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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

140: East Texas Gulf

Major Habitat Type:

temperate coastal rivers

Author:

Text modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment. Island Press, Washington, DC, USA.

Countries:

United States

Boundaries:

This ecoregion stretches from eastern New Mexico to southeastern Texas, covering most of central Texas. The ecoregion is defined primarily by the watersheds of Matagorda Bay, as well as the Brazos and Colorado rivers and their numerous tributaries.

Drainages flowing into:

The drainages of this ecoregion flow into the Gulf of Mexico.

Main rivers or other water bodies:

Major rivers in the ecoregion include the Brazos and Colorado.  

Topography:

The headwaters of the Colorado and Brazos fall within the Llano Estacado in the southern High Plains in northwestern Texas and New Mexico. This is a large gently sloping mesa with elevations reaching 1500 m. The rivers descend across the Central Lowlands and Coastal Plain formations in their middle and lower portions. To the southwest lies the Edwards Plateau, which exhibits little relief except along the stream valleys of the Colorado and Brazos. The southeastern edge of the Plateau is marked by high relief in the Balcones Escarpment (Connor & Suttkus 1986).

Climate:

The western portion of the ecoregion is semi-arid with little or no water surplus; here, at the headwaters of the Brazos and Colorado, normal annual precipitation is less than 38 cm.  The southeastern end of the ecoregion is characterized by a dry to dry-subhumid climate (Connor & Suttkus 1986).

Freshwater habitats:

The Edwards Plateau lies in the southwestern portion of this ecoregion, and also covers part of the West Texas Gulf [139] and the Lower Rio Grande/Río Bravo del Norte [135] ecoregions. This is a karst area characterized by the Edwards Aquifer, a body of groundwater that has a distinct biota associated with its caverns and springs. Freshwater and intertidal marshes are along the coast.

Terrestrial Habitats:

A broad ecotone of eastern pine-hardwood forests is the dominant vegetation in the central and southeastern portion of the ecoregion. The Edwards Plateau is dominated by juniper-oak savanna and mesquite-Acacia savanna (Küchler 1964). Short and mixed grass prairie dominate the upper Brazos and Colorado drainages (Connor & Suttkus 1986). Along the coast tallgrass coastal prairie was once the primary plant community from southeastern Louisiana to the mouth of the Rio Grande River (Smeins et al. 1991). Today, less than one percent of this original community type remains.

Fish Fauna:

The fauna of this ecoregion is largely transitional, combining eastern lowland, Great Plains and Rio Grande elements. The Brazos drainage is the largest and most distinctive drainage within the western Gulf Slope, serving as the southern limit for species such as redfin pickerel (Esox americanus) and blackspot shiner (Notropis atrocaudalis), and the northeastern limit for species such as the Guadalupe bass (Micropterus treculi). The Colorado is also distinctive, serving as the southern limit for a number of species.

Description of endemic fishes:

In the East Texas Gulf ecoregion, there are around 100 fish species, of which few are considered endemic. Examples include the burrhead chub (Macrhybopsis marconis) and smalleye shiner (Notropis buccula), in the Brazos River drainage (Connor & Suttkus 1986).

Other noteworthy aquatic biotic elements:

The Edwards Plateau alone contains 12 endemic hydrobiid snails and two endemic unionid mussels, Lampsilis bracteata, and Quincuncina guadalupensis (Bowles & Arsuffi 1993). It is also the home for the endangered Texas blind salamander (Typhlomolge rathbuni) and the Edwards Plateau shiner (Cyprinella lepida). The aquatic invertebrate fauna of the region are, as one might expect, poorly known compared to its vertebrates (Edwards et al. 1989).

Justification for delineation:

Ecoregion boundaries are modified from Abell et al. (2000), which based its units on subregions defined by Maxwell et al. (1995).  Modifications to this ecoregion were made following recommendations from the Endangered Species Committee of the American Fisheries Society. This ecoregion was considered unique because of distinct faunal breaks that result from differences in drainage size and physiographic complexity. There are marked differences in richness as one moves east to west across the Gulf Slope region, as well as distinct species clusters at the 80% level among the Nueces-San Antonio drainages (in [139]), Colorado-Brazos drainages (in [140]), and Galveston-Sabine-Calcasieu drainages (in [141]).  

References/sources:

Abell, R., Olson, D., et al. (2000). "Freshwater ecoregions of North America" Washington, D.C.: Island Press.

Bowles, D. E.,Arsuffi, T. L. (1993). "Karst aquatic ecosystems of the Edwards Plateau region of central Texas, USA: A consideration of their importance, threats to their existence, and efforts for their conservation" Aquatic Conservation: Marine and Freshwater Ecosystems 3 317-329.

Conner, J.V. and R.D. Suttkus (1986)"Zoogeography of freshwater fishes of the western Gulf slope" In Hocutt, C.H.a.E.O.W. (Ed.). The zoogeography of North American freshwater fishes. (pp. 413-456) New York, NY: John Wiley & Sons.

Edwards, R. J., Longley, G., et al. (1989). "A classification of Texas aquatic communities with special consideration toward the conservation of endangered and threatened taxa" Texas Journal of Science 41 232-240.

Master, L. L., S. R. Flack, and B. A. Stein (1998). "Rivers of life: Critical watersheds for protecting freshwater biodiversity" Arlington, VA: The Nature Conservancy.

Maxwell, J. R., Edwards, C. J., et al. (1995) "A hierarchical framework of aquatic ecological units in North America (Nearctic Zone)". St. Paul, MN. North Central Forest Experiment Station, USDA Forest Service.

Smeins, F.E., D.D. Diamond, and W. Helsanka (1991)"Coastal prairie" In Coupland, R.T. (Ed.). Ecosystems of the world: natural grasslands. (pp. 269-290) Amsterdam: Elsevier Press.

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