Lerma - Chapala
Salvador Contreras Balderas (Universidad Autónoma de Nuevo León), Jennifer Hales
Major Habitat Type
Xeric freshwaters and endorheic (closed) basins
Drainages flowing into
Lerma-Chapala consists of numerous endorheic basins around a main core of a former pluvial basin during the Pleistocene. The Río Lerma drains into Lago de Chapala. Situated in the Mexican altiplano, the lake is formed in an east-west lying depression bound by two faults. The lake sits at the bottom of the Lerma River basin and discharges into the Santiago River, which then flows to the Pacific Ocean.
Main rivers to other water bodies
The main water bodies in the ecoregion include the Río Lerma and Lago de Chapala. The Río Lerma originates in spring-fed lagoons, and empties into Lago de Chapala, Mexico’s largest lake. The main branch of the Lerma River is more than 700 km long, with numerous tributaries and aquifers along its length. The Río Santiago, which drains Lago de Chapala to the Pacific Ocean, is sometimes considered to be a unit with the Río Lerma. Other lakes include the crater lakes (Lago Alchichica, Lago Quechulac, Lago La Preciosa) in Puebla - Llanos de San Juan plateau.
The complex of Río Lerma, Lago de Chapala, and Río Santiago has suffered fragmentation. The Valley of Mexico, the San Juan Plains in Puebla (Lago Alchichica, Lago Quechulac, Lago La Preciosa), Lago de Pátzcuaro, Lago de Cuitzeo, Laguna Atotonilco, Laguna San Marcos, Laguna Sayula, Laguna Magdalena Etzatlán, Laguna Juanacatlán, and other endorheic basins are related in a complex system to either of the main rivers, Lerma and Santiago. Also, some southern headwaters of the Río Pánuco were formerly part of the Lerma basin.
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.
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.
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 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.
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