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.

The Okavango Delta, Southern Africa.

A vast wetland supporting a huge diversity of life,

with over 1,000 species of plants

and more than 400 types of bird.

It's a magnet for wildlife,

home to one of the biggest congregations

of large animals in Africa.

Fed by the Okavango river,

this incredible, lush microworld is actually an oasis.

A vast wetland in the very heart of the Kalahari desert.

And what an oasis.

At 22,000 square kilometres,

the Okavango is the world's largest inland delta.

A normal delta is formed where the river flows out to sea,

but here the river flows into the middle of the African continent,

creating an immense wetland, that can be seen from space.

Inland deltas are extremely unusual,

so just how does the Okavango function?

To understand this and how it can support all this life,

we have to take a look at it through the year.

Because for at least three months of the year,

this vast wetland transforms back into the desert that surrounds it,

making life here formidably harsh.

There is water in the desert even now,

but it's confined to a few rapidly shrinking pools.

So the animals of the delta,

seemingly trapped in this vast dry arena, have to travel long distances

to get to these remaining waterholes.

These become packed with the unlikeliest of drinking partners.

Elephants and lions.

The lions are wary.

An adult elephant could easily kill one of them.

But the elephants are also wary -

lions have been known to kill young elephants,

and they have a baby with them.

But this tense stand-off is just about getting a drink,

no matter how disgusting the water is.

The elephants have an advantage.

They have a trick to get at the better water.

They use their trunks to siphon off the cleanest water at the surface.

Regardless of the dangers, elephants have to drink every four days.

At the height of the dry season, the water is barely drinkable,

but remarkably, it still supports some life.

Trapped in these fetid, oxygen-starved pools,

catfish are survivors.

Able to gulp air from the atmosphere,

they can live in wet mud,

but this adaptation doesn't protect them from predators.

African fish eagles and Marabou storks are quick to take advantage.

The catfish are attacked from all sides.

But they are not the only ones struggling to survive out here.

It's January and all the animals far out in the desert

are searching for water, but our microworld is on the cusp of change,

because 1,000 kilometres away to the north, it's raining.

The mountains of Angola were formed millions of years ago

by the tectonic forces of the Earth's crust.

In January and February, clouds condense on their high slopes

and form rain, lots of it.

This huge deluge pours out of the highlands,

and slowly starts making its way towards our microworld.

The first place to see the benefits is the Okavango river.

Fresh water pours in and the water levels rise.

Species like the hippo, that need to be able to completely submerge

in water to survive, have retreated to these deep pools

during the worst of the dry season.

Soon they will be moving out into the seasonal swamps

when the delta floods.

But not quite yet,

it takes three months for the rain that has poured off the highlands

to finally reach the delta, 1,000 kilometres away.

What this means for the wildlife in our microworld

is that they just have to carry on as best they can,

unaware that help is on its way.

And conditions are creating conflicts.

Bull elephants get aggressive

when they are forced to share the same resources.

Buffalo will have to wait their turn,

or risk becoming the target of the elephants' frustrations.

By early March,

the Angolan rain has filled the Okavango river to overflow

and water has started pouring into the delta.

It moves painfully slowly over the sandy ground,

gradually filling up the bone-dry channels.

But it will be at least another three months

before the delta is completely transformed.

And during this time there's one species that will benefit

from the last of the dry conditions.

Lions.

The long dry grass affords them cover,

the firm sandy soil gives them purchase.

These are the lions' good times.

But the lions need to be careful who they pick on -

a kick from a buffalo could cause serious injury.

The weak and the old are an easier target for the lions.

It might seem that this place offers the lions all they could need,

but the tables will turn with the swing of the season.

As the water creeps slowly into the delta,

salvation has finally arrived for the last hardy catfish.

The lifeblood of the delta reaches them at last.

Since falling in the Angolan highlands,

it has taken nearly three months

for the water to travel 1,000 kilometres towards our microworld,

and each year 10 trillion litres of water flows into the delta.

Of course, this sheer volume of water

plays its part in the size of the delta,

but there's another factor that allows it to function

on such a huge scale.

The delta is almost completely flat.

On average, it only slopes by a metre every 3.5 kilometres.

Enough to create flow, but also create spread.

So with the ground at roughly the same level, the water fans out,

spreading across the entire Okavango basin.

And life follows the waters as it overflows into the desert.

Fish that have spent the dryer times in the main river

can expand their territories and spawn.

And the catfish can move with the water to find new areas to colonise.

The herbivores that have risked starvation and thirst

can now drink and feed again.

And for the elephants, the wait is over.

Now they relish the chance to play and swim

in the deep, newly filled pools.

However, the rain pouring off the Angolan highlands

and the shallow gradient are not the only contributing factors

to the massive scale of the Okavango delta.

There is one other pivotal piece to this puzzle.

Tectonic activity, that helped lift the Angolan highlands,

the source of the flood,

has also had a much more direct impact on the delta itself.

The Okavango sits in a geological depression.

The land that the delta sits on

has dropped between two faults in the Earth's crust,

creating the depression that the Okavango river pours into.

The fault at the other end of the Okavango

creates a natural dam across the southern end of the delta.

It is this that backs the water up,

greatly increasing the habitat in size

and creating the Okavango delta.

It's an oasis in the middle of the desert,

capable of supporting a vast and surprising array of life,

including species that you wouldn't expect to see

in the middle of a desert.

Like pelicans!

Countless water birds fly for thousands of kilometres

to take advantage of the seasonal swamps.

They know that the flood will provide them with a bounty

and they take advantage of these good times to breed,

filling the trees with nests and young.

It seems that nearly every space available fills up with birds.

The seasonal flood has a huge effect on our microworld,

but it doesn't last.

So just where does 10 trillion litres of water disappear to?

Incredibly, virtually all the water is lost to the atmosphere

through the plants and evaporation.

But this microworld has evolved over millennia,

long enough to allow life to adapt.

The wet times more than compensate for the dry,

because grasses can survive long periods of drought

and spring back into life as soon as they get wet.

The grasses cover the plains in an incredible green flush.

Their secret is that they grow from the base.

Even though they face an onslaught from the grazers,

this feature allows them to keep on growing

and provide huge amounts of food.

Enough to feed a collection of large mammals on a massive scale.

Now hippos that have been stuck in their deep water pools

through the dry season can find new territories,

the females congregating in groups.

Which leads to battles between males competing for mating rites.

Hippos can weigh over 3,000 kilos

and have forward-pointing tusks in their mouths,

so fights can be brutal, even deadly.

But these fights aren't just about females.

They are also about territory.

Although hippos rely on water, they feed on land.

An adult hippo eats around 40 kilos of grass a day,

maintaining hippo lawns that they feed on.

They're creatures of habit,

visiting the same feeding grounds along the same paths day after day.

But all this water doesn't benefit everyone.

The lions find it hard to hunt in the water of the swamps

as they can't creep up easily on their prey.

Red lechwe are swamp specialists.

They move out of the permanent swamps every year with the floods.

With long splayed hooves, they can run through the wetland easily.

The lions need to try and hunt on dry land,

where they can use the long grass for cover.

Now the water has turned the tide in favour of the buffalo

and they have young to look after.

At the first sniff of danger

the herd heads for the safety of deep water.

They can move a lot more easily in the water than the lions

and leave them in their wake.

The vast expanses of water hinder some residents in our microworld

and benefit others, but at least there's plenty of food.

And even if you don't like getting wet, like these baboons,

the feast here is a bit too appealing to miss.

The physical processes that have created the Okavango

are remarkable, but they are not the whole story.

Similar inland deltas tend to end in lifeless salty lakes,

devoid of the kind of richness seen here.

So what is it that makes the Okavango delta so different?

When trying to find an answer to that,

scientists noticed something remarkable -

this microworld is constantly changing.

Each year it floods in a slightly different way from the last.

And over time, some water courses dry up, leaving areas to dry out,

while new areas flood.

Eventually, these dry areas revert to grassland and end up providing

more food and habitat for the animals that live here.

But that is not all -

the scientists discovered that this cycle of grassland and swamp

has a crucial effect on the delta as a whole.

It stops any area from becoming stagnant,

constantly keeping the channels on the move

and keeping the water flowing and clean.

Without this process, the delta would become a saline lake,

unable to support such a huge diversity of wildlife.

Now here is the really crazy thing about this unique microworld.

The process of change is being driven by just two key players.

The first is the plants.

By the way they grow,

the plants actually manage the environment that they live in.

The plants that line the channels stabilise the banks,

stopping the sand from getting washed away.

But in areas where the flow of water is slower, the plants build up,

their roots mat together, making it harder for the water to flow.

Water will always seek the path of least resistance

so it flows away in a different direction.

So the plants help spread the water out across the delta.

They not only maintain the banks, but they also close off

slow-flowing waterways before they become stagnant.

And the sheer density of plants acts like a sponge,

slowing the flow of water and helping divert it outwards

across the delta,

thus perpetuating the constant change within our microworld,

and stopping it becoming a salty pan.

The plants are not the only ones

that affect the water's distribution.

When the land is so flat, it is not always obvious

what the path of least resistance might be.

All around, the ground is virtually exactly the same level.

Other than areas that have already been eroded, that is,

and this leads us to the incredible twist in this tale.

Hippos.

These massive lawnmowers are the other key factor

in the management of this microworld.

So key that the indigenous people of the Okavango

ascribe the origin of a major waterway to the hippos alone.

When moving about between their deep water wallows

and their feeding grounds, hippos act like four-legged trail blazers,

creating holes in the vegetation along the edge of waterways.

These paths allow water to flow to other parts of the swamps,

and therefore distribute it to new areas of the delta.

In doing so, they contribute

to the ever-changing flow of water in this microworld.

They also bulldoze clear thoroughfares in deeper water too.

Their constant use means that the vegetation doesn't get a chance

to grow inwards and block the channel.

These underwater highways play an important role

for the other creatures too, allowing them to get around

using clear, plant-free underwater paths.

But they don't stop there.

Probably the most important bit of hippo-engineering

happens when the waters reach their highest.

Their paths to feeding grounds become well-trodden walkways.

In years of lots of rain, these paths offer ready-made canals,

to transport the deluge to parts of the Okavango

that were previously dry,

diverting water away from other areas,

allowing them to become grassland again.

So the hippos are the architects of change in our microworld,

building new canals and maintaining the old ones,

perpetually shaping and reshaping the entire Okavango,

preventing it from becoming a stagnant, lifeless salt lake.

So the world's biggest inland delta

provides us with a unique look at how intricate and complex

the interlocking pieces there are that make up a microworld,

but unlike many others, not one piece holds the key.

Everything plays its role.

The water that pours into the desert each year.

The shallow pan, created by tectonic processes,

that also lifted the rain generators,

the Angolan highlands to the north.

The constant cycle of change that stops the water stagnating

and generates new habitats.

And the real surprise is that two key living inhabitants

of our microworld also play a key role in orchestrating this change.

By manipulating the way the water flows,

the plants and the hippos keep the wetlands alive and flourishing,

and in doing so, they enable this microworld to support

one of Africa's greatest congregations of wildlife,

creating the world's greatest inland delta,

a glittering oasis in the middle of a desert

that truly is a world within a world.

The Okavango.