Location and General Description
The ecoregion is located on the physiographical province known as the Coastal Gulf Plain. It begins in the eastern part of the Coahuila State, in Mexico at the base of the Sierra Madre Oriental, and then proceeds eastward to encompass the northern half of the state of Tamaulipas, and into the United States threw the south western side of Texas. Elevation increases northwesterly from sea level near the Gulf Coast to a base of about 300 m (1,000 ft) near the northern boundary of the ecoregion, from which a few hills or mountains protrude. The Anacacho Mountains and Turkey Mountain (550 m, or 1,805 ft) in Kinney County, Texas, as well as Blue Mountain (389 m, or 1,277 ft) in Uvalde Country, occur within this ecoregion.
The Rio Grande flows through this Tamaulipan/Mezquital ecoregion and once formed a broad and meandering waterway that produced numerous resacas or oxbows within its floodplain, and an extensive marshy environment at its mouth. Some of the earliest Spanish explorers called the river "Rio de las Palmas," because of the abundant palm trees that grew along its banks. These Texas palms once extended upriver as much as 80 miles, into this ecoregion. Today, the Valley’s only remaining native palm grove is the 74 hectare (172 acre) Sabal Palm Grove Sanctuary.
The native vegetation type covering much of northeastern Mexico and parts of southern Texas is mesquite-grassland, an important element of the ecoregion that plant ecologists classify as characteristic of the Tamaulipan biotic province. The Tamaulipan province extends south of the border for almost 200 miles between the coast and the deciduous woodlands on the slopes of the Sierra Madre Oriental. The Tamaulipan thornscrub, a subtropical, semi-arid vegetation type, occurs on either side of the Rio Grande. Spiny shrubs and trees dominate this thornscrub, but grasses, forbs, and succulents are also prominent (Crosswhite 1980). The slightly higher, drier, and rockier sites originally had vegetation of chaparral and cacti, whereas the flat, deep soils supported mesquite as well as taller brush and a few drought-resistant trees, often rather openly spaced and savanna-like in a grassland matrix. This region also includes elements of pastizal, a combination of grassland, savanna, and páramo-like communities (Rzedowski 1994). Leguminous shrubs and trees constitute one-third of the diverse woody flora, which the rural population uses for extensive grazing of livestock, fuelwood, and timber for fencing and construction (Reid 1990).
The two species that characterized the Mezquital communities before alteration were mesquite itself (Prosopsis glandulosa) and the curly mesquite grass (Hilaria belangeri) which grew under it. The most common shrubs were probably chaparro (Zizyphus obtusifolia) and jazmincillo (Aloysia gratissima). Parts of this region consisted of open woods of mesquite, with a pronounced understory of grasses which often contained a layer of taller species such as hooded finger grass (Chloris cucullata), and a layer of shorter grasses such as grama (Bouteloua spp.) In some places dense stands of prickly pear or nopal (Opuntia lindheimeri) would take the place of many of the shrubs and grasses (Crosswhite 1980).
Montezuma bald cypress (Taxodium mucronatum) was once common along the Rio Grande for 160 km (100 miles) from the Gulf of Mexico. The tree is now rare along the Rio Grande, though it is still the dominant tree along the Rio Corona to the south. Perhaps the most striking endemic along the lower Rio Grande is the palma de Micharos (Sabal texana), which was once common along resacas in the floodplain and held a dominant position over mesquite under certain conditions.
A few species of plants account for the bulk of the brush vegetation of the Tamaulipan/Mezquital and give it a characteristic appearance. The most important of these plants include: mesquite, various species of acacia including Acacia smallii and A. tortuosa, desert hackberry, javelina bush, cenizo (sometimes called purple sage), common bee-brush or white brush (Aloysia wrightii), Texas prickly pear, and tasajillo or desert Christmas cactus. The only exceptions to the rather arid shrub-covered landscapes are the lines of riparian vegetation within the few river valleys.
Parts of this region support grasslands not unlike those of the Great Plains, though the grasslands here are not as uniform due to the highly variable soil and moisture conditions. The flora of the Tamaulipan/Mezquital differs dramatically from the nearest desert, the Chihuahua, largely because of the Chihuahua’s higher elevations and generally colder winters. The number of species in the Tamaulipan/Mezquital that are frankly desert plants, many showing affinities with the Sonoran Desert, is startling (Crosswhite 1980).
The Tamaulipan/Mezquital constitutes a unique assemblage of mesquite grassland in northeastern Mexico and the southern United States. The Sierra de San Carlos, at 1,700 m above sea level, houses over 700 species of plants of which 11 are endemic (Martínez-Díaz et al. 1995). The ecoregion contains at least four endemic genera of woody plants including Clappia, Nephropetalum, Pterocaulon and Runyiona (Rzedowski 1978). It is also a rich zone in terms of endemic cacti, as well as the occurrence of endangered species (Suzan & Malda 1990). This ecoregion is also considered among the priority sites for conservation of succulents, proposed by Oldfield (1997).
The southern part of the ecoregion is considered an Endemic Bird Area (EBA) (Stattersfield et al. 1998). Restricted-range species include the green-cheeked amazon (Amazona viridigenalis), crimson-collared grosbeak (Rhodothaupis celaeno), Altamira yellowthroat (Geothlypis flavovelata) and Tamaulipas crow (Corvus imparatus) (Stattersfield et al. 1998; Howell and Webb 1995). The southern nesting limits of two rare birds with very limited nesting ranges, the black-capped vireo (Vireo atricapillus) and golden-cheeked warbler (Dendroica chrysoparia) are restricted to northern Tamaulipan/Mezquital, in Texas (Campbell 1995).
Some mammals of importance to this ecoregion are those that have been persecuted by humans for fear of their damage to livestock including the Mexican prairie dog (Cynomys mexicanus) one of the most endangered species of mammals in México (Miller 1994). Also the larger cats including the ocelot (Leopardus pardalis), which reaches its northern most limit in this ecoregion. Characteristic small mammals include southern plains woodrat (Neotoma micropus), and Mexican spiny pocket mouse (Liomys irroratus).
Clearing and conversion of shrubland for agriculture has had the greatest impact on altering the patterns and processes of the landscape of south Texas and northeastern Mexico. Both banks of the Rio Grande are now crowded with homes, businesses, and farms. The only remaining natural areas south of the river are the salt marshes and mud flats east of the city of Matamoros. The Rio Corona floodplain was relatively intact until recently, but since the clearing of some lands for agriculture there has been increased erosion, pollution from agrochemicals, invasion of exotic species, and general loss of native wildlife. Only about two percent of this ecoregion remains as intact habitat, and most of the remaining area has been heavily altered by human activity. This ecoregion contains no habitat blocks larger than 250 km2, and no protected areas. Only small patches of the original landscape remain. These remnants are largely isolated and provide little opportunity for species dispersal. The surrounding matrix, however, is composed of elements of the original landscape and some movement is possible.
Types and Severity of Threats
Conversion to agriculture poses the greatest threat to this ecoregion. Agricultural expansion and clearing for development may significantly alter more than a quarter of the remaining habitat over the next two decades. Degradation of habitat due to pollution and exploitation of wildlife are significant but less pressing threats. In the valley of Jaumave, the land is commonly converted to cultivable fields by burning native vegetation. Cacti and other succulents are badly damaged or die during these processes. Most of the fires burn out of control (Oldfield 1997), which adds a major threat to the plant communities of the ecoregion. Construction of dams, road building, mining and expansion of urban areas also cause habitat destruction. In addition, illegal extraction and trade of exotic plant species have endangered many species of cactus.
Justification of Ecoregion Delineation
The United States portion of the Tamaulipan Mezquital ecoregion corresponds to Küchler’s (1975) unit 54 (Mesquite-acacia savanna) and 55 (Mesquite-live oak savanna). The boundary is the same as Omernik’s (1995) unit 31 (Southern Texas Plains) and is similar to Bailey’s (1994) section 315E (Rio Grand Plain), part of the Southwest Plateau and Plains Dry Steppe and Shrub Province. In Mexico our linework follows INEGI (1996) vegetation cover maps, from which we lumped the following classifications: "mezquital", "mezquital with matorral", "spiny matorral", "halophytic vegetation", "pastizal with huizachal", "huizachal" and all enclosed agricultural areas. Linework was reviewed by experts during ecoregional priority setting workshops (CONABIO 1996 and 1997) in Mexico.
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Suzan, H. and G. Malda. 1990. Monitoreo demográfico en 5 cactáceas en peligro de extinción en Tamaulipas. XI Congreso Mexicano de Botánica, Oaxtepec Morelos del 30 sept al 5 oct de 1990. Programas y Resúmenes. Sociedad Botánica de México.
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Prepared by: T. Cook and J. Adams, Alejandra Valero, Jan Schipper, and Tom Allnutt
Reviewed by: In process