The Living Planet

In the lands between the Arctic Circle and the tropics,

each year brings a great change between winter and summer,

imposing a rhythm in the lives of animals and plants.

Up north in the great evergreen forests,

conditions in mid-winter are cripplingly severe.

Life, if it is to flourish, has three needs:

Light, warmth and moisture.

And the reason trees like these don't grow much farther north

is not only the extreme cold,

but with the long months of winter darkness,

there is not enough light in the year for them to grow.

Here in northern Norway, 300 miles, 500km north of the Arctic Circle,

there is just enough light, but it does get extremely cold.

70 degrees of frost have been measured here,

and in winter there are very heavy snowfalls.

The cold threatens to freeze the liquid within the trees,

and denies them one of their essential supplies: Water.

Although snow and ice lie all around,

the trees can't tap that water while it's frozen.

So this land is effectively as parched as a desert,

and the pine trees have as great a need to conserve water as a cactus.

All plants lose some water through their leaves,

but pine needles are protected by a near-impermeable rind.

The pores through which they breathe,

and from which water can evaporate,

are kept out of the wind by being placed

along the groove that runs the length of the needle,

each in a tiny pit ringed with a ridge.

These dry, waxy leaves are almost inedible,

but the seeds in the cones are not,

and are one of the few kinds of food available in the forest in winter.

The crossbill's special beak enables it to separate the cone's segments

and prise out the nutritious seeds.

This winter feast is never certain.

Some years every branch of the trees will be laden with cones,

in others there will only be a handful.

Then the seed-eaters must move on or die.

The few remaining cones can then shed their seeds into the snow

when there are few animals around.

Even so, there "are" some.

Voles make their runways through the snow and collect what they can.

Moose get little nourishment from pine trees,

apart from the shaggy moss that hangs from the branches.

They chew the sappy twigs and bark of birch,

but there's not enough to keep them going.

If it wasn't for the fat reserves they built up in summer, few would survive.

The winter forests can support very few plant-eaters,

but there are just enough to feed one or two hardy hunters.

The great grey owl's legs are ideal for grabbing prey in snow:

Long and covered with warm feathers.

It regularly patrols the snow,

for it can't afford to miss a single opportunity of a meal.

And this is an incautious move.

Lynx seek bigger prey.

The female has young, which, though large,

are not yet skilled enough to hunt for themselves, so they rely on her.

The cost-efficiency of hunting is precisely calculated.

If the lynx doesn't catch a hare within 200 yards,

the meat it might provide is not enough to warrant the effort,

and the lynx gives up.

Bigger prey are worth much longer chases,

and the lynx pursue roe deer with great persistence.

A single deer will provide food for the whole lynx family.

In this bleak land, even the most ferocious and capable hunters

do not scorn to scavenge.

An eagle owl will take cold deer flesh

just as eagerly as the warm bodies of voles.

A wolverine, the biggest of the weasel family,

and more than a match for an eagle owl.

The coniferous forest grows right round the globe

in a belt that, in places, is 1,200 miles across.

From Scandinavia, it extends across northern Europe and Siberia

to the shores of the Pacific.

During the last ice age, when the seas were lower,

the Bering Strait did not exist,

so the trees continued into North America,

across northern Canada to the Atlantic.

Consequently, all the trees in this vast forest

and its permanent inhabitants in America, Asia and Europe, are much the same.

But when spring comes, visitors journey up from warmer parts

and each forest takes on its own individual character.

In Scandinavia, a hawk owl,

a nomad that has spent the winter farther south,

comes cruising up north again looking for food and a nest site.

Unlike other owls, it's primarily a daytime hunter,

and relies not so much on its acute hearing as its sharp eyesight

as it waits for the melting snow to reveal rodents.

In pine trees, from Norway to Siberia,

the cock capercaillie claims his territory.

This giant grouse is one of the few creatures that eats pine needles.

His hen takes them too.

Now is the time for nesting.

The hawk owl is in search of a hole in a tree,

for it's already found its partner.

But many tree holes are occupied,

for great numbers of owls have travelled up to feed on the voles.

No owl can dig a hole for itself.

They rely mainly on woodpeckers, and none is a more expert carpenter

than the black woodpecker of northern Europe.

Their sharp beak serves as an excellent chisel,

but most prefer to work in dead trees where the wood is softer.

There are ants near this tree too,

which the woodpeckers rely on for food in winter.

Not all owls use nest holes.

The eagle owl nests on the ground, among rocks.

It already has a clutch of three eggs,

for, being a permanent resident of these forests, it paired early.

Plants now have their chance to breed.

The wood anemones are already in flower, as are the pines.

Each tree produces male and female flowers,

which mature at different times,

so the female flowers are likely to be fertilised by pollen from other trees.

Now it is as warm as it ever will be in the northern forests.

Summer visitors are arriving, and the trees echo with their song.

This willow warbler, singing so vigorously in Scandinavia,

has come all the way from the savannah country south of the Sahara.

So has the winchat.

And the lure that has brought them so far

is the sudden emergence of myriads of insects.

This bedraggled creature is hardly recognisable, for its wings have not yet expanded.

It's a pine beauty moth,

and its first priority is to leave the forest floor which is full of danger.

But not all the moths have such a clear run.

Shrews are among the first to feed on them.

Up among the pine needles, the pine beauty pumps fluid out of its body

and into the veins of its wings.

Here the moths will lay their eggs so their caterpillars can feed on the young shoots.

The wood ants have missed their chance to catch the adult moth,

but now they're looking for the caterpillars among the branches.

The colour and the pattern of the caterpillar conceals it from birds which hunt by sight,

but is no protection against ants which search by smell and touch.

Finally the body is hauled down to the nest for all to consume.

The caterpillars of the sawfly are also swarming on the pine shoots.

They do have a defence against ants: A chemical one.

As they chew, they store some of the resin from the pine needles in a pouch inside their mouth.

When a foraging ant discovers them, they dab a spot of this resin on its head, like this.

The resin damages the ant's eyes and antennae, so disorientating it that it can hardly walk.

Even if it finds its way back to the nest,

it smells so strongly and strangely that the other ants treat it as an intruder and kill it.

The ants themselves are food for others.

The wryneck is a member of the woodpecker family that has specialised in eating ants,

and particularly relishes their cocoons.

Like its cousins, the wryneck nests in holes in trees, but it doesn't excavate them for itself.

It is yet another tenant of vacated woodpecker holes.

With a long tongue you can even collect insects from the bark without leaving your nest.

Here in the far north, close to the Arctic Circle,

the sun during the summer hardly sinks below the horizon and the nights are brief.

The eagle owl hunts just as effectively in the twilight as in the dark.

It has a rabbit. The season is a good one and game is abundant.

Down in the nest on the forest floor, there is only one chick left.

The other two may have been taken by foxes.

Eagle owls often kill rival species, and this chick's last meal was a short-eared owl,

which it's not yet finished.

The single survivor has a superabundance of food.

It has grown fast and its adult feathers are already appearing through its down.

The tail of a red squirrel is left over from a previous meal,

and it even takes that too.

The voles are swarming on the forest floor.

Last winter, the pines produced great quantities of seed,

so many adult voles survived till spring

and now they're all breeding at an extraordinary rate.

This female produced her four young only three weeks ago,

but she is already pregnant again and will soon abandon this family and start a new one.

All the owls, some visitors, some residents,

scour the forest for voles.

Tengmalm's owl, up in a tree hole, has three chicks, all flourishing and all demanding voles.

The number of voles varies considerably.

It gradually builds up over a period of five to six years

until finally there are so many that they eat out their food supply and the population crashes.

These changes have their effect on the owl population.

More voles mean better-fed owls,

which produce bigger clutches of eggs and rear more chicks.

And as the number of owls increases, so they spread out into new territory.

I'm in Finland, very close to the Russian border.

In fact, those pine forests behind me are actually in Russia.

But the frontier is no barrier to the bird they call the phantom of the north,

the great grey owl,

and in years when the vole population is high,

the owl comes across these frontiers and into the Finnish pine forests.

And I know they are here already because I have just picked up this.

This is an owl pellet.

All owls, as part of their natural digestion, throw up the fur and bones of their prey.

And this, I can see, has actually got vole skulls in it.

But to discover exactly what the state of the vole population is at the moment,

I'll have to look inside the nest of a great grey owl, and to do that I'll need this.

All owls are fairly ferocious and the great grey owl certainly can be,

so as part of the standard equipment of looking for owl nests you need this.

Up there is one of their nests, and the female has just flown off.

She's perching in that tree over there, keeping a very close eye on me.

If I go up and have a look in the nest,

I may be able to get some idea as to how the vole cycle is going.

And... come on...

there is just one chick.

If the voles had been at the height of their population,

there would probably be four chicks in such a nest as this,

but the fact there is only one makes it pretty clear

that the vole population is already beginning to crash.

So it is very likely the female and her mate will soon be on their way back to Russia.

There's now just a month left of the short northern summer.

Many of the birds that came up here to harvest the insects and to breed