Boundaries

Lerma - Chapala lies in Central Mexico along the Trans-Mexican Volcanic Belt. The ecoregion can be divided into three sub-ecoregions: the Río Lerma proper, the Valle de México, and Puebla - Llanos de San Juan plateau. This interior Mexican ecoregion encompasses eastern Jalisco, northern Michoacán, most of Guanajuato, southwestern Queretaro, northern Mexico, all of the District Federal, northern Morelos, southeastern Hidalgo, northern Tlaxcala, central Puebla, and western Veracruz.

Topography

Along the Río Lerma the topography includes extensive plains, hills, and playas. The Valle de México is surrounded by extremely high mountains of the Trans-Mexican Volcanic Belt, with elevations exceeding 5300 m. The Puebla - Llanos de San Juan plateau is primarily flat and surrounded by mountains.

Freshwater habitats

Within the Valle de México, there were formerly a number of mountain creeks, springs, terminal lagoons, and brackish water lagoons, now all dry. The Puebla - Llanos de San Juan plateau is characterized by crater lakes and occasional creeks.

Terrestrial habitats

Terrestrial habitats include Bajío dry forests, Central Mexican matorral, Trans-Mexican Volcanic Belt pine-oak forests, and Tehuacán Valley matorral.

Description of endemic fishes

The endorheic basins within the ecoregion were once part of a pluvial basin (probably Pleistocene), from which derived a variety of forms, most of them full species. The ecoregion harbors a large number of endemics due to isolation and fragmentation of these basins. In the Río Lerma proper, endemics include the Lerma chub (Algansea barbata), Popoche chub (A. popoche), bulldog goodeid (Alloophorus robustus), opal allotoca (Allotoca dugesii), barred splitfin (Chapalichthys encaustus), scowling silverside (Chirostoma aculeatum), Alberca silverside (C. bartoni), smallmouth silverside (C. chapalae), Ajijic silverside (C. contrerasi), sharpnose silverside (C. labarcae), Toluca silverside (C. riojai), darkedged splitfin (Girardinichthys multiradiatus), Mexican brook lamprey (Lampetra geminis), Mexican lamprey (L. spadicea), and Chapala chub (Yuriria chapalae).Species endemic to the Valle de México include Chignahuapan silverside (Poblama ferdebueni) and Chapultepec splitfin (Girardinichthys viviparous). The Puebla - Llanos de San Juan plateau contains a closely knit taxon of endemic forms, one at each crater lake. Some of these include the Alchichica silverside (Poblama alchichica), La Preciosa silverside (P. letholepis), and Quechulac silverside (Chirostoma squamata).

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 basin has witnessed a high level of fragmentation that has allowed isolation and resulted in a high number of endemic species. Especially important is the case of purported sympatric speciation that took place in the genus Chirostoma, forming a species flock. Within the Valle de México there is a high proportion of endemics, comprising species and one genus in the assemblage, as well as a shared genus with headwaters of the Río Lerma. The Puebla - Llanos de San Juan plateau contains mostly endemics, and all of them crater lake inhabitants.

Level of taxonomic exploration

The level of taxonomic exploration is excellent for the Rio Lerma proper. The streams in the Valle de México have disappeared with their fishes due to water over-extraction, habitat destruction, and pollution, all caused by the growing water needs of Mexico City. However, the level of taxonomic exploration was relatively well known at the time that the fish fauna became extinct. The level of taxonomic exploration is excellent for the Puebla - Llanos de San Juan plateau.