Mangroves: a national wealth
Geetha Iyer
Mangrove refers to a special group of salt tolerating plants found in the tropical and sub-tropical coastal regions. The assemblage of salt tolerating plants creates a unique habitat for a wide diversity of fauna and flora. It’s a very productive ecosystem that is of both economic and ecological importance.
Where are they found?
They are found along the coasts and near estuaries. India has a coastline of 7516.6 km of which mangroves cover 4,662.56 sq.km, roughly 0.14 per cent of the country’s geographical area; worldwide, it’s a little less than one per cent. In India, West Bengal has the largest mangrove cover followed by Gujarat. Orissa, Maharashtra, Andhra and Andaman and Nicobar Islands are places with significant mangrove areas. Of these, very dense mangrove areas are found in West Bengal, Andaman and Nicobar Islands and Orissa. Mangrove vegetation is more abundant and widespread (60 per cent of mangrove forests) along the east coast of India due to the nutrient-rich alluvial soil brought in by various rivers coupled with a perennial supply of fresh water. But they remain one of the lesser understood wetland ecosystems. Being dependent on natural forces for survival, mangroves are fragile and can disappear rather quickly sometimes.
The habitat
This is a very specialized habitat occurring between the land and sea, and in estuarine areas where the river and seawater intermingle. Mangrove areas found on land are swamps and marshy, but nutrient rich. A number of natural processes are responsible for their unique characteristics. One of it is the periodic tides. Plants and animals living here have adapted to face periodic inundation by water during high tides and desiccation during low tides. Silting is another phenomenon that is common in this region. This silting process contributes to the formation of land mass which in turn allows for the mangrove region to extend itself. The formation of landmass also provides a habitat for other plant varieties to grow. Mangroves are dynamic as their existence and characteristics are governed by natural phenomena such as tides, extent of inflow of water from the rivers, silting and to a certain extent rainfall. These factors have a direct influence on temperature, salinity, pH and turbidity of water and soil which are crucial for the survival and propagation of species.
Mangroves are rich ecosystems due to the constant recycling of nutrients. For e.g., leaves are colonized by bacteria and fungi, which in turn are food for protozoans. These in turn form particulate organic matters that are consumed by invertebrates such as crabs, worms and shrimps as well as vertebrates such as fish and amphibians. Intricate and finely balanced food chains and webs are to be found in this ecosystem.
Large plants inhabiting this region form littoral forests and are described as helophilous halophytes. Halophytes are plants growing in saline soils; marsh or swamp loving ones are described as helophilous halophytes. Common among them are Avicennia, Sonneratia, Bruguiera and Rhizhophora that form the canopy of the mangrove forests. Smaller trees such as Ceriopstangal, a medicinal plant, herbs and ferns such as Acanthus and Acrostichum are a few other examples of the macro flora. The root systems of these plants play a very important role (see box).
About 40 true mangrove algal species are known, of which species from the family Myxophyceae, Chlorophyceae and Bacillariophyceae are common.
Animals may be found both in the pelagic (surface) and benthic (bottom) regions. There are species that are found in and adapted to live in terrestrial, estuarine or marine areas. Several species of poriferans and cindarians have adapted to live without moving away, whereas there are others such as shrimps, insects that freely move about. Copepods, amphipods, decapods, Cladocera and fish larvae are some of the zooplanktons found among the vegetation. Some polychaetes and molluscs may be found burrowed in soil, some other crustaceans, actinarians, etc., can be found living in the benthic regions. Gastropods and bivalves are quite common and commercial gathering of bivalves is a vital economic activity for those residing in coastal areas. The shallow areas rich in silt and nutrients are a haven for crabs and shrimps. About 98 species of crustaceans are collected which forms an important commercial activity.
A number of fishes like the mangrove snapper and groupers spend all their life in these mangrove regions. Guitar fish, hammer headed shark, saw fish, tiger shark inhabit this ecosystem. Mud skippers are a common site in these regions. Crocodiles are common in the mangrove regions of Andaman and Nicobar Islands as also water monitor lizard and several species of sea snakes.
Several endemic and endangered species inhabit this ecosystem.
Adaptations to survive
The succulent leaves have salt glands that are capable of secreting excess salts that may have crept into the plant system. In trees such as Sonneratia, there are specialized tissues in the leaves to store water.
Mangrove flowers are generally insect pollinated. The seeds and fruits are light and fruit walls spongy, storing air so it can float in water and be dispersed.
Mangroves are viviparous, the seeds begin their development while the fruit is still attached to the tree and develop into propagules from the seed. These propagules are dispersed by the sea where they can survive prolonged periods till a suitable habitat is found. Propagule is the term given to any structure that can develop into a new individual. Stem cuttings, spores, suckers are examples of propagules. The mangrove propagule consists of a large hypocotyl and a developing radicle.
Importance of mangroves
Humans derive a lot of benefits from this ecosystem. Traditionally they were and in some communities still are a major source of wood used as fuel, for building houses, ships and boats, oars, masts, hulls, carts because they are hard and strong, do not rot easily and not attacked by termites.
The barks of many species are a rich source of tannins. Many are a source of food such as the halophytic fern, Acrostichum aureum, a tasty vegetable for many. Several species of clams and gastropods are a vital food source and are of commercial importance too.
Oysters, crabs, prawns, sea weeds, certain fish species are farmed in mangrove areas as a commercial activity. Fisheries in this region sustains livelihood.
They provide a rich natural habitat and safe breeding ground for several fish species. Not merely fish but even sea birds are known to nest and raise their broods in mangroves.
Mangroves prevent the coastline erosion caused by waves and ocean currents. The tsunami that struck Southern India in 2004 caused very little damage to regions that had a healthy mangrove in place. They are like the infantry battalion in combating a war by natural disasters such as cyclones, storms, tsunamis or simply even from erosion.
In addition to performing ecological functions such as providing shelter for wildlife and protecting coastal erosion against waves and other processes, they have also now attracted ecotourism, both a valuable as well as worrying initiative.
Threats and conservation strategies
Threats to the mangrove ecosystem could be broadly grouped into two: natural and anthropogenic. Natural threats include climatic changes, cyclones and physical processes such as drought, hurricanes, erosion, etc. Pollution, grazing, agriculture, aquaculture and human encroachment, over exploitation, reclamation of vast wetland areas for agriculture, commercial and residential development are the threats from human interactions.
Recent surveys quote that deforestation is the cause which destroyed about 44 per cent and 26 per cent of mangroves along the west and east coast of India respectively. Severe threat comes from the conversion of these lands for aquaculture, agriculture and other developmental activities. Reclamations cause damage to saplings and small mangrove plants in surrounding areas.
A rapidly increasing population, especially near coastal areas, has lead to unsustainable exploitation of the resource. Developments in the field of technology and urbanization, instead of finding solutions, have only increased the stress on this fragile ecosystem.
Mangrove fauna are also at risk due to rising sea levels brought about by climate change; but if they are conserved and allowed to flourish, they can actually help minimize the effect of the rising sea levels.
There are commissions formed to assess the status and rules framed for mangrove protection. But conversion of mangrove lands for other purposes has not been prohibited except in places where an aware general public has raised their voices and helped in halting their destruction, a recent example being the mangroves in Mumbai.
Scientists have said that, “It is necessary to have site-specific information on faunal diversity and resources, more species recovery programmes, and to establish integrated management planning with the participation of local human communities for better management of faunal resources.”
Mangroves are a national wealth. A sensitive ecosystem that harbours a diverse community and therefore needs close monitoring. Mongabay-India in its latest issue has reported that although the number of mangroves in Sunderbans has not reduced significantly, the health of about 25 per cent of the mangrove trees, including the Sundari tree, has declined due to an increase in salinity. Such is the sensitive nature of this extremely productive ecosystem. Much remains to be studied from a conservation point of view.
Root system in mangroves
As mangroves grow in soft soils they need special roots to remain physically stable. So several kinds of roots may be seen in mangrove plants. Buttress roots, flying buttresses, prop roots, stilt roots, knee roots, surface roots, spreading roots, pencil roots, cable roots and cone roots may be seen for additional support in different species. A few are described for you.
The subterranean tap root system, with its extensive network, not only binds the nutrients to the soil but also helps mangroves maintain a firm grip on the soil. These roots collect capillary water.
Stilt roots arising from their main stem towards the soil are used for support, especially during the currents caused by tides.
Marshy soil is oxygen-poor; hence special roots called pneumatophores grow upwards and the many pores on them help procure oxygen from the air. Additionally they have air spaces and aerenchyma tissue, both of which store air to meet the plant’s oxygen needs during its periodic submergence. As water requirement for the plant is not met from the soil, the leaves and stems become succulent to store water.
Plants like Bruguiera have knee roots. These are roots that come out of the soil to the surface and after some growth loop back to the soil resembling a knee. Several knees are produced which help in aeration, as they have lenticels through which air enters the spongy tissues of the roots below the soil.
Mangrove facts
- Sunderbans was the first mangrove forest and wetland to be declared a World Heritage site.
- 59 species of halophytic plants, i.e., salt tolerant plants, are found in India. They remain submerged for 12 hours periodically due to high tides.
- More than 60 species of fishes have been reported from Pitchavaram mangroves.
- Propagules of Bruguiera are eaten by the Jarwa tribe of Andaman.
- In Andaman Islands a decoction made from the shoots of Ceriops tangal is used to treat malaria.
- 3091 faunal species have been reported, which does not include several invertebrates and others that are yet to be studied.
- Several species are endemic and need to be protected. Heritiera kanikensisis is found only in Bhitarkanika in Orissa. Several fauna found in mangroves fall under the category of threatened species.
References
- http://agritech.tnau.ac.in/forestry/forest_mangrove_index.html
- http://www.upsbdb.org/pdf/Souvenir2012/ch-6.pdf. Status, Biodiversity and Distribution of Mangroves in India: An overview
- https://www.researchgate.net/publication/278002811_Mangroves_of_India_Biodiversity_Conservation_and_Management
The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.
