Boundaries

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.

Topography

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.

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.

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.

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.