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 Amazon. The largest rainforest on Earth.

Feeding the mightiest river in the world.

It drains over 40% of South America

and holds a fifth of the entire world's river water.

Up to 12 metres of rain falls in parts of the Amazon every year.

But what makes the Amazon even more incredible is the life within it.

For sheer diversity of species, it's unparalleled.

It is estimated to be home to up to 3 million species.

and its plants produce 20% of the world's oxygen.

The Amazon River even holds more kinds of fish

than the whole of the Atlantic Ocean.

A complex ecosystem with infinite connections and relationships,

but as we shall discover, as in all our microworlds

there is one piece that this entire system relies on to survive.

And without it, the rainforest would look like this.

Areas that have been slashed and burned for agriculture

fail rapidly, and before long the land cannot even support crops.

All our microworlds rely on nutrients,

they are the building blocks of life.

Phosphorous, potassium and calcium,

vital elements that plants need to grow.

But these nutrients are not a stable part of any ecosystem.

Here they are either locked up and unobtainable in living tissue

or washed out by the incessant rain.

And it's so poor in available nutrients

that scientists refer to the Amazon as a wet desert.

The endless rain affects the trees, plants and soils

and are constantly washing the building blocks of life away.

Life here is actually surprisingly hard.

So how in such a nutrient-poor microworld,

is there so much diversity and life?

The answer is that in nature, adversity leads to adaptation.

Animals and plants have carved out incredible niches

and formed vital relationships,

not just to survive, but to thrive here.

But is there one that holds the key to all this success?

Understanding the Amazon, the most complex ecosystem on Earth,

is one of the greatest challenges that ecologists have ever faced.

Our only chance is by unpicking its web of connections

until we can hopefully discover how the Amazon is able to support

such diversity on such slim pickings.

The Amazon is so massive and so intricate

that we are really going to have to delve deep

and get under this microworld's skin to search for the pivotal piece.

So in our investigation we need to start small

and focus on the details first.

Like all microworlds, the Amazon is a showcase of natural selection.

The inhabitants have evolved over time to exploit every niche,

every opportunity that exists within its world.

But what makes the Amazon special

is the vast number of relationships that have developed.

Particularly compelling is the interaction between flowers

and one of the most beautiful groups of birds on the planet.

Hummingbirds.

Like bees, they are attracted to flowers

for their energy-rich nectar.

But in a world where every nutrient has to be locked up safely,

the flowers can't give their valuable resources away

unless they get something in return.

The plants need to be pollinated.

So as the birds feed, they get their heads sprinkled with pollen.

But this only works if the hummingbird delivers the pollen

to another flower of the same species.

And that's the clever bit.

Over thousands of years, the flowers and the birds have evolved together,

so that only the species of bird with the right-shaped beak

can get nectar from their species of flower.

The birds are guaranteed food

and the plant guarantees it gets pollinated.

Relationships like this are born out of necessity

but they might not hold the key to how this world works.

The pivotal piece in many microworlds is a constant,

something that doesn't change much

so the rest of the world can keep up.

So to find the key in this complicated world,

first we need to find this constant.

And there is one thing you can rely on in a rainforest -

rain.

Massive quantities of rain fall on the Amazon.

An average downpour can dump two-and-a-half centimetres of water on the forest every hour.

This rain usually comes from water evaporated from the sea.

However, the Amazon is so immense, it literally makes its own rain.

70% of the rain that falls

has evaporated from the trees themselves.

And this constant rain has had a direct effect

on the shapes of things in the forest.

Plants have developed leaves with gutters and spouts

to get rid of as much water as possible

because a leaf that doesn't dry off will end up rotting alive.

Life can't survive without water,

so the rain is an essential part of this system,

but as we've seen, all the good it does

is balanced by the damage it causes,

literally washing away the rainforest itself.

So if the rain's effect is to carry the forest away,

what happens to it after this?

Well, it ends up being transported

by the most obvious force in our microworld, the Amazon River.

It is a huge destructive force.

Each year it removes an enormous one billion tons of sediment and mud

from the forest basin on its march to the sea.

During the course of this programme, it will have dumped the equivalent

of 150,000 Olympic-sized swimming pools into the ocean.

But the river is not as constant as you might think.

It is so powerful that it scours deep channels

barging its way through the forest,

constantly changing course on its way to the Atlantic.

But as destructive as it seems, it also creates opportunities for life.

As the river loops through the forest,

these meanders become very curved

until eventually the neck of the meander touches the opposite side,

cutting off a big loop from the main river.

In creating these pools, known as oxbow lakes,

the river creates a slightly different habitat to be exploited.

These isolated bodies of water are the ideal space

for one of the river's top predators,

the two metre long giant otter.

The lakes are full of fish and the waters are calm so the otters

use them to rear their young away from the powerful main river.

So in a microworld where every available space is to be exploited,

the otters have turned this dead end feature of the river

to their advantage.

The river is actually a dynamic element in this environment.

It's not the key to its existence,

but its destructive nature does open up

a lot of opportunities to life.

Some of the sediment the river picks up gets dumped again,

forming large banks.

The minute space becomes available, something is there to exploit it.

Thousands of giant river turtles use these banks to make their nests.

So many come here that each wave digs up the nests laid moments before

and in a world short on nutrients this bounty is not missed for long.

At every turn, another opportunity is created

and the black vultures are ready to jump in and make the most of it.

Like their sea-dwelling cousins,

these turtles have a strategy that involves producing so many eggs

that they can afford to lose some casualties along the way.

But in this ever-changing world,

the turtles might have to look for a new nesting site next year.

So the river is dependent on the rains,

both are prominent players in the Amazon's ecosystem

but they both play a role in removing the most vital piece

of this puzzle, the nutrients.

So how does the forest keep hold of enough of the good stuff

to stay alive?

In the Amazon, 99% of the nutrients are locked up in living tissues,

so when any of this becomes available it's pounced upon.

Nothing goes to waste -

trees, leaves, dung and dead bodies are recycled almost immediately.

The recycling team are an ungainly rabble of floor dwellers

that lurk and skulk in the dark, damp recesses of the forest.

Like this giant metre-long earthworm.

They methodically race against the rains to reclaim as many nutrients as possible.

Even so, commodities are scarce.

Phosphorus, potassium and calcium, essential elements for life

are hard to find, and because of the rain can be gone in a heartbeat.

But the recycling teams are good,

so good that sometimes things get recycled before they're even dead.

Spores of the cordyceps fungus float through the air

and find their way inside an insect host.

The fungus attacks them from inside their bodies,

killing them and consuming them from within.

The fruiting body then erupts out of the dead insect

and releases its spores to float through the air

and infect other unfortunate individuals.

But even here, there is a dark interdependency.

Each strain of cordyceps fungus

only infects its own particular species of insect.

These recyclers play a vital role,

keeping what little nutrients there are moving around the ecosystem.

But this is certainly not enough

to have created this forest of giants in the first place.

So where would the trees normally get their nutrients from?

Here trees can grow to over 50 metres tall.

There is enough light and water in the Amazon to fuel rapid growth.

But it is not enough. These giants like everything here

need those nutrients to grow to these incredible sizes.

Trees usually get all the nutrients they need from the soil,

but what is different in this microworld

is that these are some of the poorest soils on the planet.

Only 1% of the nutrients in this ecosystem are found in the soil,

compared to 50% in temperate forests

so the Amazon trees are really up against it.

Only the top 50 cm has any nutritional value,

below this there is only clay that is no use to plants.

So in response, the trees send their roots not down,

but out through as much of the fertile top soil as they can.

The only problem with this is that it makes them pretty unstable.

One way around this is to create huge buttress roots that help stabilise them.

And they need a good foundation because they are competing with

the other trees for the other vital resource here - sunlight.

The trees are so successful in exploiting sunlight

that the forest floor is in constant shade.

Only 2% of sunlight penetrates this canopy.

So how does a young plant that's just starting out

get established here?

Cheese plant seedlings have a strange solution to this problem.

Instead of heading for light, they search for darkness.

The shadows created by a buttress root of a big tree.

Once there and using the tree for support,

they can race upwards towards the light.

The cheese plant pours its energy resources into thin and rapid growth

rather than strong stems, and leans on the tree for a helping hand.

This gives it an advantage in a world where nutrients are limited.

Other plants don't even bother to start at the bottom.

Bromeliads grow high up in the branches of trees

and so start life closer to the light.

They have ponds in their centres

that fill with up to eight litres of rain water,

offering a water source 30 metres above the ground.

And where there are ponds, there are usually frogs.

One of the most poisonous creatures on earth

relies upon the bromeliads for their rooftop pools.

This poison arrow frog carries its tadpole,

hatched from an egg laid on a leaf, to a pool in a bromeliad heart.

Here the tadpole has its own private pool, tended by its parents.

So the frogs rely on the bromeliad for a home

but give the plant nutrients in the form of faeces and leftover food,

and the bromeliad relies upon the tree to keep it near the sunlight.

The tree tops are where it's really at.

So successful are these high-rise communities

that researchers discovered a single tree in the Amazon

to be home to nearly 2,000 species of insect and spider.

Only 100 of these were already known to science.

With such limited resources and so many mouths to feed,

once the plants have secured nutrients,

they have to keep hold of them.

Many Amazon plants use poisons to protect their leaves

from the hungry vegetarians.

But again nature finds a way.

The piping guan only eats the tender new tips that hold the least poison.

Most leaf eaters eat some then move on

before they get too large a dose from any given tree.

The white faced saki monkey, however,

has evolved a highly specialised digestive system

that can cope with virtually any toxin found in the rainforest.

And there is another primate here, one of the smallest in the world,

that has found a different way around the tree's poisonous defences.

Pygmy marmosets are omnivorous, eating both plants and insects

but their favourite food is tree gum,

produced by the trees when their trunks are damaged.

The marmosets have learnt to keep this food supply going

by reopening previous wounds on the tree

and they have evolved special teeth to do so.

These tiny monkeys, which could fit inside a tea cup,

have learnt how to exploit a particular food source

at the tree's expense.

Rather than fighting back,

some plants have developed relationships with animals

that benefit both parties.

They grow tasty, energy-rich fruit.

44 different species of bird and monkey can feed on a single tree.

And the benefit for the tree is that contained inside the fruit

are seeds that cannot be digested.

So as the vegetarians move off through the forest,

what has gone in has to come out.

And the minute it hits the floor, in come the forests street cleaners.

Dung beetles detect the bounty immediately.

The males meticulously ball it up

and roll it away with their hind legs.

However, it is a lot easier to steal someone else's

than to make your own.

And the bigger the ball, the better the chances of attracting a female.

Once he's clear, the beetle rolls his ball away,

burying it in a safe place so that the female can lay her eggs inside

and the tree's seed is planted with its own source of fertiliser.

So in this relationship,

the trees are connected to both the fruit eaters and the recycling team

and they all benefit.

There is one species of plant that has taken this one stage further

and developed a much more direct and surprising relationship to get ahead.

This might appear to be a natural forest clearing,

but in fact it is inhabited by only one species of plant.

One species of plant and thousands of ants.

In this bizarre relationship, the plant has enlisted the ants' help

in a very surprising way.

The ants inject formic acid directly into the leaves and stems

of any other plants that try and grow in the clearing,

killing off all but their host plant.

And the ants don't stop there. They also provide protection.

They attack any animals that try and feed on the plant's leaves.

Even giants many thousands of times their size

are not immune to a bit of ankle biting.

This is a great service if you are a plant in this competitive environment,

but what do the ants get in return?

The plant has small openings and swellings along its branches.

These tiny cavities provide the ants with a home,

protecting them from predators and giving them a safe place

to rear their young.

It's a collaboration developed over millennia and another example

of how to survive out here, by building a successful relationship.

And it is one of these relationships that holds the key

to the success of this entire ecosystem.

But it is not colourful, or obvious, and happens completely out of sight.

Within the soil, the trees hide a symbiotic relationship

The fungus are attached to their roots

and absorb the available nutrients far faster than the trees

because they have a huge surface area.

They efficiently take elements like phosphorus, potassium

and calcium out of the soil and into the tree

before the rain gets a chance to wash it all away.

But this relationship is mutually beneficial. Both parties benefit.

In return, the tree supplies the fungus with sugars and starches

that it can't obtain itself.

One could not survive without the other

and the rainforest as we know it would not exist.

The delicacy of this relationship

and the importance it has on this microworld is clearly demonstrated

where the rainforest has been cut down for agriculture.

The rain quickly washes away the little goodness held in the soil,

and the sun bakes the ground solid.

With no trees to shade the ground or to provide essential sugars,

the fungus in the soil dies out.

Leaving the soil barren, untenable for trees to recolonise.

Without this partnership there would be no rainforest

and so many species rely on this ecosystem.

The Amazon rainforest is a complex web of inter-connectivity,

where many organisms rely on the others to survive.

The more diverse the microworld, the more robust it is to change.

But as we have discovered, even our biggest and most diverse ecosystem

relies on a fragile balance

between the environment and the species that live within it.

And without one surprising alliance between a tree and a fungus,

life in our microworld would not exist at all.

But it does and it has created

one of the most incredible and intricate microworlds on our planet.