River conservation in south-east Asia
D. Dudgeon, S. Choowaew and S.-C. Ho
Asia is the most populous region of the world, with 13% of the land area and around 50% of the people. It includes some countries with highly developed economies (e.g. Singapore), but more people live in poverty in Asia than in Africa and Latin America combined (Braatz et al., 1992). Six of the longest rivers in the world are situated in Asia south of latitude 30°N: the Yangtze (Chang Jiang), Mekong, Indus, Brahmaputra, Ganges and Irrawaddy (Figure 11.1). In addition, Bangladesh has more than 50 ‘important’ rivers; India, 400; Indonesia, 200; and Thailand, 10 (Van Der Leeden, 1975; Jalal, 1987; see also Gopal et al., this volume). One estimate is that these rivers (together with the Palaearctic Huang Ho) transport over 80% of the sediment carried by all rivers worldwide (Jalal, 1987; but see Degens et al., 1991). The ecology of tropical Asian rivers has been reviewed recently by Dudgeon (1992, 1995a, b, 1999) who emphasizes the importance of discharge seasonality on the biota. The predominant influence of monsoons gives rise to a pattern whereby predictable periods of drought and water scarcity in the dry season alternate with times of increased discharge, spates and floodplain inundation during the wet season.
Asia, and especially the south-east Asian countries of Malaysia, the Philippines, Vietnam and Indonesia, is extremely biodiverse, with tremendous species richness and high levels of endemism. Indonesia, for example, is among the world’s top 10 countries for numbers of species of flowering plants, birds, reptiles and amphibians, with more species of plants and birds than the African continent (Braatz et al., 1992). It is estimated to support an overall total of at least 15% of the species in the world and, in terms of aquatic biodiversity, has 900 species of amphibians and more dragonflies (666 species) than any other country (Caldecott, 1996). Because many of the Indonesian islands have been isolated for long periods, there are high rates of endemism among dragonflies and freshwater fish. Fish are highly diverse elsewhere in the region also, and the Indochinese Peninsula has 930 species in 87 families (Kottelat, 1989). Despite such richness, it is apparent that existing inventories of the fish fauna are far from complete (Zakaria-Ismail, 1994).
Much of Asia is characterized by large and rapidly growing urban complexes with associated problems of potable water supply, sanitation, water pollution, flooding and depletion of groundwater aquifers. As a result, ‘the tropics, environmentally and economically, are in trouble’ (Baker, 1993). It might be argued that ‘the tropics’ has no more homogeneity than ‘the nontropics’, but tropical south-east Asia provides many examples that illustrate Baker’s view. While substantial economic progress has been made by some countries in the region, this has almost invariably been associated with environmental degradation and ever-increasing demands on the resource base (Baker, 1993). A compilation of figures by Niacin (1992: Fig. 6.14) shows that total water consumption in south-east Asia has risen from 82 km3 yr−1 in 1900 to 187 in 1950, 609 in 1990 and a projected 741 km3 yr−1 in 2000. Irretrievable water losses (due to consumption in agriculture, especially on irrigated land) have increased from 65 (1990) to 142 (1950) to 399 (1990) to a projected 435 km3 yr−1 in 2000. This loss denotes a
Global Perspectives on River Conservation: Science. Policy and Practice.
Edited by P.J. Boon, B.R. Davies and G.E. Petts. © 2000 John Wiley & Sons Ltd.