Imagine a world where water is in such short supply, and sounpredictable, that it determines the pattern of life. Not just across the landscape, or through the cycle of a year, butacross a decade, a century or a millennium.
Deserts are difficult places for people becauseof their dryness, their unpredictableecology, and also because they are patchyenvironments — where food, water, and plantsare often concentrated in small pockets.True deserts are ecosystems with sparse or novegetation due to low, infrequent and largelyunpredictable rainfall and often highevaporation. Many deserts have high averagetemperatures during the day and cool nightsdue 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 areaOccur 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 atmosphereRain, 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 high250-350mm semi-arid/semi- desert<250mm arid<100mm true deserts<70mm extreme deserts
How are deserts formed? Movement of air masses High 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 Desertslocated 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 areusually in the centre of continents. They areoften high in mountainous areas. Inwinter, these deserts can be covered withsnow and ice. The Patagonian Desert inArgentina, South Antarctica is also anextreme example of a cold desert.
Coastal desertsare found on the west coast ofcontinents. They have warm summersand cool winters and are often calledcool deserts.
Coastal deserts by the seaWater does not evaporate from a coldocean as quickly as it does when thewater is warm. Air blown onto the coastfrom a cold ocean contains no moistureto fall as rain. During the day, waterevaporates from the land further helpingto make a desert. At night the land coolsand some of the moisture in the airabove the land condenses to form fog.
Semiarid desertsreceive between 250mm and 350mm ofrain. The summers are long and drywith moderate temperatures between20°C and 30°C. Any rain usually falls inthe winter months. Over one-third ofAustralia is semiarid desert.
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 theirstems or leaves; The storage of water often gives succulent plants a more swollen or fleshy appearance than other plants Salicornia
Long root systems spread out wide or go deep into theground to absorb water; The roots collect water after rain. Stored in the pleated expandable stem, the water keeps the saguaro alive until the next rain. Saguaro
Plant drought-evaders flower only in the presence ofmoisture They persist as seeds during drought periods, ready to sprout, flower, and produce seeds when moisture and temperature are favorable. Morman tea
Semi arid desert Spines to discourage animals from eating plants for water; 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. Prickly pear cactus
Leaves with hair help shade the plant, reducing waterloss The most striking feature is the long, white hairs it possesses, which serve to protect it from the sun. Old Man Cactus
Waxy coating on stems and leaves help reduce water loss. -The waxy surface of the aloe plant acts like a plastic wrapper, keeping precious water inside. -Glossy leaves reflect the suns rays, reducing leaf temperatures and evaporation rates. Aloe
Coastal desert oplants 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.
Cold deserto The plants are widely scattered.o Plant heights vary between 15 cm and 122 cm.o The main plants are deciduous, most having spiny leaves.
Animals In spite of their aridity, desert ecosystems supporta surprising diversity of animal life, including a wideassortment ofbeetles, ants, locusts, lizards, snakes, birds, andmammals.
The mammals are notably herbivores - They consume a wide range of species, plant types They feed on succulents when available and then switch to woody browse during the dry periods Desert Bighorn sheep
Desert carnivores, are opportunistic feeders;most of them have mixed diets that includeleaves and fruitsSmall herbivores—the desertrodents, particularly the ants tend to begranivores, feeding largely on seedsOmnivory seems to be the rule in desertecosystems
Hot and dry desertoThe animals include small nocturnal (active at night)carnivores.oThe animals stay inactive in protected hideawaysduring the hot day and come out to forage atdusk, dawn or at night, when the desert is cooler.
Animals make burrows to escape the heat -Kangaroo rats dig burrows to protect themselves from predators and heat. -Kangaroo Rats are nocturnal (avoiding the heat during the day). Kangaroo rat
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 andfeathers Large ears: -are densely lined with shallow blood vessels, allowing air to cool their blood as it circulates. Jackrabbit
Coastal desertAestivation -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 Spadefoot toad heavy rains.
Other adaptations: 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.
some desert animals have concentrated thebodys fat in one place, such as a hump or tail -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. Camel
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 desertanimals get their water from the food theyeat:•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 anumber of different ways. Some, like kangaroo rats andlizards, live in burrows which do not get too hot or too coldand have more humid (damp) air inside.-Other animals have bodies designed to save water.Scorpions and wolf spiders have a thick outercovering which reduces moisture loss. The kidneys of desertanimals 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 intemperature during a 24-hour period. Because there islittle moisture, deserts lack the insulating protection ofboth humidity and cloud cover. A desert that is hot duringthe day may drop to well below zero degrees at night, oncethe heat of the sun has left. Organisms that cannot adaptwell to rapid temperature fluctuations have troublesurviving 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 dont 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. Uranium mining 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.