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ВТОРЖЕНИЕ ИНТРОДУЦЕНТОВ В ВОДНО-БОЛОТНЫЕ УГОДИЯ ТЕХАСА, ГРОЗИТ ЭКОНОМИКЕ ЭКОТУРИЗМА США
© 2016
Ван Клей Джеймс Э., Ph.D., профессор
Государственный университет им. Стивена Остина, Накогдочес, Техас (США)
Аннотация. Согластно Рамсарской конвенции Озеро Каддо на границе Техаса и Луизианы в США с многочисленными деревами Taxodium distichum входит в водно-болотные угодья международного значения. Местный бюджет сильно зависит от дохода с эко-туризма этой местности: рыбной ловли, катания на лодках и других видов активного отдыха. Из-за недавней экспансии интродуцентного растения Salvinia molesta, все это подвергается опасности. Данная работа включает в себя многолетние описания водных участков Каддо, начиная с 1995 года, когда в водных растительных сообществах преобладали местные виды растений и до 2014 года, когда больших на участках озера доминирует плотным ковром Salvinia.
Ключевые слова. Болото кипарисов, Salvinia molesta, интродуценты, водные растения.
EXOTIC SPECIES INVASIONS THREATEN A SUSTAINABLE ECOTOURISM ECONOMY ASSOCIATED WITH A TEXAS, USA TAXODIUM SWAMP
© 2016
Van Kley James E., Ph.D., professor
Stephen F. Austin State University, Nacogdoches, Texas (USA)
Abstract. Caddo Lake, on the Texas-Louisiana, USA border, includes extensive Taxodium distichum swamps and is an internationally important wetland under the Ramsar Convention. The local economy is strongly dependent on income generated by eco-tourism, fishing, hunting, and boating but these activities are threatened by non-native invasive aquatic plants, especially Salvinia molesta. This study, based on repeat-sampling of wetland vegetation plots, documents vegetation changes from 1995 when most plant communities were dominated by native species to 2014 when large areas were covered by thick Salvinia-dominated mats.
Key words. Taxodium swamp, Salvinia molesta, invasive species, aquatic plants.
INTRODUCTION. Caddo Lake, on the Texas-Louisiana, USA, border (N32.7461, W-94.1085) includes extensive and scenic Taxodium distichum (L.) Rich swamps and is listed as an internationally important wetland under the Ramsar Convention (Fig. 1). The 10,521-hectare lake formed naturally within the past 300 years as a result of a massive natural log jam--the Great Raft-- that blocked the main channel of the Red river; however the lake is currently stabilized by a dam built following the removal of the Great Raft in 1873 (Dahmer, 1988, Barrett 1995). It consists largely of open water on the Louisiana side while in Texas it is a wetland complex occurring on drowned stream channels, point bar deposits, natural levees, and lacustrine delta deposits (Barrett 1995, Van Kley and Hine 1998). While the dam preserved the cypress swamp and bottomland wetlands of the formerly natural lake it profoundly changed the natural hydrology.
East Texas has a subtropical humid climate with a mean annual low temperature of 11o C, an average high temperature of 22o C, and an annual mean of 1168 mm of rainfall (Larkin and Bomar 1983). The area is largely rural with important regional industries including petroleum extraction, timber, and agriculture (especially commercial poultry rising). The immediate area of the Texas portion of Caddo Lake is home to roughly 1400 permanent residents in addition to residents of summer homes. The local economy consists of small businesses supported largely by visitors from regional cities such as Houston, Dallas and Shreveport, Louisiana as well as from other parts of the US and world who come to Caddo lake for purposes of eco-tourism, fishing, hunting, boating, canoeing and kayaking. Many visitors stay overnight in locally owned 'bed-and-breakfast' style lodging or at Caddo Lake State Park. In the early 1990's there were 250,000 annual visitors to Caddo Lake State Park who were estimated to contribute $1.4 million to the local economy during their visitations (Shellman 1999). Another study estimated the local economic impact of the park to be $1.8 million in 2004 (Walker 2005). Although most of the lands surrounding Caddo Lake are privately owned, the core area which includes the majority of the wetlands and swamps on the Texas side is in public ownership and thus freely accessible. These areas include the 2832-hectare Caddo Lake Wildlife Management Area (Van Kley and Hine 1998). Essential to the local economy is the presence of the lake and swamplands with its ponds, baldcypress trees, and canals and channels for accessing fishing and recreation opportunities.
Unfortunately, recent invasions by non-native invasive
aquatic plant species, especially giant salvinia (Salvinia molesta Mitchell) threaten these activities by rendering waterways un-navigable and eliminating fish populations. Current efforts to control Salvinia and other invasive plants include herbicide treatments and the experimental introduction of the salvinia weevil (Cyrtobagous salviniae) (Tipping and Center 2003) which has served as a viable biological control in other areas. Nonetheless these methods have yet to be proved effective as evidenced by the recent high levels of infestation. This paper reports research monitoring changes in the vegetation of Caddo Lake resulting from non-native plants over a 20 year period.
METHODS. In 1995 we established permanent plots and described wetland plant communities along a hydrological gradient and generated a map of plant community types where patches of relatively homogeneous vegetation drawn as polygons on aerial photography were interpreted on the basis of the plots sampled within them (Van Kley and Hine 1998). At the time wetland plant communities were largely dominated by native species. Subsequent observations indicated an increase in non-native species, especially at the time, Eichhornia crassipes (Mart.) Solms. Accordingly, in 2005 we re-visited the approximate locations sampled, and established GPS points marking 56 1000m2 permanent plots. We re-sampled these plots in 2005, 2009, 2011, 2012, and 2014. At each sampling period all vascular plant species in each plot were identified and their abundance estimated on a five-point cover abundance scale (Fig 2). Environmental factors recorded at each plot included the plot elevation (depth), tree canopy closure, mean and high annual water levels, and the year of sampling. The resulting sample-by-species data matrix was subjected to canonical correspondence analysis using the PCOrd software (McCune & Mefford, 1999) Maps of the study area were also generated showing the abundance of selected species on the basis of abundances for the species observed for the plots that occurred in each mapped vegetation patch.
RESULTS AND CONCLUSIONS. In 1995, nonwoody vegetation was largely dominated by native species with Eichhornia crassipes restricted to a limited area. By 2005 Eichhornia had become abundant throughout most of the study area and Alternanthera philoxeroides (Mart.) Griseb. and Hydrilla verticillata (L. f.) Royle (hydrilla), although present in 1995, had increased. By 2009, Salvinia molesta had appeared and become abundant throughout and Eichhornia remained abundant. During 2011, a severe
drought year with near record low water levels following a cold winter, Salvinia and Eichhornia declined steeply in abundance but were still widely distributed spatially. Salvinia recovered strongly in 2012 with the return of normal conditions, apparently at the expense of Eichhornia which did not regain former abundances (Fig. 3). By 2014, marked declines had been observed in several formerly dominant or abundant native species including Wolffia brasiliensis Weddell, Spridela polyrrhiza (L.) Schleid., Nuphar lutea (L.) Sm., and Nelumbo lutea Willd. while Salvinia and Alternanthera remained abundant (See correlations on Fig. 2). Additional notable non-native species that appeared during the study period included Hygrophila polysperma (Roxb.) T. Anderson (2005) and Najas minor All. (2014).
These changes were reflected in canonical correspondence analysis results where plots for each period occupied largely different regions in ordination space (Fig. 2). In addition to the expected strong correlations between vegetation as described by the ordination and the underlying hydrologic gradient, there were also relationships between vegetation, the sampling year, and annual water levels. The ordination also reflected an increase in non-native species abundances and a corresponding decline in natives over time.
Several novel plant communities not present earlier appeared over the course of the study. These included dense floating mat communities dominated by Salvinia and the marginally-native sedge Oxycaryum cubense (Poepp. & Kunth) Lye (Fig. 4) as well as a floating mat community dominated by non-natives Alternathera, Salvinia, and the formerly uncommon native Hydrocotyle ranunculoides L. f. The latter was particularly widespread during 2014. The events at Caddo Lake can be considered an unplanned landscape-scale 'experiment' in community re-assembly following introduction of multiple competitive, potentially dominant species. The ultimate outcome is unknown as marked year-to-year differences in non-woody wetland vegetation continue to be observed between each sampling period. Continued monitoring is necessary to observe longterm outcomes and may contribute to data that enhance the understanding of biogeographic changes associated with species invasions.
Current efforts to control Salvinia and other invasive plants (herbicide treatments and introduction of the salvinia weevil) have yet to be proved effective as evidenced by the recent high levels of infestation. The reduction in overall coverage of Salvinia etc. mats following unfavorable weather in the previous season (low water levels from a severe 2011 drought and a cold 2010 winter) Indicates the potential to control aquatic vegetation at Caddo Lake by manipulating water levels. Restoring a more natural hydrological regime by allowing major flood pulses from upstream dams and by modifying the Caddo Lake dam to allow periodic draw-down of water thereby mimicking natural droughts will aid in control. At a minimum, management must focus on seasonal water level manipulations in order to destroy floating mat communities each year so that they do not overwinter and must instead re-form anew each growing season. It is hoped that proper management of aquatic vegetation will enable Caddo Lake to remain an important driver of ecotourism and the local economy.
REFERENCES:
1. Barrett, M.L. Sedimentary record of a 19th century Red River Raft lake: Caddo Lake, Louisiana. // The Compass. 1995. No 72. P. 3-11.
2. Dahmer, F. Caddo Was...a short history of Caddo Lake. / Everett, Bossier City, LA. 1988. 82 p.
3. Larkin, E.J. & G.W. Bomar. Climatic Atlas of Texas. / Texas Dept. of Water Resources, Austin, Tx. 1983. 151 p.
4. McCune B. & M. J. Mefford. PC Ord: Multivariate analysis of ecological data version 4.4. MJM Software, Glenedon Beach, Or. 1999.
5. Shellman Dwight K., Jr. Caddo Lake : Helping Local People to Use Their Educational Infrastructure for Ecological Stewardship of Ramsar Wetlands. // Report. The
Caddo Lake Institute, Inc. Aspen, Colorado & Uncertain, Texas. 1999.
6. Tipping, P.W. and T.D. Center. Cyrtobagous salviniae (Coleoptera: Curculionidae) successfully overwinters in Texas and Louisiana. // Fla. Entomol. 2003. V. 86. No.1. P. 92-93
7. Van Kley J.E. and D.N. Hine. The Wetland vegetation of Caddo Lake. // Texas Journal of Science. 1998. No. 50. P. 267-290.
8. Walker J., K. Sang, and J. Compton. The Economic of Texas State Parks. Sang Kwan Lee Texas A&M University. Department of Recreation, Park and Tourism Sciences. // Report submitted to Texas Parks and Wildlife Dept. 2005.
Fig. 1. Taxodium distichum wetlands at Caddo lake, Texas USA. The surface is dominated by a thin mat of the
native Wolfia brasiliensis.
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Fig. 2. A canonical correspondence analysis of baldcypress swamps from Caddo Lake, Texas based on submerged & surface-layer plants. Axes were constrained by year of sampling, water levels, & tree canopy cover.
Fig. 3. Changes in distribution and abundance of giant salvinia (Salvinia molesta) in wetlands at Caddo Lake, Texas USA from 1995-2014.
Fig. 4. A novel floating mat community with Salvinia molesta and Oxycaryum cubense. Widespread occurrence of this plant community was first observed in 2012.
