Chapter 37
Ecological Subregions of the United States
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Southwest Plateau and Plains Dry Steppe and Shrub

Six Sections have been delineated in this Province:

These Sections are located in the New Mexico and Texas. The area of these Sections is about 160,900 mi2 (416,700 km2).

Section 315A--Pecos Valley

Geomorphology. This section is in the Great Plains physiographic province. It is located in west-central New Mexico. Major landforms are plains, hills, basins, and fans. Major landform features include the Pecos Plains and the Canadian Valley. Elevation range from 4,000 to 6,900 ft (1,200 to 2,100 m)

Lithology and Stratigraphy. There are Paleozoic, Mesozoic, and Cenozoic aged sedimentary and volcanic rocks and alluvial deposits.

Soil Taxa. Soils include Haplustalfs, Ustochrepts, Argiustolls, Haplustolls, and a few Haplargids, Calciothids, and Ustorthents.

Potential Natural Vegetation. Vegetation consists of grama and galleta grass, pinyon-juniper in mesic soil temperature regimes, and ustic soil moisture regimes, and mesquite bush in aridic soil moisture regimes.

Fauna. Reserved.

Climate. Precipitation ranges from 8 to 16 in (200 to 400 mm) annually, with less than half of the precipitation falling in the winter. Temperature ranges from 45 to 70 oF (7 to 21 oC) and winters are cold. The growing season last 160 to 200 days.

Surface Water Characteristics. Water is scarce throughout this Section because of low annual accumulation and erratic precipitation. Only a few perennial streams exist. Ground water in deep sand and gravel deposits or where limestone residuum exists provides for domestic and livestock use.

Disturbance Regimes. Fires vary in frequency and intensity, depending on fuel load and moisture.

Land Use. The land in most of this Section is in farms, ranches, or private holdings. More than 75 percent of the area is managed as rangeland with cattle and sheep grazing.

Cultural Ecology. Although there are periods of population expansion and periods of decline, sometimes approaching near abandonment, humans have occupied the Great Plains physiographic province, which partially includes the Pecos Valley Section, for at least 12,000 years. There is inconclusive evidence of substantial earlier occupation. A dependency on hunting of large herbivores which, in turn, were dependent on the vast grasslands of this Province, characterized man-ecosystem relationships for the first 11,000 years of this 12,000 year period. The earliest well-dated sites are those of mobile peoples who, early in this period, hunted mammoth but later hunted a now extinct species of bison. In time the big game hunters were followed by peoples with a more diversified subsistence based on hunting a variety of animals, but, who, no doubt, were heavily dependent on hunting modern bison and gathering a variety of plant resources. The adoption of farming about 1,000 years ago, in combination with hunting and wild plant gathering, eventually led to the appearance of settled villages located sporadically along perennial water courses.
About the beginning of the historic period, the settled village life of plains agriculturalists was supplanted by a re-appearance of groups more heavily dependent on hunting, supplemented with some farming. These relatively mobile groups included Apache and Comanche, with a later appearance of Kiowa. One practice which allowed mobile, hunting plains peoples to exist with diminished emphasis on farming was considerable trade. In the late 1700's and early 1800's, trade begin with Puebloan peoples to the west in the foothills of the Sangre de Cristo Mountains. Historic trails, including the Santa Fe Trail, passed through the Pecos Valley Section. These trails were important in the movement west of Anglo settlers and in the movement of cattle and other commodities. The advent of transcontinental railroads in the late 1800's contributed greatly to slaughter of the buffalo, after which time hunting was never again an important subsistence pursuit in the area. Military subjugation of Native American peoples, in combination with the railroads, led to relatively rapid settlement of the western plains by Anglos, who established a rural lifestyle based largely on farming and ranching, which persists to the present.

Compiled by Southwestern Region.

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Section 315B--Texas High Plains

Geomorphology. This Section is in the Great Plains geomorphic province. The predominant landform consists of a broad, extensive flat plain formed by fluvial sedimentation of continental erosional products from adjacent mountain ranges, followed by sheet erosion and transport. These processes resulted in a region of moderate dissection. Elevation ranges from 2,600 to 6,500 ft (800 to 2,000 m). Local relief in most of the Section ranges from 100 to 300 ft, however, relief in the tablelands ranges from 300 to 500 ft.

Lithology and Stratigraphy. Rocks were formed during the Paleozoic (10 percent), Mesozoic (10 percent), and Cenozoic (80 percent) Eras. Paleozoic strata consist of Permian marine deposits (sandstone, shale, and limestone). Mesozoic strata consist of Triassic continental deposits (sandstone). Cenozoic strata consist of Tertiary Period deposits (poorly consolidated silt, sand, and gravel in varying proportions).

Soil Taxa. Soils are Ustolls and Ustalfs. Paleustolls, Argiustolls, Paleustalfs, and Haplustalfs are on uplands. Calciustolls, Haplustolls, and Paleustolls are on ridges and steeper slopes. Haplustolls occur on young valley floors. Pellusterts are in clayey playa-lake basins. Calciorthids, Paleorthids, and Torriorthents are on steep slopes in breaks. These soils have a mesic or thermic temperature regime, a ustic moisture regime, and mixed or carbonatic mineralogy. Soils are deep, fine to coarse textured, well drained, and have limited soil moisture for use by vegetation during parts of the growing season.

Potential Natural Vegetation. K\"uchler classified vegetation as grama-buffalo grass and shinnery ({\it Quercus-Andropogon}). The predominant vegetation form is short grass communities composed of bunch grasses with a sparse shrub layer. Species include short grasses (blue gramma, and buffalograss), sagebrush, mesquite, and yucca.

Fauna. Typical large to medium size herbivores and carnivores include pronghorn, coyote, swift fox, ringtail, and ocelot. Typical smaller herbivores include desert shrew, desert cottontail, black-tailed prairie dog, yellow-faced pocket gopher, plains pocket mouse, silky pocket mouse, hispid pocket mouse, and white-throated woodrat. Bison are historically associated with this Section. Birds of grasslands include many species that typically occur over a wide area, such as roadrunner, house finch, yellow warbler, willow flycatcher, cedar waxwing, western kingbird, and golden eagle. The lesser prairie chicken, found here, is restricted to the more arid grasslands. Amphibians found in this Section include plains spadefoot toad, Couche's spadefoot toad, western spadefoot toad, plains leopard frog, Great Plains toad, green toad, red spotted toad, spotted chorus frog, and yellow-mud turtle. Reptiles include species such as Texas horned lizard, round-tailed horned lizard, Great Plains skink, Texas blind snake, and plains black-headed snake.

Climate. Precipitation averages 14 to 18 in (350 to 450 mm), occurring mainly in the spring and fall. Temperature averages 55 to 63 oF (13 to 17 oC). The growing season lasts 130 to 220 days.

Surface Water Characteristics. There is a low density of small intermittent streams and few associated rivers, all with low volume of water flowing at low velocity. A shallow dendritic drainage pattern has developed. Major rivers include the Canadian and Red. The Canadian River, in north Texas, is deeply incised into the Great Plains plateau and has developed a broad area (up to 50 mi wide) of complex topography locally known as "The Breaks." Playa lakes are common in the western part of this Section.

Disturbance Regimes. Fire and drought have probably been the principal historical disturbances.

Land Use. Natural vegetation has been converted to agricultural crops or pasture on about 90 percent of the area.

Cultural Ecology. Reserved.

Compiled by Southern Region.

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Section 315C--Rolling Plains

Geomorphology. This Section is in the Central Lowlands geomorphic province. Landforms originated from platform uplift of continental sediments deposited previously into a shallow inland sea, followed by a long period of erosion. These processes resulted in a moderately dissected landscape. About 80 percent of this Section is equally divided between irregular plains and tablelands. Smaller areas of smooth plains and plains with hills are also present. Elevation ranges from 1,640 to 2,950 ft (500 to 900 m). Local relief in most of the Section ranges from 100 to 300 ft. Smaller areas are present where local relief ranges from 300 to 500 ft.

Lithology and Stratigraphy. Rocks were formed during the Paleozoic and Mesozoic Eras. Geologic strata consist of about equal amounts of Permian marine deposits and Triassic continental deposits (sandstone). A small area of Permian continental deposits (sandstone, shale, and limestone) is also present.

Soil Taxa. Soils are Ustolls, Ustalfs, and Ochrepts. Most soils are on uplands and include Argiustolls, Paleustolls, and Natrustolls, Haplustalfs, Paleustalfs, and Ustochrepts. Localized areas of Ustifluvents are on flood plains. These soils have a thermic temperature regime, a ustic moisture regime, and mixed mineralogy. Most soils are deep, well drained, variable in texture, and have limited moisture supplies for use by vegetation during part of the growing season.

Potential Natural Vegetation. K\"uchler classified vegeataion as mesquite-buffalo grass. The predominant vegetation form is medium-tall grassland with a sparse shrub cover. The vegetative community consists of sand and little bluestems and sagebrush.

Fauna. The faunal community consists of species suited to a semi-arid environment. Large to medium-size mammals include coyote, ringtail, ocelot, and collared peccary. Typical smaller herbivores include desert cottontail, hispid pocket mouse, Texas kangaroo rat, Texas mouse, desert shrew, and rock squirrel. Bison and black-footed ferret are historically associated with this Section. Domesticated cattle are the most common large herbivore. Birds of thickets and grasslands include black-capped vireo, Harris' sparrow, scaled quail, golden-fronted woopecker, and pyrrhuloxia. Amphibians include Couche's spadefoot toad, Great Plains narrow-mouthed frog, green toad, red-spotted toad, and Texas toad. The spotted chorus frog, yellow-mud turtle, and Texas map turtle are in wetter areas. Common reptiles include lesser earless lizard, crevice spiny lizard, Texas spotted whiptail, Great Plains skink, prairie skink, four-lined skink, western hook-nosed snake, Harter's water snake, and plains black-headed snake.

Climate. Precipitation averages 18 to 24 in (450 to 600 mm). Temperature averages 57 to 64 oF (14 to 18 oC). The growing season lasts 185 to 230 days.

Surface Water Characteristics. There is a low density of small intermittent streams and few associated rivers, all with low volume of water flowing at low velocity. A dendritic drainage pattern has developed. Major rivers include the Colorado and Brazos.

Disturbance Regimes. Fire and drought have probably been the principal historical disturbances.

Land Use. Natural vegetation has been converted to agricultural crops or pasture on about 90 percent of the area.

Cultural Ecology. Reserved.

Compiled by Southern Region.

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Section 315D--Edwards Plateau

Geomorphology. This Section is in the Great Plains geomorphic province. The predominant landform consists of a broad, extensive flat plain formed by fluvial sedimentation of continental erosional products from adjacent mountain ranges, followed by sheet erosion and transport; these processes resulted in a region of moderate dissection. About 90 percent of this Section consists of landforms equally divided between smooth plains and tablelands having moderate relief. Also included are smaller areas of open high hills, high hills, and plains with hills. Elevation ranges from 650 to 4,000 ft (200 to 1,200 m). Local relief in most of the Section ranges from 100 to 300 ft (30 to 90 m). In a small area of hills, relief ranges from 300 to 500 ft (90 to 150 m).

Lithology and Stratigraphy. Rock units in this Section were formed during the Precambrian (10 percent), Paleozoic (30 percent), and Mesozoic (60 percent) Eras. Precambrian strata consist of metamorphic rocks of paragneiss and schist structures and plutonic and intrusive rocks of granitic composition. Paleozoic strata consist of a mixture of Cambrian (carbonates) and lower Ordovician marine deposits (carbonates). Mesozoic strata consist of Cretaceous marine deposits (limestone and sandstone).

Soil Taxa. Soils are mostly Ustolls. Calciustolls are on limestone hills and plateaus. Chromusterts are on outwash plains and broad plateaus. Ustochrepts are on marl and chalk hills. Haplustolls are on stream deposits of valley floors. These soils have a thermic temperature regime, a ustic moisture regime, and carbonatic or montmorillonitic mineralogy. Soils are generally shallow, fine textured, and have limited soil moisture for use by vegetation during parts of the growing season.

Potential Natural Vegetation. K\"uchler classified vegetation as juniper-oak savanna and mesquite-acacia-savanna. The predominant vegetation form is mid to short grasslands and evergreen scale-leaved woodlands with a sparse cover of drought-deciduous shrubs. A mixture of species may occur, including blackjack oak, red cedar, mesquite, live oak, and species of mid and short grass grasslands.

Fauna. Common large to medium size herbivores and carnivores include coyote, ringtail, coati, hog-nosed skunk, ocelot, and collared peccary. Smaller herbivores include Mexican ground squirrel, white-ankled mouse, and prairie vole. Bison are historically associated with this Section. Domesticated cattle are the most common large herbivores. Birds of thickets typically found here include scaled quail, golden-fronted woodpecker, golden-cheeked warbler, pyrrhuloxia, and long-billed thrasher. Amphibians include Couche's spadefoot toad, Rio Grande leopard frog, Great Plains narrow-mouthed frog, green toad, Texas toad, spotted chorus frog, barking frog, cliff chirping frog, and Texas map turtle. A number of salamanders in this Section have a very restricted range: San Marcas, Texas, Cormal blind, Valdina Farms, and Texas blind. Typical reptiles include Mediterranean gecko, spot-tailed earless lizard, keeled earless lizard, Texas spiny lizard, Great Plains skink, and four-lined skink.

Climate. Annual precipitation ranges from 15 to 30 in (375 to 750 mm). Average temperature is 64 to 68 oF (18 to 20 oC). The growing season lasts 230 to 270 days.

Surface Water Characteristics. A low density of small intermittent and occasional perennial streams occurs here. All generally have a low volume of water flowing at low velocity, except along the plateau escarpment, where flow rates can be high. A dendritic drainage pattern has developed. Major rivers include the Brazos and Colorado.

Disturbance Regimes. Fire and drought have probably been the principal historical disturbances.

Land Use. Natural vegetation has been changed to agricultural crops or pasture on about 90 percent of the area.

Cultural Ecology. Reserved.

Compiled by Southern Region.

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Section 315E--Rio Grande Plain

Geomorphology. This Section is in the Coastal Plains geomorphic province. The predominant landform in this Section is a flat, weakly dissected alluvial plain formed by deposition of continental sediments onto submerged, shallow continental shelf, which was later exposed by sea level subsidence. Elevation ranges from 80 to 1,000 ft (25 to 300 m). Local relief in most of the Section ranges from 100 to 300 ft (30 to 90 m).

Lithology and Stratigraphy. Rocks formed during the Cenozoic Era. These strata consist of Tertiary marine deposits (glauconitic, calcareous, fossiliferous layers with lignitic sandy and argillaceous deposits).

Soil Taxa. Soils are Usterts, Torrerts, and Ustalfs. Pellusterts are on plains over clayey marine sediments. Paleustalfs are on eolian plains. Torrerts, Haplustolls, Calciustolls, Paleustalfs, and Haplustalfs are on plains. Calciustolls and Calciorthids are on plains over marine sediments. Soils have a hyperthermic temperature regime, a ustic or aridic moisture regime, and mixed mineralogy. Soils are mostly deep, fine to coarse textured, well drained, and have limited soil moisture for use by vegetation during the growing season.

Potential Natural Vegetation. K\"uchler classified vegetation as mesquite-acacia-savanna and ceniza shrub. The predominant vegetation form is short grassland with a sparse cover of drought deciduous shrubs. Species include mesquite, cactus, and tall and mid grasses. Live oaks and cottonwoods may be present along stream banks.
Fauna. Typical large to medium size herbivores and carnivores include coyote, ringtail, hog-nosed skunk, and ocelot. Smaller herbivores include Mexican ground squirrel, Texas pocket gopher, and southern plains woodrat. Bats typical of this Section include the ghost-faced and Sanborn's long-nosed. Bison, jaguar, and jaguarundi are historically associated with this Section. This Section and adjacent 315E form the northern range of a number of birds common to Mexico and South America. Typical birds include chachalaca, green kingfisher, pauraque, elf owl, white-winged dove, red-billed pigeon, black-headed oriole, kiskadee flycatcher, yellow-green vireo, Lichtenstein's oriole, tropical kingbird, beardless flycatcher, buff-bellied hummingbird, green jay, long-billed thrasher, and white-collared seedeater. Amphibians include Mexican burrowing toad, Rio Grande leopard frog, sheep frog, giant toad, spotted chorus frog, Mexican tree frog, Rio Grande chirping frog, and Berlandier's tortoise. Reptiles include Texas banded gecko, reticulate collared lizard, spot-tailed earless lizard, keeled earless lizard, blue spring lizard, mesquite lizard, rose-bellied lizard, Laredo striped whiptail, black-striped snake, indigo snake, speckled racer, and cat-eyed snake.

Climate. Precipitation ranges from 17 to 30 in (420 to 750 mm), decreasing from east to west and occurring mostly during May and June. Temperature averages 70 to 72 oF (21 to 22 oC). The growing season lasts 260 to 310 days.

Surface Water Characteristics. A sparse density of small to medium intermittent streams is present in a dendritic drainage pattern. Major rivers include the Rio Grande and Nueces.

Disturbance Regimes. Drought has probably been the principal historical disturbance.

Land Use. Natural vegetation has been converted to dry-land pasture for cattle grazing on about 90 percent of the area.

Cultural Ecology. Reserved.

Compiled by Southern Region.

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Section 315F--Southern Gulf Prairies and Marshes

Geomorphology. This Section is in the Coastal Plains geomorphic province. The predominant landform consists of a flat, weakly dissected alluvial plain formed by deposition of continental sediments onto a submerged, shallow continental shelf, which was later exposed by sea level subsidence. Along the coast, fluvial deposition and shore-zone processes are active in developing and maintaining beaches, swamps, and mud flats. Elevation ranges from sea level to 160 ft (0 to 50 m). Local relief ranges from 0 to 50 ft (0 to 18 m).

Lithology and Stratigraphy. Rock units formed during the Cenozoic Era. These strata consist of Quaternary marine deposits of non-glacial sand, silt, and clay.

Soil Taxa. Soils are Aquents, Aqualfs, Aquolls, and Aquepts. Psammaquents, Udipsamments, Fluvaquents, and Salorthids are on barrier islands and long bays. Haplaquolls, Natraqualfs, Pelluderts, and Pellusterts are on low coastal terraces. Ochraqualfs, Albaqualfs, and Paleudalfs are found on plains. Haplaquolls, Haplaquents, and Fluvaquents are on coastal flats and flood plains. These soils have a hyperthermic and thermic temperature regime, an aquic moisture regime, and montmorillonitic, mixed, or siliceous mineralogy. Soils are fine to coarse textured, saline, and mostly poorly drained with high water tables.

Potential Natural Vegetation. K\"uchler classified vegetation as bluestem-sacahuista prairie and southern cordgrass prairie. The predominant vegetation form is tall grassland with little tree cover. Grasslands dominate areas inland from the coast and consist of little bluestem, indiangrass, switchgrass, and big bluestem. Occasional areas of live oak are present. Poorly drained areas along the coast support freshwater and saltwater marsh vegetation of sedges, rushes, saltgrass, and cordgrass.

Fauna. The faunal communities typically include coyote, ringtail, hog-nosed skunk, ocelot, and collared peccary. Smaller mammals include Mexican ground squirrel, Texas pocket mouse, northern pygmy mouse, and southern Plains woodrat. Birds of freshwater marshes, lakes, ponds, and rivers include reddish egret, white-faced ibis, black-billed whistling duck, white-fronted goose, and olivaceous cormorant. Reptiles and amphibians include eastern spadefoot toad, Gulf coast toad, American alligator, diamondback terrapin, spiny-tailed iguana, Texas horned lizard, Texas spotted whiptail, and indigo snake.

Climate. Precipitation ranges from 25 to 55 in (620 to 1,400 mm). Temperature averages 68 to 70 oF (20 to 21 oC). The growing season lasts 280 to 320 days.

Surface Water Characteristics. A low density of small to medium perennial streams is present in this Section. The water table is high in many areas, resulting in poor natural drainage and abundance of wetlands. A poorly defined drainage pattern has developed on very young alluvial plains. There is an abundance of palustrine systems with seasonally high water levels. This Section adjoins the West Indian Marine and Estuarine Provinces.

Disturbance Regimes. Ocean tides and grazing have probably been the principal historical disturbance. Climatic influences include occasional hurricanes.

Land Use. Natural vegetation has been changed for agricultural crops on about 40 percent of the area.

Cultural Ecology. Reserved.

Compiled by Southern Region.

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