5 Aquifer Ecology and Endangered Species

One of the unique characteristics of the Edwards Aquifer is the great variety of organisms that live in the aquifer (called stygofauna) or are directly dependent upon flows from its springs. With 91 described stygobite species and at least another 14 awaiting taxonomic description, the subterranean aquatic fauna of Texas is globally renowned (Longley, 1981; Culver and Sket, 2000). The Edwards Aquifer species diversity ranks among the highest recorded for any aquifer worldwide. The Edwards (Balcones Fault Zone) Aquifer contains significant diversity, including at least 65 described species (Krejca and Reddell, 2019, Table 1 therein), 34 described species at the artesian well in San Marcos, and 14 described species at nearby Ezell’s Cave. The Edwards-Trinity Plateau Aquifer has the second greatest diversity in Texas aquifers.

Aquifer development created the opportunity for a splendid variety of aquatic fauna to invade the subsurface. The carbonates were deposited in a shallow sea. Marine invertebrates of those seas were the first precursors to stygobite fauna. With uplift and erosion, some of these marine species stayed with the land. Some adapted to living in caves and to life in freshwater. Marine invertebrates [e.g., isopods (Cirolanidae), amphipods (Hadziidae, Ingolfiellidae, and Sebidae), shrimp-like species (Bogidiellidea), worm-like crustaceans (Bathynellacea), and crustaceans found in thermal springs (Thermosbanacea)] were some of the earliest stygobites in Texas (Krejca and Reddell, 2019). During and after uplift and karstification, freshwater invertebrates, including crangonyctid amphipods, asellid isopods, palaemonid shrimp, copepods, ostracods, snails, fish, flatworms, arachnids, and insects including elmid and dytiscid beetles, were introduced.

In addition to this variety of species available for cave adaptation, the history of uplifts, faulting, karstification, stream incision, and springs formed many ecological niches within the aquifer as well as barriers to communication. For instance, stream incision by the Colorado River divides the Barton Springs and the Northern segments. In the recharge zone of the San Antonio segment, the evolutionary development of cave spiders is a function of when fault blocks became available as vadose zone habitats (White et al., 2009). These barriers isolate subterranean populations and create the opportunity for speciation and endemism (Krejca, 2005).

The aquifer’s remarkable biological diversity also includes two groups of highly troglomorphic (cave-adapted) vertebrates, Eurycea salamanders (13 species, including some spring and some cave forms) and Ictalurid catfish (three species). Given this diversity, it is surprising how little is known about the biology of the species. The invertebrates, in particular, span the range of taxa, including snails, flatworms, ostracods, copepods, amphipods, isopods, shrimp, mites, beetles, and even an unstudied and perhaps the world’s only aquifer-adapted leech (Mooreobdella species). Many of these groups lack taxonomic experts to describe new species or assign names to the existing collections (Krejca and Reddell, 2019). There is concern that spring ecosystems might fail before taxonomists can describe the species present (Gary et al., 2019; Smith et al., 2020), with the looming issues of water quality and quantity essential to the continued survival of these ecosystems. The known endangered species of the aquifer (discussed in Section 5.1) create challenges to the future development and management of the aquifer. Payne and others (2019) discuss the efforts to maintain habitat for endangered species in Comal and San Marcos springs. In addition, the microbial ecosystems of the aquifer have only recently started to receive attention. Some of them are unique, such as those along the aquifer’s brackish-water line, which is discussed in Section 5.2.