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Risk
Animals in U.S. freshwater ecosystems are much more likely to become extinct than those found elsewhere. The NatureServe nonprofit conservation organization states that of the more than 4000 animals that depend on freshwater habitats, 35.3 percent are at risk. The Vice President of Programs for NatureServe, Bruce Stein, said that "Nearly every group of animals that depends on freshwater habitat is in trouble --- fishes, turtles, frogs, mussels, crayfish." According to the EPA, the major threats to freshwater biodiversity include agricultural and urban runoff, the introduction of invasive species and the creation of dams and water diversions.
Wetlands
Marshes, swamps and bogs are examples of wetlands --- areas of standing water that support aquatic plants and animals. According to the University of California's Museum of Paleontology, wetlands contain the highest species diversity of all the planet's ecosystems. Enchanted Learning says that the biggest freshwater marsh in the United States is the Florida Everglades. Marshes such as this contain beavers, common egret alligators, copepods, cranes, crayfish, shrimp, turtles, river otters, raccoons, toads and newts.
Rivers and Streams
Rivers and streams are moving bodies of water that flow in one direction only. They start at springs, lakes or from melted snow and travel to larger water channels or the sea. The types of animals found in rivers and streams depend on where the water mass is observed. Near the source, the water is cooler, clearer and more oxygenated, supporting fish such as heterotrophs and trout. Near the stream or river's mouth, the water is murkier and lower in oxygen, providing the perfect habitat for carp and catfish.
Ponds and Lakes
These diverse ecosystems can range in size from a few square meters to thousands of square kilometers. While ponds can be seasonal, lakes can survive for centuries and are kept full by streams, springs and rainfall. The University of California's Museum of Paleontology claims that ponds and lakes may contain a limited number of species due to their relatively closed nature. In a lake, most life is found in the warmer areas near the surface and shore. Typical animals include crustaceans, such as crayfish and scuds; fish, such as bass, minnow and sculpin; amphibians, such as frogs; and reptiles, such as snakes and turtles.