How Nature Works

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...

the tropical rainforest

Where the mighty brazil nut tree...

owes its life to the tiny agouti.

And extraordinary relationships have created more creatures than anywhere else on earth.

Panama.

Famous for its canal that connects the Caribbean sea in the north...

...to the great ocean of the Pacific in the south.

Flanking the canal a very special area of rainforest thrives.

Pipeline Road.

It runs next to an old pipeline from World War ll

that carried oil and carves through pristine rainforest.

Like rainforests everywhere it has an enormous number of plants and animals.

Birds particularly flourish here.

Volunteers set a new world record in the area,

spotting 357 different types of bird in just 24 hours.

Making it a Mecca for birdwatchers.

Even within one group of birds, an amazing amount of variety exists.

Hummingbirds.

They feed on nectar and find these dishes of sugar solution irresistible.

Within this family alone Panama boasts 59 different species.

Their wings beat at 50 times a second.

And their acrobatic flight means they can dart between flowers and hover while they feed.

In fact, they're the only group of birds able to fly backwards.

They need such good flying skills

because their legs are so small and weak, they can't walk.

But constant flying has a cost, and these birds must eat every ten minutes to stay alive.

Some of the variety between them is bill size and shape.

While they all feed on flowers

their different bills allow them to specialise in feeding on different shaped flowers.

Jungles show amazing variety in all different animal and plant groups,

from hummingbirds to poison arrow frogs.

It's called biodiversity - the number of different living things.

This extraordinary amount of biodiversity makes jungles unique.

Their location holds the key.

The tropics.

In Asia, rainforests cover hundreds of islands.

They include some of the remotest places on Earth.

Across the Indian ocean: Africa and the rainforests of the Congo basin.

Further west, the Americas and the Amazon rainforest,

harbouring the largest unbroken expanse of trees in the tropics.

Together these jungles form an emerald band, that circles our globe.

Here in the tropics, heat and moisture stay high all year round -

and this climate holds the key to biodiversity.

2% metres of rain falls every year.

And humidity hovers at 80%.

Plants get food by combining carbon dioxide and water, in the process of photosynthesis.

No shortage of water means no shortage of photosynthesis.

Energy from the sun powers this reaction.

Being on the equator, these forests receive more intense sunshine than anywhere else -

a steady 12 hours of daylight every day.

The sun's energy captured by rainforests in one year alone

could power California for over a 2000 years.

All this energy fuels photosynthesis on a scale unseen anywhere else.

Thriving rainforest plants provide an abundance of food for animals that can eat leaves.

Leaf cutter ants are the main herbivore in these forests.

Plentiful food allows each nest to contain as many as 7 million individuals.

Female workers collect leaves all clay long.

Their sharp jaws vibrate at over a thousand times a second to slice off pieces of leaf.

But leaf cutter ants can't digest leaves on their own.

They now have to transport something over three times their bodyweight back to the nest.

They deposit the leaf fragments deep within the nest.

Specialised smaller ants feed the leaves to a fungus,

which the ants then eat.

This fungal farm grows very large -

the nest can become as big as a small truck.

The gigantic leaf cutter queen stays in the heart of the nest,

laying up to 30,000 eggs a day.

But this starts a war with the plants.

These eggs will form more worker ants, which will spend all clay attacking leaves.

The climate means rainforests don't have winters, so these pests are never killed off by cold.

For the plants, this means relentless attack.

These leaf cutter ants alone destroy 20% of the leaves in the area where they forage.

And many more plant pests join the assault.

Grasshoppers.

Caterpillars.

Slugs and snails.

Almost every leaf shows signs of pest attack.

To stand any chance of survival, plants have to defend themselves.

Every single plant in the rainforest has taken arms.

Some have evolved hooks, others vicious thorns.

And some have taken to chemical weaponry, filling their leaves with poisons.

Plant eaters have had to adapt to find a way around these defences.

Like this female three toed sloth.

She has long, strong front legs to help her climb around for food,

20 metres up in the canopy.

Ligaments bind her toes together, turning her claws into large hooks,

so she hangs from a branch without spending much energy.

This bizarre body has developed as one big adaptation to a specialised diet.

She eats just a few kinds of tree.

These tough leaves contain inedible toxins that most animals can't digest.

Her gut has developed a special system of pouches to separate food in various stages of digestion

and it hosts specialised bacteria that break down the leaves.

But digesting can take weeks.

This means she only has space to eat a very small helping of leaves a clay -

the weight of a small egg.

She gets so little energy from this diet, that she must keep movement to a minimum.

This explains the famous slowness of sloths.

She has almost become part of the forest furniture

and other creatures have started to move in.

She has become the ideal home for a colony of sloth moths.

She looks harmless but this sloth's leaf eating makes her part of the plant pest battle too.

The way she looks, the way she moves and the way she eats

all help her overcome the plant's defences.

To survive in this war, plant eaters need to have a specialised mode of attack.

Each plant and pest becomes locked in an ongoing battle, and all must adapt, or die.

The continuous warfare creates even more biodiversity.

This passionflower vine uses chemicals to fight back - lacing its leaves with cyanide.

And cyanide poses just as much danger to insects as it does to humans.

But not all insects.

These caterpillars have a counter attack.

They're from a Heliconius butterfly, called Heliconius Doris.

Doris has evolved a unique internal body chemistry that can counteract the plant's deadly cyanide.

As an added bonus, they can keep this cyanide in their bodies throughout their lives,

which means they stay toxic and largely safe from predation.

Being one of the few animals that can eat this vine,

these caterpillars have this food source pretty much to themselves.

The downside is Doris must specialise to find a way through the vine's armour.

She can't eat anything else in this forest, and will only lay her eggs on this vine.

Because plants with different armoury can resist her attack.

A different species of passionflower vine grows in the same patch of rainforest.

It has a slightly different cocktail of cyanide.

But these subtle changes make it toxic to Heliconius Doris.

A victory for the vine ...perhaps'?

Except this battle never ends.

A different plant offers an opportunity for a different plant eater.

And another species of Heliconius butterfly has evolved to take advantage.

The two butterflies have different wing patterns,

but the most important differences go on internally.

This butterfly can cope with this set of toxins.

The battle between plant defences and caterpillar attack never stops.

An evolutionary arms race has begun.

This arms race results in a proliferation of

different species of both plant and butterfly.

500 different types of vines, and 73 types of heliconius butterfly have evolved.

Each caterpillar specialises in feeding on one or two very specific food plants.