Boundaries

The ecoregion spans most of the Mexican state of Sonora from the Gulf of California to the west to the Sierra Madre Occidental to the east, the Río Colorado and Río Gila to the north, and the Río del Fuerte to the south. This coastal ecoregion consists of several disjunct regions in southern Arizona, most of Sonora, and a small portion of eastern Chihuahua. Isla Tiburón is also part of this ecoregion.

Topography

The topography ranges from extensive plains to elevations over 2700 m asl in the Sierra Madre Occidental. The northern extent of the ecoregion is defined by the series of mountain ranges, including the Gila, Cabeza Prieta, Sierra Pinta, Comobabi, and Baboquivari in Arizona. The Continental Divide forms the eastern limit of the ecoregion. Much of the bedrock is comprised of granite and gneiss, with areas of volcanic origin. The largest volcanic mountain is Pinicate Peak. The ecoregion is also noted for the Gran Desierto in northwestern Sonora, the most extensive desert dune system in North America.

Freshwater habitats

Freshwater habitats include high mountain streams and creeks, usually with boulder, gravel, or sandy bottoms. Silty lowland small streams also occur.

Terrestrial habitats

Ironwood and bristebush form the dominant desert scrub communities within the interior of the ecoregion. Other notable species include organ pipe and Saguaro cacti, the latter of which reaches its southern limit near Hermosillo. Along the coast sparsely vegetated communities with copal, torchwood, cardón, ironwood, palo verde, mesquite, and ocotillo, are common. The coast also contains a high degree of plant species endemism.

Description of endemic fishes

Species that are strictly endemic to the ecoregion include the Opata sucker and desert chub. Other species that are near-endemic include the Sonora chub, Sonora sucker, headwater livebearer, Gila topminnow, blackstripe livebearer, green guapote, Sonoyta pupfish, and Mexican roundtail chub.

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 delineation of this ecoregion was based on species endemism as well as speciation in Poeciliopsis, where females give birth only to females, although incorporating genes from available Poeciliopsis males.

Level of taxonomic exploration

Poor in the mountainous region of the upper Río Yaqui, below the plateau.