Channel Fragmentation by Dams: How It Interacts with Species Diversity and a Changing Climate, From Global to Regional Scales

The majority of the world’s large river systems are fragmented and have their flow altered by dams. Exceptions to this tend to lie in regions inhospitable to hydropower development, such as northern tundra, or in the least economically active regions. The biogeographic distribution of dam impact is widespread, both at terrestrial and freshwater scales, representing significant threat to global biodiversity. Nearly half of the world’s freshwater ecoregions are internally fragmented by dams, and ecoregional distinctions may be artifically imposed by dams in many cases. Freshwater ecoregions with the highest counts of total and endemic species remain relatively unobstructed, representing significant conservation potential. Diadromy is one of the few fish traits indicative of vulnerability to dams for which data are sufficient for global scale analysis. Lampreys (Lampetra spp.), Eels (Anguilla spp.) and Shad (Alosa spp.) are examples of genera particularly vulnerable to dams because their distributions coincide with the most heavily fragmented freshwater ecoregions, and a large proportion of the coincident species for each genera are diadromous. Coupled with expected changes in discharge and water stress, the area of large river basins in need of management interventions to protect ecosystems or people will be much greater for basins fragmented by dams than for basins with free-flowing rivers. Proactive measures that restore the natural capacity of rivers to buffer global-change impacts are more desirable than reactive actions since they may also lead to environmental benefits such as higher water quality and restored fish populations – benefits which may later be unattainable.