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 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.