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.

On a plateau high in the Rocky Mountains of North America

sits an iconic wilderness.

Yellowstone National Park.

Cataclysmic forces have shaped this natural wonder.

Its unique geology means that

volcanic heat from the Earth's core comes right to the surface.

It is the most extensive geothermal area on Earth,

with more than 10,000 thermal wonders.

Established in 1872, Yellowstone is the world's first national park,

covering more than 8,000 square kilometres.

Aside from its awe-inspiring landscape,

it's home to an array of North America's most charismatic wildlife.

BEAR GROANS

WOLVES HOWL

ELK CALLS

BISON SNORTS

These dwell in vast forests and grasslands

as well as rivers and wetlands.

Isolated by sheer mountain peaks,

this stunning microworld is a stage upon which scientists

discovered surprising connections between different species.

To fully understand the key to the rich complexity of life

in Yellowstone today, we must look at how the presence of one animal

can affect that of another.

At the very top of the food chain in Yellowstone

is in an iconic predator.

The wolf.

The park is currently home to 98 wolves, in ten packs.

These survive by hunting the herds of large grazing mammals.

At the other end of the spectrum is a humble tree-felling rodent.

The beaver.

On the face of it,

wolves and beavers live very independent lives.

And there appears to be no connection between them.

So it came as a surprise to find

that their fates were somehow intertwined.

This was discovered after the National Parks and Wildlife Service

launched a landmark project to return wolves to Yellowstone.

Wolves were absent from the park for 70 years.

They were eradicated as part of a predator control campaign

aimed at protecting livestock.

But in 1995, 31 wolves were released in a last-ditch effort

to save the species from extinction in the wild.

The releases were surrounded with controversy.

Some parties claimed that wolves would unleashed devastation

upon valuable stocks of America's rare grazing mammals.

What was to happen was even more remarkable.

Soon after the reintroduction of wolves,

Yellowstone's rivers took a turn for the better.

Flows improved, fish spawned more successfully...

..and numbers of water birds increased.

This improvement in river habitat

was down to the tireless work of beavers.

Beaver dams are key to rich wetlands.

As well as providing habitat for fish,

birds and other animals, their precious reserves of water

improve flows through the dry summers and remove sediments.

The timing of the beavers' return was too closely linked

with that of the wolf to be a coincidence.

So what is the link between wolves and beavers?

How are their lives interconnected?

Before we answer these questions, let's take a look

at what makes a wolf pack a force to be reckoned with.

Winter is the wolf's time.

The relentless cold weakens their prey.

Even so, this elk is a formidable quarry,

standing 1.5 metres at the shoulder, and weighing over 300 kilos.

A wolf in comparison, only weighs about 50 kilos.

But wolves, as well as being highly intelligent,

have the advantage of numbers and teamwork.

The odds are stacked in their favour.

Despite the wolves' apparent advantage, the elk is in his prime.

And is smart.

He knows that wolves will not risk following him

into the freezing water.

Crossing the river, he makes good his escape...

..to the disappointment of the pack.

WOLF WHINES

A wolf pack is a close family unit.

It's led by a dominant pair of breeding wolves,

known as the alpha pair.

As a general rule, the other pack members

consist of their siblings and offspring.

In a pack, only the alpha male and female are allowed to breed.

The function of the rest of the pack is only to help in hunting

and looking after young.

But it doesn't stop them all playing together.

Pack hierarchy is maintained through these ritualised play-fights.

Rank is based on attitude rather than size or physical strength.

And there is one other way through which they strengthen their bond.

WOLVES HOWL

Howling is used as a means of long-distance communication.

It maintains cohesion within the pack, even if they are separated.

And, most importantly, it advertises their presence,

making a stamp on the territory,

and letting other wolf packs know where their boundaries lie.

Apart from the fact that beavers exist within wolf territory,

they have little to do with them.

Why, then, have their numbers increased?

First, we need to look at what Yellowstone was like

before the wolves were brought back.

Before 1995, Yellowstone's top dog was a close cousin to the wolf.

HOWLING

The coyote.

Before wolves returned,

Yellowstone had the largest coyote population in North America.

This had a profound impact on the ecosystem.

Weighing in at 12 kilos,

Yellowstone's coyotes are less than half the size of the wolves

and are not built to tackle big prey.

They exist mainly on a diet of small animals.

Voles and ground squirrels are high on their menu.

They're also scavengers.

Winter casualties are a welcome feast.

For bald eagles, winter carrion is a welcome addition to the diet.

But, during the reign of the coyote,

they were up against serious competition.

Thievery is another coyote trait.

These otters have hidden their hard-earned winter catch

in a burrow under deep snow.

But it doesn't go unnoticed.

The coyote's excellent sense of smell leads it to the prize.

They don't call coyotes wily for nothing.

In the absence of wolves,

coyotes moved around in the open without fear.

This fearlessness had a devastating impact on the wider ecosystem.

And especially on one of Yellowstone's rarest mammals...

..the pronghorn antelope.

The pronghorn is the only surviving member of an ancient group

of animals that arose in North America 20 million years ago.

They're quite small, standing about one metre at the shoulder.

But what they lack in height, they make up for in speed.

They are able to maintain speeds of over 70 kilometres an hour

for long periods.

Coyotes are not big or fast enough to take down an adult pronghorn,

but they do have a taste for their calves.

Before wolves returned,

the average life expectancy of a pronghorn fawn was just 35 days.

Coyotes swept through the herds, taking calves,

and the pronghorn population fell to dangerously low levels.

By the early 1990s, there was serious concern for the pronghorn.

But all that was to change.

When the wolf returned, coyotes were dealt a shocking blow.

Through systematic killing and direct competition,

wolves showed the coyote who was boss.

With coyotes firmly back in their place,

pronghorns have been given a respite.

Wolves pay little attention to them.

The adults are too fast

and the young are not substantial enough to feed a pack.

So wolves set their sights on bigger prey.

This peculiar symbiosis has seen pronghorn numbers increase

over the wolf's range.

Coyote populations, on the other hand,

have fallen to a fraction of what they were before.

A more natural level.

These days, coyotes have reverted back

to being more secretive and nervous.

They have to look over their shoulder at every turn.

So by, policing the coyotes,

wolves have unwittingly benefited the pronghorn,

redressing a balance.

An unforeseen consequence of the reintroduction programme.

Were coyotes also the reason why beavers were absent in Yellowstone?

Do beavers feature in a coyote's diet?

Beavers don't live in the dam itself,

but within a nearby mound built from the same material, known as a lodge.

The lodge can only be accessed via an underwater entrance.

Inside, safe from the attention of predators,

they can rear their young.

With all this protection,

it's very unlikely they'd ever encounter a coyote.

So coyotes are not the link between wolves

and the recent success of the beaver.

But there is one Yellowstone predator

that is comfortable in the water and has been known to prey upon beavers.

The bear.

There are up to 150 grizzly bears in Yellowstone.

Big males weigh almost 300 kilos.

They are, without a doubt, the largest predator here.

So what is the grizzly's relationship with the wolf?

And has this had an impact on the beaver?

Bears are omnivores,

feeding on a diet of berries, nuts and herbs,

as well as scavenging and hunting other animals.

They will sometimes dig open beaver lodges and prey upon beavers.

If they really need meat,

they're more than capable of chasing big prey like elk.

Despite their enormous bulk, bears are surprisingly fast.

However, bears are also quite happy to let someone else do all the work.

And, luckily for them, wolves are a willing workforce.

Since wolves rarely eat all of their kill in one sitting,

the carcass becomes an abandoned picnic just waiting to be exploited.

Before wolves,

only the hardest winters would bring a glut of carrion to the table.

With wolves, the supply has not only increased

but is spread more evenly throughout the year.

For bears, this is manna from heaven.

And if they don't find an abandoned carcass,

bears are more than capable of stealing one.

Fear of wolves is most definitely not on this bear's agenda.

Even a young bear will stand up to the pack.

Despite strength in numbers, wolves know to respect bears -

their huge jaws and powerful claws mean business.

When the stakes are even, bears and wolves will eat at the same table.

Wolf kills have brought an unlikely bounty to Yellowstone.

As a result, bear cub survival has increased.

And, despite the obvious competition,

the bear population is on the up.

If bear predation had any impact on beaver populations,

we'd not expect beaver numbers to increase alongside bears'.

But they have.

So bears are not the key to the return of the beaver.

So wolf reintroductions and the success of beavers in Yellowstone

and not connected with either the coyote or the bear.

With two of Yellowstone's largest predators

removed from our list of suspects, we must think again.

What other wolf relationships would lead, ultimately,

to an increase in beaver numbers?

Our attention must now turn to those animals whose lives

are far more intertwined with those of wolves.

Their prey.

One of the animals on their menu

is Yellowstone's most iconic grazer, the bison.

Bison are built for the freezing Yellowstone winter.

Bulls can weigh over 1,000 kilos. They have an attitude to match,

and will happily stand their ground against a pack.

These giants have been roaming the plains of North America

since the last ice age.

Their bulk allows them to stay warm from within.

They can also slow their metabolism right down to save energy.

Deep snow is no barrier to these enormous beasts.

Their mighty neck muscles allow them to bulldoze the snow away

to reach the grass beneath.

It's estimated that 50 million bison once lived in North America...

before Europeans settled here.

Overharvesting caused total numbers across the continent to crash

to less than 2,000 in 1900.

GUNSHOTS

Now, thanks to careful management, their numbers across North America

have swelled again to half a million animals.

Today, almost 4,000 live in Yellowstone itself.

Only weak and old bison have anything to fear from wolves.

But, during the summer months, it's the young that are most vulnerable.

But, for wolves, it's still a risky business.

Their strategy is to charge the herd

and try to separate the calves from their mothers.

But the bison are quick to react.

A large bull joins the fray.

Working together, they form an impenetrable wall.

Any attempt at snatching a calf

could bring fatal consequences for a wolf.

The pack realise they've missed their chance.

The threat of serious injury or death is always a good deterrent.

They must be successful soon,

for they themselves have young to rear at this time of year.

It's very rare for pups to be seen out in the open.

Spending nearly all of their time in the den,

they only emerge when under the protection of the pack.

A brief foray to the river gives them a chance to play

and to view the kingdom they'll soon rule themselves.

The alpha female will have around six pups.

She must feed well so that they can stay strong.

Weaned on her milk,

they're five weeks old before they can share in the spoils of the hunt.

But for now, they must not stay out in the open for too long.

Here, they're at risk from bears and coyotes,

so they're soon led back to the den.

Breeding success in wolves is strongly linked to hunting success.

If their kill rate declines, then the wolf population soon follows.

Bison provide plenty of meat

but hunting them pushes wolves to their limits.

Because of this, bison are not the preferred prey of the wolf.

With bison numbers changing little in response to wolves,

it's unlikely that they've had a major influence

on the fortunes of beavers and river habitat within the park.

So bison are not the link we're looking for.

But there is one prey animal that's inextricably linked to the wolf.

The elk.

Numbering tens of thousands,

elk are the most numerous grazer in Yellowstone.

Late summer is the time for the rut.

The dominant male calls to advertise his status to females

and to other males in the vicinity.

His antlers are a sign of age and status.

ELK WAILS

Made of bone, antlers take a lot of energy to produce -

they're shed and regrown every year.

So they're a clear and honest indication

of the health of an individual.

The dominant male is clearly in his prime.

But there are challengers to the throne and he must prove his worth.

The challenge starts with a ritualised display

designed to size up and intimidate an opponent.

If nobody backs down, then contact is made.

Fights may last many minutes. It's a dangerous time.

A well-placed antler could mean certain death.

This time, the competition is seen off...

..and the victor can claim his prize.

A large male may not eat for weeks during the rut.

All he'll do is fight.

And, if he's lucky, he'll mate with all the females in the vicinity.

Exhausted at the end of the rut, he has very little time to feed

and regain condition before thick snow covers the lands once again.

And when the snows arrive, the wolves come into their own.

Wolves hunt elk by chasing them across open country.

This gives them time to select their quarry.

By running the herd, the pack can spot the weak and the slow.

Many hunts end in failure.

But, with teamwork and a bit of luck, they can secure a meal.

With wolves,

the elk population consists of only the healthiest animals.

In this way, wolves and elk are interdependent.

However, before the time of the wolf,

Yellowstone's burgeoning elk population

grazed in open country without fear.

And here lies the key to our puzzle.

Without fear of predation by wolves, the elk had free range

and their grazing habits radically changed the landscape.

Especially on the river banks where everything was grazed to the ground.

Shoots of bank-side trees were continually pruned

and never gained a foothold.

Without trees and shrubs strengthening the riverbanks,

erosion was rife.

This was exacerbated again by the trampling of the herds.

Without roots,

the water-holding properties of the soil are diminished.

And summer flows decrease.

This in turn encourages silt to build up in fish spawning beds.

As a whole, biodiversity decreased.

Yellowstone was left with less habitat for birds and insects,

and less fish for otters and birds of prey.

Then along came the wolf.

The culture of fear instilled by the wolf

returned the elk to their naturally flighty state.

Elk no longer stay exposed in one place for too long.

Instead, the herds stay on the move.

In their absence, vegetation once chewed to its roots

have had a chance to gain a foothold.

Willows, aspen and cottonwood trees started to re-colonise river banks.

This is prime habitat for much of Yellowstone's wildlife.

And absolutely essential for one in particular.

The beaver.

Beaver dams encourage the growth of aquatic plants.

These, in turn, provide food and habitat for aquatic insects,

which themselves provide food for birds and fish.

The enriched soil encourages further growth of trees,

creating more nesting and feeding sites for squirrels and birds.

The resulting ponds act as reservoirs,

increasing water flow during the summer months.

This provides a much better habitat for fish to breed in.

And fish provide an important resource for ospreys and otters.

All this is the result of the most unlikely of connections.

That between Yellowstone's top predator

and a shy, industrious rodent.

Yellowstone is once again a landscape

shaped by fear of the wolf.

By saving the wolf, human intervention has had

a far more beneficial impact than was ever thought possible.

They control predator and prey alike,

and have unleashed a cascade of changes,

increasing biodiversity at all levels of the food chain.

With wolves reinstated as apex predator,

Yellowstone is once again one of Earth's great microworlds.

WOLF HOWLS