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Context: The United States National Aeronautics and Space Administration (NASA) recently highlighted the loss of mangrove cover on Katchal island, a part of India’s Nicobar archipelago. The orange portion shows the area where mangrove cover was lost

 

More on the news:

• After the magnitude-9.2 Aceh-Andaman earthquake in December 2004, the islands experienced up to 3 meters of land subsidence. This submerged many mangrove ecosystems, resulting in a loss of more than 90 percent of mangrove extent in some areas.
• The mangrove cover on Katchal will not come back. But in other places, mangroves have reappeared since they propagate themselves through propagules.
• A propagule is a vegetative structure that can become detached from a plant and give rise to a new plant. Examples include a bud, sucker, or spore.
• The study also noted that mangroves had the highest ratio of loss to gain among the three types of tidal wetlands it studied. The other two were tidal flats and marshes.
• Some 27 per cent of the losses and gains were directly caused by human activity. Humans can alter wetlands through development, water diversion projects, or by converting the land to agriculture or aquaculture. But they can also expand wetlands through restoration projects.
• Other causes of wetland change were sea level rise, shoreline erosion, storms, altered sediment flow and subsidence. These can be either indirectly caused by humans or the result of natural coastal processes.
• The study also found that outside of Asia, tidal wetlands in Africa had the highest ratio of loss to gain.

 

What is a mangrove?

• A mangrove is a small tree or shrub that grows along coastlines, taking root in salty sediments, often underwater.
• The word ‘mangrove’ may refer to the habitat as a whole or to the trees and shrubs in the mangrove swamp.
• Mangroves are flowering trees, belonging to the families Rhizophoraceae, Acanthaceae, Lythraceae, Combretaceae, and Arecaceae.

Why mangroves matter?

• The upper trunk, including the branches and leaves, of a mangrove tree lives completely above the waterline, while the lower trunk and the large root system are partly covered by seawater.
• Many species have roots diverging from stems and branches and penetrating the soil some distance away from the main stem (like banyan trees).

 

What are some of the special features of mangroves?

• Saline environment:A speciality of mangroves is that they can survive under extreme hostile environment such as high salt and low oxygen conditions. Mangrove trees contain a complex salt filtration system and complex root system that filter out 90% of the salt when come into contact with saline and brackish water.
• Low oxygen:In a mangrove environment, the oxygen in soil is limited or nil. Hence the mangrove root system absorbs oxygen from the atmosphere. Mangroves have special roots for this purpose called breathing roots or pneumatophores. These roots have numerous pores through which oxygen enters the underground tissues.
• Mangroves, like desert plants, store fresh water in thick succulent leaves. A waxy coating on the leavesseals in water and minimises evaporation.
• Viviparous – their seeds germinate while still attached to the parent tree. Once germinated, the seedling grows into a propagule. The mature propagule then drops into the water and gets transported to a different spot, eventually taking root in a solid ground.

 

How do mangrove forests help protect against strong cyclones?

• Mangrove forests act as natural barriers against storm surge, coastal flooding and sea level rise. Their intricate root system stabilises the coastline, reducing erosion from storm surges.Together with the tree trunks, they work like speed-breakers to slow down the tides.
• They protect shorelines from damaging winds and waves.
• Mangroves also help prevent erosion by stabilising sediments with their tangled root systems.

 

What are the other benefits to the environment?

• Mangrove thickets maintain water quality by filtering pollutants and trapping sediments originating from land.
• They provide habitat for a diverse array of terrestrial organisms. Their branches provide homes for lizards, snakes and nesting birds. Many species of coastal and offshore fish and shellfish rely exclusively on mangroves as their breeding, spawning, and hatching grounds.
• Mangroves also have a big impact on climate. Mangroves are powerhouses when it comes to carbon storage.Mangroves can sequester (lock away) greater amount of carbon than other trees in the peat soil beneath. They store this carbon for thousands of years.
• Many people living in and around mangroves depend on them for their The trees are a source of wood for construction and fuel. The ecosystem provides local fishermen with a rich supply of fish, crabs and shellfish. The ecosystem also supports tourism.

 

Where are mangrove ecosystems found?

• Mangroves can be found in the tropical and subtropical regions of the world.
• Asia has the largest coverage of the world’s mangroves, followed by Africa, North and Central America, Oceania and South America.

 

In India:

• The deltas of the Ganges, Mahanadi, Krishna, Godavari, and the Cauveryrivers contain mangrove forests.
• The backwaters in Kerala have a high density of mangrove forest.
• The Sundarbans in West Bengal is the largest mangrove region in the world and a UNESCO World Heritage Site. It spans from the Hooghly River in West Bengal to the Baleswar River in Bangladesh.
• The Bhitarkanika mangrove system in Odishais India’s second largest mangrove forest. Pichavaram in Tamil Nadu has a vast expanse of water covered with mangrove forests. It is home to many aquatic bird species.

 

What are the threats to mangroves?

• At least one third of all mangrove forests has been lost during the last few decades.
• Coastal development, including construction of shrimp farms, hotels, and other structures, is the primary threat to mangroves.
• Mangrove forests are cleared to make room for agricultural land and human settlements.
• Mangrove trees are used for firewood, construction wood, charcoal production, and animal fodder. In some parts of the world, there has been overharvesting which is no longer sustainable.
• Overfishing, pollution, and rising sea levels are the other threats to mangrove forests and their ecosystem.

https://www.downtoearth.org.in/news/environment/nasa-highlights-loss-of-mangrove-cover-on-katchal-island-in-the-nicobars-84236