Bird's eye view of the spring-fed pools (pozas) on the valley floor. (Photo from National Geographic October 1995). |
Isolation and stability are key factors in evolutionary change (Cox & Moore).The Cuatro Ciénegas basin has been an isolated oasis in the midst of the arid Chihuahuan desert throughout the Pleistocene by the periodic uplifting and warping of the plateau. Although access to the basin occurred during pluvial periods, the majority of the Pleistocene saw the region isolated from the surrounding regions (Minckley 1969). For example, Minckley (1969) theorized that uplift in the north eastern part of the basin occurred early in the Pleistocene, spilling an ancestral lake. An uplift in the western side of the basin reversed this trend later in the Pleistocene and closed the basin again. Minckley also cites evidence of an uplift on the south and west sides of the basin that connected the valley (for a short time) to the Rio Grande basin. It is suspected that during these pluvial periods, a variety of organisms migrated to the valley habitats.
Eggs are visible in the abdominal pouch of this inch-long female grass shrimp, an endemic organism in Cuatro Ciénegas. (Photo from National Geographic, October 1995). |
With the exception of these pluvial periods, the flora and fauna of the moist basin has been and is currently isolated from the arid regions surrounding the unique desert environment. Because of this long-term isolation, the basin's diversity of habitats have produced a highly endemic biota. Endemism is a product of the history of an area, its subsequent isolation, and the present day environment of the region, The number of endemic species is generally considered a clue to the length of time that the area has been isolated. A number of studies cite varying numbers of Cuatro Ciénegas endemics, ranging from 60 to 100 plants and animals out of the approximately 1,000 species found in the basin. Endemism in the fauna of Cuatro Ciénegas is found especially in fish, reptiles, snails and crustaceans (Pinkava 1987:9). For example, of the 20 fish species found in the basin, 12 are found only in the valley waters (Minckley) Although there are varying reports of the number of endemic species in the basin, all studies agree that the degree of endemism in the basin is extremely high. The Cuatro Ciénegas region has been compared to the Galapagos Islands and the Rift Lakes of Africa because of the high incidence of endemism found in many of its groups (Taylor and Minckley 1966).
|