8.6 THREATENED BIODIVERSITY IN SOUTHEAST
ASIAN RIVER BASINS

Riverine birds / Mammals of floodplain grasslands / River dolphins / Riverine forest mammals / Crocodilians / Turtles

Mekong molluscs--
The biodiversity of invertebrates in Asian rivers has been less studied than that of fishes, but some taxa are certainly highly speciose (Dudgeon 1999). For example, the mollusc fauna in the lower Mekong Basin includes species-flocks of stenothyrid and pomatiopsid snails (>110 endemic species); a similar radiation has occurred in the Yangtze River, China. The Mobile Bay Basin in the southeastern United States once supported the most diverse freshwater mollusc assemblage in the world (Groombridge and Jenkins 1998), but one third of them have been driven to extinction by flow regulation and habitat alteration. As a result, with over 160 species, the lower Mekong now supports the richest fauna. This is despite the fact that much of the river course has not been surveyed for molluscs. The richness of stenothyrids and pomatiopsids is related to their dietary and habitat specialism (Attwood 1995a): some species prefer muddy substrates, others occur on sand, on rocks or on submerged wood. Flow regulation in the Mekong would lead to changes in food availability and substrate characteristics that might impact some snails while favouring others – in particular, Neotricula aperta, a host of the human blood fluke parasite Schistosoma mekongi (Attwood 1995b).

Riverine birds--
Although Asia is home to around half the global total of specialist river bird species, fewer than 10% of scientific publications on these animals deal with Asian species (Ormerod 1999). Although this limits our knowledge of river bird biodiversity in Southeast Asia, many species are declining in range and abundance. The White-eyed river martin (Pseudochelidon sirintarae) is already extinct.

Data from Lao PDR (Lekagul and Round 1991; Duckworth et al., 1998; Thewlis et al., 1998) illustrate the fate of river birds in Asia. Lao PDR retains a higher proportion of forest cover than its neighbours, and substantial lengths of riverine forest. It is therefore likely to support a more complete assemblage of river birds. In addition, because Lao PDR is land-locked, these birds are not able to use estuaries or mangroves as alternative wetland habitats, and their population status accurately reflects the extent of human impacts on rivers.

Twenty-four Laotian bird species associated with rivers have declined over the last 50 years and are now absent from large areas of suitable habitat. Thirty-one of the 44 species classified as at risk in Lao PDR are wetland birds. Species such as the Indian skimmer (Rhychops albicolis) are globally endangered (Collar et al., 1994; IUCN 2000), while the Black-bellied tern (Sterna acuticauda), Great thick-knee (Esacus magnirostris), and Oriental darter (Anhinga melanogaster) are globally near-threatened.

Birds associated with sandbars on large rivers in Lao PDR and elsewhere have declined greatly due, in part, to human disturbance of nesting sites. The River lapwing (Vanellus duvaucelii), Great thick-knee, River tern (Sterna aurantia), Black-bellied tern and Little tern (Sterna albifrons) are approaching national extinction in Lao PDR, and the Great thick-knee, River tern, Black-bellied tern, and Indian skimmer are already extinct in Thailand (Lekagul and Round 1991). The Plain martin (Riparia paludicola), which has declined in Lao PDR and Thailand, nests in burrows on sandbars – a habit it probably shared with the extinct White-eyed river martin.

A new species of river bird endemic to the lower Mekong basin – Motacilla samveasnae, the Mekong wagtail – was described in 2001. It is restricted to wide braided channels in lowland areas subject to seasonal flooding, and concentrates along earthen banks and overhanging vegetation when water levels are high. Its restricted distribution makes it vulnerable to habitat alteration. Modification of flow and sedimentation patterns (caused by, for example, flow regulation) would put the Mekong wagtail at risk (Davidson et al., 2001).

The wetlands of the lower Mekong support a number of globally-threatened species, and The Great Lake (Lake Tonlé Sap) holds the largest breeding colonies of large water birds in Asia (Edwards 2001). Among them are the critically endangered Giant ibis (Pseudibis gigantea); the endangered Greater adjutant (Leptoptilos dubius), White-shouldered ibis (Pseudibis davisoni), White-winged duck (Cairina scutulata), Bengal florican (Eupodotis bengalensis), and Nordmann's greenshank (Tringer guttifer); the vulnerable Spot-billed pelican (Pelecanus philippensis), Lesser adjutant (Leptoptilos javanicus), Milky stork (Myteria cinerea), and Masked finfoot (Heliopais personata).

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