Center for Biological Diversity

Protecting endangered species and wild places through
science, policy, education, and environmental law.

For immediate release: December 15, 2003

Contact: Noah Greenwald: 503-243-6643 or 503-484-7495 (cell)



The Center for Biological Diversity filed a petition today to list the Mexican garter snake as endangered under the Endangered Species Act (ESA). The species is an aquatic garter snake with a range-wide distribution in Arizona, southwest New Mexico, and Mexico. It is one of hundreds of native riparian species that are threatened by the destruction and degradation of rivers and streams in the Southwest. Under the ESA, the U.S. Fish and Wildlife Service has one year to respond to the petition and determine if listing of the snake is warranted.

The Mexican garter snake has been extirpated from most of its U.S. range, including the Colorado, Gila, and much of the Santa Cruz and San Pedro Rivers. The decline of the Mexican garter snakes is closely linked to the deteriorating quality of streamside habitats and the disappearance of native frogs and native fishes. “Widespread degradation of southwest rivers and introduction of dozens of exotic species necessitates protection of the Mexican garter snake under the ESA,” states Noah Greenwald, conservation biologist for the Center for Biological Diversity.

Populations of the Mexican garter snake are severely fragmented and isolated due to loss and destruction of suitable habitat, which consists of riparian areas with permanent water, streamside vegetation for cover, and native prey species. Livestock grazing, urbanization, pollution, loss of native prey species, and exotic species have resulted in the loss of greater than 90% of the Southwest’s riparian habitat and the listing of 30 species under the ESA. “Southwest rivers have been under massive assault for over a century,” states Greenwald. “To protect southwest riparian species, livestock must be removed from all southwest rivers and streams, instream flows must be established, and further introduction of non-native species must be prohibited.”

“Without ESA listing and critical habitat designation, the Mexican garter snake population will continue in its dangerous trajectory of local extirpations and decreased range,” concludes Greenwald. Listing of the Mexican garter snake will facilitate conservation of riparian habitats and species by prohibiting activities that result in destruction of habitat, such as livestock grazing and groundwater pumping, directing federal funding towards removal of non-native species, and encouraging additional research on the status of the species.

Mexican garter snakes reach a maximum length of one meter, range in color from olive to olive-brown to olive-gray, and have three stripes that run the length of the body. They feed primarily on native frogs and fish, but will also occasionally eat lizards and mice. A picture of the garter snake and a map of its range are available on request.


The Mexican garter snake, Thamnophis eques megalops (T. e. megalops), is an aquatic garter snake found in Arizona, New Mexico, and Mexico. T. e. megalops is a subspecies of T. eques first documented by R. Kennicott in 1860. The species is an olive-brown color with three bright lateral stripes. Habitat requirements include: permanent water, vegetative cover, and native prey species.

Thamnophis eques megalops qualifies as a Distinct Vertebrate Population Segment (DPS) under the Endangered Species Act. The U.S./Mexico international border creates a discreet population with international differences in regulatory mechanisms, quality of information, habitat occupied, and types of threat. The Mexican garter snake in the U.S. is significant because (1) their loss would create a significant gap in the species distribution, including the entire distribution of the species in the U.S. (2) the U.S. populations are likely genetically distinct (3) the Mexican garter snake is an indicator species for the health of southwest riparian ecosystems. The USFWS may also list the Mexican garter snake as Endangered or Threatened throughout its entire range.

T. e. megalops ranges from central and southern Arizona, to southwestern New Mexico, and into Mexico in the states of Sonora, Chihauhua, Durango, Guanajuato, Hidalgo, and San Luis Potosi. Exact population numbers are unknown, but population decline is documented in New Mexico and Arizona. Populations of Mexican garter snakes have been extirpated from Tucson, Phoenix, along the Colorado River, Lower Santa Cruz River, Salt River, Gila River, San Pedro Valley, and along the Gila River and Duck Creek in New Mexico.

Current threats to the Mexican garter snake include: (1) destruction, modification, and curtailment of its habitat and range, (2) illegal collection or intentional take, (3) predation from non-native species, (4) inadequacy of existing regulatory mechanisms, and (5) competition with Checkered garter snakes.

The destruction, modification, and curtailment of its habitat and range are caused by urbanization, loss of wetlands, loss of native prey species, livestock grazing, and climate change. Development of the southwest has caused a significant loss of wetlands and riparian areas and degraded suitable habitat. Ciénegas, the ideal habitat for T. e. megalops in Arizona, are known to be in decline. Climate change and drought are negatively affecting the riparian habitat of the Mexican garter snake. The Mexican garter snake is particularly sensitive to the modification of habitat from livestock grazing and the loss of native prey species.

Non-native bullfrogs are known to prey on the Mexican garter snake and extirpate local populations. In addition, the bullfrog is a predator to the Mexican garter snake’s native prey. Bullfrogs have been known to prey on and extirpate leopard frogs, one of the Mexican garter snake’s key food sources. The Mexican garter snake is particularly sensitive to the loss of native prey species.

Currently, the Mexican garter snake receives no federal protection. In New Mexico, the species is listed as Endangered and in Arizona, the Mexican garter snake is listed as a “Species of Special Concern.” It is listed as “vulnerable to extirpation or extinction” in the Natural Heritage Database. However, these listings are inconsequential as neither state has produced any conservation programs or recovery plans or taken substantial action to protect the species’ habitat. The Mexican garter snake is co-existent with other species listed on the ESA, yet this co-occurrence has not halted decline.

Additional conservation measures are recommended by the Center for Biological Diversity to insure recovery of the Mexican garter snake. These measures include: (1) increase surveys of the Mexican garter snake to accurately determine population status and distribution, (2) eliminate non-native predators including bullfrogs and predatory fish, (3) fence wetlands with existing Mexican garter snake populations to sustain and propagate for reintroduction, (4) protect and conserve the species’ food base including native leopard frogs and native fish, (5) manage lands to preserve vegetative cover and manage perennial flows for lakes, rivers, streams, and ciénegas in current and historical habitat, (6) create an international agreement with Mexico to protect and conserve T. e. megalops.

Petitioners request critical habitat designation concurrent with ESA listing. Critical habitat should be connected vegetated riparian areas that include floodplains and watersheds in current and historic habitat of the Mexican garter snake.


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