FAUNA DIVERSITY AND ECONOMIC VALUES OF THE MANGROVE ECOSYSTEMS IN SRI LANKA

Abstract: Mangroves are of great ecological importance and socio-economic significance as a hub for tropical marine biotope. The mangroves are also one of the world’s richest storehouses of biological and genetic diversity. Furthermore, 90 % of the marine organisms spend part of their life in this ecosystem and 80 % of the global fish catches are dependent on mangroves. Mangrove forests are considered as a highly productive ecosystem, they provide important ecological and economic services. Besides, mangroves and their associated biota are identified as a promising source of natural and novel drugs.

Introduction

Mangrove is a salt-tolerant plant that occurs within inter-tidal zones where a river enters into the sea, especially in the coastal belt. They are a highly productive, tropical coastal ecosystem encompassing estuaries, creeks, lagoons, backwaters, mud-flats, salt-pans, and islands which has both great aquatic and terrestrial biodiversity and plays an integral role in coastal ecosystem functions at the interface between terrestrial, freshwater and marine systems. Mangrove fauna is animal communities that inhabit or utilize mangrove habitats, such as coastal inter-tidal zones, estuaries, and riverine areas where rivers drain into the sea, and to fulfill their need for survival and reproduction. The vegetation structure and composition of mangrove areas may vary depending upon soil texture and structure, rainfall pattern, and inflow of fresh water from rivers to the sea.

Fig. 1 The distribution of mangrove eco-system in Sri Lanka

Mangrove forests are considered as a highly productive ecosystem, and they provide important ecological and economic services. They play a crucial role in providing suitable habitats for fauna, safe breeding and chick-rearing grounds, nurseries for a diversity of fishes and shellfishes, as well as ideal foraging grounds for animals such as fishes, birds, and aquatic invertebrates and refuge from predators. All the organisms living therein are well-adapted to encounter higher salinity, wind speed, temperature, muddy anaerobic soils, and high tidal interference. Mangroves are a rare forest type in the world, built by a small group of 73 species of trees and shrubs. Global mangroves have an estimated cover of 15.2 million hectares (0.4 % of all forest, 1 % of tropical forests) in 123 countries and territories. The mangroves are largely restricted to the tropics and few warm temperate regions at the latitudes between 30oN and 30oS. Mangroves grow better in wet equatorial climates than they do in seasonally monsoonal or arid climates. The growth and biomass production of mangroves decrease with increasing latitudes. Mangroves in Sri Lanka shows a wide distribution covering most parts of the island, which estimated about 0.34% of the total land area or 6000 – 7000 ha is a very important ecosystem which provides a wide array of benefits to humans as well as for the balance of nature. Many mangrove ecosystems in Sri Lanka have been, and to a large extent are, indiscriminately exploited for commercial, aquacultural, agricultural, residential, tourism, mining, and industrial development.

Mangrove Biodiversity

This unique ecosystem is home for over 29 true mangrove species in Sri Lanka. These species grow under a wide range of soil and hydrological conditions and are widely distributed in Sri Lanka. The common category of mangrove species includes Aegiceras corniculatum, Avicennia officinalis, Bruguiera cylindrica, Bruguiera sexangula, Ceriops tagal, Heritiera littoralis, Pemphis acidula, Sonneratia alba, and Nypa fruticans. Although these species are widely distributed in Sri Lanka, they are low in abundance. Rare species of mangroves that are few in numbers restricted to a few locations include Lumnitzera littorea, Xylocarpus granatum, and Scyphiphora hydrophyllaceae in Sri Lanka. 

Mangrove areas are a favorable habitat for a variety of water birds (i.e., the bird, species that entirely depend on water for a variety of activities such as foraging, nesting, loafing, and moulting) as well as terrestrial birds. This is due to the diversity of habitats such as mangroves, mudflats, estuaries, and richness of food resources which includes fishes, turtles, snake, amphibians, mammals, and invertebrates such as gastropods, bivalves, prawn, nekton, crabs and insects. Noske reported that mangroves support more than 200 bird species that utilize mangrove forests, mudflats, estuaries, and adjacent areas. For example, Pelicans, Fish Eagles, Kingfisher, Woodpeckers, Pigeons, Pittas, etc. Bird species are a bioindicator of a mangrove ecosystem and play a significant role in the management of vegetation. They control the population of insect pests that cause the defoliation among trees and reduce their growth and also cause damage to the seeds.

Fig. 2 Juvenile of Great White Pelican

Fig. 3 Common Kingfisher

Mangroves are an ideal habitat and are rich in reptile fauna, which includes snakes, turtles, crocodiles, and alligators.

Fig. 4 Saltwater/Estuary Crocodile

Fig. 5 Mangrove Monitor Lizard

Only a few species of frogs occur in mangrove forests including Tree Frog and Mangrove Frog. The occurrence of such a few numbers of amphibians could be due to the high salt contents of the water. Mangrove frogs are predators that may eat almost every small living thing such as insects (e.g., beetles, bees, ants, termites, crickets and bugs, snails, smaller toads, prawns, and fishes). Mangrove forests are rich in mammal species such as 3 Species of monkeys, Loris, Wild cats, Otters, and Bats (Flying fox, Fulvous fruit bat, Short-nosed fruit bat). Mammals are a major source of food for a variety of animals such as birds, snakes, crocodiles, and a significant component of mangrove ecosystems. Frugivorous mammals such as monkeys, squirrels, and bats are also important as seed dispersal agents. Herbivorous mammals browse on young shoots of trees, shrubs, and other vegetation; hence, they control the growth of shrubs and bushes that may compete for nutrition with trees.

Mangrove areas are rich in fish fauna. For example, Mono argenteus, Scatophagus argus, Terapon jarbua, Etroplus suratensis, etc. Variety of fish species use mangrove areas for foraging, i.e. feed on amphipods, isopods, crabs, snails, insects, spiders, copepods, shrimp, and organic matter. Many scientists have reported that an array of fish species extensively use mangrove areas as breeding and nursery sites, especially during early juvenile stages. This could be due to the abundance and richness of food resources such as invertebrates that inhabit the vegetated area, and richness of benthic fauna. Aquatic invertebrates play an important role in the ecology of mangrove because they break down leaf litter that acts as a fertilizer, increases the surface area of mud through burrowing, and increasing the diffusion rate of gases that ultimately affect the growth and productivity of the mangrove vegetation.

Fig. 6 Terapon jarbua

Fig. 7 Tree Frog

Fig. 8 Flying fox

Economic benefits of mangroves

The diverse vegetation structure and composition of mangroves with denser foliage have created different layers of vegetation that offer heterogeneous habitats that support a variety of marine, freshwater, and terrestrial wildlife species. The coastal zone accounts for 33% of the population of Sri Lanka, possessing 24% of the total land area. Rural poverty persists in coastal areas and the majority of the coastal communities depend on fishing and agriculture for their livelihoods. Mangroves are of benefit to man both directly and indirectly. Some mangrove tree species (especially Rhizophora, Bruguiera, and Sonneratia) provide wood suitable for construction of houses and canoes. But today the main use of mangrove wood is for piles and temporary house constructions. The bark of some mangrove trees, especially R. apiculata, and C. tagal contain a high percentage of tannin (20.7% and 17.3%) respectively and extracts of this tannin can be used for tanning leather. It has also been used to dye cotton fishnets and sails of the boats and ships. But with the introduction of nylon fishing nets and mechanized boats, the use of mangrove tannin extracts has decreased in importance. The mangrove palm N. fruticans provides alcohol, sugar, and vinegar in Malaysia, but is not widely used in Sri Lanka. Nypa leaves are used as thatching material for the houses in the vicinity of the mangrove areas. Leaves of some mangrove species, especially Avicennia spp. are preferred as fodder by goats and cattle. In addition to this, mangrove leaves are used to a limited extent as green manure. Fisherman in some parts of Sri Lanka, particularly those living around the Negombo Lagoon use branches of some mangrove trees (especially Lurnnitzera, and Rhizophora sp.) for the brush pile fisheries (locally known as masathu). Fruits of Sonneratia caseolaris is used to make a soft drink, and tender leaves of Acrostichum aurerum are eaten as a curry. Also, mangroves and their associated biota are identified as a promising source of natural and novel drugs.

Mangrove communities contribute significantly to the food chain in coastal areas of the tropics. Numerous species of tropical finfish and shellfish depend upon the mangrove derived material for food. Many of these are commercially important. The clams and crabs occurring in mangrove swamps are also an important source of protein for a large section of low-income people in many tropical countries. Most of the peaneid shrimp species depend upon mangroves for shelter and food during the larval and juvenile stages of their development. These shrimp species are highly valued for their taste (as a gourmet dish) and subjected to intensive fishery all over the tropics. The mangrove ecosystem forms nursery grounds not only for marine species of fish and shellfish but also for some freshwater species. In addition to that many mangroves, resources are harvested for subsistence purposes. These include medicinal herbs, vegetables, poles for fences, and posts.

Mangroves also play an important role in shoreline protection as their well-developed root system prevents erosion of coastal and estuarine areas. Decaying organic residues and other particles carried by tides as well as runoff and drainage sediments are trapped by the closely set root systems of genera such as Rhizophora, Avicennia, and Sonneratia. This often leads to an accretion of existing land areas but they are partly balanced by erosion taking place from other areas resulting in dynamic and changing coastal geomorphology.

Conclusion and Future Perspectives

The current review indicated that mangrove areas are ideal habitat for a variety of fauna and aquatic as well as terrestrial invertebrates. These faunas are an important component of the food web and play a significant role in the mangrove ecosystem. In this review focused on the various fauna of mangrove and adjacent area, and economic benefits of mangroves. Therefore, where mangroves have been destroyed, they can be restored through careful planning and replanting procedures. Mangroves of Sri Lanka are disappearing fast and immediate action should be taken to stop the destruction of this valuable ecosystem.

References

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