BBC How Nature Works: Waterworlds

1O million species live on Planet Earth.

But none can survive on its own.

All life depends on connections.

Often hidden and unexpected relationships between millions of plants and animals.

They form great communities of life.

This time,

why this tiger needs a crab to survive.

Why this giant otter needs a snail.

And why this shark needs a sponge.

Connections like these power life in our Watervvorlds.

##HOW NATURE WORKS

One of the most volatile and powerful environments on the planet.

Vital ingredients for all life on earth begin in these raging torrents.

Millions of gallons of water relentlessly carve through rock and soil

grinding away, picking up a precious cargo.

Sediment

and oxygen.

Together, these are potent ingredients.

In these fast flowing waters just a few specially adapted creatures can survive.

But when the raging torrent slows down this cargo fuels a transformation.

Nowhere shows this more dramatically than here in South America.

Water from the Brazilian plateau floods into a vast lowland bowl the size of California.

To create the greatest wetland on Earth,

the Pantanal.

This is home to the largest cat in the Americas,

the Pantanal Jaguar.

Weighing in at over 160kgs twice as heavy as an average man.

These famously elusive cats are so numerous here

that people travel from around the world just to see them.

But other predators also thrive in this wetland.

The biggest snake in the world, the green anaconda.

It can grow up to 9 metres long.

And around every corner lurks a caiman.

10 million caimans.

The largest concentration of crocodilians anywhere on earth.

And all these deadly predators hunt the largest rodent in the world,

the Capybara.

One hundred times heavier than a guinea pig,

the Capybara is the dominant plant eater in the Pantanal.

They gather in huge numbers.

Everything here is big.

This is the land of the giants.

For all this life, one creature rules supreme,

the apex predator of the river,

the giant otter.

Up to 2 metres long and capable of killing a man,

the giant otter is one of the deadliest predators of the Pantanal.

This land of the giants exists, thanks to the cargo that mountain streams carry.

Oxygen and sediment packed with nitrogen and phosphorous.

When it reaches the Pantanal, the water slows down and finally releases this valuable cargo.

Gradually, this sediment has built up year on year,

over millions of years, to create this vast, fertile, paradise.

But the river only brings the ingredients.

For life to flourish the Pantanal depends on the contribution of one animal in particular.

But it's not one of the big giants.

To find it, we have to look beneath the surface.

The apple snail.

Out of the sight of predators he finds safety on the riverbed.

Like a fish he has a gill that can breathe dissolved oxygen in the water.

And here he finds plenty of food.

His teeth saw through rotting flesh and tough plants,

and his gut can break down cellulose,

the tough parts of plants indigestible to other animals.

After so much eating he relieves himself.

This is when the magic happens.

He has recycled all that dead matter back into nitrogen and phosphorous.

Pure fertiliser that all the plants here need in order to grow fast and big.

And no plant here is as big as the giant water lily.

It has up to 40 leaves which can grow up to 3 metres wide.

Each one acts as a giant solar panel.

They gather energy from the sun to power photosynthesis in order for the plant to grow.

The giant water lily not only grows big but it has energy to burn.

At night it opens its flowers and does something extraordinary.

As the thermal camera reveals this flower heats itself like a furnace.

Burning sugar raises its temperature 10 degrees higher than its surroundings.

Using this heat it's radiating its scent in the night air.

And attracting Scarab beetles.

Inside they warm up and feed on the sugars.

In return these beetles pollinate the flower and help it to reproduce.

The rich bounty of plants in the Pantanal all benefit from the snails constant feeding and recycling.

But the snail's banquet comes with a price.

It's August, and the annual drought

transforms the wetland into a vast grassy plain dotted with small muddy pools.

The snails are left high and dry, exposed and tasty.

They make a meal for millions of predators, especially caimans.

The calcium in the snails shell helps the caimans to thicken their skins.

Being on the menu is bad enough,

but to continue their vital role of recycling,

the surviving snails have to overcome a much bigger challenge.

One which puts the whole ecosystem of the Pantanal on a knife edge.

Heavy rains return to the highlands of South America.

The swollen rivers burst their banks and once again flood across the grassy plains.

Surprisingly, this is the most dangerous time for the ecosystem.

Millions of tons of swamped grass begin to rot,

rapidly using up the most important ingredient for life,

oxygen.

The Pantanal is suffocating.

It needs the snail to recycle the dead grass and unlock the nutrients.

But without oxygen, the snail can't survive to do its job.

Thankfully, the snail has a secret weapon.

Uniquely, apple snails possess a specially adapted, telescopic appendage.

Which can extend to eight centimetres.

A snorkel!

He's pumping air from the surface, straight to a special lung.

Now he has enough oxygen to return to what he does best,

recycling dead matter.

Out of sight, oxygen or no oxygen the constant feeding of millions of apple snails

keeps the Pantanal alive and rich with nutrients,

no matter what the vast wetland throws at it.

Young aquatic plants flourish pumping oxygen back into the water.

Millions of fish arrive to spawn.

Enough food to satisfy the king of the river.

The giant otter.

These constantly screeching calls

warn all other animals that they own the river and all the fish in it.

Any rival is met with aggression.

This family is patrolling a 15 kilometre stretch of the river.

Nothing frightens them, not even a caiman.

These giant otters hunt large prey co-operatively.

And even caiman is on the menu.

But to an otter nothing tastes better than fish.

Their appetites are insatiable.

Each otter needs to eat a tenth of its bodyweight, every day.

Luckily in the Pantanal, there's plenty for everyone,

thanks to a snail that keeps on eating and recycling

no matter what this vast every-changing wetland throws at it.

All over the planet, from the Florida Everglades to the Fens of Britain,

to the Nile,

freshwater snails and other wetland creatures sustain watery ecosystems.

And the importance of wetlands to the planet is huge.

At one end, the water from mountain streams slows down

and millions of plants soak it up like a sponge.

At the other end, water is released as a steady flow.

Ensuring that the rivers and their inhabitants further downstream

get a reliable supply of life-giving water, all year round.

But as the river reaches the coast...

Here it creates an environment so inhospitable that in order to survive

animals and plants have had to evolve some spectacular solutions.

The end of the mighty river Ganges.

This is one of the biggest swamps on earth,

the Sundarbans on the coast of Bangladesh.

The biggest challenge life here faces is the constantly changing environment.

Twice a day, with the rise and fall of the tide,

the whole place floods.

Precious nutrients like these leaves wash out to sea.

And worst of all, salty water, toxic to most plants, drenches everything.

The tide retreats leaving behind thick mud, so thick that not even air can penetrate it.

Sulphur breathing bacteria thrive here.

They're pumping out toxic hydrogen sulphide

making the air thick with the stench of rotten eggs.

But the Sundarbans is not the Hell on Earth that it might seem.

The mud ripples with the movement of strange looking fish, air-breathing mudskippers.

250 species of bird flourish here.

Black-capped kingfisher,

striated heron,

brahmany kite.

And a large fishing industry finds plenty to catch.

Herds of large mammals roam the forests,

30 thousand chital deer

and thousands of macaque monkeys.

All these animals bring this muddy swamp to life.

And provide enough prey for one of the world's most elusive and dangerous predators.

The Royal Bengal Tiger.

Scientists are only now discovering that as many as one fifth of the worlds tiger population live here.

For the Sundarbans to support so many predators the plants and animals that live here

have some unique solutions to the problems of living in a swamp.

It all starts with a very strange tree.

The mangrove.

In this thick mud most plants would quickly suffocate,

but the mangrove has a spectacular solution.