Download The Physical World Of Deserts

The Physical World Of Deserts
The image of a desert is distinctive. An ocean of sand stretches as
far as the eye can see, the sun beats down mercilessly, and there are
no clouds; it is a dry,Inhospitable, and often beautiful environment.
But the deserts of the world also present dramatic contrasts of
landscape and weather- and most are not sandy.
Deserts are typically empty, barren places (the word comes from the
Latin deserere, meaning “to abandon”) that are characterized above all
by their dryness. A desert will form if the rate at which water
evaporates is greater than the rate of precipitation (rain, snow, for,
or dew). As it seldom rains in arid areas, this is measured as the
amount of water that will evaporate during a stated period from an open
water surface. This is called the potential evaporation and is
compared with the precipitation over the same period. The higher the
temperature, the greater the potential evaporation will be. But
regardless of the temperature, a desert is likely to form if the
average amount of precipitation is less than 10 inches (250 mm) a year.
The largest desert is the Sahara, which is about the size of
the United States and stretches across North Africa, joining
the Arabian Desert. The American deserts include the
Colorado Desert and the Mojave Desert with its Death
Valley – one of the hottest places on Earth. The Atacama
Desert in northern Chile is one of the driest deserts in the
World.
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Most deserts are not sandy. More often, the desert
Major Deserts Of The World
Surface consists of stones, small rocks, or gravel.
Deserts may also consist of salt pans, formed by lakes
AREA
SQUARE MILES
Or rivers that no longer exist, or large areas of dried
North America
Mud flats. Mountain ranges can also produce deserts.
Air is forced to rise as it crosses the mountains. As air
Mojave
15,000
Rises it cools. Cold air does not hold water vapor very
Well so clouds on a mountain often produce rain or, South America
snow or fog that moistens everything that it touches.
Patagonian
260,000
By the time the air crosses the mountain, the air has
Atacama
140,000
Become fairly dry.
Africa
The Natural World Of Deserts
Sahara
Kalahari
Namib
3,500,000
275,000
97,000
Middle East
Deserts are harsh places, but they are far from lifeless.
Arabian
In some the ground is quite densely covered with plants
Syrian
that are adapted to the dry conditions. Other deserts
Asian
“bloom” whenever it rains, with plants that can flower
and set seed before the water evaporates. Wherever
Turkestan
there are plants, there are animals to eat them. And
Gobi
wherever there are animals that graze, there are other
Thar
animals that hunt them.
Iranian
Takla Makan
Plants produce sugars through photosynthesis- the formation
Australian
of organic compounds using water, carbon dioxide, and
light energy. Because green plants can make their own
food
Australian
from simple chemical substances, they are called primary
Producers. They form the basis of every ecosystem.
1,600,000
200,000
750,000
374,420
231,600
150,000
115,000
979,700
In a desert plants can grow only where the sand is no longer shifting,
and even among stable dunes they are scarce. Seeds that germinate in
shifting sand are soon buried, and the young plants are killed.
Animals cannot manufacture food for themselves the way plants can.
Ecologists call them consumers because the only way they can obtain
food is by eating the producers.
In the desert consumers include herbivores (such as rodents and
gazelles), which are animals that feed upon the plants. Because there
are so few plants, the bigger herbivores must travel to find enough
food for their needs, often covering considerable distances.
Carnivores (such as cats, dogs, foxes, snakes, and eagles) feed on
other animals. They are also consumers, but they feed on herbivorous
consumers, they are called secondary consumers. The distinction is not
as sharp as it seems, because many animals eat both plant and animal
foods, so they are both primary and secondary consumers.
Not all the consumers are as big as gazelles or eagles, nor even the
size of mice. Insects such as ants, bees, grasshoppers, and locusts
feed on the plants, and wasps, spiders, and scorpions hunt them.
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The desert soil has no covering of fallen leaves or other plant
material, but plants do shed leaves, even in the desert. This, and the
organic remains of dead plants and animals, provides food for another
group of organisms, the decomposers. Beetles bury organic material and
consume it below ground. In addition to the beetles and their young,
or larvae, there are mites and still smaller animals, along with fungi
and bacteria, all finding food among the organic litter lying on the
ground. The decomposers are in turn hunted by such animals as
centipedes and spiders, and their wastes , too, are eaten.
Desert Plant Survival
During the long wait between rains, desert plants must somehow stay
alive. Plants must cope not only with drought, but also with the
extreme heat, which can kill them.
Deserts can have a wide range of
temperatures within a single day. It is not uncommon for nights to
reach temperatures below freezing (32 F) then have daytime
temperatures reach 115 F. Death Valley, California, one of the
hottest desert areas will attain average daytime temperatures in the
hottest month of July of 166 F. Because of this temperature variance
plants can die quickly unless they develop special survival
characteristics.

Some plants, especially trees and shrubs, have adapted to the
dryness by having long roots that seek water far below ground.
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Some plants have a shallow system of lateral roots, which can
absorb water as soon as the ground becomes moist.
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Many desert plants develop both root systems at the same time.
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Some plants survive by allowing their leaves, stems, and roots to
die away, so all that remains are their seeds or bulbs. Which can
remain dormant for several years waiting for the next rain.
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Most seeds germinate when the surrounding soil warms. Desert
seeds only germinate when sufficient moisture penetrates their
seed coat
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Some plants produce seeds, which germinate when temperatures are
warm and also produce other seeds that germinate with moisture
allowing them to take advantage of either condition.
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Some plants germinate, produce leaves, flower, produce fruit and
then seeds in as little as 2 weeks, allowing them to take
advantage of very little moisture.
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Plants avoid water loss (Transpiration through their leaves) by
developing very small leaves which reduce their surface area.
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Plants lose moisture through small openings in the leaf called
stomata. Many desert plants have stomata on the bottom of the
leaf- out of direct sunlight.
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Some plants keep their stomata closed during the daytime, only
opening at night when temperatures are lower to save moisture.
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Some plants have a thick waxy cuticle on tops of their leaves to
protect against extreme temperatures.
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Unique Plants of the Desert
 Saguro or giant cactus (Carnegiea gigantean) grows in the
southwestern United States and in northern Mexico. This is the tallest
of all cacti, growing 65 ft and weighing more than 12 tons. It has a lifesp
of about 200 years. The water stored in their trunk is toxic to animals.
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Prickly Pear cactus (Opuntia engelmannii) has no true
a
Flattened, swollen stem that grows in short, jointed section
covered
spines. These spines prevent water loss and deter grazing he
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Date Palm (Phoenix dactylifera) grows up to 80 feet tall
and has been cultivated since at least 3000 B.C. The da
are the fruit, yellow when they are ripe but brown when
they are dried, they form in huge bunches hanging from
the crown. A sugary sap is also obtained from the crown
of the tree and boiled to produce sugar. This sap
also fermented to make an alcoholic drink called toddy o
Palm wine. The dates may also be dried and ground into
type of flour.
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Stones (Lithops) are succulents- they store
avoid being eaten by herbivores disguising t
stones. They appear as other desert stones
show no
Signs of life until rains saturate the ground.
There are 37 species of Living Stones.
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Welwitschia (Welwitschia mirabilis) discovered in 1860 is one of
the worlds strangest plants as it derives all of its moisture from
The plant grows only two leaves from a short stem, but the leaves
grow 5 inches /year throughout the plants Lifetime. It can live
for 500 or more years.
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Desert Rhubarb (Rheum palaestinum) uses large funnel shaped
leaves to capture 16 times more water than other plants. It grows
four leaves that grow to a length of 30 inches. These leaves channel
rainwater to its one large root. This method allows it to live in the
driest areas of the world.
Desert Animal Survival
Desert survival requires the ability to shelter from or endure the intense heat, and if
necessary, to go for long periods without drinking. Smaller animals can burrow
underground or rest in the shade of small desert shrubs to avoid heat and thus water lo
Large animals cannot bury themselves so must resort to spending daytime hours in caves
Or other shelters. Large mammals enjoy some advantages, however. They can cover much
greater distances in search of food and drink. Also larger animals have a smaller surf
area in relation to their volume than do small animals. This means that a big animal a
heat more slowly than a small one, and it takes longer for its temperature to rise.
All animals lose water in their urine. Desert animals excrete highly concentrated urin
very dry feces to avoid losing too much water. Drinking only occasionally, or not at a
brings another advantage: security. Predators await victims at watering holes where
ambush the thirsty animal. Desert animals also tend to have large appendages filled wi
blood vessels which allow them to release body heat and thus keep them cool. Many have
large ears or tails just for this purpose
Herbivores of the Desert
 Camels are mainly used for transportation and have
earned the nickname “ships of the desert.” They feed on
thorny plants and grasses. The hump on their backs contain
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fat, which is used as an energy reserve. Camels can travel 300
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miles, taking up to three weeks in between drinks. A camel
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can drink 44 gallons of water in less than 10 minutes.
 Gazelles (Gazella dorcas) obtain most of their moisture from the plants they eat.
They have the ability to eat large amounts of grass in a short period of time after
Later they can regurgitate food back into their mouth for chewing when grass gets sc
Hermann’s Tortoise (Testudo hermanni) supplements its
vegetarian diet with earthworms and snails. Tortoises shelter
in deep burrows and emerge around dawn and dusk to feed.
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Arabian Tahr (Hemitragus jayakari)is a goat-antelope, it looks and behav
like a goat, climbing and leaping easily over mountainous terrain. In t
morning they lie on the ground, which has cooled overnight. Their thinl
furred underside and legs are pressed against the ground exposing only
their highly protected backs to the sun.
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 Kangaroos and Wallabies can travel great distances on only two
feet in search of water. They have a unique way of cooling themselves.
They salivate profusely, then by licking their tails, bellies and feet
are “cooled” as the saliva evaporates from their body.
 African Ground Squirrel (Xerus inaurus) provides itself with shade by lifting its
out tail to cover its body. Nocturnal by nature but will search for food during the
leaving its underground burrow.
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Jerboa (Jaculus jaculus) nocturnal rodent which uses large back
legs to hop great distances. Build burrows with 4 entrances to
help avoid predators. Plug openings during daytime to prevent
detection by scavengers. Resemble kangaroo rats.
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that are
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 Spotted Sandgrouse (Pterocles senegallus) produce young
able to feed on seeds immediately. Avoid thirst by soaking
their feathers in water, which they can carry throughout the da
and simply access by preening.
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Desert Locust (Schistocera gregaria) potentially the most
dangerous insect due to the ability to form huge swarms
(100 million per square mile) that travel great distances.
Live a solitary life until it rains which triggers females
lay large numbers of eggs. Developing quickly thro
larval stages they become adults as vegetation forms.
Can travel 150 miles per day looking for food.
 Dung Beetle (Phangeus vindex) growing up to an inch
these feces eating insects are extremely valuable to desert
organisms. By eliminating animal feces they control fly
populations which can carry disease to other organisms.
Remove about 80% of animal feces within a desert environment.
They eat the partially digested vegetation with the animal
droppings.
Carnivores of the Desert
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Sand Cat (Felis margarita) discovered 150 years ago has large
ears that are widely spaced. This suggests keen hearing, but
large ears help keep the cat cool. It hunts at night eating small
rodents, birds reptiles and insects. It is the size of a
Domestic housecat.
Desert Lynx (Felis caracal) is recognizable by the turfs of hair at the tips of their e
It is larger than a Sand Cat. It is nocturnal and hunts birds and mammals, including
gazelles and hares.
Fennec Fox (Vulpes zerda) prefers to eat small mammals but
will eat almost anything, including, worms, insects, and fruit.
One of the smallest foxes in the world it is only 16 inches long
with an 8 inch tail. It spends the day in its burrow and only
hunts at night.
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 Scorpions (Hadrurus arizonensis) are insect eaters
which hunt from their underground burrows or the shade of large
rocks where they spend most of their day. They attack passing
insects, grabbing them with their pincers, and stinging them with
deadly poison from their tails.
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Camel Spiders are large spiders growing up to 8 inches in
length and able to Travel at speeds of up to 10 miles per hour.
These desert animals hunt scorpions, lizards and mice.
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Roadrunner (Geococcyx californianus) spends most of its time
on the ground but can actually fly. Can reach running speeds
of 15 mph while growing to about 2ft in eight. Diet consists of
insects, other bird eggs, rodents and reptiles.
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Great Horned Owl (Bubo virginianus) tends to perch during
daylight hours on tree limbs or rocky alcoves preferring to hunt at
night. Using keen hearing it locates prey where it will silently
swoop down using its talons to take the victim. Diet comprised of
rodents, hares, insects, reptiles and other small birds.
 Red-Tailed Hawk (Buteo jamaicensis) most abundant flying predator of the desert
due to its flexibility in nesting which ranges from cliffs and ledges to shrubs and
Diet consists of hares, mice small birds and reptiles. Has excellent vision and wil
Swoop at a speed of 100mph to catch pray.
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 Egyptian Vulture (Neophron percnopterus) also known as
“Pharoahs chicken”. Not a carnivore but more of a scavenger eating
anything from lion dung to snails, lizard eggs, decaying flesh, and
locusts. Often soar over watering holes waiting for a carnivore to kil
a large animal, then moves in to eat any leftover meat from the kill.
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Desert Reptiles
Reptiles are cold-blooded vertebrates. The fact that they are cold-blooded
does not mean that they are cold, it means that they cannot generate or regulate
their body temperatures. Animals that cannot regulate their own temperature are
called ectotherms. There is an advantage to being an ectotherm. Warm-blooded
animals use 90% of their food to provide the energy needed to maintain their body
Desert reptiles – must control their body temperature by finding areas within their
Environment that are appropriate for different times of day. They bask to warm their m
At dawn, then use shade and the wind to cool them during the day. As the graph shows,
The lizard has warmed up it maintains a fairly constant body temperature throughout the
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practice a reptiles need only eat one-tenth of the amount of food required to sustain a
bird or mammal of the same size. This allows them to live in places where food is scar
Early in the morning reptiles will bask on rocks in the desert. As soon as the sun has
it, the animal will disappear into shade in order to cool itself. The basking animal
presses its body against the warm rock to raise its temperature. It presses against co
objects in order to lower its temperature.
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Examples of Desert Reptiles
Horned Viper (Cerastes cerastes) is a 4 – 5 foot long snake
With horns above its eyes living in stony type deserts where it
eats small rodents.
Sand Monitor (Varanus gouldi ) is a large lizard with a forked
Tongue. Voracious predators which hunt by day but seek shelter
when temperatures get too high. About 5 ft in length they
feed on other reptiles, birds, insects and their eggs.
Sand Skink (Neoseps reynoldsi ) swim through desert sand
like a fish. They hunt for insects and dive underground when
they feel threatened.
Desert Iguana (Dipsosaurus dorsalis ) grows to 16 inches in
Length and they spend most of their day eating insects and
Flowers from desert plants. They are unique in that they can b
Active even when temperatures reach 115o F, where most
Desert animals would need to seek shelter from the sun.
Being active during day time makes them diurnal
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