Freshwater rivers, lakes and wetlands are among the most threatened ecosystems on the planet, facing growing pressures from an expanding human population and increased socioeconomic development (Ormerod et al., 2010; Vörösmarty et al., 2010; Carpenter et al., 2011). This pressure on freshwater ecosystems is accompanied by correspondingly high levels of threat to freshwater biodiversity (Dudgeon et al., 2006; WWF, 2010; Thieme et al., 2011; Collen et al., 2014), as is clearly demonstrated by the high species extinction rates and levels of threat recorded on the IUCN Red List of Threatened Species (www.iucnredlist.org), hereafter referred to as the IUCN Red List. North American freshwater bivalves are, for example, notable for having the greatest proportion of extinct species worldwide. In Europe, freshwater species top the IUCN Red List with the highest proportion of threatened species. Remarkable twenty-first-century extinctions such as the baiji (Lipotes vexillifer), the golden toad (Incilius periglenes) and the Alaotra grebe (Tachybaptus rufolavatus), just to name a few, are all freshwater species. However, no-one has yet comprehensively assessed the level of threat and extinction rates for freshwater fishes at the global scale – and it is likely that many fishes are disappearing without record.
The twenty-first century is a critical time for the future of freshwater fishes. Human actions have a serious impact on freshwater ecosystems around the world and the freshwater fish species face ever increasing risks. Unless actions are taken rapidly to reduce the multiple threats facing freshwater fishes, many species will be lost. A sobering example, which serves well to demonstrate the severity of the threat, is the perilous state of the world's sturgeons and paddlefishes (Acipenseriformes). Sturgeon have survived on this Earth for 250 million years, but now face the serious possibility of becoming extinct in this century as a direct result of human activities. Illegal fishing, overfishing, obstructions to migratory routes and pollution has resulted in 23 of the 27 sturgeon species being assessed as threatened on the IUCN Red List. Of these, 17 species are Critically Endangered and four are possibly Extinct, including the Chinese paddlefish (Psephurus gladius), the world's longest freshwater fish for which only two adult specimens (both females) have been recorded since 2002. Human exploitation of freshwater ecosystems and the fishes within them must operate within sustainable limits, and critical sites for freshwater species must be identified and protected before it is too late for many species.
The huge diversity of freshwater fishes is concentrated into an area of habitat that covers only about 1% of the Earth's surface, and much of this limited area has already been extensively impacted and intensively managed to meet human needs (Dudgeon et al., 2006). As outlined in Chapter 1, the number and proportions of threatened species tend to rise wherever fish diversity coincides with dense human populations, intensive resource use and development pressure. Of particular concern is the substantial proportion of the global diversity of freshwater fishes concentrated within the Mekong and Amazon Basins and west-central Africa (Berra, 2001; Abell et al., 2008; Dudgeon, 2011; Chapter 1) with extensive exploitation of water resources planned to accelerate in future years (Dudgeon, 2011; Chapter 1). If current trends continue, and the social, political and economic models that have been used to develop industrialised regions of the world over the past two centuries prevail, then the future of a significant proportion of global diversity of freshwater fish species is clearly uncertain.
Understanding why so many freshwater fish species are threatened requires some understanding of their biology, diversity, distribution, biogeography and ecology, but also some appreciation of the social, economic and political forces that are causing humans to destroy the natural ecosystems upon which we all ultimately depend. To begin to understand the diversity of freshwater fishes, we first need to consider the processes that generated and continue to sustain the diversity of species we see today. Based on an understanding of how freshwater fish diversity is generated and sustained, we consider how vulnerable or resilient various freshwater fishes are to the range of anthropogenic impacts that impinge on freshwater ecosystems. Finally, we discuss how social, political and economic drivers influence human impacts on natural systems, and the changes needed to current models of development that can lead to a sustainable future for humans and the diverse range of freshwater fish species with which we share our planet. The aim of this chapter is to provide an overview of the key issues and threats driving the declines in freshwater fish diversity identified in Chapter 1; subsequent chapters provide more detail on the key issues and address our options for developing a sustainable future for freshwater fishes.
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