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Within the range of similar environments around the world, the springs and streams of the Chihuahuan Freshwater ecoregion constitute one of the world's most outstanding subtropical desert aquatic systems. It contains many species that have adapted to highly localized conditions, with some species living in only one small spring or stream and nowhere else in the world. Imagine walking across the desert and coming upon a lush paradise of freshwater pools that are teeming with fish and other forms of wildlife. This would be unlikely to happen in many desert environments, but the Chihuahuan Freshwater ecoregion is an exception.
The Rio Grande is the heart of the Chihuahuan Freshwater ecoregion and its only major river system. Rising in the San Juan Mountains in southwestern Colorado and fed by its two main tributaries, the Pecos and Río Conchos, the river flows nearly 2,000 miles (3,219 km) through New Mexico and the Big Bend canyons of Texas to the Gulf of Mexico. The isolation and challenging conditions of the Chihuahuan’s freshwater springs and streams has promoted evolution of species adapted to the unique habitats. Many species are found in only one portion of a stream or in one spring. Cuatro Ciénegas has been called the freshwater Galapagos of the Americas because of the extraordinary diversity of freshwater species and numbers of species that are found only there.
Many species of cichlid and cyprinid fishes, aquatic snails, and aquatic reptiles are found in the Chihuahuan Freshwater ecoregion and nowhere else in the world. For example, the world’s only known population of the Leon Spring pupfish lives in the Diamond Y Spring, adjacent to the Pecos River. Some aquatic species of this ecoregion have the unique ability to tolerate extremely high temperatures in thermal springs. For example, in San Diego Springs, the bighead pupfish and an undescribed Gambusia species survive in water temperatures as high as 111° F (44° C), the highest known temperatures in the world inhabited by freshwater fish. Springs aren't just for fish, either: Snails, crustaceans, and crayfish need these habitats, too. Cuatro Ciénegas, unparalleled worldwide for its aquatic species richness in a desert environment, supports endemic fish, crustaceans, and snails, as well as many species of riparian semi-aquatic or aquatic turtles, lizards, skinks, snakes, frogs and toads.
The streams and springs of the Chihuahuan Freshwater ecoregion are precious in such a dry landscape, but some springs are drying up from the unregulated extraction of groundwater, and habitat is being fragmented by the building of canals to irrigate crops. The Rio Grande is considered one of North America’s most endangered rivers as a result of degraded water quality and water withdrawals. Livestock trample streamside vegetation when they graze, causing soils to muddy the streams and springs. Both mining and agriculture cause pollution. And alien species have been introduced and now compete with native species for food and habitat. In the future, large-scale tourism in Cuatro Ciénegas and elsewhere could disrupt the natural systems.
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