All language subtitles for BBC.Natures.Microworlds.02of13.Serengeti.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 Serengeti -

without doubt one of the most famous
wildlife destinations on Earth.

It's a vast grassland, covering
over 23,000 square kilometres,

which is pretty much the same area
as Wales.

THUNDERCLAP

It's a landscape exposed
to unpredictable rains...

..fires...

even occasional droughts.

But this microworld supports
the greatest density of herbivores

found anywhere in the world...

..grass-eaters
of all shapes and sizes.

While they rely on the vegetation,
they themselves are essential

for the survival of some of the
world's most magnificent predators.

The big cats...

..supreme athletes whose hunting
abilities have been honed

over millennia out here
on the open plains.

But despite their predatory prowess,
not every hunt leads to a kill.

This place, with these
daily life and death battles,

seems as old as time itself,

but a million years ago, this whole
area was completely different.

The Serengeti
was once covered with woodland.

The grasslands we see today
were formed just 100,000 years ago.

Sitting between a chain of volcanoes
along its eastern border

and the shores of Lake Victoria
to the west,

this microworld is a comparatively
new, yet complicated, ecosystem.

Untangling the web
of its many working parts

is not going to be easy.

We'll need to explore the connections
and relationships between animals

and also between them
and the world they live in.

Because by doing so, we can
hopefully discover the single key

that unlocks
the entire Serengeti ecosystem.

In terms of spectacle and scale,

there's one species that eclipses all
others on the Serengeti grasslands.

Wildebeest.

There are an estimated 1.5 million
making a living here,

and evidence suggests
their numbers are increasing.

It's one of the largest congregations

of a single species of mammal
found anywhere on Earth.

Wildebeest is an Afrikaans name
meaning "wild cattle",

but, actually, these bizarre-looking
creatures are antelope -

not cows at all.

The herds are so massive
they have to constantly move around

the entire Serengeti system,

driven by the need for food.

Such a vast concentration
of grazing animals

has an enormous impact
on the environment.

But they themselves are
a vital part of the Serengeti story -

a living larder
for Africa's killer carnivores.

WILDEBEEST LOW

Lions are the largest predators
in Africa,

and out here, they're
a constant threat to the herds.

Lions are special.

Unlike other feline predators,
they're social,

hunting together cooperatively
as a pride.

It's believed that this social
behaviour evolved so that together,

in a team, they could hunt
mega-herbivores like buffalo.

Living in a pride
has other advantages too.

The females, usually sisters,

are able to help look after
each others' cubs.

So this social behaviour also means
they can raise more offspring.

But more young means
more hungry mouths to feed.

The spoils of the hunt
will have to be shared.

But as an adult wildebeest
weighs as much as a lioness,

there should be enough to go around.

With so much prey available,
the Serengeti is home

to some of the largest prides of lion
found anywhere in the world.

They make for
an imposing, intimidating sight,

strutting the Serengeti...

..with characteristic swagger.

But they don't get things
entirely their own way.

These are spotted hyena.

They're stocky predators

with perhaps the most potent
bite force of any mammal.

Though they're fearsome hunters,

they're better known
as being scavengers and thieves.

LIONESS GROWLS

And they're not beyond
taking on a pride of lion,

particularly if they're fat and full.

The hyenas potent stomach acids
are capable of dissolving even bones,

so they clear up
what the lions leave behind.

Predators like lion and hyena

play a valuable role
in balancing the ecosystem.

Without them,
the herd could decimate the plains,

leaving little food
for other foragers...

..like elephants.

The largest land mammal
has a vast appetite

and can be extremely destructive.

Elephants are one of only
a few animals that have the ability

to fundamentally alter
their surroundings.

This leviathan beast is a powerful
force in the Serengeti's ecosystem.

But the working parts
to any microworld

are not just the creatures
that live within it.

Other factors also influence
these self-contained systems.

The weather, water,
natural disasters, disease,

even the very bedrock beneath,
all play their part.

And in the Serengeti,

there's one invisible factor
with an enormous influence -

the equator.

The close proximity of the equator

has a variety of effects
on a variety of worlds.

In the Serengeti,
it plays havoc with the weather.

The rainfall not only varies
drastically year-on-year,

but also from the wetter North
to the drier South.

The more rain,
the better the grazing.

This unpredictability
with the weather

has a tangible effect
on the wildebeest.

THUNDER RUMBLES

They follow the rain clouds
like storm chasers.

This ensures
there's always enough food

to support one and a half million
stomachs.

But there are even more
subtle connections between rain

and life on the plains.

Even the lion are affected.

It's been shown
that their success rate in hunting

is not just determined by the number
of prey animals that are present,

but by the amount of cover there is.

The longer grass,
produced by plenty of rain,

gives the lions a great advantage.

The very grass the lions need to
cover them as they stalk their prey

is the same grass that attracts
animals here in the first place.

The more food the lions secure,
the more cubs they can have.

So the lions' breeding success
is not just dependent on the herds,

but on the long grass
fed by the rains.

Of course, the grass
has a much more direct connection

with the herbivores that live here.

It's the food source that
supports the vast wildebeest herds.

But it's not just them.

With 28 species of herbivores,

the Serengeti supports the greatest
diversity found anywhere in Africa.

The herbivores may appear to be in
direct competition with one another,

all trying to eat
as much as they can,

but on closer inspection,

the grass-eaters all fill
an ever so slightly different role.

Elephants tear out trunkfuls
of long grass.

Wide-faced wildebeest and zebra
crop the grass like lawn mowers.

And behind them,
the smaller Thomson's gazelle,

with their small, pointed faces,

clip stalks ever shorter.

With all these animals eating grass,
it's remarkable there's any left.

If the grasses were to recede,

the trees would return
to take over the plains.

But these trees
are also subject to attack.

On the western edge
of the Serengeti plain,

there are a few hardy acacia trees.

They're resilient trees with
inch-long spines to deter browsers,

but the animals have learnt
to get around these defences.

At almost ground level, the dik-dik,
one of Africa's smallest antelopes,

picks at the tiny leaves.

With a small, narrow face,

it daintily plucks between
the lethal-looking spines.

Impala also have narrow faces

that allow them to feed
from the higher branches.

But there's another
similar-sized antelope

which employs a different trick
so as not to compete with the impala.

The gerenuk
has a specially adapted pelvis,

which allows it to stand straight up.

So they're able to get to the leaves
that are out of reach to the impala.

But none of these can compete
with nature's leggy champion -

the giraffe.

With a long leathery tongue
and thick rubbery lips,

they browse the untouchable leaves
at the top of the trees,

seemingly impervious
to the trees' spiny defences.

So the trees are under attack
from every level,

with none of the species
actually competing directly

with one another for food.

This niche feeding is what allows

such a vast array of life
to succeed here.

How important
the grass is to this system

becomes most evident
when it disappears

from over-grazing and lack of rain.

The migrating herds move on,
following the rains,

leaving behind in their shadow
drought.

For the resident territorial lion,
unable to migrate with the herds,

these are desperate times.

But it's these harsh conditions
that keep the lion numbers

on the Serengeti under control.

Only the fittest will survive.

Without cover, food is hard to come
by and it's about to get worse.

Fire.

But even this has a valuable part
to play in this microworld.

In other parts of the world,
grasslands are found on soils

that do not have enough nutrients
to support trees.

The Serengeti, though, is different.

The rich soils here
can support trees, and once did.

So where are they now?

This was a great puzzle to scientists

when they first started studying
the Serengeti.

Do these fires keep the trees at bay?

FIRE CRACKLES

The answer to that lies in how the
fires are started in the first place.

Unlike many other parks in Africa,

where as much as 50% of all fires
are caused by lightning strikes,

the fires in the Serengeti
are mostly down to another factor...

..people.

To create grazing for their cattle,
they set fires to burn down the trees

and allow the fast-growing grasses
to become established.

Grass grows from its roots,
not just its tips,

so even with the surface
completely burnt away,

new prolific growth
can begin underground.

People's deliberate modification
of the landscape through fire

has had massive effects.

But it's not the only,
or the most important, factor

in the Serengeti we see today.

The wildebeest herd
certainly play their part.

They keep the grass so short that
there's less fuel for fire to spread.

Fire plays an active role in keeping
the trees at bay in some areas,

but there must be something else
that's managing the tree population.

Elephants decimate the few trees
there are, especially in droughts.

In one scientific study,

six male elephants entered a stand
of acacia trees covering an area

of less than a square mile.

And within 24 hours,

34% of the trees were destroyed and
a further 22% damaged beyond repair.

Six elephants, one day,

one square mile and 50% destruction.

It's a significant amount of damage,

and it's not just adult trees.

Elephants target and eat young
saplings as they grow in the grass,

kind of like gardeners
weeding their borders.

But fire and elephants are not the
reason grasses reign on the plain.

That's down to something
even bigger than an elephant.

What makes the Serengeti so special

is down to what lies
on the eastern border -

volcanoes.

When this region was a dry woodland
half a million years ago,

it had yet to feel the full force
of the Earth's fiery power.

First came eruptions from Kerimasi,
which is now extinct.

But it was followed
by Ol Doinyo Lengai,

a volcano that first came to life
half a million years ago

and continues to regularly erupt.

These volcanoes spew out hot lava,
ash and burning gases

from the Earth's core.

The existing woodland didn't stand
a chance and burnt to the ground.

In their place, fast-growing grasses
sprang forth and flourished.

Over time, more and more layers of
nutrient-rich volcanic ash piled up.

They created a very unusual soil.

The combination of ash and rain
has created together something unique

just below the surface
of the Serengeti...

..a thick impenetrable crust that's
less than a metre below the surface

called the hardpan.

Made from calcium carbonate, the same
mineral as found in cave stalactites,

it's as hard as concrete and,
like a protective underground layer,

it covers the eastern grasslands.

This hardpan is just deep enough
below the surface for grasses

and shallow roots to thrive,

but too shallow to allow tree roots

to reach the water table
deep underground.

So, like an underground fence,
it keeps out the trees,

leaving the Serengeti's grasses
to flourish unhindered.

So this hard underground pan holds
the key to the inner-workings

of the Serengeti's system.

But this pivotal piece
has another vital role to play

in this microworld...

..one that will make the wildebeest
risk their very lives.

Every year,
the wildebeest migration swings back

to these eastern grasslands,

even though lethal obstacles
lie in their path.

African rivers hide a real danger -

Nile crocodiles.

These huge reptiles can measure
nearly six metres in length.

Crocodiles, like the wildebeest,
are creatures of habit,

and they know the herd is coming.

The tired and thirsty migrating herds

are desperate to reach the rich
grasslands on the eastern side.

They're willing to face danger
just to get a drink.

But there is method
to this seeming madness.

They have to cross the river
regardless of the danger,

because what lies on the other side

is lush pasture that holds
the key to their survival.

In their numbers,
the herd find a kind of safety.

The predators
are hopelessly outnumbered.

For every one or two dragged down
to their doom,

a thousand cross unhindered.

In a herd of over
a million individuals,

comparatively few will perish.

It's as if the few that are taken

are sacrificed for the greater good
of the herd.

And in the land of plenty
on the other side of the waters,

the wildebeest have sufficient bounty
to give birth to their young.

The hardpan has created
not just rich fertile grassland

almost devoid of trees,

but it's also rich in nutrients,
especially phosphates.

Mammals are distinct
from other groups of animals

as they suckle their young with milk.

And to produce milk,
mammals need phosphates -

lots of them.

And these eastern plains
are loaded with it.

So the wildebeest come here
in vast numbers.

They risk crocodiles,
lion and hyena to give their young

the very best start in life.

The female wildebeest eat
the phosphate-rich grasses

and in turn produce
the richest and best milk

for their growing offspring.

These vital phosphates get passed on

through the food chain
to their young,

and then on to the predators.

CUB SQUEAKS

Life here seems, at times,
almost too easy.

These minerals could ultimately
be said to be the reason why

these plains support
some of the largest

and most diverse concentrations

of large mammals found
anywhere in the world.

So in a world within a world,
where everything plays its role

and life seems
completely interconnected,

the pivotal piece lies out of sight

just below the surface. The hardpan,

rich in vital nutrients
that feed the grass,

which then feed the herds,
who in turn feed the predators.

And the results are plain to see -

a thriving, bustling microworld

packed with life.

The greatest and most famous
grassland on our planet -

the Serengeti.