♪ [Birds chirping] David Attenborough: The relationship between plants and humans is extraordinary.

♪ We've been adapting to each other for longer than we've been on the planet.

[Birds chirping] We rely upon plants for almost everything-- the air we breathe, the water we drink, the food we eat, much of the clothes we wear, and in some parts of the world, the very buildings in which we live.

But that relationship is now changing.

How it changes next will shape the future of our green planet.

[Birds chirping] ♪ Some plants have the ability to live alongside us... even when we make it extremely difficult for them to do so.

This is Piccadilly Circus in the heart of London.

It would be difficult to imagine a more hostile place for a plant.

And yet, even here, plants will find a way.

♪ Plants like this may seem to follow us wherever we go.

♪ We call them, perhaps a little unkindly, weeds.

♪ In fact, these plants are pioneers, the ones that are most able to take advantage of new habitats, even very harsh ones.

♪ This wall is at least 100 years old.

It's like a sheer cliff face with no sign of soil.

And yet, this ivy-leaved toadflax thrives here.

How?

The plant grows towards the light with its flowers facing outwards to attract pollinators.

But once they've succeeded in doing that and the seed pods start to develop, its behavior changes.

♪ For now, the pods turn and grow away from the light, seeking the darkest place they can find-- a crevice, perhaps, like this one.

♪ It's all the toadflax needs.

It can now germinate and start to produce a new plant.

♪ Other pioneers have a different strategy.

They, instead, distribute their seeds far and wide in great numbers.

Sow thistles are masters of this trick.

Each of its seeds is equipped with a tiny downy parachute which will catch the slightest breeze.

[Wind blowing] And they can travel extraordinary distances.

They can rise a mile high in the sky and reach places hundreds of miles away from the parent plant.

♪ The seeds only need to find the tiniest chink, and they can take hold.

♪ You barely notice many of these plant invaders... but here in the heart of Hong Kong, strangler fig seeds that landed on a stone wall generations ago have spread their flexible branching roots far and wide.

♪ They can be anchored so firmly, that they become a part of the city's architecture.

♪ This is, perhaps, the ultimate weed... but there are places where the extraordinary abilities of fig trees have created a very different relationship with human beings.

♪ [Birds chirping] ♪ Meghalaya, in northeast India.

a high-altitude plateau riven by dizzyingly steep valleys.

♪ It's home to the Khasi people... ♪ who've learned how to use the remarkable properties of fig trees to help them live in such difficult country.

[Water flowing] ♪ [Birds chirping] As in Hong Kong, the tree's numerous roots anchor it firmly, here to steep mountainsides.

♪ These roots can grow up to an inch a day.

They are unusually strong, flexible, and can develop into a multitude of shapes.

The Khasi persuade them to provide them with stairs.

♪ The monsoon season brings a seemingly insurmountable challenge for Khasi communities.

[Thunder] ♪ This is the wettest place in the world.

♪ Nearly 12 inches of rain can fall in a day.

♪ Rivers become lethal torrents, dividing communities from each other.

This is when the relationship between fig trees and the Khasi becomes most important.

♪ They deliberately plant fig trees near the rivers.

When they're mature enough to have developed many hanging roots, Shining Star Kongthaw begins working with them.

♪ ♪ ♪ Attenborough: He guides the roots through bamboo tubes that are pointed towards the opposite riverbank.

♪ These strands will combine as they grow, sharing nutrients and resources and becoming stronger than a lone strand could ever be.

♪ ♪ ♪ As the years pass, the growing roots become a living bridge.

♪ As they grow, they become ever stronger and more stable.

♪ [Kongthaw speaking] ♪ Attenborough: This unusual relationship has allowed the people here to thrive in an otherwise challenging landscape.

♪ But there are plants that have found a deeper partnership with us, and, in doing so, have changed landscapes across the globe.

♪ It began over 10,000 years ago with plants that we found especially good to eat.

At first, this relationship created challenges of its own.

Many of the ancestral grasses, like these wild oats from Israel, have a trick to ensure that their seeds are planted in the perfect place.

Each seed head at the top carries 2 long bristles called awns.

And when a seed drops to the ground, these awns do something truly extraordinary.

♪ They walk!

♪ The awns twist as they dry out during the day, and then when they get wet... they untwist.

♪ Tiny hairs grip onto the ground, keeping the individual moving forward.

♪ In this way, the seeds work their way along the ground until eventually they find a rock to hide under or a crack to drill into.

♪ This adaptation is useful for the plant, but not for us.

It's hard to collect seeds when they drop off and walk away.

So our ancestors selected plants whose seeds don't drop off, don't have legs, and don't bury themselves out of reach.

♪ They also selected those individuals which put their energy into developing much larger seeds.

♪ Close relationships like this have developed all over the world, producing the plants that are now our crops.

♪ This may seem a poor deal from the point of view of plants, but not so.

We eat their seeds, but in return, we cherish and cultivate them, and now they are widespread and far more abundant than their wild ancestors.

♪ You might call this a bargain between ourselves and plants.

And over the years, it's proved extraordinarily powerful.

[Buzzing] ♪ In the beginning... the bargain operated on a small scale.

♪ Plant, tend, and harvest by hand in small patches.

♪ Over time, these plants produced more food increasingly efficiently.

♪ And the partnerships became more exclusive.

♪ We started doing more and more for a small number of chosen species.

♪ These few persuaded us to eliminate their competitors, cure their diseases, poison their enemies... and keep them well-watered, even when other species face drought.

Fewer and fewer plant species, like soy, wheat, and rice, now occupy more and more land.

So, now, whole landscapes are dominated by a single species of plant.

♪ A monoculture.

♪ This is the Central Valley of California, the biggest orchard in the world, a million acres of just one type of tree: almonds.

♪ Each flower, if it's fertilized by pollen from a different almond tree, may produce an almond nut.

♪ The flower's patterns and scent have evolved to attract insects and bribe them to do that job for them.

The pollen doesn't have to move far.

It just needs to reach a neighboring tree.

♪ There are about 20,000 flowers on each tree... ♪ and 140 million trees.

♪ That is billions of flowers all calling out at once.

♪ But here, the beauty of the blossom is wasted.

To make way for these almond trees, the land was, in effect, wiped clean.

♪ Countless wild species of plants and animals were removed, including, critically, pollinators.

So now the flowers need help and lots of it.

[Engine starts] [Machine humming] [Buzzing] 40 billion honeybees... trucked in from all over the United States.

[Buzzing continues] The orchard is only in bloom for a few weeks, so the almonds need the bees to get to work immediately.

[Buzzing continues] Each bee can visit thousands of blossoms a day.

And while they take most of the pollen they collect back to the hive, they also drop some pollen at each stop.

And so, with luck, the flowers are all eventually fertilized.

Day after day, the process is repeated... ♪ flower after flower... ♪ tree after tree.

♪ [Buzzing continues] ♪ By the time the petals fall, 2.5 trillion flowers have been successfully pollinated and will now grow into 2.5 trillion almonds.

This type of intensive, streamlined agriculture produces amazingly high yields.

♪ But monocultures are fragile.

While they can function effectively when conditions remain stable, it only takes a small change to create catastrophe.

♪ Lodgepole pine-- a very valuable timber tree grown extensively in Western Canada.

♪ Millions of acres of the same species, all the same age and the same size.

♪ For centuries, lodgepole forests have lived in a natural balance with their enemies, including this one: the Mountain Pine Beetle.

♪ In summer, female beetles start hunting for a suitable nursery in which to lay their eggs.

♪ The ideal site is a mature lodgepole pine, with bark thick enough to feed a female's brood and, critically, to protect them from the bitter cold of the coming winter.

Once under the bark, she tunnels upwards... ♪ laying her eggs as she goes.

[Crunching] ♪ A single female beetle can lay 100 eggs in a season.

♪ When the larvae hatch, they grow by feeding on the inside of the bark.

As they do, they damage the channels that transport water and nutrients between the roots and needles.

♪ For centuries, the freezing northern winters killed the majority of larvae, so tree and beetle remained in balance.

But now, with the climate changing, the winters aren't cold enough to control the beetle numbers.

♪ Needles turning red are a sure sign that the trees are dying.

♪ The beetle plague spreads like wildfire across a landscape covered by a monoculture of similarly vulnerable trees.

Since the first mass outbreak 40 years ago, trillions of trees in North America have been killed by the Mountain Pine Beetle.

Loss of plant diversity makes any habitat more vulnerable to changing conditions.

Now with climate changing so fast, we're losing plant diversity just when we need it the most.

2 out of 5 plants are now facing extinction.

Of course, the loss of any one species is, in itself, a tragedy.

But such a loss erodes the stability of a whole ecosystem, and that should be of great concern for all of us.

We need an insurance policy, a hedge against extinction.

This is Kew's Millennium Seedbank.

♪ Packets of seeds arrive here from all over the world.

These are from a relative of the yucca plant, which grows in Central Mexico on the slopes of volcanoes and nowhere else.

♪ Most of the seeds are from plants that are threatened.

Some, indeed, have gone extinct since their seeds arrived here, but here, at least, those seeds are safe.

♪ When they arrive, they're processed... and sealed into airtight jars.

Seeds are then brought underground to be stored in a vault like this one at -20 degrees Centigrade.

♪ Since the bank was founded, 2 billion seeds from 40,000 different species have been brought and stored here.

♪ Because a seed contains everything it needs to start a new plant, each and every one of them represents a little grain of hope... ♪ hope that one day we will make it possible for the seeds of these rarities to grow in the wild once again.

♪ The Seedbank certainly gives us options when a species becomes rare or even extinct... ♪ but around the world, people are trying ways to keep natural plant populations healthy where they should be... in the wild.

[Waves crashing] [Birds chirping] Hawaii.

90% of the plant species here are found nowhere else in the world.

Many are threatened by a plant invader called Miconia.

♪ It was brought to the island of Maui in the seventies as an ornamental plant.

In its native Mexico, it grows in balance with a rich variety of predators, competitors, and diseases.

♪ It grows taller than most native Hawaiian vegetation and little can grow beneath it.

♪ On the island of Maui, with nothing to keep it in check, it creates a stifling monoculture.

♪ Miconia, it would appear, is doing very well for itself.

♪ But today, it's under attack.

[Snap] ♪ On level ground, a single dedicated team can keep Miconia in check.

♪ But unfortunately, this plant can spread to places impossible for people to reach on foot.

♪ Just one plant can produce around 10 million seeds a year, enough to cover this entire landscape.

With so much at stake, the team have come up with an extraordinary plan.

♪ [Chains rattling] ♪ [Helicopter blades whirring] ♪ It's too dangerous to land here... but they don't need to.

♪ A marksman with great skill can shoot the Miconia with paint balls full of herbicide.

[Gun firing] A perfect shot is one that hits the stem, so ensuring that the herbicide spreads throughout the plant.

♪ [Man speaking indistinctly] The method is so precise, that will careful flying and accurate aim, they can kill the intruders... without damaging any wild plants.

♪ [Gun firing] [Snap] ♪ Destroying alien invaders is not the only way to help the native plants here.

♪ This is Waikamoi Preserve, the last-surviving fragment of a high-altitude Hawaiian rainforest and home to one of the world's rarest plants... ♪ Holokea.

There are only 57 fully grown individuals left in the wild.

♪ Their peculiar flowers evolved to suit the beak of a bird found only in Hawaii: the I'iwi.

The bird is now so rare itself that today, these partners seldom, if ever, meet.

♪ So the chances of Holokea getting pollinated are very slim, but a strange new partnership might just save it.

Man: I look at losing a plant as flying in a plane and taking a screw out, and, yeah, the plane'll fly and you might be able to take a couple of screws out and keep flying, but eventually, you're gonna crash.

♪ Attenborough: Hank knows where to find every one of the last 57 plants, and he returns every year to act as their pollinator.

Oppenheimer: We have to step in and play that role as pollinator matchmaker.

♪ Hank collects the pollen from the male flower and dusts it onto a female flower of a different plant.

♪ He's currently their life support, but his goal is for Holokea to thrive without him.

So, before he leaves, he plays the call of the I'iwi bird.

[Playing I'iwi bird call] Hank hopes the sound will attract the living birds and if they come that they will reconnect with the last Holokea and rekindle their vital relationship.

[Recorded I'iwi calls continue] Oppenheimer: I believe all species are important.

And I believe we have a duty to act.

Attenborough: We need to act not only in the wild places, but even in those where we live and where we farm.

[Water flowing] Like much of the world, Kenya is losing thousands of native trees annually, ones that local people rely on for so much, especially fuel.

But here, people have come up with an ingenious way to reverse some of that loss around them.

At a factory in the outskirts of Nairobi, workers collect and sort waste charcoal dust.

[Machine humming] The seeds of carefully selected native trees... are mixed with the dust.

This carbon coat will protect the seeds from hungry animals until the rains arrive.

These are seedballs.

In the village of Kebwezi, acacia trees are becoming scarce, cut down by previous generations to make charcoal.

[Children talking, drum beating] [Clap hands] But the students at this school make forest restoration child's play.

♪ The nutrients in the dissolving char dust will give each little seed a good start.

♪ If only a handful of these seeds grow into a tree, the effort has been well worth it.

Every mature acacia tree can itself produce thousands of seeds a year.

♪ 13 million seedballs have been distributed in Kenya alone.

♪ And the methods by which they are dispersed... ♪ are ever more inventive, to say the least.

♪ This technique is being repeated around the globe.

Seedball-making workshop.

People are choosing seeds of local native plants, giving them a little initial help, and bringing some wild plants back into the world around them.

♪ For those wild plants that we use, is it possible to take what we need without damaging whole ecosystems?

♪ This plateau in the highlands of Ethiopia is covered by an extraordinary plant.

♪ Guassa, a grass uniquely adapted to thrive at high altitudes on the Equator.

♪ Its blades contain silica, which makes it very tough and therefore very valuable to the people who live here.

♪ [Speaking native language] With so many uses, it would be only too easy to use too much... ♪ something the people here understand very well.

♪ Attenborough: They don't cultivate Guassa, but instead, for over 400 years, they have protected the area... ♪ making sure no one harvests more than the natural grassland can sustain.

♪ This approach means the people get a resource that will endure.

♪ But they're not the only ones who benefit from this approach.

♪ Rare animals such as the geladas, the world's only grass-eating monkey, thrive here.

♪ And the natural variety of plants in this alpine community also thrive.

♪ The practices of Guassa may seem irrelevant for our most extreme monocultures.

♪ But is it possible even here to invite a bit of wildness in and with it a bit of resilience?

♪ Woman: I have 2 different kinds of forage growing in my orchard.

The yellow that you see behind me is a variety of different mustards.

And then, after the bloom, we're gonna see some clover blossom.

Attenborough: This mix of plants means the bees can feed before and after the brief almond bloom.

And they also get a more balanced, healthier diet.

Gemperle: In the meantime, it's gonna provide habitat for all sorts of insects and the bees, probably some rabbits.

Attenborough: It's a step towards establishing a better, more stable system.

It's about finding balance.

That's what we're trying to do here.

Find a balance.

♪ Attenborough: Can humanity globally find a new balance between wild plants and those we have domesticated?

♪ Around half the usable land on Earth is taken up by agriculture.

Do we really need that much?

Maybe not.

It's a remarkable fact, but around 80% of all cultivated land is used for raising livestock for us to eat.

Raising animals can be a sustainable way to create food, but in many places, plants can produce the same amount of protein on a fraction of the land that animals need.

What could it mean for wild plants if the global balance between plant eating and meat eating shifted?

It may sound odd, but the more plants we eat, the more space there will be for wild plants.

♪ Remarkably, it's possible to restock even highly degraded land with wild plants.

♪ In Brazil, the needs of cattle ranching drive most deforestation.

♪ 30 years ago, the owners of one former cattle ranch wanted to restore their land to the Atlantic rainforest that once covered it.

They had no idea if it could be done.

The land was so bare and eroded, but they were determined to try.

First, they cleared the introduced African cattle grasses, invasive plants that outcompete most of the native plants.

♪ The grasses were replaced by seedlings grown from seeds gathered in nearby remnants of Atlantic rainforest.

♪ The Earth Institute, or Instituto Terra, as the ranch is known now, was encouraged by early success and expanded its scope and ambition.

♪ In 5 years, trees covered the land again... and within 10, it was clear something remarkable was happening.

[Insects chirping, animals calling] [Buzzing] [Thunder] When rain falls, it now no longer simply runs off the land, leaving it parched.

Instead, it clings to the plants, to every root, stem, and leaf, and then slowly filters to the forest floor.

♪ And so, previously dry streams burst into life for the first time in decades.

♪ The Institute never introduced animals or plants other than the trees.

♪ The animals came back on their own.

Small ones came first... ♪ and then, very recently, camera traps left in the forest captured images beyond the hopes of everyone involved.

A maned wolf... an animal that has been driven from its native habitat by deforestation, drawn to its favorite plant, the wolf fruit tree.

Not only that... [High-pitched cries] a puma... ♪ with cubs!

♪ The arrival of these precious top predators and their young shows what can happen when we make space for wild plants.

♪ We have for centuries robbed wild plants of the space and time they need to thrive.

That has certainly not been to their benefit, nor, ultimately, is it to ours.

♪ Our relationship with plants has changed throughout history.

And now, it must change again.

Whether it's what we eat and cultivate or whether it's what we like, we must now work with plants and make the world a little greener a little wilder.

♪ If we do this, our future will be healthier and safer and in my experience, at any rate, happier.

Plants are, after all, our most ancient allies, and together, we can make this an even greener planet.

(dramatic music) - [David] Throughout "The Human" episode, "The Green Planet" crew sought to capture the lives of plants confronted with the human world.

(dramatic music) But they also heard the stories of people deeply connected to the plants around them, (dramatic music) such as those at the frontline of the battle to save Hawaii's native endangered plants from invasive species.

- If we don't get a handle on miconia we could lose everything that makes Hawaii special and unique.

- [David] The team first focus on filming the rare holokea plant, (dramatic music) accessible only on foot.

(dramatic music) There are few pristine areas like this left in Hawaii.

(dramatic music) Only 40% of the land still has native vegetation, (dramatic music) most of it wiped out by invasive species like miconia.

(dramatic music) (birds tweet) Next on the list was to film the team waging war on miconia, known locally as the purple plague.

For crew member Aja, the work is not just a physical battle against this invasive species, it's deeply personal.

- I've been to the pristine areas, so I know what it looks like and feels like, you know, to be in the presence of all that manna and spirituality.

(dramatic music) Those aren't just plants.

Those are ancestor spirits, you know.

- [David] Controlling these invasive plants is a relentless job and Aja is not just doing this for native Hawaiian ecosystems, but also to protect her ancestral relationship to this fragile landscape.

But Aja believes that her fight is worthwhile.

- Everything we do is for the next generation just like our ancestors and kupunas, it wasn't for them is for the next generation.

And just to have my kids go into areas, like, where they can see there's still natives, where we don't have to go through all this invasive vegetation.

You know, it's not just my responsibility, it's everyone's responsibility.

You know, the Earth takes care of us, so we have to do the same.

We can't just let it go by.

We gotta take care too.

(relaxing music)