BBC How Nature Works: Grasslands

10 million species live on planet earth.

But none can survive alone.

All life depends on connections.

Unexpected and complex relationships between millions of plants and animals.

This time...

grasslands.

Where this monkey depends on the giraffe.

And bizarre connections allow some of the most extraordinary animals to thrive against all odds.

Grasslands cover a quarter of the land on earth.

The Prairies of North America.

The Steppes of Asia.

And the savannah of Africa.

Life here must fight to survive.

The lioness must eat large amounts of meat to produce enough milk to feed her cubs.

This milk provides them with vital energy and vitamins.

But it also contains another essential ingredient.

Protein.

The cubs can't grow without it.

They need protein to make every structure in their bodies, new muscle, bone, and hair.

Their mother needs it to maintain and repair her tissues.

She gets protein from the animals she consumes which in turn get it from the grass they eat.

Making these proteins requires an essential element.

Nitrogen.

Without this nutrient, life would be impossible.

But grasslands run critically short of this vital ingredient.

And competition for nitrogen is more intense in grasslands

than almost any other terrestrial ecosystem.

Yet they support some of the largest animals on the planet.

The story of how these top predators survive here

starts at the bottom of the food chain with the grass itself.

Grass may look simple but don't be deceived.

It's a very special plant.

It can cope with a nitrogen shortage that other plants simply can't tolerate because of its structure.

Grass roots grow up to 3 metres long delving deep to gather up all the nitrogen compounds they can.

And grasses can produce leaves with little nitrogen.

This allows it to survive, providing food for the next level,

the plant eaters.

And Africa alone has millions of them.

But the grass itself doesn't contain much nitrogen,

so grazing animals have a challenge,

to squeeze enough nitrogen out of the grass.

Impala are one of Africa's commonest antelopes and eat mostly grass.

They survive this nutrient poor diet with a special strategy.

She swallows the grass, just as you'd expect,

but watch what happens next.

She regurgitates a ball of partly digested grass and processes it all over again.

It's called chewing the cud.

By digesting grass not once but twice they can extract every last drop of nitrogen.

Wildebeest chew the cud too,

but have another strategy to find nitrogen

that gives rise to one of the greatest wildlife spectacles on earth.

Every year, one and a half million wildebeest migrate 1500 kilometres across east Africa,

in search of the best grass.

Because some grass is richer in nitrogen.

As grass gets longer, the nitrogen it holds becomes diluted.

Eventually the grass holds so little nitrogen per gram it becomes useless to the wildebeest.

The strategy of staying constantly on the move means the wildebeest find the very best grass.

Having done the hard work, obtaining nitrogen from across the grassland,

each wildebeest forms one concentrated package of protein.

And now the predators can take advantage.

The cheetah waits for the wildebeest to start to run before she charges

because the momentum of moving prey makes them easier to knock over.

The mother will have to hunt for her cubs until they're at least a year old.

Even so, only 5 percent of cheetah cubs avoid starvation and predation to make it to adulthood.

Competition for precious protein is fierce, so they must eat fast,

before the vultures arrive in enough numbers to bully them off the kill.

4 cheetahs can devour a prey animal of this size in just 15 minutes.

But cheetahs aren't strong enough to hide or guard their catch,

so eventually the vultures get their turn.

The wildebeest and most of the cheetahs other prey feed in comparatively high nitrogen grass areas.

And the cheetah follow their prey.

But it's not just the best grass that supports Africa's famous wildlife.

Even in the lowest nitrogen areas, big animals like this white rhino still manage to thrive.

She achieves this with a different strategy that actually relies on her size.

From head to rump she measures about 3 metres and weighs the same as a truck.

This makes her one of the world's biggest animals.

Her huge body houses an enormous fermentation chamber.

This allows her to compensate for the low quality of the grass by eating a lot of it

and allowing it to ferment slowly.

In fact she spends 12 hours a day, solidly eating.

But this gentle giant also provides a food supply for other animals.

Red-billed oxpeckers hitch a ride, while feeding on her ticks.

And cattle egrets follow the rhino across the grassland,

feasting on the insects flushed out by its huge feet.

Remarkably, rhinos also help other grazers by creating nutrient hotspots.

When rhinos defecate and urinate, they concentrate a lot of nitrogen in one spot.

Here new nitrogen-rich grass can sprout.

And cropping the long grass down encourages the growth of shorter grass species.

Now that it's more nutritious, the fussy impala can come here to feed.

The rhinos have changed areas of poor quality grass into nitrogen-rich meadows,

which makes them important ecosystem engineers.

But in some areas, no such meadows can be found.

Brazil.

The grasslands here in Emas National Park are very low in nitrogen.

And no large grazing herds like bison or wildebeest exist.

So the top predator in these grasslands

has developed different survival tactics to those of lions or cheetahs.

The maned wolf.

Its long legs help it see over tall grass,

and its large ears help it find its prey.

Other wolves hunt in packs.

But here, the pickings are so poor that two wolves only come together briefly to breed.

Otherwise one of them has to go.

This wolf spends a lot of energy catching his dinner for a small reward.

He tries to provoke a mouse into running by jumping up and thumping his feet on the ground.

The wolf listens and waits for the tiniest sound until the mouse moves and blows its cover.

He can walk thirty kilometres a day to find enough morsels of food.

This lifestyle provides just enough nitrogen for the maned wolf to grow and maintain its tissues.

But to get enough energy, the wolf has put itself on an extraordinary dietary supplement.

Maned wolves do most of their hunting at night.

Surprisingly, these lobeira fruit are exactly what he's looking for.

This fruit contains protein and carbohydrates

giving the wolf the extra nitrogen and energy he needs.

Lobeira fruit is so important, it forms up to half the wolf's diet.

And more surprising still, the wolves effectively farm their own fruit.

Maned wolves need to mark their territory.

They do it on high spots where other wolves will be able to see, and smell it, from much further away.

On the flat Cerrado, leaf-cutter ant nests form one of the few raised areas.

Before long, the leaf cutter ants that live in the nest below find the wolf's faeces.

And those partly digested remains of fruit are irresistible.

They even try bits of the mouse.

But the ants are searching for something else and one of them has just found it.

The round seed of a lobeira fruit.

Over half the faeces are made up of these seeds.

And the ants carefully carry each seed into the nest.

Later, for some unknown reason,

many of the seeds will then be ejected from the nest.

These are dumped, along with the ant's faeces and their digging refuse,

together it all acts as compost.

Not only has the wolf helped deliver the seeds to the perfect place for germination,

its faeces will provide fertiliser to help the young fruit tree grow.

Those seeds that have passed through the wolf are more likely to germinate than those that haven't.

It works so well that in some areas

every single lobeira fruit tree grows from a leaf cutter ant nest.

Providing the wolf with a regular supply of fruit.

Complex webs of relationships like these allow animals to survive the nitrogen shortage of grasslands.

But interfering with these links can cause changes that ripple through the whole ecosystem.

Nowhere shows this better than Australia.

These grasslands have some of the lowest levels of nitrogen on the planet.

Kangaroos are well adapted to survive in such a challenging place.

But one group of kangaroos shows what happens when their environment changes.

This baby eastern grey kangaroo weighed less than a small coin when he was born.

From birth, he will increase in weight a staggering 60 thousand times, to reach the size of his dad.

This requires a lot of protein.

Luckily for him, he lives in a rather special place.

Anglesea Golf Club near Melbourne.

Since the course was built 60 years ago, the kangaroo population has exploded.

At the last count there were 359 kangaroos,