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The Balcones Escarpment :

the Balcones Fault Zone as a Major Zoogeographic Feature, p.35-40

by Raymond W. Neck

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The Balcones Fault Zone is a major influence on the geographic distribution of animals in central Texas. This influence is most evident in species which are directly influenced by edaphic factors or are closely associated with plants which are similarly influenced by edaphic factors.

Figure 1 : Biotic provinces of Texas according to W. Franik Blair (1950).

Black-capped Vireo

Golden-cheeked Warbler


In an attempt to understand the non-random distribution of animal species on the face of the earth, zoogeographers have long attempted to delineate discrete geographical areas that possess significant internal homogeneity of faunal assemblages, especially in relation to adjacent areas with a different set of assemblages. Lines are drawn between such discrete areas after analysis of geographical ranges of individual species. Exact delineation of the boundary between adjacent regions is usually not possible, because zoogeographers realize (if only on a subconscious level) that these lines are an invention of the human mind. The resulting set of biotic regions is simply a model of the real biological world--not an exact picture of that world.

Meaningful analysis of the North American fauna from a zoogeographical viewpoint dates to the efforts of Dice (1943). He published a major contribution with cartographic delineations and verbal descriptions of many biotic provinces in North America from the Arctic frontiers to the Tropics of Panama. Dice placed portions of biotic provinces within the boundaries of Texas.

The treatment by Dice of the biotic provinces of Texas was illuminating but fell short of being satisfactory. Such a lack of demonstration of faunal reality in the 1940's was not unexpected. Details of the distribution of animal species in Texas were unknown to most workers outside the boundaries of the state, as Texas was far removed from the intellectual centers of the time.

The worker of the early twentieth century with the best knowledge of distribution of animal species in Texas was self-trained and had died in early 1933. Yet most areas of southern and western Texas were unknown to John K. Strecker of the Baylor University Museum. Hence, the total picture of the Texas fauna was unavailable to most biological workers in Texas at that time.

However, studies by one of the new breed of field biologists of the mid-twentieth century were sufficient to allow a refinement of Dice's biotic provinces with the resultant creation of a new biotic province, one that was totally enclosed by the capricious geographical boundaries of Texas. W. Frank Blair came to The University of Texas in Austin in 1946. His early training was in mammals, but Blair later was to concentrate on amphibians and especially the true toads of the genus Bufo. Blair's analysis of the biotic provinces of Texas was published in 1950. This paper marks the beginning of the study of zoogeography in Texas. No significant alterations of Blair's biotic provinces have been forthcoming. The original analysis (Blair 1950) remains the most often cited reference in the zoogeographic literature of Texas. The creation of the Balconian Biotic Province by Blair (1950) was brought about by the realization that the Balcones Fault Zone was a major physical factor in the distribution of animals in central Texas.

Personal field studies and perusal of pertinent literature by this author for over a decade have demonstrated variation in the relative distinctiveness of adjacent biotic provinces. Of all the biotic provinces now accepted by zoogeographers to occur in Texas (except for the Guadalupe Mountains, which are included in the Navahonian Biotic Province), the Balconian Biotic Province is by far the most distinct. Clearly related to the surface exposure of an enormous (though highly faulted) block of Lower Cretaceous calcareous sedimentary rocks, the Balconian Province is most distinct along its eastern and southern edges. Boundary lines drawn on its western edge to delineate it from the Chihuahuan Biotic Province are related to increasing xericity. Similarly, the boundary along the northwestern and northern margins are rather vague but are related to changes in geological substrate.

The sharper delineation of the Balconian Biotic Province along its eastern (Texan Biotic Province) and southern (Tamaulipan Biotic Province) is a reflection of the sharper break in geological substrate. Biotic provinces are artifacts of human desires to order and classify the natural world; in reality, animals and their environments know no such tidy boundaries. This reality is the result of adjustments by animal species in response to variations in substrate, microclimatic, and vegetative environments along and to either side of the Balcones Escarpment.

The purpose of this paper is to analyze the distributions of animal species along the eastern edge of the Balconian Biotic Province. In this zone of complex geological changes, the environmental factors most important in controlling the distributions of animal species should be easily discernible. The zone in the area from Austin southwestward to New Braunfels and then west into the Texas Hill Country is the area discussed herein because of greater scientific knowledge in this area. Faunal range changeovers appear to be more dramatic in shorter distances here than elsewhere.

In recognition of his seminal work in the field of Texas zoogeography and his creation of the Balconian Biotic Province, I am pleased to dedicate this paper to the memory of W. Frank Blair (1912-1985). 1 can hope only that Frank would have enjoyed this analysis and, further, that this analysis may encourage future workers to devote additional intellectual investments to this area.


Lists of vertebrate species and selected invertebrate groups which occur in central Texas have been generated. Species (or subspecies) have been designated as limited (east or west) or nonlimited in reference to the effect of the Balcones Fault Zone on geographical range.

Analyses of the effect of the Balcones Escarpment upon species distributions for several faunal groups have been published (Buechner, 1946; Smith and Buechner, 1947). However, the narrative thrust of this paper is aimed more toward specific examples than a comprehensive numerical analysis of the faunal changes exhibited on either side of the Balcones Escarpment. A summary numerical analysis of the influence of the Balcones Escarpment is presented for various faunal groups, but more effort has been expended toward providing examples of paired species and subspecies. Detailed numerical analyses will be presented in a later paper.


Analysis of the herpetofauna of central Texas by Smith and Buechner (1947) revealed that the majority of species (77%) of reptiles and amphibians are limited (either eastward or westward) by the Balcones Escarpment. The percentage of species thus limited varied among the major orders of the herpetofauna from 100% (salamanders) to a mere 67% (chelonians) Other orders had intermediate percentages of limited species as follows: snakes, 70%; anurans (frogs and toads), 74%; and lizards, 95%.

The dynamic nature of the boundaries of the geographical ranges of birds complicates any zoogeographical analysis of the effect of the Balcones Escarpment upon specific birds. Buechner (1946) reported that geographical ranges of 56% of the bird species occurring in central Texas were limited by the Balcones Escarpment.

Of the 128 species of nonmarine mammals listed by Davis (1974) from Texas, a total of 65 species (50.8%) occur along the Balcones Escarpment. Of these 65 forms, 34 species (52.3%) are limited by the escarpment while 31 species (47.7%) have an overlapping distribution. Of the 34 limited species, 18 (52.9%) are found only west of the Balcones Escarpment while 12 (35.3%) are found only east of this line.


Parapatric Species

Taxa whose geographical ranges meet with no significant overlap have parapatric ranges. Recognition of such pairs is dependent upon the ability to pair taxa as ecological analogues and/or phylogenetic relatives.

Several pairs of birds can be placed in this category. The eastern tufted titmouse (Parus bicolor bicolor) and the black-crested titmouse Parus bicolor atricristatus) form an east-west pair of subspecies which have a very narrow zone of intergradation along the eastern margin of the Balcones Escarpment (Dixon 1978). Also forming an east-west pair are the red-bellied woodpecker (Melanerpes carolinus) and the golden-fronted woodpecker (Melanerpes aurifrons). The eastern taxa of these two pairs (and additional unlisted pairs) are typically found in broadleaf, deciduous woodlands, while the western taxa are more typical of the more open, xeric woodlands of the Texas Hill Country (both broadleaf and coniferous woodlands).

An exceptionally interesting species pair is given by two salamanders that are members of two different families. The wide-mouthed salamander (Ambystoma texanum) of the family Ambystomidae ranges to the east of the Balcones Escarpment. While individuals of this species spend dry periods under logs in protected areas, eggs are deposited in water where larval development occurs. The slimy salamander (Plethodon glutinosus) of the family Plethodontidae is found in mesic canyons, caves, and limestone slopes in the Texas Hill Country. Dry periods are spent deep in talus slopes, rocky terraces, or caves; larval development is terrestrial but must occur in very mesic microhabitats. These two species of salamanders with differing phylogenetic and ecological histories meet along the eastern face of the Balcones Escarpment. Overlap is very narrow and occurs as isolated populations in a series of mesic canyon environments in the Texas Hill Country.

Another notable parapatric species pair involves the eastern-ranging southern leopard frog (Rana sphenocephala) and the southern and western ranging Rio Grande leopard frog (Rana berlandieri). The boundary zone between these two forms is very narrow and is a definite example of true parapatry. However, this boundary zone is actually about 30 kilometers east of the Balcones Escarpment. The range boundary zone of these two species is apparently related to substrate and moisture relationships which are unrelated to the presence of a major physiographic break to the west.

Ground squirrels provide a mammalian example of a parapatric range pair. The Mexican ground squirrel (Spermophilus mexicanus) is found east of the escarpment in well-drained, generally non-rocky soils especially in open terrace habitats without significant wood vegetation. West of the escarpment, the rock squirrel (Spermophilus variegatus) is found in talus slopes, canyons, and rocky uplands.

Eastern Sympatry - Western Allopatry

Other pairs of taxa exhibit overlapping geographical ranges (sympatry) east of the Balcones Escarpment, but only one species occurs westward of the escarpment (allopatry). In other words, one taxa of each pair ranges on both sides of the escarpment while the other pair occurs only east of the Balcones Escarpment.

Several examples of this distribution pair-type are exhibited by reptilian species. The eastern box turtle (Terrapene carolina) ranges over most of the eastern United States; a western subspecies, the three-toed box turtle (Terrapene carlina triunguis), ranges westward to within 30 kilometers of the Balcones Escarpment. The three-toed box turtle is typically found in open woodlands dominated by hardwoods (post oak/black hickory). Occurring over much of eastern Texas westward through central Texas, the ornate box turtle (Terrapene ornata ornata) is found in prairies, savannahs, and open woodlands.

Another congeneric pair of species with a similar range is provided by the broad-banded copperhead (Agkistrodon contortrix laticinctus) and the western cottonmouth (Agkistrodon piscivorus leucostoma). The western cottonmouth is found in the southeastern United States as far west as the larger streams of the eastern Hill Country of Texas, although this snake is very rare in the Balcones Escarpment area. On the other hand, the broad banded copperhead ranges from eastern Texas westward through central Texas. In this western portion of the range, the broad-banded copperhead is found in riparian areas as it approaches the aquatic habitat of the cottonmouth.

A pair of congeneric aquatic species is provided by the yellow mud turtle (Kinosternon flavescens flavescens) and the Mississipp mud turtle (Kinosternon subrubrum hippocrepis). The yellow mud turtle ranges from southern Nebraska to northern Mexico; in central Texas, it exists both east and west of the Balcones Escarpment. The Mississippi mud turtle ranges from the bottomlands of the Mississippi River westward to the creeks with more dependable flow, such as those occurring along the Balcones Escarpment.

The large, vociferous jays provide another species-pair example. The blue jay (Cyanocitta cristata) occupies mesic, broad-leave woodlands from the Atlantic coast westward through central Texas to the well-watered canyons in the Texas Hill Country. The scrub jay (Aphelocoma coerulescens) occupies xeric, evergreen woodlands from the Pacific coast to the western margins of the Balcones Escarpment Zone.

The narrow-mouthed toads are small anurans that feed on ants and other small insects. The eastern narrow-mouthed toad (Gastrophryne carolinensis) ranges from the southeastern United States westward to about 30 kilometers east of the Balcones Escarpment. A related species, the Great Plains narrow-mouthed toad (Gastrophryne olivacea), ranges from western Mexico through central Texas almost to the Texas-Louisiana boundary. While both species are found in terrestrial microhabitats during the dry season, the eastern narrow-mouthed toad is found in areas of more constant water supply while the Great Plains narrow-mouthed toad is found in areas without persistent water supplies.

Western Sympatry - Eastern Allopatry

As an analogue to species discussed in the previous section, another type of species-range pairs includes species that are sympatric west of the Balcones Escarpment, with only one species of each pair exists east of the escarpment.

The phylogenetically varied group of mammals that are classified as "squirrels" provide a species pair of this type. The wide-ranging fox squirrel (Sciurus niger) is certainly most common east of the Balcones Escarpment, where it is found in bottomland and upland woods, i.e. woodlands consisting entirely or largely of broad-leaved trees. However, a significant portion of the natural range of this species exists in riparian and canyon woodlands of the Texas Hill Country. The pairing of the fox squirrel, a "true squirrel," with the previously mentioned rock squirrel, a "ground squirrel," may seem anomalous. However, these two species have substantial overlap in their diet, and, thus, form a pair of ecological analogues. Ecological interactions occur in ecotonal areas of mesic canyon systems in the Texas Hill Country where the fox squirrel and rock squirrel are found in mesic and xeric areas, respectively.

Two scorpions also occur sympatrically west of the Balcones Escarpment, whereas only one occurs east of that area. The striped scorpion, Centruoides vittatus, ranges over most of Texas where it is found in mesic and xeric woodlands, savannahs, and prairies. A rock scorpion, Vejovis reddelli, is found over much of the Texas Hill Country but is unknown east of the Balcones Escarpment.

Similarly, two congeneric woodland snails have partially sympatric ranges. Polygyra texasiana ranges from eastern Texas through central Texas to the western Hill Country, where it is found in woodlands, savannahs, and those prairies with sufficient downed wood to provide cover for reduction of water loss. The similar-appearing, but smaller Polygyra mooreana, is found in the Texas Hill Country, where it occurs in xeric broadleaf and coniferous woodlands.

Endemic Species

A number of aquatic species are endemic to thermally and physicochemically stable waters in spring-run streams of the Balcones Escarpment and the Texas Hill Country. Freshwater mussels (family Unionidae) endemic to the Hill Country include Quincuncina mitchelli, Lampsilis bracteata, Quadrula petrina, and Quadrula aurea. The freshwater gastropod, Elimia comalensis, is restricted to spring-run streams of the Balconian Biotic Province.

Two bird species are endemic to the central Texas area, at least as far as breeding range is concerned. Both the black-capped vireo (Vireo atricapilla) and golden-cheeked warbler (Dendroica chrysoparia) breed nowhere but in xeric woodlands of central Texas. Substantial portions of the breeding range occur north of the true Hill Country in the Lampasas Cut Plains. The Lampasas Cut Plains also are located west of the Balcones Escarpment, although the vertical displacement is not visible at the surface. Interestingly, these two endemic breeders are found in very distinct but often adjacent (even anastomosing) habitats. The black-capped vireo nests in thickets of scrub deciduous oaks, while the golden-cheeked warbler nests in woodlands of mature Ashe juniper trees with long bark strands.

Endemic amphibians include a variety of species found in rocky epigean habitats and hypogean (cave) habitats. Species include the cliff frog (Syrrophus marnocki), Texas blind salamander (Typhlomolqe rathbuni), San Marcos salamander (Eurycea nana), and several blind salamanders (Eurycea latitans, E. troglodytes, E. tridentifera).

A large number of species of aquatic snails are found in aquifer habitats of both the Edwards and Trinity-Edwards aquifers (Hershler, 1986). Other invertebrate and vertebrate species are also restricted to such subterranean habitats (Longley, 1981). At least one terrestrial gastropod is endemic to the Balcones Fault Zone; Mesodon leatherwoodi is restricted to very mesic box canyons.

Range-Limited Single Species

A number of species which limited geographically by the Balcones Fault Zone have no obvious ecological analogue on the opposite side of this zone. Eastern species in this classification include American alligator (Alligator mississippiensis), southern flying squirrel (Glaucomys volans), and red-headed woodpecker (Melanerpes erythrocephalus). Western species in this grouping include cliff frog, barking frog (Hylactophryne auqusti latrans), red-spotted toad (Bufo punctatus), green toad (Bofo debilis), and black-tailed rattlesnake (Crotalus molossus).

A small group of western species is known to have outlier populations east of the Balcones Escarpment. Both the Texas alligator lizard (Gerrhonotus liocephalus infernalis) and the woodland snail (Polygyra mooreana) are known to have populations in an area of calcareous sandstone down stream on the Colorado River (Oakville Cuesta at La Grange, Fayette Co.). Polygyra mooreana also lives on Goliad Sandstone at Goliad State Park, Goliad Co. (downstream on San Antonio River).


Prior to the initiation of major faulting activity in what is presently central Texas, no sharp environmental boundary is believed to have existed. Certainly, environmental conditions were quite different from modern conditions due to the closer proximity of the coast as well as differences in global climate and continental configuration. The historical origin of the Balconian Biotic Province occurred when the environment on either side of the newly created (and probably yet developing) escarpment became differentiated sufficiently to restrict animal species to only one side of the escarpment. This general statement merely states the obvious and demonstrates ignorance of the temporal genesis and development of the Balconian Biotic Province. While faulting began during the Miocene, the length of the activity period is unknown as is the degree and constancy of the rate of vertical displacement. Whether significant effects on geographical ranges of animal species commenced in Late Miocene, Pliocene, or Pleistocene times is unknown. Portions of the aquifer fauna may date to Late Cretaceous time when an extensive cave system developed in Lower Cretaceous sediments.

Fossil Quaternary faunas known from sediments in the Texas Hill Country and Edwards Plateau reveal that significant biological changes have occurred since the beginning of the Holocene. Unfortunately, correlative faunas from east of the Balcones Escarpment are not available (and may not exist). As recently as 8000 B.P., the eastern chipmunk (Tamias striatus) was living in the western Hill Country (Schultze Cave, Edwards County Dalquest et al. 1969). This species is not known to live anywhere in Texas today. Historical records of the pine vole (Pitymys pinetorum) are known from Kerr County which is separated from its main range by at least 450 kilometers. Persistence of this species to the present indicates that a portion of the mesic-adapted fauna survived in the Balconian Biotic Province to the present.


Alteration of the natural environments along the Balcones Escarpment by human activities in the last 150 years has also affected the distribution of numerous animals in this area. Eastern species, such as the bluejay, fox squirrel and the snail Polygyra texasiana, are more common now or are now present west of the Balcones Escarpment after major human utilization of central Texas. Alteration of intact natural communities in the Texas Hill Country has caused general xerification of the environment. This xerification has allowed certain western species, e.g., the house finch (Carpodacus mexicanus), to extend eastward. In this instance, the western-ranging house finch now exhibits a narrow sympatry with the eastern-ranging purple finch (Carpodacus purpureus).


A large percentage of the animal species occurring in central Texas are limited to either the eastern or western side of the Balcones Escarpment. Going from east to west, general environmental conditions change in several ways. Ambient conditions become less humid and somewhat cooler. Precipitation decreases in total amount but shortterm rainfall totals increase. Substrate changes abruptly at the Balcones Escarpment, although some substrate changes occur east of the escarpment. Species most likely to have geographical ranges limited by the escarpment are those whose life cycle is totally or partially associated directly with the substrate, e.g., soil burrows, or hydrologic effects of faulting (the spring fauna). Other species may be associated with plant communities whose distribution is limited directly by the escarpment.


Blair, W. F., 1950, The biotic provinces of Texas: Texas Journal of Science, v. 2, no. 1, p. 93-117.

Buechner, H. K., 1946, Birds of Kerr County, Texas: Transactions of the Kansas Academy of Science, v. 49, no. 3, p. 357-364.

Dalquest, W. W., Roth, E., and Judd, F., 1969, The mammal fauna of Schultze Cave, Edwards County, Texas: Bulletin of the Florida State Museum, Biological Sciences, v. 13, no. 4, p. 204-276.

Davis, W. B., 1974, The mammals of Texas: Texas Parks and Wildlife Department, Bulletin 41, 294 p.

Dice, L. R., 1943, The biotic provinces of North America: Ann Arbor, University of Michigan Press, 78 p.

Dixon, K. L., 1978, A distributional history of the black-crested titmouse: American Midland Naturalist, v. 100, no. 1, P. 29-42.

Hershler, R., and Longley, G, 1986, Phreatic hydrobiids (Gastropoda: Prosobranchia) from the Edwards (Balcones Fault Zone) Aquifer region, south-central Texas: Malacologia, v. 27, no. 1, p. 127-172.


Longley, G., 1981, The Edwards Aquifer: earth's most diverse groundwater ecosystem?: International Journal of Speleology, v. 11, no. 1, p. 123-128.

Smith, H. M., and Buechner, H. K., 1947, The influence of the Balcones Escarpment on the distribution of amphibians and reptiles in Texas: Bulletin of the Chicago Academy of Sciences, v. 8, no. 1, p. 1-16.

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 in Abbott, Patrick L. and Woodruff, C. M., Jr., eds., 1986,

The Balcones Escarpment, Central Texas: Geological

Society of America, p. 35-40