Fresh waters—rivers, streams, lakes, ponds, wetlands—cover less than 1 percent of the earth’s surface, yet their biodiversity is unrivaled. Fully 10 percent of all known animal species and a third of all vertebrate species, including about 40 percent of the world’s fishes, live in fresh waters. Other well represented groups include insects, crustaceans, mites, and mollusks (table 17-1). Further, an estimated 20,000-200,000 freshwater animal species (mostly invertebrates, including those cryptic species inhabiting ground waters) have yet to be described (Strayer, 2006). Despite this rich diversity, extinction risk of freshwater species has been largely overlooked (Strayer and Dudgeon, 2010).