Original subtitles

One continent on our planet changes

more dramatically than any other.

North America.

Whole landscapes here can be transformed in a matter of minutes.

And all life has to be ready to seize the moment.

Getting it wrong can be disastrous.

But time it correctly, and there can be great rewards.

This is a land of opportunity.

Over 100 times the size of Great Britain,

North America stretches from just above the equator almost to the North Pole.

Temperatures can range from 50 degrees Celsius to minus 40.

The coldest wilderness of all lies in the Arctic north.

Here, in Canada's Yukon Territory, winter is brutal.

Many animals have travelled south to avoid the worst of the cold.

For those which stay behind,

life becomes very hard indeed.

A Canadian lynx.

No other kind of cat, anywhere,

lives farther north than this one.

The search for food could take him hundreds of miles.

A snowshoe hare.

This could be a chance.

He's lost the element of surprise.

And now there's a tangle of branches between him and his prey.

Missed again.

More of his energy wasted.

Another hare.

This time, out in the open.

And another failure.

If he doesn't kill soon, he may not survive these winter months.

But in North America,

change is never far away.

April - and billions of tonnes of snow and ice are starting to melt,

bringing new opportunities.

Much of the water drains southwards.

10,000 creeks join, forming North America's biggest river system.

The meltwaters travel 2,000 miles down the Mississippi,

right through the heart of North America.

In the streams of Tennessee, river chub are getting ready to breed.

This male is busy building a mound of stones, in which,

if he's lucky, a female will lay her eggs.

As the river flows faster, the work gets harder.

It's a good start, but rivals are doing the same thing

and want some of his pebbles.

The intruder seen off,

and it's back to business.

A few finishing touches.

And then, with the last of some 7,000 stones,

his nest is complete.

And right on time, here is a female.

But the mound now needs one last feature.

A trench where she can lay her eggs.

And with a gentle shimmy of his body, he nudges her into position.

And she deposits her eggs

where they will be safe from both currents and predators.

H is work is done.

The eggs will hatch, and then his mound will be slowly dismantled

by the current.

So next year, he'll have to start all over again.

Spring is now well on its way.

North America is wonderfully rich in wild flowers.

There are over 10,000 species of them.

Hummingbirds fly up from the south to feast on the nectar.

Along the North American coast, the tides are the largest in the world.

The fragmented shoreline forces sea water to rush up narrow inlets.

And each time the tide retreats, it opens a freshly stocked larder.

7,000 square miles of extra coastline

is uncovered in the space of just two hours.

A mother black bear is looking for something suitable for her cubs.

This is their first-ever trip to the seaside.

In a few hours, the tide will return.

So, they must keep up with Mum.

Here's something tasty -

crabs.

Big crabs can give a nasty nip.

So, it's best to start off with smaller ones.

During spring, three quarters of the bears' food comes from the beach.

But now, this family is not alone.

An adult male.

He's double her size, and they're in his territory.

The cubs know that call.

It's time to head for safety.

Bears have poor eyesight, but their sense of smell is acute,

and the male has detected intruders.

He knows exactly which tree they are in.

If the cubs stay up there, they'll be safe,

but they're losing precious feeding time.

For now, he's content to leave his scent mark.

A warning note for trespassers.

The family moves on.

For the cubs, lunch today will have to be a takeaway.

The tide comes in.

And within minutes, feeding time is over for another day.

As the continent warms,

on a few special nights,

when the temperature is just right...

...the forests of North America put on a show.

Male fireflies are taking to the air.

These insects are not, in fact, flies, they're a kind of beetle.

The light produced by the males is the most energy-efficient on Earth.

Some species synchronise their flashes

to increase the visibility of the message.

If a female sees something she likes,

she returns the signal.

It looks as if the message is working.

But the forest is booby trapped...

...by orb-weaver spiders.

The firefly continues to glow, even in death.

Other males see the flashes, but not the dangers.

The spider's larder is filling up.

But for every firefly that gets caught,

thousands avoid the traps.

And this male is lucky.

He's found a mate.

And over the course of the night,

millions will pair up.

There are over 150 different species of fireflies in North America,

and each has its own flash code.

Once they're mated, the females deposit their eggs in the ground.

And for those that didn't find a mate,

their one chance has come and gone.

Little rain reaches the heart of the continent.

For over a million square miles, there's not a tree in sight.

Many of those living here spend much of their time underground.

A prairie dog - a kind of large, burrowing squirrel.

Sentinels keep watch, ready to warn others of any danger.

And at this time of year is a new generation to protect.

This mother has six pups.

The youngsters constantly beg for food.

After all, the pups aren't great at sharing.

But the prairie dogs don't have the prairies to themselves.

An American badger.

The pups have never seen one before.

And badgers eat pups.

A burrowing owl.

It also knows that badgers are dangerous.

It, too, has young and does not want the badger anywhere near them.

Eventually, the badger gives up.

He sets off to look elsewhere.

If he can find a place where the grass is higher than he is,

he will have a better chance.

The mother sounds the alarm.

Adults know the danger all too well.

Four pups make it to safety below.

But their mother dare not wait above ground any longer.

Half of all prairie dog pups don't survive to adulthood.

Predators are not the only problem faced by animals

living on these great plains.

Warm air sweeps up from the Gulf of Mexico and across the open prairies.

And here, with no mountains to block it,

it meets the frigid air from the Arctic.

The results are spectacular.

As the air currents mix and swirl,

they build into one of nature's most fearsome forces.

Tornados.

These twisters spin at speeds of up to 300 miles an hour,

the world's fastest wind.

Over 1,000 tornadoes touch down here every year.

These drenching summer storms

are prevented from spreading westwards by the Rocky Mountains.

Some peaks are over four kilometres high.

So, beyond them, to the west, much of the land remains parched.

The great American deserts cover over a million square miles.

Here, roasted by the sun and blasted by the wind, the rocks disintegrate,

and mountains, particle by particle, are reduced to sand.

These pillars are all that remain of a plateau where dinosaurs once roamed.

Few animals can now survive here.

In summer, as in all deserts,

the enemy is heat,

and it returns every day.

At seven in the morning,

the temperature is already 25 degrees Celsius.

The clock is ticking

for one unusual descendant of the dinosaurs.

A roadrunner.

Found only in the deserts of North America.

Built for a life on the ground.

He can run at over 20 miles an hour.

But the prey he seeks are one step ahead.

The roadrunner's challenge is picking the right target.

A Gila monster -

too big.

By mid-morning, it will be 40 degrees Celsius.

Even the smallest lizards will soon head for cover.

A centipede.

Slim pickings for a morning's work.

Hunting should get easier

and prey will stay out longer

once the summer's peak begins to fade.

On the East Coast, summer is coming to an end.

Cold currents are sweeping along the coast of Florida.

And with them, come grey mullet on their migration

to their spawning grounds in the south.

Millions of them -

and packed so tightly, they stain the water like an oil slick.

Few people on the beach are even aware of them.

The mullet keep as close to the shore as possible,

for in deeper water, there are predators.

Close to the beach,

they have to keep clear of human swimmers.

But now hunters from the open ocean have detected them.

Tarpon - over two metres long and weighing over 100 kilograms.

To get close to the mullet in the shallows,

the tarpon turn on their sides.

Now the mullet can't see their shining silver flanks

until it's too late.

And feasting begins.

Black-tipped shark arrive.

Pelicans join in.

But these attacks have little effect on the immense shoals.

The mullet outnumber their enemies by 10,000 to one.

So, the great procession travels on, into the Northern Atlantic Ocean,

where, at last, they will be able to spawn in safety.

Autumn arrives in North America.

No mountain range spans the continent from east to west,

so there is nothing to stop cold air from the Arctic

sweeping down unhindered as far south as Louisiana.

Autumn brings colour to the southern swamps.

Here, it has been hot and humid for most of the year,

and now these sub-zero temperatures are something of a trial

for the local inhabitants.

An American alligator.

The alligator reacts to the chill by slowing its heart rate

to barely one beat per minute.

It's a kind of energy-saving winter sleep.

But other creatures don't have this ability.

The largest inhabitants of the swamp.

A relative of the elephant.

A manatee.

A female provides milk for her calf from a teat behind her flipper.

She might be able to survive the chill...

...but her calf cannot.

So every autumn, they need to find warmer waters.

They have a long journey ahead -

it could be hundreds of miles.

And she is not alone.

The warm waters they need to survive come from an unusual source.

An immense underground river

flows through subterranean channels in Florida's rocks.

In places, it comes to the surface, creating over a thousand springs.

Now, in winter,

these springs are ten degrees warmer than the surrounding waters.

So manatees come here every year from far and wide.

And here they can relax.

They can get their backs scrubbed.

And they socialise.

No-one is in much of a hurry.

And young manatees have a chance to meet some surprising neighbours.

In summer, playing with an alligator might be a dangerous game.

But now, at any rate,

the giant reptiles are still too chilled to react with any vigour.

Without these warm pools,

few of Florida's manatees would be able to survive the winter.

But the waterways here are now very popular with people, too,

and many manatees carry scars inflicted by the propellers of boats.

Her calf has already had its first scrape.

Every year, more than 100 manatees die from human causes.

As the human population of Florida grows,

so wildlife is coming under increasing pressure.

Across North America,

a million acres of wilderness are lost every decade.

But it is in the far north of the continent

that human beings are affecting wildlife most critically.

Canada is warming faster than any other country on Earth.

Polar bears have always relied on sea ice for their hunting.

But summers are now getting longer and hotter.

For most polar bears,

this is a time of starvation.

On the shores of Hudson Bay,

some bears are finding a new source of food.

They're all looking for the same thing,

and they have their own ways of catching it.

First, they must find the right rock.

Now they must wait.

The tide comes in, bringing with it other northern giants.

Beluga whales.

Close to the shore,

their calves should be safe from ocean-going predators such as orca.

Over 3,000 whales have gathered here.

A young male makes his move.

Polar bears are skilled marine hunters...

...and beluga are slow swimmers.

As long as a whale can keep an eye on a hunting bear,

it should be safe.

This young bear is still learning.

More experienced bears know to be patient.

Success will depend on split-second timing.

A bite at the back of the head, and the kill is swift.

This extraordinary behaviour has only been reported here,

in this remote corner of North America...

...and only in the last few years.

This one small group of bears has found an ingenious way

of surviving the lean summer months.

But for others, it is not so easy.

We continue to transform our planet,

and the seasons are becoming less predictable.

Will the wildlife of North America be able to adapt?

Not only on this continent, known for its change,

but in a rapidly changing world.

Next time...

...the continent where wildlife puts on the greatest show on Earth.

Africa.