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Ecoregion Description

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Species Richness

# of Endemic Species


169: Rio Balsas

Major Habitat Type:

tropical and subtropical coastal rivers


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




The ecoregion extends from the Pacific coast northward through central Michoacán, northward into eastern Jalisco, then eastward through northern Guerrero, southern Mexico, most of Morelos, southern Tlaxcala, and southern Puebla, reaching its eastern limit in western Oaxaca. It includes the basins of Río Balsas, Río de Coalcoman, and Río Nexpa.

Drainages flowing into:

Pacific Ocean

Main rivers or other water bodies:

The extent of this coastal ecoregion is largely defined by the Río Balsas and its tributaries, including the Río Tepalcatepec. Other rivers that drain into the Pacific Ocean include the Coalcoman and Nexpa. The largest lake is Presa del Infiernillo.


This is a highly mountainous ecoregion. The Sierra Madre Occidental and coastal mountains separate this ecoregion from the Sierra Madre del Sur [170] ecoregion. The northern edge is bordered by the Trans-Mexican Volcanic Belt.


The mean annual temperature is 22 ºC, and mean annual precipitation is 1000 mm.

Freshwater habitats:

The Balsas River is one of the longest in Mexico. Along its course are rapids and fast flowing waters. The river’s bottom is primarily rocky.

Terrestrial Habitats:

Jalisco Dry forests line the coast. The Balsas River valley is characterized by Balsas dry forests, with dominant species including Bursera longipes, B. morelensis, and B. odorata. Pine-oak forests characterize higher elevations of the ranges surrounding this valley.

Fish Fauna:

The Balsas ecoregion harbors a relatively small number of fish species, but a high percentage of those are endemic to this ecoregion. This pattern is consistent with the distribution of fish in the other ecoregions of the mountainous, subtropical Pacific Coast of Mexico (Sinaloa [162], Santiago [164], Ameca-Manantlan [168], and Sierra Madre del Sur [170]).

Description of endemic fishes:

Seven endemic fish are found in the Balsas ecoregion. These are the Balsas splitfin (Ilyodon whitei), the Catarina allotoca (Allotoca catarinae), the Balsas livebearer (Poeciliopsis balsas), the Balsas molly (Poecilia maylandi), the Balsas silverside (Atherinella balsana), the Balsas catfish (Ictalurus balsanus), and the Balsas shiner (Hybopsis boucardi). This ecoregion does not have the richness of cichlid species that most of southern Mexico exhibits. Only one of these fish, the redside cichlid (Cichlasoma istlanum), appears in the Balsas ecoregion.

Other noteworthy aquatic biotic elements:

This ecoregion is remarkable for its populations of endemic frogs and other aquatic herpetofauna, such as Rana madrensis, Rana sweifeli, and Rana forrers. In total, there are about one hundred aquatic reptile and amphibian species.

Evolutionary phenomena:

Speciation of the goodeid genus Allotoca.

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 harbors a highly endemic fish fauna that has adapted to fast flowing waters.

Level of taxonomic exploration:

There has been good taxonomic exploration conducted within the ecoregion, but no documentation has been published on the total holdings.


Abell, R., Olson, D., et al. (2000). "Freshwater ecoregions of North America" Washington, D.C.: Island Press.

Alvarez del Villar, J. (1972). "Ictiología Michoacana V. Orìgen y distribución de la ictiofauna dulceacuìcola de Michoacán" Anales de la Escuela Nacional de Ciencias Biologicas Mexico 19 155-161.

Alvarez del Villar, J.,Cortéz, T. (1969). "Ictiología michoacana I. Claves y catálogo de las especies conocidas" Anales de la Escuela Nacional de Ciencias Biologicas Mexico 11(1-4) 85-142.

Paulo, J.,Ramírez, A. (1997). "Distribución espacio-temporal de la ictiofauna del Río Cutzamala, Michoacán, México" Rev. Biol. Trop. 45(2) 84-88.

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