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MANGROVE ECOSYSTEM - Its Threats and Conservation

Sachin O. Khairnar, Bhavesh.V. Solanki and Li Junwei

College of Fisheries, Ocean University of China,

Qingdao, Shandong, P.R.C-266003


Introduction:

The term "mangrove" refers to a tidally influenced wetland ecosystem within the intertidal zone of tropical and subtropical latitudes. Mangrove also designates the marine tidal forest that includes trees, shrubs, palms, epiphytes and ferns (Tomlinson, 1986). The distinctive community of plants and animals associated with mangroves is sometimes referred to as the 'mangal' (Macnae, 1968). The origins of the terms for mangrove are well researched by Vanucci (1989). She concludes that the word mangue is from West Africa, Senegal, Gambia and Guinea. The English word mangrove is a derivation of the Portuguese or Spanish meaning grove made of mangue. Mangrove ecosystems are heterogeneous habitats with an unusual variety of animals and plants adapted to the environmental conditions of highly saline, frequently inundated, soft-bottomed anaerobic mud. (Khairnar et al., 2009). Not all mangroves are obligated to live in saline intertidal areas (Clough, 1992). Plants that are confined to the mangrove are called true mangroves; plants that can also occur elsewhere are called mangrove associates (Tomlinson, 1986). Mangrove associates never grow in true mangrove communities and may occur in terrestrial vegetation. The mangrove fauna includes terrestrial, marine, temporary and permanent animal species, all of which have different adaptations to cope with the mangrove environment. The diversity of mangroves is high, but the variety of mangrove ecosystems also makes it difficult to produce general guidelines for conservation and management of mangroves because each system is unique.

Mangroves are found in depositional coastal environments where fine sediments, often with high organic content, collect in areas protected from high energy wave action. Mangroves are salt-tolerant plants of tropical and subtropical intertidal regions of the world. The specific regions where these plants occur are termed as 'mangrove ecosystem'. These are highly productive but extremely sensitive and fragile. Besides mangroves, the ecosystem also harbours other plant and animal species.

It has been proved that the presence of mangrove ecosystems on coastline save lives and property during natural hazards such as cyclones, storm surges and erosion. These ecosystems are also well known for their economic importance. They are breeding, feeding and nursery grounds for many estuarine and marine organisms. Hence, these areas are used for captive and culture fisheries. The ecosystem has a very large unexplored potential for natural products useful for medicinal purposes and also for salt production, apiculture, fuel and fodder, etc.

Global warming is expected to result in the global sea level rise affecting various marine habitats in the low-lying regions. Mangrove ecosystems would be more vulnerable to the climatic changes, as directly influenced by tides. Mangroves are believed to keep pace with rising sea level by continuous sedimentation enriching the ambient environment with organic matter. Due to sea level rise the intertidal and supralittoral zones are likely to be extended farther inland causing destruction of existing mangroves and associated biota. The changes in the salinity concentration may cause genetic erosion as well as change in the species composition. There may be a total new assemblage of species in response to the changing salinity regime. Climate changes coupled with anthropogenic pressures and poor management practices may reduce productivity and loss of mangrove dependent biodiversity, adversely impacting ecological and socioeconomic benefits (Jagtap & Komarpant, 2003). Recognition of the environmental, social and economic impacts associated with the decline and degradation of mangroves are now being addressed through legislative, management, conservation and rehabilitation efforts aimed at mitigating the negative impacts of development on mangrove ecosystems.

Ecology:

Tidal action is a primary factor that determines mangrove ecology, i.e. ecosystem processes. Once established, roots of mangrove plants provide a habitat for oysters and help to impede water flow, thereby enhancing the deposition of sediment in areas where it is already occurring. Usually, the fine, anoxic sediments under mangroves act as sinks for a variety of heavy (trace) metals which are scavenged from the overlying seawater by colloidal particles in the sediments. In areas of the world where mangroves have been removed for development purposes, the disturbance of these underlying sediments often creates problems of trace metal contamination of seawater and biota. Mangroves protect the coast from erosion, surge storms (especially during hurricanes), and tsunamis. Their massive root system is efficient at dissipating wave energy. Likewise, they slow down tidal water enough that its sediment is deposited as the tide comes in and are not re-suspended when the tide leaves, except for fine particles. As a result, mangroves build their own environment. Because of the uniqueness of the mangrove ecosystems and their protection against erosion, they are often the object of conservation programs including national Biodiversity Action Plans.

Despite their benefits, the protective value of mangroves is sometimes overstated. Wave energy is typically low in areas where mangroves grow, so their effect on erosion can only be measured in the long-term. Their capacity to limit high-energy wave erosion is limited to events like storm surges and tsunamis. Erosion often still occurs on the outer sides of bends in river channels that wind through mangroves, just as new stands of mangroves are appearing on the inner sides where sediment is accreting.

Mangroves support unique ecosystems, especially on their intricate root systems. The mesh of mangrove roots produces a quiet marine region for many young organisms. In areas where roots are permanently submerged, they may host a wide variety of organisms, including algae, barnacles, oysters, sponges, and bryozoans, which all require a hard substratum for anchoring while they filter feed. Shrimps and mud lobsters use the muddy bottom as their home. Mangrove crabs improve the nutritional quality of the mangal muds for other bottom feeders by mulching the mangrove leaves. In at least some cases, export of carbon fixed in mangroves is important in coastal food webs. The habitats also host several commercially important species of fish and crustaceans.

The major factors, governing the mangrove ecosystem

Climatic factor:

Climate plays major role in inter-tidal region for natural growth, development and succession of the mangroves. Among these climatic factors

  • Fluctuation of temperature

  • Rainfall

  • Regular wind flow

  • Frost free

  • Radiation

Sedimentation play very major role for the mangrove viability in a holistic manner.

Soil: The soil structure and soil salinity are the main agents, controlling the distribution of mangroves. The soil salinity of the supralittoral zone, where water reaches once or twice in a month during highest high-water tide, is comparatively high because of the rate of evaporation.

Tidal Amplitude: The tidal fluctuation has the major role for mangrove habitats, as most mangroves grow well in between the Mean High Water Spring Tide (MHWST) and Mean Sea Level (MSL).


Threats to Mangrove ecosystem:

The threats to the mangrove ecosystem could be broadly grouped into two: Natural and Anthropogenic


Natural threats:

  • Climate change;

  • Cyclones;

  • Infestation by barnacles

  • Damage by Crustaceans

  • Insect pests such as woodborers, caterpillars

  • Drying of mangrove trees

Anthropogenic threats:

  • Tree felling for fuel wood and wood products;

  • Grazing by cattle;

  • Reclamation for agriculture and aquaculture;

  • Urban development

  • Industralisation

Conservation of the Mangrove ecosystem:

  • Afforestation

  • Legislation (including laws and policies); 

  • Monitoring and Surveys (land and aerial, etc.);

  • Protection (including conservation, parks and reserves development, etc.)

Sustainable use of Mangrove ecosystem:

  • Culture (Agriculture, Aquaculture -capture fisheries, culture fish), 

  • Natural products useful for medicinal purposes, drugs, etc.);

  • Other products (timber, salt production, honey, etc.); 

  • Socio-economic aspects;

  • Tourism

  • Traditional medicines

References

Clough, B.F., 1992. Primary productivity and growth of mangrove forests. In: A.I. Robertson and D.M. Alongi (Eds.) Tropical Mangrove Ecosystems. American Geophysical Union, Washington, D.C. pp.225-249.

Jagtap & Komarpant, 2003. Evaluation of mangrove ecosystem of India for assessing its vulnerability to projected climatic changes. Assessment of climate change in India and mitigation policies, World Wide Fund for Nature-India eds. Dash, S.K.; Rao P. 39-51p.


Khairnar S. O., Sathe, A.R., Mhatre V.D., Jadhav S. S and Desai K.B., 2009. Mangroove Biodiversity of India. Aqua International, Vol.16. No.12, pp- 29-30.


Macnae, E., 1968. A general account of the fauna and flora of mangrove swamps and forests in the Indo-West Pacific Region. Advances in Marine Biology 6: 73-270.


Tomlinson, P.B., 1986. The botany of mangroves. University Press, Cambridge, 419 pp.


Vanucci, M., 1989. The mangroves and us: a synthesis of insights. Indian Association for the advancement of Science, New Delhi, India.



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