0:00:04 > 0:00:07Ten million species
0:00:07 > 0:00:08live on planet Earth.
0:00:10 > 0:00:12Each one is remarkable.
0:00:13 > 0:00:15But none can survive on its own.
0:00:20 > 0:00:22All life depends upon connections.
0:00:24 > 0:00:27Unexpected, invariably complex,
0:00:27 > 0:00:32beautiful relationships between millions of plants and animals.
0:00:37 > 0:00:39In our waterworlds,
0:00:39 > 0:00:42I want to show you why
0:00:42 > 0:00:45- this crab needs a tiger... - TIGER GROWLS
0:00:46 > 0:00:50..why this giant otter needs a snail...
0:00:52 > 0:00:55..and why this shark needs a sponge.
0:00:58 > 0:01:04Connections like these form the planet's great ecosystems.
0:01:04 > 0:01:07They're vital for all life.
0:01:07 > 0:01:11I want to show you our world as you've never seen it before.
0:01:32 > 0:01:37This is the Gullfoss waterfall in Iceland.
0:01:37 > 0:01:41100 cubic metres of water are falling here every second.
0:01:49 > 0:01:52So this is just about as close as I want to get,
0:01:52 > 0:01:56because this is a dangerous and volatile environment.
0:01:56 > 0:02:00But for all of that danger, these raging torrents contain
0:02:00 > 0:02:04an ingredient which is absolutely vital for life.
0:02:04 > 0:02:08And the clue is in the name.
0:02:10 > 0:02:14"Gullfoss" means "Golden Falls".
0:02:14 > 0:02:19The colour is produced by millions of tonnes of raging water,
0:02:19 > 0:02:23relentlessly carving through rock and soil.
0:02:27 > 0:02:30And accumulating that golden sediment.
0:02:32 > 0:02:36And there's something else vital being carried in this water...
0:02:36 > 0:02:38oxygen.
0:02:38 > 0:02:42Together, these are incredibly potent ingredients.
0:02:45 > 0:02:49At the moment, all of this is just cargo,
0:02:49 > 0:02:52being swept along by this very fast-moving water.
0:02:53 > 0:02:56But with ingredients like this,
0:02:56 > 0:03:02if the environment changes, then the potential for life is huge.
0:03:13 > 0:03:15It doesn't matter where you are.
0:03:15 > 0:03:20Most life on Earth depends on the simple ingredients
0:03:20 > 0:03:23that start upstream.
0:03:27 > 0:03:29Places where, normally,
0:03:29 > 0:03:32just a few specially adapted creatures can survive.
0:03:53 > 0:03:57So, how do mere oxygen and sediment
0:03:57 > 0:04:02ignite such a richness of life downstream?
0:04:05 > 0:04:09To find out, I'm going to witness
0:04:09 > 0:04:12one of the greatest explosions of life on Earth.
0:04:15 > 0:04:20Over millions of years, waters from the Brazilian highlands
0:04:20 > 0:04:23have flooded into a vast lowland basin...
0:04:37 > 0:04:39..the Pantanal.
0:05:00 > 0:05:02Look at this!
0:05:02 > 0:05:05These murky waters are virtually boiling with fish.
0:05:15 > 0:05:22I've never seen so much life so densely packed into one place.
0:05:40 > 0:05:44Every river and every tributary that we've paddled up in the Pantanal...
0:05:44 > 0:05:47- CAIMANS SPLASH - ..has been lined with these animals.
0:05:51 > 0:05:55An estimate suggests there might be as many as ten million caiman living in the Pantanal.
0:05:57 > 0:06:02That would make it the largest concentration of land vertebrates anywhere on Earth.
0:06:08 > 0:06:11But the really staggering thing about the animals in the Pantanal
0:06:11 > 0:06:14is their sheer size.
0:06:14 > 0:06:16Look at this wonderful bird.
0:06:18 > 0:06:20It's called the jabiru stork.
0:06:21 > 0:06:24The tallest flying bird in South America.
0:06:30 > 0:06:35This really is a land of the giants.
0:06:35 > 0:06:38The world's largest snake, the green anaconda,
0:06:38 > 0:06:41which can grow to nine metres long.
0:06:50 > 0:06:53Even the plants are monsters.
0:06:56 > 0:07:00Look at this splendid spread of giant water lilies.
0:07:00 > 0:07:02Absolutely fabulous things.
0:07:04 > 0:07:08A single plant produces around 40 leaves
0:07:08 > 0:07:11and each leaf can grow to three metres wide.
0:07:17 > 0:07:19BIRD CHEEPS
0:07:19 > 0:07:21Even the rodents here are the largest in the world.
0:07:21 > 0:07:23These are capybara.
0:07:23 > 0:07:27They're the dominant herbivore in the Pantanal,
0:07:27 > 0:07:29and they occur here in huge numbers.
0:07:32 > 0:07:36And what does it take to catch such an overgrown rat?
0:07:40 > 0:07:43(Look at that! Look at that!)
0:07:47 > 0:07:51We're about six metres from a wild jaguar. It's unbelievable.
0:07:52 > 0:07:55The jaguars in the Pantanal are the biggest cats
0:07:55 > 0:07:57anywhere in the Americas.
0:07:58 > 0:08:00(Oh, my goodness!)
0:08:12 > 0:08:17And then there's the apex predator -
0:08:17 > 0:08:19the king of the river.
0:08:19 > 0:08:22- OTTER SQUEAKS - Giant otters.
0:08:22 > 0:08:24Remarkable animals.
0:08:24 > 0:08:27OTTERS SQUEAK
0:08:31 > 0:08:35They're supremely adapted for their aquatic lifestyle.
0:08:44 > 0:08:48And given the size of these animals and their abundance here,
0:08:48 > 0:08:52this has to mean that this water is literally full of fish.
0:09:04 > 0:09:08It's like an Eden, it's just packed - packed with life!
0:09:09 > 0:09:14So, how did that cargo from those barren mountain streams
0:09:14 > 0:09:16help create this magical place?
0:09:21 > 0:09:23Here's a jar of water.
0:09:23 > 0:09:25I've just collected it from the creek behind me.
0:09:25 > 0:09:29And look, if I shake it, to mimic the action of a waterfall,
0:09:29 > 0:09:33swirling and frothing in a violent eddy,
0:09:33 > 0:09:36you can see that all of the material here
0:09:36 > 0:09:38is now held in suspension.
0:09:38 > 0:09:41But what's so important about that material?
0:09:41 > 0:09:44What's so important about that cargo?
0:09:44 > 0:09:47Well, when water tumbles down from the mountains,
0:09:47 > 0:09:50through the fast-flowing streams, along the giant rivers,
0:09:50 > 0:09:55it's constantly grinding away at the bedrock and the soil,
0:09:55 > 0:09:59picking up material, so that when it arrives here in the Pantanal,
0:09:59 > 0:10:03it's filled full of silt, sediment and detritus -
0:10:03 > 0:10:07a heady cocktail of inorganic and organic material.
0:10:07 > 0:10:10And it's filled full of things which are essential for life -
0:10:10 > 0:10:13things like nitrogen and phosphorus.
0:10:13 > 0:10:18And look - now the water in my jar has begun to slow down,
0:10:18 > 0:10:21the material in it has started to settle out.
0:10:21 > 0:10:26And this exactly replicates what's happening here in the Pantanal.
0:10:26 > 0:10:28In fact, it's settled out right here,
0:10:28 > 0:10:34and what I'm standing on is a great plain of fertility.
0:10:34 > 0:10:36BIRDS CHIRP
0:10:36 > 0:10:42This land of the giants is exceptionally fertile
0:10:42 > 0:10:45because that cocktail of nutrients deposited by the rivers
0:10:45 > 0:10:46has been trapped here.
0:10:49 > 0:10:54Gradually, they've built up, year-on-year,
0:10:54 > 0:10:56over millions of years.
0:11:16 > 0:11:22But this hugely productive process has only been possible
0:11:22 > 0:11:27thanks to the tireless work of one species of animal.
0:11:33 > 0:11:36It's so important that even the giant otter
0:11:36 > 0:11:39depends upon it for its survival.
0:11:46 > 0:11:49Now, you might expect my hero of the Pantanal
0:11:49 > 0:11:52to be one of the large creatures that we've already seen,
0:11:52 > 0:11:55but brace yourselves... My hero...
0:11:55 > 0:12:00is this, the apple snail.
0:12:00 > 0:12:03And I can tell you that, without this humble animal,
0:12:03 > 0:12:08the Pantanal wouldn't - couldn't - be such a rich environment.
0:12:10 > 0:12:14But how on earth can a snail be so important?
0:12:34 > 0:12:38The apple snail spends much of its life on the river bed,
0:12:38 > 0:12:42safe from most of its predators.
0:12:44 > 0:12:49It has a fish-like gill that can remove oxygen from the water,
0:12:49 > 0:12:51so it can breathe beneath the surface.
0:12:54 > 0:12:57Here, there's plenty of food...
0:13:01 > 0:13:04..because there's nothing that an apple snail likes more to eat
0:13:04 > 0:13:08than dead and decaying vegetation.
0:13:12 > 0:13:16Its teeth can saw through the toughest plant fibres
0:13:16 > 0:13:20and, unlike most animals, it can digest cellulose.
0:13:21 > 0:13:27But like all animals, it needs to relieve itself
0:13:27 > 0:13:29and that's when the magic happens.
0:13:31 > 0:13:35It's miraculously recycled all that dead material
0:13:35 > 0:13:38back into accessible nitrogen and phosphorous.
0:13:39 > 0:13:44Five-star fertiliser for the next generation of plants.
0:13:46 > 0:13:49But that's not the only way this species
0:13:49 > 0:13:52recycles essential nutrients.
0:13:53 > 0:13:58For hundreds of predators, apple snails make a very tasty meal.
0:14:10 > 0:14:13Young caimans particularly enjoy them.
0:14:19 > 0:14:20And each mouthful
0:14:20 > 0:14:26helps spread the vital ingredients of all life around the Pantanal.
0:14:28 > 0:14:32It's thanks to all of this recycling by the apple snail
0:14:32 > 0:14:36that the Pantanal is so full of so many giants.
0:14:42 > 0:14:46But, in order to carry out this pivotal role,
0:14:46 > 0:14:51the snail has to overcome one huge challenge -
0:14:51 > 0:14:56one which puts the whole ecosystem of the Pantanal on a knife edge.
0:15:01 > 0:15:03THUNDERCLAP
0:15:06 > 0:15:12April brings rains to the highlands across central South America.
0:15:23 > 0:15:26For months, the Pantanal has been drying into a patchwork
0:15:26 > 0:15:30of small pools and grass.
0:15:38 > 0:15:41But now, the rivers swell.
0:15:41 > 0:15:45They burst their banks and drown the grassy plains.
0:15:51 > 0:15:56This is the most dangerous time for this ecosystem,
0:15:56 > 0:16:01because the most important ingredient for life is running out.
0:16:06 > 0:16:09Clearly, there's no shortage of sunshine here.
0:16:10 > 0:16:13Nor is there any shortage of nutrients.
0:16:13 > 0:16:19No, what's in surprisingly short supply in these waters is oxygen.
0:16:21 > 0:16:26As billions of tonnes of grass is swamped, it begins to decay...
0:16:28 > 0:16:32..stagnating the water and robbing it of that oxygen.
0:16:34 > 0:16:37The Pantanal is suffocating.
0:16:39 > 0:16:44It will only survive if that rotting grass
0:16:44 > 0:16:48is rapidly recycled into new forms of life.
0:16:50 > 0:16:56The Pantanal now needs the apple snail more than ever.
0:16:59 > 0:17:02But without oxygen, not even it can survive.
0:17:08 > 0:17:12Thankfully, it has a secret weapon.
0:17:23 > 0:17:25Uniquely amongst snails,
0:17:25 > 0:17:31it possesses a beautifully evolved, telescopic appendage...
0:17:33 > 0:17:35..a snorkel!
0:17:39 > 0:17:44The snail pumps air straight into a special lung.
0:17:53 > 0:17:57And when it's breathed enough, it returns to what it does best -
0:17:57 > 0:18:01processing that rotting grass.
0:18:07 > 0:18:10So whatever this place throws at them,
0:18:10 > 0:18:14billions of apple snails keep munching away
0:18:14 > 0:18:16and fertilising the Pantanal.
0:18:35 > 0:18:39The aquatic ecosystem flourishes,
0:18:39 > 0:18:44supporting enough fish to satisfy the king of the river.
0:18:44 > 0:18:46OTTERS SCREECH
0:18:51 > 0:18:54The giant otter.
0:18:54 > 0:18:57OTTERS SNORT AND SCREECH
0:19:02 > 0:19:03SCREECHING CONTINUES
0:19:03 > 0:19:05Just listen to that!
0:19:05 > 0:19:09Their vocalisations are constant, this family party,
0:19:09 > 0:19:12constantly in touch with one another.
0:19:14 > 0:19:16These screeching calls are territorial,
0:19:16 > 0:19:21warning other animals that this group owns this stretch of river,
0:19:21 > 0:19:22and all of the fish in it.
0:19:22 > 0:19:25Any rivals are aggressively dealt with.
0:19:34 > 0:19:40The whole family regularly patrols a stretch of up to 20km.
0:19:42 > 0:19:45And there's nothing that frightens them -
0:19:45 > 0:19:48not even caimans.
0:19:59 > 0:20:01Now that they've got the river to themselves,
0:20:01 > 0:20:05it's down to the business of catching fish.
0:20:22 > 0:20:25And their appetites are insatiable.
0:20:25 > 0:20:31Each animal needs to eat a tenth of its bodyweight every day.
0:20:35 > 0:20:40In the Pantanal, there really is plenty for everyone,
0:20:40 > 0:20:42and it's all thanks to a mollusc.
0:20:43 > 0:20:49For me, the most important species in the Pantanal is the apple snail.
0:20:49 > 0:20:51It's a true keystone species,
0:20:51 > 0:20:54involved in everything that's going on here.
0:20:54 > 0:20:57It converts fish and other matter into grass.
0:20:57 > 0:21:00Grass and aquatic vegetation into food.
0:21:00 > 0:21:03It's even food itself,
0:21:03 > 0:21:07and it can survive anything that this place can throw at it.
0:21:07 > 0:21:10It shapes and structures the Pantanal.
0:21:10 > 0:21:14Therefore, it's what we call an ecosystem engineer.
0:21:14 > 0:21:20Without it, all those millions of caimans, the capybara,
0:21:20 > 0:21:23the jaguar, and especially the giant otter,
0:21:23 > 0:21:26would struggle to live here.
0:21:26 > 0:21:31And that's why the giant otter needs the snail.
0:21:33 > 0:21:37I like that. I love the connectedness!
0:21:40 > 0:21:44The tireless work of apple snails over millions of years
0:21:44 > 0:21:48has helped to create the Pantanal.
0:21:53 > 0:21:57And all over the planet, from the Florida Everglades,
0:21:57 > 0:22:01to the Fens of East Anglia,
0:22:01 > 0:22:07to the Nile, freshwater snails and other wetland creatures
0:22:07 > 0:22:10recycle material on an epic scale,
0:22:10 > 0:22:16sustaining watery ecosystems, and making them rich in life.
0:22:18 > 0:22:22And thanks to these animals, the impact of wetlands
0:22:22 > 0:22:24on the planet is huge.
0:22:26 > 0:22:31At one end of the wetland, the raging water from mountain streams
0:22:31 > 0:22:33is soaked up like a giant sponge.
0:22:33 > 0:22:38At the other, it's released in a steady flow.
0:22:41 > 0:22:44In doing so, these wetlands ensure
0:22:44 > 0:22:47that the rivers and the animals downstream
0:22:47 > 0:22:50get a reliable supply of water.
0:22:59 > 0:23:02But as the river makes its way to the sea,
0:23:02 > 0:23:06life in the water faces a much bigger challenge.
0:23:26 > 0:23:32This is the end of the line for the mighty Ganges.
0:23:38 > 0:23:41It's the swamp of the Sunderbans, on the coast of Bangladesh.
0:23:45 > 0:23:51On the ground here, it feels and smells like an alien world.
0:23:56 > 0:24:00The whole place is pervaded by the stench of rotten eggs,
0:24:00 > 0:24:04generated by sulphur-belching bacteria.
0:24:04 > 0:24:10It's certainly a lot less inviting than the Pantanal.
0:24:12 > 0:24:15It's surprising that anything survives here at all,
0:24:15 > 0:24:17because life in this place
0:24:17 > 0:24:20has to survive some pretty tough challenges.
0:24:20 > 0:24:23For a start, the silt.
0:24:23 > 0:24:26You see, when the water reaches the coast,
0:24:26 > 0:24:29only the finest particles are held in suspension
0:24:29 > 0:24:32and when they drop out, they form this...
0:24:35 > 0:24:39..thick, gloopy mud. Whoa!
0:24:42 > 0:24:46It's so thick that not even air can penetrate it,
0:24:46 > 0:24:50so no oxygen can get into this soil.
0:24:50 > 0:24:54And, as if that wasn't bad enough, twice a day,
0:24:54 > 0:24:58with the rise and fall of the tide,
0:24:58 > 0:25:00this whole place floods.
0:25:05 > 0:25:11Precious nutrients, in the form of leaves, are flushed out to sea.
0:25:11 > 0:25:17And, worst of all, everything is drenched in bitter salty water,
0:25:17 > 0:25:19which very few plants can tolerate.
0:25:30 > 0:25:35But the Sunderbans is not the hell on Earth that it might seem.
0:25:44 > 0:25:47Look at this beauty! A black-capped kingfisher.
0:25:50 > 0:25:51A brahminy kite.
0:25:58 > 0:26:02There are enough fish here to support millions of people.
0:26:08 > 0:26:10Macaque monkeys...
0:26:14 > 0:26:17..chital deer too.
0:26:22 > 0:26:27And there's one creature that is very special.
0:26:27 > 0:26:32But it's extremely well hidden.
0:26:32 > 0:26:34It's also a man-eater.
0:26:37 > 0:26:38Tiger!
0:26:44 > 0:26:46I can see the stripes on its back.
0:26:47 > 0:26:49It's got its rump facing towards me
0:26:49 > 0:26:52and its head's lying on the ground.
0:26:53 > 0:26:55Wow!
0:26:55 > 0:26:59It's about the worst view of an animal that I've ever had,
0:26:59 > 0:27:01and yet it's one of the best!
0:27:04 > 0:27:06It's getting up.
0:27:10 > 0:27:12Which way did it go?
0:27:17 > 0:27:23After that tantalising and remarkable view of an animal,
0:27:23 > 0:27:28we've been able to follow a diary of its movements, here in the mud.
0:27:28 > 0:27:31And, if you look, you can see that the tiger has come here.
0:27:31 > 0:27:36Here's a pug mark, here's another, here are three more.
0:27:36 > 0:27:39But I think what's happened here
0:27:39 > 0:27:42is that the tiger has come to this point, it's changed its mind,
0:27:42 > 0:27:46and it's turned around, headed back this way.
0:27:46 > 0:27:48There's another print there.
0:27:48 > 0:27:50And it's gone off, into the forest.
0:27:52 > 0:27:55And this is not a one-off.
0:27:55 > 0:27:57Pictures from our camera traps reveal
0:27:57 > 0:28:00that living in this salty, drowned forest
0:28:00 > 0:28:03is a large population of Bengal tigers.
0:28:08 > 0:28:13Surely one of nature's most magnificent predators.
0:28:15 > 0:28:18And despite the fact that population estimates vary,
0:28:18 > 0:28:21we think that a quarter of the world's wild tiger population
0:28:21 > 0:28:24might be living here in the Sunderbans.
0:28:24 > 0:28:26So, there has to be enough food for them,
0:28:26 > 0:28:28this has to be a productive ecosystem.
0:28:28 > 0:28:34But how can a muddy, salty, sulphurous bog support so much life?
0:28:35 > 0:28:41Well, the secret of the Sunderbans lies in beautiful relationships
0:28:41 > 0:28:45that have evolved between the most unlikely species,
0:28:45 > 0:28:48including the tiger.
0:28:48 > 0:28:52And it all starts with a very peculiar plant...
0:28:54 > 0:29:00..the mangrove - the only trees that can survive in salty water.
0:29:00 > 0:29:05They even expel some of that salt through their leaves.
0:29:05 > 0:29:09And as for the lack of oxygen in the soil,
0:29:09 > 0:29:12mangroves have a spectacular solution,
0:29:12 > 0:29:15not unlike that of the apple snail.
0:29:19 > 0:29:24Now, normally, plants access oxygen through their roots
0:29:24 > 0:29:28from tiny pockets of air in the soil.
0:29:28 > 0:29:30But in this sticky ooze,
0:29:30 > 0:29:34these pockets are virtually non-existent.
0:29:34 > 0:29:39But then the mangrove is a pretty special plant.
0:29:39 > 0:29:44All of these spikes sticking out of the soil here are roots,
0:29:44 > 0:29:50and they act a bit like snorkels, sucking in oxygen out of the air
0:29:50 > 0:29:54when the plant can't get it out of this thick mud.
0:29:54 > 0:29:58But then the mangrove doesn't just rely on its snorkels.
0:29:58 > 0:30:00There's something else going on here,
0:30:00 > 0:30:03something you can only appreciate at low tide.
0:30:18 > 0:30:20Millions of crabs!
0:30:23 > 0:30:27Leaf-eating crabs and fiddler crabs.
0:30:31 > 0:30:33They're called fiddler crabs
0:30:33 > 0:30:36because they have this vastly-enlarged front claw.
0:30:38 > 0:30:42And when they're feeding, it looks like they're playing the fiddle.
0:30:44 > 0:30:47They also wave them at any adversaries
0:30:47 > 0:30:50in a relative show of size and strength.
0:30:50 > 0:30:54In fact, when they're fully grown, that claw can represent up to 65%
0:30:54 > 0:30:56of the crab's body weight -
0:30:56 > 0:30:58quite an investment for something to wave around
0:30:58 > 0:31:01at your enemies.
0:31:01 > 0:31:05These fiddlers are displaying to defend their territories.
0:31:23 > 0:31:28And their most valuable real estate is underground.
0:31:29 > 0:31:34This little fiddler is excavating mud to create a burrow.
0:31:34 > 0:31:35When he's finished,
0:31:35 > 0:31:38it's going to be more than half a metre deep.
0:31:42 > 0:31:46His burrow gives him somewhere to hide from predators,
0:31:46 > 0:31:48like this stork.
0:31:48 > 0:31:52And when the tide comes in, from predatory fish.
0:31:56 > 0:31:59These leaf-eaters live in communal burrows,
0:31:59 > 0:32:04and together, their tunnels form an underground labyrinth.
0:32:06 > 0:32:11All of these burrows are vital for the mangrove.
0:32:11 > 0:32:16At low tide, they channel an air supply through the mud,
0:32:16 > 0:32:18direct to the roots.
0:32:18 > 0:32:22And it's not just oxygen.
0:32:22 > 0:32:25The crabs even supply the trees with food.
0:32:25 > 0:32:30The first ingredient is all those smelly bacteria.
0:32:30 > 0:32:36Look really carefully, and you can see this crab feeding.
0:32:36 > 0:32:41It's picking up particles of soil and passing them to its mandibles.
0:32:44 > 0:32:47When it gets enough, it forms them into a ball,
0:32:47 > 0:32:51and it gradually removes all of the organic material,
0:32:51 > 0:32:53detritus and bacteria,
0:32:53 > 0:32:56and then it discards the ball.
0:32:58 > 0:33:02And you can see those that it's processed lying on the surface here.
0:33:02 > 0:33:06And if the crabs didn't do this, this mud wouldn't be very nice -
0:33:06 > 0:33:09a nasty, sulphurous ooze.
0:33:12 > 0:33:14Racing against the tide,
0:33:14 > 0:33:19this fiddler is taking bacteria-rich mud back to his burrow.
0:33:19 > 0:33:24Here, he'll recycle it and release nutrients
0:33:24 > 0:33:25for the roots of the mangrove.
0:33:29 > 0:33:34Further up the beach, this leaf-eater is also working hard
0:33:34 > 0:33:38to gather his food before the tide steals it.
0:33:51 > 0:33:55These crabs collect a staggering 80% of the leaves
0:33:55 > 0:33:58that fall here in the Sunderbans
0:33:58 > 0:34:01and they store them at the bottom of their burrows,
0:34:01 > 0:34:06where they too will essentially fertilise the mangrove.
0:34:08 > 0:34:09But best of all,
0:34:09 > 0:34:13the burrows even help control the saltiness of the swamp.
0:34:15 > 0:34:17When the tide comes in,
0:34:17 > 0:34:20toxic seawater flows into the burrows
0:34:20 > 0:34:22and mixes with fresh water.
0:34:25 > 0:34:28And this allows the mangrove
0:34:28 > 0:34:31to expend less of its energy excreting salt,
0:34:31 > 0:34:35and more on actually growing.
0:34:38 > 0:34:40Without these burrows,
0:34:40 > 0:34:42the Sunderbans simply couldn't survive.
0:34:42 > 0:34:45Together, the crabs make a vast network -
0:34:45 > 0:34:48a sort of Sunderbans tube system.
0:35:18 > 0:35:23The scale of the tube system is unbelievable.
0:35:23 > 0:35:28Just one square metre can have 300 tunnels.
0:35:32 > 0:35:36Crabs are ecosystem engineers.
0:35:36 > 0:35:39Without the many millions of them,
0:35:39 > 0:35:44living in this mangrove, the Sunderbans simply couldn't work.
0:35:45 > 0:35:50That's why the tiger needs the crab.
0:35:58 > 0:36:04So the tiger needs the crab. But it's more magical than that.
0:36:04 > 0:36:08And there's an even more unusual relationship...
0:36:10 > 0:36:15..one that protects the Sunderbans from a lethal threat.
0:36:22 > 0:36:25Thanks to the crabs gardening the mangroves,
0:36:25 > 0:36:28the Sunderbans support some large herbivores.
0:36:28 > 0:36:33But too many eating too much would soon damage the forest,
0:36:33 > 0:36:35so it needs protection.
0:36:47 > 0:36:50The monkeys have sounded a warning.
0:37:00 > 0:37:03This family of chital deer won't be staying much longer.
0:37:17 > 0:37:20It's the very presence of these terrifying predators
0:37:20 > 0:37:21that protects the Sunderbans.
0:37:26 > 0:37:28You see, in any ecosystem,
0:37:28 > 0:37:33top predators exert what we call "an ecology of fear".
0:37:33 > 0:37:38And this influences the behaviour and movement of their prey.
0:37:38 > 0:37:44Here, that might be monkeys, or deer, or humans.
0:37:44 > 0:37:46In the Sunderbans, the tigers keep
0:37:46 > 0:37:49large numbers of people out of the forest,
0:37:49 > 0:37:53and they also keep all of the herbivores on the move,
0:37:53 > 0:37:55so they don't damage the trees.
0:37:55 > 0:38:00So, in a way, the tiger needs the crab to help build this place,
0:38:00 > 0:38:06but then the crab needs the tiger to help protect it.
0:38:06 > 0:38:10You've got to admit, that's pretty neat.
0:38:15 > 0:38:18And the result is this -
0:38:18 > 0:38:24the largest mangrove forest in the world!
0:38:31 > 0:38:37This mangrove ecosystem is dependent upon a complex web of relationships
0:38:37 > 0:38:43between species as diverse as crabs and tigers to make it functional.
0:38:43 > 0:38:48But surprisingly, these connections don't end here,
0:38:48 > 0:38:53because what happens on the coast, where the river meets the sea,
0:38:53 > 0:38:59actually has a profound effect on what happens out there.
0:39:06 > 0:39:09Across the planet, coastal ecosystems, like the Sunderbans,
0:39:09 > 0:39:13are essential for both the land and the sea.
0:39:16 > 0:39:20They act as barriers, protecting the land from storms.
0:39:20 > 0:39:26And they provide vital nurseries for ocean-going fish.
0:39:28 > 0:39:33But, more importantly, they trap much of the silt and sediment,
0:39:33 > 0:39:37so that clean water flows out to sea.
0:39:37 > 0:39:42And in the tropics, this has a profound effect
0:39:42 > 0:39:45on the world's richest marine habitats...
0:39:48 > 0:39:50..coral reefs.
0:39:54 > 0:39:59And one of the finest on Earth is here - the Maldives.
0:40:14 > 0:40:18There's something very odd about coral reefs.
0:40:18 > 0:40:25Look at the water - it's clear. It's absolutely crystal clear.
0:40:32 > 0:40:37I can see a vast and colourful coral city.
0:40:40 > 0:40:42And across the world's oceans,
0:40:42 > 0:40:46these are home to a quarter of all marine species...
0:40:54 > 0:40:55..from tiny clown fish...
0:40:58 > 0:41:02..to the black-tipped reef shark.
0:41:02 > 0:41:03This really is
0:41:03 > 0:41:06the ocean equivalent of a rainforest.
0:41:06 > 0:41:09But it's also a puzzle.
0:41:09 > 0:41:13The waters around this reef aren't just low in sediment,
0:41:13 > 0:41:16they're consequently low in nutrients.
0:41:16 > 0:41:21So, how on earth can they support so much life?
0:41:23 > 0:41:27It was a puzzle that stumped the world's most famous biologist,
0:41:27 > 0:41:29Charles Darwin.
0:41:29 > 0:41:33And thus it became known as Darwin's Paradox.
0:41:33 > 0:41:37And it took science more than 100 years to figure it out.
0:41:37 > 0:41:40And guess what the key was.
0:41:40 > 0:41:45Connections. Wonderful connections between the species that live here.
0:41:47 > 0:41:49Take the coral itself.
0:41:49 > 0:41:51It's not made from one,
0:41:51 > 0:41:54but from two organisms.
0:41:56 > 0:42:00First, tiny creatures, just a few millimetres in length,
0:42:00 > 0:42:03called polyps.
0:42:03 > 0:42:06Polyps spend their lives filtering
0:42:06 > 0:42:08microscopic particles in the clear waters.
0:42:11 > 0:42:17But up to 90% of their food comes from their coral partners.
0:42:17 > 0:42:20Sheltering within the safety of the polyps,
0:42:20 > 0:42:23are colourful specks -
0:42:23 > 0:42:25algae.
0:42:31 > 0:42:35Like plants, these tiny algae get most of their food
0:42:35 > 0:42:41through photosynthesis, to make sugars, powered by the sunlight.
0:42:48 > 0:42:52So, whilst the polyps provide the algae with protection,
0:42:52 > 0:42:57in return, the algae supply the polyps with food.
0:43:04 > 0:43:08But this ecosystem just doesn't add up.
0:43:13 > 0:43:18As Darwin knew, this fabulous diversity of life here
0:43:18 > 0:43:22can't be sustained by just sunlight alone.
0:43:22 > 0:43:26It also needs vital nutrients, things like nitrogen,
0:43:26 > 0:43:28phosphorous and potassium.
0:43:28 > 0:43:31And if it doesn't get these from silt
0:43:31 > 0:43:32then where does it get them from?
0:43:36 > 0:43:38Well, all of the animals
0:43:38 > 0:43:42that live in these waters excrete valuable nutrients.
0:43:46 > 0:43:52But the constant tidal currents quickly wash them away.
0:43:56 > 0:44:03So, what the reef needs is something that can hold on to those nutrients.
0:44:05 > 0:44:06Well, there is such a thing,
0:44:06 > 0:44:09and there's also a very special creature
0:44:09 > 0:44:12that's going to lead me straight to it.
0:44:27 > 0:44:28The hawksbill turtle.
0:44:37 > 0:44:39They're strong swimmers,
0:44:39 > 0:44:42but I need to keep up with it, to see where it goes.
0:44:58 > 0:45:02And I'm really hoping that this one is hungry.
0:45:39 > 0:45:44It's a rather odd-looking meal - a sponge.
0:45:46 > 0:45:49It's tough, but the sharp beak of the hawksbill
0:45:49 > 0:45:51can bite through its sinuous flesh.
0:45:56 > 0:45:57What's clearly much harder
0:45:57 > 0:46:01is actually keeping hold of it in the swirling currents.
0:46:17 > 0:46:19So turtles love to eat them,
0:46:19 > 0:46:23but why does the reef need the sponge?
0:46:25 > 0:46:30Well, sponges are creatures that live embedded amongst the coral.
0:46:30 > 0:46:34And there are thousands of species.
0:46:38 > 0:46:43They are amongst the most bizarre animals on the planet.
0:46:43 > 0:46:48They don't have eyes, a heart, or a nervous system.
0:46:48 > 0:46:52But the weirdest thing about a sponge is the way that it feeds.
0:46:58 > 0:47:03The sponge sucks the coloured water out of this syringe.
0:47:03 > 0:47:06The plunger isn't even being touched.
0:47:09 > 0:47:12As sponges siphon water through their bodies,
0:47:12 > 0:47:14they extract the nutrients.
0:47:16 > 0:47:20Although those nutrients are in tiny concentrations,
0:47:20 > 0:47:24if the sponge pumps quickly, it can get enough.
0:47:26 > 0:47:28Their secret is the scale with which they can do this.
0:47:28 > 0:47:35A sponge like this one can pump five times its own volume of water
0:47:35 > 0:47:39through its feeding canal in just one minute.
0:47:39 > 0:47:42And a sponge 60cm in length,
0:47:42 > 0:47:45can filter the equivalent of an Olympic-size swimming pool
0:47:45 > 0:47:47in just five days.
0:47:53 > 0:47:56So, as the animals on the reef excrete nutrients,
0:47:56 > 0:48:01it's the sponges that capture and concentrate these as viable food.
0:48:03 > 0:48:07It's these nutrients that help feed the reef,
0:48:07 > 0:48:11benefiting everything that lives here,
0:48:11 > 0:48:15from the coral, right up to the top predator.
0:48:19 > 0:48:22This large-scale recycling of nutrients
0:48:22 > 0:48:27helps keep these nutrients around the reef for longer,
0:48:27 > 0:48:30delaying the inevitable leaking away into the open ocean.
0:48:32 > 0:48:38For that reason, it's the sponges that are my coral reef heroes.
0:48:51 > 0:48:55What I've learned here is nothing short of a revelation, really.
0:48:55 > 0:49:01Everything is connected - the fish, the turtles, the corals.
0:49:01 > 0:49:04But it's not just these animals - it's the sponges too.
0:49:04 > 0:49:08And further upstream, the tiger and the snails.
0:49:08 > 0:49:11And when all of these things come together,
0:49:11 > 0:49:15the connections make this place work.
0:49:15 > 0:49:16And it really does work,
0:49:16 > 0:49:21because this is one of the richest ecosystems on our planet.
0:49:41 > 0:49:46Around the world, ecosystems in shallow seas like these
0:49:46 > 0:49:50convert scarce nutrients in the water
0:49:50 > 0:49:55to provide a haven for a huge variety of sea life.
0:49:56 > 0:50:02But the most miraculous place of all is further out to sea...
0:50:08 > 0:50:10..in the deep ocean.
0:50:15 > 0:50:17And in this endless expanse,
0:50:17 > 0:50:20it appears there's nothing living here
0:50:20 > 0:50:22and nothing to eat.
0:50:25 > 0:50:29On the face of it, it's devoid of life.
0:50:36 > 0:50:39But, of course, it's not.
0:50:39 > 0:50:43It's home to the world's largest animals.
0:50:45 > 0:50:50Thanks to connections that lead back to those wetlands upstream,
0:50:50 > 0:50:53all the way back to that apple snail.
0:50:58 > 0:51:02All of the silt, the sediment, the recycled organic matter
0:51:02 > 0:51:06that's washed down from the wetlands, the mangroves
0:51:06 > 0:51:09and the coral reef, where's it all gone?
0:51:09 > 0:51:12Has it just washed out into the open ocean,
0:51:12 > 0:51:13to be lost for ever?
0:51:13 > 0:51:18And if it has, what do the animals that live here feed upon?
0:51:18 > 0:51:22Well, potentially, it could have been a great waste of food,
0:51:22 > 0:51:27if it weren't for the way that the water moves.
0:51:37 > 0:51:39All of those valuable nutrients
0:51:39 > 0:51:44fall like marine snow on the sea bed, far below.
0:51:44 > 0:51:46But they're not lost for ever.
0:51:50 > 0:51:53Deep sea currents of unimaginable power,
0:51:53 > 0:51:57stir up the oceans on a global scale.
0:52:01 > 0:52:06It may take centuries, but carried by these upwelling currents,
0:52:06 > 0:52:10many of these lost nutrients eventually resurface.
0:52:12 > 0:52:17A sudden bounty of all the ingredients needed to sustain life.
0:52:19 > 0:52:25And a feast for all of the microscopic algae - phytoplankton.
0:52:28 > 0:52:33The plankton that live here on the surface are dependent
0:52:33 > 0:52:35on these upwellings of nutrients.
0:52:35 > 0:52:37And when they're able to combine them
0:52:37 > 0:52:41with bright sunlight, their population explodes.
0:52:45 > 0:52:48These multiplying plankton soon attract millions
0:52:48 > 0:52:52of small crustaceans, krill, larvae of all kinds
0:52:52 > 0:52:54and many other creatures.
0:52:54 > 0:52:56And together, they combine
0:52:56 > 0:53:01to create the biggest frenzy of life on our planet...
0:53:02 > 0:53:04..a plankton bloom.
0:53:09 > 0:53:13And plankton blooms attract some awe-inspiring creatures.
0:53:15 > 0:53:17Here, in the Indian Ocean,
0:53:17 > 0:53:21I've come to witness one of the most enchanting...
0:53:23 > 0:53:26..the manta ray.
0:53:36 > 0:53:40They fly through the water,
0:53:40 > 0:53:43filtering and feeding on the plankton.
0:53:43 > 0:53:47They can eat 30kg a day.
0:54:11 > 0:54:16Astonishing! Astonishing! Just so graceful!
0:54:42 > 0:54:45And it's not just rays.
0:54:45 > 0:54:50The plankton bloom has attracted the world's largest fish.
0:54:50 > 0:54:56This whale shark might have swum thousands of kilometres
0:54:56 > 0:54:59just to feast on this plankton bloom.
0:55:21 > 0:55:27And this great spectacle of life is all thanks to connections
0:55:27 > 0:55:31that stretch back, right across our planet.
0:55:36 > 0:55:41All of the debris of life on Earth ultimately ends up here,
0:55:41 > 0:55:43in the ocean.
0:55:43 > 0:55:46And that's why the marine environment
0:55:46 > 0:55:49is so dependent on healthy terrestrial ecosystems -
0:55:49 > 0:55:55places like the Pantanal wetlands and the mangroves in the Sunderbans.
0:55:55 > 0:55:59That's why the ray needs the snail.
0:55:59 > 0:56:04A giant fish needs a moderately sized mollusc,
0:56:04 > 0:56:06thousands of miles away.
0:56:06 > 0:56:12Unexpected, undeniably complex, but a certainly beautiful connection.
0:56:15 > 0:56:19But this is really only the beginning.
0:56:19 > 0:56:23Because the presence of this plankton affects
0:56:23 > 0:56:28not just life in the ocean, but all life on Earth.
0:56:28 > 0:56:33And that's because plankton blooms are so dramatic,
0:56:33 > 0:56:36they can even affect the weather.
0:56:37 > 0:56:42When the blooms reach their peak, they alter the temperature
0:56:42 > 0:56:43of the ocean surface,
0:56:43 > 0:56:48driving weather systems across the whole planet -
0:56:48 > 0:56:53systems that create rain.
0:57:02 > 0:57:05So, here we are, back at the beginning!
0:57:05 > 0:57:09Some of the water that's evaporated from the oceans,
0:57:09 > 0:57:12is now pouring down on these highlands,
0:57:12 > 0:57:15and beginning its long journey back to the sea.
0:57:15 > 0:57:18It's remarkable to think that this rain,
0:57:18 > 0:57:22falling in this remote corner of the North Atlantic,
0:57:22 > 0:57:26is actually dependent upon the activity
0:57:26 > 0:57:29of microscopic plankton in the sea.
0:57:29 > 0:57:32And that those plankton, in turn, in order to flourish,
0:57:32 > 0:57:35are dependent upon the interconnectedness
0:57:35 > 0:57:39of all of our waterways and the life that lives in them.
0:57:39 > 0:57:43And that is truly remarkable!
0:58:23 > 0:58:27Subtitles by Red Bee Media Ltd