Mangroves are the salt-tolerant forest ecosystems that evolve in the transition zones between terrestrial and marine environments of tropical and sub-tropical areas of the world. Mangrove forests currently occupy 14,650,000 ha of coastline globally. The Bangladesh coast supports about 587,380 ha of natural mangroves and a further 100,000 ha of planted mangroves, locally known as Perabon, and mostly distributed in the intertidal zone.
There are numerous ecological and economic benefits of the mangrove ecosystem. The fringe-like root system of mangroves act as a coastal soil stabilizer and binder of sediment and so aid in preventing erosion in the coastal areas. Moreover, mangroves provide goods and services among the local community, viz., household necessities (firewood, housing materials, and boat making materials), herbal plants for traditional medicines, honey and also protect lives and property from cyclonic storms.
From Sundarban mangrove forest, an average of 6000 ton/ha mangrove litter is released per year. After decomposition of this huge quantity of litter through mineralization process, enormous amount of organic nutrients are released in coastal ecosystem. This is a great source of natural food for aquatic organisms.
According to United Nations Environmental Programme (UNFP), the annual economic value of mangroves, estimated by the cost of products and services they provide, has been estimated to be US$200,000-9000,000 per ha. Another economic survey by World Conservation Union (IUCN) indicates that intact and healthy mangroves can have an overall yearly use value of as much as US$14,000/ha/household. The yearly protection value of mangroves is estimated at around US$2,000 per household.
On 26 December 2004 the largest earthquake in 40 yrs (seismic magnitude MW =9.0) produced the most devastating tsunami in recorded history, killing more than 283,000 people throughout the Indian Ocean region. The earthquake was so powerful it wobbled the Earth’s rotation. The tsunami triggered by seismic event swept across the Indian Ocean at speeds upto 800 km/hr, with succeeding waves reaching heights of upto 30 m. Along with vast numbers of people, man-made and natural structures and habitats were destroyed or damaged, including coral reefs, mangroves, beaches, seagrass beds, and other coastal vegetation.
Several reports based on initial post-impact surveys in southeastern India, the Andaman Islands, and Sri Lanka indicated that mangroves offered a significant defence against the full impact of the tsunami. The presence of mangroves saved lives along the Tamil Nadu coast of southeast India. Measurement of wave forces and modeling of fluid dynamics suggested that mangrove vegetation may shield coastlines from cyclone, storm surge and tsunami damage by dissipating incoming wave energy and reducing the erosion rates. Besides, the wave-driven, wind-driven, and tidal currents also reduce due to the dense network of trunks, branches and above ground roots of the mangroves.
Analytical models show that 3000 trees/ha in a 100 m wide mangrove belt may reduce the maximum tsunami flow pressure by more than 90 percent. These benefits are not found in artificial coastal protection structures. The artificial sea defences were not only expensive to build and repair, but they were also, in many cases, ineffective.
But the mangrove forest has been subjected to heavy human interference. A recent survey by the Institute of Marine Sciences and Fisheries indicates that mangrove areas are becoming smaller or fragmented and their long-term survival is at risk. Their satellite image and Geographical Information System (GIS) based research revealed the spatial distribution of only about 37,600 ha mangrove forest in lower Meghna islands, Noakhali, Feni, Chittagong and Coxs Bazar coast. The factors responsible for the destruction of mangrove forest are unplanned shrimp farming, salt production, removal of forest produces for fuel wood, grazing pressure, and human settlement.
In addition to these, the fishermen build dams in the mouth of the creeks, thereby disrupting tidal inundation and causing water stagnation. For this change in hydrology, the seedlings in stagnant water fail to survive, which seriously affect natural regeneration of mangrove forest.
Given the backdrop of increasing natural disaster around the globe it is imperative to fully enhance conservation of its coastal belts of shelter woods so as to improve protection against typhoons, storm tides, tsunami and other catastrophic consequences. Action has to be taken for plantation, restoration and development of mangroves in the required coastal areas.