Minckley W. L. & Unmack, P. J. 2000. Western springs: their faunas, and threats to their existence. Pp: 52-53. In: Freshwater Ecoregions of North America. Eds. Abell, R. A., Olson, D. M., Dinerstein, E., Hurley, P. T. et al. Island Press, Washington DC.
Springs, Their Faunas, and Threats to Their Existence
W. L. Minckley and Peter J. Unmack
Springs occur where water from subsurface aquifers rises to the surface. They are more reliable than most other aquatic habitats, more constant in environmental conditions, and often form sources for surface streams and lakes. With increasing aridity, aquatic habitats shrink and springs soon become isolated islands or archipelagos in seas of desert. If replenished from recharge areas or fed by large subsurface reservoirs, they continue to rise long after perennial lakes and streams are gone, so in arid parts of the World, springs may frequently act as the only refuges as desertification advances. A substantial proportion of aquatic life, as well as terrestrial inhabitants reliant on perennial water (as in many endorheic basins of western United States and México) are intimately associated with springs and spring-fed systems.
Springs have several unusual physical characteristics not seen in other aquatic habitats. Due to the characteristics of aquifers, water rises to the surface with constant physiochemical conditions and flow rates. As water travels downstream spring outflows physiochemical conditions change along a gradient. At any single point in the outflow conditions tend to vary little, i.e., temperatures vary by 0-2oC from day to day or throughout the year. The larger spring discharge is the slower changes occur downstream. Very small springs tend to have some opposite characterists. They still have a constant water supply, however, due to their shallow nature daily temperature fluctuations can be some of the most extreme for perennial aquatic habitats, rising and falling as much as ~20oC, yet highly consistant in this variable pattern from day to day. Another unique characteristic is when springs first emerge from an aquifer they are often saturated with carbonates which results in a series of complicated chemical reacations as carbonates precipitate and carbon dioxide is evolved.
Along with unique physical characteristics there is a suite of organisms typically associated with springs throughout the world. Springs also harbor a disproportionate percentage of endemic species probably due to their fragmented nature, long term persistance, and unusual physical characteristics. The best known organisms are fishes. Most fishes inhabiting springs tend to be small, have short generation times, and some of the broadest physiochemical tolerances of any fish. These species tend to be found throughout spring outflows. All of the remaining spring fauna tends to be restricted to head spring environments and their upper outflows. A few fishes also fit this pattern. Hydrobiid snails tend to be extremely abundant and display tremendous diverity. The number of hybrobiid species (and likely other organisms) is often proportionate to the number and closeness of springs. Individual springs may only have 1 species, smaller groups of springs frequenty have between 1-4 species from 1 or 2 genera, while larger groups such as Ash Meadows and Cuatro Cienegas have 11 and 13 species from 2 and 9 genera respectively. Amphipods are also common to springs. These, along with all other smaller animal and microscopic life is virtually unstudied or recorded.
Springs and their biotas are critically threatened by human intervention into arid lands. When water quality is suitable for irrigation or domestic use, excessive pumping soon stops surface flow, killing surface ecosystems. Overuse of subterranean water also often is insidious and difficult to detect since effects at a spring outflow may appear only after years of pumping several 10s or 100s of kilometers away. Extraction of subsurface flow by agricultural pumping destroyed some of the largest springs in Texas (Brune 1975); water extraction for agricultural and residential development in Ash Meadows, Nevada, was stopped only by a favorable judgement in the U.S. Supreme Court (Deacon & Williams 1991); and entire biotas were destroyed in less than a decade in Valle de Sandia, Nuevo León, where four pupfishes and unknown numbers of endemic invertebrates disappeared with the surface water before it could be fully recorded (Contreras-B. & Lozano-V. 1996). Mexican springs are especially endangered because of new availability of electricity in formerly remote areas and expanding irrigation to support burgeoning human populations. Almost 100 known springs in México have been dried in the last few decades (Contreras-B. & Lozano-V. 1994). Unfortunately, there is also a lack of strong environmental will to conserve springs and the legal avenues are not well enforced (Contreras-B. & Lozano-V. 1994).
Springs not simply pumped dry are often capped, diverted, or if large enough, converted for recreation such as bathing and swimming, all to the detriment of their biota (Contreras-B. 1991; Sheppard 1993). Intensive grazing by domestic animals significantly degrades springs through vegetation removal, trampling, fecal contamination, and dead carcasses. Once disturbed, fencing to protect them from grazing may have the opposite effect, as vegetation often invades to choke the spring pool and outflow. They also suffer from extensive introductions of non-native species. Continuing problems with sportfish stocking and bait items such as crayfish jeopardize the native biotas of Owens Valley, California (Minckley et al. 1991), Ash Meadows and White River system, Nevada (Courtenay, et al. 1985; Pister 1991), and Cuatro Ciénegas, Coahuila, México (orig. data). Introduced aquarium fishes and invertebrates, mostly poeciliids, cichlids, and Melanoides snails are common in many southern Nevada and Mexican springs (Williams et al. 1985; Contreras-A. et al. 1995). Mosquitofish have also been widely planted around the world to supposedly control mosquito larvae (Courtenay and Meffe 1989).
Due to their small size and isolation, absolute numbers of species inhabiting springs are small. They nonetheless usually contain or support a dispropportionate amount of biodiversity as they may represent the only existing surface water. Because of this, habitat loss and alteration are highly destructive of biodiversity on a relative scale. This is especially true since one-of-a-kind populations of endemic species, as well as their whole ecosystems, can be lost at a single, relatively sudden stroke. Only public demand for conservation coupled with swift and definitive action can reverse the imminent extinction of a large percentage of arid-land springs and their irreplacable unique biotas.
Brune, G. 1975. Major and historical springs of Texas. TX Water Development Board Reptort 189: 1-94.
Contreras-Arquieta A, G. Guajardo-Martinez, & S. Contreras-Balderas. 1995. Thiara (Melanoides) tuberculata (Müller, 1774) (Gastropoda: Thiaridae), su probable impacto ecológico en México. Publiciones Biologicas FCB-UANL, Mexico. 8: 17-24.
Contreras-Balderas, S. 1991. Conservation of Mexican freshwater fishes: some protected sites and species, and recent federal legislation. Pp. 191-197 in W. L. Minckley & J. E. Deacon, eds., Battle Against Extinction: Native Fish Management in the American West, Univ. AZ Press, Tucson.
Contreras-Balderas, S. & M. L. Lozano-Vilano. 1994. Water, endangered fishes, and development perspectives in arid lands of Mexico. Conserv. Biol. 8: 379-387.
Contreras-Balderas, S. & M. L. Lozano-Vilano. 1996. Extinction of most Sandia and Potosi valleys (Nuevo Leon, Mexico) endemic pupfishes, crayfishes and snails. Ichthyol. Explor. Freshwat. 7: 33-40.
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Courtenay, W. C., Jr., & G. K. Meffe. 1989. Small fishes in strange places: A review of introduced poeciliids. Pp. 319-332 in G. K. Meffe & F. F. Snelson, Jr., eds., Ecology and Evolution of Livebearing Fishes (Poeciliidae), Prentice-Hall, Englewood Cliffs, NJ.
Deacon, J. E. & C. D. Williams. 1991. Ash Meadows and the legacy of the Devils Hole pupfish. Pp. 69-90 in W. L. Minckley & J. E. Deacon, eds., Battle Against Extinction: Native Fish Management in the American West, Univ. AZ Press, Tucson.
Minckley, W. L., G. K. Meffe & D. L. Soltz. 1991. Conservation and management of short-lived fishes: The cyprinodonts. Pp. 247-282 in W. L. Minckley & J. E. Deacon, eds., Battle Against Extinction: Native Fish Management in the American West, Univ. AZ Press, Tucson.
Pister, E. P. 1991. The Desert Fishes Council: Catalyst for change. Pp. 55-68 in W. L. Minckley & J. E. Deacon, eds., Battle Against Extinction: Native Fish Management in the American West, Univ. AZ Press, Tucson.
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Williams, J. E., D. B. Bowman, J. E. Brooks, A. A. Echelle, R. J. Edwards, D. A. Hendrickson & J. J. Landye. 1985. Endangered aquatic ecosytems in North American deserts with a list of vanishing fishes of the region. J. AZ-NV Acad. Sci. 20: 1-62.