Mangroves our line of defense and the cradle of biodiversity

My recent visit to Kerala via the Konkan route reminded me of my Konkan visit in Ratanagiri, Maharashtra I had two months back to study the role of the Mangrove ecosystem along the coast. Mangroves are an interesting and important forest rich in biodiversity I have seen so far with its interesting pipe like roots that stick out of the water to breathe just like the the pipes used by swimmers during snorkeling and another thing that was quite amazing to witness is that the seeds of the tree germinates still when they are hanging on to the branches as like a baby in the womb of a mother. These trees have features almost like humans but the roots have anchored themselves to the soggy muds stabilizing and protecting the coast line.  They have come to light in India after their role in protecting the Indian coast against Tsunami and cyclones.

Mangrove forest along the estuary in Ratnagiri, Maharashtra. (Photo credit: Abhijit  Gandhi)

Mangroves are 'Evergreen Xerophytes' that grow in the inter-tidal areas and estuary mouths between land and sea. Mangroves provide critical habitat for a diverse marine and terrestrial flora and fauna. Mangrove forests fix more carbon dioxide per unit area than phytoplankton in tropical oceans. Full carbon credits for that!!

They are salt-tolerant forest ecosystems found in specific regions mainly in tropical and sub-tropical inter-tidal regions of the world. They grow in shallow and muddy salt water or brackish waters, especially along quiet shorelines and in estuaries. Typically they produce tangled masses of arching roots that are exposed during low tides. The Ecosystem is highly fragile, subjected to long duration of intrusion as well as incessant physiological drought and morphological stresses, salinity effect, aeration and onslaught of wave action.

Mangrove Ecosystem is distributed all over the coast lines of the world and their occurrence is largely limited to the regions between 30° north and south of the Equator. According to estimate made by F.A.O./ UNDP a total area of 7.1 million hectare is covered under the mangrove formation in the world.

Map showing distribution of Mangroves around the globe. (Map Courtesy: Britannica Encyclopedia)

The total area of mangroves in India is about 6,740 sq. km, which is about 7% of the world's total area of mangroves. Luxuriant patches of mangroves are found on all the other continents but the best mangroves are found in Asia, especially in India and Bangladesh - the Sunderbans are the largest mangrove forest in the world both in size as well as biodiversity. 

Stilt roots of a Mangrove tree connected to the Cable roots below. (Photo credit: Abhijit  Gandhi)

       During my Kerala journey I could see large stretches of Mangrove forest especially along the coast of Kasaragod, Kannur and Kozhikode that have anchored and stabilized the coastal area with their highly developed roots. It is a bit strange that they haven't appeared in the State/UT wise Mangrove Cover Assessment 2009.

       The stilt roots are the main organs for breathing especially during the high tide. This very roots of these plants help in binding the soil and also help the establishment of micro-organisms which further help in stabilizing the area. Stabilization starts from the land side and gradually shifts towards the sea. These pioneer plants are slowly replaced by other mangrove plants and then mangroves gradually spread towards the sea. Once mangroves grow, the submerged banks are fully stabilized; after this, mangroves slowly reach climax vegetation stage. Climax vegetation is represented by the complete circle of life where there are different species of plants, animals (both terrestrial and aquatic) and micro-organisms forming an ecosystem called the tropical salt marsh or the mangrove ecosystem. In case the sediments are not stabilized, submerged banks are washed out. Like in Gangetic delta, thousands of deltas are formed and washed out every year before they can be stabilized.

       The major plant species forming the mangrove ecosystem have aerial roots, commonly known as prop roots or even stilt roots. Stilt roots serve to anchor the plants, but also are important in aeration, because the mangrove mud tends to be anaerobic. Rhizophora spp. (Red mangroves) have prop roots descending from the trunk and branches, providing a stable support system. Other species, including the white mangroves (A. marina) obtain stability with an extensive system of shallow, underground ”cable roots” that radiate out from the central trunk for a considerable distance in all directions and the pneumatophores extend from these cable roots. 

Breathing roots (Pneumatophores). (Photo credit: Abhijit  Gandhi)

      Breathing roots are special vertical roots, called Pneumatophores, form from lateral roots in the mud, often projecting above soil permitting some oxygen to reach the oxygen-starved submerged roots.

Stages of Development of a Mangrove tree. (Photo credit: Abhijit  Gandhi)

     Virtually all mangroves share two common reproductive strategies they are dispersal by means of water and vivipary. Vivipary means that the embryo develops continuously while attached to the parent tree. They may grow in place, attached to the parent tree for one to three years, reaching length upto one meter, before breaking off from the parent plant & falling into the water, these seedling then lodged in the mud where they quickly produce additional roots and begin to grow.

     The first line of defense for many mangroves is to prevent much of the salt from entering by filtering it out at root level. The leaves of many mangroves have special salt glands. Another method is the retention of water in the leaves giving rise to leaf succulence in many species. Third method of coping with salt is to concentrate it in bark or in older leaves which carry it with them when they drop.

  

   The Indian mangroves are represented by approximately 59 species (inclusive of some mangrove associates) from 29 families. These forest are the breeding grounds for many faunas especially the fishes. There are lots of birds that use the Mangroves for roosting like the western reef egret, purple heron, black kite, red wattled lapwing, Black-tailed Godwit, Common Greenshank, Brown-headed Gull. Other important faunas are the grasshoppers, crocidiles, snakes, crabs.

       Mangrove forests are one of the world’s most threatened tropical ecosystems. More than 35% of the world’s mangroves are already gone. The figure is as high as 50% in countries such as India, the Philippines, and Vietnam, while in the Americas they are being cleared at a rate faster than tropical rainforests. 12 species of mangroves found in India are considered to be ‘Critically Endangered’ and a total of 57 mangrove and mangrove-associated species are considered threatened. The main reasons for the destruction in the area are mainly due to population pressure in and around the mangrove belts. There are several other specific reasons for degradation of mangrove area. 

      Natural threats like change in climate can have effects on Mangrove forests as they require stable sea levels for long-term survival. They are therefore extremely sensitive to current rising sea levels caused by global warming and climate, Cyclones, typhoons and strong wave action. Infestation by barnacles that interfere with respiration and photosynthesis can delay in seedling growth. Oysters, Crabs, Gastropods also damage the plant parts. Browsing and trampling by wildlife damages young seedling, leaves, flowers, roots and fleshy tissues. Insect pests such as Wood borers, Caterpillars, beetles which eat the mangrove foliage do damage the wood. Infestation by weeds such as Acrostichum aureum and Acanthus species, which often occupy deforested mangrove areas and restrict the re-growth of mangrove tree species.

       Destructive activities by human like the use of mangrove land for urban development. 

• This has been one of the most visible reasons for accelerated disappearance of the mangrove. Mangrove forests are cleared to make room for agricultural land, anchoring of boats, human settlements, resettlement activities, infrastructure, and industrial areas. More recently, mangrove clearing for tourist development, shrimp aquaculture, and salt farms has also taken place.

• Over harvesting of Mangrove trees for firewood, construction wood, wood chip and pulp production, charcoal production, and animal fodder. While harvesting has taken place for centuries, in some parts of the world it is no longer sustainable, threatening the future of the forests.

• Dams and irrigation have reduced the amount of water reaching mangrove forests, changing the salinity level of water in the forest. If salinity becomes too high, the mangroves cannot survive. Freshwater diversions will to lead to mangroves drying out. In addition, increased erosion due to land deforestation has massively increased the amount of sediment in rivers. This can hamper the mangrove forest’s filtering ability, leading to the forest being smothered.

• Coral reefs provide the first barrier against currents and strong waves. When they are destroyed, the stronger-than-normal waves and currents reaching the coast can undermine the fine sediment in which the mangroves grow. This can prevent seedlings from taking root and wash away nutrients essential for mangrove ecosystems.

• Pollution is the major problem through out the world. Fertilizers, pesticides, discharge of industrial effluent, solid waste dumping, pollutants, and sewage into creeks, rivers, and estuaries has several consequences, such as blocking of the mangrove pneumatophores, decreased oxygen level in the surrounding water bodies. Other toxic man-made chemicals carried by river systems from sources upstream can kill animals living in mangrove forests, while oil pollution can smother mangrove roots and suffocate the trees.

• Due to many anthropogenic (Commercial, religious, cultural) activities, the inlet water channels to the mangrove patches have been stopped or diverted. This diversion of water flow has resulted in the elevation of the surrounding lands, deposition of one way flow of salt water & intrusion of less or non-salt tolerant grass species resulting in disappearance of many of the mangrove dense patches.

Ecological & Economical importance of mangroves

       Ecological significance: -

    Mangrove forests are among the most productive terrestrial ecosystems and are natural and are a renewable resource. Mangroves are not a marvel just for their adaptations but also for the significant role they play in our environment.

• Mangrove ecosystem act as Buffer Zone between the land and sea.

• Mangroves protect the coast against erosion due to wind, waves, water currents and protect coral reefs, sea-grass bed and shipping lanes against siltation. They are also known to absorb pollutants. Mangroves host a number of threatened or endangered species, different animal species- mammals, reptiles, amphibians and bird- offer nutrients to the marine food web and provide spawning grounds to a variety of fish and shellfish, including several commercial species

• It has been suggested that the large loss of life (300,000 to 500,000 lives) in Bangladesh during the 1970 typhoon was partly due to the fact that many of the mangrove swamps protecting those populated coastal regions had been removed and replaced by rice paddies. 

• In mangrove areas water level is shallow, warm water temperatures due to various decaying activities, water current is slow (nearly stagnant) hence ideal place for growing of sea algae and for spawning for fish and marine animals. They are breeding, feeding and nursery grounds for many estuarine and marine organisms.

• Purify the water by absorbing impurities and harmful heavy metals and help us to breathe a clean air by absorbing pollutants in the air.

• The tidal swamp is an ideal sanctuary for avifauna some of which are migratory

• Mangrove forests are also important in terms of aesthetics and tourism. Many people visit these areas for sports fishing, boating, bird watching, snorkeling, and other recreational pursuits.

       In addition to these ecological roles, mangrove forest possess attributes that are specifically important to humans:

• Mangroves are also a source of a vast range of wood and non-wood forest products including timber, fuelwood, charcoal, fodder, honey, pulp, tannin, medicine and thatch etc.

• The ecosystem has a very large unexplored potential for natural products useful for medicinal purposes & also for salt production, apiculture, fisheries products fuel and fodder, etc.

• Mangroves also provide opportunities for education, scientific research and eco tourism.

• It is essential to systematically conserve the biodiversity in the mangrove ecosystem and manage well for the use of mankind.

         The Government of India had set up the National Mangrove Committee at the Ministry of Environment and Forests in 1976 to advise the government about mangrove conservation and development and also a legislative framework for the conservation and management of mangroves is already in place like the Indian Forest Act, 1927; The Wildlife (Protection) Act, 1972; The Forest Conservation Act, 1980; Coast  Guard Act, 1978; The Environmental (Protection) ACT, 1986. We cannot survive without mangroves. Despite their importance, mangrove forests are being neglected by a majority of the population. Despite all the legal framework there has been incidences of encroachment and violations by humans on to these lush mangroves for the development of various housing societies, laying of railway lines, sports complex, constructing of golf courses, pipelines, jetties, sewerage treatment plants, garbage disposal project….etc and this story is not just from India but also from other Mangrove forests in the world. 

      We tend to forget that life first originated in water and these very forest has been protecting and providing the required nutrients  and providing organisms with shelter and food for them to grow and evolve. If we continue to destroy this precious forest then we will be left with no life on our Planet. We will be breaking the very foundations of the food Pyramid.

Lets protect our Mangrove forest!! (Photo credit: Abhijit  Gandhi)

P.S: Most of the information provided is accessed from ENVIS Newsletter 2008, ENVIS Centre, Environment Department, Government of Maharashtra, Mumbai
Mangrove Ecosystem in Maharashtra, Retrieved on 30th April, 2012 from  <http://envis.maharashtra.gov.in/envis_data/files/Mmain.html>