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ECOLOGY OF MANGROVE FORESTS

The term mangrove refers to salt-tolerant species of tree or shrub which grow on sheltered shores and in estuaries in the tropics and some sub-tropical regions.
There are about 60 species which occur exclusively in this habitat, and many non-exclusive species. Mangroves are outstandingly adapted to growing in sea water, which they desalinate by an ultrafiltration process. Mangrove roots typically grow in anaerobic sediment and receive oxygen through aerating tissue which communicates to the air through small pores (lenticels) on the aerial roots and trunks.
Mangroves may occur as narrow fringes on steeper shores and river banks, or as extensive forests on flat delta-land. Within any area of mangroves there may be zones or mosaics of different biological communities, depending upon many factors. These include height of sediment surface relative to tidal water movements, and salinity and nutrient supply (which in turn are influenced by freshwater inputs from the catchment area).
Mangrove forests in optimum conditions are one of the most productive ecosystems; for example a net primary productivity of 23.3 tonnes/ha/year and litter productivity of 10 tonnes/ha/year was measured for a 15-year-old stand of Rhizophora at Matang, Malaysia. The litter (such as fallen mangrove leaves) is broken down by bacteria, fungi and herbivores, and the resulting detritus supports food webs including large populations of invertebrates and fish.

The calm waters in the forests are ideal breeding and nursery grounds for young fish and shrimps, while the aerial roots, lower trunks and mud surface usually support a varied fauna of oysters, snails, barnacles, crabs and other invertebrates. The upper part of the mangrove trees is an essentially terrestrial environment with a fauna of birds, mammals and insects. Mangroves are affected by the freshwater and nutrient supply which they receive from their catchment area, and on the other hand have a strong influence on the adjoining coastal waters and associated ecosystems such as coral reefs, seagrass beds and tidal marshes. For example, they trap and stabilize sediment which might otherwise limit the growth of corals.

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