Planet Earth (2006) s01e05 Episode Script

1
A third of the land
on our planet is desert.
These great scars
on the face of the Earth
appear to be lifeless,
but surprisingly none are.
In all of them,
life manages somehow
to keep a precarious hold.
Not all deserts are hot.
Fifty-mile-an-hour winds
blowing in from Siberia
bring snow to the Gobi Desert
in Mongolia.
From a summer high
of 50 degrees centigrade,
the temperature in midwinter
can drop to -40,
making this one
of the harshest deserts of all.
Few animals can survive
these extreme changes.
Wild Bactrian camels —
one of the rarest mammals
on the planet.
And perhaps the hardiest.
Their biggest problem
is the lack of water,
particularly now in winter,
when the little there is,
is locked up as ice.
Surprisingly, snow here never melts.
The air is just too cold
and too dry for it to do so.
The sun's rays turn it
straight into vapour.
It evaporates.
But it is the only source
of water,
so Bactrian camels eat it.
Elsewhere in the world,
a camel at a waterhole can drink as much
as 200 litres during a single visit.
Here, the strategy is
to take little and often.
And with good reason,
for filling the stomach
with snow could be fatal.
The camels must limit themselves
to the equivalent
of just 10 litres a day.
Winter is the time for breeding.
This extraordinary performance
is a male Bactrian camel's way
of attracting the attention
of a passing female.
In summer, the camels can't stray
far from waterholes.
But now, with mouthfuls
of snow lying everywhere,
they can travel widely
in search of mates.
Today, less than a 1,000 of these
desert specialists remain in the wild.
The Gobi, hostile though it is,
is their last stronghold.
There's no other desert
quite like the Gobi,
but why is this place a desert?
There is one simple
and massive cause —
the Himalayas.
Clouds blowing from the south
hit this gigantic barrier.
As they're forced upwards,
so they empty their moisture
on the mountain slopes,
leaving little for the land
on the other side.
From the space,
deserts are very conspicuous.
Dunes of sand, hundreds of miles long,
streak their surface.
With no cloak of vegetation
to conceal them,
strange formations are exposed
in the naked rock.
Africa's Sahara is
the largest desert of all.
It's the size
of the United States
and the biggest source of sand
and dust in the entire world.
Sandstorms like these
appear without warning
and reduce visibility for days
over areas the size of Britain.
Dromedaries, single-humped camels,
take these storms in their stride.
The heaviest sand rises only
a few metres above the ground,
but the dust can be blown
5,000 metres out into the sky.
The ferocious wind,
armed with grains of sand,
is the agent that shapes all deserts.
Reptiles have armoured scaly skins
that protect them from the stinging grains.
For insects, the bombardment
can be very severe indeed.
The only escape
is below the surface.
As the winds rise and fall,
swallow and eddy,
so they pile the sand into dunes.
These sand scenes can be
hundreds of miles across.
In Namibia, the winds have built some
of the biggest dunes in the world.
Star dunes like these
can be 300 metres high.
Grains, swept up the flanks, are
blown off the crests of the ridges,
so it's only the tops
that are moving.
The main body of these dunes
may not have shifted for 5,000 years.
Few rocks can resist the continuous
blast of the sand carrying wind.
These outcrops are standing
in Egypt's White Desert.
But they will not do so
for much longer.
They're being inexorably
chiseled away
and turned into more sand.
Now lumps of heavily eroded rocks
have been marooned in a sea of sand.
These jagged pyramids
a hundred metres tall,
were once part
of a continuous rocky plateau.
The blasting sand will eventually
eliminate them altogether.
The relentless power of the
wind
ensures that the face of a desert
is continually changing.
But there is one constant presence —
the desert sun.
The sun's heat and power
to evaporate water
has had a profound effect on the bodies
and habits of everything that lives here.
This sun potentially is a killer.
And the red kangaroos
must acknowledge that.
Right now, while the sun is low,
there's no immediate cause
for concern.
But this situation won't last long.
Australia is the world's most arid continent
with blistering daytime temperatures.
Every hour, the temperature rises
by 5 degrees centigrade.
Soon the heat will reach
a critical point.
Any kangaroo out in the open
is in serious danger of overheating.
In the full sun, the temperature
on the ground soars to 70 degrees.
By midday, the radiation is
so intense they must take shelter.
In the shade they're shielded
from much of the sun's energy,
but their body temperature
can still rise.
So they lick saliva
on to their forearms
where there is a network of blood vessels
close to the surface of the skin
and, as the saliva evaporates,
their blood is cooled.
This thermal image shows
just how effective the process is.
The blue areas on the body
are the cooler parts.
As the saliva dries
it has to be replaced,
and this is a real drain
on the kangaroo's body fluids.
Even in the shade
the earth is baking hot,
so the kangaroos dig away
the warmed topsoil
to get at the cooler ground beneath.
By staying in the shade and licking
to control their body temperature,
kangaroos manage to get through
the hottest part of the day
without heat stroke.
But for the majority
of desert animals,
this strategy would not be
enough for survival.
The extraordinary ears of the
fennec foxes of Africa radiate heat,
but the animals have
another way of keeping cool.
They spend their days underground
and only emerge at sunset.
Darkness brings huge changes.
In the Sahara, the temperature can drop
as much as 30 degrees during the night,
so it's cool enough to allow
these desert fox cubs to play.
All sorts of creatures now appear
including some really unexpected ones.
Toads have permeable skins
and would quickly die from desiccation
out in the daytime heat.
It's only now that
they can leave shelter.
The same is true for scorpions,
even though their shells
are actually watertight.
In fact, most small desert
creatures are nocturnal —
so it's only now
that you can judge
just how much life
there can be in the desert.
But moisture, lost even at night,
has to be replaced sometime somehow,
and that problem dominates
the lives of all desert dwellers.
The Atacama in Chile.
This is the driest desert
in the world.
Some parts may not see rain
for 50 years,
and with such a record
you'd expect the place
to be completely barren.
These are South America's camels,
guanacos.
They're very good
at conserving moisture,
but they nonetheless
need a regular supply of water.
They get it partly
from cactus flowers,
but that explanation
raises another question.
How do the cacti
survive without rain?
Hot winds suck all the moisture
from the surface of the land.
Clearly, there must be something else
that takes the place of rain.
The secret is a cold sea current
that runs parallel to the land.
The cold water cools
the moist warm air above it
and that produces banks of fog.
At the same time, wind blowing
on to the shore, sweeps the fog inland.
Before long, the cacti
are dripping with dew.
The fog is so regular,
that moisture loving lichens
are able to grow on the cacti
and they absorb liquid
like a sponge.
In the land of almost no rain,
these precious drops are life-savers
for many different creatures.
Further inland, the air remains so warm
that its moisture does not condense,
so this slender strip of desert is
virtually the only part of the Atacama
where life can exist.
And without the fog,
this land, too, would be empty.
The guanacos make
the most of the dew,
but it will not remain
for long.
In an hour or two,
the sun will have burnt it off
and dried the surface of the cacti.
The Sonoran desert in Arizona,
is not quite so dry as the Atacama —
some rain does fall.
But it is infrequent,
and when it does arrive,
animals and plants have
to be ready to make the most of it.
And it's coming.
When the summer monsoon blows in,
the giant saguaros,
one of the biggest of all cacti,
are ready to take
full advantage of it.
After a rainstorm, the saguaro's long
shallow root system sucks up the water,
and the pleats on its trunk
enable it to expand rapidly.
When full, a saguaro stem can store
up to 5 tonnes of water,
and that's enough to see it
through many months of drought.
The trunks of these huge plants
provide homes for the gila woodpecker.
But birds are not the only animals
to benefit from the presence of the cacti.
During four weeks of the summer,
the saguaros bloom at night
to attract visitors.
The pollen and nectar with which
these flowers are loaded,
attract long-nosed
and long-tongued bats.
The bats left Mexico a few days earlier
to escape the heat of summer
and are on their way north
to the southern United States.
To get there, they have
to cross the Sonoran desert.
But the desert is so big
that for most of the year
they would be unable to cross it.
Now, with the saguaro in bloom,
they can refuel on the way.
So the saguaro's success in
developing a way to store water
is now crucial to most of the animals
that live or even travel through this land.
The scarcity of rain determined
the shape of this icon of the desert,
but water, scarce thought it is,
has also, like the wind,
shaped the land itself.
In the deserts of Utah,
ancient rivers flowing across sandstone
country, steadily widen their canyons
until now the land between them
has been reduced to spires and pinnacles.
With little or no soil to retain
the water on the surface of the land,
life here is scarce indeed.
And when resources are limited,
conflict is never far away.
These are Nubian ibex,
and they are squaring up
for a duel.
And when trouble starts,
a smart ibex knows that the best thing
to do is to gain higher ground.
These are actually
subordinate male ibex,
but their fights
are nonetheless serious.
Losing one might mean
never getting the chance to breed ever.
When competitors are evenly
matched as they are here,
duels can last for an hour.
In this heat the effort
is truly exhausting.
But victory here will gain important
ranking points on a male's way to the top.
There's so much at stake,
that not all play fair.
The battle has produced the winner,
but the ultimate prize
is not his yet.
That currently belongs
to the dominant male ibex.
His rank earns him the loyalty
of a harem of females
and they follow him closely
as he travels across this desert
searching for food and water.
He doesn't have to waste time
looking for mates —
they're his for the taking,
so he can concentrate with them
on keeping fit and healthy.
Lizards are desert specialists.
But here, their numbers
are extraordinary.
These crevices in South Africa
contain the highest density
of lizards in the world.
They're called flat lizards
for obvious reasons,
and they flaunt their multi-coloured
bellies in territorial disputes.
He's made his point,
and now it's time
to find some food.
As the day warms up,
the lizards move away
from their cracks
and head down
to the bottom of the gorge.
Their goal is the river.
There is no food at the edge,
but this desert river
holds a secret.
Each day, blackfly rise from
turbulent stretches of the river.
This is what the lizards
have come for.
The black fly never land,
so the lizards have to leap
for their food.
In one day, each of these acrobatic
little lizards may catch 50 flies.
There are plenty of flies
to go round,
even with hundreds of lizards
competing for them.
Away from these rapids,
flat lizard populations
are found in much smaller numbers.
But here, one unusual abundance
has produced another.
Deserts are created
by the lack of water,
but what actually kills animals
here is not heat or thirst,
but lack of food.
So, how on earth does
a plant-eater this size
survive in a place apparently
totally devoid of vegetation?
Elephants in Namibia
are the toughest in Africa.
And they need to be.
What little food exists
is so dispersed,
that these elephants walk
up to 50 miles a day
as they travel up the dry river channels
searching for something to eat.
At times the task
looks truly helpless.
Elephants may seem out of place
in this landscape,
but they're not the only ones.
Amazingly, lions live here, too.
In savanah country, huge herds
of games support prides
containing 20 lions or more.
But to live here, lions have had
to change their habits —
prides are much smaller
and their home ranges
are very much bigger.
And there's an added problem —
their food is always on the move.
Like the elephants,
the lions must travel great distances
to find enough to live on.
But lions can't go everywhere —
they won't attempt to cross
this field of sand dunes
and the oryx know it.
The lions must wait for the oryx
to leave the safety of the dunes,
which eventually they must
to find food and water.
And then the lions
will ambush them.
The elephants have found
some of their favourite food.
Grasses are the staple diet
of all elephants,
but this herd concentrates
on digging up the roots,
which have more nutrition
and moisture than the stems.
It's the sort of behaviour
that can make all the difference
in a place of serious shortages.
Yet all this can change
in an instant.
The fortunes of many deserts
are ruled by distant rains.
This water fell as rain in mountains
more than a hundred miles away.
It's known as a "flash flood"
and called that because the water
may run for just a single day.
It's an event that only happens
once or twice a year at the most.
The sandy riverbed acts like a
giant strip of blotting paper
sucking up the water
as soon as it appears.
But every square metre of soil
moistened by this river
will increase the chances
of survival for those that live here.
Waterholes are filled temporarily.
Elsewhere in Africa,
elephants drink every day,
but the lack of water here,
means that desert elephants
can only refill their tanks
once every 4 or 5 days.
Within a week, the flash flood
has produced a flush of green,
more than enough to draw
the oryx out of the dunes.
It's a rare chance for them
to build up their food reserves.
The flood has made life easier
for the lions, too.
The flesh of this oryx will keep the
family going for a week at the most.
But for a while,
the hunting will be easier,
now that river channel
has turned green.
The good times
for lions and oryx are brief,
but these are
the short moments
that make it possible to live
in deserts the year round.
"Death Valley" is the hottest
place on Earth.
Yet even this furnace
can be transformed by water.
A single shower can enable
seeds
that have lain dormant
for 30 years or more
to burst into life.
And there hasn't been a bloom
like this one for a century.
The periods of boom
in Death Valley are short.
but they're just frequent enough
to keep life ticking over.
A sudden flush of vegetation is
what every desert dweller waits for,
and when it happens,
they must make the most of it.
There is no other species
on the planet
that responds as quickly and as
dramatically to the good times
as the desert locust.
Eggs, that have remained in the ground
for 20 years, begin to hatch.
The young locusts
are known as hoppers,
for at this stage
they're flightless.
They find new feeding grounds by
following the smell of sprouting grass.
Normally, it takes 4 weeks
for hoppers to become adults,
but when the conditions
are right as now,
their development switches
to the fast track.
As the vegetation in one place
begins to run out,
the winged adults release
pheromones — scent messages,
which tell others in the group
that they must move on.
And when groups merge,
they form a swarm.
An adult locust eats
its entire body weight every day,
and a whole swarm
can consume literally hundreds
of tonnes of vegetation.
They have to keep on moving.
The swarm travels with the wind —
it's the most energy-saving
way of flying.
Following the flow of wind
means that they're always heading
toward areas of low pressure,
places where wind meets rain
and vegetation starts to grow.
As they fly,
swarms join up
with other swarms
to form gigantic plagues
several billions strong
and as much as 40 miles wide.
They will consume every edible
thing that lies in their path.
This is one of planet Earth's
greatest spectacles.
It's rarely seen on this scale
and it won't last long.
Once the food is gone,
the steady roar of a billion
beating locust wings,
will once again be replaced
by nothing more than the
sound of the desert wind.