Ecology

Mangroves are salt-tolerant evergreen forests found in intertidal environments at the land–sea interface. They grow at tropical and subtropical latitudes in areas along sheltered coastlines, shallow-water lagoons, estuaries, rivers and deltas, mainly on soft substrates. Mangrove ecosystems represent an interphase between terrestrial and marine communities, which receive daily inputs of water from the ocean and often freshwater, sediments, nutrients and silt deposits from upland rivers.

The term “mangrove” describes both an ecosystem type and the group of woody plants with specialized physiological and morphological adaptations for living in intertidal environments (Tomlinson, 1986). These adaptations include aerial roots for respiration and anchorage in waterlogged muddy substrates; the ability to cope with salinity (e.g. through salt exclusion at the roots and the elimination of excess salt by excretion); propagules adapted to tidal dispersal (i.e. seed vivipary); and highly efficient nutrient-retention mechanisms (Ball, 1988; Hogarth, 2015).

The stature and composition of mangroves vary according to climate, salinity, topography and the edaphic features of the area in which they exist. Mangrove forests may occur as isolated patches of dwarf stunted trees in very-high-salinity or disturbed conditions and, on more favourable sites, as lush forests with canopies exceeding 40 m in height.

Most mangrove species have wide distributions, although some have restricted ranges. The highest species diversity is in South and Southeast Asia, with minor diversity centres in southern Central America and the Western Indian Ocean (Spalding et al., 2010). Mangrove diversity diminishes quickly at the geographical limits of mangrove growth in the subtropics and in arid zones, where they often appear as small trees. Nevertheless, such mangrove areas may still play essential roles for local people.

Mangroves provide habitats for a vast variety of species. One reason for this is the diversity of habitat structure provided by mangroves at the interface between land and sea. In addition to the usual canopy habitat and underground root structure of a forest ecosystem, the complex root structures (pneumatophores are lateral roots) aboveground which are inundated at high tides provide a habitat that many fish and other marine life depend on as habitat and nursery ground.

A variety of marine species, including threatened species, are attracted to mangrove forests for the high food availability, cooler water with higher oxygen content and the refuge they provide. In addition, a number of mammals and reptiles live, forage or hunt in mangrove ecosystems, including the endangered Bengal tiger (Panthera tigris tigris) which lives in the Sundarbans mangrove ecosystem in India and Bangladesh. As well, a number of migratory bird species also rely on mangroves as wintering and roosting sites along their migratory routes. Mangroves also provide a key habitat and support a number of endemic, restricted-range and migratory bird species.

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