Demography and Population Structure of a Rio Grande Endemic Emydid, The Big Bend Slider
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The once mighty Rio Grande has become one of the most challenged river systems in North America, and has been included in a list of the 10 rivers most at risk globally. Knowledge of the status and structure of current populations is critical for management and conservation of native species. The Big Bend slider, Trachemys gaigeae gaigeae, is a Rio Grande endemic on the IUCN red list and has apparently been extirpated from most of its historic range in the Rio Grande. This species is also facing competition and hybridization with introduced individuals of its sister taxon, Trachemys scripta elegans (Red-eared slider). Both mitochondrial DNA and 13 nuclear microsatellite markers were employed to examine these issues. Analysis of mitochondrial data from Trachemys have shown that hybridization is indeed occurring in extant T. g. gaigeae populations. Microsatellite data also supports occurrence of hybridization, viability of hybrids, and supports introduced origin of T. s. elegans in Big Bend National Park but also revealed potential range expansion of south Rio Grande native Trachemys scripta elegans. Microsatellite analysis results for Trachemys show evidence of historic gene flow between New Mexico and Texas populations, though they are now well differentiated. Genetic diversity is lower in T. g. gaigeae than in T. s. elegans. Mark-recapture data collected over a five-year span for T. gaigeae has demonstrated that the species has a greater than anticipated ability for dispersal yet shows a high degree of site fidelity even after large floods. Abundance estimates from mark-recapture data show that the Big Bend population is small, and this is corroborated by genetic estimates of effective population size. Results provided by this study allow T. g. gaigeae populations to be monitored in the future and demonstrate that T. g. gaigeae is a species of high conservation concern.