Biological diversity represents the web of life: in addition to the biological species themselves it involves the interaction of species within and between ecosystems. The most significant human-based reason for preserving biological diversity is the role that micro-organisms, plants and animals play in providing free ecosystem services without which society in its present form could not persist (Erlich, 1988).

The Mekong River with its monsoon based annual flood pulse supports a biological diversity second only to the Amazon in numbers of fish, mammals and birds. A number of the most sensitive and charismatic species, however, are critically endangered. In addition, the Mekong’s flood pulse underpins the most productive and arguably most complex ecosystem services regime in the world, producing fish that sustains the majority of the protein and fatty oil needs of 55 million natural resource-dependent floodplain inhabitants in the Lower Mekong Basin of Laos, Thailand, Cambodia and Vietnam.

Recent estimates of the biota of the greater Mekong region put the species count numbers at  20,000 plant, 430 mammal, 1200 bird, 800 reptile and amphibian, and 850 fish species (see new species here). The inland fisheries yield was estimated at 2.6 million tonnes per year in the LMB,  which is the world’s largest fresh water yield. 

Wetland types and coverage in the Lower Mekong Basin: 

Wetland type

Area (km2)

Percent of total

Seasonally inundated forest



Seasonally inundated grassland



Marshes, swamps, lakes & ponds



Mangrove forest



Rice field










Enivormental protected areas Wetlands and reservoirs

















                                             Photo credit: Sokrith Heng

Three major bio-regions define the Lower Mekong Basin. Each region has different habitats and unique ecological associations that provide interdependent connectedness of critical habitat for different life stages of numerous fish. These fish are consumed by other river and wetland dependent species, such as birds and mammals -- including humans.

River of Giants
The Mekong’s assemblage of tributaries and dry and wet season habitats has fostered a high level of fish species diversity and endemism. As many as 1,300 species of fish may be found, making it 1st in species per unit area and 3rd in total number after the Amazon and perhaps the Congo (Rainboth, 1996). One quarter of all giant fish species -- the most species of giant freshwater fish in the world (WWF, 2010) -- are found in the central river bio-region and the Tonle Sap bio-region. The giant freshwater stingray (Himantura chaophraya), possibly the largest freshwater fish on Earth, grows to 5 meters in length. The Mekong giant catfish (Pangasianodon gigas) is the world’s heaviest recorded freshwater fish -- up to 300 kg and the Giant Silver Barb is only slightly smaller. Additionally, the infamous "dog-eating" catfish (Pangasius sanitwongsei) is found in these bio-regions. The freshwater Irrawady dolphin (Orcaella brevirostris) is also found in the river’s main channel (Mattson et al. 2002). At the other end of the size spectrum, as many as 223 species of planktonic rotifers are estimated to live in the central river bio-region (Malay et al., 2011).
The Great Lake, Tonle Sap
The most conspicuous elements of the Tonle Sap are the birds and the fish, as well as the flood pulse, the driving force of the ecosystem processes supporting the system’s huge natural productivity. Knowledge of the full biodiversity, and the specific flood pulsed ecosystem processes that support it, is considered poor by most scientists (Santa Barbara, 2011). Yet, by value of its biological richness, it is a Biosphere Reserve as part of UNESCO’s Man and the Biosphere Network (1997) and on the IUCN Ramsar list of Wetlands of International Importance (1999). In 2001 the Cambodian Ministry of Environment reported a count of 885 species of historical floodplain plants and animals (Mok et al., 2001), while in 2005 only 471 species - including 200 plants, 46 mammals and 225 birds - were recorded in the Lake region (ADB, 2005).
A major Tonle Sap site for large colonial water birds is Prek Toal, on the lake’s northwestern tip. Breeding species here include the globally threatened greater adjutant (Leptoptilos dubius), lesser adjutant (Leptoptilos javanicus), Bengal florican ibus (Houbaropsis bengalensis), milky stork (Mycteria cinerea) and the globally near-threatened grey-headed fish eagle (Ichthyophaga ichthyaetus), darter (Anhinga melanogaster), painted stork (Mycteria leucocephala), black-necked stork (Eppypiorhynchus asiaticus) and black-headed ibis (Threskiornis melanocephalus) (Goes & Hong 2002, IUCN 2004).
Fish are the largest vertebrate group in the Tonle Sap ecosystem, both in number of species and in biomass. Some species remain permanently in the Tonle Sap while others use the lake and the floodplain only temporarily and migrate back and forth to the Mekong. The biogeography of most migrating fish species is based on scant quantitative research.
Four species of water snake from the Lake provide a significant harvest tonnage that mainly is used to feed cultured Siamese crocodile, of which there may still be a remnant presence.
Mekong Delta
The delta once provided a rich habitat for Mekong anadromous fish species, those species that require or endure the estuary’s salinity transitions. Today, however, this abundant delta is one of the top three most threatened deltas in the world from the impacts of climate change, from reduction of Mekong sediment flows, and from human land use modification. Only 1.3% of the once biodiversity-rich Mekong Delta now remains in a semi-natural condition and the few remaining wetland species are wholly reliant on these remnant patches. The region is intensively cultivated for rice and for cage-cultured Pangasius catfish.
1.      Asian Development Bank, 2005. Tonle Sap Basin Strategy. Phnom Penh, Cambodia.
2.      Biosphere Reserves – World Network. 2008. Tonle Sap Biosphere Reserve, 1997. Publication Date: 03-11-2008. UNESCO –MAB Secretariat, Paris. Retrieved from UNESCO website:, 10 August 2011.
3.      Erlich, P.R. 1988. The loss of diversity: Causes and consequences. Pages 21-27. in E. O. Wilson, ed. Biodiversity. National Academy Press, Washington DC.
4.      Goes, F. and C. Hong, 2002. The status and significance of waterbirds of the Tonle Sap. Cambodia Bird News 8: 3–19.
7.      Maylay, M, Ken K. Y. Wong, Meas Seanghun, 2011. Rotifer fauna in samples from the upper Cambodian Mekong River Basin. 2011, No. 1, Cambodian Journal of Natural History. Phnom Penh.
8.      Mok, M., Neou, B. & Lane, B.D., 2001. Biodiversity conservation and social justice in the Tonle Sap watershed: the Tonle Sap Biosphere Reserve. Paper presented at the International Conference on Biodiversity and Society, UNESCO, 22–25 May, Columbia University, New York.
9.      Rainboth, W. J. 1996. Fishes of the Cambodian Mekong. Food And Agriculture Organization of the United Nations. Rome.
10. Rainboth, W.J., 1996. FAO species identification field guide for fishery purposes. Fishes of the Cambodian Mekong. Rome: FAO.
11. WWF, 2010. River of Giants. Giant fish of the Mekong. Greater Mekong Program. Christian Thompson, Washington, DC.
  Changes in flows in the Mekong River and its tributaries will have a direct impact on the biodiversity in the region.