Mangroves. Adaptations. Associated Species

Mangroves are woody plants that grow between the land and the sea, in the intertidal zone. The plants belong to a number of unrelated families, but have all developed special adaptations to the particular needs of life between the tides. About seventy species of mangrove plants have been recognized. Most are shrubs and trees, but they include ferns and a palm.

Many experts list only thirty to forty species as "core" members of the mangrove community, which typically dominate. These include tall red mangroves from the genus Rizophora with highly distinctive stilt roots and the black and gray mangroves from the genus Avicenna, smaller trees which surround themselves with upward pointing pencil roots and which are capable of surviving in very harsh environments. The Nipa palm, which can only survive in salty waters, is commonly found on the banks of tropical estuaries, and its eaves can reach nine meters in length and are often used to build roofs.

These plants often grow in abundance, forming large forests of mangrove habitat along coastlines and into estuaries in the tropics and near tropics. These areas are variously known as mangrove forests, mangrove swamps, or mangal.

Numerous other species are found within mangrove forests, including nonmangrove plants, which grow on the leaves and branches of the mangroves (epiphytes), and animals from both land (birds, reptiles, and insects) and sea (fish, mollusks, and crustaceans). These forests are also of considerable importance to people who use them, among other things, for food and timber.

Adaptations. The intertidal zone is a harsh environment, where species have to adapt to regular inundation, interspersed with drying out as the tide rises and falls. The salinity is high, creating significant physiological challenges. In many cases this salinity is also highly variable, ranging from almost fresh to hypersaline. Adaptations that mangroves use include: Aerial roots. The soils around mangrove roots are usually waterlogged.

At high tides waters may rise two meters over the root surface. In order to maintain a supply of oxygen to the roots, many mangrove species have developed specialized aerial roots. These include pneumatophores—upward extensions of roots into the air—which may be long and pointed, such as the pencil roots of Avicennia, or the more conical forms of Sonneratia. Root-knees are another form of pneumatophore, where more rounded knobs extend up from the soil.

Equally characteristic are the prop or stilt roots, typified by Rhizophora. Some species also have buttress roots, which may extend into longer sinuous roots known as plank roots. In all cases these aerial elements of the root system provide a critical means of obtaining oxygen and are endowed with large internal spaces for gas storage and a large area of lenticels where gaseous exchange occurs.

Salt exclusion or extrusion. All mangroves have adopted mechanisms to reduce salt uptake. Some may use ultrafiltration at the cellular level to keep salt out from the roots. Others can secrete salt from their leaves using special glands, and salt crystals can often be seen on the leaf surface of species such as Avicennia.

Dispersal. Living in the sea it is difficult for small seeds to be dispersed, to find their way into the soil, and to take root. All mangrove seeds or seedlings float, and rely on water to disperse them to new areas. A number of mangrove species have adapted still further and undergo initial development whilst remaining attached to their parent tree. This is known as vivipary, or "live-birth." In species of Rhizophora the embryo grows out of the seed coat, and out of the fruit while still attached to the tree. The resulting long propagule is effectively a seedling. It floats upright in the water and is designed to become trapped into the mud on a falling tide.

Associated Species. A broad wealth of species is found living in mangrove communities. These include animals and plants coming from land, sea, and fresh waters. The sheer physical complexity of the mangrove environment, with its tangled roots and with a great network of twisting channels between the trees, makes the mangrove forest an important area for many animal species. These include fish that come in from the open sea to breed, with their young utilizing the shelter and the abundant food as a nursery area.

Mudskippers are a familiar resident in mangrove forests from West Africa east to the Pacific Ocean. These extraordinary fish (a type of goby) are capable of gulping air and spend much of their lives out of water. Many climb onto the base of trees, but they always remain close to water and will race across the water surface if frightened.

Some of the most commonly observed animals in the mangrove forest are crabs. In one Malaysian forest a density of seventy crabs per square meter has been recorded. The larger mangrove crabs take shelter in burrows during periods of low tide. Many are edible and are widely caught. Fiddler crabs are another characteristic group.

The males of this group have different sized claws; one highly enlarged claw is used in territorial displays and fighting. Mollusks are also abundant, including mangrove snails, which feed on detritus, but also oysters and mussels, often attached to the mangrove roots, which filter the water for plankton. These are often collected for human consumption.

Microscopic life within mangrove mud is abundant, though poorly known. Many species, including some crabs and mudskippers, feed by filtering through this mud. At the other end of the spectrum, very large animals are rare in the mangrove forest. The estuarine crocodile is a regular inhabitant in mangrove forests of Australia and the islands of Southeast Asia. In the Sundarbans, perhaps the largest continuous mangrove forest in the world, between India and Bangladesh, there are still tigers. And on the island of Borneo the proboscis monkey has a specialized digestive system enabling it to subsist largely on a diet of mangrove leaves.

Importance to People. Although mangroves were mostly probably likely familiar to the ancient civilizations from Egypt to China, the first written record of mangroves were probably those of Nearchus, a commander under Alexander the Great, who undertook a voyage from the Indus River through the Arabian Sea and the Arabian (Persian) Gulf in 325 все. Theophrastus, the Greek philosopher and botanist, described mangroves in greater detail, based on observations in both the Arabian Gulf and the Red Sea. It is not so strange that these Europeans should have written down their observations of these trees—for them the concept of a tree growing in the sea was entirely alien.

Mangroves are of considerable importance to people today, most obviously through their role in supporting fisheries and forestry, but also in a less appreciated role as a form of coastal protection.

Fisheries. Crabs, oysters, and shrimps are among the very high value species regularly caught in mangrove forests. In some places farms have been established within the mangrove channels themselves. Even away from the mangrove forests, many fishermen gain considerable benefits, as the fish they catch often use the mangroves as a place to breed. Unfortunately the location of mangrove forests has placed them in direct competition with many entrepreneurs seeking to develop shrimp aquaculture.

In many places, but notably across Southeast Asia, mangroves have been cleared to make way for the building of shrimp-ponds. Profits from such ponds are often considerable, but poor design has led to many being abandoned within a few years. The disruption of the ground prevents the return of mangroves to these abandoned farms.

Forestry. Mangrove wood has been used for many years, and is known for its high density and resistance to rotting. Although trees rarely reach very large sizes, smaller trees are used for poles, typically used in the construction industry, while in many areas the wood is also used in the production of charcoal. In a number of places this use of mangrove has been developed as a sustainable industry with regular harvesting on a fifteen-to twenty-year cycle.

Coastal protection. Few people, even amongst those living adjacent to mangroves, are aware of the important role they play in protecting the land from erosion by the sea. Although they are not absolute barriers, mangrove roots bind the sediments and prevent them from being transported by currents and waves. During storms they also reduce the impact of large waves on coastlines.

Despite their importance, it would appear that vast areas of mangroves have been lost. A recent estimate suggested that there are probably 181,000 square kilometers of mangroves in the world, but it seems likely that 100 years ago there may have been double this area. Efforts to protect mangroves remain limited, although there are now over 850 protected areas with mangroves. In some areas mangroves are being actively planted, in recognition of their important commercial value.