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# of Endemic Species
Major Habitat Type:
xeric freshwaters and endorheic (closed) basins
Salvador Contreras Balderas, Universidad Autónoma de Nuevo León. Additional text was modified from Abell et al. 2000. Freshwater Ecoregions of North America: A Conservation Assessment
Mexico; United States
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.
Drainages flowing into:
Gulf of California
Main rivers or other water bodies:
The ecoregion contains fragmentary or small basins, running from Río Sonoyta to Río Mayo. The large central basin is Río Yaqui (this does not include the headwaters east of the Sierra Madre that were former tributaries of the Río Grande).
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.
The ecoregion experiences dry, warm winters, and moderately warm, wet summers, with temperatures moderated by the Gulf of California. The mean annual temperature ranges between 11-26 ºC, and mean annual precipitation ranges between 59-1060 mm. Rainfall occurs in a bimodal pattern, with frontal systems in the late fall and early winter, and sporadic thunderstorms in the late summer. Rainfall is particularly sporadic along the coast, and generally increases as one moves west to east, with greater rainfall at higher elevations.
Freshwater habitats include high mountain streams and creeks, usually with boulder, gravel, or sandy bottoms. Silty lowland small streams also occur.
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.
The Sonoran Desert is one of four deserts in North America. Unlike the Chihuahuan ecoregions further to the east, the Sonoran does not exhibit the same remarkable aquatic biodiversity and endemism. In fact, the freshwater biota of this region is quite depauperate. Characteristic fish species include primary species confined to freshwater, such as the longfin dace (Agosia chrysogaster), Mexican stoneroller (Campostoma ornatum), beautiful shiner (Cyprinella Formosa), Sonora chub (Gila ditaenia), desert chub (G. eremica), and Mexican roundtail chub (G. minacae) (Cyprinidae); Yaqui sucker (Catostomus bernardini), Sonora sucker (C. insignis), and Opata sucker (C. wigginsi) (Catostomidae); and Yaqui catfish (Ictalurus pricei) (Ictaluridae). Secondary, or euryhaline, species include the headwater livebearer (Poeciliopsis monacha), Gila topminnow (P. occidentalis), and blackstripe livebearer (P. prolifica) (Poeciliidae); desert pupfish (Cyprinodon macularius) and Sonoyta pupfish (C. eremus) (Cyprinodontidae); and green guapote (Cichlasoma beani) (Cichlidae). Periphreal species (of marine ancestry) include the Pacific gizzard shad (Dorosoma smithi) (Clupeidae). For the most part, the aquatic fauna of the Sonoran closely resembles that of the Rio Grande complex. The most likely reason for this is that at some time in the past, the former headwaters of the Río Conchos were captured by the Yaqui system.
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.
Other noteworthy aquatic biotic elements:
There are at least three endemic snails: the San Bernardino springsnail (Fontelicella sp.), the Yepomera springsnail (Fontelicella sp.), and the Yepomera tryonia (Tryonia sp.). There are no known endemic crayfish or aquatic herpetofauna.
Through speciation, the genera Catostomus, and particularly Poeciliopsis, have developed all-female strains or species, and are able to incorporate paternal traits. Also, a fish fauna that inhabits the upper headstreams on the plateau (mostly derived from Río Grande stocks) is distinct from the one along the middle and lower reaches (related to Río Colorado).
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.
Abell, R. A., Olson, D. M., et al. (2000). "Freshwater Ecoregions of North America: A Conservation Assessment" Washington, DC, USA: Island Press.
Campoy-Favela, J., Varela-Romero, A., et al. (1989). "Observaciones sobre la ictiofauna nativa de la cuenca del Río Yaqui, Sonora, México" Ecológica (Hermosillo, Sonora, México) 1 1-14.
Hendrickson, D. A. (1983). "Distribution records of native and exotic fishes in Pacific drainages of Northern Mexico" Journal of the Arizona-Nevada Academy of Science 18(2) 33-38.
Hendrickson, D. A., Minckley, W. L., et al. (1980). "Fishes of the Rio Yaqui Basin, Mexico and United States" Journal of the Arizona-Nevada Academy of Science 18(2) 65-106.
Juárez-Romero, L., Varela-Romero, A., et al. (1988). "Observaciones preliminares sobre la ictiofauna del Río Mátape, Sonora, México" Mem. IX Cong. Nal. Zool., Villahermosa, México 2 27-33.
Miller, R. R., Minckley, W. L., et al. (2005). "Fishes of México" Chicago Univ. Press.