Boundaries

This ecoregion is defined primarily by the Rio Grande, or Río Bravo del Norte, from its confluence with the Río Conchos to the Gulf of Mexico. The ecoregion covers a small part of southwestern Texas, and in Mexico parts of eastern Chihuahua, northern Coahuila, the extreme northern section of Nuevo León, and northern Tamaulipas. The Río Conchos borders the western part of the ecoregion, and the Gulf of México borders the east.

Topography

Gently undulating hills characterize the upper part of the ecoregion, tapering to lowlands towards the sea. Elevations range from sea level to over 3500 m, although relief rarely exceeds 100 m.

Freshwater habitats

Freshwater habitats of this large river system include floodplains in the middle and lower basin, small creeks within the central portion, springs, and caves. Within the lower basin large diversified springs are habitats for a variety of fish, many of which are endemic.

One outstanding feature of the Lower Rio Grande is the area known as Zona Carbonifera. This is a subterranean aquifer with an extensive system of springs and caves associated with it. A distinctive cave fauna occurs here, which is imperiled by overuse of the aquifer’s water.

The Rio Grande/ Río Bravo basin is being dewatered and salinized, suffering from a heavy invasion of brackish water. As a result, lowland fish fauna has expanded up to 800 km upstream, forcing the extirpation of freshwater species.

Terrestrial habitats

The ecoregion is dominated by three terrestrial ecoregions: Chihuahuan Desert in the northwest, Tamaulipan mezquital in the central and lower portion, and Western Gulf coastal grasslands along the coast.

Tamaulipan mezquital, which covers a majority of the ecoregion, is a mesquite grassland that includes mesquite (Prosopsis glandulosa), various species of acacia such as Acacia smallii and A. tortuosa, desert hackberry (Celtis pallida), javelina bush (Condalia ericoides), cenizo (Leucophyllum frutescens), common bee-brush or white brush (Aloysia wrightii), Texas prickly pear (Opuntia lindheimeri), and tasajillo or desert Christmas cactus (Cylindropuntia leptocaulis) (WWF 2001).

Description of endemic fishes

Species endemic to the ecoregion include the Devil’s River minnow (Dionda diaboli), Big Bend gambusia (Gambusia gaigei), and spotfin gambusia (Gambusia krumholzi). Near-endemics include the Rio Grande silvery minnow (Hybognathus amarus), phantom shiner (Notropis orca), and bluntnose shiner (N. simus).

Justification for delineation

Ecoregion delineations were based on qualitative similarity/dissimilarity assessments of major basins, using the standard administrative hydrographical regions of the Mexican federal government.This ecoregion has numerous strict endemics, many of which are shared with neighboring areas, or are endemic to the Rio Grande basin. The relict groups of Neotropical taxa make this ecoregion distinct from other areas of the Rio Grande basin.

Level of taxonomic exploration

The level of taxonomic exploration along the Rio Grande/Río Bravo is reasonable along middle and lower courses, but poor in most headwaters.