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00:00To be continued...
00:30The Tatlamakan Desert in northern China.
00:36A constantly shifting landscape of sand.
00:43Temperatures that swing from minus 20 to more than 40 degrees centigrade.
00:52And most critically, almost entirely, without rain.
01:00The Tatlamakan Desert in northern China.
01:06Yet, here on the dunes...
01:15...a Euphrates poplar tree.
01:25And it's not alone.
01:29Some of these trees have lived here for a thousand years.
01:36They have exceptionally long roots with which to collect water.
01:42And what is more, those roots are connected to neighboring trees.
01:45So that if one strikes water, others can share it.
01:54In every desert, across the planet, plants have found ways to not only survive, but flourish.
02:02This is the Grand Desierto of Mexico and the United States.
02:20The Grand Desierto of Mexico and the United States.
02:24These dunes may appear to be totally barren.
02:40In fact, they are full of life.
02:42In the sands beneath my feet, there are seeds of many different kinds.
02:49In fact, you could say that the dune itself is one great seed bank.
02:54And when it rains, it bursts into life.
02:57But rain may come only once a decade.
03:08And even then, the long-awaited storm may be very brief.
03:12So seeds must respond immediately.
03:20This is sand verbena.
03:22This is sand verbena.
03:36It can grow from a seed to a sweetly-scented flowering plant in just a few weeks.
03:42Primroses and many other plants soon join the race to flower before the sand dries.
04:00Desert blooms like this, however, are rare.
04:07This is the first year for 20 years.
04:12The combination of vibrant color and powerful scent attracts migrating pollinators,
04:23such as these painted lady butterflies,
04:26which fly into the middle of what were only recently barren dunes.
04:33Everything is rushing to complete their lives before the moisture has gone.
04:43Such spectacular blooms transform deserts all around the world.
04:51From the Atacama in South America...
04:56...to the dusty plains of southern Africa...
04:58...to the dusty plains of southern Africa.
04:59From the Atacama in South America...
05:04...to the dusty plains of southern Africa...
05:07to the dusty plains of Southern Africa.
05:29The rain in deserts, however, never last long,
05:32and all too soon the flowers wither and die.
05:42But not before they've produced the next generation.
05:51The seeds that will now wait in the sand for the next rains.
05:55In the Sonoran desert of North America,
06:03the huge saguaro cacti have a different strategy.
06:08They store water in quantity and can live to a great age.
06:14But in their early years, they are extremely vulnerable.
06:20This little saguaro cactus is about ten years old.
06:28When they're really small and growing out in the open,
06:32there's a real chance that they may shrivel up and die.
06:36But this one has been lucky.
06:39It's been growing in the shade of this mesquite tree.
06:42And it's got a very good chance of surviving to maturity.
06:49The young saguaro is protected by the mesquite's branches.
06:54They halve the amount of scorching sunlight reaching the cactus,
06:59and so keep it cool.
07:04And the mesquite's extremely long roots draw up water,
07:08bringing it within reach of the young saguaro.
07:16So the mesquite is known as a nurse plant,
07:20and a very effective one it is too.
07:23In fact, this particular mesquite has already nurtured
07:28seven young saguaros over the past 30 years.
07:31As a young cactus grows, it needs the protection of its nurse plant,
07:41not only from the heat,
07:43but from the other hazards of desert life.
07:49Temperatures can drop to minus ten degrees overnight.
07:53And, very occasionally, it even snows.
08:05If the water stored inside a young cactus should freeze,
08:11the cactus will die.
08:13But the nurse plant traps a blanket of slightly warmer air around it,
08:21just enough to keep it alive.
08:26Eventually, saguaros outgrow their nurses.
08:30But by that time, they are robust enough to face the elements by themselves.
08:35No matter how old a desert plant is, water is always precious.
08:45Whether gathered from melting snow...
08:52...or a shower of rain.
08:54So, cacti have developed extraordinary adaptations
09:03that enable them to not only collect water,
09:06but to retain it.
09:13Instead of leaves, which would lose precious moisture through evaporation,
09:17they have spines.
09:19The spine has a tiny pad at its base, where the water is absorbed.
09:28And then stored in the great swollen trunk.
09:35A large saguaro can hold 5,000 litres of water,
09:40and is able to do so because it has another special adaptation.
09:49The ridges on its surface are like the pleats on an accordion.
09:59They allow the saguaro to change its shape.
10:04After rain has fallen, the pleats expand,
10:08and the saguaro fills up its water tank.
10:10In the dry times, it uses its water to grow, produce flowers,
10:21and eventually, siemens.
10:27Fully loaded with thousands of litres of water,
10:30this saguaro won't need to drink a single drop for another year.
10:34But such valuable stores of water attract thieves.
10:40Now the spine's function changes from collection,
10:46to guarding.
10:47To guarding.
10:51The spines of some species are a quarter of a metre long.
11:02Others are needle-like barbs that grow in clusters
11:06and easily break off in the skin of any animal that touches them.
11:09But perhaps the most vicious cacti belong to a group called the Choyas.
11:17This is called a teddy bear Choya,
11:22because of the thick coating of spines on it.
11:26But don't be deceived by the name.
11:29There is nothing cuddly about this particular teddy bear.
11:32In fact, it's the most dangerous plant in the desert.
11:37And I wouldn't dream of putting my hand anywhere near it
11:41without proper protection.
11:45Brush against it.
11:47This can happen.
11:49Ow!
11:50This can happen, even with this glove on.
11:52One of them has just gone through, I can feel it.
11:54It's quite painful.
11:57Look closely at the spine,
11:58and you can see very clearly why they're so dangerous.
12:02Each is like a splinter of glass,
12:06sharp enough to pierce flesh.
12:09And they're covered with backward-pointing barbs.
12:13So getting them out, even with a pair of pliers,
12:16is quite hard.
12:18This is not pleasant at all.
12:21It won't come off or without.
12:25Oh, look at that.
12:26It's hard to imagine a more aggressive defence than this.
12:33And it makes both the plant and its buds virtually invulnerable.
12:50Most animals know to keep clear.
12:52Cholla buds grow like tiny barrels from the top of the adult plant,
13:01and then drop off.
13:03If the young cholla put down roots here,
13:14it would compete with its parent for water.
13:22Night falls.
13:25And this one is on the move.
13:27A pack rat.
13:38She knows how to deal with the cholla.
13:47She avoids the spines by gripping it at the place where it broke off from its parent.
13:51from its parent.
13:58And she works fast.
14:02There are pack rat hunters here.
14:04She uses the cholla to build a spiny wall around her nest.
14:18The flesh of the cholla supplies her with water.
14:25And the severed spines further reinforce the defences.
14:30The flesh of the cholla supplies her with water.
14:36The flesh of the cholla supplies her with water.
14:40And the severed spines further reinforce the defences.
14:44This cholla bud might be next.
15:00Might be next.
15:05But one accidental nudge.
15:12And it escapes.
15:13And it escapes.
15:14And it escapes.
15:17It is still an object.
15:19And the an object.
15:20It melts.
15:22It takes time out of her.
15:23And it escapes.
15:29Up to the test.
15:31And it escapes.
15:33All the long time.
15:34And this path will be found
15:35It escapes if there are no more.
15:38It is, it is.
15:40It is.
15:42The bud starts to put down roots.
15:53So the choya, thanks to the pack rats,
15:56finds new territory and sets about claiming it.
16:01A few plants deal with the problems of desert living better than cacti.
16:17There are almost 2,000 different species of them.
16:21They're spread across the deserts of the American West,
16:24from Arizona all the way to Mexico and beyond.
16:31In South America, the ice-covered peaks of the Andes act as a rain barrier,
16:42beyond which lies the world's driest desert,
16:46the Atacama.
16:57In the desert world, water thieves can come in many forms.
17:01to exploit even the smallest chink in a plant's defense.
17:11One of the strangers travels within the gut of a fruit-eating mockingbird.
17:16These are the seeds of trysteryx.
17:28They're kind of mistletoe.
17:29They're kind of mistletoe.
17:30They're kind of mistletoe.
17:31They're kind of mistletoe.
17:32They're kind of mistletoe.
17:33Their goal is the water inside this hedgehog cactus.
17:41Using the spines as anchors, the seeds start to germinate.
17:45Each produces a long probe with which to try and locate the cactus's skin.
17:57For most, that's a stretch too far, and they perish.
18:02But for this one, the cactus's surface is within reach.
18:10It's clamped onto it with a special sucker.
18:25And then waits for darkness.
18:27At night, the cactus opens its pores in order to respire.
18:38Oxygen goes out.
18:40Carbon dioxide goes in.
18:42And so does trysteryx.
18:51Once within, its tissues spread throughout the body of the cactus,
18:56sustained by the precious store of water that they find there.
19:06Then, a year later, it breaks through the cactus's skin
19:12and bursts into flower.
19:37Hummingbirds come to drink their nectar
19:39and pollinate them as they do so.
19:53And then, to complete the cycle,
19:56trysteryx produces hundreds of white, eye-catching seeds,
20:00ready to be carried away by a bird to invade another cactus.
20:09The Karoo Desert in southern Africa.
20:22And although it may look bare,
20:24its rocky ground contains an unrivaled variety of plants that,
20:28one way or another, store water in their tissues.
20:32They belong to many different families,
20:37but, as a group, they're known as succulents.
20:40Some are small and low,
20:47and barely distinguishable from their surroundings.
20:50These look like little pebbles.
20:53They resemble them so closely,
20:56that animals, which might be only too glad to steal their water,
21:00just pass them by.
21:01They resemble them so closely,
21:02that animals, which might be only too glad to steal their water,
21:06just pass them by.
21:20When rain does fall, they absorb it and quickly expand.
21:28But even this doesn't spoil their disguise.
21:34They just look like larger pebbles.
21:38Nor are they green.
21:40The cells on their top surface are transparent,
21:43and allow sunlight to pass through.
21:48Deep within and out of sight are the green cells,
21:51where photosynthesis occurs.
21:54The process which uses this light to make food for the plant.
22:05When the time comes to reproduce, however,
22:08the stone plant abandons its disguise.
22:23And now, it blooms.
22:43The flowers open and close every 24 hours.
22:53So, for a few dangerous days,
22:59the plant advertises for pollinators
23:01before returning to life as a pebble.
23:04Some desert plants have developed a very different way of attracting pollinators.
23:26This is Stapelia.
23:28It produces what is perhaps the desert's strangest disguise.
23:35It uses water stored in its stems to grow buds the size of tennis balls.
23:49The flower once opened is called a desert starfish.
24:08Instead of releasing millions of loose pollen grains, as most flowers do,
24:13the desert starfish produces them packed in five tiny sacks.
24:21But if its strategy is successful,
24:24just one of them will produce hundreds of seeds.
24:27And this depends on deception.
24:37The flower appears to have hair.
24:42Wrinkly skin.
24:43And it produces a stench, like the carcass of a dead animal.
25:05And when a carrion fly investigates,
25:07the flower clamps a tiny sack of pollen to its proboscis.
25:22It's not easy to feed with such encumbered mouth parts.
25:28But try as it might, the fly can't get rid of it.
25:31And it's still there when the fly leaves to try and feed from another bogus carcass.
25:46This time, however, when its clamped-up proboscis slots into the flower,
25:50the pollen sack is released.
25:52With pollination complete, the fly is no longer needed.
26:08And just as well.
26:15Some deserts can be so dry that plants must find techniques of surviving for long periods
26:21without any water whatsoever.
26:25One of them is to grow extremely slowly.
26:30And few plants grow more slowly than this one, the creosote bush.
26:37It is inactive for most of its life, and only wakes up and grows for a brief period,
26:44if and when there is a fall of rain.
26:46I've seen evidence of this grow slow strategy for myself.
26:56Forty years ago, I came here to California's Mojave Desert to visit one particular plant.
27:04An individual creosote bush tends to spread not by setting seeds and producing a new generation,
27:11but by sending out new stems around its base.
27:16This plant started growing between 10 and 12,000 years ago.
27:23That was in 1982.
27:29Since then, careful measurement has shown that it has increased its size by less than one inch.
27:37It's ability to endure is truly extraordinary.
27:49So efficient is creosote collecting what little rain falls here, that few other plants can compete with it.
27:56As a result, over the last 12,000 years, it's come to completely dominate this landscape.
28:07The Chihuahuan Desert in North Mexico.
28:25Here, one particular plant plays the waiting game so well,
28:29that it spends much of its life looking dead, and certainly not worth eating.
28:38And it can survive like this for a decade.
28:50This is the resurrection plant.
28:52It's a kind of moss.
28:58It barely has roots, and it certainly can't store much water.
29:03But it can travel.
29:04After a particularly long drought,
29:15it breaks away from its roots
29:19and becomes a tumbleweed.
29:27Blowing across the desert, it can travel a mile in a week.
29:34With luck, it may find water.
29:44Just a shower of rain can bring it back to life.
29:53As its fronds soak up the water, they unfurl.
30:04In its protected center, it still has green cells which absorb both the water and sunlight
30:18and rapidly produce the food it needs to resume its growth.
30:22It will grow for just as long as there is moisture.
30:36But when that disappears, it closes up once more and resumes its travels.
30:52Here in the canyon lands of Utah, lives a plant that has developed a finely balanced relationship
31:01with the animals with which it shares this dramatic desert.
31:10Rain does occasionally fall here and turns dust into mud.
31:16But that doesn't last long.
31:25A brief window of opportunity opens.
31:28A brief window of opportunity opens.
31:38Seeds that have been buried for years may now be exposed to light
31:44and come to life.
31:45Seeds that have been buried for years may have been buried for years.
32:03This is Coyote Tobacco.
32:15In just a few weeks, it grows a meter tall and produces dozens of flowers.
32:32The night air becomes heavy with their fragrance.
32:37Soon, they attract hawk moths which sip their nectar and in doing so, pollinate them.
32:46But the moths also lay their eggs on them.
33:06Soon, their caterpillars have hatched and are munching the leaves.
33:11Their nipples expose the plant's sap to the drying air.
33:21But the tobacco plant has a defense.
33:27The leaves under attack produce nicotine.
33:35This chemical sedates the caterpillars and slows them down.
33:41And what is more, it makes them give off a particular scent.
33:53One that summons others to come to the plant's aid.
33:59Big-eyed bugs.
34:01Miniture assassins only two millimeters long.
34:04And whip-tailed lizards.
34:10Big or small, they make a meal of the caterpillars.
34:13It's certainly effective.
34:25But there's more to this strategy than meets the eye.
34:32Big or small, they have a need of the caterpillar.
34:34When the leaf of a tobacco plant is attacked by a caterpillar, all the rest of the leaves
34:41prepare to defend themselves.
34:44But how does this leaf know that that leaf there is under attack?
34:49Well, scientists here in the United States have specially genetically modified these
34:57tobacco plants so that under special lighting conditions this microscope can show us exactly
35:04what is going on.
35:08I'm going to attack one of the leaves of this plant with these tweezers so the plant
35:14will seem as if it's being nibbled by a caterpillar.
35:32Signals are being transmitted along the veins that link the leaf to the rest of the plant.
35:39It's rather like a very simple nervous system.
35:46From that initial injury the whole of this little plant is aware that something has happened.
35:57This signal warns each leaf of the danger so that it is ready to produce nicotine the
36:04moment it's attacked.
36:09With this defense at the ready the tobacco plant can continue to grow until eventually it produces seeds.
36:16It's particularly important in desert for seeds to be distributed as widely as possible so
36:37that some will have a chance of reaching moisture.
36:44And deserts have an excellent agent to help them do that.
36:50The wind.
36:54Many seeds have adaptations to help them exploit it.
36:58They have shells to protect the seeds within from abrasion.
37:04Or wings to help them catch the air.
37:11As the temperature rises throughout the day desert winds increase in strength.
37:19The wind.
37:41Here in Arizona the land is regularly swept by what is known as a haboob.
37:46It's a giant sandstorm but also in effect a seed storm.
38:01Countless millions of them are swept up into the air.
38:11Some seeds can travel thousands of miles on the wind.
38:18So that plants may eventually reach even the most isolated desert.
38:32Some have landed on an island in the middle of the world's largest salt flat in Bolivia.
38:38In the Galapagos they sprout on fields of recently erupted lava.
38:57They've even reached one of the most inhospitable of all sites.
39:01The tiny island of San Pedro Matia.
39:05A scorched lonely rock off the coast of Mexico.
39:14This is the home of the giant Cardone.
39:17A species of huge cactus that can weigh up to 12 tons.
39:23They're able to thrive here because of an extraordinary partnership.
39:39With brown and blue-footed boobies.
39:42The Cardones here can become so broad that they provide cooling shade for nesting birds.
39:55As the booby chicks get older, they repay the Cardones.
40:12With their droppings.
40:18With their droppings.
40:21Guano.
40:23The digested remains of vast shells of fish.
40:28This guano is of such strength and quantity that most plants would be poisoned by it.
40:35These Cardones, however, have evolved the ability to tolerate the toxins in the guano and digest the nutrients.
40:52As a result, the cacti now grow in a dense forest over a million strong.
40:57But such relationships are very finely balanced and can only too easily tip into catastrophe.
41:12As is now happening in northern Zimbabwe.
41:20For six months of the year, the savannah here is kept lush and green by daily rains.
41:25But when the rainy season is over, it becomes as dry as any desert.
41:45So to survive here, trees must be able to tolerate both conditions.
41:50And these giants are adapted to do just that.
41:57They are baobabs.
41:59This one might be over a thousand years old.
42:10It survives here in part thanks to its ability to store thousands of litres of water within the spongy wood of its trunk.
42:23But its battered surface is evidence of a very finely balanced relationship.
42:28These huge trees are a focus for animals of all kinds.
42:41And they're particularly important for elephants.
42:44In the wet season, they eat the baobabs' fruit and disperse the seeds in their dung.
42:55Now, as the dry season begins, they migrate to distant watering holes.
43:09The baobabs have damp inner wood.
43:14And the elephants use it to quench their thirst on the journey.
43:20This relationship can only work because baobabs have a remarkable ability to heal themselves.
43:39This relationship can only work because baobabs have a remarkable ability to heal themselves.
43:49This relationship can only work.
43:50This relationship can only work.
43:51This relationship can only work.
43:55Between each damaging attack, they expand their spongy wood and grow new skin.
44:14And they've done this time and time again over centuries.
44:27Today, however, it's harder for the baobabs to recover as dry seasons become longer and drier due to climate change.
44:36Not only that, but the elephants are forced to take ever more wood from the trees in order to survive.
44:51In some parts of Africa, many of the largest and oldest baobabs have fallen in the last decade.
44:57The loss of a vital species like the baobab strikes a blow at all life in the desert.
45:18In these hostile lands, few living organisms can survive without help from others.
45:33You can find an extraordinary illustration of this in Arizona's saguaro country.
45:39If you wander off the beaten track here, you may be lucky enough to find one of these.
45:53It might look like an old boot, but in fact, it's part of a saguaro cactus.
46:00Almost every saguaro has one.
46:03There's one up there.
46:09It has been produced, indirectly, by woodpeckers, which regularly dig homes for themselves in the bloated trunks of the saguaros.
46:18In the next six months, the cactus heals the wound, and so creates a safe, cool, and watertight nest hole.
46:29Its tough lining will persist for years, even after the cactus is destroyed.
46:32A single saguaro may hold several of these extraordinary homes.
46:33A single saguaro may hold several of these extraordinary homes.
46:34A single saguaro may hold several of these extraordinary homes.
46:35In the next six months, the cactus heals the wound, and so creates a safe, cool, and watertight nest hole.
46:44Its tough lining will persist for years, even after the cactus itself has died and rotted away.
46:51A single saguaro may hold several of these extraordinary homes.
47:02So over its lifetime, it may provide accommodation for some 3,000 chicks of several different species.
47:09But in the long term, the saguaro benefits as their lodgers repay the cactus by pollinating its flowers and dispersing its seeds.
47:29Its relationships like these that enable life to flourish in some of the world's harshest landscapes.
47:44Over millions of years, plants have become superbly adapted to hostile desert conditions.
48:05But it's a very finely balanced existence, and one that makes them uniquely vulnerable.
48:12However, our growing understanding of the complex ways by which desert animals and plants rely on one another
48:22is now helping us to understand how best we can protect them.
48:31Ninety years ago, a photograph was taken from this very spot
48:35that shows a population of saguaro cactus that was very different from what it is today.
48:42In the last 50 years, the population of saguaros here has greatly diminished.
48:49Not because of a direct assault on the cactus, but because many of the shade-giving nurse trees
48:56were harvested for firewood, leaving young saguaro to die in the sun.
49:01Now that this relationship is understood and the nurse trees are protected, there are already signs that the saguaro are recovering.
49:16Wherever there's a desert, plants have evolved to meet its challenge.
49:23But everywhere, they need our help.
49:28As we understand more about them and their intimate and complex relationships,
49:35we will be better able to protect them and all life in these beautiful but increasingly fragile worlds.
49:46The most remote shoot for the desert team is to the island of San Pedro Martir in the Sea of Cortez, Mexico.
50:09They are to film the giant cacti that have found a unique way to thrive here.
50:18It takes 48 hours to travel the 260 miles over unpleasantly choppy seas.
50:33It's not made some of us feel very well.
50:37I think one of us is quite seasick.
50:40There's nothing but big blue out there and the island is somewhere in that direction.
50:46Can't see it yet.
50:48As they draw close to the island, there's a spectacular reminder of how rich life in the sea is here.
50:57Oh, look in front of us! Look out for a pot!
51:02Oh, they're everywhere!
51:09Show us the way!
51:18Meeting them here is desert scientist Ben Wilder.
51:22It's through his research that the team first heard about the island's peculiar residents.
51:29I'll never forget the first time I came to actually exactly where we're standing right now.
51:35It was April of 2006 and saw this view and it both settled in my heart and captivated my mind.
51:45And so that started a process of trying to understand, you know, what can produce this.
51:55The secret is in the relationship between the cactus and a type of seabird called a booby.
52:05All the boobies that we need to film are right up there at the top of the island.
52:09So it's going to be a bit of a scramble.
52:14The crew soon discover how harsh the conditions are here.
52:18Pretty hot. It's pretty hot. It's a great view.
52:22There's very few places on Earth where you're going to see this many cactus.
52:33I mean, it's absolutely amazing.
52:36I would say this is the only place you're going to see this.
52:39Well, there you go then.
52:42Ben's research is uncovering the ingenious ways the cacti have adapted to the conditions here.
52:48The waters just offshore here are some of the most productive marine waters in the world.
52:55And so it's kind of an ideal habitat for seabirds to roost.
52:58But when they do so, they deposit tons of guano.
53:02And so those nutrients, really high in nitrogen and phosphorus,
53:06actually are toxic to most plant species.
53:09The Cardones thrive because they're uniquely able to process the guano
53:13and extract what they need to fuel their growth.
53:20One of the first shots the crew need to get is of the boobies nesting under the cacti.
53:27The lack of predators means the birds aren't afraid of people.
53:31It should make filming them up close a bit easier.
53:35That's the theory.
53:36So we better position this camera here to try and get a good perspective on the chick, on the nest.
53:41It's taken quite a liking to our camera.
53:44Let's hope it doesn't break here.
53:53Bullseye. It's pooed right on the front of the lens.
53:56It's kind of a shot we need, but unfortunately,
53:59my camera wasn't rolling at the time it did it, so now I've just got a dirty lens.
54:02After a thorough lens clean, Oli eventually gets the shots to reveal the extraordinary relationship between bird and plant.
54:19But it's not just the bird guano that influence how the Cardones grow.
54:23They're way shorter. They're dwarfed here.
54:26Throughout the rest of their range, they usually get an upwards of 50, 60 feet.
54:31But here, on average, they're 20, 24 feet in height.
54:36They stop growing up, and they grow out.
54:41Ben's research suggests that they grow wide here as an adaptation to the violent winds.
54:48Too tall, and they'd blow over.
54:51A gusting wind isn't helping the drone crew, either.
54:54We really need it to calm down a bit, otherwise it's going to be impossible.
54:57We're going to be quick.
55:03Come on, keep an eye on it.
55:05It's not safe.
55:07Bring it out.
55:08On the Sonoran mainland, where the Cardone is also found, you have on average between 50 to 150 plants per hectare.
55:29Here, on this island, you have over 2,500 plants per hectare.
55:36A lull in the wind, and the team get a chance to reveal the remarkable density of the cacti.
55:43Almost 20 times greater than anywhere else.
55:47It appears that here they're doing very well.
55:49But even this island isn't isolated from the effects of a changing planet.
55:57Every cactus you see there, its body is filled with nutrients from the sea.
56:03Given that we know that the Cardones are linked to the ocean, what happens in the ocean affects what happens to the Cardone.
56:10So a concern we have right now is that there's a lot of overfishing.
56:16And we know there are less seabirds here than there were 20 years ago.
56:21And we have reason to believe that that will ripple and affect the nutrients that fuel the Cardones as well.
56:27This finely balanced relationship is at risk.
56:33And knowing that makes leaving the island particularly thought-provoking for the crew.
56:40The Cardones look beautiful. I've never seen so many cactus in my life.
56:45It's absolutely amazing.
56:47I'm just hoping that when we leave this place, it stays as it is, and things don't affect it, they're negative.
56:52It's an absolutely wonderful thing, and, uh, yeah, I don't wanna go. I don't wanna stay.
56:58This remote cactus forest is a reminder of the adaptability of plants
57:05that enables them to establish green worlds almost anywhere on Earth.
57:15Next time on the Green Planet, our extraordinary relationship with plants.
57:23From those we eat...
57:28To those we help...
57:31Plants are our greatest allies.
57:38The Open University has produced a poster that explores the vital role that plants have for our planet.
57:46To order your free copy, call 0300 303 4200, or go to bbc.co.uk forward slash greenplanet and follow the links to The Open University.
58:01Be taken on a journey to calm with beautiful music and Sir David's wonderful voice.
58:09The Green Planet, Mindful Mix, listen now on sounds.
58:13An emotional odyssey, The Caribbean with Andy and Makita continues with songs, swimming and surprises on BBC 2 at 9.
58:19On BBC 4 now, a celebration of classic MGM music.
58:23Of course...
58:24The Green Planet...
58:25...m 짱...
58:26...m 짱...
58:27...m 짱...
58:28...m 짱...
58:29...m g ja...
58:30...m ch j m m j m m m g j c i m i m m g j c.