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 Scottish Highlands.

An ancient landscape of startling beauty.

Craggy peaks peer over a terrain

of thick forest and wide open space.

Two contrasting habitats united by their battle with the elements.

Presided over by Scottish emblems

like golden eagles

and red stags.

And it's home to some of the rarest animals in Britain.

The hand of man has been felt

throughout the history of the Highlands,

and yet its wildlife remains iconic.

How have the Highlands remained a home for these species

despite the pressure of the modern world?

To find out, let's explore its deep forest and open heather moors,

discover the impact of the elements on these landscapes,

and the role of their key characters

in keeping this remote corner of the British Isles wild.

There's a famous saying about Scotland -

it doesn't have a climate, just weather.

Nowhere is this more true than in the Highlands.

It's a landscape at the mercy of gale-force Atlantic winds.

Winters have been known to hit -20.

And when it rains, it pours.

Have the elements had a hand in keeping this place wild?

Up to five metres of rain fall here every year -

that's the same as in some rainforests.

With most other parts of Britain

receiving less than a quarter of this,

it's easy to see why humans might have chosen

to set up shop elsewhere.

But the Highlands' wild residents

are made of sterner stuff.

In fact, some rely on rain.

From the soggy earth emerges a creature

feared throughout the Highlands.

The scourge of Scotland -

the midge.

This tiny millimetre-long vampire

hatches from eggs laid in the rain-soaked ground.

As the midge feeds,

it sends off a chemical signal

inviting others in the area to join the feast.

Cutting the skin with their scissor-like mouthparts

and sucking up the pooling blood.

And with an estimated 40,000 capable of landing on a deer

or similar-sized animal within an hour,

they can make life unbearable.

Midges die off before winter,

but the change in seasons

doesn't make life in the Highlands any easier,

as the rain turns to snow.

On cue, the mountain hare population turns from russet brown

to brilliant white.

The new coat is thicker and warmer,

but it also helps to keep the hare safe from predators.

A brown hare on a white background

would stick out like a sore thumb.

The ptarmigan employs the same colour-changing tactics.

Its feathery feet increase the surface area,

acting like snow shoes,

preventing the ptarmigan from sinking into the deep snow.

Humans would struggle to live in these cold conditions.

But surprisingly, these hardy animals can only survive here

because of the Highland's extreme weather.

As Britain melted from its most recent ice age,

around 12,000 years ago,

animals adapted for arctic conditions were forced back

to the very highest, coldest points of the British Isles.

While humans busily colonised the rest of the newly temperate UK,

the Highlands provided a climate

in which these animal refugees could survive.

So, has the tough climate kept the Highlands wild?

While it's true that, as a rule,

the highest, coldest, wettest parts of the world

are some of the least developed,

actually, the Highlands share a similar climate

with major cities in other parts of the world.

So, if weather alone does not keep a place wild,

the Highlands must have another secret up their sleeve.

There's less than 50 people per square mile here

compared with 600 in the rest of the UK.

Perhaps the landscape has a part to play.

There's one very special, ancient habitat

that's found nowhere else in the United Kingdom.

The Caledonian forest.

Dominated by the gigantic Scots pine.

Growing up to 20 metres high

and a solid three metres in girth.

Each tree can live for a very respectable 250 years...

..during which time, it supports an array of Scottish wildlife.

It's the backbone of the Caledonian community.

A familiar character is on the hunt for food among its branches.

The red squirrel's agility allows it to reach

the very top branches of the pine,

home to some of the choicest cones.

It's been estimated that a single squirrel can eat the seeds

from as many as 20,000 pine cones in a year.

Squirrels have the sharp incisors

common with the rest of the rodent family,

which they use to tear the tough cone apart.

Perfect tools for the job.

But even so, these teeth will need to grow

as much as 15 centimetres a year

to keep up with wear and tear.

The squirrel knows just how tough the winters can be,

so it plans ahead,

burying stores for leaner times.

Scent glands in its cheeks help

to guide it back to the cache.

But it might be months

before the squirrel needs to excavate its hoard

and, unsurprisingly, some seeds are forgotten

or dropped by messy eating.

On the forest floor,

they have a chance of germinating,

bolstering the pine population.

It's a mutually beneficial relationship

that's been going strong for millennia.

Food is not the only reason the squirrel prefers the Scots pine.

The bark blends perfectly with its rusty red fur

and camouflage is all important

when you have a predator that's equally at home in the branches.

The pine marten rivals the squirrel's agility...

..and in a high tree top chase,

is one of few predators able to catch it.

A smaller male wants in on the meal,

but he's testing his luck.

This pine marten is in no mood for sharing...

IT SQUEALS

..and chases the chancer out to a thin branch.

IT SQUEALS

The squirrel makes a good meal

in an environment where every calorie counts.

The young male will just have to find his own food.

And by the looks of it,

he could be going hungry.

Pine martens are agile enough

to also prey on the pine's resident birds.

Like the Scottish crossbill,

an endemic species found in Highland Caledonian forests.

The bird's shape reveals its close relationship with the pine.

Its curved, slightly crossed beak is perfectly adapted

to prise seeds from tough cones.

It eats little else.

The tiny crested tit,

one of Britain's rarest birds,

is equally at home in the Caledonian forest.

The tit supports the pine

by winkling insects from crevasses in the bark

and preventing burrowing larvae from doing serious damage.

But birds are not the only species that groom the tree.

The caterpillars of the pine looper moth and sawfly

mimic the needles that they feed on.

These caterpillars can defoliate a whole tree.

But the Scots pine has an ally.

Wood ants, on patrol.

They detect their prey by vibration,

stalking and attacking.

The larvae may be twice the size of the ant,

but this poses little problem,

because these ants are team players...

..dragging the larvae from the needles back to their nest.

The largest ant in Britain has a suitably impressive home.

Each metre-high mound holds up to 100,000 ants at a time.

The nest is constructed almost entirely from pine needles.

With a queen ant living for anything up to 15 years,

the nest must be constantly repaired and improved to see out her reign.

Each one is a feat of engineering.

With the Scots pine providing

both food and construction material for the wood ant,

it's no wonder that their nests are a common site

throughout the Caledonian forest.

In fact, so many of the forest's species

are interconnected with the Scots pine,

relying on it for a food source,

a nursery or a home,

that it's known as a keystone species.

The backbone of the forest community

and where many trees grow together,

the forest habitat supports larger species.

CLICKING SOUND

IT CLICKS

This strange call belongs to the capercaillie,

the largest member of the grouse family.

The turkey-sized bird lives across much of Europe,

but, in Britain, is predominantly found in the Highlands.

IT CLICKS

This is actually a mating call.

It might not sound particularly enticing,

but it's widely believed that the subtleties of the love song

are too low for human ears to register.

Capercaillies have been breeding in the Highlands

since the ice age gave way to forest.

Stands of Scots pine set a stage for their mating displays

from the Highlands to Western Asia

and the males are dressed to impress.

With the affection of the more drably coloured hens at stake...

..testosterone-fuelled tussles regularly break out.

CLICKING

The chance of injury is very real.

The victor gets his girl

and the chance to mate.

IT CLICKS

The Caledonian forests and their Scots pine

are synonymous with the Highland wilderness.

But they're only one habitat within this microworld.

The forest exists side by side

with another completely contrasting landscape...

..moorland, as open as the forest is dense.

Scotland's largest living mammal,

the red deer,

needs both these habitats.

Deer feed on new pine shoots,

but need open space to breed.

IT BELLOWS

Large males bellow across the heather,

advertising their superiority.

The stag rounds up a herd of females.

By keeping them under his watch,

he can ensure that no other male

breeds with them and that only his genes are passed on.

But that's easier said than done.

Other males will want the females for themselves

and are ready to fight for the right.

THEY BELLOW

These are ferocious battles

to prove strength

and determine dominance.

The rut has been fought in the open heather moorlands

for more than 11,000 years.

It's the perfect arena, stretching for miles without a single tree,

let alone sign of human habitation.

Without natural shelter,

it's no wonder humans avoided the moors,

but deer are not the only animals to thrive.

Despite first appearances,

the open moorland is as rich with life as the pine forest.

The red grouse is as dependent on the heather

as its cousin, the capercaillie, is on the pine.

IT CLUCKS

Feeding on seeds, flowers and shoots as the seasons change.

The heather, like the Scots pine,

relies on the relationships with other species.

Its striking purple flowers are pollinated by bees,

while passing animals help

to distribute its seeds.

And it creates a territory

for one of the most powerful birds on Earth.

The golden eagle,

a symbol of the wild Highlands.

It's a predator, adapted for purpose.

Keen eyesight allows it to scour the heather for movement

from up to two kilometres away.

Powerful talons and beak help it to dispatch prey

many times its body weight.

And this formidable bird needs these open spaces...

..to hunt.

Following a particularly cold winter, the snow's melted,

but the mountain hare's fur hasn't quite caught up.

The white that provided camouflage a month before

is now more of an advertisement.

With its two metre wing span,

the eagle soars on wind currents,

constantly spotting for movement below.

While the hare is unaware of the danger overhead,

the eagle locks in.

But the hare is no sitting target.

Once it spots the threat,

it will sprint through the heather.

By using pre-practiced routes that it knows are obstacle free,

the hare is capable of hitting 60 kilometres an hour.

But with the eagle topping out

at three times this,

it needs something more than speed to survive.

So the hare dodges and weaves,

throwing itself out of the eagle's trajectory.

A last-minute zigzag saves the hare by a whisker.

Heather plants can grow for around 30 years.

After this time, they degenerate and die back.

Bacteria in the soil will break them down.

The increased nutrients that this breakdown provides

is just enough for larger, hardy plants to begin to grow.

Over time, heather will be replaced

by trees and, eventually, forest.

But something doesn't quite make sense.

If this natural succession was taking place,

the Highlands should be full of trees.

Instead, Caledonian forest only exists in small pockets.

This explains why some of the species that it supports

are so rare in Britain,

as there's just not enough habitat to go around.

But it hasn't always been this way.

Some 10,000 years ago,

after the end of the last ice age,

Scotland was covered in trees.

These primordial forests would have been rich with life,

wolves, bears, wild boar,

animals missing from Scotland today.

WOLF HOWLS

So what has happened to the Highlands?

How does a forest just disappear?

What's allowed the heather to take hold in such vast areas?

And how has the natural succession of the trees been halted?

The soil below the heather reveals the answer to this conundrum.

By studying remnants of the habitats from thousands of years back,

scientists have been able to determine

that there was still forest in the Highlands up to 5,000 years ago.

Before disaster struck.

The climate changed.

A small fluctuation compared with the chill of the ice age,

but enough to make the Highlands wetter.

The deluge that followed was far more serious

than the worst of modern-day Scottish winters.

The forests literally drowned.

It's thought that three quarters

of the original Caledonian forest rotted away,

creating vast bare spaces.

Which suited not only the tough heather, but another species.

Humans.

Open land allowed communities to farm crops, build homes

and raise livestock.

For the first time, humans became a dominant Highland species.

The climate eventually settled,

but only a tiny 10% of the once vast Caledonian forest remained.

The landscape that we recognise as the Scottish Highlands today

had been created.

This was, of course, great news for open-space-loving species

like the grouse and mountain hare.

IT HOWLS

But many of the animals

that relied on the forest -

wolves,

the capercaillie,

red squirrel

and bears became extinct.

Those species that could survive were excluded

to small pockets of woodland.

Over the centuries, the land use of the Highlands shifted.

Farms were cleared to make way for deer and grouse hunting.

The Highlands were kept bare.

Trophy species relied on healthy heather,

and so patches were burnt to encourage new growth

and extend the plants natural 30-year cycle.

A new habitat was created,

a patchwork of old and young heather growing side by side,

in which not only the grouse, but all moorland species could flourish.

But this wasn't a natural habitat.

Man was artificially managing the Highland's wild populations.

Humans had become an intrinsic part of the ecosystem.

Scotland's heather moorlands were thriving,

but the few remaining areas of forest and their inhabitants

were still isolated.

It was an environmental disaster that triggered the demise

of the Caledonian forest 5,000 years before,

so it's fitting that another would start to bring it back.

But this time it wasn't brought about by natural forces.

Two world wars left Britain's natural resources ravaged.

New trees needed planting

and, although their primary purpose was for timber,

these forests also encouraged species back into Scotland

that hadn't been seen in decades.

Red squirrels re-colonised,

supporting greater numbers of their predators - pine martens.

Pine-loving populations of the rare crested tit and the crossbill grew.

Landowners began to take responsibility for the wildlife.

The focus shifted to forest protection and regeneration.

But this posed a dilemma.

Left to its own devices,

the forest would naturally replace the heather habitat

that now supported many more species than just the grouse.

The regeneration of one couldn't come at the expense of the other.

For these two habitats to co-exist in the Highlands,

then humans would need to be actively involved.

They'd have to site manage.

Today, the Scottish Highlands are celebrated

as the last truly wild place in Britain.

But it is a wilderness under supervision.

Forests are being planted and the heather managed.

Species have been re-introduced.

It is a microworld under reconstruction.

One in which humans are as fundamental to success

as any of its other animals or plants.

The Scottish Highlands are, at once, both wild and controlled.

People have had to learn how to go about regenerating an ecosystem.

A long and delicate process that's still not complete.

But the hope is that, under careful guardianship,

these iconic characters and ice age survivors

will have a place in the Highland landscape for centuries to come.