Biodiversity conservation in African inland waters: Lessons of the Lake Victoria region

Fresh waters are among the most seriously threatened ecosystems on the planet, having suffered intense human impacts over the past century (SALA et al. 2000, COWX & COLLARESPEREIRA 2002, DUDGEON et al. 2005). In their direct comparison of rates of species loss in freshwater vs. terrestrial ecosystems, RICCIARDI & RASMUSSEN (1999) projected a future extinction rate of 4% per decade for North American freshwater faunas. This is 5 times greater than that for terrestrial faunas and matches estimates for tropical rainforest communities. Although rates of species loss from tropical freshwaters are less known (DUDGEON et al. 2005), the limited data on North America is a serious alarm call for tropical faunas. These distressing trends reflect salient characteristics of both freshwater ecosystems and humans. People have settled disproportionately near waterways for millennia, and their impacts on inland waters have followed development and population growth throughout history. Fresh water is a resource that may be extracted, diverted, contained, eutrophied, or contaminated by humans; the effects are intensified by the landscape position of many inland waters as “receivers” of effluents (SALA et al. 2000). The result of these multiple stressors on inland waters has been a massive reshaping of aquatic communities, with the pace of change quickening in the tropics. With respect to vertebrate faunas, there is little doubt that freshwater fishes represent the most threatened set of vertebrates in the world due to both extensive and intensive human impacts, but also because of the disproportionate richness of inland waters (DUDGEON et al. 2005). Over 10 000 fish species live in freshwater, representing approximately 41% of global fish diversity (LUNDBERG et al. 2000). Despite this extraordinary richness of species, surface freshwater habitats contain a relatively small proportion of the earth’s water supply. The combination of species-rich faunas, high endemism, and the disproportionate richness of the inland waters as habitat has led to a global biodiversity crisis for fresh waters. Of the native freshwater species of North America, 21.3% (217 species) are imperiled (WILLIAMS et al. 1989). A significant challenge to conservation of freshwater fishes results from their patchy nature, imposed by catchment divides and saltwater barriers (LUNDBERG et al. 2000, STIASSNY 2002a, DUDGEON et al. 2005). This same heterogeneity that has fostered wild allopatric speciation – hundreds of species are known only from one drainage or lake system – severely limits possibilities for faunal rescue. Fish like to be in water, and most cannot easily move from one aquatic system to another without it; they therefore have to adapt to changes in situ or disappear. An additional challenge to freshwater conservation is our limited knowledge of fish diversity (STIASSNY 2002a, 2002b). This problem is particularly serious in tropical waters where less intensive exploration and accelerating environmental change in these species-dense systems rob us of potential future discoveries. Some of the most poignant cases are found in equatorial Africa where several fish species have suffered anonymous extinction, the most notable example being Lake Victoria, the largest tropical lake in the world.