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# of Endemic Species
135: Lower Rio Grande - Bravo
Major Habitat Type:
temperate floodplain rivers and wetlands
Salvador Contreras Balderas, Universidad Autónoma de Nuevo León. Additional text was modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment
Mexico; United States
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.
Drainages flowing into:
Gulf of México
Main rivers or other water bodies:
The Rio Grande/Río Bravo is one of the largest rivers worldwide (2900 km), where it is 22nd in size globally, and 5th in North America. It is one of the three longest bi-national frontiers (2004 km), forming the border between the US and México. Its basin covers an area of about 457,700 km2. This ecoregion comprises the drainage area of the Lower Rio Grande starting just above the mouth of the Río Conchos, with the exception of the basins of the Pecos River , Río Salado , and Río San Juan , each of which support distinct freshwater faunas. Also included in this ecoregion are the watersheds of the San Fernando River, located south of the Rio Grande in Mexico and draining to the Gulf of Mexico, and the Devil’s River, a tributary to the Rio Grande directly to the east of the Pecos River.
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.
This ecoregion experiences a hot arid steppe climate (Köppen 1936). Mean annual temperature is around 21 ºC and mean annual precipitation is 475 mm, with the highest precipitation occurring toward the coast.
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.
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).
The fish fauna of the lower Rio Grande/Río Bravo contain nearly twice as many species as the Río Conchos and three times as many species as the upper Rio Grande. Species that are found within the larger basin include the Conchos pupfish (Cyprinodoneximius), Mexican stoneroller (Campostoma ornatum), ornate shiner (Codoma ornata), Río Grande chub (Gila pandora), Tamaulipas shiner (Notropis braytoni), Chihuahua shiner (N. chihuahua), Río Grande shiner (N. jemezanus), and Conchos pupfish (Cyprinodon eximius).
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).
Other noteworthy aquatic biotic elements:
Associated with this ecoregion are one endemic unionid mussel species and one endemic crayfish species.
The cyprinid genus Dionda, the poecilid genus Gambusia, the lepida section of Cyprinella, and the pupfishes are usually represented by one species each on the major sub-basins. The Neotropical species of Xiphophorus and Gambusia represent relict distributions.
During pluvial stages the Rio Bravo/Rio Grande included the now isolated basins or groups of basins such as the Samalayuca complex (Río Casas Grandes, Río Santa María, Río Carmen, Laguna Bavícora), the Mayrán-Viesca complex (Río Nazas, Río Aguanaval, Laguna de Santiaguillo, Laguna Tlahualilo, Parras), and the Río Conchos (Laguna de Bustillos, Rio Saúz and Laguna Encinillas, Laguna de los Mexicanos). Some high tributaries of Pacific basins are also derived from the Old Río Bravo most likely by way of stream capture, like those of Río Yaqui, Río Fuerte, Río Mayo, Río Piaxtla, and probably others inhabited by assemblages also derived from Río Bravo/Rio Grande stocks.
Examples of allopatric speciation related to fragmentation that took place in the Old Rio Grande/Río Bravo are the complexes of the cyprinids Cyprinella and the cyprinodontid Cyprinodon eximius. Usually allopatric speciation develops mostly through barriers and dispersals. The divergence may be exemplified by a number of cases. The series of the cyprinid Cyprinella is represented by forms in different major tributaries of the Río Bravo/Rio Grande: Río Conchos (C. panarcys), Río Salado (Cyprinella cf. rutila), Río San Juan (C. rutila), some minor tributaries asociated to Rio Pecos (C. proserpina), and the Río Nazas (C. alvarezdelvillari), with one extraterritorial species in central Texas (C. lepida).
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.
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