Boundaries

The Mayrán-Viesca ecoregion is located in north-central Mexico, and crosses the states of Chihuahua, Coahuila, Durango, Zacatecas, San Luis Potosí, and Nuevo León. It is bounded to the west by the Sierra Madre Occidental and to the east by the Sierra Madre Oriental.

Topography

The topography of the ecoregion ranges from flat plains in the interior of the ecoregion to peaks reaching upwards of 3500 m in the Sierra Madre Occidental. Additionally, the Continental Divide borders the ecoregion in the southwest.

Freshwater habitats

This ecoregion is characterized by closed-basin streams and spring environments, all associated with high levels of endemism. Unique and isolated environments encourage the development of highly specialized biota and speciation by extant species to fill ecological niches.

Terrestrial habitats

Most of the ecoregion occurs within the Chihuahuan desert, which is one of the most biologically diverse arid ecoregions in the world. The Sierra Madre Occidental pine-oak forests in the western part of the ecoregion are also exceptionally diverse, and are characterized by species such as Lumholtz's pine or pino triste (Pinus lumholtzii). Pockets of Sierra Madre Oriental pine-oak forest also occur throughout the ecoregion. The southern portion of the ecoregion is characterized by Meseta Central matorral.

Description of endemic fishes

Endemics to the ecoregion include the Nazas sucker (Catostomus nebuliferus), bold characodon (Characodon audax), Tepehuan shiner (Cyprinella alvarezdelvillari), Gibbous shiner (C. garmani), Mezquital pupfish (Cyprinodon meeki), Nazas pupfish (C. nazas), Nazas chub (Gila conspersa), Durango shiner (Notropis aulidion), and Nazas shiner (N. nazas). The bold characodon, Mezquital pupfish, and Nazas shiner are restricted to the upper Rìo Mezquital. Other species, such as the Nazas pupfish and Gibbous shiner, are shared with the Parras basin. The stumptooth minnow (Stypodon signifier) and Parras pupfish (Cyprinodon latifasciatus) are extinct, but were once located in the Parras basin.

Ecological phenomena

This ecoregion includes fish fauna of the upper Rìo Mezquital above the canyons (composed of Rio Grande derived fishes [cyprinoids, catostomids, a pupfish, and one percid]) and the Parras basin. Species divergence has taken place due to the fragmentation and isolation of the basins, particularly in the Parras basin, and the derived upper Rìo Mezquital sections.

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 includes a high number of endemic taxa, including one genus (Stypodon). The basins of this ecoregion formed a tributary to the former pluvial Rio Grande basin, where its fauna is considered derived.

Level of taxonomic exploration

Rather complete, except in higher headwaters.