Boundaries

Part of the Rio Grande complex, this ecoregion encompasses most of central and southern Chihuahua, extending into northern Durango. It is bounded to the north by the Río Grande proper, west by the Río Casas Grandes complex and the summit of the Sierra Madre Occidental, to the east by the plains between Coahuila and Chihuahua, and to the south by the Río Nazas basin complex.

Topography

Originating along the eastern slopes of the Sierra Tarahumara, the Río Conchos runs along plains and hills towards its junction with the Río Grande/Río Bravo. Elevations range from 800 to over 3000 m above sea level.

Freshwater habitats

Freshwater habitats of the ecoregion include mountain headwaters in Sierra Tarahumara, the middle and lower basin of the Río Conchos, some isolated interior basins that were formerly part of the basin (Laguna Mexicanos, Laguna Bustillos, Laguna Encinillas-Sauz), and semi-isolated springs (San Gregorio, La Hacienda, San Diego).

The Río Conchos is the only free-flowing large river environment left in the Rio Grande basin. It still harbors an impressive assemblage of species because the ecology of this system has not been impacted by channel modifications. This ecoregion is important because it not only supports surface water biota, but also fauna in its specialized spring and cave habitats. These spring habitats contribute most to the ecoregion’s high endemism.

Terrestrial habitats

The dominant plant species throughout the Chihuahuan desert is creosote bush (Larrea tridentata), whereas Pinus lumholtzii dominates the eastern slopes of the Sierra Madre Occidental.

Description of endemic fishes

The Río Conchos ecoregion exhibits remarkable endemism in its fish species, with roughly a third of its native fish endemic. Species strictly endemic to the ecoregion include the Conchos shiner (Cyprinella panarcys), largescale pupfish (Cyprinodon macrolepis), bighead pupfish (C. pachycephalus), Bocochi pupfish (C. salvadori), Conchos darter (Etheostoma australe), Chihuahua darter (E. pottsii), yellowfin gambusia (Gambusia alvarezi), and crescent gambusia (G. hurtadoi).

Ecological phenomena

The Codoma ornata complex represents an example of stream capture. It attains different levels of divergence between forms in the Río Yaqui, Upper and Lower Río Conchos, headstreams of the Río Fuerte, Río Mayo, Río Mezquital, the interior Laguna de los Mexicanos, Río Nazas and Río Aguanaval. Such divergence seems to have a hub in the Lower Río Conchos that may represent the true Codoma ornata s.s.

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 has numerous strict endemics not shared with the rest of Río Grande/Río Bravo complex, and a handful that are endemic to the broader Río Grande/Río Bravo complex. The lower portion of the basin is ecologically “separate” from the rest by Cañón de Peguis, where the fish fauna of the Rìo Conchos stop, and below where the fish fauna of the Río Grande proper begin. Historically, the sierra has isolated the Conchos from the rest of the Río Grande, allowing divergence of fish fauna. 

Level of taxonomic exploration

Reasonable along the middle and lower courses. Not well known in most headwaters on Sierra Tarahumara.