain Fall500mm arid-like conditions if the temperature and evaporation rates are high

250-350mm semi-arid/semi-desert

<250mm arid<100mm true deserts<70mm extreme deserts

How are deserts formed?

Movement of air massesHigh pressure area alters the course of rain, deflects rainstorms preventing moisture from reaching certain areas

Winds blowing over cold waters become cold carry little moisture & produce little rain

How are deserts formed?Mountain ranges

Forms rain shadow A rain shadow is an area of dry land that lies on the

leeward (or downwind) side of a mountain. Winds carry air masses up and over the mountain range and as the air is driven upward over the mountain, falling temperatures cause the air to lose much of its moisture as precipitation. Upon reaching the leeward side of the mountain, the dry air descends and picks up any available moisture from the landscape below. The resulting profile of precipitation across the mountain is such that rainfall and moist air prevails on the windward side of a mountain range while arid, moisture-poor air prevails on the leeward side of the mountain range.

Examples: The Gobi Desert lies in the rain shadow of the

Himalayas. The Atacama Desert lies in the rain shadow of the

Andes. The Patagonia region lies in the rain shadow of the

Andes. Death Valley lies in the rain shadow of the Pacific Coast

Ranges of California and the Sierra Nevada Mountains. The city of Spokane in the state of Washington lies in

the rain shadow of the Cascade Mountain Range (Spokane receives little rainfall). Seattle, Washington lies on the windward side of the Cascades (it receives generous amounts of rainfall).

Types of Deserts

Deserts come in all shapes and sizes and there are lots of different ways of looking at them – location, climate, ground cover and main types of plants

We usually think of deserts as being hot but this is not always the case. There are also cold, coastal and semiarid deserts.

Hot Deserts

located near the equator. Day temperatures can reach more than 38ºC but the nights are usually very cold.

generally have no cloud cover allows them to heat up and cool down very quickly.

Great Sandy Desert / Simpson Desert

Cold Desertsare found further from the equator and are usually in the centre of continents. They are often high in mountainous areas. In winter, these deserts can be covered with snow and ice. The Patagonian Desert in Argentina, South Antarctica is also an extreme example of a cold desert.

Coastal desertsare found on the west coast of continents. They have warm summers and cool winters and are often called cool deserts.

Coastal deserts by the sea

Water does not evaporate from a cold ocean as quickly as it does when the water is warm. Air blown onto the coast from a cold ocean contains no moisture to fall as rain. During the day, water evaporates from the land further helping to make a desert. At night the land cools and some of the moisture in the air above the land condenses to form fog.

Semiarid desertsreceive between 250mm and 350mm of rain. The summers are long and dry with moderate temperatures between 20°C and 30°C. Any rain usually falls in the winter months. Over one-third of Australia is semiarid desert.

Biotic

Plants

•Desert Plants are exposed to extreme temperatures and drought conditions. 

• Plants must cope with extensive water loss.

How do you think plants grow in a place that is very, very dry? 

Hot and dry desert

oSome plants have no leaves or small seasonal leaves that only grow after it rains. 

-The lack of leaves helps reduce water loss during photosynthesis.  Leafless plants conduct photosynthesis in their green stems.

Cactus Garden Asparagus

Some plants, called succulents, store water in their stems or leaves;

Salicornia

The storage of water often gives succulent plants a more swollen or fleshy appearance than other plants

Long root systems spread out wide or go deep into the ground to absorb water;

Saguaro

The roots collect water after rain. Stored in the pleated expandable stem, the water keeps the saguaro alive until the next rain. 

Plant drought-evaders flower only in the presence of moisture

Morman tea

They persist as seeds

during drought periods,

ready to sprout, flower,

and produce seeds when

moisture and temperature

are favorable.

Spines to discourage animals from eating plants for water;

Prickly pear cactus

Since many desert plants store water in their spongy tissue, animals will eat them for the moisture. The thorns keep them safe from many animal predators. 

Semi arid desert

Leaves with hair help shade the plant, reducing water loss

Old Man Cactus

The most striking feature is the long, white hairs it possesses, which serve to protect it from the sun.

Waxy coating on stems and leaves help reduce water loss.

Aloe

-The waxy surface of the aloe plant acts like a plastic wrapper, keeping precious water inside. 

-Glossy leaves reflect the sun's rays, reducing leaf temperatures and evaporation rates.

Coastal desertoplants with thick and fleshy leaves or stems can take in large quantities of water when it is available and store it for future use.

oSome plants have extensive root systems close to the surface where they can take advantage of any rain showers.

Buckwheat bush Saltbush

Cold desert

oThe plants are widely scattered.

o  Plant heights vary between 15 cm and 122 cm.

oThe main plants are deciduous, most having spiny leaves.

Animals

In spite of their aridity, desert ecosystems

support a surprising diversity of animal life,

including a wide assortment of beetles, ants, locusts,

lizards, snakes, birds, and mammals.

The mammals are notably herbivores

- They consume a wide range of species, plant types

Desert Bighorn sheep

They feed on succulents when available and then switch to woody browse during the dry periods

Desert carnivores, are opportunistic feeders; most of them have mixed diets that include leaves and fruits

Omnivory seems to be the rule in desert ecosystems

Small herbivores—the desert rodents,

particularly the ants tend to be granivores, feeding largely on seeds

Hot and dry desert

oThe animals include small nocturnal (active at night) carnivores.

oThe animals stay inactive in protected hideaways during the hot day and come out to forage at dusk, dawn or at night, when the desert is cooler.

Animals make burrows to escape the heat

Kangaroo rat

-Kangaroo rats dig burrows to protect themselves from predators and heat.

-Kangaroo Rats are nocturnal (avoiding the heat during the day).

Semi arid desert

oDuring the day, insects move around twigs to stay on the shady side;

ojack rabbits follow the moving shadow of a cactus or shrub.

large ears, long legs, Pale-colored fur and feathers

Jackrabbit

Large ears:

-are densely lined with shallow blood vessels, allowing air to cool their blood as it circulates.

Aestivation

Spadefoot toad

-during hot dry spells, the spade-foot toad (Scaphiopus sp.) covers itself with a substance to stay moist and then enters an underground burrow, where it can survive for many months until heavy rains signal it to wake. -After mating and laying eggs in temporary pools, the toads return to their burrows and resume estivation until the next heavy rains.

Coastal desert

Long legs :

Some small creatures, such as beetles and lizards,

reduce the amount of heat they absorb from the desert surface by having long legs to keep them high up and to

disperse heat.

Pale-colored Fur and Feathers:

- help others to keep cool by reflecting sunlight. 

Other adaptations:

some desert animals have concentrated the body's fat in one place, such as a hump or tail

Camel

-This animal has the ability to store large amounts of water for a long time, and because of that, it is able to survive in the dry and harsh desert.

-These animals have transparent eyelids which enable them to keep sand out of their eyes.

Tolerate Hyperthermia

Some animals can survive body temperatures that would be dangerous to humans (e.g., this antelope squirrel tolerates body temperatures over 104 degrees Farenheit).

Antelope squirrel

Where do animals in the desert get their water from?

Since water is so scarce, most desert animals get their water from the food they eat:

•succulent plants,

•seeds, or

•the blood and body tissues of their prey.

How do desert animals prevent water from leaving their bodies?

-Desert animals prevent water leaving their bodies in a number of different ways. Some, like kangaroo rats and lizards, live in burrows which do not get too hot or too cold and have more humid (damp) air inside.

-Other animals have bodies designed to save water. Scorpions and wolf spiders have a thick outer covering which reduces moisture loss. The kidneys of desert animals concentrate urine, so that they excrete less water.

Abiotic

• Rainfall• One of the most distinct abiotic factors in a desert is the

amount of rain that falls per year. Typically, deserts receive less than 20 inches of rain per year. This means that animals and plants looking to survive in the desert must be able to live with little water for extended periods of time. For example, cacti have evolved to store water in their stems to help them through dry spells.

•TemperatureDeserts typically go through huge fluctuations in temperature during a 24-hour period. Because there is little moisture, deserts lack the insulating protection of both humidity and cloud cover. A desert that is hot during the day may drop to well below zero degrees at night, once the heat of the sun has left. Organisms that cannot adapt well to rapid temperature fluctuations have trouble surviving in the desert.

• Soil• The type of soil in an ecosystem determines what plants can

grow, which in turn defines which animals can survive. Desert soil types can vary greatly, but most have excellent drainage. This means that the soil is loose instead of compacted. When rain does fall in the desert, it seeps quickly into the soil. Rapid drainage can have the side effect of washing nutrients out of the soil.

• Light• Because deserts typically don't have many shade-giving

plants, almost all of their surface area is given full exposure to direct sunlight during the day. Extreme light exposure limits the types of plants that can grow in the desert, and makes it difficult for many animals to see. Imagine being surrounded by light-colored soil on a very bright day. It might be blinding. This may be one reason that so many animals in the desert are nocturnal.

Human Impact on deserts• Deserts are being destroyed by development. Some build industries and

factories in the deserts. Many people also think the deserts sunny weather is a perfect reason to build a home there.

• As populations in desert cities rise, so does the demand for water.• Many desert streams, rivers, and lakes once used by animals have been

drained for industries, people, and farms.

• Another simple activity is Off roading.

• Their vehicles leave tracks on the soil, which will scar the land for decades. • These tracks also kill off vegetation, when vegetation is harmed, so are the

animals.

• Animals that hide in the sand can also be harmed. The sand viper, which hides so well in the soil, can become an accidental victim of off roading mayhem.

• Animal and plant collectors have also devastated the desert. People who collect rare species of cacti have completely wiped out entire populations.

• Chuckwalla lizards have also been targeted to be sold in the pet trade. Their numbers have also been reduced dramatically.

• Mining - not only do the mining operations themselves disturb the desert habitat and plants and animals that live there, the roads built to get needed equipment into the mines also destroy the desert. Uraniummining creates a host of radioactive hazard.

• Ranching - overgrazing is one of the major threats to the desert habitat. Mismanagement and overuse of land is putting severe pressure on desert habitat areas.

• Farming - as the population on our planet continues to increase at an alarming rate,the demand for more agricultural products, needed to feed these billions of people, will also increase. With modern technology, it is becoming increasingly easier to turn dry, arid desert habitat into productive farmland.

• Deserts are being used as dump sites for nuclear waste and as nuclear testing grounds. Recently, a site was opened in the desert of New mexico to be used for the storage of nuclear waste.