Edwards Plateau savanna

• Category: Grassland, Tundra, and Human Biomes.
• Geographic Location: North America.
• Summary: These brushlands are regionally unique in geology and terrain. Historically used for livestock production, human population is exerting pressure on its habitats and species.

The Edwards Plateau Savanna biome encompasses 20 million acres (8 million hectares) in central Texas. The Cretaceous-epoch limestone forming the bedrock of the area creates a region that is distinctively different from surrounding areas in geology, terrain, and ultimately its land-use culture. The region has shallow soils and receives low annual precipitation of 16–32 inches (406–813 millimeters). Despite these limitations, the savanna has been extensively used for livestock production, generally goats, sheep, and cattle. The vegetation that existed here before livestock grazing is unclear but is believed to have consisted of mixed woodlands, oak savannas, and grasslands. Overgrazing has increased the spread of unpalatable brush species, and many areas are experiencing increased population growth and residential development.

Soil and Water Regimes

Soils in the savanna are often very shallow, with areas of exposed bedrock. The limestone bedrock also causes the soils to have high clay content. Caliche (redeposited calcium carbonate) is likely to be found in areas with deeper soils; this makes agricultural production difficult.

Several plant communities occur in the Edwards Plateau savanna, including mixed woodland, tree and shrub savanna, cedar brake, bottomland forest, and riparian forest. Except on steep, rocky slopes, the historical vegetation in the region is considered to be open savanna or grassland, and the brush species present are encroaching on the area due to changes in historic grazing and fire regimes.

The eastern Edwards Plateau savanna is commonly referred to as the Texas Hill Country. Many rivers and streams in this region have cut through the layers of limestone to produce steep hillsides and valleys. Major rivers in the region include the Colorado of Texas, Pecos, Nueces, Guadalupe, and Rio Grande. The limestone layer allows water from streams to lose water into underground caves and springs, thereby recharging the Edwards Aquifer that lies beneath the ground layers.

The Edwards Plateau watershed and Edwards Aquifer provide municipal water resources for several of the largest cities in the region, including Austin, Del Rio, San Antonio, and San Marcos. There are no natural lakes in the region, but many human-made reservoirs have been built to provide flood control, municipal water sources, and recreational opportunities.

Fauna and Flora

Underneath the limestone bedrock are many caves, sinkholes, and springs that are home to endemic (found nowhere else on Earth) vertebrate species, including the widemouth blindcat (Satan eurystomus), San Marcos salamander (Eurycea nana), comal blind salamander (Eurycea tridentifera), Texas blind salamander (Typhlomolge rathbuni), blanco blind salamander (Typhlomolge robusta), and Texas salamander (Eurycea neotenes). Invertebrate species endemic to the region include the Tooth Cave pseudoscorpion (Microcreagis texana) and the Kretschmarr Cave mold beetle (Texamaurops redelli).

Many bat species roost in the caves; the area is world renowned for the millions of Mexican free-tailed bats (Tadarida brasiliensis) that gather in hordes in the caves to raise their young. Among avians, the endangered golden-cheeked warbler (Setophaga chrysoparia) and black-capped vireo (Vireo atricapilla) nest in the region, which is listed in the Top 20 Most Threatened Bird Habitats in the United States by the American Bird Conservancy.

Endemic plants here include netleaf swampprivet (Forestiera reticulata), plateau milkvine (Matelea edwardsensis), basin bellflower (Campanula reverchonii), Lindheimer crownbeard (Verbesina Lindheineri), low loosestrife (Lythrum ovalifolium), Buckley's fluffgrass (Tridens buckleyanus), Texas snowbells (Stryas texana), bracted twist-flower (Streptanthus bracteatus), and cliff bedstraw (Galium correllii).

Effects of Human Activity

Before settlement by people of European and African descent in the mid-1800s, Native American peoples inhabited the area. After the introduction of horses to the region, Apache and Comanche tribes developed a culture based on horses and bison. Due to the tough conditions for growing crops, settlers depended on grazing livestock for subsistence and the trade economy. Today, the region remains one of the top sheep-producing areas in the United States.

Recreational hunting of white-tailed deer, turkey, and quail provides significant revenue to landowners in the region. Various antelope and deer species of African and Indian countries have been introduced to provide further hunting opportunities. Bird-watching also brings revenue into the region, with thousands of visitors annually arriving to see the black-capped vireos and golden-cheeked warblers. The U.S. Fish and Wildlife Service established the Balcones Canyonlands National Wildlife Refuge to protect 80,000 acres (32,375 hectares) of Edwards Plateau habitat.

Many changes have occurred in the Edwards Plateau savanna since the time of settlement, some of which threaten its rustic foundation. Rapid population growth in the region has resulted in increased urban and suburban development in formerly rural areas. As much as 90 percent of the Edwards Plateau savanna has been converted either to urban development or agricultural uses. King Ranch bluestem (Bothriochloa ischaemum var. songarica) has become a serious problem in the area as it outcompetes native grasses.

This rapid and pervasive growth has led to a loss of natural ecological processes, such as fire, and has led to changes in vegetation. Changes in water use and runoff from development have altered the dynamics of riparian ecosystems. Overgrazing of domestic livestock and high white-tailed deer populations have led to an increase in shrub and tree species at the cost of losses in herbaceous vegetation throughout many areas. Invasive exotic species introduced for agricultural and domestic purposes affect native plants and animals.

All of these factors, coupled with warming temperatures, are projected to alter plant growth cycles, as well as increase and expand the naturally occurring fire regime, and change the landscape over time. The opposite is true, too, as the absence of natural fires, compounded by overgrazing, oil and gas development, and tillage has already spread many species of woody plants across most of the Edwards Plateau. How this newly altered and still-reshaping ecosystem responds to long-term lower precipitation and higher temperatures remains to be seen—but the dynamics of habitat impact on the native flora and fauna here seem to rest upon the changing balance between surface conditions and the underground aquifer and cave infrastructure that caused many of theses species to evolve.

Bibliography

Chapman, Brian. R. The Natural History of the Edwards Plateau: The Texas Hill Country. Texas A&M University Press, 2020.

Fowler, Norma. “An Introduction to the Vegetation and Ecology of the Eastern Edwards Plateau (Hill Country) of Texas.” University of Texas, 2005, www.yumpu.com/en/document/view/26038465/an-introduction-to-the-vegetation-and-ecology-of-the-eastern. Accessed 13 Nov. 2024.

Johnson, E. H. “Handbook of Texas Online—Edwards Plateau.” Texas State Historical Association, 2012, . Accessed 13 Nov. 2024.

Hiers, Quinn A. et.al. "Grass Bud Responses to Fire in a Semiarid Savanna System." Ecology & Evolution, 2021, vol. 11, no, 11, pp. 6620-33, doi.org/10.1002/ece3.7516. Accessed 13 Nov. 2024.

Lockwood, Mark. Birds of the Texas Hill Country. Austin: University of Texas Press, 2001.