What are Mangroves
History and evolution of mangroves
As is usual when one enters the realms of science one must first come to term with the terminology. Scientists often tend to extend the mystique of their subject by divising an elaborate set of terms. The treatment of mangroves has not been immune to this approach.
The term "mangrove" has been applied historically to plants which live in muddy, wet soil in tropical or subtropical tidal waters. In the nineteen sixties the term "mangal" was used for a community of mangrove plants and the term "mangrove" for the plant species making up the forest. The terminology has tended to fall into disuse recently and term such as "mangrove forest", "tidal forest" and "coastal woodland" have begun to appear from groups of evergreen plants possessing marked similarities in their physiological characteristics and structural adaptations to habitats influenced by the tides. The scientific literature is divided broadly into studies of the biology of individual species of plants or animals in the mangroves and the study of communities that may involve just plants or the relationship between plants and animals. The present intention is to sketch the most important features of mangroves and mangrove communities in such a way that they can be understood by the interested user.
Mangroves can be trees, shrubs, palms or ground ferns growing in the zone between high and low tide. Every kind of plant has a Latin name or latinised scientific name and mangroves are no exception.
The Swedish naturalist, Carl von Linne (Linnaeus) in 1735 devised a system for classifying plants and animals in systematic way. Linnaeus assigned each different kind of organism a latinised double name consisting of a genus name followed by an exclusive species name. This system is known as binomial nomenclature.
An example of the taxonomic classification of a particular mangrove is as follows :
Division : Spermatophyta
Class : Dicotyledonae
Order : Rhizophorales
Family : Rhizophoraceae
Genus : Rhizophora
Species : stylosa
Scientific name : Rhizophora stylosa Griff.
Common name : Spider mangrove
There are nineteen plant families with mangrove representatives and only two families which are exclusively mangrove. There are no order or higher ranks that are exclusively mangroves. Mangroves are not a single genetic group but represent genetic adaptation of a large variety of plant families to a particular environment. In case of plant family, Rhizophoraceae, often considered to be a true mangrove family, only four of its 16 genera inhabit a mangrove habitat.
Extensive mangrove stands require a layer of earth or sand, usually deposited by rivers and flood tides and shores free of strong wave and tidal action. The also require salt and brackish water. Mangroves are often characterized by aerial roots, seedling that germinate on the tree and buoyant seeds that can be dispersed by water. Mangroves are often found in regions such as estuaries, embayments and broad muddy tidal flats where the local terrain has led to the build up of soil. They prefer sheltered places where tidal and wind are not too destructive. The conditions in which mangroves grow also influence their characteristics for survival, their size and the pattern in which they congregate. On a global scale mangrove distribution is influenced by the presence of warm and cold oceanic currents.
Mangrove shores and forest
Mangrove forests are best developed on tropical shorelines where there are large areas available between high and low tide points. Large mangrove formation are typically found in sheltered muddy shorelines that are often associated with the formation of deltas at the mouth of a river system. Mangroves can also be found growing on sandy and rocky shores, coral reefs and oceanic islands. There are instances where islands can be completely covered by mangroves. It is impossible to describe a typical mangrove forest, as the variation in height and girth, even for the same species, is immense, depending on the many factors that control growth
All plants require various mineral elements to survive and these are absorbed by the roots from the soil. Plants require nitrogen, phosphorus, potassium, calcium, magnesium, sulphur and iron. Sodium chlorides required only in trace quantities and this poses certain problems for mangroves due to high abundance of these two elements in the sea water surrounding their roots. Other trace elements required by the plants for successful growth are boron, manganese, zinc, copper and molybdenum.
Mangrove soils are quite different from those that most other terrestrial plants grow on. They are poorly drained, lacking in oxygen and are often fine grained and rich in organic matter. In appearance the soils are often clayey mud or sand.
Mangroves grow on waterlogged soils that are often lacking in oxygen. These are known as anaerobic soils, literally, soil without air. The lack of oxygen in the soil is due to the slow rate of diffusion of oxygen in water and the biological activity of microorganisms in the soil which consumes oxygen. The amount of oxygen in the soil varies according to how often and for how long tides cover the mud, how well drained the areas are, and whether there are chemicals in the soil that absorb oxygen. Oxygen in the soil could be expected to increase in proportion to the amount of time that the soil is exposed to the air and the soil is covered by water. Extreme lack of oxygen in the soil can lead to the formation of gas, hydrogen sulphide, which has rotten egg smell often associated with mangrove swamps.
Over wash mangrove forests : These are small islands covered with mangroves that are frequently washed by the tides. The dominant species is Rhizophora mangle or the red mangrove.
Fringing mangrove forests : These strips of mangrove found along waterways and covered by daily tides. The dominant species is Rhizophora mangle.
Riverine mangrove forests : These are luxuriant stands of mangrove along tidal rivers and creek with a good input of fresh water. Often composed of Rhizophora, Avicennia.
Basin mangrove forests : These are stunted mangroves located in places such as the interior of swamps. Often dominated by Avicennia.
Hammock mangrove forests : Similar to basin mangrove forests but are found in more elevated sites.
Scrub mangrove forest : A dwarfed stand
of mangroves found on flat coastal fringes.
Major constituents of this group in the mangrove environment of India are 105 species of fishes, 20 species of shell fished and more than 225 species of crustaceans. Among these, commercially important are Meretrix sp., Crassostrea sp., Penaeus sp., Scylla serrata and Mugil cephalis
Many crustaceans in the mangroves make burrows which are used for refuge, the feeding, as a source of water or for establishing a territory necessary for mating. Some may filter water through their burrows, feeding on suspended detritus and plankton while others may breed there. These burrows play and important role in the mangroves, aerating, draining and turning the dense waterlogged soil - a direct benefit to the plants which in turn give them shelter.
There is a limit to how many burrows can be dug in any one area. It seems that when there are too many, homeless crabs may try to take over occupied ones. Some fiddler crabs and ghost crabs have been observed filing in the burrows of their neighbors to maintain their territories.
Scylla serrata, the large
edible swimming crab, inhabits the muddy bottom of mangrove estuaries,
as well as