Photo: Long-stemmed water lilies reach for the hot Mexican sun while schools of fish feed in their shade

Photograph by George Grall

Lakes, ponds, rivers, and streams are freshwater habitats. So too are swamps, bogs, and marshes. Lumped together, these ecosystems contain all the water in the world that is not frozen, essentially salt-free, and accessible to humans.

It amounts to precious little water, less than half a percent of the world's water supply. Yet it is vital to the survival of humans and a wide range of critters from dragonflies and piranhas to beavers and bullfrogs.

Lakes and ponds are basins of water surrounded by land, whereas rivers and streams are the arteries that move freshwater from land, such as mountain peaks, to the oceans. At the edge of many lakes and rivers are saturated wetlands—the swamps, bogs, and marshes—that support myriad types of plant and animal life, prevent floods, retain sediments, and purify drinking water. All the freshwater ecosystems can be found within a watershed—a set of habitats that drain into a single body of water such as a big lake or the ocean.

From scum on ponds to water celery on river beds and reeds swaying in a wetland breeze, freshwater algae and plants provide oxygen to breathe and food to eat. Plants like duckweeds float on the surface of still and slow-moving waters, while those with roots and flexible stems can stand in swift-flowing streams. Wetlands are full of vegetation that feeds everything from fish to people. In fact, about half the world's human population depends on rice, a wetland plant.

Freshwater habitats contain about 12 percent of the world's known animals, including 40 percent of the fish species. Many insects, amphibians, and crustaceans (such as freshwater shrimp) are also found in freshwater habitats. And wetlands are rich with birds, offering them a place to breed, nest, and rear young, and a source of food and shelter during long migrations. Some waterfowl such as grebes never leave this habitat.

Yet despite—and because of—the importance of freshwater, humans have manipulated these habitats more than any other on Earth. Sewage and chemicals pollute entire watersheds. Wetlands are routinely drained for strip malls, homes, and plantations. Rivers are dammed to supply electricity and water to cities and irrigation for farms. In fact, freshwater siphoned for agriculture accounts for 70 percent of all global water use. More than half of it is wasted.

These habitat alterations have destroyed half the world's wetlands in the past 100 years, dammed and diverted thousands of river miles, and left behind a polluted legacy for generations to come. At least 20 percent of all freshwater species are either extinct or threatened with extinction. As freshwater habitats are further degraded and stressed, wars over access to freshwater may darken the world's future.

Hope lies with cooperation among nations to better manage, conserve, and share the planet's scarce freshwater resources. Positive actions include resistance to wetland development, efficient agricultural use of water, and a switch to solar and wind energy from hydroelectric dams.

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