Thamnophis eques megalops

Large range extending from Arizona and New Mexico to central Mexico; populations in the United States have undergone a major decline; status in Mexico is poorly documented but available information suggests an ongoing decline; major threats are loss and degradation of habitat and detrimental effects of non-native species.

The range extends from central and southeastern Arizona and southwestern New Mexico southward through the Mexican states of Sonora, Chihauhua, Durango, Guanajuato, Hidalgo, and San Luis Potosi (Rosen and Schwalbe 1988, Rosen et al. 2001, 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 experiencing declines or are characterized by low density (Rosen et al. 2001). Camp Verde and Sonoita Grassland-Canelo Hills-San Raphael Valley are the only areas with substantial populations (Rosen et al. 2001).

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). A record from a single locality along Mule Creek is the only recent evidence of the presence of this species in New Mexico, but the current status of that population is unknown (Center for Biological Diversity 2003).

Roughly 20 occurrences remain in the United States (Rosen et al. 2001, Center for Biological Diversity 2003); many more exist in the major portion of the range in Mexico.

The decline 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). Collecting currently does not appear to be a significant threat, but the small isolated populations of the species are vulnerable to illegal collection or intentional take (Painter 2000).