Alamo Chapter Master Naturalist Gary Poole is the author of this article. All photos are by him except where noted.
WHAT ARE ECOREGIONS?
Ecoregions are geographic areas within which environmental resources are similar in type, quantity and quality. These resources include both living (biotic) and non-living (abiotic) factors, such as climate, hydrology, vegetation, wildlife, soils, geology, landforms and land use.
WHY IS AN UNDERSTANDING OF ECOREGIONS IMPORTANT?
This knowledge is critical for evaluating the current condition and prognosis of ecosystem resources and for implementing ecosystem management and restoration programs. As Master Naturalists an understanding of ecoregions also gives a context for our expectations of ecological relationships in an area. For example, knowing that the basement rock is limestone we can predict that the soils will be alkaline or if sandstone and shale then we expect soils to be acidic. And just as plants follow soil type, so certain fauna follow plants. Similar, predictable relationships are caused by rainfall, hydrology, and topography.
WHAT ECOREGIONS ARE FOUND IN SOUTH-CENTRAL TEXAS?
There are four ecoregions that occupy parts of Bexar County or the surrounding counties: Edwards Plateau, Blackland Prairie, Post Oak Savannah, and South Texas Plains. Figure 1 illustrates the distribution of ecoregions across the state of Texas while Figure 2 shows a closeup of the ecoregions in our part of Texas. One can see that the Blackland Prairie takes up the middle half of Bexar County, the Edwards Plateau the northern fourth and the Post Oak Savannah and South Texas Plains occupy the southern fourth and a little in the west.
For a larger version of the map in Figure 1 follow this link.
WHAT ARE THE LIMITATIONS ON ECOREGIONS?
There are several caveats when talking about ecoregions. First, not all ecoregion maps of Texas are identical though all are very similar. Some maps identify 10 ecoregions and some have more. This ecoregion map is used for two reasons. First, it is based on Gould et al (1960) and is the most commonly used map. Second, it is the map primarily used by Texas Parks and Wildlife Department, a sponsor of the Texas Master Naturalists.
Second, the borders between ecoregions are not hard and fast. Rather, these contacts demonstrate gradations where environmental resources from both juxtaposed regions mix and transition.
Third, within a particular ecoregion, there often exist smaller areas with significant variations from the overarching environmental description of that ecoregion.
Finally, ecoregions are not static. They can and do change and some of these changes are essentially irreversible. Ecologists often refer to the condition of the ecoregion when settlers first arrived. And while that is useful information, it is of limited utility when attempting to manage and assess the environmental resources as they exist in the present day. The primary source of change is humanity with its land use practices and introduced species.
OUR NEIGHBORHOOD ECOREGIONS
The Blackland Prairie (Photo 1) is a long, narrow wedge extending from Bexar County almost to the Red River. The underlying geology of soft limestones, shales and sandstone have produced soils which are dark, alkaline, clay-rich and fertile with patches of acidic, sandy loam.
The original vegetation consisted of the four tall grass prairie species: Little Bluestem (Schizachyrium scoparium), Big Bluestem (Andropogon gerardii), Yellow Indiangrass (Sorghastrum nutans), and Switchgrass (Panicum virgatum). River and stream drainages contained woodlands: Cedar Elm (Ulmus crassifolia), Pecan (Carya illinoinensis), Bur Oak (Quercus macrocarpa), Blackjack Oak (Quercus marilandica), and Cottonwood (Populus deltoides).
Original fauna which is no longer present included American Bison and Pronghorn Antelope. Over 320 species of birds are found in the Blackland Prairie though 15 of these are considered at risk. The predominant and most impactful mammal currently found is cattle. There is a limited population of White-tailed Deer and the usual small mammals: skunk, possum, coyote, and rabbit.
The Blackland Prairie ecosystem was an example of a disturbance-maintained system where disturbance was provided by the intense grazing of, primarily, American Bison and episodic, extensive fire. These two factors, along with climate, maintained the savannah and inhibited the expansion of wooded areas. Only about 1 per cent of the original cover remains, a consequence of agriculture, highways and urban/suburban construction. This original prairie is extremely fragmented.
The topography is gently rolling hills to mostly flat terrain. Rainfall varies by latitude with the northern portion receiving 40 or more inches annually and the southern tip in Bexar County receiving only around 31 inches.
The Edwards Plateau (Photo 2) is a highland area in central Texas consisting of low rolling to high hills, deep rocky canyons and numerous springs. Rainfall varies from east to west with the portion of the Edwards Plateau in Bexar County receiving about 32 inches annually but the western margins receiving only 15 inches or so. A thin, alkaline soil covers the hills and slopes though deeper soils can be found in the drainages of rivers and creeks.
The Edwards Plateau is an example of a karst landscape and contains caves, sinkholes, swallets, and other solution features. The shallow basement rock for almost the entire area is limestone. There are relatively few perennial streams on the surface as most of the hydrological drainage occurs underground through these cavernous limestones.
The extreme conditions and isolation of these caves and solution features give rise to fauna which is unique in the world and can only live in caves (troglobites). Most of these animals are invertebrates which over millennia have adapted to the absolute darkness and extreme ecosystems of caves. Nine troglobitic beetles, spiders, and harvestman found in Bexar County have been listed as endangered by the Fish and Wildlife Service. (More information available here.) There are a few troglobitic vertebrates found in the Edwards Plateau: salamanders (none in Bexar County) and two species of catfish which are found in the deep artesian section of the aquifer under Bexar County. (More info here.)
Another distinctive faunal feature of Edwards Plateau karst is the presence of some of the largest maternal bat populations in the world. These are mostly Mexican Free-tailed Bat (Tadarida brasiliensis) colonies where females bear and raise their young in some of the regions many limestone caves.
Other significant fauna in this ecoregion includes the Golden-cheeked Warbler (Dendroica chrysoparia) and White-tailed Deer (Odocoileus virginianus). The warbler is a migratory song bird which nests almost exclusively in mature Ashe Juniper brakes on the Edwards Plateau. It is threatened by habitat loss and Cowbird (Molothrus ater) nest parasitism. White-tailed Deer populations have surged over the past century as predators have been reduced or eliminated and hunting has been regulated. The result has been pressure on certain species of native plants as they have been preferentially browsed by deer and the proliferation of Ashe Juniper which is not palatable to the deer.
Vegetation regimes are very complex in this ecoregion. In the upland areas and on slopes with thin, rocky soils one finds Plateau Live Oak (Quercus fusiformis), Shin Oak (Quercus vaseyana), Ashe Juniper (Juniperus ashei), Texas Mountain Laurel (Sophora secundiflora), Cenizo (Leucophyllum frutescens) Texas Persimmon (Diospyros texana), Texas Sotol (Dasylirion texanum) and Agarita (Mahonia trifoliata) as well as short-grass prairie representatives, primarily Sideoats Grama (Bouteloua curtipendula), Hairy Grama (Bouteloua hirsuta), Texas Cupgrass (Eriochloa sericea) and Texas Wintergrass (Nassella leucotricha).
A very different flora is found in the canyonlands with deeper soils and more water: Lacey Oaks (Quercus laceyi), Plateau Live Oaks (Quercus fusiformis), Cedar Elms (Ulmus crassifolia ), walnuts, hackberry, Bigtooth Maple (Acer grandidetatum) , Chinkapin Oak (Quercus muehlenbergii), Pecan (Carya illinoinensis), and Bald Cypress (Taxodium distichum). Grasses found here are the four tall grass prairie species mentioned above.
The original landscape was mostly grasslands with dispersed Live Oak (Quercus virginiana) and Shin Oak (Quercus vaseyana) dominated mottes in the flat or rolling terrain and Ashe Juniper (Juniperus ashei) found on thin-soiled slopes. Only about two percent of this original habitat is intact. This is a result of overgrazing, the introduction of non-native grasses, the suppression of fire, soil loss, urbanization, and fragmentation of due to land use policies. These cultural practices have spurred the expansion of juniper/oak woodlands, woody thickets and Mesquite (Prosopis glandulosa) savannahs throughout the Edwards Plateau.
Post Oak Savannah:
The Post Oak Savannah (Photo 3) roughly parallels and is east of the Blackland Prairie but in contrast contains deep, mostly acidic, sandy loams or sands. The first Europeans found the four mentioned species of the tall grass prairie. This prairie was cut by creek and river drainages with woodlands containing Post Oak (Quercus stellata), Blackjack Oak (Quercus marilandica), Pecan (Carya illinoinensis) and hackberry.
Like the Blackland Prairie, the Post Oak Savannah is an example of a disturbance-maintained system where grazing animals and episodic fire maintained long-term ecosystem stability. The suppression of fire and the elimination of the migratory American Bison and their replacement with the intensive grazing of cattle resulted in the decline of native grasses and other native prairie plants.
Fauna found in the Post Oak Savannah is largely the same as that found in the Blackland Prairie.
Agriculture, mostly ranching, has greatly modified the ecoregion. Now much of the land is covered by introduced grasses and oak, hackberry and elm thickets. The understory of these woodlands contains Yaupon Holly (Ilex vomitoria), American Beautyberry (Callicarpa americana), and Greenbrier (Smilax bona-nox). Most of Post Oak Savannah consists of gently rolling to hilly terrains receiving between 28 and 40 inches of rain annually.
South Texas Plains
Originally the South Texas Plains (Photo 4) were covered by grasslands with extensive woodlands in the river valleys. These grasses were mostly Little Bluestem (Schizachyrium scoparium), Curley Mesquite (Hilaria belangeri), Buffalograss (Bouteloua dactyloides) and bristlegrasses. Pecan (Carya illinoinensis), Cottonwood (Populus deltoides) and Mexican Palm (Sabal mexicana) were found in the river and stream drainages, especially the Rio Grande River. The palm was found primarily in the southern reach of the Rio Grande.
Today the South Texas Plains consists of thorny brush, prickly pear cactus and trees as a consequence of overgrazing and fire suppression. The palm and hardwood forests of the river valleys have been mostly displaced by agriculture and reduced to small, isolated populations. Primary examples of current vegetation are Mesquite (Prosopis glandulosa), Blackbrush Acacia (Vachellia rigidula), Spiny Hackberry (Celtis ehrenbergiana), Huisache (Vachellia farnesiana), Whitebrush (Aloysia gratissima) and prickly pear. There are also extensive areas of introduced grasses such as Kleingrass (Panicum coloratum), Buffelgrass (Pennisetum ciliare), Bermudagrass (Cynodon dactylon) that have significantly displaced native grasses.
Due to its proximity to Mexico and Central America, the South Texas Plains contains distinctive fauna found nowhere else in the United States. These species include numerous birds as well as several reptiles and mammals. Notable among the birds is the Green Jay (Cyanocorax yncas) which is only found in South Texas to South America. Similarly, the Texas Tortoise (Gopherus berlandieri) ranges from South Texas to Northern Mexico. Finally, the Ocelot (Leopardus pardalis) is found only in the Rio Grande Valley in the United States and requires a habitat of thick brush cover. With the clearing of land for urban and suburban development and agriculture, the population of this wild cat has been reduced to an estimated 80 individuals in the US.
Soils are alkaline to slightly acidic and the high temperatures and rates of evaporation have produced extensive areas of caliche and hardpan dominated by short, dense brush. Rainfall ranges from 20 inches annually in the west to 32 inches in the east and south. Despite the impact of agriculture and urbanization on the ecosystem, the South Texas Plains remains an important region where plant and animal species from northern Mexico, the Gulf Coast, the Hill Country, and the Chihuahuan Desert mix.
Follow this link for an eight-minute video on the South Texas Plains.
The four ecoregions that converge in South Central Texas contain widely variable floral and faunal regimes, possess very different conditions, and demonstrate distinct ecological relationships. This article has given a brief overview of the conditions and relationships that exist throughout these large ecoregions. More specific information on the natural history of Bexar County can be found in the Talking Points articles that follow. Additional information can be found in the book, Texas Master Naturalist Statewide Curriculum and at the Texas Parks and Wildlife Department website which can be viewed here.
Gould, F. W., G. O. Hoffman, and C. A. Rechenthin, 1960. Vegetational areas of Texas. Leaflet 492. College Station: Texas Agricultural Experiment Station.
Haggerty, Michelle M. and Mary Pearl Meuth, eds. Texas Master Naturalist Statewide Curriculum. Texas A&M University Press, April 2019.