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About the Concho Valley

Definition of the Region

The Concho Valley, defined here as a combination of ecological and political boundaries, occupies about 11,000 square miles of the northwestern Edwards Plateau and the southern margin of the Rolling Plains. Counties included in the area are Coke, Concho, Irion, Menard, Reagan, Runnels, Schleicher, Sterling, Tom Green and portions of Crockett and Glasscock counties.

Present-Day Description

The Concho Valley is a rural, sparsely populated region of Texas. About 140,000 people live in the area, with 60 percent of the population living in the only large city, San Angelo. About 20 percent of the people live in 11 surrounding small towns.

Ranching, oil and gas mining and farming dominate the economy of the region. At least 80 percent of the region is rangeland. When traveling through most of the region, the wide-open spaces and scarcity of human habitations are apparent (Maxwell, 1995).

Soils and Climate

Permian sandstone in the east and Cretaceous limestone in the west and south dominate the Concho Valley surface geology. The San Angelo area, which is centrally located, has a mixture of the two surfaces.

Limestone is the parent material for most of the region’s moderately alkaline soils. Slope soils are shallow, cobbly clay loams, outwash plain soils and moderately deep clay loams. The deepest soils (often 20 feet) are alluvials in flood plains.

The climate is continental and sub-humid to the east to semiarid in the west, but both areas experience moisture deficiency year-round. Over the past 40 years, average annual precipitation has totaled 19-20 inches with frequent droughts. The average January minimum temperature is 14.2°F and the average August maximum temperature is 103.7°F.

The growing period is roughly 232 freeze-free days. Average annual wind speed is 11 mph, typically out of the southwest.

Biological Description

Two major biomes, as defined by Aldrich (1967), are present in the Concho Valley.
The Pinyon-Juniper Woodland (Pygmy Conifer Woodland of some authors) is found on the shallow, rocky soils overlying Edwards Plateau limestone. The most conspicuous plant species are junipers (Juniperus pinchotii and J. ashei) and live oak (Quercus virginiana) which are found in densest growth in the southern part of the region and along the eroded margins of the Edwards Plateau in the central and north central areas.

Mesquite (Prosopis glandulosa) and short grasses (i.e. Aristida purpurea, Bouteloua breviseta, B. curtipendula, Buchloe dactyloides, Hilaria mutica) dominate the Mesquite-Grassland and are found on the deeper soils of more level terrain. With disturbance, this vegetation type often develops into a dense brushland with mesquite, agarita (Mahoniatrifoliolata), lotebush (Ziziphusobtusifolia) and pricklypear cactus (Opuntia spp.).
Representatives of a third biome, the Desert Scrub, enter the region from the southwest. A few representative Desert Scrub plant species in the Concho Valley include catclaw (Acacia greggii), creosote (Larrea tridentata), white brush (Aloysia gratissima) and guajillo (Acacia berlandieri).

Biodiversity – a narrow band of forest along the rivers and streams – is of major significance. This riparian forest is dominated by pecan (Carya illinoinensis) and bur oak (Quercus macrocarpa) along permanent waterways and the more aridity-tolerant hackberry (Celtis spp.) and river walnut (Juglans microcarpa) along more temporary watercourses.

Biological History

Prior to the establishment of Angelo State University in 1965, the remoteness of the region isolated it from intensive biological surveys or investigations. Incidental accounts of some terrestrial species are mentioned as early as the 17th century (Mendoza expedition, translated and reported by Bolton, 1908).

Early settlers, military men, explorers and travelers (1849-1900) frequently wrote of their observations of conspicuous mammals (prairie dogs, bison) and birds (prairie chickens, quail, turkey, ducks), but took less interest in writing about other types of organisms. The earliest scientific investigations in Concho Valley counties were those of J. Reverchon and W. Lloyd. Some of their collections are preserved in museums today.

Between 1892 and 1939, field biologists employed by the U.S. Bureau of Biological Survey (now the Fish and Wildlife Service) visited the area, often briefly, while en route to points in the west. Among them were B. H. Dutcher in 1892, Vernon Bailey in 1899 and 1918, J. S. Ligon in 1900-1905, H. C. Oberholser in 1901, J. E. Gaut in 1904, D. A. Gilchrist in 1915-1916, A. K. Fisher in 1921, S. E. Aldous 1932-1936, N. Hotchkiss in 1936, T. D. Burleigh in 1937, F. C. Lincoln in 1937 and K. W. Kenyon in 1939 (Oberholser, unpublished manuscript, U. S. Fish and Wildlife Service files) (Maxwell, 1995).

The establishment of ASU ushered in a new era in biodiversity investigations. The university’s Biology Department has always had, and continues to maintain, a strong interest in natural history among both its faculty and students. This interest led to the development of the Angelo State Natural History Collections.

References Cited