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Ecoregion Description

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Species Richness

# of Endemic Species


138: Rio San Juan (Mexico)

Major Habitat Type:

xeric freshwaters and endorheic (closed) basins


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




Part of the Rio Grande complex, the Río San Juan stretches from southeastern Coahuila through central Nuevo León, and northwestern Tamaulipas to the Rio Grande (Río Bravo) River.

Drainages flowing into:

The Río San Juan originates in the mountains of the Sierra Madre Oriental and the Altiplanicie Mexicana, and flows eastward onto the coastal plains of eastern Mexico, where it converges with the Rio Grande (Río Bravo) on the U.S.-Mexico border.

Main rivers or other water bodies:

Main rivers in the ecoregion include the Río San Juan and Río Álamo, both which flow into the Rio Grande/Río Bravo.


Headwaters of the Río San Juan originate in the Sierra Madre Oriental (mostly within Área Natural Protegida Cumbres de Monterrey). The topography throughout the lower course is hilly in the piedmont and grades to plains toward the confluence with the Río Grande/Río Bravo.


This ecoregion has a semiarid or steppe climate. Total precipitation in this climate zone ranges between 250 – 500 mm, although some areas in the ecoregion exceed 900 mm.

Freshwater habitats:

Freshwater habitats in the ecoregion include springs, mountain creeks, plains rivers, and a few lagoons. There are some semi-isolated springs (such as Apodaca, Mezquital, Infiernillo, Huasteca, and others that have dried out) with strictly Neotropical species. However, most headwaters are Nearctic, with only one or two Neotropical species.

Terrestrial Habitats:

The ecoregion includes the following terrestrial ecoregions: Sierra Madre Oriental pine-oak forests, Chihuahua desert, Tamaulipan mezquital, and Tamaulipan matorral.

Fish Fauna:

With over forty species, this ecoregion is the second richest xeric freshwater ecoregion in North America. The ecoregion is noted for its large number of cyprinids, including Dionda, Cyprinella, Notropis, Gila, and Xiphophorus. The Salinas chub (Gila modesta) is critically endangered and found in one stream of the Salinas River System in Coahuila, Mexico. Other species include the alligator gar (Atractosteus spatula), spotted gar (Lepisosteus oculatus), and longnose gar (L. osseus) (Lepisosteidae); mosquitofish (Gambusia affinis), Tex-Mex gambusia (G. speciosa), Amazon molly (Poecilia formosa), sailfin molly (P. latipinna), shortfin molly (P. Mexicana), and Xiphophorus couchianus (Poeciliidae); and Rio Grande cichlid (Herichthys cyanoguttatus) (Cichlidae).

Description of endemic fishes:

Like many xeric regions, the Río San Juan exhibits high endemism among its freshwater fauna. The only strict endemic to the ecoregion is Membras vagrans, although the Mexican red shiner (Cyprinella rutila), Salinas chub (Gila modesta), and Monterrey platyfish (Xiphophorus couchianus) are near-endemic. There are no known endemics from the Río Álamo sub-basin, although there is one distinctive form of ghost shiner (Notropis buchanani), as well as the northernmost population of shortfin molly (Poecilia Mexicana).

Other noteworthy aquatic biotic elements:

Other endemic species include one crayfish (Procambarus regiomontanus) and two isopods (Sphaerolana spp).

Evolutionary phenomena:

Some species are endemic members of radiation complexes, like Dionda, Cyprinella, Notropis, Gila, and Xiphophorus. The series of the cyprinid Cyprinella is represented by various forms in different major tributaries of the Río Bravo/Rio Grande. The Mexican red shiner (C. rutila) is located in Río San Juan.

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. The delineation of this ecoregion was based on several endemics, as well as a high presence Notropis species.

Level of taxonomic exploration:

Excellent for southern tributaries, but poor for northern ones.


Abell, R. A., Olson, D. M., et al. (2000). "Freshwater Ecoregions of North America: A Conservation Assessment" Washington, DC, USA: Island Press.

Contreras-Balderas, S. (1967). "Lista de Peces del estado de Nuevo León" Monterrey, México: Cuad. Inst. Invest. Cient., Universidad de Nuevo León.

Miller, R. R.,Smith, M. L. (1986)"Origin and geography of the fishes of central Mexico" In Hocutt, C.H.;Wiley, E.O. (Ed.). The zoogeography of North American freshwater fishes. (pp. 487-517) New York, New York, USA: Wiley.

Smith, M. L.,Miller, R. R. (1986)"The evolution of the Rio Grand Basin as inferred from its fish fauna" In Hocutt, C.H.;Wiley, E.O. (Ed.). The zoogeography of North American freshwater fishes. (pp. 457-485) New York, New York, USA: Wiley.

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