All language subtitles for BBC.Natures.Microworlds.05of13.Okavango.720p.HDTV.x264.AAC.MVGroup.org Eng

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.