Western Gulf coastal grasslands
Attwater's Greater Prairie Chicken, Western Gulf coastal grasslands. USFWS
The geophysical province of this the Western Gulf coastal grasslands ecoregion is a distinct ecosystem in the southern USA, due to its more temperate climate, proximity to the Gulf of Mexico and associated natural processes (e.g., tropical storms), and geological origin with subsequent succession since the Pleistocene era inundation.
Middleton Prairie, Texas, United States. (Photograph by Robert Parvin) This ecoregion is critical to many species of vertebrates, especially grassland birds, both resident and migrant, that are declining on a continental scale. Several endemic species of this ecoregion have become extinct in the wild, including the red wolf, or are critically imperiled such as the Attwater’s Greater Prairie Chicken (Tympanuchus cupido attwateri), and Whooping Crane (Grus americana). Intertidal, estuarine marsh habitats are less threatened elements of the ecoregion; however, emergent palustrine wetlands within this ecoregion are the most threatened habitat type along the Gulf Coast.
Contents
Location and General Description
The Western Gulf Coastal Grasslands ecoregion follows the coast of the Gulf of Mexico encompassing the wetlands of Louisiana and Texas, in the USA, west of the Mississippi Delta then south into Mexico to just past the Laguna Madre. Grasslands of the northern part of Tamaulipas State, in Mexico have developed on a portion of sandy plains that gently slope to the waters of the Laguna Madre, a sound off the Gulf Coast, which forms the most important hydrographic feature of the ecoregion.
Source: WWF
The delta of the lagoon consists of an interlaced network of resacas; each bordered by a loamy ridge or natural (Natural environment) levee. In the past, the levees were covered by mesquite brush. Clay dunes near the coast were also covered with brush on their leeward sides, while the windward sides held a thick growth of Wright's Dropseed grass (Sporobolus wrightii). Most of this Tamaulipas ecoregion was originally covered with mesquite brush, with some areas of prairie in the Loreto sand plain area. The soil is a reddish sandy loam that varies in depth from approximately two to twenty centimeters. It contains calcareous matter; the sandy soil grades down to a thick arenaceous caliche which is a layer of sand cemented into a limestone by interstitial deposition of calcium carbonate. The climate is semi-arid with precipitation levels of less than 300 millimeters (mm) per year.
The tallest grassland vegetation in this ecoregion ( higher than one meter) grows at near sea level elevation atop of reddish [[soil]s]; Texas Fluffgrass (Tridens texanus), Shortleaf Crabgrass (Digitaria hitchcockii), Purple Threeawn (Aristida purpurea), Slim Fluffgrass (Tridens muticus) and Purple Grama (Bouteloua radicosa) are some of the most common grass species. Climax grasses include tall bunchgrasses such as Seaside Bluestem (Schizachyrium scoparium var. littorale), Eastern Gamagrass (Tripsacum dactyloides), Southern Hairgrass (Mulenbergia capiallaris var. filipes), and several species of panicum.
Towards the Gulf Coast, the topography shifts to lower elevations and more saline soils. Concomitantly, the prairie becomes more intermixed with Gulf Cordgrass (Spartina spartinae), sedges (Carex spp., Cyperus spp.), rushes (Junicus spp.), bulrushes (Scirpus spp.), and Inland Saltgrass (Distichlis spicata). Sporadic shrubland communities dominated by Honey Mesquite (Prosopsis glandulosa), Prickly Acacia (Acacia farnesiana), lime Prickly Ash (Zanthoxylum fagara), and Texas Persimmon (Diospyros texana) may be found in the lower third of the Texas coast and into Tamaulipas. The vegetation in the southern edge of the Laguna Madre consists of scrubs of Prosopis, Acacia, Cordia, Neopringlea, Capparis, as well as perennial grasses and other herbaceous plants.
The tallgrass [[coast]al] prairie region of Texas is generally thought of as a continuum of the north-south range of tallgrass communities in Texas. In contrast, the coastal sand plain of Texas is sufficiently distinct such that it cannot be considered and extension of the true prairie continuum. In general, grasslands eventually meld into freshwater and intertidal marsh habitat at the interface with Gulf bays and estuaries.
Biodiversity Features
Gulfcoast rat (Dipodomys compactus), Padre Island, Texas, USA. (Photograph by E.H. Smith /USFWS) About 700 species of vertebrates have been found in this ecoregion; approximately 145 of these taxa require immediate protection, and some 86 are endangered or threatened. There are 342 birds (with three near-endemics) and 86 mammals (including two endemics) recorded.
Birdlife
Near-endemic birds include the Red-crowned Amazon (Amazona viridigenalis), and the Tamaulipas Crow (Corvus imparatus). The Yellow-crowned Yellowthroat (Geothlypis flavovelata) may be restricted to this ecoregion, but this ecoregion endemism is not fully confirmed. Other notable avifauna found in the Western Gulf coastal grasslands are the Red-billed Pigeon (Patagioenas flavirostris), Brown Jay (Cyanocorax morio), Audubon’s Oriole (Icterus graduacauda), White-tipped Dove (Leptotila verreauxi), and the White-collared Seedeater (Sporophila morelleti).
Reptiles
The nesting beach of the ecoregion endemic Critically Endangered Kemp’s Ridley Sea Turtle (Lepidochelys kempii), that was unknown until its discovery about half a century ago at Rancho Nuevo in Tamaulipas, where virtually the entire population nests. In 1942 there were 42,000 turtle nests, but fewer than 1500 turtles were nesting by the mid 1990s.
Mammals
Other characteristic and distinctive vertebrate species include: Ocelot (Leopardus pardalis), Jaguarondi (Puma yagouaroundi), Lesser Yellow Bat (Scotophilus borbonicus), Mexican Spiny Pocket Mouse (Liomys irroratus);
Amphibians
There are a number of amphibians found in the ecoregion including the Río Grande Chirping Frog (Eleutherodactylus cystignathoides), a sometimes fossorial taxon also known to hide in moist spaces; Mexican White-lippedFrog (Leptodactylus fragilis), often found near marshes or ephemeral lentic ponds; Cope's Gray Treefrog (Hyla chrysoscelis), an anuran, when inactive, hides under tree roots or leaf litter; Couch's Spadefoot Toad (Scaphiopus couchii), a fossorial species that can create burrows, or borrow the burrow of another species; and Southern Crawfish Frog (Lithobates areolatus); Eastern Narrowmouth Toad (Gastrophryne carolinensis).
Current Status
Middleton Prairie, Texas, United States. (Photograph by Robert Parvin) Less than one percent of these grasslands remains in near pristine condition. Conversion to agricultural production has caused the greatest loss. In addition, overgrazing, conversion to tame grasses, fragmentation, and woody encroachment have affected the area.
The National Wildlife Refuge System possesses several important sites within the USA portion of this ecoregion, although the main emphasis of their acquisition was development of refuges specifically for waterfowl. Tallgrass prairie on these refuges were generally not considered key to acquisition priorities with few exceptions (e.g. Attwater’s Prairie Chicken National Wildlife Refuge). The refuges include: Delta National Wildlife Refuge, Lacassine National Wildlife Refuge, Cameron Prairie National Wildlife Refuge, Rockefeller National Wildlife Refuge, Sabine National Wildlife Refuge, Texas Point National Wildlife Refuge, McFaddin National Wildlife Refuge, Anahuac National Wildlife Refuge, Moody National Wildlife Refuge, Attwater’s Prairie Chicken National Wildlife Refuge, Brazoria National Wildlife Refuge, San Bernard National Wildlife Refuge, Big Boggy National Wildlife Refuge, Whitmire Division of Aransas National Wildlife Refuge, Aransas National Wildlife Refuge, Matagorda Island National Wildlife Refuge, Laguna Atascosa National Wildlife Refuge, Santa Ana National Wildlife Refuge, Padre Island National Seashore (National Park Service), and other significant areas owned or managed by state or NGOs.
Types and Severity of Threats
Fragmentation of remaining habitat via subdividing large tracts into more marketable ranchettes leads to other degrading factors such as overgrazing, alien plant expansion, lack of fire as a natural or prescribed process, and modification of local hydrological features by means of land leveling. Urbanization around larger metropolitan areas such as Houston, Texas has been another direct cause of habitat loss. This will proceed with the continued proliferation of suburban development.
Coastal wetlands are less suited, in most cases, for high-density development and agricultural conversion. However, channelization projects with all the associated damage to overland sheet flow and hydrological function, continue to impact this portion of the grasslands. Subsidence, erosion, and loss of emergent wetlands are significant problems that are ongoing in this ecoregion.
Further Reading
- Bailey, R.G. 1994. Ecological classification for the United States. Washington, DC: USDA Forest Service.
- CONABIO Workshop, 17-16 September, 1996. Informe de Resultados del Taller de Ecoregionalización para la Conservación de México.
- CONABIO Workshop, Mexico, D.F., November 1997. Ecological and Biogeographical Regionalization of Mexico.
- Diamond, D. D. 1993. Plant communities of Texas. Texas Natural Heritage Program, Texas Parks and Wildlife Department, Austin.
- Diamond, D. D., and T. E. Fullbright. 1990. Contemporary plant communities of upland grasslands of the Coastal Plain. Southwestern Nature 35: 385-392.
- Diamond, D. D., and F. E. Smeins. 1984. Remnant grassland vegetation and ecological affinities of the upper coastal prairie of Texas. Southwestern Nature 29:321-334.
- Gould, F. W. 1962. Texas plants: A checklist and ecological summary. Texas Agriculture Experimental Station Leaflet 492. Texas A&M University, College Station. ISBN: 0521357780
- Gould, F. W., G.O. Hoffman, and C.A. Rechenthin. 1960. Vegetational areas of Texas. Texas Agriculture Experimental Station Leaflet 492. College Station: Texas A&M University. ISBN: 0521357780
- Howell, S.N.G. and S. Webb. 1995. A Guide to the Birds of Mexico and Northern Central America. Oxford Univ. Press, England. ISBN: 0198540124
- Jahrsdoerfer, S.E. and D.M. Leslie. 1988. Tamaulipan Brushland of the Lower Rio Grande Valley of South Texas. Description, human impacts and management options U.S. Fish & Wildlife Service, Washington, D.C.
- Johnston, M.C. 1963. Past and present grasslands of Southern Texas and Northeastern Mexico. Ecology 44: 456-466.
- Knopf, F. L. 1994. Avian assemblages on altered grasslands. Studies in Avian Biology 15: 247-257.
- Márquez-M., R. 1994. Synopsis of biological data on the Kemp´s ridley turtle , Lepidochelys kempi. NOAA Technical Memorandum, NMFS-SEFSC-343.
- The Nature Conservancy. 1997. Designing a geography of hope: Guidelines for ecoregion-based conservation in The Nature Conservancy. Arlington, VA.
- Smeins, F. E., D. D. Diamond, and C. W. Hanselka. 1991. Coastal prairie. Pages 269-290 in R. T. Coupland (editor), Ecosystems of the world: Natural grasslands---introduction and western hemisphere. Elsevier, New York:
- Stattersfield, A.J., M.J. Crosby, A.J. Long and D.C. Wege. 1998. Endemic Bird Areas of the World: Priorities for Biodiversity Conservation. Birdlife Conservation Series No. 7, Cambridge, UK.
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