Review of Globally Important Ecosystems and Their Biodiversity of the Lower Mekong Basin
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Deltaic Formations and the Plain of ReedsThe Mekong Delta is a classic fan delta commencing just downstream from Phnom Penh and extends over 300 km to the coast. The northern part of the Delta, commonly referred to as the Plain of Reeds, was a vast wetland depression of about 1.3 million ha encompassing the provinces of Dong Thap, Tien Giang, and Lang Sen in Vietnam, and parts of Svay Reang in Cambodia. A large area (368,000 ha) of the Plain in Vietnam (known as "Dong Thap Muoi") is composed of acid-sulphate soils. Except for areas with relatively high ground near the Cambodian border and the river levees along the main branches of the Mekong, the Plain lies in a flat lowland region subject to seasonal flooding between July to January. At its peak between late September and the end of October, the Plain becomes practically a vast lake with some areas flooded to a depth of nearly 4m. Except for scattered ponds and swamps, during the dry season the Plain dries out. The original plain was covered in dense vegetation with small natural streams. However, agricultural expansion over the past 40 years has led to most natural areas being converted to rice production. Of the initial 4 million hectares of the Delta, only 1.3 % now remains in a natural or semi-natural state. The key wetland habitats of the Mekong Delta are described below:
Melaleuca forests now dominate the Mekong Delta, particularly in areas of acid sulphate soils. A high proportion of them are of relatively recent origin, and many are within wood production reserves. In Lang Sen, Plain of Reeds, there are patches of the former riverine delta forest comprising Melaleuca leucodendron associated with Syzgium sp., Elaeocarpus hygrophilus, Ficus microcarpa and Cassia grandis. Peat swamps are associated with melaleuca forest in U Minh. The remaining, scattered patches of forest are an impoverished relic of a more extensive and variable forest cover. Although today’s melaleuca forest is low in plant biodiversity, they act as one of the few sources of fish, amphibian, reptilian and bird biodiversity in the delta. These melaleuca forests are of prime important for their breeding colonies of large waterbirds and are one of the few refuges in the Delta for freshwater species such as turtles.
Formerly, extensive grasslands occurred throughout the upper Mekong Delta, today, much of this habitat is now under rice cultivation. The remaining grasslands form communities dominated by Phragmites sp., Elocharis dulcharis, E. ochrostachys, Lepironia articulata, Hymenanachne pseudo-interrupta, wild rice Oryza rufipogon (one of the few remaining sources for the natural stock of this species) and Ischaemum rugosum common towards Vam Co River. The grasslands are important for the presence of Elocharis spp., the favoured food of the Sarus Crane. In wetter areas, permanent and seasonal emergent vegetation dominates. Species include the lotus Nelumbo nucifera, Nymphacea nouchali, N. pubescens and N. tetragona. The remaining grasslands are important for water birds including the Sarus Crane, White-shouldered Ibis, Greater and Lesser Adjutants and the Bengal Florican.
Intertidal mangrove forests cover about 280,000 hectares of the coastal portion of the Mekong Delta. Less than 40 species of mangroves are found. Successive communities are dominated by Avicennia, Brugiera. Rhizophora, Sonneratia caseolaris and Nypa fruticans. This is an ecologically important area as a breeding ground for many species of fish, crabs and shrimps. Over 300 species of fish have been recorded in the Delta. There are no reports of vertebrates endemic to the mangrove areas, but the wildlife is diverse with mammals such as fishing cats, otters and crab-eating macaques. Salt-water crocodile are still reported in the Delta. Many fish and shrimp species depend upon the estuaries of the delta for their breeding and nursery areas. Some marine species of fish of the families Polynemidae (threadfins) and Tachysuridae ascend the rivers to spawn in the gradient or freshwater zone of the estuaries, while the larvae of many economically important shrimp species, spawned in the shallow coastal areas, are moved by tides into the brackish water zone where they stay as juveniles for 2-4 months amongst abundant food and safe from predators.
Apart from the main communities above, the flora of the delta is quite complex due to the variety of flood and water availability regimes. The following floral communities can be distinguished, roughly according to the descending order of water depths - submergent plants e.g. Hydrilla verticillata, Limnophila sp., are present in quite large, deeply flooded areas. Floating plants found in low-acid and shallow water bodies are dominated by water hyacinth Eichhornia crassipes; water ferns Salvinia cucullata, Pistia stratiotes and Azolla pinnata; and duckweeds Spirodela polyrhiza and Wolffia schleidenii. Emergent plants, particularly common in stagnant, low-acid canals, include Nymphaea spp. and Nymphoides sp. (water lilies). The Polygonum tomentosum community is very common along the banks of low-acid, slow-flowing canals, and the Phragmites karka and Saccharum spontaneum community is found on islets scattered in the area. 3.Biodiversity ReviewThe wetlands of the Lower Mekong Basin support almost 100 species classified as Globally Threatened (Critically Endangered, Endangered, Vulnerable, Near-Threatened and Data Deficient). This reflects the vital contribution of the Lower Mekong Basin to global wetland biodiversity. However, it warns of the grave threats confronting many of these species in this region. Comprehensive information on all the biodiversity of the Lower Mekong Basin is not available. Large mammals and birds are quite well documented, while other groups are less so. Thus of necessity, this review will focus on documented taxa, e.g. mammals and birds, and will consider those groups whose information base is increasing, e.g. fish and reptiles. Little information exists for groups such as amphibians and invertebrates. |