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The Taklamakan Desert
in northern China,
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00:00:37,400 --> 00:00:41,680
a constantly shifting landscape
of sand.
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WIND WHISTLES
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Temperatures that swing from -20
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to more than
40 degrees centigrade...
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..and, most critically,
almost entirely without rain.
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Yet, here on the dunes...
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..a Euphrates poplar tree.
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And it's not alone.
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Some of these trees have lived
here for 1,000 years.
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They have exceptionally long roots
with which to collect water,
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and what is more, those roots are
connected to neighbouring trees,
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so that if one strikes water,
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others can share it.
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In every desert across the planet,
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plants have found ways
to not only survive,
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but flourish.
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This is the Gran Desierto
of Mexico and the United States.
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These dunes may appear
to be totally barren.
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In fact, they are full of life.
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In the sand beneath my feet
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there are seeds
of many different kinds.
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In fact, you could say that the dune
itself is one great seed bank,
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and when it rains,
it bursts into life.
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WIND WHISTLES
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THUNDER CRACKLES
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But rain may come only once
a decade, and even then,
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the long-awaited storm
may be very brief.
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So seeds must respond immediately.
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This is sand verbena.
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It can grow from a seed to a
sweetly-scented flowering plant
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in just a few weeks.
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Primroses and many other plants
soon join the race
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to flower before the sand dries.
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Desert blooms like this,
however, are rare.
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This is the first year for 20 years.
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The combination of vibrant colour
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and powerful scent
attracts migrating pollinators,
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such as these
Painted Lady butterflies,
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which fly into the middle of what
were, only recently, barren dunes.
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Everything is rushing
to complete their lives
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before the moisture has gone.
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Such spectacular blooms transform
deserts all around the world.
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From the Atacama in South America...
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..to the dusty plains
of southern Africa.
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Rain in deserts, however,
never lasts long,
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and all too soon,
the flowers wither and die.
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But not before they've produced
the next generation...
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..the seeds that will now wait
in the sand for the next rains.
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00:06:02,680 --> 00:06:05,600
In the Sonoran Desert
of North America,
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the huge saguaro cacti
have a different strategy.
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They store water in quantity
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and can live to a great age.
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But in their early years,
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they are extremely vulnerable.
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This little saguaro cactus
is about ten-years-old.
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When they're really small and
growing out in the open,
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there's a real chance that they may
shrivel up and die.
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But this one has been lucky.
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It's been growing in the shade
of this mesquite tree,
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and it's got a very good chance
of surviving to maturity.
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The young saguaro is protected
by the mesquite's branches.
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They halve the amount of scorching
sunlight reaching the cactus...
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..and so, keep it cool.
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And the mesquite's extremely
long roots draw up water,
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bringing it within reach
of the young saguaro.
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So the mesquite
is known as a 'nurse plant',
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and a very effective one it is, too.
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In fact, this particular mesquite
has already nurtured
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seven young saguaros
over the past 30 years.
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As a young cactus grows,
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it needs the protection of its nurse
plant, not only from the heat,
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but from the other hazards
of desert life.
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Temperatures can drop
to -10 degrees overnight...
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..and very occasionally...
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..it even snows.
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If the water stored inside a young
cactus should freeze,
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the cactus will die.
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But the nurse plant traps a blanket
of slightly warmer air around it,
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just enough to keep it alive.
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Eventually, saguaros outgrow
their nurses,
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but by that time,
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they are robust enough to face
the elements by themselves.
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No matter how old a desert plant
is,
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water is always precious,
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whether gathered
from melting snow...
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THUNDER BOOMS
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..or a shower of rain.
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So, cacti have developed
extraordinary adaptations
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that enable them
to not only collect water...
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..but to retain it.
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Instead of leaves,
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which would lose precious
moisture through evaporation,
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they have spines.
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Each spine
has a tiny pad at its base
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where the water is absorbed...
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..and then stored
in the great swollen trunk.
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A large saguaro can
hold 5,000 litres of water
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00:09:41,880 --> 00:09:46,920
and is able to do so because it
has another special adaptation.
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The ridges on its surface
are like the pleats on an accordion.
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They allow the saguaro
to change its shape.
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After rain has fallen,
the pleats expand
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and the saguaro fills
up its water tank.
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In the dry times,
it uses its water to grow,
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produce flowers and,
eventually, seeds.
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Fully-loaded with thousands
of litres of water,
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this saguaro won't need to drink
a single drop for another year.
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But such valuable stores
of water attract thieves.
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Now the spine's function
changes from collecting...
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..to guarding.
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The spines of some species
are a quarter of a metre long.
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Others are needle-like barbs
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that grow in clusters and easily
break off in the skin of any animal
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that touches them.
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But perhaps the most vicious cacti
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belong to a group
called the chollas.
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This is called a teddy-bear cholla
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because of the thick coating
of spines on it.
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But don't be deceived by the name,
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there is nothing cuddly
about THIS particular teddy bear.
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In fact, it's the most dangerous
plant in the desert,
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and I wouldn't dream of
putting my hand anywhere near it
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without proper protection.
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Brush against it...
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..this can happen.
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Ow!
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This can happen,
even with this glove on.
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One of them has just gone through,
I can feel it.
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It's quite painful!
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Look closely at the spine
and you can see very clearly
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why they're so dangerous.
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Each is like a splinter of glass,
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sharp enough to pierce flesh...
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..and they're covered
with backward-pointing barbs.
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So getting them out, even with a
pair of pliers, is quite hard.
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This is not pleasant at all.
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It won't come off, or without.
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Oh, look at that!
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It's hard to imagine a more
aggressive defence than this,
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and it makes both the plant and its
buds virtually invulnerable.
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Most animals know to keep clear.
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Cholla buds grow like tiny barrels
from the top of the adult plant
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and then drop off.
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BARKS
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If the young cholla
put down roots here,
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it would compete with
its parent for water.
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Night falls...
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..and this one is on the move.
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A pack rat. She knows how to deal
with a cholla.
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She avoids the spines by gripping
it at the place
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where it broke off from its parent.
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And she works fast.
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There are pack rat hunters here.
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She uses the cholla to build a spiny
wall around her nest.
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The flesh of the cholla
supplies her with water...
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..and the severed spines
further reinforce the defences.
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This cholla bud...
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..might be next.
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But one accidental nudge...
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..and it escapes.
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DRAMATIC MUSIC PLAYS
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The bud starts to put down roots.
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So, the cholla,
thanks to the pack rats,
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finds new territory
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and sets about claiming it.
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Few plants deal with the problems
of desert living better than cacti.
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There are almost 2,000 different
species of them.
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They're spread across the deserts
of the American West,
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from Arizona,
all the way to Mexico and beyond.
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In South America, the ice-covered
peaks of the Andes
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act as a rain barrier...
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..beyond which
lies the world's driest desert -
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the Atacama.
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In the desert world, water thieves
can come in many forms
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to exploit even the smallest chink
in a plant's defence.
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One of the strangest
travels within the gut
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of a fruit-eating mockingbird.
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These are the seeds
of Tristerix,
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a kind of mistletoe.
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Their goal is the water
inside this hedgehog cactus.
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Using the spines as anchors,
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the seeds start to germinate.
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Each produces a long probe
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with which to try and locate
the cactus' skin.
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For most,
that's a stretch too far,
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and they perish.
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But for this one, the cactus'
surface is within reach.
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It clamps onto it
with a special sucker...
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..and then waits for darkness.
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At night, the cactus opens its pores
in order to respire.
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Oxygen goes out,
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carbon dioxide goes in,
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and so does Tristerix.
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Once within, its tissues spread
throughout the body of the cactus,
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sustained by the precious store
of water that they find there.
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Then, a year later,
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it breaks
through the cactus' skin...
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..and bursts into flower.
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Hummingbirds come to drink
their nectar
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and pollinate them as they do so.
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And, then, to complete the cycle,
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Tristerix produces hundreds
of white, eye-catching seeds,
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ready to be carried away by a bird
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to invade another cactus.
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00:20:18,040 --> 00:20:20,840
The Karoo Desert in southern
Africa.
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00:20:23,080 --> 00:20:25,360
And although it may look bare,
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its rocky ground contains an
unrivalled variety of plants that,
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one way or another,
store water in their tissues.
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00:20:36,160 --> 00:20:39,280
They belong to many different
families, but as a group,
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they're known as succulents.
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Some are small and low
and barely distinguishable
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from their surroundings.
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These look like little pebbles.
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They resemble them so closely
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that animals which might be only too
glad to steal their water
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just pass them by.
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00:21:24,800 --> 00:21:29,240
When rain does fall,
they absorb it and quickly expand.
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00:21:31,600 --> 00:21:34,800
But even this
doesn't spoil their disguise.
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They just look like larger pebbles.
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Nor are they green.
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00:21:41,200 --> 00:21:44,480
The cells on their top surface
are transparent
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and allow sunlight to pass through.
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Deep within and out of sight
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00:21:50,840 --> 00:21:54,880
are the green cells
where photosynthesis occurs -
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the process which uses this light
to make food for the plant.
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00:22:05,680 --> 00:22:08,560
When the time comes to reproduce,
however,
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the stone plant
abandons its disguise.
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And now, it blooms.
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The flowers open and close
every 24 hours.
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So for a few dangerous days,
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the plant advertises for pollinators
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00:23:02,400 --> 00:23:05,240
before returning to life
as a pebble.
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Some desert plants
have developed a very different way
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of attracting pollinators.
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This is a Stapelia.
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00:23:30,360 --> 00:23:35,840
It produces what is perhaps
the desert's strangest disguise.
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It uses water stored in its stems
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to grow buds
the size of tennis balls.
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00:24:01,800 --> 00:24:06,320
The flower, once opened,
is called a desert starfish.
240
00:24:09,360 --> 00:24:12,800
Instead of releasing millions
of loose pollen grains
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as most flowers do,
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00:24:14,760 --> 00:24:19,560
the desert starfish produces them
packed in five tiny sacks.
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But if its strategy is successful,
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just one of them
will produce hundreds of seeds...
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00:24:32,240 --> 00:24:34,880
..and this depends on deception.
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00:24:38,200 --> 00:24:40,440
The flower appears to have hair...
247
00:24:42,360 --> 00:24:45,280
..wrinkly skin...
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00:24:47,400 --> 00:24:51,600
..and it produces a stench
like the carcass of a dead animal.
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And when a carrion fly
investigates...
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00:25:11,360 --> 00:25:15,640
..the flower tempts a tiny sack
of pollen to its proboscis.
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It isn't easy to feed
with such encumbered mouthparts.
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00:25:29,560 --> 00:25:32,680
But try as it might,
the fly can't get rid of it.
253
00:25:36,000 --> 00:25:38,960
And it's still there when the fly
leaves to try and feed
254
00:25:38,960 --> 00:25:41,560
from another bogus carcass.
255
00:25:47,040 --> 00:25:50,000
This time, however,
when its clamped-up proboscis
256
00:25:50,000 --> 00:25:54,040
slots into the flower,
the pollen sack is released.
257
00:26:00,320 --> 00:26:02,600
With pollination complete,
258
00:26:02,600 --> 00:26:04,960
the fly is no longer needed...
259
00:26:08,840 --> 00:26:11,000
..and just as well.
260
00:26:16,120 --> 00:26:20,080
Some deserts can be so dry
that plants must find techniques
261
00:26:20,080 --> 00:26:24,320
of surviving for long periods
without any water whatsoever.
262
00:26:25,560 --> 00:26:30,400
One of them
is to grow extremely slowly,
263
00:26:30,400 --> 00:26:34,800
and few plants grow more slowly
than this one -
264
00:26:34,800 --> 00:26:37,000
the creosote bush.
265
00:26:38,080 --> 00:26:40,480
It is inactive for most of its life
266
00:26:40,480 --> 00:26:44,640
and only wakes up and grows
for a brief period,
267
00:26:44,640 --> 00:26:47,600
if and when
there is a fall of rain.
268
00:26:51,160 --> 00:26:55,440
I've seen evidence of this
"grow slow" strategy for myself.
269
00:26:56,640 --> 00:27:00,640
40 years ago, I came here
to California's Mojave Desert
270
00:27:00,640 --> 00:27:03,320
to visit one particular plant.
271
00:27:04,560 --> 00:27:07,000
ARCHIVE: An individual
creosote bush
272
00:27:07,000 --> 00:27:10,040
tends to spread
not by setting seeds
273
00:27:10,040 --> 00:27:12,280
and producing a new generation,
274
00:27:12,280 --> 00:27:16,520
but by sending out new stems
around its base.
275
00:27:16,520 --> 00:27:22,280
This plant started growing between
10,000 and 12,000 years ago.
276
00:27:23,960 --> 00:27:26,240
NOW: That was in 1982.
277
00:27:29,400 --> 00:27:34,720
Since then, careful measurement
has shown that it has increased
278
00:27:34,720 --> 00:27:38,280
its size by less than one inch.
279
00:27:40,800 --> 00:27:44,560
Its ability to endure
is truly extraordinary.
280
00:27:49,080 --> 00:27:53,240
So efficient is creosote
at collecting what little rain
281
00:27:53,240 --> 00:27:56,680
falls here, that few other plants
can compete with it.
282
00:27:56,680 --> 00:28:01,120
As a result,
over the last 12,000 years,
283
00:28:01,120 --> 00:28:05,200
it's come to completely
dominate this landscape.
284
00:28:21,000 --> 00:28:25,520
The Chihuahuan Desert
in north Mexico.
285
00:28:25,520 --> 00:28:30,440
Here, one particular plant
plays the waiting game so well
286
00:28:30,440 --> 00:28:33,560
that it spends much of its life
looking dead
287
00:28:33,560 --> 00:28:35,840
and certainly not worth eating.
288
00:28:39,120 --> 00:28:42,960
And it can survive like this
for a decade.
289
00:28:50,600 --> 00:28:53,520
This is the resurrection plant.
290
00:28:56,600 --> 00:28:59,080
It's a kind of moss.
291
00:28:59,080 --> 00:29:03,520
It barely has roots, and it
certainly can't store much water,
292
00:29:03,520 --> 00:29:05,760
but it can travel.
293
00:29:12,320 --> 00:29:14,520
After a particularly long drought...
294
00:29:15,960 --> 00:29:17,920
..it breaks away from its roots...
295
00:29:20,480 --> 00:29:21,640
..and becomes...
296
00:29:23,280 --> 00:29:24,760
..a tumbleweed.
297
00:29:27,520 --> 00:29:29,520
Blowing across the desert,
298
00:29:29,520 --> 00:29:32,600
it can travel a mile in a week.
299
00:29:35,440 --> 00:29:38,080
With luck, it may find water.
300
00:29:41,320 --> 00:29:43,440
THUNDER RUMBLES
301
00:29:45,440 --> 00:29:49,080
Just a shower of rain
can bring it back to life.
302
00:29:54,000 --> 00:29:57,920
As its fronds soak up the water,
they unfurl.
303
00:30:12,040 --> 00:30:14,000
In its protected centre,
304
00:30:14,000 --> 00:30:16,000
it still has green cells
305
00:30:16,000 --> 00:30:18,760
which absorb both the water
and sunlight
306
00:30:18,760 --> 00:30:24,000
and rapidly produce the food
it needs to resume its growth.
307
00:30:28,920 --> 00:30:31,920
It will grow for just as long
as there is moisture...
308
00:30:36,720 --> 00:30:38,680
..but when that disappears...
309
00:30:40,280 --> 00:30:42,560
..it closes up once more
310
00:30:42,560 --> 00:30:45,440
and resumes its travels.
311
00:30:55,080 --> 00:30:57,680
Here in the Canyonlands of Utah
312
00:30:57,680 --> 00:31:00,200
lives a plant that has developed
313
00:31:00,200 --> 00:31:02,360
a finely-balanced relationship
314
00:31:02,360 --> 00:31:06,120
with the animals with which
it shares this dramatic desert.
315
00:31:11,080 --> 00:31:13,960
Rain does occasionally fall here
316
00:31:13,960 --> 00:31:17,760
and turns dust into mud.
317
00:31:22,240 --> 00:31:24,680
But that doesn't last long.
318
00:31:26,400 --> 00:31:29,760
A brief window of opportunity opens.
319
00:31:39,480 --> 00:31:41,760
Seeds that have been buried
for years
320
00:31:41,760 --> 00:31:44,840
may now be exposed to light
321
00:31:44,840 --> 00:31:47,280
and come to life.
322
00:32:03,960 --> 00:32:07,320
This is coyote tobacco.
323
00:32:20,080 --> 00:32:23,480
In just a few weeks,
it grows a metre tall
324
00:32:23,480 --> 00:32:26,280
and produces dozens of flowers.
325
00:32:32,760 --> 00:32:36,280
The night air becomes heavy
with their fragrance.
326
00:32:38,360 --> 00:32:40,600
Soon, they attract hawk moths
327
00:32:40,600 --> 00:32:42,560
which sip their nectar,
328
00:32:42,560 --> 00:32:44,840
and in doing so, pollinate them.
329
00:32:47,200 --> 00:32:50,280
But the moths also
lay their eggs on them.
330
00:33:06,640 --> 00:33:09,520
Soon, the caterpillars have hatched
331
00:33:09,520 --> 00:33:11,280
and are munching the leaves.
332
00:33:14,680 --> 00:33:19,200
Their nibbles expose the plant's
sap to the drying air.
333
00:33:22,200 --> 00:33:25,840
But the tobacco plant has a defence.
334
00:33:28,200 --> 00:33:31,760
The leaves under attack
produce nicotine.
335
00:33:36,120 --> 00:33:38,760
This chemical
sedates the caterpillars
336
00:33:38,760 --> 00:33:41,080
and slows them down.
337
00:33:46,560 --> 00:33:50,080
And what is more, it makes them give
off a particular scent...
338
00:33:53,280 --> 00:33:55,600
..one that summons others
339
00:33:55,600 --> 00:33:58,240
to come to the plant's aid.
340
00:34:00,120 --> 00:34:01,880
Big-eyed bugs,
341
00:34:01,880 --> 00:34:05,200
miniature assassins,
only two millimetres long...
342
00:34:07,080 --> 00:34:08,880
..and whiptail lizards.
343
00:34:10,680 --> 00:34:14,160
Big or small, they make
a meal of the caterpillars.
344
00:34:24,680 --> 00:34:26,840
It's certainly effective...
345
00:34:29,640 --> 00:34:33,160
..but there's more to this strategy
than meets the eye.
346
00:34:35,360 --> 00:34:40,280
When the leaf of a tobacco plant
is attacked by a caterpillar,
347
00:34:40,280 --> 00:34:44,960
all the rest of the leaves
prepare to defend themselves.
348
00:34:44,960 --> 00:34:48,040
But how does this leaf know
349
00:34:48,040 --> 00:34:51,280
that that leaf there
is under attack?
350
00:34:51,280 --> 00:34:54,960
Well, scientists here
in the United States
351
00:34:54,960 --> 00:34:58,600
have specially genetically modified
these tobacco plants
352
00:34:58,600 --> 00:35:01,440
so that under special
lighting conditions,
353
00:35:01,440 --> 00:35:05,880
this microscope can show us
exactly what is going on.
354
00:35:08,480 --> 00:35:11,920
I'm going to attack one
of the leaves of this plant
355
00:35:11,920 --> 00:35:15,400
with these tweezers which,
to the plant,
356
00:35:15,400 --> 00:35:18,240
will seem as if it's being nibbled
by a caterpillar.
357
00:35:32,880 --> 00:35:35,640
Signals are being transmitted
along the veins
358
00:35:35,640 --> 00:35:39,480
that link the leaf
to the rest of the plant.
359
00:35:39,480 --> 00:35:42,520
It's rather like
a very simple nervous system.
360
00:35:47,280 --> 00:35:49,200
From that initial injury,
361
00:35:49,200 --> 00:35:53,120
the whole of this little plant is
aware that something has happened.
362
00:35:57,720 --> 00:36:02,320
This signal warns each leaf
of the danger,
363
00:36:02,320 --> 00:36:05,240
so that it is ready
to produce nicotine
364
00:36:05,240 --> 00:36:07,400
the moment it's attacked.
365
00:36:10,160 --> 00:36:12,160
With this defence at the ready,
366
00:36:12,160 --> 00:36:14,840
the tobacco plant
can continue to grow
367
00:36:14,840 --> 00:36:17,720
until eventually, it produces seeds.
368
00:36:31,280 --> 00:36:34,000
It's particularly important
in deserts
369
00:36:34,000 --> 00:36:38,520
for seeds to be distributed as
widely as possible
370
00:36:38,520 --> 00:36:42,600
so that some will have a chance
of reaching moisture.
371
00:36:44,800 --> 00:36:48,840
And deserts have an excellent
agent to help them do that -
372
00:36:50,880 --> 00:36:52,560
the wind.
373
00:36:54,840 --> 00:36:59,640
Many seeds have adaptations
to help them exploit it.
374
00:36:59,640 --> 00:37:04,720
They have shell to protect
the seeds within from abrasion
375
00:37:04,720 --> 00:37:07,920
or wings to help them catch the air.
376
00:37:12,360 --> 00:37:14,720
As the temperature rises
throughout the day,
377
00:37:14,720 --> 00:37:16,960
desert winds increase in strength.
378
00:37:41,360 --> 00:37:43,280
Here in Arizona,
379
00:37:43,280 --> 00:37:48,480
the land is regularly swept by what
is known as a haboob.
380
00:37:48,480 --> 00:37:50,680
It's a giant sandstorm,
381
00:37:50,680 --> 00:37:54,480
but also, in effect, a seed storm.
382
00:38:02,040 --> 00:38:06,080
Countless millions of them
are swept up into the air.
383
00:38:12,000 --> 00:38:16,080
Some seeds can travel
thousands of miles on the wind...
384
00:38:19,080 --> 00:38:24,080
..so that plants may eventually
reach even the most isolated desert.
385
00:38:33,360 --> 00:38:36,120
Some have landed on an island
386
00:38:36,120 --> 00:38:39,560
in the middle of the world's
largest salt flat in Bolivia.
387
00:38:45,120 --> 00:38:50,360
In the Galapagos, they sprout
on fields of recently-erupted lava.
388
00:38:57,720 --> 00:39:03,240
They've even reached one of the most
inhospitable of all sites -
389
00:39:03,240 --> 00:39:06,960
the tiny island of San Pedro Martir,
390
00:39:06,960 --> 00:39:09,520
a scorched, lonely rock
391
00:39:09,520 --> 00:39:11,360
off the coast of Mexico.
392
00:39:15,600 --> 00:39:19,000
This is the home of
the giant cardon,
393
00:39:19,000 --> 00:39:21,840
a species of huge cactus
394
00:39:21,840 --> 00:39:24,400
that can weigh up to 12 tonnes.
395
00:39:32,520 --> 00:39:37,400
They're able to thrive here because
of an extraordinary partnership...
396
00:39:40,600 --> 00:39:44,640
..with brown and blue-footed
boobies.
397
00:39:49,560 --> 00:39:52,360
The cardones here
can become so broad
398
00:39:52,360 --> 00:39:56,600
that they provide cooling
shade for nesting birds.
399
00:40:08,120 --> 00:40:10,960
As the booby chicks get older,
400
00:40:10,960 --> 00:40:13,560
they repay the cardones...
401
00:40:19,360 --> 00:40:20,840
..with their droppings...
402
00:40:22,400 --> 00:40:23,800
..guano -
403
00:40:23,800 --> 00:40:28,040
the digestive remains
of vast shoals of fish.
404
00:40:29,680 --> 00:40:33,200
This guano is of such
strength and quantity
405
00:40:33,200 --> 00:40:36,360
that most plants
would be poisoned by it.
406
00:40:42,760 --> 00:40:46,080
These cardones, however,
have evolved the ability
407
00:40:46,080 --> 00:40:50,800
to tolerate the toxins in the
guano and digest the nutrients.
408
00:40:52,920 --> 00:40:56,720
As a result, the cacti
now grow in a dense forest
409
00:40:56,720 --> 00:40:58,320
over a million strong.
410
00:41:03,800 --> 00:41:07,160
But such relationships
are very finely balanced
411
00:41:07,160 --> 00:41:10,280
and can only too easily
tip into catastrophe...
412
00:41:13,720 --> 00:41:16,720
..as is now happening
in northern Zimbabwe.
413
00:41:20,920 --> 00:41:22,760
For six months of the year,
414
00:41:22,760 --> 00:41:27,000
the savannah here is kept lush
and green by daily rains.
415
00:41:32,280 --> 00:41:33,760
ELEPHANTS TRUMPET
416
00:41:35,480 --> 00:41:38,360
But when the rainy season is over,
417
00:41:38,360 --> 00:41:41,080
it becomes as dry as any desert.
418
00:41:46,480 --> 00:41:51,440
So to survive here, trees must be
able to tolerate both conditions.
419
00:41:53,760 --> 00:41:58,680
And these giants
are adapted to do just that.
420
00:41:58,680 --> 00:42:00,760
They are baobabs.
421
00:42:08,640 --> 00:42:12,200
This one might be over
1,000 years old.
422
00:42:12,200 --> 00:42:15,800
It survives here in part
thanks to its ability
423
00:42:15,800 --> 00:42:18,800
to store thousands
of litres of water
424
00:42:18,800 --> 00:42:21,400
within the spongy wood of its trunk.
425
00:42:25,080 --> 00:42:28,280
But its battered surface is evidence
426
00:42:28,280 --> 00:42:31,120
of a very finely-balanced
relationship.
427
00:42:33,000 --> 00:42:34,920
These huge trees
428
00:42:34,920 --> 00:42:38,880
are a focus for animals
of all kinds.
429
00:42:42,560 --> 00:42:46,200
And they're particularly
important for elephants.
430
00:42:50,720 --> 00:42:54,480
In the wet season,
they eat the baobab's fruit
431
00:42:54,480 --> 00:42:56,960
and disperse the
seeds in their dung.
432
00:43:02,480 --> 00:43:04,880
Now as the dry season begins...
433
00:43:04,880 --> 00:43:06,480
ELEPHANT RUMBLES
434
00:43:06,480 --> 00:43:09,640
..they migrate
to distant watering holes.
435
00:43:13,960 --> 00:43:16,520
The baobabs have damp inner wood...
436
00:43:18,000 --> 00:43:23,680
..and the elephants use it to
quench their thirst on the journey.
437
00:43:23,680 --> 00:43:25,480
ELEPHANT ROARS
438
00:43:42,320 --> 00:43:46,080
This relationship can only
work because baobabs
439
00:43:46,080 --> 00:43:49,320
have a remarkable
ability to heal themselves.
440
00:44:08,200 --> 00:44:11,200
Between each damaging attack,
441
00:44:11,200 --> 00:44:14,920
they expand their spongy
wood and grow new skin.
442
00:44:19,320 --> 00:44:23,440
And they've done this
time and time again over centuries.
443
00:44:28,120 --> 00:44:32,320
Today, however, it's harder
for the baobabs to recover
444
00:44:32,320 --> 00:44:37,280
as dry seasons become longer
and drier due to climate change.
445
00:44:39,440 --> 00:44:43,280
Not only that, but the elephants
are forced to take ever more wood
446
00:44:43,280 --> 00:44:46,120
from the trees in order to survive.
447
00:44:51,760 --> 00:44:53,520
In some parts of Africa,
448
00:44:53,520 --> 00:44:56,400
many of the largest
and oldest baobabs
449
00:44:56,400 --> 00:44:58,840
have fallen in the last decade.
450
00:45:11,840 --> 00:45:15,040
The loss of a vital species
like the baobab
451
00:45:15,040 --> 00:45:18,080
strikes a blow at all life
in the desert.
452
00:45:23,080 --> 00:45:24,960
In these hostile lands,
453
00:45:24,960 --> 00:45:29,480
few living organisms can survive
without help from others.
454
00:45:34,440 --> 00:45:37,880
You can find an extraordinary
illustration of this
455
00:45:37,880 --> 00:45:40,400
in Arizona's saguaro country.
456
00:45:47,800 --> 00:45:50,160
If you wander off
the beaten track here,
457
00:45:50,160 --> 00:45:53,640
you may be lucky enough
to find one of these.
458
00:45:53,640 --> 00:45:56,400
It might look like an old boot,
459
00:45:56,400 --> 00:46:00,640
but in fact,
it's part of a saguaro cactus.
460
00:46:00,640 --> 00:46:03,920
Almost every saguaro has one.
461
00:46:03,920 --> 00:46:05,520
There's one up there.
462
00:46:09,960 --> 00:46:13,920
It has been produced
indirectly by woodpeckers,
463
00:46:13,920 --> 00:46:18,040
which regularly dig homes for
themselves in the bloated trunks
464
00:46:18,040 --> 00:46:19,760
of the saguaros.
465
00:46:32,960 --> 00:46:34,880
In the next six months,
466
00:46:34,880 --> 00:46:37,760
the cactus heals the wound
467
00:46:37,760 --> 00:46:42,000
and so creates a safe,
cool, and watertight nest hole.
468
00:46:44,800 --> 00:46:47,480
Its tough lining
will persist for years,
469
00:46:47,480 --> 00:46:51,560
even after the cactus itself
has died and rotted away.
470
00:46:55,680 --> 00:47:00,600
A single saguaro may hold several
of these extraordinary homes.
471
00:47:03,440 --> 00:47:09,360
So over its lifetime, it may provide
accommodation for some 3,000 chicks
472
00:47:09,360 --> 00:47:12,440
of several different species.
473
00:47:12,440 --> 00:47:14,440
BIRDS SCREECH
474
00:47:18,640 --> 00:47:21,280
But in the long term,
the saguaro benefits,
475
00:47:21,280 --> 00:47:24,160
as their lodgers repay the cactus
476
00:47:24,160 --> 00:47:27,120
by pollinating its flowers...
477
00:47:28,560 --> 00:47:31,080
..and dispersing its seeds.
478
00:47:38,560 --> 00:47:40,680
It's relationships like these
479
00:47:40,680 --> 00:47:42,680
that enable life to flourish
480
00:47:42,680 --> 00:47:45,760
in some of the world's
harshest landscapes.
481
00:47:59,280 --> 00:48:01,280
Over millions of years,
482
00:48:01,280 --> 00:48:04,400
plants have become superbly adapted
483
00:48:04,400 --> 00:48:06,640
to hostile desert conditions.
484
00:48:06,640 --> 00:48:10,120
But it's a very
finely-balanced existence
485
00:48:10,120 --> 00:48:13,240
and one that makes them
uniquely vulnerable.
486
00:48:15,800 --> 00:48:19,520
However, our growing understanding
of the complex ways
487
00:48:19,520 --> 00:48:23,160
by which desert animals and plants
rely on one another
488
00:48:23,160 --> 00:48:27,400
is now helping us to understand
how best we can protect them.
489
00:48:32,240 --> 00:48:36,640
90 years ago, a photograph
was taken from this very spot
490
00:48:36,640 --> 00:48:39,960
that shows a population
of saguaro cactus
491
00:48:39,960 --> 00:48:42,920
that was very different
from what it is today.
492
00:48:45,680 --> 00:48:47,120
In the last 50 years,
493
00:48:47,120 --> 00:48:51,520
the population of saguaros
here has greatly diminished,
494
00:48:51,520 --> 00:48:55,080
not because of a direct
assault on the cactus,
495
00:48:55,080 --> 00:48:57,920
but because many of the
shade-giving nurse trees
496
00:48:57,920 --> 00:49:02,760
were harvested for firewood, leaving
young saguaro to die in the sun.
497
00:49:07,920 --> 00:49:10,880
Now that this relationship
is understood
498
00:49:10,880 --> 00:49:13,000
and the nurse trees are protected,
499
00:49:13,000 --> 00:49:17,880
there are already signs
that the saguaro are recovering.
500
00:49:21,320 --> 00:49:26,360
Wherever there's a desert, plants
have evolved to meet its challenge.
501
00:49:28,080 --> 00:49:30,560
But everywhere, they need our help.
502
00:49:32,720 --> 00:49:34,440
As we understand more about them
503
00:49:34,440 --> 00:49:37,440
and their intimate and
complex relationships...
504
00:49:38,960 --> 00:49:43,520
..we will be better able to
protect them and all life
505
00:49:43,520 --> 00:49:48,160
in these beautiful but
increasingly fragile worlds.
506
00:50:02,480 --> 00:50:05,640
The most remote shoot
for the deserts team
507
00:50:05,640 --> 00:50:08,280
is to the island of
San Pedro Martir
508
00:50:08,280 --> 00:50:10,600
in the Sea of Cortez, Mexico.
509
00:50:14,000 --> 00:50:16,400
They are to film the giant cacti
510
00:50:16,400 --> 00:50:19,080
that have found a unique way
to thrive here.
511
00:50:21,880 --> 00:50:25,880
It takes 48 hours
to travel the 260 miles...
512
00:50:27,240 --> 00:50:29,800
..over unpleasantly choppy seas.
513
00:50:36,120 --> 00:50:38,160
It's not made some of
us feel very well.
514
00:50:38,160 --> 00:50:41,280
I think one of us is quite seasick.
515
00:50:42,760 --> 00:50:45,280
It's nothing but big blue out there,
516
00:50:45,280 --> 00:50:47,920
and the island is
somewhere in that direction.
517
00:50:47,920 --> 00:50:49,400
Can't see it yet.
518
00:50:51,240 --> 00:50:53,400
As they draw close to the island,
519
00:50:53,400 --> 00:50:58,520
there's a spectacular reminder of
how rich life in the sea is here.
520
00:50:58,520 --> 00:51:01,480
Oh, look in front of us!
Look at that for a pod!
521
00:51:02,960 --> 00:51:04,760
Oh! They're everywhere!
522
00:51:10,760 --> 00:51:12,040
Show us the way.
523
00:51:12,040 --> 00:51:13,480
HE CHUCKLES
524
00:51:19,000 --> 00:51:22,920
Meeting them here
is desert scientist Ben Wilder.
525
00:51:24,360 --> 00:51:27,400
It's through his research
that the team first heard
526
00:51:27,400 --> 00:51:30,720
about the island's
peculiar residents.
527
00:51:30,720 --> 00:51:33,320
I'll never forget
the first time I came
528
00:51:33,320 --> 00:51:35,960
to, actually, exactly where
we're standing right now.
529
00:51:35,960 --> 00:51:38,400
It was April of 2006,
530
00:51:38,400 --> 00:51:40,960
and saw this view.
531
00:51:40,960 --> 00:51:46,760
And it both settled in my
heart and captivated my mind.
532
00:51:46,760 --> 00:51:52,240
And so that started a process
of trying to understand,
533
00:51:52,240 --> 00:51:54,760
you know, what can produce this?
534
00:51:56,800 --> 00:52:00,640
The secret is in the
relationship between the cactus
535
00:52:00,640 --> 00:52:04,280
and a type of
sea bird called a booby.
536
00:52:06,440 --> 00:52:09,080
All the boobies that we need
to film are right up there
537
00:52:09,080 --> 00:52:10,320
at the top of the island.
538
00:52:10,320 --> 00:52:13,200
So, it's going to be
a bit of a scramble.
539
00:52:14,840 --> 00:52:18,400
The crew soon discover how
harsh the conditions are here.
540
00:52:20,080 --> 00:52:22,480
I'm pretty tired. It's pretty hot.
541
00:52:22,480 --> 00:52:23,800
It's a great view.
542
00:52:30,760 --> 00:52:32,240
There's very few places on Earth
543
00:52:32,240 --> 00:52:34,120
where you're going to
see this many cactus.
544
00:52:34,120 --> 00:52:37,320
I mean, it's absolutely amazing.
545
00:52:37,320 --> 00:52:39,680
I would say this is the only
place you're going to see this.
546
00:52:39,680 --> 00:52:41,400
Well, there you go, then.
547
00:52:43,240 --> 00:52:47,160
Ben's research is uncovering
the ingenious ways the cacti
548
00:52:47,160 --> 00:52:49,760
have adapted to the conditions here.
549
00:52:51,240 --> 00:52:54,160
The waters just offshore here
are some of the most productive
550
00:52:54,160 --> 00:52:55,720
marine waters in the world.
551
00:52:55,720 --> 00:52:58,720
And so it's kind of an ideal
habitat for sea birds to roost.
552
00:52:58,720 --> 00:53:02,720
But when they do so,
they deposit tonnes of guano.
553
00:53:02,720 --> 00:53:06,760
And so those nutrients, really high
in nitrogen and phosphorus,
554
00:53:06,760 --> 00:53:10,600
actually are toxic
to most plant species.
555
00:53:10,600 --> 00:53:13,800
The cardones thrive because
they're uniquely able
556
00:53:13,800 --> 00:53:18,360
to process the guano and extract
what they need to fuel their growth.
557
00:53:21,080 --> 00:53:23,640
One of the first shots
the crew need to get
558
00:53:23,640 --> 00:53:26,480
is of the boobies
nesting under the cacti.
559
00:53:28,080 --> 00:53:31,680
The lack of predators means
the birds aren't afraid of people.
560
00:53:31,680 --> 00:53:36,160
It should make filming
them up close a bit easier.
561
00:53:36,160 --> 00:53:37,800
That's the theory.
562
00:53:37,800 --> 00:53:40,680
So we've positioned this camera here
to try and get a good perspective
563
00:53:40,680 --> 00:53:42,880
on the chick on the nest.
564
00:53:42,880 --> 00:53:45,760
It's taken quite a
liking to our camera.
565
00:53:45,760 --> 00:53:48,280
Let's hope he doesn't break it.
566
00:53:53,960 --> 00:53:55,120
Bull's-eye.
567
00:53:55,120 --> 00:53:57,800
It's pooed right on
the front of the lens!
568
00:53:57,800 --> 00:54:00,200
It's kind of a shot we need,
but unfortunately,
569
00:54:00,200 --> 00:54:02,080
my camera wasn't rolling
at the time it did it.
570
00:54:02,080 --> 00:54:04,120
So now, I've just got a dirty lens.
571
00:54:07,080 --> 00:54:11,200
After a thorough lens clean,
Ollie eventually gets the shot
572
00:54:11,200 --> 00:54:15,640
to reveal the extraordinary
relationship between bird and plant.
573
00:54:19,720 --> 00:54:22,240
But it's not just the
bird guano that influence
574
00:54:22,240 --> 00:54:23,920
how the cardones grow.
575
00:54:25,000 --> 00:54:28,480
They're way shorter,
they're dwarfed here,
576
00:54:28,480 --> 00:54:30,080
throughout the rest of their range,
577
00:54:30,080 --> 00:54:32,680
they usually get in
upwards of 50-60 feet.
578
00:54:32,680 --> 00:54:36,760
But here, on average,
they're 20-24 feet in height.
579
00:54:36,760 --> 00:54:40,400
They stop growing up,
and they grow out.
580
00:54:42,080 --> 00:54:45,800
Ben's research suggests
that they go wide here
581
00:54:45,800 --> 00:54:49,040
as an adaptation
to the violent winds.
582
00:54:49,040 --> 00:54:52,160
Too tall, and they'd blow over.
583
00:54:52,160 --> 00:54:56,000
A gusting wind isn't
helping the drone crew, either.
584
00:54:56,000 --> 00:54:57,640
We really need it
to calm down a bit,
585
00:54:57,640 --> 00:54:59,640
otherwise it's
going to be impossible.
586
00:55:05,880 --> 00:55:07,240
WIND GUSTS
587
00:55:18,560 --> 00:55:20,360
BIRD CHIRPS
588
00:55:23,880 --> 00:55:27,280
On the Sonoran mainland, where the
cardon is also found,
589
00:55:27,280 --> 00:55:31,360
you have on average between
50 to 150 plants per hectare.
590
00:55:31,360 --> 00:55:37,160
Here on this island, you have
over 2,500 plants per hectare.
591
00:55:37,160 --> 00:55:41,080
A lull in the wind, and the
team get the chance to reveal
592
00:55:41,080 --> 00:55:43,920
the remarkable density of the cacti,
593
00:55:43,920 --> 00:55:48,120
almost 20 times greater
than anywhere else.
594
00:55:48,120 --> 00:55:51,240
It appears that here
they're doing very well.
595
00:55:52,520 --> 00:55:55,520
But even this island isn't isolated
596
00:55:55,520 --> 00:55:57,960
from the effects
of a changing planet.
597
00:55:59,240 --> 00:56:01,040
Every cactus you see there,
598
00:56:01,040 --> 00:56:05,200
its body is filled with
nutrients from the sea.
599
00:56:05,200 --> 00:56:09,240
Given that we know that the
cardones are linked to the ocean,
600
00:56:09,240 --> 00:56:12,680
what happens in the oceans
affects what happens to the cardon.
601
00:56:12,680 --> 00:56:16,680
So a concern we have right now is
that there's a lot of overfishing,
602
00:56:16,680 --> 00:56:21,400
and we know there are less sea birds
here than there were 20 years ago.
603
00:56:21,400 --> 00:56:25,640
And we have reason to believe
then that will ripple and affect
604
00:56:25,640 --> 00:56:28,960
the nutrients that fuel
the cardones as well.
605
00:56:30,920 --> 00:56:34,520
This finely-balanced relationship
is at risk,
606
00:56:34,520 --> 00:56:37,240
and knowing that
makes leaving the island
607
00:56:37,240 --> 00:56:39,920
particularly thought provoking
for the crew.
608
00:56:41,320 --> 00:56:43,480
The cardones look beautiful.
609
00:56:43,480 --> 00:56:45,680
I've never seen so
many cactus in my life.
610
00:56:45,680 --> 00:56:47,600
It's absolutely amazing.
611
00:56:47,600 --> 00:56:50,240
I'm just hoping that when
we leave this place,
612
00:56:50,240 --> 00:56:54,280
it stays as it is, and things
don't affect it that are negative.
613
00:56:54,280 --> 00:56:57,560
It's an absolutely wonderful thing,
and, yeah, I don't want to go.
614
00:56:57,560 --> 00:56:58,880
I want to stay.
615
00:57:00,200 --> 00:57:05,040
This remote cactus forest is
a reminder of the adaptability
616
00:57:05,040 --> 00:57:09,320
of plants that enables them
to establish green worlds
617
00:57:09,320 --> 00:57:11,600
almost anywhere on Earth.
618
00:57:16,240 --> 00:57:19,000
Next time on The Green Planet...
619
00:57:19,000 --> 00:57:22,360
..our extraordinary relationship
with plants.
620
00:57:24,680 --> 00:57:26,560
From those who eat...
621
00:57:29,920 --> 00:57:31,680
..to those we help...
622
00:57:33,720 --> 00:57:36,480
..plants are our greatest allies.
623
00:57:39,440 --> 00:57:42,920
The Open University
has produced a poster
624
00:57:42,920 --> 00:57:47,440
that explores the vital role
that plants have for our planet.
625
00:57:47,440 --> 00:57:49,760
To order your free copy call...
626
00:57:53,800 --> 00:57:55,600
Or go to...
627
00:57:59,760 --> 00:58:02,240
..and follow the links
to the Open University.