HOT DESERT BIOMEHOT DESERT BIOMEOPTION 7: GEOECOLGY

WHAT IS A BIOMEWHAT IS A BIOMEBiomes 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) soil3) plants4) animals

Definition of a desertDefinition 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 Distribution of Hot DesertsDeserts

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 tempSuns rays very concentrated so hot in dayNo cloud cover so cold at nightAll deserts are dry- some 1cm a yearOther rain in short heavy burstsEvaporation and run-off can lead to gullyingCold deserts can get snow which is no good

to plants

3 desert types3 desert typesExtremely Arid – no precipitation

Arid – less than 250mm annually

Semi-Arid – 250-500mm annually

Factors causing desert Factors causing desert climatesclimates

1) Latitude: Located at the tropics : 15 - 30

north 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 winds2) Dry prevailing windsLie in the path of dry trade windsWinds 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 currents3) Cold ocean currentsDeserts 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 ranges4) High mountain rangesRain 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 the land in the leeward (sheltered side) receives little rainfall as the air descends

Relief rainfall:Relief rainfall:

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 soilDesert soilAridsoils.

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 exfoliationFiner soils are moved to lowland areas by

wind and torrential rain – do not favour growth

Desert soilDesert 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 floraDesert 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 floraDesert flora

Desert plants have a number of adaptations;1. Well spread out to conserve the available water.

Leading to large open spaces between the plants2. 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)

3. 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, and Skins 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. cactiTap 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 plantsDesert 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 Desert flora: Creosote BushBushone 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 soilsCreosote bush and soilsThe 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: Creosote Desert flora: Creosote BushBush

Desert flora and soilsDesert 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 hollyDesert hollyThe 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 soilsFlora 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 faunaDesert 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 Desert fauna: how they adapt to climate to climate 1. Few large animals life in desert climates2. Many animals ( especially mammals and

reptiles) are only active at dawn and dusk 3. Others like bats, many snakes, and

mammals like foxes are nocturnal, sleeping in a cool den, cave or burrow by day.

4. Some smaller animals (insects & reptiles) burrow below the surface of the soil or sand.

5. 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.

6. Many mammals have large ears to dissipate heat. These large ears have many blood vessels which release heat when the animal is resting.

7. Reptiles and birds excrete body waste in the form of uric acid, wasting very little water in the process.

8. Adaptations can be made to absorb water ie. Camels and kangaroo rats.

Desert fauna: the camel 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: camelDesert fauna: camel

Desert fauna and soilsDesert 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.

Slide winder snakeSlide winder snake

Human interaction in the Human interaction in the desert biomedesert biomevery fragile environment because

of a shortage of water and plants. Plants bind the soil in the desertIf plants destroyed so is the soilThe soil will quickly erode away

in the strong winds. Positive and negetive effects

(1) Recreational Use;(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;(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 warming2) Global warmingMan 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 warming2) Global warmingAs 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 3) Collection of plants and animalsanimalsPeople 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 3) Collection of plants and animalsanimalsvehicles used by the collectors

also damage and destroy fauna and flora and can lead to soil erosion.

invasion of non-native plants

4) Development 4) Development Deserts are also being destroyed by

development buildings plus access roads etc leads

to the clearing away of the natural vegetation

increase in tourism eg in the Nile Valley, pressure on water resources

The development of agriculture in these arid areas necessitates irrigation

4) Development4) Developmentvaluable natural water supplies

reduces the amount available for local fauna and flora.

The clearing of the land for crops also destroys native species.

Mining and ranching have also decreased many plant species

5) Hunting 5) Hunting Laws have also been passed

against plant and animal collecting.

Illegal trade of restricted plants and animals still occur, but it is not as common as it used to be.

6) Agriculture6) AgricultureIrrigation has developed farming

with in these areas. Countries such as Saudi Arabia,

the USA, Israel and Egypt have benefited greatly from the use of irrigation.

Crops grown include cotton, rice, nuts, olives, peaches and fruit etc

6) Agriculture6) Agriculture

impacts are as follows:damage to soil due to increased

salinity pollution of groundwater and

oasisland subsidencedestruction of natural habitat

Thar desert (India)Thar desert (India)3162 square kmApril, May and June hottestAvg temp in this time between 24°C and

41°CDec-Feb temp avg 28°C to 9°CTemp affected by lattitude, altitude and

distance from the seaRainfall is 100mm-500mm – mainly july-septWater gathers in small ponds (tobas) / only

source of water in Thar Desert

SoilSoilArid, sandy10% is shifting sand dunes90% fixed dunes, rock outcrops

and salt pansHard pan existsSalination – ph 7- 9.5Lack of organic matter due to lack

of plants leads to lack of humusSoil light grey colour

FloraFloraClassed as Northen Desert Thorn

forestVegetation increases from west

to east due to rainfallDrought resistant700 plant species with 107 being

grassDeep rooted and tenacious

FaunaFauna23 species of lizard25 species of snakeIndian gazelle and wild assSmaller than similar animals in

other climatesIndian Desert Eagles are the

largest flying predator

HumanHumanMost populated desert, 84 per km2Depends on agriculture and animal husbandryPop increase has led to damaging of fragile

soilLow living standardsNomadic people move from one tobas to

anotherLivestock are sheep, goats, camels etc Irrigation projects – Indira Gandhi Nahar Canal

ProjectDesert increasing by ½ km per year