Original subtitles

Our planet is the greatest living puzzle in the universe.

A collection of worlds within worlds.

Each one a self-contained ecosystem bursting with life.

But how do they work?

The intricate web of relationships

and the influence of natural forces

makes each microworld complex and unique.

So, to discover their secrets,

we need to explore them one by one,

untangle their interlocking pieces

and, ultimately, reveal the vital piece,

the key to life itself

hidden deep within each of nature's microworlds.

And we may think it's brimming with life from shore to shore,

but, actually, that's not quite true.

In fact, in most of this ocean, life can be hard to come by.

Instead, most marine species are condensed into intense pockets,

crammed along the coasts.

And, in tropical seas, these colourful coastlines

are the most diverse ecosystems in the ocean -

the coral reef.

These coral reefs are found across the world,

but one of them is the master of them all.

It is simply the largest

living structure on Earth.

The Great Barrier Reef.

Of all the reefs on the planet,

how has this one managed to achieve this incredible size?

But scale isn't the only success of this microworld.

The water that this reef basks in

lacks many of the nutrients vital for life.

So such a vibrant ecosystem should have struggled

to get a foothold here.

So how has this enormous place managed to survive?

To understand this,

we first need to find out

what it was that allowed the Great Barrier Reef to get so big.

The reef stretches along the north-eastern coast of Australia

for over 2,000 kilometres -

a distance equivalent to that between Britain and North Africa.

Underwater, it's a marine metropolis

and home to a hustling, bustling whirlwind of life.

But this microworld encompasses more than just a reef.

All along its lengths are lagoons,

islands and seagrass meadows.

On the western boundary is the Australian mainland.

And to the east,

the open Pacific Ocean,

where the outer wall of the Great Barrier Reef

meets the deep blue sea.

In our search for the key factors that make this place so big,

we need to look at some of these habitats in more detail.

And, to begin, the one that dominates all others...

..the underwater world of coral,

home to the majority of species on the Great Barrier Reef.

The fish are perhaps the most conspicuous of all the animals here

and there are some 1,500 species,

each with their own way of making a living.

Clownfish live amongst anemones.

There are more than seven species of these fish

that live along this reef

and each species depends on the presence

of their host anemone to survive.

Spiny damselfish are another reef resident.

And they face the challenges of parenthood in a watery world.

They keep their school of miniature offspring in sight at all times,

only venturing away from the safety of the reef to grab a quick snack

before returning back to the cover of their coral cave.

This diversity of fish is only the beginning

of the Great Barrier Reef story.

This microworld is jam-packed with thousands of other species,

like mantis shrimp

and feather stars.

All these lives play out against a backdrop

of hundreds of coral species,

which can stretch as far as the eye can see.

This huge array of life may be impressive,

but it doesn't set the Great Barrier Reef apart from other reefs.

Amongst all of these players, it isn't the fish,

the invertebrates or even the corals

that explain why this reef is so big.

We need to look further.

We need to visit the other environments

that make up the Great Barrier Reef.

And one of these is seagrass.

Remarkably, seagrass is actually a flowering plant -

the only one adapted to live in the ocean.

And like huge grasslands, such as the Serengeti,

seagrass provide important feeding grounds for herbivores.

More related to the elephant than to any dolphins or whales,

these are herbivores that can hold their breath.

This is a dugong.

Meadows of seagrass stretch right along the Great Barrier Reef

and cover up to 40,000 square kilometres of the sandy bottom.

This huge area allows impressive numbers of dugongs

to congregate in these southern stretches of the coast.

But what part do these vast areas of seagrass play

in the Great Barrier Reef's size?

Well, the seagrass supports massive creatures

and provides additional habitat...

..but this seagrass is just a product of the reef, not its cause.

In fact, the seagrass actually depends on the reef

for the material in which it grows.

Sand.

And some of that sand comes from animal life.

The aptly-named parrotfish is a conspicuous sight

to come across on any reef.

And, by following these fish,

we soon see where some of this sand comes from.

The fish's teeth are fused into a parrot-like beak

that crunches its way through the calcium structure

of dead and living coral alike.

But what seems to be pure destruction has a surprising twist.

The digested bits of coral skeleton

are ground down inside the humphead parrotfish

into a fine calcium powder

and excreted as sand...

..sand that builds up with other bits of calcium rubble

and, with wind and waves, becomes mounds that push up above the water.

The beginnings of islands

that allow other creatures to prosper on the reef.

There are some 600 islands dotted all along this coast.

Oases of life that have been colonised by plants

and become vibrant ecosystems of their own.

Some of the islands in the southern reaches of the reef

have become central to generation after generation of green turtles.

Every year, 38,000 female turtles

come to the sandy beaches of the Great Barrier Reef

to lay their eggs.

This female was born on this same beach over 45 years ago

and she's now returned to lay her own clutch of around 100 eggs.

It might sound like a lot but of each 1,000 eggs laid,

only one turtle might survive to adulthood.

So the reef's protected sandy beaches

provide vital nesting habitat for an ancient and iconic reptile.

But the islands have brought other life to the reef too.

They have become important breeding grounds

for the 20 or so species of seabirds.

And white-capped noddies are here in their thousands.

They produce a single egg

and build their nests among the trees and shrubs

of coral sand islands.

Without the sand that built these islands,

these birds would never be able to nest here in such great numbers.

So sand plays a vital role

in creating the islands and seagrass

that allow a rich diversity of animals to make the reef their home.

But sand is just one of the reasons it's so vast.

The Great Barrier Reef has 2,900 reef systems,

600 tropical islands

and more than 270,000 square kilometres of coral.

To find the ultimate reason for this immense size,

we need to look wider still.

And the answer is right under its feet.

Seen from space,

the reef sticks out from the coast.

It's sitting up on a huge platform.

This is an extension of the continental shelf -

over 2,000 kilometres long

and up to 200 kilometres wide -

that provides the reef

with the perfect shallow-water stage on which to grow.

But there's another reason why it's so massive.

In the northern section of the reef,

a gap in the coral allows a huge current

to pour in from the Coral Sea.

It's the East Australian Current

and it's far from just a trickle.

This movement of water can be up to 100 kilometres wide

and 500 metres deep.

It carries over 30 million cubic metres of water per second.

And the current is a vehicle that carries life.

Combined with wind-driven currents,

planktonic animals are transported for hundreds of kilometres

up and down the reef.

It's a great highway that has spread life

to all its individual parts

and it's joined up this massive ecosystem

to make the longest reef on the planet.

So the Great Barrier Reef's immense size

is a product of the huge shallow-water platform

created by Australia's continental shelf,

combined with the effects of a great current

that spreads colonising life right along the coast.

And it's this unique set of circumstances

that has led to such success.

But survival on this coast is far from guaranteed.

The Great Barrier Reef is an ecosystem in an impoverished ocean.

So how does it actually work?

It's almost unbelievable

but all of this is created by a tiny creature.

An animal just one millimetre across,

which barely looks alive at all.

The coral polyp.

This is a reef-building coral

and it literally manufactured the Great Barrier Reef

from the ground up.

It's made by a creature you can barely see with the naked eye,

and it's created enough coral reef

to cover the entire land area of the United Kingdom.

270 billion square metres.

How does it do it?

Reef-building corals are made up of tiny polyps

and they each live in a little box of calcium carbonate.

This stony structure is called their skeleton

and they deposit it beneath them in order to grow.

Each colourful structure here is a colony

and each has slowly built up their fortified-calcium castle

of all shapes and sizes.

They look like sturdy rock giants

but, remarkably, the living tissue forms a thin veneer on the outside

of sometimes metres of inanimate coral rock.

This tiny layer of life makes corals incredibly fragile

and vulnerable to attack.

The dainty mouths of angelfish might appear harmless

but they actually eat coral.

And, for the coral under attack,

the daytime is best spent hiding indoors.

But, at night, when the reef goes quiet,

the coral polyps emerge,

and, finally, we get to see the animals within.

All across the tropics, stone coral statues come to life.

The coral animals are relatives of the anemone

and are equipped with stinging tentacles

that they use to gather the plankton floating by.

These corals are clearly far from lifeless rocks.

And, on some of these crammed reefs,

these tiny animals will even wage war on their neighbours.

The growing corals send out digesting tentacles

that, over hours, can eat away at their neighbours

in a quest for dominance.

And their thin layer of living tissue is soon stripped back

to their limestone skeleton.

But, for the most part, these corals can grow peacefully.

And, in good conditions,

will deposit new layers of their calcium skeleton

at up to five centimetres per year.

This remarkable process is what lays down

the limestone building blocks of the reef.

Between them, the polyps of the Great Barrier Reef

lay down one billion tonnes of calcium carbonate every year,

creating the foundations

for this awe-inspiring shallow-water ecosystem.

But building their skeletons is demanding

of both energy and nutrients

and the coral animals can't manage such a feat alone.

The crystal-clear water of the Great Barrier Reef

might seem like the stuff of postcards and dreams...

..but this bright blue sea is missing something vital for life.

It lacks the nutrients

that microscopic plants and animals need to grow -

the nitrates and phosphates

that fuel plankton blooms,

which are the base of the food chain.

So, despite feeding every night,

the coral simply can't get enough energy to survive here

from plankton alone.

But deep in its evolutionary history,

coral developed a clever trick to get around this.

The key to this is a remarkable association

between the coral and a single-celled algae,

called zooxanthellae.

These tiny single-celled algae are incorporated into the coral tissues,

at up to five million per square centimetre.

The coral's waste products feed the algae with the nutrients they need.

And, in return, the algae develops inside the coral,

photosynthesising, using sunlight to create oxygen and sugars.

These much-needed sugars are the vital energy

that the coral needs to grow.

So this unique partnership allows coral to thrive

in the nutrient-poor waters along this coast.

But this relationship also sets the rules for this entire ecosystem.

By having an algae within its tissue,

the coral effectively becomes half plant and half animal.

And so, like plants, coral has become a slave to the sun.

It needs the sunlit waters of the tropics to grow.

But these shallow tropical waters

leave the coral exposed

because each Australian summer,

destructive forces move towards the coast.

150kmph winds.

Destructive waves.

Tropical cyclones develop over the warm seas

of the South-Western Pacific

and hit parts of the Great Barrier Reef every year.

Huge waves smash the fragile coral skeletons,

breaking them up into rubble

which tumbles down the reef edge...

..leaving behind a wasteland of coral skeletons.

This coral takes the brunt of the storm

and, in doing so, protects other parts of the reef.

The seagrass is sheltered in protected lagoons

and the rich island ecosystems are kept intact.

The sturdy outer wall of coral protects this coastline

from the wild forces of the Pacific Ocean that roll in every year.

But if life in these shallow waters

is threatened on such a regular basis,

how has this coral reef managed to survive?

The secret to this reef's success

is that it's bigger than any storm.

And to see how this saves the reef from ruin,

we need to take a look at what happens when the dust settles.

This coral graveyard is not an untimely death but an opportunity.

An opportunity for new life.

Not all of the reef was damaged in the storm.

Even in the worst cyclones,

85% of it can escape damage

and new life comes from these undamaged parts each November

when the reef experiences an incredible reproductive event.

In the moonlit night,

the corals of the Great Barrier Reef

start to release eggs and sperm into the water

and it becomes alive.

Tiny coral nomads take shape.

But they don't just settle on the reef where they were released.

This is where the ocean current comes into play.

It carries the larvae of coral, fish and invertebrates

all along the reef.

This allows bits of damaged reef to be re-colonised with life.

A single coral polyp.

The beginning of a new colony.

And a renewed stretch of reef.

The new life created by the annual coral spawn

helps rebuild damaged parts of the reef...

..allowing them to flourish once more.

This process repairs the ramparts

and can help replenish the reef edge

before the next cyclone arrives.

It might seem like a pointless reconstruction

but the re-growth of coral on a bedrock of dead calcium skeleton

is vitally important to the continuation of life here.

And, to see just why this is,

we need to look back in time.

Just some 10,000 years ago,

this whole stretch of coastline was completely different.

Since the end of the last ice age,

sea levels have been rising

and as they rise,

flooded land gives way to shallow seas.

The sea surface was about 100 metres lower than it is today

and this shallow-water ecosystem used to be on dry land.

And the sea hasn't stopped there.

It continues to rise

and threatens to flood the reef too,

leaving it in the dark depths of the seabed.

This could have spelled the end for the Great Barrier Reef

but not yet.

In the same way that it recovers from cyclones,

the reef builds up on itself,

layer by layer.

The dead coral skeletons of the past are cemented together

by colourful coralline algae.

A living mortar that binds the calcium reef together.

From here, new life is brought on the currents to colonise

and the reef spreads ever upwards.

Keeping up pace with the rising seas

and creating the shallow-water ecosystem that we see today.

This exceptional process not only ensures

that the coral stays near the surface

so that its algae can photosynthesise

but, in doing so,

it allows this whole shallow-water ecosystem to exist.

The Great Barrier Reef is more than just coral and fish.

It's a network of thousands of islands, reefs and other habitats

that allow animals both large and small to prosper.

The reef is remarkably resilient.

Not only can it withstand some of the most ferocious forces of nature

but it has also managed to exist in nutrient-poor waters

that could otherwise be lifeless.

But the reason this reef has got bigger than any other on Earth

is the massive shallow-water platform

of the Australian continental shelf,

the great current that runs right along the coast

and the simple partnership between a one-millimetre coral

and a single-celled algae.

These are the secrets of the Great Barrier Reef.