Paleohydrology of the Edwards Aquifer karst and the evolution of rare and endangered Cicurina cave spiders, south-central Texas

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, structurally controlled karstic aquifer system, the current stage of which is the modern Edwards Aquifer, which follows the base of the escarpment. Today, caves, springs and canyons in the area provide habitat for more than 25 federally listed endangered species. Discrete cave habitats in differentially rotated, en-echelon fault blocks were progressively exhumed across approximately 10 million years of aquifer evolution. Invertebrates that have adapted to life within these caves contain a genetic record of their epigean ancestors at the time each cave system became available for colonization. The biological evolution of cave spiders of the Genus Cicurina appears to have followed in the wake of the hydrologic development of the Edwards Aquifer. Mitochondrial genetic variability is statistically correlated with discrete structural components of the fault zone and with the degree of relative uplift, which is a measure of the relative age of exhumation of karstic strata.;In general, troglobites (terrestrial cave species) are more restricted in range than stygobites (aquatic cave species) because terrestrial habitat is fragmented where connections between cave systems occur below the aquifer base level. Five major phylogenetic clades in the study area have geographic distributions that are much smaller than the range proposed for stygobitic catfish known from the confined aquifer. Cicurina biodiversity is a product of vicariance whereby a barrier-rich habitat mosaic was colonized by isolated demes of a common surface ancestor across 10 million years of its evolutionary trajectory.;Previous species distribution data and geological mapping accumulated over the past decade have failed to support many conclusions drawn in the management model for endangered Bexar County karst invertebrates. The phylogeography of troglobitic Cicurina suggests that alternative geographic management units may be more appropriate than those currently considered by the U.S. Fish and Wildlife Service. Mutually informative geologic and genetic data have also identified several troglobites that may be more vulnerable to stochastic events than the listed taxa.