THREATS TO DESERT SPRINGS
Peter J Unmack
Why should we be concerned about fishes in desert springs? Most fish species of the desert springs are not found in any other habitat type. As a result, some of these fishes have developed unique adaptations. In the majority of cases, individual fish species are found in only one or just a few springs. This isolation provides evolutionary biologists with an excellent opportunity to study local processes of speciation and evolution not readily available elsewhere. Desert springs are tiny oases that harbor many aquatic plants and animals in the midst of arid conditions. Thus much of the fauna is often considered relictual, or left over from wetter periods. As a result, the fauna may provide clues as to what once existed in the area.
Unfortunately the long-term survival of fishes in desert springs is under serious threat. The purpose of this review is to provide a summary of the types of threats to the springs and their fishes. These threats include unsustainable exploitation of groundwater, introduced species, diversion and destruction of water sources and waterways for irrigation, and damage by stock grazing.
A serious threat to desert springs is extraction of groundwater at unsustainable levels. An unsustainable level occurs when more groundwater is extracted than can be replaced in an aquifer by recharge. Sustainable groundwater extraction does not significantly change groundwater level and or pressure in an aquifer.
Springs becoming significantly reduced in discharge are an early warning sign that groundwater extraction is at unsustainable levels. If excessive extraction continues, an aquifer will eventually fail completely. Groundwater is usually extracted for agricultural, industrial and/or domestic purposes. Immense pressure is being placed upon aquifers due to increasing human populations that are primarily dependent upon them for water supplies. Despite water extraction being unsustainable in many areas at current population levels, most populations continue to increase exponentially, which places increased pressure on aquifers. For example, the population of northern Mexico grew from 5.5 to 8.3 million, a 53% increase between 1980 and 1990 (Contreras Balderas & Lozanzo-Vilano 1994).
Unsustainable groundwater extraction results in the cessation of spring discharge. The extinction of springs has a severe impact on spring fauna; they are obliterated. Springs from many of the world’s arid zones are failing due to over-extraction from aquifers. In northern Mexico at least 92 springs are now dry. This figure is considered to be an underestimate, as many sites have not been recently re-examined (Contreras Balderas & Lozanzo-Vilano 1994). In Texas, Brune (1975) reports that 63 of 281 major or historically significant springs have completely failed; many more will follow if present trends continue. In Australia, most springs in the Springvale, Bourke, Eulo and Bogan groups, and several springs in the Flinders group are now extinct or virtually so (Ponder 1986). In Saudi Arabia, Al Kahem and Behnke (1983) report that several springs have recently become extinct. They collected Cyprinion sp. from a spring in Khaybar City. When they returned four years later, the spring was dry. They predict many springs will disappear in the future as groundwater extraction increases.
Decreases in Spring Discharge
Besides those desert springs which have become extinct, most have suffered some reduction in discharge. The degree of spring discharge reduction is determined by the quantity of groundwater exploitation. (Impacts of decreased discharge are discussed later under spring diversions.) A number of Mexican, Texan, and Nevadan springs vary considerably in discharge at times due to groundwater exploitation. Many Australian springs have undergone a decrease in discharge since the late 1800’s. The extent of change in the discharge of most springs is not accurately known, as adequate early records for comparison are limited. The only well-documented example is Elizabeth Springs. It is the only remaining spring of the Spring-vale supergroup. It has undergone at least a 95% reduction. Further developments, such as the Roxby Downs uranium mine in the Lake Eyre super-group are likely to cause a further decrease in spring discharges. Groundwater extraction rates for Roxby Downs are presently at 15ML/d, but are planned to increase to 33ML/d once the mine moves into full production (Harris 1992).
Introduced Organisms: Fishes
Introduced fishes are present in arid zone springs around the world. Most introductions have apparently been due to the illegal disposal of aquarium fishes, escapees from aquaculture/fish hatchery ventures, intentional releases for angling purposes, or mosquito control. The primary impacts of introduced species are predation, competition for space or resources, and hybridization. Meffe (1985) demonstrated that dambusia (Gambusia affinis) cause a decline in Gila topminnows (Poeciliopsis occidentalis) populations due to predation. It appears that the loss of a number of spring-fish populations has been caused by introduced piscivorous salmoniids and largemouth bass (Micropterus salmoides). Evidence for predation is usually anecdotal because it is rare for introduced species to be detected before the springfish present have disappeared. No doubt various levels of competition exist between native and introduced species; however, these are difficult to prove, since such relationships are rarely clear. Despite this, there can be no doubt that introduced species have major impacts on springfish. A secondary impact of introductions is the hybridization between native and introduced species. Leon Springs pupfish (Cyprinodon bovinus) was seriously threatened when a sheepshead minnow (C. variegatus) was introduced. Pure populations of both Owens tui chub (Gila bicolor snyderi) and Mohave tui chub (G. b. mohavensis) are only found in isolated springs, as all other populations consist of hybrids with other introduced tui chubs (G. bicolor ssp.). The Bighead pupfish (C. pachycephalus) is restricted to the upper 20 or so metres of a springhead where the temperature is above 38oC; below this only hybrids with Conchos pupfish (C. eximus) are found. An associated problem with introduced fish is the introduction of diseases or parasites along with them. Williams et. al. (1985) suggested that anchor-worm (Lernea sp.) had been introduced into populations of Hiko springfish (Crenichthys baileyi grandis) at Hiko and Crystal Springs, Nevada, along with introduced fish. It may be contributing to the Hiko springfishes’ decline. While few introduced diseases have been recorded, they are a significant potential threat.
Aquatic and Terrestrial Plants
Only one aquatic plant, water hyacinth (Eichornia crassipes) is reported to be causing problems in springs. Contreras Balderas (1991) found it choking spring-heads and outflows after it was introduced into Cuatro Date palms (Phoenix dactylifera) have been introduced to Dalhousie Springs. Introduced terrestrial plants form very dense stands which shade out any vegetation underneath them. They also clog springs and extract large amounts of water through evapotranspiration. Cattails (Typha sp.) cause problems in North American springs, especially when grazing is removed. They overgrow springs, resulting in the removal or reduction of open water habitat, and they also increase evapotranspiration. Typically they need to be controlled at a number of springs and refuges, eg. Corn Creek Spring, a refuge for Manse Ranch poolfish (Empetrichthys latos latos) (Minckley et. al. 1991).
Other Introduced Organisms
Several other aquatic organisms have been introduced into springs including, crayfish (Procambarus clarkii), bullfrogs (Rana catesbeiana), and snails (typically Melanoides) in Ash Meadows and other North American springs (Williams et. al. 1985). The impact of these species is likely to be important, although evidence to demonstrate this has not been obtained.
Diversion of Water:
Water is primarily diverted from spring outflows for agriculture, aquaculture, and human recreation. There are few springs in North America that do not have the spring outlet converted into a channel to collect and divert water from the spring outflow. Two factors that determine the severity of impact are what proportion of the flow is diverted, and how far downstream from the springhead diversion occurs. Most of the larger springs in Nevada have had their outflows diverted (Williams et. al. 1985). In Mexico, virtually every spring described by Contreras Balderas (1991) has been diverted. Minckley (1969, p33-34) graphically displays Poso de la Becerra, in Cuatro Cienegas before and after being diverted. In Australia, virtually no springs have been diverted. One attempt was made to divert one of the Dalhousie Springs but was abandoned. Australian springs occur in areas with very low populations, and irrigation has never been a feasible option presumably due to unsuitable soils and/or water. Also, most Australian springs are too small to warrant diverting.
The are several impacts to springs when flow is diverted. Destruction of the complete natural lower out-flow and associated cienegas or wetlands occurs. The severity of this is determined by the distance from the springhead of the diversion and the quantity diverted. Many springs in Soldier Meadows inhabited by desert dace (Eremichthys acros) are diverted. The desert dace cannot survive over 38oC; it lives in 8 warm springs which need to cool down before they can be inhabited. If too much water is diverted, the only areas remaining are too hot for the fish (Ono et. al. 1983). Water diverted may travel further than was previously possible and is channeled into other habitats, providing other species access to a spring. For instance, the Conchos pupfish is thought to have invaded the spring containing bighead pupfish through a diversion channel where it has since hybridized with bighead pupfish.
Fish farms have been established at a few springs in North America. A trout hatchery is present on Hot Creek Springs, Owens Valley, California. Forest Spring, Ash Meadows, had an illegal tropical fish farm operating in it in the 1960’s (Soltz and Naiman 1978). Big Warm Spring in Railroad Valley is presently utilized for channel catfish (Ictalurus punctatus) aquaculture (Williams et. al. 1985). In most cases, fish hatcheries divert water or channelize spring outflows. Cultured organisms inevitably escape and impact
Physical Destruction of Springs
Springs may be dug out or dynamited in an attempt to increase discharge, concentrate the water flow for irrigation, provide water for stock, or some other purpose. Once a spring is dug out or filled in, most of the endemic fauna disappears. The only population of Raycraft poolfish (E. l. concavus) was destroyed when Raycraft Spring, Pahrump Valley, Nevada, was bulldozed in 1955 (Soltz & Naiman 1978). Point of Rocks Spring, Ash Meadows, was dug out to improve water flow (see photos, Soltz & Naiman 1978, p68-69). Numerous small springs in Queensland, Australia, have been dug out to provide enhanced watering point for stock.
Bores on Springs
A number of bores or wells have been sunk into springs in Australia, particularly in New South Wales and South Australia around Marree. This results in the extinction of the spring.
Inundation of Springs
Equally as destructive as filling in a spring is flooding one. Goodenough Spring, one of the three largest Texan springs, was the only habitat of the Amistad gambusia (G. amistadensis). In 1968, the newly constructed Amistad Reservoir started to fill, subsequently inundating and destroying the spring. This species is now extinct (Minckley et. al 1991). Springs close to Lake Eyre, Australia, that are occasionally flooded when the lake naturally fills also lack any of the typical spring fauna.
Carson Slough, a large springfed marsh in Ash Meadows was completely drained and destroyed so that the abundant peat present could be mined. This resulted in the loss of habitat for thousands of Ash Meadows speckled dace (Rhinichthys osculus nevadensis) and Ash Meadows pupfish (C. nevadensis mionectes) (Soltz & Naiman 1978). The USFWS is going to attempt to restore this slough over the next few years.
Removal of Riparian Vegetation
Some springs have had all their surrounding vegetation removed to decrease water loss through evapotranspiration. This reduces the available shade, which increases the water temperature and the available food sources. Both insects and leaves falling into the water provide a major component of the available food for the fish and other invertebrates, which in turn are eaten by the fish.
Removal of Natural Barriers
A few springs are isolated by barriers such as waterfalls which prevent non-endemic fishes from gaining access to them. Removal of these barriers has a major impact on the endemic fishes. Invading fishes can either displace or hybridize with them. Tecopa pupfish (C. n. calidae) once occurred in a spring separated by small impassable waterfalls before draining into the Amargosa River in Death Valley. These waterfalls became altered due to disturbance from the public baths situated on the springs. This gave Amargose pupfish (C. n. amargose) access to the spring, where it hybridized with the Tecopa pupfish, resulting in its extinction (Soltz & Naiman 1978). Natural barriers are not always physical structures. They can be caused by significant differences in habitat. The Big Bend gambusia (G. gaigei), from Texas is adapted to flowing environments. It almost became extinct when the remaining habitat was dammed in 1953 to provide a pond for fishing. Dambusia gained access to the spring, resulting in the elimination of the Big Bend gambusia by 1956. In 1972 the pond started leaking and the spring returned to a flowing environment. Big Bend gambusia re-invaded the spring from nearby refuges during a flood in 1983. By 1985 they were re-established in the spring.
Changes in Grazing Regimes:
Introduced Animal Grazing
Introduced grazing animals, such as cattle, sheep, pigs, goats, etc, tend to focus on springs for food and water, as these are limited in arid zones. In Australia and North America, the majority of springs have introduced animals grazing them. These animals trample springs and eat palatable vegetation. Trampling destroys vegetation, decreases habitat heterogeneity, and destroys discrete spring outflows, reducing them to bogs. Grazing also changes the original structure of vegetation communities. The loss of vegetation may increase water temperatures, decrease nutrient utilization and reduce food inputs. The presence of animals increases inputs of organic waste, resulting in increased quantities of ammonia, nitrate, and bacteria. Animals may also become bogged in the soft soil present at the head of some springs and die. This can have disastrous impacts. At Edgbaston, Australia, it has been observed that when a sheep dies in a small spring, it kills everything in the spring; in larger springs it reduces the total number of organisms present. At Elizabeth Springs, Australia, the largest out-flow is too soft to walk upon. As a result of this, there were numerous cattle bones scattered in it. Animals walking from spring to spring may also translocate organisms on their hooves (eg. hydrobiid snails).
Changes in Grazing Regimes:
Removal of Grazing
Most springs were either originally grazed by native animals, or they had no grazing at all. The flora and fauna has evolved in conjunction with these grazing pressures. If the grazing regime is changed, then the vegetation community will also change. Numerous springs have been fenced to protect them from adverse grazing impacts. Unfortunately, the impacts of removing grazing can be more devastating. With the removal of grazing pressure, vegetation levels increase and may choke spring outflows. Alternatively, small springs may dry out through increased evapotranspiration. Mexican Spring, Ash Meadows, probably had the smallest self-sustaining fish or vertebrate population in the world, between 20 and 47 individual warm springs pupfish (C. n. pectoralis). This spring went dry in 1973 due to increased evapotranspiration caused by an increase in vegetation when the spring was fenced (Soltz & Naiman 1978). Grazing may also increase the flow rate by decreasing evapotranspiration, increase productivity through increased organic inputs, and it may increase fish habitat in springs.
What can be done to reduce the increasing trend of these threats? We need to undertake field experiments to determine how much groundwater can be extracted before it has a significant impact on springs. Groundwater extraction is inevitable and can be sustainable in most cases. Some compromise must be made to allow groundwater extraction without the loss of the fish fauna. Tighter legislation and broader education need to be enacted to reduce the introduction of non-native species. Again, education of landholders as to the impacts of habitat disturbance would go a long way to avoiding it. Most people will not deliberately destroy the habitat if they are provided with the appropriate information and alternatives. Legislation to protect springs from certain disturbances may also be necessary in some situations. Finally, the problems with changed grazing regimes and fencing urgently require systematic study to determine what the impacts are and the best strategies for managing stock grazing on springs.
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Reprinted from The Advocate, April 1995 Vol. 2 No. 2, the quarterly newsletter for Tropical FishKeepers Exchange USA, a captive maintenance study group.