In 1292, after 17 years of service Marco Polo, the Venetian traveller, finally took his leave of the great emperor Kublai Khan of China. The boat journey back to Europe took three years, by way of Indonesia, Southern India and Persia. From his records, we know that Marco Polo stopped at Palembang, then a busy port on the Sumatran coast. Today, Palembang still thrives as the capital of South Sumatra province but it now lies more than 50 km inland. Its change of position is due, at least in part, to natural land-building by coastal forests of mangrove trees.
The importance of mangroves
In recent decades the acreage of mangrove swamps worldwide has been much reduced: yet the mangrove forests are of enormous ecological importance. They act as a nursery ground for edible fish and shellfish, a haven for these vulnerable larvae, without which there would be no future generations of adults. The mangroves are also home to endangered species such as the Atlantic ridley sea turtle and the brown pelican.
They stabilise coasts, protect against storm and hurricane damage, support a large marine food web, and act as a giant filter system. In trapping muddy riverine sediments, they may aid the establishment of corals in the clearer silt-free waters offshore. Additionally, in recent geological history, mangroves have played a crucial land-building role.
As a bridging point between land and sea, mangroves may also have special evolutionary significance. It is easy to envisage how life forms moved from sea to land and back again in the mangrove swamps of times gone by. Those life forms may have included air-breathing fish that we count among our own ancestors. The true historical importance of the mangrove communities, which date back at least 60 million years, can still only be guessed at.
The march of mangroves
In temperate climates, sandy estuaries or gently sloping single shorelines are often fringed by salt marshes. The salt-marsh vegetation consists of the most part of small, hardy land plants moving seawards, which have evolved the ability to tolerate the brackish water and tough conditions of the coastal environment.
In the tropics, similar types of coastline have mangrove swamps instead of salt marshes. The mangroves form a far richer and more complex habitat, with the dense thickets of trees straddling the gap between sea and land.
Both salt-marsh plants and mangroves slow down the passing water currents, causing them to drop their load of sediment, which then becomes trapped between the mesh of plant roots. Mangroves, being larger, have a more powerful impact, accruing sediment at a remarkable speed. As the layers accumulate, so new land emerges above sea level, which in turn is slowly colonised by other plants staking out a tenuous claim on the new ground. Their presence ousts the mangroves that progress farther seaward, claiming land from the sea at a high rate.
The term mangrove embraces an unrelated mixture of species, ranging from sparse bushes to dense trees that grow very high. They are drawn together under the same label only by their shared ability to meet the challenge of life in the muddy brine, where oxygen is in short supply. Most plants cannot withstand the salty tides, lapping waves and shifting substrate. The mangroves, however, have evolved in such a way as to give them a competitive edge in this exacting environment.
Suffocating mud
The silty mud that is deposited among the mangrove roots is fine, oozing and noxious. It originates in the rivers that discharge their load into the sea, where it is carried back and forth until it settles at the slack of the tide. Below the surface layer, bacteria use up all the available oxygen as they decompose loose plant material. The mud is made acidic by bacterial action, and contains poisonous Sulphur compounds. Such conditions would challenge any plant root that attempted to establish itself here, but for a tree, which requires especially stable anchorage, it presents particular difficulties.
Mangroves have adapted in a number of ways. Above ground, many species have arching prop roots. Which emanate form the trunk and boughs to balance and support the tree. Some are branched hoops, others flange-like blades.
Underground, the roots are very shallow, and spread horizontally rather than downwards. Thus, they form a broad base, which rests in the upper layers of the mud, rather than threading through it at depth. Fine feeding-roots sap nutrients from the uppermost layer of silt, supplies of which are replenished with each incoming tide.
This layer is relatively innocuous, but a little deeper down the mangroves face the problem of sustaining their living roots in a medium that is almost devoid of oxygen. The problem is solved in a variety of ways. In some mangroves, the upper sections of the prop roots are above ground and are punctured by tiny spongy patches in the bark, which allow air to pass into the root.
For those mangroves without prop roots, the porous patches are situated on elbow-like protuberances from the horizontal roots which jut through the surface of the mud.
Ventilating those roots that are deeper down, and permanently immersed in the stifling mud, is more difficult. Some mangroves send up vertical, finger-like extensions from the roots called pneumatophores or “breathing roots”.
These roots project upwards, through the surface, as a carpet of spikes. Each has a porous tip to take in air and acts as a tiny snorkel, providing the root below with a means to breathe freely.
Besides a shortage of oxygen, the mangroves must contend with an excess of salt. Land plants, like people, need a little salt, but too much salt disrupts the vital reactions of their internal chemistry. Most mangroves have more than one strategy to combat the salt. Some have a filter system in the roots to keep as much out as possible. Other let salt enter, but sequester it away safely in special stores until it can be shed in old leaves as they fall.
Some species have glands to secrete salt onto the surface of living leaves, where it is washed away by the rain.
Around the roots, salt tends to draw the essential life-sustaining water out of the plant.
The result is a tug-of-war over every molecule of water, with the root cells pulling it inwards and the salty mud pulling it outwards. The result is a paradox in which the mangroves, while growing in a swamp, are constantly thirsty for fresh, revitalising water. Consequently, they have various water conserving mechanisms similar to those found in desert plants, including thick, succulent leaves covered by a waxy cuticle to reduce evaporation in the hot sun.
Life in the swamp
It is their ability to thrive in such salty conditions that really sets mangroves apart from other plants. Mangroves do not need salt to survive. In fact, they grow better without it.
However, they have evolved so that they can tolerate the salt, and live on the fringes of the shore, where the briny conditions keep out less specialised plants.
Further inland, where the soil is sweeter, the mangroves lose ground to other species: their extreme specialisation to brackish mud has left them less competitive on ordinary terrain, where they must battle with other plants for nutrients, space and light.
To the human visitor, a mangrove swamp offers a fearsome tangle of snare-like stems, curving stilt roots and fetid and glutinous mud. By night, it may be eerily lit with the synchronous flashing lights of fireflies. By day, biting insects whine persistently in the malodorous air, which is heavy with water vapour, sulphureous fumes and the smell of decomposition.
Yet this inhospitable swamp is home to countless small creature, which move between the knobbly roots, over the mud and up the slippery stems with ease and agility. In this uncertain zone that is neither land nor sea, a unique combination of terrestrial and marine creatures that exist side by side.
Most are adapted to a lifestyle that swings with the tides, the rise and fall of the sea shaping life among the mangroves.
When the tide is in, the distinction between marine and terrestrial life seems relatively well defined.
Above the water level, mangrove trees hold their own communities of animals, many of which have no immediate contact with the sea beneath, and show no special adaptations to this particular environment.
Crickets, ants, beetles, thrips, caterpillars, centipedes and spiders all crawl across the leaves. Lizards, snakes, bats and monkeys find shelter in the mangrove branches.
Meanwhile, the rare proboscis monkey, native to Borneo, feeds on the tender, young leaves. Large numbers of noisy sea birds, such as pelicans, frigate birds, egret, herons, cormorants, and kingfishers, arrive to roost and nest in the trees.
Unlike some of the other tree inhabitants, these birds are directly dependent on the sea for food. Out of sight, below water level, encrusting marine organisms coat the submerged prop roots.
Oysters, sponges, barnacles and sea squirts jostle for space on the crowded woody pillars, while small colonies of coral might be found attached to underwater mangrove stilts. Five-armed brittle stars clamber over the sedentary animals.
Translucent shrimps dart between the tangled roots, where there may also be a hidden octopus, a water snake or a jellyfish brought in with the tide. Weaving through the maze are crowds of immature fish, such as mangrove snappers, which use the mangroves as nursery grounds. Other valuable visitors include young bass, grouper, mullet, milkfish, spiny lobster, blue crab and prawns.
With the turn of each tide, the departing sea deposits sediment and carries away seeds and leaf litter. The distinction between marine and terrestrial life becomes more blurred.
As the waters subside, the mud is alive with an army of scurrying creatures picking over the newly revealed morsels: crabs and snails leave the sea to scour the sediment, while birds, monkeys and snakes descend from the tress to forage.
Birds probe for worms and shellfish. Snakes take crabs, fish small birds and rodents. There is even a crab-eating monkey, the long-tailed macaque, that seizes crabs as they emerge from their holes, supplementing its diet of fruits, insects and leaves.
