The three most dominant biological groups that typically form stream communities are fish, aquatic invertebrates (chiefly arthropods, mollusks, and segmented worms), and attached algae (the primary producers). The fish and invertebrates include species specialized as primary consumers (of the algae), as detritivores (shredders of terrestrial debris entering the stream or filterers and gatherers of fine organic particles), and as predators. Other species are omnivorous—opportunistic consumers of several food sources. Aquatic species also have specific habitat requirements; a stream community will be determined largely by the available habitat (stony riffle, sandy run, or soft-bottom pool). The condition of the habitat (including embeddedness of stones, amount of cover from instream structures and streambank features, and contaminants in bottom sediments and food) and the quality of the water (temperature, light, pH, conductivity, dissolved oxygen, nutrients, and dissolved and suspended solids) can affect the distribution of aquatic organisms. Other factors that affect the distribution of organisms include dispersal (proximity of colonization areas or downstream barriers such as dams), predation and competition from native and introduced species, food sources from upstream and terrestrial inputs, and hydrologic conditions such as floods and droughts.
The resident stream biota reflect both the current and recent conditions of the habitat, water-quality, and hydrologic factors. In general, algae integrate the previous days to weeks of conditions, and invertebrates can reflect conditions during their lifespans of several months to a year; and fish can reflect previous conditions for as much as several years. The biological groups also differ in the specificity of the environment they reflect: The less-mobile algae and invertebrates reflect recent conditions within a specific pool, run, or riffle, and the more-mobile fish can integrate conditions over much greater distances.
Thirty-two studies of vertebrates (primarily fish) in streams in the SCTX study unit published mostly during the 1980s and 1990s are summarized in table 1. The locations of many of the studies (those with specifically identifiable sampling sites) are shown in figures 2 and 3. The majority of the reports list taxa and numbers of fish for the study sites. When coupled with the report by Young and others (1973), an extensive amount of fish data are available for the study unit. About 140 fish species from the SCTX study unit are listed by the Texas System of Natural Laboratories, Inc. (1994) (table 2).
Hubbs (1957) suggested that the distribution of fish closely follows climatological and geologic factors because these factors affect the chemical and physical properties of aquatic systems. The SCTX study unit is a highly diverse assemblage of environments controlled by the wide variety of climatic, topographic, soil, and biotic factors in the region (Blair, 1950; Gould, 1975). In the analysis of fish collections from eastern and central Texas in 1953 and in 1986, Anderson and others (1995) showed the relative region-wide stability in species diversity during 33 years. Despite this encouraging trend, the report indicated that, for localized areas, several species had become extinct or endangered.
Fifty-four studies of aquatic invertebrates published during 1971–97 are summarized in table 1. The locations of most of the studies (those with specifically identifiable sampling sites) are shown in figures 4 and 5. Surveys, species composition reports, and theses make up the majority of studies concerning invertebrates in the study area. Most of the reports list taxa and numbers of invertebrates collected at the study sites. Although aquatic vertebrates in Texas have been well documented (Hubbs and others, 1991), complete species inventories for most groups of aquatic invertebrates are lacking (Bowles and Arsuffi, 1993). One of the more complete recent studies catalogued invertebrates from the least-impacted streams of the Texas ecoregions (Bayer and others, 1992). About 180 aquatic invertebrate species were collected from the SCTX study unit (table 3).
Thirteen studies of plant communities published during 1940–97 are summarized in table 1. The majority of the reports are listings of taxa or distributions within the study unit. There also are secondary references to plant communities and their role as habitat for, or impact on, endangered species (Bowles and Arsuffi, 1993; Power, 1996). Macrophytes are the focus of all the plant studies listed in table 1 partly because of the importance of macrophytes in Comal and San Marcos Springs, the two largest springs in Texas. About 30 aquatic plants have been identified in the study unit (table 4).
Threats to the continued existence of aquatic endemics (native species unique to the area) typically are anthropogenic and include "agricultural practices, impoundments and flood-control projects, siltation from erosion, ground-water pumping, introduction of non-native species, recreational activities, wastewater discharge, and general pollution" (Bowles and Arsuffi, 1993, p. 320). Allan and Flecker (1993, p. 35) listed six factors as critical in flowing water systems: "habitat loss and degradation, the spread of non-native species, overexploitation, secondary extinctions (loss of a species resulting from loss of one or more other species), chemical and organic pollution, and climate change." With respect to non-native species, they suggest that more tolerant invaders might gain a foothold because of favorable conditions, reduction of the native fauna population, or alteration and degradation of habitat.
Non-native species, that is, those species introduced into an area outside their natural range (or in the case of "exotic" species, from outside the continent) present a threat to Federally listed (endangered or threatened) species and their habitat (U.S. Fish and Wildlife Service, 1995). Competition, predation, hybridization, and habitat modification by non-natives have been identified as major factors threatening endemic organisms (Bowles and Arsuffi, 1993; Ono and others, 1983; U.S. Fish and Wildlife Service, 1995). The SCTX study unit has more than 30 known non-native aquatic species (table 5), many of tropical origin. Their ability to survive cold winters often relates directly to the minimum annual water temperature. For example, exotic giant ramshorn snails (Marisa cornuarietis) have been shown to withdraw into their shells and collect on the bottom substrates at 19 degrees Celsius (°C). These organisms die within 5 hours upon exposure to a temperature of 8 °C (Robins, 1971), thereby effectively limiting their range. Most spring-fed streams in the study unit have temperature ranges within 1 °C and thus provide suitable habitat for these snails and many other tropical species that would otherwise die from minimum winter temperatures (Hubbs, 1995). The introduction and subsequent survival of the blue tilapia (Tilapia aurea) into heated power-plant effluent reservoirs and into the spring-fed upper reaches of the San Marcos, Comal, and San Antonio Rivers (Hubbs and others, 1991) is an excellent example of this phenomenon.
Endangered and Threatened Species
Hubbs and others (1991) estimated that 20 percent of the 169 native Texas freshwater species are in potential danger of extirpation (range reduction) or extinction. Of the 19 aquatic species listed in Texas by the USFWS as endangered or threatened, 8 are associated with the SCTX study unit. Table 6 lists the species considered to be of concern (proposed for listing, endangered, or threatened) in Texas by the USFWS, TPWD, or by the Texas Organization for Endangered Species (TOES). Endangered and threatened species are at the center of a complex battle over water rights within the study unit. The San Marcos and Comal Springs and Associated Aquatic Ecosystems Recovery Plan (revised) was developed to ensure the survival of listed species in their native systems through an ecosystem approach to the recovery of multiple species (U.S. Fish and Wildlife Service, 1995). The San Marcos gambusia (Gambusia georgei) is presumed extinct (Miller and others, 1989), and its case is indicative of the problems of many of the endemics throughout the study unit. Its habitat was restricted to the upper San Marcos River (spring run) where, even historically, the organism was rare and difficult to find. Ono and others (1983) list habitat alteration, pollution, and competition with the introduced Gambusia affinis as the probable causes for the extinction of G. georgei. This species has not been collected in the wild since 1982 (Hubbs and others, 1991).
All of the Federally listed endangered species in the study unit are associated with springs and spring runs, thereby emphasizing the importance of conservation of these habitats. Hubbs (1995) noted that spring fish seldom are found at any substantial distance from the springs and that the area occupied by the endemics is related directly to the volume of water flowing from the springs. Hubbs also reported that droughts reduce available habitats.
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