Thamnophis gigas


Taxonomy [top]

Kingdom Phylum Class Order Family

Scientific Name: Thamnophis gigas
Species Authority: Fitch, 1940
Common Name/s:
English Giant Gartersnake, Giant Garter Snake
Taxonomic Notes: Thamnophis hammondii, T. gigas, and T. atratus (including subspecies hydrophilus and aquaticus) formerly were included in Thamnophis couchii but now are recognized as distinct species (Rossman et al. 1996; Crother et al. 2000, 2003; Ernst and Ernst 2003).

Assessment Information [top]

Red List Category & Criteria: Vulnerable B1ab(ii,iii,iv,v) ver 3.1
Year Published: 2007
Date Assessed: 2007-03-01
Assessor/s: Hammerson, G.A.
Reviewer/s: Cox, N., Chanson, J.S. & Stuart, S.N. (Global Reptile Assessment Coordinating Team)
Listed as Vulnerable because its area of occupancy is less than 2,000 km², its distribution is severely fragmented, and there is continuing decline in its area of occupancy, the extent and quality of its habitat, the number of locations, and the number of mature individuals.
1996 Vulnerable
1994 Rare (Groombridge 1994)
1990 Rare (IUCN 1990)
1988 Rare (IUCN Conservation Monitoring Centre 1988)
1986 Rare (IUCN Conservation Monitoring Centre 1986)

Geographic Range [top]

Range Description: This species is endemic to the west of the United States. Historically the range included much of the floor of the Central Valley (Sacramento and San Joaquin valleys) of California, from Butte County in the north to Kern County in the south, at elevations from near sea level to 122 m (Rossman et al. 1996, Ernst and Ernst 2003, Stebbins 2003). The species apparently is now extirpated from most of the range in the San Joaquin Valley (Rossman et al. 1996, USFWS 1993, Stebbins 2003). Extant populations are distributed in portions of the rice production zones of Sacramento, Sutter, Butte, Colusa, and Glenn counties; along the western border of the Yolo Bypass in Yolo County; and along the eastern fringes of the Sacramento-San Joaquin Delta from the Laguna Creek-Elk Grove region of central Sacramento county southward to the Stockton area of San Joaquin County (Hansen, cited by USFWS 1993). As of 1992, there were 13 known populations as follows (population = cluster of locality records in a contiguous habitat): (1) Butte Basin, (2) Colusa Basin, (3) Sutter Basin, (4) American Basin, (5) Yolo Basin-Willow Slough, (6) Yolo Basin-Liberty Farms, (7) Sacramento Basin, (8) Badger Creek-Willow Creek, (9) Caldoni Marsh, (10) East Stockton-Diverting Canal and Duck Creek, (11) North and South Grasslands (extirpated?), (12) Mendota (extirpated?), and (13) Burrell-Lanare (probably extirpated) (USFWS 1993). These population largely coincide with historical riverine flood basins and tributary streams. Populations 1 to 4 are associated with rice production zones; populations 5 to 13 mainly are in small, isolated patches of valley floor habitat.
United States (California)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: As of 1992, there were 13 known populations (population = cluster of locality records in a contiguous habitat) (USFWS 1993). Not all of the known populations have good viability. The adult population size is unknown but presumably is at least a few thousand. Estimates of population size for three local populations in the mid-1990s were in the low 100s (USFWS 1999). The species is now apparently extirpated or very rare in most of the former range in the San Joaquin Valley. Surveys in the 1970s and 1980s yielded some previously unknown localities and several cases of extirpation or at least severe population declines (USFWS 1993). The area of occupancy, number of subpopulations, and population size are probably continuing to decline, but the rate of decline is unknown.
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: The habitat of this highly aquatic species includes primarily marshes and sloughs, sometimes low-gradient streams, ponds, and small lakes, with cattails, bulrushes, willows, or other emergent or water-edge vegetation usually present and used for basking and cover (California Department of Fish and Game 1990, USFWS 1993, Rossman et al. 1996, Stebbins 2003). Because of the direct loss of natural habitat, this snake now relies heavily on rice fields in the Sacramento Valley, but it also uses managed marsh areas in various national wildlife refuges and state wildlife areas (USFWS 1999). "Essential habitat components consist of: (1) adequate water during the snake's active season (early spring through mid-fall) to provide adequate permanent water to maintain dense populations of food organisms; (2) emergent, herbaceous wetland vegetation, such as cattails and bulrushes, for escape cover and foraging habitat during the active season; (3) upland habitat with grassy banks and openings in waterside vegetation for basking; and (4) higher elevation upland habitats for cover and refuge from flood waters during the snake's inactive season in the winter" (from USFWS 1999). The Giant Garter Snake is absent from large rivers and other waters with populations of large, introduced, predatory fishes, and from wetlands with sand, gravel, or rock substrates (see USFWS 1993). Riparian woodlands do not provide suitable habitat because of excessive shade and inadequate prey resources (Hansen and Brode 1980).
Systems: Terrestrial; Freshwater

Threats [top]

Major Threat(s): Loss and fragmentation of wetland habitats have extirpated the Giant Garter Snake from the majority of its historical range (USFWS 1999). Loss and degradation of habitat remain the greatest threat to the survival of the species (USFWS 1999). Activities that may degrade habitat include maintenance of flood control and agricultural waterways, weed abatement, rodent control, discharge of contaminants into wetlands and waterways, and overgrazing in wetland or streamside habitats. Factors that may be significant in some areas include predation by and competition with introduced species, parasitism, and road kills (USFWS 1999, Carpenter et al. 2002). USFWS (1993) listed threats as habitat loss (e.g., through large-scale urbanization in the American Basin, dewatering of habitat through water diversions and impoundments), flooding (in rice production areas), contaminants (e.g., selenium and salinity in North and South Grassland areas), agricultural and vegetation maintenance activities (e.g., on levees and canal borders), vehicular traffic (on levees and roads along canals), livestock grazing, and introduced predators (e.g., house cats, bullfrogs, perhaps bass). See USFWS (1993) for information on threats to specific populations.

Conservation Actions [top]

Conservation Actions: The draft recovery plan (USFWS 1999) listed the following needed conservation actions: 1. Protect existing populations and habitat. 2. Restore populations to former habitat. 3. Survey to determine species distributions. 4. Monitor populations. 5. Conduct necessary research, including studies on demographics, population genetics, and habitat use. 6. Develop and implement incentive programs, and an outreach and education plan. Protection of waterfowl habitats may allow this species to survive in the Delta and Sacramento Valley regions (Hansen and Brode 1980, Rossman et al. 1996).
Citation: Hammerson, G.A. 2007. Thamnophis gigas. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <>. Downloaded on 19 April 2014.
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