HOT DESERT BIOME OPTION 7: GEOECOLGY
WHAT IS A BIOME • Biomes are unique natural world regions which are controlled by climate. • Examples include tropical rainforest, deciduous forest biome, the desert and the tundra. • There are four main parts of any biome • 1) climate • 2) soil • 3) plants • 4) animals
Definition of a desert • Deserts are dry places, where the availability of water is the major factor that determines the type of organisms that can live there. Typically deserts have less than 250mm of precipitation per year. But a more important factor is that deserts have a strong tendency to lose water by evaporation, and this potential for water loss exceeds the annual rain. • A desert therefore can be defined as; “ an arid region characterised by little or no rainfall, in which vegetation is scant or absent unless specially adapted or where groundwater conditions are favourable,” • Contrary to popular opinion, deserts are not regions that are entirely covered by sand. Deserts are predominately regions of boulders, gravel or bare rock, with only about 10% of deserts worldwide being sandy.
Distribution of Hot Deserts • Hot Deserts generally occur between latitudes 15 & 30 North and South of the equator. They are common on the western sides of continents where may be called Coastal Hot Deserts eg the California/North Mexican in North America, the Atacama in South America, the Sahara in North Africa and the Kalahari in South Afriica.They are also found in the heart of continents eg the Sahara/Arabian and the Great Australian.
Not all deserts are hot – Gobi Desert is -40 degrees in winter • Large Diurnal range= night v day temp • Suns rays very concentrated so hot in day • No cloud cover so cold at night • All deserts are dry- some 1cm a year • Other rain in short heavy bursts • Evaporation and run-off can lead to gullying • Cold deserts can get snow which is no good to plants
3 desert types • Extremely Arid – no precipitation • Arid – less than 250mm annually • Semi-Arid – 250-500mm annually
Factors causing desert climates 1) Latitude: • Located at the tropics : 15 - 30north and south of the equator • Influence of high global pressure belts • Year round high pressure means air sinks to the ground and warms up • Heat leads to low levels of moisture in the atmosphere (DRY AIR)
2) Dry prevailing winds • Lie in the path of dry trade winds • Winds become warm as they blow towards the equator from sub tropical (high) pressure belts • Winds absorb moisture rather than forming rain clouds so air remains clear and dry
3) Cold ocean currents • Deserts on the western edges ( as seen in distribution slide) are located beside cold ocean currents ie. Canary currents (Sahara) Peruvian current (Atacama) • Currents lead to a cooling in the prevailing winds moving across the ocean leading to heavy rainfall • When the winds reach the land they are dry • Annual precipitation is less than 250mm.
4) High mountain ranges • Rain shadow effect : the reason for the desert in the continent of Australia, The Great Dividing Range has caused the onshore winds from the east to drop its moisture in the form of relief rainfall before it reaches the desert. • As a result theland in the leeward (sheltered side) receives little rainfall as the air descends
All of the above factors lead to highly irregular rainfall • Rain, when it occurs, does so in short violent storms three to fifteen times a year.
Desert soil Aridsoils. • Coarse grained aridisols found in low mt slopes have a low water-holding capacity – they cannot retain all the water that falls onto them, hence infertile • Created by rock exfoliation • Finer soils are moved to lowland areas by wind and torrential rain – do not favour growth
Desert soil • Lack of Vegetation: • poorly developed • high content of mineral particles, little organic matter. • low plant productivity, restricts the soil-building properties • Lack of Rainfall: • Can be fertile after heavy rainfall but it doesn’t last long Soil forming factors: • Capillary action bring minerals to surface • Intense evaporation of water from desert soils leaves layer of minerals which hinders growth • The high surface content of sodium and calcium ions can lead to extensive saltpans where little or nothing will grow (salinisation) • Because the calcium carbonate content is not leached through the soil because of limited rainfall, most desert soil are alkaline with a PH level from7 to14. • This can harden to calcite to form an impermeable hardpans
Desert flora • Hot Deserts are home to many living things. • Second only to the tropical rainforests in the variety of plants and animal species that live there. • Many of the fascinating features of desert plants ( xerophytes) are adaptations
Desert flora Desert plants have a number of adaptations; • Well spread out to conserve the available water. Leading to large open spaces between the plants • Short life cycle - Many of the scrub like plants have the ability to lie dormant for years and to burst into life with the advent of rain, only to die away shortly after the rain stops – (ephemerals) • Succulents – Ability to store water, fleshy sponge like interior, groves in skin to allow it to expand and act as funnel to bring water to roots, andSkins tend to be waxy, leaves small & spiky to prevent loss of moisture through evapotranspiration.
4 Roots are radial or tap in nature. Radial root are shallow and spread out over large area near the surface e.g. cacti Tap roots have one long root up to 50m long that reaches the water table below e.g. mesquite bush 5. Defensive: Spiky leaves or unpleasant smell to deter animals
Desert flora: Cactus plants • drought resistant plants due to their absence of leaves • ability to store water in their stems (Succulents) • waxy skin seal in moisture. • extensive shallow root system radiates out from the base, allowing for the quick acquisition of water when it rains. • Adaptaion of the rose family (short thorny leaves) • roots can survive years of drought on the water collected from a single rainfall. • they store water both in the core and stems
Desert flora: Creosote Bush • one of the most successful of all desert species • Instead of thorns, it relies for protection on a smell and taste wildlife find unpleasant. • It has tiny leaves that close their stomata (pores) during the day to avoid water loss and open them at night to absorb moisture. • extensive double root system- both radial and deep – to accumulate water from both surface and ground water.
Creosote bush and soils • The Creosote bush has a very shallow root system that spreads out near the surface to avail of any moisture that is available near the surface and so it is again not dependant on water in the soil.
Desert flora and soils Desert plants have adapted to their environment in a number of ways • The Saguaro Cactus due to the hardpan has a shallow root system and does not depend on the arid soil for water. Instead the plant itself absorbs up to one ton of water through its roots when it rains.
Desert holly • The Desert Holly absorbs salt from the saline soil and coats its leaves. The white colour reflects the direct rays of the sun during the day and so reduces evapotranspiration
Flora and soils • Other plants have very long roots that penetrate through the dry soil to the water table to soak up sufficient moisture for life.
Desert fauna Many desert animals have evolved mechanisms to solve the heat and water problems the desert environment creates. Among the thousands of desert animal species, there are almost as many remarkable behavioural and structural adaptations developed for avoiding excessive heat. Equally ingenious are the diverse mechanisms various animal species have developed to acquire, conserve, recycle, and actually manufacture water.
Desert fauna: how they adapt to climate • Few large animals life in desert climates • Many animals ( especially mammals and reptiles) are only active at dawn and dusk • Others like bats, many snakes, and mammals like foxes are nocturnal, sleeping in a cool den, cave or burrow by day. • Some smaller animals (insects & reptiles) burrow below the surface of the soil or sand. • Some like Desert Toads, remain dormant deep in the ground until the rain arrives. Then they emerge ,breed lay eggs and replenish their body reserves of food and water for another long period.
Many mammals have large ears to dissipate heat. These large ears have many blood vessels which release heat when the animal is resting. • Reptiles and birds excrete body waste in the form of uric acid, wasting very little water in the process. • Adaptations can be made to absorb water ie. Camels and kangaroo rats.
Desert fauna: the camel • Found in Sahara and Middle – East deserts • Excellent example of adaptation. • Don’t pant and perspire very little = conserving body fluids &avoiding unnecessary water loss. • Ears lined with fur to filter out sand and dust. • eyes are protected by a double row of long curly eyelashes that helps to keep out sand and dust. • Thick bushy eyebrows shield the eyes from the desert sun. • Broad, flat, leathery pads with two toes on each foot. When the camel places its foot on the ground the pads spread, preventing the foot from sinking into the sand. • They can go 5 – 7 days with little food or water, and can lose a quarter of body weight without impairing its normal functions.
Desert fauna and soils • Some animals/ insects e.g. scorpions bury themselves in the sand to escape the intense heat of the day. • The Sidewinder snake uses muscles along its body to move quickly sideways over the sand and so reduces friction with the hot surface sand and so does not get burned. • The Marsupial Mole lives a subterranean life, rarely emerging on the surface. They are totally blind and are fast burrowers, using their hard nose pad and front feet to excavate sand, while their luxuriant fur helps reduce the friction of the sand. • Desert toads after birth bury themselves in the still moist soil and hibernate for a number of years until the rains appear. Then they emerge, mate and breed in the temporary ponds that appear and the cycle begins again.
Human interaction in the desert biome • very fragile environment because of a shortage of water and plants. • Plants bind the soil in the desert • If plants destroyed so is the soil • The soil will quickly erode away in the strong winds. • Positive and negetive effects
(1) Recreational Use; • Recreational use of off road vehicles eg dune buggies and motorcycles destroys vegetation and compact the soil. • reduces the soils ability to absorb water. • landscape more susceptible to wind and water erosion, resulting in dust storms, sand storms and muddy flash floods
(1) Recreational Use; • Tourism: This results in the building of Interpretive Centres, restaurants, rest rooms etc. • demand for water rises. Many desert streams, rivers, and lakes once used by animals have been drained • exhaust fumes from vehicles also damage and destroy fauna and flora.
2) Global warming • Man made emissions of CO2 resulting in global warming has affected both rainfall levels and temperatures • last 25 yrs overall temperatures in Hot Desert areas has risen between 0.5 and 2.0ºC, (Global average of 0.45ºC) • Precipitation: deserts of the Middle East has seen a drop 16%, the Kalahari of South Africa a 12% decrease.
2) Global warming • As plant species die out so too will the animals that depend on them. • Ultimately local soils will be eroded by wind and water further damaging this fragile environment.
3) Collection of plants and animals • People who collect rare species of cacti have completely wiped out entire populations • As the cacti become extinct so also does the natural habitat for certain insects, birds etc and can lead to their extinction. • species of lizards have also been targeted to be sold in the pet trade
3) Collection of plants and animals • vehicles used by the collectors also damage and destroy fauna and flora and can lead to soil erosion. • invasion of non-native plants