The Living Planet

This is a battle ground.

In many places, the sea is forcing the land to retreat, cutting back its cliffs

and leaving islands and towers as markers of the territory that the land has lost

The debris is swept away

and strewn on beaches farther down the coast as sand and gravel.

In some places, the land is advancing.

In the tropics, mangroves are moving out into the sea, gathering mud

and building new territory for land-living creatures.

Even in the mouths of rivers,

where fresh water laden with sediment mingles with the salt water of the sea,

new land is being created of a sort.

I'm in an estuary in the west of England.

You might think that this mud is not the most attractive stuff in which to live.

Certainly, animals that do live in it have to face some severe problems.

Part of their time they're out of water like this,

part of the time they're underwater.

The saltiness of the water, too, varies.

Fresh water comes down from the land, the tides bring in salt water.

And then there's the nature of this extraordinarily sticky mud itself.

It is so glutinous that little oxygen gets into it

but the rewards for enduring these unpromising conditions are high.

Edible particles deposited on the surface of the mud

are cautiously sucked up by the searching siphon of Scrobicularia,

a mollusc whose main body, enclosed in a shell, hides in the mud for safety.

A tiny crustacean, Corophium, half an inch long,

grazes on the bacteria which proliferate in millions,

breaking down rotting organic matter in the mud.

Ragworms live in burrows

and will tackle Corophium, algae, bacteria, almost anything that's around.

The puddles are flecked with floating mucus.

It is produced by spire shells, no bigger than grains of wheat.

The mucus attracts bacteria, and the spire shells eat the lot.

The peacock worm fans out its tentacles from the top of its tube

to gather food particles before they settle.

Beating threads on each filament of the fan

transport the catch down to the mouth at the centre.

While it feeds, it also disgorges a cement of mud and mucus

and builds up the margin of its tube.

The cockle lies with its shell agape,

filtering the water by sucking it in through one siphon...

...and blowing it out through another.

Mussels use the same technique, collecting within their shells

substantial quantities of the abundant and nutritious drifting particles.

When the tide goes out, they clamp their shells tightly together

to keep in their moisture and to keep out attackers,

but some creatures know how to deal with that.

Each oyster-catcher has its favourite technique for dealing with mussels.

It is usually the same as that used by its parents

though a bird needs years of practice before it becomes really expert.

Some hunt in the shallow waters for mussels that have not yet shut their shells.

Others carry unattached shells away from the main flock

so they've got a little privacy.

They skilfully place the mussel in such a position that they can cut it open along its hinge.

Other individual birds resort to brute force.

They hammer their way in through the shell itself.

As the tide retreats still further, spire shells are exposed,

as many as 35,000 buried within a single square yard.

All these mud feeders together constitute a rich prize,

and there are abundant claimants.

Sandpipers, on migration, depend on them,

but at all times of the year, wading birds come to the estuaries to feed.

The godwit, equipped with long legs and a long bill,

can wade in water several inches deep

and collect food before it can be reached by other birds.

The curlew prefers to work out of water.

Its long bill enables it to probe deep into the mud for a worm,

and serves equally well as a pair of forceps.

The dunlin is a smaller bird and goes for smaller prey:

Ragworms and insect larvae.

It feels for its food with its short bill.

The ringed plover, with a very short bill,

can only collect food from the surface and locates it by sight.

It works alone so that its prey won't be disturbed by pattering feet

and withdraw before being spotted.

The scything action of the avocet

collects creatures that live in the liquid mud.

Their bills are very sensitive. As soon as they close on something edible,

the bird can juggle it up into its mouth.

The quantities of food taken by wading birds from estuaries is enormous.

Some species consume every day about a third of their own weight in food.

In a year, a single oyster-catcher

can consume the flesh over half a ton of cockles,

and many an estuary supports tens of thousands of wading birds,

so these places are rich indeed.

As the river brings down more and more particles of mud,

so the flats grow bigger and higher,

and on their surface they develop a slimy skin,

and that's formed by microscopic plants, algae.

They start the process of consolidation.

But soon, bigger plants get root, like this glasswort,

and now the process really speeds up.

As the high tide brings in more mud particles, they clog around the stems of the glasswort

and don't swill back to the sea when the tide fall

So with each new tide, the flats grow higher and higher.

Glasswort is a plant of the cold estuaries of Europe.

In the tropics, the colonisers of mud are not small plants but trees:

Mangroves.

This mud is the pulverised remains of rocks eroded from the Himalayas

that has been carried down by the Ganges for 1,000 miles

and dumped on the edge of the Bay of Bengal.

This is the biggest intertidal forest of all, the Sunderbans, 4,000 square miles of it,

and here roam many animals that usually live in dry-land forests.

Axis deer.

Woodpeckers: The Indian golden-banded.

And wild boar.

But mangrove forests also harbour creatures that live nowhere else at all.

The proboscis monkey eats almost nothing but mangrove leaves.

It developed that specialism on the island of Borneo,

and has never spread overseas, trapped by its own specialised requirements.

Mangroves themselves are distributed widely through the tropics,

for they have evolved from many different plant families

and today there are some 40 different species of them.

The flowers of this pioneering mangrove are pollinated by the wind.

The seed doesn't immediately leave the parent tree.

It starts to grow while it is still attached,

producing a green shoot a foot long with a sharp end to it.

If it falls when the tide is in,

it floats horizontally in the buoyant salt water

and may be carried for miles before being stranded.

If the tide is out, it stabs the mud and stays in that position when the tide returns.

It puts out rootlets from the bottom and leaves from the top,

and within a few days, it's firmly established.

Just as in cold-water estuaries,

there's a lot of organic matter in this mud.

Because it's so sticky, it isn't stirred up, so there's little oxygen in it,

and the process of rotting produces within the mud itself

an acid, smelly, poisonous chemical: Hydrogen sulphide.

So these roots don't go down far into the mud.

Instead, they support the trees by their sheer number.

But what about the other things that normal roots do for normal trees,

like gathering nutrients and water and oxygen?

Well, these roots deal with the nutrient problem like this.

It has this cluster of very fine roots

which don't go more than an inch or so below the surface of the mud,

but it is on the surface of the mud that the bulk of the nutrients are found.

As for water, there's plenty of it here, but it's salty.

Some mangroves have a special membrane around the cells in the root hairs

which filters off the salt.

Others absorb the salt but then excrete it from the leaves,

or concentrate it in the leaf and then the leaves are shed.

And oxygen, well, there are several different solutions to that problem.

This mangrove has pores actually in these prop roots

which absorb the oxygen directly.

This one has roots which actually grow upwards,

so keeping pace with the rising surface of the accumulating mud.

It's not only plants in the mangrove swamps that have difficulty in getting oxygen.

So do animals, and this time, low tide, is a period of particular difficulty.

Many molluscs, like cockles and mussels elsewhere,

shut their shells to keep what moisture they have

and wait for the food-and-oxygen-bearing water to return.

For them, it's a period of inactivity, but for other creatures, it's just the opposite.

The mudskipper, of course, is a fish.

There are several different kinds.

This one lives near high-water mark,

and is the sort that spends most time out of water.

It has to keep its skin moist for it absorbs oxygen through it.

It also keeps its mouth full of water swilling over its gills.

It feeds on the little crabs that graze on the mud

And having got one, it needs another mouthful of water.

A second kind lives close to low-water mark,

so it is only out of water for an hour or so each day.

It sifts the liquid mud for small crustaceans and worms.

In between these two kinds lives the largest of the three.

It is a vegetarian, collecting algae and other microscopic plants from the mud.

And it, too, nips back every now and then for a wet.

It guards its grazing rights with vigour,

building walls around its territory.

And when neighbours meet, there's trouble.

On clear mud, their territories form a patchwork of walled ponds.

These flats are very flat, so when a male starts to advertise for a mate,

he has to be a bit of a gymnast.

When a female is enticed into his private pond,

he can continue his courtship at close quarters

in a more conventionally fish fashion,

with flexed fins, waggling tail and enormous excitement.

They'll spawn in a burrow at the bottom of the pond.

This crab is too big to be intimidated by mudskippers,

even when it does wander through their territories.

Its scissoring mouthparts not only sort out its food but help it to breathe.

On top of its shell, there is a puddle of water,

and as its mouthparts move,

they circulate this into a gill chamber within the shell,

out again and up to the reservoir on the top.

Eventually, the oxygen in the water is exhausted

and the crab has to return to the sea, tip it off and get a fresh supply.

Close by the edge of the sea, the tiny soldier crabs feed with frantic haste.

No one else will steal their mud, but they have to eat an enormous quantity

to extract the few particles necessary to keep alive.

They have to work at it pretty well non-stop and have no time to waste.

High up, beyond the reach of all but the highest tides,

lives the large mangrove crab.

It keeps moist by boring its hole as much as six feet deep to reach water.

The lure that tempts it out is a newly fallen mangrove leaf.

And quickly back to safety.

Among the air-absorbing roots of the mangroves, fiddler crabs are busy.

The females collect mud with both pincers,

working with the same frantic speed as the soldier crabs.

The males need to munch just as much mud as the females,

but work with one hand only,

for one of their claws is so big that it is useless for feeding.