Thamnophis eques

This species exhibits significant morphological variation. For example, based on morphological differences in coloration and pattern, Conant (2003) described seven new subspecies from the transvolcanic belt in Mexico, each from a different lake or from the remnants of a former large lake. However, the phylogeography of this species, based on patterns of genetic variation, has not yet been elucidated, so the validity of nominal subspecies as significant lineages is uncertain.

Wide range from Arizona and New Mexico to Oaxaca, Mexico; documented declines in the number of U.S. populations and abundance, with substantial range contractions in Arizona; threats are high and ongoing in the United States; likely declining in Mexico, but status there is poorly documented; major threats are loss and degradation of habitat and detrimental effects of non-native species.

Range extends from central and southeastern Arizona (now rare; Rosen et al. 2001) and extreme southwestern New Mexico (Degenhardt et al. 1996) south through the highlands of western and southern Mexico to Oaxaca and Veracruz, at elevations of 53-2,590 meters (175-8,500 feet) (Rosen and Schwalbe 1988, Rossman et al. 1996, Stebbins 2003). The Mexican distribution extends down the Sierra Madre Occidental and western edge of the Chihuahuan Desert to the southern mountains of the Mexican altiplano around Mexico City. There is an isolated population in Sierra Madre del Sur (Rosen and Schwalbe 1988). See De Queiroz and Smith (1996) for evidence of the former occurrence of this species along the Colorado River as far north as southern Nevada (and presumably adjacent Arizona). There is an unsubstantiated report from southwestern Texas (Rosen and Schwalbe 1988), but there is no evidence that this represents a valid natural occurrence of this species.

In New Mexico, this snake is known from the lower Gila River basin, along Duck and Mule creeks in Grant County and near Virden in Hildago County (Hubbard and Eley 1985, cited by New Mexico Department of Game and Fish 1997); it may now be eliminated from Duck Creek (New Mexico Department of Game and Fish 1997), and a record from a single locality along Mule Creek is the only recent record (Center for Biological Diversity 2003).

In southeastern Arizona, populations occur at the San Bernardino National Wildlife Refuge, Finley Tank (Audubon Society's Appleton-Whittell Research Ranch), Scotia Canyon (Huachuca Mountains), San Raphael Valley, Canelo Hills, Sonoita Grasslands, Babocomari, Ciénega Creek, Arivaca Cienega, sites on the San Pedro River, and Huachuca Mountain bajada. However, most of these populations are small or declining (Rosen et al. 2001).

This species is represented by a large number of occurrences or subpopulations, most of which are in Mexico. In the United States, roughly 20 populations may be extant, virtually all in Arizona (Rosen et al. 2001, Center of Biological Diversity 2003).

Habitat loss and degradation and detrimental effects of introduced non-native species are the primary threats (USFWS 2008). Populations of T. e. megalops in Mexico "are likely suffering from human impacts on wetlands such as lakes drying up, pollution, and alteration of shoreline (Manjarrez and Drummond, pers. comm.)" (Center for Biological Diversity 2003).

The decline in the United States (and likely Mexico) is closely associated with the loss, alteration, dewatering, siltation, and pollution of streams, wetlands, and riparian zones (resulting from urbanization, water diversion, groundwater pumping, cattle grazing, and other factors). Cover is a critical habitat component. Removal of cover by grazing for even a single season could eliminate a population. Especially vulnerable to grazing are small, isolated populations in areas of limited habitat (Rosen and Schwalbe 1988).The decline of native frogs and native fishes, which serve as critical food resources for the snake, is another significant factor in the decline of the gartersnake. Additionally, the gartersnake is negatively affected by predation and competition, particularly from the non-native bullfrog and also by predation by domestic geese and introduced fishes (e.g., catfish, bass, green sunfish, pike) (Rosen and Schwalbe 1988, Kulby 1995, New Mexico Department of Game and Fish 1997, Center for Biological Diversity 2003, USFWS 2008).