DELINEATION OF CAVERNOUS POROSITY AND ENDANGERED SPECIES HABITAT WITHIN THE VADOSE ZONE OF THE EDWARDS AQUIFER OUTCROP USING CAVE SPIDER DNA AS AN ENVIRONMENTAL TRACER

The Balcones Escarpment of South-central Texas is a rolling landscape of Cretaceous carbonates where stratal continuity was complicated by more than 350 meters of Miocene displacement along the coincident Balcones Fault Zone. Faulting initiated the development of a coastward-evolving karst system, the current stage of which is represented by the modern Edwards Aquifer, located along the southeastern margin of the fault escarpment. Caves, springs and canyons along the aquifer outcrop provide habitat for more than 25 federally listed endangered species. Six of these species are terrestrial troglobites that occur only within vadose zone caves in the rapidly urbanizing San Antonio outskirts. The degree of connectivity between adjacent cave systems, and associated terrestrial troglobite habitats, is poorly understood, but has important implications for the evolutionary history, potential range, and management and conservation of endangered karst biota. Individual terrestrial karst invertebrate habitats are defined by and isolated from one another by structural and stratigraphic controls on cavernous porosity development within the paleoaquifer and by the cumulative effects of vadose zone alteration that reduce and segment primary cavernous porosity. Recent mapping of the spatial distribution of such isolating mechanisms within the karst indicates that they are more numerous than assumed by current management models and that individual habitat sites may be confined to the individual fault block or smaller scale. Analysis of DNA sampled between and among cave spider populations of the Genus Cicurina shows a positive correlation between the occurrence of interpreted isolating mechanisms and the amount of genetic drift between populations. The spatial distribution of genetic diversity among isolated populations shows poor correlation with a point-of-origin predictive model of troglobite phylogeny. A vicariant model of troglobite speciation is indicated whereby geologic complexity and isolating mechanisms have replicated numerous similar habitats each occupied by an independent evolutionary unit.