Threats Like canaries in coal mines, aquatic animals such as stream-dwelling insects, mollusks, and crustaceans serve as indicators of environmental quality. W hile the Southeast and West stand out as epicenters of endangerment, aquatic organisms are under stress throughout the United States. Of special concern are the sediments from soil erosion, which can smother stream bottoms and render them unsuitable for many aquatic creatures. Dams and water diversions for agriculture, industry, and domestic use seriously alter the flow, temperature, and nutrient content of waterways.
Dams also create physical barriers to species migration and dispersal.
The nation's 75,000 large dams and 2.5 million small dams have widespread and pervasive effects on aquatic species. Non-native species accidentally or purposefully introduced into aquatic systems compete with native species for space and resources. The combined effects of competition, predation, and hybridization by non-native species further tax native plants and animals already jeopardized by chemical and physical threats. Direct habitat destruction or degradation caused by the alteration or conversion of wetlands, stream channels, and riparian corridors also threatens aquatic biodiversity. Copyright 1996, The Nature Conservancy.