Mangrove ecosystems provide a unique and valuable range of resources and services
The collective noun mangrove designates a tidal wetland ecosystem formed by a very special association of plants and animals that live in the intertidal areas of low lying tropical and sub-tropical latitudes. It is also used to designate halophytic marine tidal forests comprising trees, shrubs, palms, epiphytes, ground ferns and grasses. They are one of the easiest tropical forest types to generate because of their reproductive biology and adaptation to intertidal conditions.
Mangroves can be classified in to three broad categories. Firstly, true mangroves are mainly restricted to intertidal areas between the high water levels of neap and spring tides. Plant species from true mangroves belong to at least 17 different families. About 80 species of true mangrove trees/shrubs are recognized, of which 50~60 species make a significant contribution to the structure of mangrove forests. Species diversity is much higher in the southeast region, where approximately two-thirds of all species are found while approximately 15 species occur in Africa and 10 in the Americas (Field, 1995). The species composition and structure of the mangrove forest varies as a function of geophysical, geographical, geological, hydrographic, biogeographical, climatic, and edaphic factors and the environmental conditions.
Rhizophora species occur in all three regions. Secondly, minor species of mangroves are distinguished by their inability to form conspicuous elements of the vegetation and they rarely form pure communities. The third category, the mangal associates, are not found exclusively in the proximity of mangroves and may occur only in transitional vegetation, landwards and seawards. However, they do interact with true mangroves. These are salinity tolerant plant species such as Terminalia, Hibiscus, Thespesia, Ficus, Calophyllum, Casuarina, some legumes and milk weeds (Aslepiadaceae and Apocynaceae). Some of them even have their roots in salt water.
Collectively they fulfill at least some of the ecological roles of mangrove associations, through litterfall and root exudates for instance, dissolved nutrients and the droppings of bats, birds and other animals that nest and live in the canopy or among the roots. On the seaward fringe, beach and dune fixing plants like Ipomoea pes-caprae, Sesuvium portucalastrum and species of Salicornia Arthrocnemum consolidate the sandy, sea front. Species such as Porteresia (= Oryza) coarctata tolerate some degree of salinity. On the landward side thrive the coconut (Cocos nucifera) the sagu palm, the pepper wine, and species of Dalbergia, Pandanus and Hibiscus tiliaceus. The only other group of vascular plants that has successfully adapted to sea water are a few species of sea grasses, which thrive on the sea front and salt marshes, and in the tropics are often found associated with mangroves and coral reefs.
The epiphytes are quite abundant in the most humid areas of mangroves. They belong to different families, most notably two species of semi parasitic Loranthaceae and a true parasite of the genus Viscum (mystletoe). Lichens, mushrooms, ferns other than Acrostichum, occur in the branches and trunks. In the drier areas exist the Bromeliaceans, prominent among them Tilandsia usneoides, that occur in the Americas.
It is a common feature of tropical estuarine brackish waters bordered by mangroves, that the standing stock of phytoplankton is dense in the lower reaches where it is dominated by diatoms, specially those of the genera Coscinodiscus, Pleurosigma, and Biddulphia. The zooplankton is representcd by almost all aquatic groups of animals from protozoa to fish eggs and fingerlings as well as larvae of most zoological groups except Echinoderms. Pathogenic bacteria such as Shigella, Aeromones, Vibrio can survive in the nutrient rich mangrove water, sometimes contaminated with noxious chemicals (such as flavonoids, tannic acid and derivatives), pesticides, fertilizers and untreated domestic sewage and industrial waste. Some of these lignolythic, cellulolytic, proteolytic bacteria and other micro-organisms can break down large organic molecules such as tannins and cellulose into useful smaller fragments. Higher algae are common, specially on pneumatophores and stilt roots.
Inventory of the world's Mangrove forests
World-wide - 15.5
Indo-Pacific - 6.9
Americas - 4.1
Africa - 3.5
Important elements of the mangrove soils are the microbes, bacteria, fungi, and blue-green algae (Cyanobacteria).
Approximately one-fourth of the world's tropical coastline is dominated by mangroves and they extend over 15.5 million ha world-wide. The most extensive and luxurious mangroves extend across the Indo-Pacific regions where they are best developed in the delta system of major rivers. The largest single area of mangroves in the world lies in the Bangladesh part of Sunderbans, covering an area of almost 600,000 ha including waterways. There are about 6.9 million ha in the Indo-Pacific region, 3.5 million ha in Africa, 4.1 million ha in the Americas including the Caribbean. Mangroves also penetrate some temperate zones, but there is a rapid decrease in the number of species with increasing latitude (Chapman, 1977; Tomlinson, 1986).
The earliest references to the uses of mangroves dates back to the year 1230. Reference is made to the use of Rhizophora seedlings as food in times of famine, to cure sore mouth, to produce fuel, tannin and dye and wine having an aphrodisiac effect when ingested and of their use as philters in Arabia. The uses of mangroves are many and varied. Mangroves are of great importance to many people who live along tropical shorelines. In southeast Asia mangroves have been managed as a sustained yield forest crop for more than a century. In recent years the ecological, environmental and socio-economic importance of mangroves has been emphasised by international agencies, governments, local authorities' non government organizations, coastal communities and scientists.
Large areas of land formally occupied by mangroves have been reclaimed and planted with cash crops such as rice, coconut and palm oil
Though mangrove ecosystems provide a unique and valuable range of resources and services, huge areas of mangrove have been lost, especially in southeast Asia and most parts of south Africa, due to wood extraction, conversion to agriculture, coastal aquaculture and salt production, coastal industrialization and urbanization. Recently, shrimp farming has caused large scale losses of mangrove habitats in several countries, the worst cases being Ecuador, Indonesia and the Philippines among others. Large areas of land formally occupied by mangroves have been reclaimed and planted with cash crops such as rice, coconut and palm oil. In drier areas, mangroves are converted to salt pans. The pressures of increase in population, food production and industrial and urban development have led to a large proportion of the world's mangrove resource being threatened by destruction. In Papua New Guinea, Thailand and Malaysia the mangroves are destroyed for mining for tin and other minerals.