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Upper San Marcos River Exotics

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Upper San Marcos River Exotics A Review of the Exotic Species Inhabiting the Upper San Marcos River, Texas, U.S.A David E. Bowles & Beth Davis Bowles May 28, 2001 Recommended citation: Bowles, D. E., and B. D. Bowles. 2001. A review of the exotic species inhabiting the upper San Marcos River, Texas, U.S.A. Texas Parks and Wildlife Department, Austin, TX. 30 pp. A Review of the Exotic Species Inhabiting the Upper San Marcos River, Hays County Texas, U.S.A. David E. Bowles Texas Parks and Wildlife Department. 4200 Smith School Road, Austin, TX 78744, USA Beth Davis Bowles Molecular Cell and Developmental Biology, School of Biological Sciences, University of Texas, Austin, TX 78712, USA Abstract.—The occurrence and current status of aquatic and semi-aquatic exotic species and their real or potential impacts on native species are assessed for the San Marcos River, Texas. Exotic species were found to be abundant in the river including 16 species of plants, four species of invertebrates, and 28 species of vertebrates. These species were introduced through a variety of means, but released aquaria specimens and stocking for the purpose of supplementing the sport fishery of the river are the primary sources. The geographic origin of the species reported here is diverse with most being native to tropical and subtropical areas of Asia, Central and South America, and other areas of the southern United States outside of central Texas. Solutions to address the problem of exotic species introductions are presented including some management and control options, regulatory mechanisms, and public education and outreach. Public education, and, when possible, management and removal of exotic species are proposed as the best means of alleviating threats to the native fauna and flora of the river. 1 1 There has been a continual increase in the problematic because they are difficult, if rate of species invasions into freshwater not impossible, to control or manage once habitats in the United States (U.S.), and established (Courtenay and Robins 1975; the rate that mankind is intentionally Simberloff 1996). introducing species to new habitats also is The upper San Marcos River, from its increasing (Lachner et al. 1970; source at San Marcos Springs (impounded Courtenay and Hensley 1980; Schwartz as Spring Lake) in the City of San 1997). Introductions of non-native Marcos, Hays County, Texas to its species may seem harmless in some confluence with the Blanco River some 11 circumstances, or even desirable in the km downstream (Fig. 1), has been case of certain sport fishes. Indeed, impacted by exotic species for over 100 certain introduced species play major years. San Marcos Springs are the second roles in our culture and many have largest spring system in Texas producing socioeconomic benefits (e.g., sport fishes, some 4,500 liters per second average flow aquacultural enterprises). However, (Brune 1981). The San Marcos River is introductions of many non-native species an ecologically unique system that serves can have severe negative effects on as habitat to several threatened, ecosystem structure and functioning and endangered, and endemic species (United ultimately result in the loss of biological States Fish and Wildlife Service 1995). diversity (Lodge 1993). Exotic species The warm subtropical to Mediterranean compete with native species for food and climate of central Texas and the thermally space, serve as predators, introduce constant (~23oC) flows of the San Marcos disease and parasites, modify habitats, and Springs and River provide an excellent sometimes interbreed with native species. medium for establishment of non-native There also are aesthetic considerations species. No previous attempt has been associated with introduced species such as made to fully address the extent to which recreational and sport value degradation. exotic species have invaded the river. For instance, lakes and streams clogged In this paper, we present a list of the with Hydrilla are not popular for known exotic species that have been swimming and other aquatic sports reported from the San Marcos River because the abrasive texture of this ecosystem and address the current status invasive weed scratches the skin, and its of these species and their real and voluminous growth habits make potential impacts on the native flora and movement by swimmers and watercraft fauna. A survey of the exotic flora and difficult. Other activities such as hunting fauna of the San Marcos River is an and fishing also can be impacted. Exotic important step toward developing relevant species can cause serious economic losses management plans for protecting and when they detrimentally impact conserving rare, threatened and recreational, commercial and agricultural endangered species that naturally occur in enterprises, or pose risks to human health the river (U.S. Fish and Wildlife Service and welfare (Pimentel et al. 2000). To 1995). confound this issue further, invasive aquatic species are particularly 1 2 Methods the species reported here are diverse with most coming from Asia, Central and The terminology for exotic species used South America, and from other locations herein follows Shafland and Lewis (1984), within North America north of Mexico. and we thus define exotic species as those Under Texas law, possession, sale, and entering an ecosystem from beyond their distribution of several of the species historic range through non-natural means. discussed here are prohibited due to the The scope of this paper is restricted to real or potential risks they pose to native aquatic and semiaquatic species, but we species (Howells 1999). recognize that there are numerous terrestrial and riparian exotic species in the San Marcos River watershed that may have impacts on the ecology of the river. Aquatic Vegetation To the extent known, we also assess the current relative population status of these We found 16 species of aquatic and species and consider their potential semi-aquatic plants have been introduced impacts on native species and habitats. into the river and its headwaters at Spring Based on this information, we have Lake, and most of these have established assigned a relative ecological impact populations. Many of these species are (high, medium, low) to each species. considered noxious weeds in Texas and Known or suspected ecological impacts elsewhere in the world (Barrett 1989). are based on published information and Introduction of aquatic vegetation into our own observations. We also discuss large spring ecosystems in Texas presents the geographic origin of each species, several potentially significant problems. their probable means and dates of For example, invasive aquatic vegetation introduction, and summarize pertinent can impact human activity and safety, literature relating to their biology and interfere with navigation, increase risk and management in the river. severity of flooding, and disrupt ecosystem structure and functioning Results and Discussion (Gurnell and Midgley 1994; Madsen 1997). The many threats to aquatic The exotic species that presently occur ecosystems from invading hydrophytes or have been reported from the San were addressed by Cronk and Fuller Marcos River are presented in Table 1. (1995) and Barrett (1989). Common Forty-eight species including 16 plants, names for plants presented here follow four invertebrates, 18 fish, one reptile, Brako et al. (1995) and Correll and eight birds, and one mammal are listed. Correll (1975). Thirty-two of these species are established in the river and have breeding populations Arundo donax L., giant reed. —This while the remaining 14 species either have species apparently was introduced from localized populations or apparently did the Mediterranean and is now widely not become established following naturalized in the southern U.S. (Correll introduction. The geographic origins of and Correll 1975). Giant reed commonly 2 3 grows along the banks of the river and on along the upper river although the species elevated, partially dewatered areas and has been present there for nearly 100 sandbars within the stream channel. Once years. The species apparently was established, giant reed grows to several introduced sometime after 1907, and the meters in height and produces first herbarium record is dated 1929 considerable shading. In addition, the (Akridge and Fonteyn 1981). Elephant “islands” formed by giant reed can cause ear dominates the shoreline areas of accumulation of debris and sediments Spring Lake and the shallow, near bank ultimately forming larger islands that alter zone of the river to well downstream of its stream channel morphology. The date of confluence with the Blanco River. introduction of this species to the river is Elephant ear is considered highly unknown, but the most probable time was detrimental to the ecological functioning before 1900 given that it was widely of the river (Akridge and Fonteyn 1981; distributed in the southern U.S. in the U.S. Fish and Wildlife Service 1995). 1800s. Cryptocoryne beckettii Thw. ex R. Ceratopteris thalichtroides (L.) Brongn., Trim.—This hydrophyte is a native of floating fern.—Cook et al. (1974) stated southeast Asia (Cook et al. 1974). A this species is a native of Asia where it is population of Cryptocoryne becketti commonly cultivated for food in Japan. occurs in the river near the outfall of the The first report of this species from the San Marcos Wastewater Treatment river was by Morton (1967) from a Facility. The occurrence of this species in collection made in 1963, who further the river likely resulted from an reported the plant to be native to South unintentional release by an aquarist given America in addition to Asia. Hannen [sic. the popularity of this plant in the Hannan] (1969) followed Morton’s work aquarium industry. The year of documenting the establishment of C. introduction of C. becketti into the river is thalichtroides in the river, and he stated unknown, but biologists with the Texas that this species was intentionally Parks and Wildlife Department first introduced for purposes of commercial observed this species at its current production by the now defunct Texas location in 1993 (DEB, personal Aquatic Plants Company.
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