The Living Planet

Blue water covers most of our planet, but in it are set tiny specks of land,

some the tips of volcanoes, some mere rings of coral.

They're miniature enclosed worlds

where animals and plants become transformed into new species with extraordinary speed.

If you wanted to pick a really remote desert islan cut off from the rest of the world,

you might well choose this one.

This is Aldabra in the Indian Ocean.

The nearest land in that direction is the coast of Africa, about 250 miles away.

Over there, about the same distance, is Madagascar,

and if you sailed in that direction, you wouldn't hit much

until you got to the coast of Australia 4,000 miles away.

The island is the tip of an extinct submarine volcano

that rises 15,000 feet from the bottom of the Indian Ocean and is capped with coral rock.

When it finally rose above the surface of the sea about 50,000 years ago, it was lifeless,

but now, a mere 50,000 years later, well, just look.

Frigate birds, thousands of them, circle above one end of the island.

They've come from all over the Indian Ocean, even from India itself 2,000 miles away,

to nest on this particular island in the mangroves.

The white-headed birds among them are immatures,

and there are two different species of them, one bigger than the other.

The males inflate their scarlet throat pouches to show that the site is taken,

and to attract the female.

When she arrives, he persuades her to stay with ecstatic shakes of his head.

Red-footed boobies are here, too.

They're great travellers, and their chicks, which are already fledging,

may well be fishing 3,000 or 4,000 miles away within a year.

Noddies nest not on Aldabra but on a neighbouring atoll,

building platforms of seaweed in the Pisonia trees,

and beneath, on the open coral sand, two million sooty terns lay their eggs.

Their vast numbers are an indication of the richness of the surrounding sea.

Every day, the birds take from it many tons of small fish,

little squid and other marine creatures.

The atoll itself provides no food for them.

All a pair of sooty terns seek from it

are a few square inches of dry land on which to place their single egg,

and an absence of cats, rats and all other egg-stealers and chick-eaters

that plague nesting sites on the mainland.

Such security is important to these terns,

for not only do they lay their eggs exposed and unprotected on the ground,

but their young remain flightless for several weeks after hatching

and a hungry cat could cause havoc among them.

So terns find it well worthwhile, for the sake of such security,

to fly hundreds of miles to this island.

The plants that grow on remote islands like Aldabra... how do they get here?

Well, some certainly come by sea.

In a short walk along this high-water mark,

I've picked up already three different kinds of seeds.

Here's the biggest floating seed of them all. This is a coconut.

There's the familiar nut which contains the white flesh,

and this husk, from which we sometimes make coconut mats, is the flotation device.

Nuts like this can float in the sea for up to four months. This one is dead...

...but here is one that's alive and still sprouting

The green stem springing from the top, a white rootlet striking down underneath.

Under natural conditions, coconuts establish themselves at the head of the beach.

As they grow taller, they lean out over the sand so that when they're full-grown,

their nuts will drop within reach of the high tide

and be washed out to sea to spread to other islands.

A land-living animal also reached here by sea.

The time and place to find it is at night among the coconut groves.

It travelled here as a larva in the same way as the coconuts, floating in the surface waters.

One or two in a million were washed up on the beach

and crawled ashore to live on land among the coconuts, feeding on them.

It's almost the only creature here likely to give you a painful bite,

so it needs tackling with care.

It's the coconut crab.

Its legs are so long that it can embrace the trunk of a coconut palm,

and it has no difficulty in clambering up to the top.

There it cuts down young nuts with its pincers,

and returns to the ground to feed on the soft white coconut flesh.

Crabs as a group are sea-living creatures and breathe in water by means of gills.

To breathe in air, the coconut crab has developed large pouches within its shell

that have moist linings and can act as simple lungs.

But when it breeds, it has to return to the sea.

There it releases its eggs and sperm into the water at high tide,

so that its larvae will circulate through the sea,

and may be washed up on some new island.

One exceptional land animal made the voyage to Aldabra as an adult:

Its most famous inhabitant, the giant tortoise.

Most tortoises are naturally buoyant.

If one on the coast of mainland Africa, grazing among the mangroves, were swept out to sea,

it might survive long enough to be carried by currents to the islands of the Indian Ocean,

and later to spread among them.

That, almost certainly, is how ancestors of the Aldabran giant tortoise reached here.

It's not a very hospitable place for animals like tortoises

that feed on land-living plants.

The coral rock which forms the substance of the island

erodes into a honeycomb of wickedly sharp blades and spikes.

Any creature moving over it has to step with care if it's not to cut itself badly.

Here and there, the rock forms deep pits into which tortoises sometimes tumble.

When that happens, there is no escape,

and the trapped animals, even if they survive the fall, die from starvation

Quite apart from such traps, the island is a harsh, taxing place in which to live.

The tropical sun, beating down on the animals, threatens to bake them alive inside their shells,

and the remains of casualties are common.

So as the day heats up,

the tortoises head determinedly for the few trees that can provide shade.

Here and there on some beaches grow low, windswept Guettarda trees.

By noon, the ground beneath their branches is packed with refugees from the sun,

waiting for the temperature to fall so that they can search for edible leaves.

Birds, too, can overheat.

The frigates swoop over the one almost permanent lagoon of rainwater on the island,

snatching sips from its surface.

Tortoises, too, must have fresh water.

Although they don't drink every day,

they must do every week or so if they're to survive.

Water can also cool an overheated body.

As the dry season progresses,

the water evaporates and the pools get smaller and more crowded.

Many that came here for relief are near the end of their strength.

Some are unable to drag themselves out of the mud, and so die of starvation.

And yet, in spite of all these hardships, the tortoises breed and proliferate.

There are some 150,000 of them on the atoll.

Their staple food is vegetation and they crop the grass right down to the rootstock.

But as island animals everywhere tend to do,

they've broadened their taste in food to include almost anything that is edible,

including the carcasses of their dead companions.

Flesh is too nutritious to be allowed to rot and go to waste

in this land where there is so little to eat.

50,000 years, which is the time, apparently, that Aldabra has been above the sea,

is not a very long time in terms of evolution.

Nonetheless, 50,000 years of isolation on the island

has brought changes to many plants and animals that live here.

They've begun to take on their own character,

so now they differ slightly both from the ancestors which colonised the island

and from their nearest relations elsewhere in the world.

For example, this close-cropped withered turf around me

contains about 20 different species of plants.

All have been relentlessly cropped by giant tortoises like that.

And look, for example, at this little sedge.

Most sedges bear their flowers at the top of stems that rise quite high above the leaves.

Flowers sticking up like this would not survive long on Aldabra. The tortoises would eat them.

These Aldabran sedges bear their flowers and develop their seeds

close to the rootstock where the jaws of the hungry tortoises can't reach them.

The changes that take place in an island species are not always directly useful like that.

Another of Aldabra's plants has changed in a way

that seems to have no practical significance at all.

This is a lily called Lomatophyllum.

It's slightly different in colour from Lomatophyll growing elsewhere, but that's all.

The difference is very trivial.

But some island plants are spectacularly different from their nearest relatives.

Very, very rarely, extraordinary double nuts like this

are washed up on the shores of the coral islands of the Indian Ocean.

For centuries, nobody knew where they came from.

Some said they were produced by fantastic palm trees

that grew under the surface of the sea, so they were called coco-de-mer.

People believed that their kernels could be made into irresistible love potions

and that their shells, when turned into a cup,

would render the most powerful poison harmless.

A single nut like this was literally worth a king's ransom.

It wasn't until the 18th century

that people discovered that the palms that produced these nuts

grew in one tiny group of islands in the Seychelles, some 700 miles from Aldabra.

The largest surviving group of these trees stands on the little island of Praslin.

There are male and female trees.

The males produce small yellow flowers on long spikes,

and on them lives a little gecko, feeding on their nectar and pollen.

Once again, it's an island original,

slightly different in colour from others in neighbouring islands.

The female flowers start as small reddish buds, no bigger than a man's fist,

but they will develop into the biggest seed produced by any plant.

It takes seven years for the nuts to develop,

and when they are mature, they are so large and so heavy

that almost the only way of opening them is with a saw.

Inside, you can see how very different they are from coconuts.

Not only do they have two lobes to them,

but the nut itself is full solid with flesh.

Flesh that is so heavy that these mature nuts won't float in sea water.

Indeed, sea water kills them.

And that means two things.

First of all, that these palms have never been able to spread to other islands,

and secondly, that they must have actually evolved here.

Isolation changes not only plants but animals.

On Aldabra, wandering among the tortoises are sacred ibis with light blue eyes.

Others elsewhere have dark eyes.

The Aldabran ibis breed among themselves and feed on small shore creatures.

Land crabs are far too big to be eaten,

but they have to be pecked to clear them out of the way.

Several species of Aldabran birds have developed slight variations that make them unique.

The kestrel here is slightly smaller than the Madagascar species, but otherwise the same.

The Aldabran sunbird, however, is a little darker than its African relations.

But perhaps the most dramatic and certainly the most endearing quality

brought to some of the birds of Aldabra by isolation is this.

Not only extreme tameness, but flightlessness.

This is the Aldabran rail.

Flying takes a lot of energy.

It's of obvious value when escaping ground-living enemies,

but there are no such enemies on Aldabra or other remote islands.

So some birds that reach such islands by air have given up flying.

Their wing muscles have dwindled and they can't fly even if they wanted to.

The Aldabran rail is only one example.

A kind of pigeon once lived on another island in the Indian Ocean: Mauritius.

It, too, became flightless and grew as big as a turkey.

It was so tame that European sailors were able to kill it with clubs.

They called it the dodo, and in less than 200 years after finding it,

they'd exterminated it.

Grazing alongside the dodo in Mauritius,

and living in other islands in the Indian Ocean as well, were giant tortoises.

They, too, were taken for food by seamen and were exterminated.

But Aldabra is so remote that few ships come near it,