56228 Federal Register / Vol. 71, No. 186 / Tuesday, September 26, 2006 / Proposed Rules 

DEPARTMENT OF THE INTERIOR 

Fish and Wildlife Service 

50 CFR Part 17 

Endangered and Threatened Wildlife 

and Plants; 12-Month Finding on a 

Petition To List the Northern Mexican 

Gartersnake (Thamnophis eques 

megalops) as Threatened or 

Endangered With Critical Habitat 

AGENCY: Fish and Wildlife Service, 

Interior. 

ACTION: Notice of 12-month petition 

finding. 

SUMMARY: We, the U.S. Fish and 

Wildlife Service (Service), announce a 

12-month finding on a petition to list 

the northern Mexican gartersnake 

(Thamnophis eques megalops) as 

threatened or endangered with critical 

habitat under the Endangered Species 

Act of 1973, as amended (Act). The 

petitioners provided three listing 

scenarios for consideration by the 

Service: (1) Listing the United States 

population as a Distinct Population 

Segment (DPS); (2) listing Thamnophis 

eques megalops throughout its range in 

the United States and Mexico based on 

its rangewide status; or (3) listing 

Thamnophis eques megalops 

throughout its range in the United States 

and Mexico based on its status in the 

United States. After thorough analysis 

and review of all available scientific and 

commercial information, we find that 

listing of the subspecies, under any of 

the three scenarios, is not warranted. Of 

the three listing scenarios specified 

above, we found scenario two provided 

the most rigorous evaluation of the 

status of the northern Mexican 

gartersnake and herein provide detailed 

discussion of our conclusions in that 

context. We also provide additional 

discussion of our evaluation of 

scenarios (1) listing the United States 

population as a DPS and (3) listing 

Thamnophis eques megalops 

throughout its range in the United States 

and Mexico based on its status in the 

United States. 

DATES: The finding announced in this 

document was made on September 26, 

2006. 

ADDRESSES: The complete supporting 

file for this finding is available for 

inspection, by appointment, during 

normal business hours at the Arizona 

Ecological Services Office, 2321 West 

Royal Palm Road, Suite 103, Phoenix, 

AZ 85021–4951. Please submit any new 

information, materials, comments, or 

questions concerning this species or this 

finding to the above address. 

FOR FURTHER INFORMATION CONTACT: 

Steve Spangle, Field Supervisor, 

Arizona Ecological Services Office (see 

ADDRESSES) 602–242–0210. 

SUPPLEMENTARY INFORMATION: 

Background 

Section 4(b)(3)(B) of the Act (16 

U.S.C. 1531 et seq.), requires that, for 

any petition to revise the Lists of 

Threatened and Endangered Wildlife 

and Plants that contains substantial 

scientific and commercial information 

that listing may be warranted, we make 

a finding within 12 months of the date 

of receipt of the petition on whether the 

petitioned action is (a) not warranted, 

(b) warranted, or (c) warranted, but that 

the immediate proposal of a regulation 

implementing the petitioned action is 

precluded by other pending proposals to 

determine whether any species is 

threatened or endangered, and 

expeditious progress is being made to 

add or remove qualified species from 

the Lists of Endangered and Threatened 

Wildlife and Plants. Section 4(b)(3)(C) of 

the Act requires that a petition for 

which the requested action is found to 

be warranted but precluded be treated 

as though resubmitted on the date of 

such finding, i.e., requiring a 

subsequent finding to be made within 

12 months. Each subsequent 12-month 

finding will be published in the Federal 

Register. 

On December 19, 2003, we received a 

petition dated December 15, 2003, 

requesting that we list the northern 

Mexican gartersnake as threatened or 

endangered, and that we designate 

critical habitat concurrently with the 

listing. The petition, submitted by the 

Center for Biological Diversity, was 

clearly identified as a petition for a 

listing rule and contained the names, 

signatures, and addresses of the 

requesting parties. Included in the 

petition was supporting information 

regarding the species’ taxonomy and 

ecology, historical and current 

distribution, present status, and actual 

and potential causes of decline. We 

acknowledged the receipt of the petition 

in a letter to Mr. Noah Greenwald, dated 

March 1, 2004. In that letter, we also 

advised the petitioners that, due to 

funding constraints in fiscal year (FY) 

2004, we would not be able to begin 

processing the petition at that time. 

On May 17, 2005, the petitioners filed 

a complaint for declaratory and 

injunctive relief, challenging our failure 

to issue a 90-day finding in response to 

the petition as required by 16 U.S.C. 

1533(b)(3)(A) and (B). In a stipulated 

settlement agreement, we agreed to 

submit a 90-day finding to the Federal 

Register by December 16, 2005, and if 

positive, submit a 12-month finding to 

the Federal Register by September 15, 

2006 [Center for Biological Diversity v. 

Norton, CV–05–341–TUC–CKJ (D. Az)]. 

The settlement agreement was signed 

and adopted by the District Court of 

Arizona on August 2, 2005. 

On December 13, 2005, we made our 

90-day finding that the petition 

presented substantial scientific 

information indicating that listing the 

northern Mexican gartersnake 

(Thamnophis eques megalops) may be 

warranted, but we did not discuss the 

applicability of any of the three listing 

scenarios that were provided in the 

petition. The finding and our initiation 

of a status review was published in the 

Federal Register on January 4, 2006 (71 

FR 315). We are required, under the 

court-approved stipulated settlement 

agreement, to submit to the Federal 

Register our 12-month finding pursuant 

to the Act [16 U.S.C. 1533(b)(3)(B)] on 

or before September 15, 2006. This 

notice constitutes our 12-month finding 

for the petition to list the northern 

Mexican gartersnake as threatened or 

endangered. 

Previous Federal Actions 

The Mexican gartersnake 

(Thamnophis eques) (which included 

the subspecies) was placed on the list of 

candidate species as a Category 2 

species in 1985 (50 FR 37958). Category 

2 species were those for which existing 

information indicated that listing was 

possibly appropriate, but for which 

substantial supporting biological data to 

prepare a proposed rule were lacking. In 

the 1996 Candidate Notice of Review 

(February 28, 1996; 61 FR 7596), the use 

of Category 2 candidates was 

discontinued, and the northern Mexican 

gartersnake was no longer recognized as 

a candidate. In addition, on January 4, 

2006, we published a 90-day finding on 

a petition to list the northern Mexican 

gartersnake (71 FR 315), as discussed 

above. 

Biology 

Species Description. The northern 

Mexican gartersnake may occur with 

other native gartersnake species and can 

be difficult for people without 

herpetological expertise to identify. 

With a maximum known length of 44 

inches (in) (112 centimeters (cm)), it 

ranges in background color from olive to 

olive-brown to olive-gray with three 

stripes that run the length of the body. 

The middle dorsal stripe is yellow and 

darkens toward the tail. The pale yellow 

to light-tan lateral stripes distinguish 

the Mexican gartersnake from other 

sympatric (co-occurring) gartersnake 

species because a portion of the lateral 


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stripe is found on the fourth scale row, 

while it is confined to lower scale rows 

for other species. Paired black spots 

extend along the olive dorsolateral 

fields and the olive-gray ventrolateral 

fields. A conspicuous, light-colored 

crescent extends behind the corners of 

the mouth. The two dark brown to black 

blotches that occur behind the head of 

several gartersnake species may be 

diffuse or absent in the Mexican 

gartersnake. The coloration of the venter 

is bluish-gray or greenish-grey. The 

dorsolateral scalation is keeled, the anal 

plate is single, and there are eight or 

nine upper labial scales (Rosen and 

Schwalbe 1988, p. 4; Rossman et al. 

1996, pp. 171–172). 

Taxonomy. The northern Mexican 

gartersnake is a member of the family 

Colubridae and subfamily Natricinae 

(harmless live-bearing snakes) (Lawson 

et al. 2005, p. 596). The taxonomy of the 

genus Thamnophis has a complex 

history partly because many of the 

species are similar in appearance and 

scutelation (arrangement of scales), but 

also because many of the early museum 

specimens were in such poor and faded 

condition that it was difficult to study 

them (Conant 2003, p. 6). There are 

approximately 30 species that have been 

described in the gartersnake genus 

Thamnophis (Rossman et al. 1996, p. 

xvii–xviii). De Queiroz et al. (2002, p. 

323) identified two large overlapping 

clades (related taxonomic groups) of 

gartersnakes that they called the 

‘‘Mexican’’ and ‘‘widespread’’ clades 

which were supported by allozyme and 

mitochondrial DNA genetic analyses. 

Thamnophis eques is a member of the 

‘‘widespread’’ clade and is most closely 

related taxonomically to, although 

genetically and phenotypically distinct 

from, the checkered gartersnake 

(Thamnophis marcianus) (De Queiroz 

and Lawson 1994, p. 217). 

Rossman et al. (1996, p. 175) noted 

that the current specific name eques was 

not applied at the time of the original 

description of the holotype because the 

specimen was mistakenly identified as a 

black-necked gartersnake (Thamnophis 

cyrtopsis). In recent history and prior to 

2003, Thamnophis eques was 

considered to have three subspecies, T. 

e. eques, T. e. megalops, and T. e. 

virgatenuis (Rossman et al. 1996, p. 

175). T. eques displays considerable 

phenotypic variability (variation in its 

physical appearance) across its 

distribution, and all subspecific 

descriptions under T. eques have been 

based on morphometrics or 

morphological characters. The 

subspecies T. e. eques and T. e. 

megalops are distinguished by average 

differences in sub-caudal scale counts, 

while T. e. virgatenuis is distinguished 

from T. e. megalops based on having a 

darker background color and a narrower 

vertebral stripe (Rossman et al. 1996, p. 

175). Rossman et al. (1996, p. 175) also 

noted that the discontinuous 

distributions of high-elevation and lowelevation T. e. virgatenuis and T. e. 

megalops, respectively, are 

‘‘zoogeographically peculiar and unique 

among gartersnakes.’’

Rossman et al. (1996, p. 172) describe 

the distribution of T. e. eques as 

occurring from southern Nayarit 

eastward along the Transverse Volcanic 

Axis to west-central Veracruz, and 

identified an additional disjunct 

population in central Oaxaca. T. e. 

virgatenuis is distributed in three 

isolated, high-elevation populations in 

southwestern Durango and in westcentral and northwestern Chihuahua 

(Rossman et al. 1996, p. 172). 

In 2003, an additional seven new 

subspecies were identified under T. 

eques: (1) T. e. cuitzeoensis; (2) T. e. 

patzcuaroensis; (3) T. e. inspiratus; (4) 

T. e. obscurus; (5) T. e. diluvialis; (6) T. 

e. carmenensis; and (7) T. e. scotti 

(Conant 2003, p. 3). These seven new 

subspecies were described based on 

morphological differences in coloration 

and pattern; have high endemism 

(degree of restriction to a particular 

area) with highly restricted 

distributions; and occur in isolated 

wetland habitats within the 

mountainous Transvolcanic Belt region 

of southern Mexico, which contains the 

highest elevations in the country 

(Conant 2003, pp. 7–8). We are not 

aware of any challenges within the 

scientific literature of the validity of 

current taxonomy of any of the 10 

subspecies of T. eques. 

The most widely distributed of the 10 

subspecies under Thamnophis eques is 

the northern Mexican gartersnake 

(Thamnophis eques megalops), which is 

the only subspecies that occurs in the 

United States and the entity we address 

in this finding. In Mexico, T. e. 

megalops historically occurred 

throughout the Sierra Madre Occidental 

south to Guanajuato, and east across the 

Mexican Plateau to Hidalgo, which 

comprised approximately 85 percent of 

the total rangewide distribution of the 

species (Rossman et al. 1996, p. 173). 

Robert Kennicott first described the 

northern Mexican gartersnake in 1860, 

as Eutenia megalops from the type 

locality of Tucson, Arizona (Rosen and 

Schwalbe 1988, p. 2). In 1951, Dr. 

Hobart Smith renamed the subspecies 

with its current scientific name (Rosen 

and Schwalbe 1988, p. 3). A summary 

of this species’ lengthy taxonomic 

history can be found in Rosen and 

Schwalbe (1988, pp. 2–3). Several 

common names have been applied to 

the northern Mexican gartersnake in the 

United States over the years, such as the 

Arizona ribbon snake, the Emory’s 

gartersnake, and the Arizona gartersnake 

(Rosen and Schwalbe 1988, p. 2). 

In summary, while the taxonomic 

history of Thamnophis eques is robust, 

we found no indication in the 

significant body of taxonomic literature 

we reviewed that its current taxonomy 

is in doubt or in any way invalid (Rosen 

and Schwalbe 1988, pp. 2–3; De Queiroz 

and Lawson 1994, pp. 215–217; Liner 

1994, p. 107; Rossman et al. 1996, pp. 

171, 175; Conant 2003, p. 6; Crother et 

al. 2000, p. 72; 2003, p. 202; De Queiroz 

et al. 2002, p. 327). 

Habitat. Throughout its rangewide 

distribution, the northern Mexican 

gartersnake occurs at elevations from 

130 to 8,497 feet (ft) (40 to 2,590 meters 

(m)) (Rossman et al. 1996, p. 172). The 

northern Mexican gartersnake is 

considered a riparian obligate (restricted 

to riparian areas when not engaged in 

dispersal behavior) and occurs chiefly 

in the following general habitat types: 

(1) Source-area wetlands [e.g., cienegas 

(mid-elevation wetlands with highly 

organic, reducing (basic, or alkaline) 

soils), stock tanks (small earthen 

impoundment), etc.]; (2) large river 

riparian woodlands and forests; and (3) 

streamside gallery forests (as defined by 

well-developed broadleaf deciduous 

riparian forests with limited, if any, 

herbaceous ground cover or dense grass) 

(Hendrickson and Minckley 1984, p. 

131; Rosen and Schwalbe 1988, pp. 14–

16; Arizona Game and Fish Department 

2001). Vegetation characteristics vary 

based on the type of habitat. For 

example, in source-area wetlands, dense 

vegetation consists of knot grass 

(Paspalum distichum), spikerush 

(Eleocharis), bulrush (Scirpus), cattail 

(Typha), deergrass (Muhlenbergia), 

sacaton (Sporobolus), Fremont 

cottonwood (Populus fremontii), 

Goodding’s willow (Salix gooddingii), 

and velvet mesquite (Prosopis velutina) 

(Rosen and Schwalbe 1988, pp. 14–16). 

In riparian woodlands consisting of 

cottonwood and willow or gallery 

forests of broadleaf and deciduous 

species along larger rivers, the northern 

Mexican gartersnake may be observed in 

mixed grasses along the bank or in the 

shallows (Rossman et al. 1996, p. 176; 

Rosen and Schwalbe 1988, p. 16). 

Within and adjacent to the Sierra Madre 

Occidental in Mexico, it occurs in 

montane woodland, Chihuahuan 

desertscrub, mesquite-grassland, and 

Cordillera Volca ́nica montane woodland 

(McCranie and Wilson 1987, pp. 14–17). 


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In small streamside riparian habitat, 

this snake is often associated with 

Arizona sycamore (Platanus wrightii), 

sugar leaf maple (Acer grandidentatum), 

velvet ash (Fraxinus velutina), Arizona 

cypress (Cupressus arizonica), Arizona 

walnut (Juglans major), Arizona alder 

(Alnus oblongifolia), alligator juniper 

(Juniperus deppeana), Rocky Mountain 

juniper (J. scopulorum), and a number 

of oak species (Quercus spp.) (McCranie 

and Wilson 1987, pp. 11–12; CirettGalan 1996, p. 156). 

Behavior, Prey Base, and 

Reproduction. The northern Mexican 

gartersnake is surface active at ambient 

temperatures ranging from 71 degrees 

Fahrenheit (°F) to 91 °F [22 degrees 

Celsius (°C) to 33 °C] and forages along 

the banks of waterbodies. The northern 

Mexican gartersnake is an active 

predator and is believed to heavily 

depend upon a native prey base (Rosen 

and Schwalbe 1988, pp. 18, 20). 

Generally, its diet consists 

predominantly of amphibians and 

fishes, such as adult and larval native 

leopard frogs [e.g., lowland leopard frog 

(Rana yavapaiensis) and Chiricahua 

leopard frog (Rana chiricahuensis)], as 

well as juvenile and adult native fish 

species [e.g., Gila topminnow 

(Poeciliopsis occidentalis occidentalis), 

desert pupfish (Cyprinodon 

macularius), Gila chub (Gila 

intermedia), and roundtail chub (Gila 

robusta)] (Rosen and Schwalbe 1988, p. 

18). Auxiliary prey items may also 

include young Woodhouse’s toads (Bufo 

woodhousei), treefrogs (Family Hylidae), 

earthworms, deermice (Peromyscus 

maniculatus), lizards of the genera 

Aspidoscelis and Sceloporus, larval tiger 

salamanders (Ambystoma tigrinum), and 

leeches (Rosen and Schwalbe 1988, p. 

20; Holm and Lowe 1995, pp. 30–31; 

Degenhardt et al. 1996, p. 318; Rossman 

et al. 1996, p. 176; Manjarrez 1998). To 

a much lesser extent, this snake’s diet 

may include nonnative species, 

including juvenile fish, larval and 

juvenile bullfrogs, and mosquitofish 

(Gambusia affinis) (Holycross et al. 

2006, p. 23). 

Sexual maturity in northern Mexican 

gartersnakes occurs at 2 years of age in 

males and at 2 to 3 years of age in 

females (Rosen and Schwalbe 1988, pp. 

16–17). Northern Mexican gartersnakes 

are ovoviviparous (eggs develop and 

hatch within the oviduct of the female). 

Mating occurs in April and May in their 

northern distribution followed by the 

live birth of between 7 and 26 neonates 

(newly born individuals) (average is 

13.6) in July and August (Rosen and 

Schwalbe 1988, p. 16). Approximately 

half of the sexually mature females 

within a population reproduce in any 

one season (Rosen and Schwalbe 1988, 

p. 17). 

Distribution 

Historical Distribution. The United 

States comprises the northern portion of 

the northern Mexican gartersnake’s 

distribution. Within the United States, 

the northern Mexican gartersnake 

historically occurred predominantly in 

Arizona with a limited distribution in 

New Mexico that consisted of scattered 

locations throughout the Gila and San 

Francisco headwater drainages in 

western Hidalgo and Grant counties 

(Price 1980, p. 39; Fitzgerald 1986, 

Table 2; Degenhardt et al. 1996, p. 317; 

Holycross et al. 2006, pp. 1–2). 

Fitzgerald (1986, Table 2) provided 

museum records for the following 

historical localities for northern 

Mexican gartersnakes in New Mexico: 

(1) Mule Creek; (2) the Gila River, 5 

miles (mi) ( 8 kilometers (km)) east of 

Virden; (3) Spring Canyon; (4) the West 

Fork Gila River at Cliff Dwellings 

National Monument; (5) the Tularosa 

River at its confluence with the San 

Francisco River; (6) the San Francisco 

River at Tub Spring Canyon; (7) Little 

Creek at Highway 15; (8) the Middle Box 

of Gila River at Ira Ridge; (9) Turkey 

Creek; (10) Negrito Creek; and (11) the 

Rio Mimbres. 

Within Arizona, the historical 

distribution of the northern Mexican 

gartersnake ranged from 130 to 6,150 ft 

(40 to 1,875 m) in elevation and spread 

variably based on the relative 

permanency of water and the presence 

of suitable habitat. In Arizona, the 

northern Mexican gartersnake 

historically occurred within several 

perennial or intermittent drainages and 

disassociated wetlands that included: 

(1) The Gila River; (2) the Lower 

Colorado River from Davis Dam to the 

International Border; (3) the San Pedro 

River; (4) the Santa Cruz River 

downstream from the International 

Border; (5) the Santa Cruz River 

headwaters/San Rafael Valley and 

adjacent montane canyons; (6) the Salt 

River; (7) the Rio San Bernardino from 

International Border to headwaters at 

Astin Spring (San Bernardino National 

Wildlife Refuge); (8) Agua Fria River; (9) 

the Verde River; (10) Tanque Verde 

Creek in Tucson; (11) Rillito Creek in 

Tucson; (12) Agua Caliente Spring in 

Tucson; (13) the downstream portion of 

the Black River from the Paddy Creek 

confluence; (14) the downstream 

portion of the White River from the 

confluence of the East and North forks; 

(15) Tonto Creek from the mouth of 

Houston Creek downstream to Roosevelt 

Lake; (16) Cienega Creek from the 

headwaters to the ‘‘Narrows’’ just 

downstream of Apache Canyon; (17) 

Pantano Wash (Cienega Creek) from 

Pantano downstream to Vail; (18) 

Potrero Canyon/Springs; (19) Audubon 

Research Ranch and vicinity near Elgin; 

(20) Upper Scotia Canyon in the 

Huachuca Mountains; (21) Arivaca 

Creek; (22) Arivaca Cienega; (23) 

Sonoita Creek; (24) Babocomari River; 

(25) Babocamari Cienega; (26) Barchas 

Ranch, Huachuca Mountain bajada; (27) 

Parker Canyon Lake and tributaries in 

the Canelo Hills; (28) Big Bonito Creek; 

(29) Lake O’Woods, Lakeside area; (30) 

Oak Creek from Midgley Bridge 

downstream to the confluence with the 

Verde River; and (31) Spring Creek 

above the confluence with Oak Creek 

(Woodin 1950, p. 40; Nickerson and 

Mays 1970, p. 503; Bradley 1986, p. 67; 

Rosen and Schwalbe 1988, Appendix I; 

1995, p. 452; 1997, pp. 16–17; Holm and 

Lowe 1995, pp. 27–35; Sredl et al. 

1995b, p. 2; 2000, p. 9; Rosen et al. 

2001, Appendix I; Holycross et al. 2006, 

pp. 1–2, 15–51; Brennan and Holycross 

2006, p. 123; Radke 2006; Rosen 2006; 

Holycross 2006). 

One record for the northern Mexican 

gartersnake exists for the State of 

Nevada, opposite Fort Mohave, in Clark 

County along the shore of the Colorado 

River (De Queiroz and Smith 1996, p. 

155); however, any populations of 

northern Mexican gartersnakes that may 

have historically occurred in Nevada 

pertained directly to the Colorado River 

and are likely extirpated. 

Within Mexico, northern Mexican 

gartersnakes historically occurred 

within the Sierra Madre Occidental and 

the Mexican Plateau in the Mexican 

states of Sonora, Chihuahua, Durango, 

Coahila, Zacatecas, Guanajuato, Nayarit, 

Hidalgo, Jalisco, San Luis Potosı ́, 

Aguascalientes, Tlaxacala, Puebla, 

Me ́xico, Veracruz, and Quere ́taro, which 

comprises approximately 70 to 80 

percent of its historical rangewide 

distribution (Conant 1963, p. 473; 1974, 

pp. 469–470; Van Devender and Lowe 

1977, p. 47; McCranie and Wilson 1987, 

p. 15; Rossman et al. 1996, p. 173; 

Lemos-Espinal et al. 2004, p. 83). 

Status in the United States. Holycross 

et al. (2006, p. 12) included the northern 

Mexican gartersnake as a target species 

at 33 sites surveyed within drainages 

along the Mogollon Rim. A total of 874 

person-search hours and 63,495 traphours were devoted to that effort, which 

resulted in the capture of 23 snakes total 

in 3 (9 percent) of the sites visited. This 

equates to approximately 0.03 snakes 

observed per person-search hour and 

0.0004 snakes captured per trap-hour 

over the entire effort. For comparison, a 

population of northern Mexican 

gartersnakes at Page Springs, Arizona, 


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that we consider stable yielded 0.22 

snakes observed per person-search hour 

and 0.004 snakes captured per trap-hour 

(an order of magnitude higher) 

(Holycross et al. 2006, p. 23). Survey 

sites were selected based on the 

existence of historical records for the 

species or sites where the species may 

occur based on habitat suitability within 

the historical distribution of the species. 

Holycross et al. (2006, p. 12) calculated 

the capture rates for the northern 

Mexican gartersnake as 12,761 traphours per snake and 49 person-search 

hours per snake. Northern Mexican 

gartersnakes were found at 2 of 11 (18 

percent) historical sites and 1 of 22 (4 

percent) sites where the species was 

previously unrecorded (Holycross et al. 

2006, p. 12). When compared with 

extensive survey data in Rosen and 

Schwalbe (1988, Appendix I), these data 

demonstrate dramatic declines in both 

capture rates and the total number of 

populations of the species in areas 

where multiple surveys have been 

completed over time. However, these 

data may be affected by differences in 

survey efforts and drought. 

In 2000, Rosen et al. (2001, Appendix 

I) resurveyed many sites in southeastern 

Arizona that were historically known to 

support northern Mexican gartersnake 

populations during the early to mid1980s, and also provided additional 

survey data collected from 1993–2001. 

Rosen et al. (2001, pp. 21–22) reported 

their results in terms of increasing, 

stabilized, or decreasing populations of 

northern Mexican gartersnakes. 

Three sites (San Bernardino National 

Wildlife Refuge, Finley Tank at the 

Audubon Research Ranch near Elgin, 

and Scotia Canyon in the Huachuca 

Mountains) were intensively surveyed 

and yielded mixed results. The northern 

Mexican gartersnake population on the 

San Bernardino National Wildlife 

Refuge experienced ‘‘major, 

demonstrable declines’’ to near or at 

extirpation over the span of a decade. 

That population is now considered 

extirpated (Radke 2006). The status of 

the population at Finley Tank is 

uncertain. Scotia Canyon was the last 

area intensively resurveyed by Rosen et 

al. (2001, pp. 15–16). In comparing this 

information with survey data from Holm 

and Lowe (1995, pp. 27–35), northern 

Mexican gartersnake populations in this 

area suggest a possible decline from the 

early 1980s, as evidenced by low 

capture rates in 1993 and even lower 

capture rates in 2000. 

The remaining 13 sites in 

southeastern Arizona resurveyed by 

Rosen et al. (2001, pp. 21–22) also 

yielded mixed results. Population trend 

information is difficult to ascertain 

given the variability of survey sample 

design and effort used by Rosen et al. 

(2001). However, the survey results 

suggested population increases at one 

site (lower Cienega Creek), possible 

stability at two sites (lower San Rafael 

Valley, Arivaca), and negative trends at 

many other sites [Empire-Cienega Creek, 

Babocomari, Bog Hole, O’Donnell Creek, 

Turkey Creek (Canelo), Post Canyon, 

Lewis Springs (San Pedro River), San 

Pedro River near Highway 90, Barchas 

Ranch Pond (Huachuca Mountain 

bajada), Heron Spring, Sharp Spring, 

and Elgin-Sonoita windmill well site 

(San Rafael Valley)] (Rosen et al. 2001, 

pp. 21–22). While this survey effort 

could not confirm any specific 

extirpations of northern Mexican 

gartersnake populations on a local scale 

in southeastern Arizona, most sites 

yielded no snakes during resurvey 

(Rosen et al. 2001, Appendix I). 

Our analysis of the best available data 

on the status of the northern Mexican 

gartersnake distribution in the United 

States indicates that its distribution has 

been significantly reduced in the United 

States, and it is now considered 

extirpated from New Mexico (Nickerson 

and Mays 1970, p. 503; Rosen and 

Schwalbe 1988, pp. 25–26, Appendix I; 

Holm and Lowe 1995, pp. 27–35; Sredl 

et al. 1995b, pp. 2, 9–10; 2000, p. 9; 

Rosen et al. 2001, Appendix I; Painter 

2005, 2006; Holycross et al. 2006, p. 66; 

Brennan and Holycross 2006, p. 123; 

Radke 2006; Rosen 2006; Holycross 

2006). Fitzgerald (1986, pp. 9–10) 

visited 33 localities of potential habitat 

for northern Mexican gartersnakes in 

New Mexico in the Gila River drainage 

and was unable to confirm its existence 

at any of these sites. The New Mexico 

Department of Game and Fish State 

Herpetologist, Charles Painter, provided 

several causes that have synergistically 

contributed to the decline of northern 

Mexican gartersnakes in New Mexico, 

including bullfrog and nonnative fish 

introductions, modification and 

destruction of habitat, commercial 

exploitation, direct human-inflicted 

harm, and fragmentation of populations. 

The last known observation of the 

northern Mexican gartersnake in New 

Mexico occurred in 1994 on private 

land (Painter 2000, p. 36; Painter 2005). 

Our analysis of the best available 

information indicates that the northern 

Mexican gartersnake has likely been 

extirpated from a large portion of its 

historical distribution in the United 

States. We define a population as 

‘‘likely extirpated’’ when there have 

been no northern Mexican gartersnakes 

reported for a decade or longer at a site 

within the historical distribution of the 

species, despite at least minimal survey 

efforts, and natural recovery at the site 

is not expected due to the presence of 

known threats. The perennial or 

intermittent stream reaches and 

disassociated wetlands where the 

northern Mexican gartersnake has likely 

been extirpated include: (1) The Gila 

River; (2) the Lower Colorado River 

from Davis Dam to the International 

Border; (3) the San Pedro River; (4) the 

Santa Cruz River downstream from the 

International Border at Nogales; (5) the 

Salt River; (6) the Rio San Bernardino 

from International Border to headwaters 

at Astin Spring (San Bernardino 

National Wildlife Refuge); (7) the Agua 

Fria River; (8) the Verde River upstream 

of Clarkdale; (9) the Verde River from 

the confluence with Fossil Creek 

downstream to its confluence with the 

Salt River; (10) Tanque Verde Creek in 

Tucson; (11) Rillito Creek in Tucson; 

(12) Agua Caliente Spring in Tucson; 

(13) Potrero Canyon/Springs; (14) 

Babocamari Cienega; (15) Barchas 

Ranch, Huachuca Mountain bajada; (16) 

Parker Canyon Lake and tributaries in 

the Canelo Hills; and (17) Oak Creek at 

Midgley Bridge (Rosen and Schwalbe 

1988, pp. 25–26, Appendix I; 1997, pp. 

16–17; Rosen et al. 2001, Appendix I; 

Brennan and Holycross 2006, p. 123; 

Holycross 2006; Holycross et al. 2006, 

pp. 15–51, 66; Radke 2006; Rosen 2006). 

Information pertaining to the cause or 

causes of extirpation of these sites is 

summarized in Table 1 below. 

Conversely, our review of the best 

available information indicates the 

northern Mexican gartersnake is likely 

extant in a fraction of its historical range 

in Arizona. We define populations as 

‘‘likely extant’’ when the species is 

expected to reliably occur in 

appropriate habitat as supported by 

recent museum records and/or recent 

(i.e., less than 10 years) reliable 

observations. The perennial or 

intermittent stream reaches and 

disassociated wetlands where we 

conclude northern Mexican gartersnakes 

remain extant include: (1) The Santa 

Cruz River/Lower San Rafael Valley 

(headwaters downstream to the 

International Border); (2) the Verde 

River from the confluence with Fossil 

Creek upstream to Clarkdale; (3) Oak 

Creek at Page Springs; (4) Tonto Creek 

from the mouth of Houston Creek 

downstream to Roosevelt Lake; (5) 

Cienega Creek from the headwaters 

downstream to the ‘‘Narrows’’ just 

downstream of Apache Canyon; (6) 

Pantano Wash (Cienega Creek) from 

Pantano downstream to Vail; (7) Upper 

Scotia Canyon in the Huachuca 

Mountains; and (8) the Audubon 

Research Ranch and vicinity near Elgin 


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(Rosen et al. 2001, Appendix I; Caldwell 

2005; Brennan and Holycross 2006, p. 

123; Holycross 2006; Holycross et al. 

2006, pp. 15–51, 66; Rosen 2006). 

The current status of the northern 

Mexican gartersnake is unknown in 

several areas in Arizona where the 

species is known to have historically 

occurred. We base this determination on 

mostly historical museum records for 

locations where survey access is 

restricted, survey data are unavailable or 

insufficient, and/or current threats 

could preclude occupancy. The 

perennial or intermittent stream reaches 

and disassociated wetlands where the 

status of the northern Mexican 

gartersnake remains uncertain include: 

(1) The downstream portion of the Black 

River drainage from the Paddy Creek 

confluence; (2) the downstream portion 

of the White River drainage from the 

confluence of the East and North forks; 

(3) Big Bonito Creek; (4) Lake O’Woods 

near Lakeside; (5) Spring Creek above 

the confluence with Oak Creek; (6) Bog 

Hole Wildlife Area; (7) Upper 13 Tank, 

Patagonia Mountain bajada; (8) 

Babocamari River; and (9) Arivaca 

Cienega (Rosen and Schwalbe 1988, 

Appendix I; Rosen et al. 2001, 

Appendix I; Brennan and Holycross 

2006, p. 123; Holycross 2006; Holycross 

et al. 2006, pp. 15–51; Rosen 2006). 

In summary, after consultation with 

species’ experts and land managers, and 

based upon our analysis of the best 

available scientific and commercial 

data, we conclude that the northern 

Mexican gartersnake has been extirpated 

from 85 to 90 percent of its historical 

distribution in the United States. 

Status in Mexico. Throughout this 

finding, and due to the significantly 

limited amount of available literature 

that addresses the status of and threats 

to extant populations of the northern 

Mexican gartersnake in Mexico, we rely 

in part on (1) information that addresses 

the status of and threats to both riparian 

and aquatic biological communities 

within the historical distribution of the 

northern Mexican gartersnake in 

Mexico; and (2) information that 

addresses the status of and threats to 

native freshwater fish within the 

historical distribution of the northern 

Mexican gartersnake in Mexico, which 

we use as ecological surrogates due to 

their similar habitat requirements and 

their role as important prey species 

utilized by the northern Mexican 

gartersnake. Observations on the status 

of riparian and aquatic communities in 

Mexico are available but limited in 

comparison to our knowledge of these 

communities in the United States. The 

current distribution of the northern 

Mexican gartersnake in Mexico is also 

not well understood, although its status 

is believed to be in decline in many 

areas due to historical and continuing 

threats to its habitat and prey base, as 

discussed below. A large number of 

springs have dried up in several 

Mexican states within the distribution 

of the northern Mexican gartersnake, 

namely, Chihuahua, Durango, Coahila, 

and San Luis Potosı ́ (Contreras Balderas 

and Lozano 1994, p. 381). Contreras 

Balderas and Lozano (1994, p. 381) also 

stated that several streams and rivers 

throughout Mexico and within the 

distribution of the northern Mexican 

gartersnake have dried up or become 

intermittent due to overuse of surface 

and groundwater supplies. We further 

acknowledge that northern Mexican 

gartersnakes were historically 

distributed in several regions within 

Mexico that have remained roadless and 

isolated and, according to the 

information we were able to obtain 

regarding the status of the northern 

Mexican gartersnake in Mexico, few 

ecological investigations have occurred 

in these areas due to their remote nature 

and the logistical difficulties that face 

research in such areas. However, 

Mexican biologists Ramirez Bautista and 

Arizmendi (2004, p. 3) were able to 

provide general information on the 

principal threats to northern Mexican 

gartersnake habitat in Mexico which 

included the dessication of wetlands, 

improper livestock grazing, 

deforestation, wildfires, and 

urbanization. In addition, nonnative 

species, such as bullfrogs and sport and 

bait fish, have been introduced 

throughout Mexico and continue to 

disperse naturally, broadening their 

distributions (Conant 1974, pp. 487–

489; Miller et al. 2005, pp. 60–61). 

Given the lack of specific data on the 

status of the northern Mexican 

gartersnake in Mexico, we cannot 

conclude with any degree of certainty 

its overall status in Mexico. 

Northern Mexican Gartersnake Distinct 

Population Segment 

In the petition to list the northern 

Mexican gartersnake, the petitioners 

specified several listing options for our 

consideration, including listing 

northern Mexican gartersnake in the 

United States as a DPS. Under the Act, 

we must consider for listing any species, 

subspecies, or DPSs of vertebrate 

species/subspecies, if information is 

sufficient to indicate that such action 

may be warranted. To implement the 

measures prescribed by the Act and its 

Congressional guidance, we developed a 

joint policy with the National Oceanic 

and Atmospheric Administration 

(NOAA) Fisheries entitled Policy 

Regarding the Recognition of Distinct 

Vertebrate Population (DPS Policy) to 

clarify our interpretation of the phrase 

‘‘distinct population segment of any 

species of vertebrate fish or wildlife’’ for 

the purposes of listing, delisting, and 

reclassifying species under the Act (61 

FR 4721; February 7, 1996). Under our 

DPS policy, we consider three elements 

in a decision regarding the status of a 

possible DPS as endangered or 

threatened under the Act. The elements 

are: (1) The population segment’s 

discreteness from the remainder of the 

taxon to which it belongs; (2) the 

population segment’s significance to the 

taxon to which it belongs; and (3) the 

population segment’s conservation 

status in relation to the Act’s standards 

for listing (i.e., when treated as if it were 

a species, is the population segment 

endangered or threatened?). Our policy 

further recognizes it may be appropriate 

to assign different classifications (i.e., 

threatened or endangered) to different 

DPSs of the same vertebrate taxon (61 

FR 4721; February 7, 1996). 

Discreteness 

The DPS policy’s standard for 

discreteness requires an entity given 

DPS status under the Act to be 

adequately defined and described in 

some way that distinguishes it from 

other populations of the species. A 

population segment may be considered 

discrete if it satisfies either one of the 

following conditions: (1) Marked 

separation from other populations of the 

same taxon resulting from physical, 

physiological, ecological, or behavioral 

factors, including genetic discontinuity; 

or (2) populations delimited by 

international boundaries within which 

differences in control of exploitation, 

management of habitat, conservation 

status, or regulatory mechanisms exist 

that are significant in light of 4(a)(1)(D) 

of the Act. 

Marked Separation from Other 

Populations of the Same Taxon as a 

Consequence of Physical, Physiological, 

Ecological or Behavioral Factors. We do 

not have any information to indicate 

that a marked separation exists between 

the United States and Mexico that 

would distinguish populations of 

northern Mexican gartersnake in the 

United States from those in Mexico. 

There is no information to indicate that 

a marked separation exists as a result of 

physical, physiological, ecological, or 

behavioral factors. 

There has been no genetic analysis 

completed for the northern Mexican 

gartersnake. Thus, we have no 

information to indicate that genetic 

differences exist. 


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Populations Delimited by 

International Boundaries Within Which 

Differences in Control of Exploitation, 

Management of Habitat, Conservation 

Status, or Regulatory Mechanisms Exist 

that are Significant. In terms of the 

conservation status of the northern 

Mexican gartersnake, despite the 

significantly limited amount of 

monitoring and/or survey data for the 

northern Mexican gartersnake in 

Mexico, we believe there is a higher 

probability that the subspecies is fairing 

better overall in Mexico in terms of 

having more total populations, because 

a larger percentage of the overall range 

of the subspecies (approximately 70 to 

80 percent of it historical distribution) 

occurs in Mexico. However, we have no 

information to indicate that the 

populations on either side of the United 

States-Mexico border have a more stable 

or better conservation status. 

We recognize that differences in 

management regulatory protection of 

northern Mexican gartersnake 

populations may exist between 

populations within Mexico and those 

within the United States. These 

differences primarily pertain to 

protections afforded to occupied habitat. 

In Mexico, any activity that 

intentionally destroys or adversely 

modifies occupied northern Mexican 

gartersnake habitat is prohibited 

[SEDESOL 2000 (LGVS) and 2001 

(NOM–059–ECOL–2001)]. Neither the 

Arizona Game and Fish Department or 

the New Mexico Department of Game 

and Fish can offer protections to 

occupied habitat. Instead, these agencies 

regulate take in the form of lethal or live 

collection of individuals which is 

prohibited in both states. However, any 

conclusions that may be drawn with 

reference to differences in management 

across the United States-Mexico border 

are largely speculative due to the lack of 

information available as to the efficacy 

and protections of these regulations in 

practice. Because we determine in the 

following section that populations of the 

northern Mexican gartersnake in the 

United States are not significant to the 

subspecies as a whole, we need not 

address further the ‘‘discreteness’’ test 

of the DPS policy. For further 

information on regulatory 

considerations, please see our 

discussion under Factor D below. 

Significance 

Under our DPS policy, a population 

segment must be significant to the taxon 

to which it belongs. The evaluation of 

‘‘significance’’ may address, but is not 

limited to, (1) evidence of the 

persistence of the discrete population 

segment in an ecological setting that is 

unique for the taxon; (2) evidence that 

loss of the population segment would 

result in a significant gap in the range 

of the taxon; (3) evidence that the 

population segment represents the only 

surviving natural occurrence of a taxon 

that may be more abundant elsewhere as 

an introduced population outside its 

historic range; and (4) evidence that the 

discrete population segment differs 

markedly from other populations of the 

species in its genetic characteristics. 

Ecological Setting. Throughout its 

rangewide distribution, the northern 

Mexican gartersnake occurs at 

elevations from 130 to 8,497 ft (40 to 

2,590 m) (Rossman et al. 1996, p. 172). 

The northern Mexican gartersnake is 

considered a riparian obligate (restricted 

to riparian areas when not engaged in 

dispersal behavior) and occurs chiefly 

in the following general habitat types in 

both the United States and Mexico: (1) 

Source—area wetlands [e.g., cienegas 

(mid-elevation wetlands with highly 

organic, reducing (basic, or alkaline) 

soils), stock tanks (small earthen 

impoundment), etc.]; (2) large river 

riparian woodlands and forests; and (3) 

streamside gallery forests (as defined by 

well-developed broadleaf deciduous 

riparian forests with limited, if any, 

herbaceous ground cover or dense grass) 

(Hendrickson and Minckley 1984, p. 

131; Rosen and Schwalbe 1988, pp. 14–

16; Arizona Game and Fish Department 

2001). Based on this information, we 

determine that populations of the 

northern Mexican gartersnake in 

Arizona do not occupy an ecological 

setting differing enough from 

populations that occur in Mexico to be 

considered unique for the subspecies. 

Gap in the Range. The Service can 

determine that a gap in a taxon’s range 

caused by the potential loss of a 

population would be significant based 

on any relevant considerations. One 

factor which may support such a 

determination is whether the loss of a 

geographic area amounts to a substantial 

reduction of a taxon’s range and this 

reduction is biologically important. The 

United States comprised the most 

northern portion of the northern 

Mexican gartersnake’s range and 

constituted approximately 20–30 

percent of its rangewide historical 

distribution. Because we do not 

currently know exactly what the status 

of the northern Mexican gartersnake is 

in Mexico at this time, we are unable to 

ascertain what percentage of extant 

populations occur in the United States 

as compared to Mexico. However, this is 

not sufficient evidence to support a 

determination that loss of the northern 

Mexican gartersnake in the United 

States represents a substantial reduction 

in the subspecies’ range based on the 

geographic area which would be lost. 

Furthermore, no area that is uniquely 

biologically significant to the northern 

Mexican gartersnake is located within 

the United States as compared to 

Mexico. 

Another factor relevant to 

determining whether a gap is significant 

is the biological significance of the 

number of total individuals of the taxon 

in the population that may be lost. 

Although we have no data on the 

absolute numbers of northern Mexican 

gartersnakes in the United States or 

Mexico, the best available science 

suggests that there are far more 

individuals in Mexico than in the 

United States, based on the more 

extensive range in Mexico and the 

current low density and number of 

extant populations in the United States. 

Therefore, we have no information to 

indicate that the loss of between 8 and 

17 populations of northern Mexican 

gartersnakes known in the United States 

is biologically significant to the taxon as 

a whole. 

In conclusion, we have determined 

that the gap in the range of the northern 

gartersnake that would be caused by the 

loss of the United States population 

would not be significant because: (1) 

Loss of the United States population 

would not constitute a substantial and 

biologically important reduction of the 

range of the subspecies; (2) the loss of 

the individuals in the United States 

would not be biologically significant to 

the subspecies; and (3) we have not 

identified any other reason why loss of 

the United States population would 

result in a significant gap in the range 

of the subspecies. 

Marked Differences in Genetic 

Characteristics. Within the distribution 

of every species there exists a peripheral 

population, an isolate or subpopulation 

of a species at the edge of the taxon’s 

range. Long-term geographic isolation 

and loss of gene flow between 

populations is the foundation of genetic 

changes in populations resulting from 

natural selection or change. Evidence of 

changes in these populations may 

include genetic, behavioral, and/or 

morphological differences from 

populations in the rest of the species’

range. We have no information to 

indicate that genetic differences exist 

between populations of the northern 

Mexican gartersnake at the northern 

portion of its range in the United States 

from those in Mexico. Therefore, based 

on the genetic information currently 

available, the northern Mexican 

gartersnake in the United States should 

not be considered biologically or 

ecologically significant based simply on 


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genetic characteristics. Biological and 

ecological significance under the DPS 

policy is always considered in light of 

Congressional guidance (see Senate 

Report 151, 96th Congress, 1st Session) 

that the authority to list DPS’s be used 

‘‘sparingly’’ while encouraging the 

conservation of genetic diversity. 

Whether the Population Represents 

the Only Surviving Natural Occurrence 

of the Taxon. As part of a determination 

of significance, our DPS policy suggests 

that we consider whether there is 

evidence that the population represents 

the only surviving natural occurrence of 

a taxon that may be more abundant 

elsewhere as an introduced population 

outside its historic range. The northern 

Mexican gartersnake in the United 

States is not the only surviving natural 

occurrence of the subspecies. 

Consequently, this factor is not 

applicable to our determination 

regarding significance. 

Conclusion 

Following a review of the available 

information, we conclude that the 

northern Mexican gartersnake in the 

United States is not significant to the 

remainder of the subspecies. We made 

this determination based on the best 

available information, which does not 

demonstrate that (1) these populations 

persist in an ecological setting that is 

unique for the subspecies; (2) the loss of 

these populations would result in a 

significant gap in the range of the 

subspecies; and (3) these populations 

differ markedly from populations of 

northern Mexican gartersnake in Mexico 

in their genetic characteristics, or in 

other considerations that might 

demonstrate significance. Further, 

available information does not 

demonstrate that the life history and 

behavioral characteristics of the 

northern Mexican gartersnake in the 

United States is unique to the 

subspecies. Therefore, on the basis of 

the best scientific and commercial 

information available, we find that 

proposing to list a DPS for the northern 

Mexican gartersnake in the United 

States is not warranted; these 

populations do not meet the definition 

of a distinct population segment. We are 

not addressing the third prong of the 

DPS policy (i.e. the population 

segment’s conservation status in relation 

to the Act’s standards for listing) since 

we find that the United States portion 

of the range of the northern Mexican 

gartersnake does not qualify as a listable 

entity pursuant to our DPS policy, as 

discussed above. 

Significant Portion of the Range 

In the petition to list the northern 

Mexican gartersnake, the petitioners 

also requested that we consider listing 

the species throughout its range based 

on its status in the United States. As 

required by the Act, we have considered 

in this finding whether the northern 

Mexican gartersnake is in danger of 

extinction ‘‘in all or a significant portion 

of its range’’ as defined in the terms 

‘‘threatened species’’ and ‘‘endangered 

species’’ pursuant to section 3 of the 

Act. In order to determine if Arizona 

constitutes a significant portion of the 

range of the subspecies, we evaluate 

whether threats in this geographic area 

imperil the viability of the subspecies as 

a whole due to any biological 

importance of this portion of the 

subspecies range. Based upon the best 

scientific information available, we find 

that the extant populations in the 

United States are not considered a 

stronghold for the subspecies, they do 

not represent core or important breeding 

habitat, we are not aware of any unique 

genetic or behavioral characteristics, 

and we are not aware that threats in this 

portion of its range threaten the whole 

subspecies with extinction. Therefore, 

we determine that the extant 

populations of the northern Mexican 

gartersnake in Arizona do not constitute 

a significant portion of the range of the 

subspecies because there is no 

particular characteristic to any segment 

within this portion of its range that 

would render it biologically more 

significant to the taxon as a whole than 

other portions of its current range. 

We note that the court in Defenders of 

Wildlife v. Norton, 258 F.3d 1136 (9th 

Cir. 2001), appeared to suggest that a 

species could be in danger of extinction 

in a significant portion of its range if 

there is a ‘‘major geographical area’’ in 

which the species is no longer viable 

but once was. Although we do not 

necessarily agree with the court’s 

suggestion, we have determined that the 

historical range of the subspecies within 

the United States does not constitute a 

‘‘major geographical area’’ in this 

context. The portion of the northern 

Mexican gartersnake’s historical range 

in United States (20 to 30 percent) 

constitutes a small percentage of the 

total range of the subspecies. 

The petitioners also requested that we 

consider listing the species throughout 

its range based on its rangewide status. 

Below we respond to the petitioners 

request through our analysis of the five 

listing factors for the United States and 

Mexico. 

Summary of Factors Affecting the 

Northern Mexican Gartersnake 

Section 4 of the Act (16 U.S.C. 1533), 

and implementing regulations at 50 CFR 

424, set forth procedures for adding 

species to the Federal Lists of 

Endangered and Threatened Wildlife 

and Plants. Under section 4(a) of the 

Act, we may list a species on the basis 

of any of five factors, as follows: (A) The 

present or threatened destruction, 

modification, or curtailment of its 

habitat or range; (B) overutilization for 

commercial, recreational, scientific, or 

educational purposes; (C) disease or 

predation; (D) the inadequacy of 

existing regulatory mechanisms; or (E) 

other natural or manmade factors 

affecting its continued existence. In 

making this finding, information 

regarding the status of, and threats to, 

the northern Mexican gartersnake in 

relation to the five factors provided in 

section 4(a)(1) of the Act is discussed 

below and summarized in Table 1 

below. 

TABLE 1.—SUMMARY OF NORTHERN MEXICAN GARTERSNAKE STATUS AND THREATS BY POPULATION IN UNITED STATES

[All locations in Arizona unless otherwise specified.] 

Population locality Current status Regional historical/current threats 

Gila River ........................................ Extirpated ....................................... Considered extirpated by nonnatives, improper grazing, recreation, 

development, groundwater pumping, diversions, channelization, 

dewatering, road construction/use, wildfire, intentional harm, dams, 

prey base reductions. 

Gila and San Francisco Headwaters in New Mexico.Extirpated ....................................... Considered extirpated by nonnatives, improper grazing, recreation, 

prey base reductions. 

Lower Colorado River from Davis 

Dam to International Border.

Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, recreation, development, road construction/use, borderland security/undocumented immigration, intentional harm, dams. 


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TABLE 1.—SUMMARY OF NORTHERN MEXICAN GARTERSNAKE STATUS AND THREATS BY POPULATION IN UNITED STATES—

Continued 

[All locations in Arizona unless otherwise specified.] 

Population locality Current status Regional historical/current threats 

San Pedro River in United States ... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, groundwater pumping, road construction/use, borderland 

security/undocumented immigrants, intentional harm. 

Santa Cruz River downstream of 

the Nogales area of the International Border.

Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, development, groundwater pumping, diversions, channelization, road construction/use, borderland security/undocumented 

immigrants, intentional harm, contaminants. 

Salt River ......................................... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, development, diversions, wildfire, channelization, road construction/use, intentional harm, dams. 

Rio San Bernardino from International Border to headwaters at 

Astin Spring (San Bernardino National Wildlife Refuge).

Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, borderland security/undocumented immigration, intentional harm, competition with Marcy’s checkered gartersnake. 

Agua Fria River ............................... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, development, recreation, dams, road construction/use, 

wildfire, intentional harm. 

Verde River upstream of Clarkdale Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, development, groundwater pumping, diversions, channelization, road construction/use, intentional harm. 

Verde River from the confluence 

with the Salt upstream to Fossil 

Creek.

Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, groundwater pumping, diversions, channelization, road construction/use, wildfire, development,intentional harm, 

dams. 

Potrero Canyon/Springs .................. Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing. 

Tanque Verde Creek in Tucson ...... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, development, groundwater pumping, road construction/use, intentional harm. 

Rillito Creek in Tucson .................... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, development, groundwater pumping, road construction/use, intentional harm. 

Agua Caliente Spring in Tucson ..... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, development, groundwater pumping, road construction/use, intentional harm. 

Babocamari Cienega ....................... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing. 

Barchas Ranch, Huachuca Mountain bajada.Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, borderland security/undocumented immigration, intentional 

harm. 

Parker Canyon Lake and tributaries 

in the Canelo Hills.

Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, road construction/use, borderland security/undocumented immigration, intentional harm, dams. 

Oak Creek at Midgley Bridge .......... Extirpated ....................................... Considered extirpated by nonnatives, prey base reductions, improper 

grazing, recreation, development, intentional harm. 

Santa Cruz River/Lower San Rafael 

Valley (headwaters downstream 

to International Border).

Extant ............................................. Nonnatives, prey base reductions, improper grazing, borderland security/undocumented immigration, intentional harm. 

Verde River from the confluence 

with Fossil Creek upstream to 

Clarkdale.

Extant ............................................. Nonnatives, prey base reductions, improper grazing, recreation, development, groundwater pumping, diversions, channelization, road 

construction/use, intentional harm, dams. 

Oak Creek at Page Springs ............ Extant ............................................. Nonnatives, prey base reductions. 

Tonto Creek from mouth of Houston Creek downstream to Roosevelt Lake.Extant ............................................. Nonnatives, prey base reductions, improper grazing, recreation, development, diversions, channelization, road construction/use, wildfire, intentional harm, dams. 

Cienega Creek from headwaters 

downstream to the ‘‘Narrows’’

just downstream of Apache Canyon.

Extant ............................................. Nonnatives, prey base reductions, improper grazing. 

Pantano Wash (Cienega Creek) 

from Pantano downstream to Vail.

Extant ............................................. Nonnatives, prey base reductions, improper grazing, wildfire. 

Upper Scotia Canyon in the 

Huachuca Mountains.

Extant ............................................. Nonnatives, prey base reductions, wildfire. 

Audubon Research Ranch and vicinity near Elgin.Extant ............................................. Nonnatives, prey base reductions, improper grazing. 

Downstream portion of the Black 

River drainage from the Paddy 

Creek confluence.

Unknown ........................................ Nonnatives, prey base reductions, improper grazing, recreation, intentional harm. 


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TABLE 1.—SUMMARY OF NORTHERN MEXICAN GARTERSNAKE STATUS AND THREATS BY POPULATION IN UNITED STATES—

Continued 

[All locations in Arizona unless otherwise specified.] 

Population locality Current status Regional historical/current threats 

Downstream portion of the White 

River drainage from the confluence of the East/North.

Unknown ........................................ Nonnatives, prey base reductions, improper grazing, recreation, road 

construction/use, intentional harm. 

Big Bonito Creek ............................. Unknown ........................................ Nonnatives, prey base reductions, improper grazing. 

Lake O’ Woods (Lakeside) ............. Unknown ........................................ Nonnatives, prey base reductions, recreation, development, road construction/use, intentional harm. 

Spring Creek above confluence 

with Oak Creek.

Unknown ........................................ Nonnatives, prey base reductions, development. 

Bog Hole Wildlife Area .................... Unknown ........................................ Nonnatives, prey base reductions. 

Upper 13 Tank, Patagonia Mountains bajada.Unknown ........................................ Nonnatives, prey base reductions, improper grazing. 

Babocamari River ............................ Unknown ........................................ Nonnatives, prey base reductions, improper grazing. 

Arivaca Cienega .............................. Unknown ........................................ Nonnatives, prey base reductions, improper grazing, borderland security/undocumented immigration, intentional harm. 

Note: ‘‘Extirpated’’ means that there have been no northern Mexican gartersnakes reported for a decade or longer at a site within the historical 

distribution of the species, despite survey efforts, and there is no expectation of natural recovery at the site due to the presence of known or 

strongly suspected causes of extirpation. ‘‘Extant’’ means areas where the species is expected to reliably occur in appropriate habitat as supported by museum records and/or recent, reliable observations. ‘‘Unknown’’ means areas where the species is known to have occurred based on 

museum records (mostly historical) but access is restricted, and/or survey data is unavailable or insufficient, or where threats could preclude occupancy. The information used to develop this table can be found in the sources listed below. 

Sources: Hyatt undated, p. 71; Nickerson and Mays 1970, pp. 495, 503; Hulse 1973, p. 278; Vitt and Ohmart 1978, p. 44; Hendrickson and 

Minckley 1984, p. 131, 138–162; Meffe 1985, pp. 179–185; Rosen 1987, p. 5; Ohmart et al. 1988, pp. 143–147, 150; Rosen and Schwalbe 1988, 

Appendix I; 1995, p. 452; 1996, pp. 1–3; 1997, p. 1; 2002b, pp. 223–227; 2002c, pp. 31, 70; Bestgen and Propst 1989, pp. 409–410; Clarkson 

and Rorabaugh 1989, pp. 531–538; Marsh and Minckley 1990, p. 265; Medina 1990, pp. 351, 358–359; Sublette et al. 1990, pp. 112, 243, 246, 

304, 313, 318; Abarca and Weedman 1993, pp. 2, 6–12; Girmendonk and Young 1993, pp. 45–52; Sullivan and Richardson 1993, pp. 35–42; 

Stefferud and Stefferud 1994, p. 364; Bahre 1995, pp. 240–252; Hale et al. 1995, pp. 138–140; Holm and Lowe 1995, pp. 5, 27–35, 37–38, 45–

46; Rosen et al. 1995, p. 254; 1996b, pp. 8–9; 2001, Appendix I; Sredl et al. 1995a, p. 7; 1995b, p. 9; 1995c, p. 7; 2000, p. 10; Degenhardt et al. 

1996, p. 319; Fernandez and Rosen 1996, pp. 6–19, 52–56; Stromberg et al. 1996, pp. 113–114, 123–128; Yuhas 1996; Drost and Nowak 1997, 

p. 11; Weedman and Young 1997, pp. 1, Appendices B, C; Inman et al. 1998, Appendix B; Rinne et al. 1998, pp. 75–80; Nowak and Spille 

2001, pp. 11, 32–33; Esque and Schwalbe 2002, pp. 161–193; Nowak and Santana-Bendix 2002, p. 39; Stromberg and Chew 2002, pp. 198, 

210–213; Tellman 2002, p. 43; USFWS 2002a, pp. 40802–40804; 2002b, Appendix H; 2006, pp. 91–105; Voeltz 2002, pp. 40, 45–81; Krueper et 

al. 2003, pp. 607, 613–614; Bonar et al. 2004, pp. 1–108; Forest Guardians 2004, p. 1; Unmack and Fagan 2004, p. 233; Fagan et al. 2005, pp. 

34–41; Olden and Poff 2005, pp. 75, 82–87; Painter 2005; Philips and Thomas 2005; Webb and Leake 2005, pp. 302, 305–310, 318–320; 

ADWR 2006; American Rivers 2006; Brennan and Holycross 2006, p. 123; Holycross et al. 2006, pp. 15–61; McKinnon 2006a, 2006b, 2006c, 

2006d, 2006e; Paradzick et al. 2006, pp. 88–93, 104–110; Segee and Neeley 2006, Executive Summary, pp. 5–7; 10–12, 15–16, 21–23. 

In the discussions of Factors A 

through E below, we describe the 

known factors that have contributed to 

the current status of the northern 

Mexican gartersnake. The majority of 

this assessment is specific to those 

factors that have contributed to its status 

in the United States. The following 

discussion of these factors that pertain 

to the status and threats to the northern 

Mexican gartersnake in Mexico are 

mainly regional, or statewide, in scope 

because in many cases we were unable 

to find specific information 

documenting that populations of the 

northern Mexican gartersnake in Mexico 

are directly affected by these threats. In 

some instances, we do include 

discussion on more refined geographic 

areas of Mexico when supported by the 

literature. However, many of the threats 

that affect the northern Mexican 

gartersnake in the United States are also 

present in Mexico. Thus, the 

relationship between the threats to the 

habitat and species in Mexico may be 

similar to what we have documented in 

the United States. 

A. The Present or Threatened 

Destruction, Modification, or 

Curtailment of Its Habitat or Range 

In the following discussion, we 

elaborate on the physical threats to 

northern Mexican gartersnake habitats 

(i.e., riparian and aquatic communities) 

that have occurred and continue to 

occur within the distribution of the 

species in the United States and Mexico. 

Various threats that have affected and 

continue to affect riparian and aquatic 

communities include dams, diversions, 

groundwater pumping, introduction of 

nonnative species (vertebrates, plants, 

and crayfish), woodcutting, mining, 

contaminants, urban and agricultural 

development, road construction, 

livestock grazing, wildfires, and 

undocumented immigration 

(Hendrickson and Minckley 1984, p. 

161; Ohmart et al. 1988, p. 150; Bahre 

1995, pp. 240–252; Medina 1990, p. 351; 

Sullivan and Richardson 1993, pp. 35–

42; Fleischner 1994, pp. 630–631; 

Hadley and Sheridan 1995; Hale et al. 

1995, pp. 138–140; DeBano and Neary 

1996, pp. 73–75; Rinne and Neary 1996, 

p. 135; Stromberg et al. 1996, pp. 124–

127; Girmendock and Young 1997, pp. 

45–52; Rinne et al. 1998, pp. 7–11; 

Belsky et al. 1999, pp. 8–12; Esque and 

Schwalbe 2002, pp. 165, 190; Hancock 

2002, p. 765; Voeltz 2002, pp. 87–88; 

Webb and Leake 2005, pp. 305–308; 

Holycross et al. 2006, pp. 52–61; 

McKinnon 2006a, 2006b, 2006c, 2006d, 

2006e; Paradzick et al. 2006, pp. 88–93; 

Segee and Neeley 1996, Executive 

Summary, pp. 10–12, 21–23). These 

activities and their effects on the 

northern Mexican gartersnake are 

discussed in further detail below. 

It is important to recognize that in 

most areas where northern Mexican 

gartersnakes historically or currently 

occur, two or more threats may be acting 

synergistically in their influence on the 

suitability of those habitats or on the 

northern Mexican gartersnake itself. In 

our assessment of the status of these 

habitats, discussion of the role that 

nonnative species introductions have 

had on habitat suitability is critical. 

However, we provide that discussion 

under ‘‘Factor C. Disease and Predation’’

due to the intricate and complex 

relationship nonnative species have 

with respect to direct and indirect 

pressures applied to the northern 


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Mexican gartersnake and to its native 

prey base. 

Threats to Riparian and Aquatic 

Biological Communities in the United 

States. The modification and 

destruction of aquatic and riparian 

communities in the post-settlement arid 

southwestern United States is well 

documented and apparent in the field 

(Medina 1990, p. 351; Sullivan and 

Richardson 1993, pp. 35–42; Fleischner 

1994, pp. 630–631; Stromberg et al. 

1996, pp. 113, 123–128; Girmendock 

and Young 1997, pp. 45–52; Belsky et 

al. 1999, pp. 8–12; Webb and Leake 

2005, pp. 305–310; Holycross et al. 

2006, pp. 52–61). Several threats have 

been identified in the decline of many 

native riparian flora and fauna species 

through habitat modification and 

destruction as well as nonnative species 

introductions. Researchers agree that the 

period from 1850 to 1940 marked the 

greatest loss and degradation of riparian 

and aquatic communities in Arizona, 

which were caused by anthropogenic 

(human) land uses and the primary and 

secondary effects of those uses 

(Stromberg et al. 1996, p. 114; Webb and 

Leake 2005, pp. 305–310). Many of 

these land activities continue today and 

are discussed at length below. An 

estimated one-third of Arizona’s presettlement wetlands have dried or have 

been rendered ecologically 

dysfunctional (Yuhas 1996). 

Modification and Loss of Cienegas in 

the United States. Cienegas are 

particularly important habitat for the 

northern Mexican gartersnake and are 

considered ideal for the species (Rosen 

and Schwalbe 1988, p. 14). Hendrickson 

and Minckley (1984, p. 131) defined 

cienegas as ‘‘mid-elevation [3,281–6,562 

ft (1,000–2000 m)] wetlands 

characterized by permanently saturated, 

highly organic, reducing soils.’’ Many of 

these unique communities of the 

southwestern United States, and 

Arizona in particular, have been lost in 

the past century to streambed 

modification, improper livestock 

grazing, cultural impacts, stream flow 

stabilization by upstream dams, 

channelization, and stream flow 

reduction from groundwater pumping 

and diversions (Hendrickson and 

Minckley 1984, p. 161). Stromberg et al. 

(1996, p. 114) state that cienegas were 

formerly extensive along streams of the 

Southwest; however, most were 

destroyed during the late 1800s, when 

groundwater tables declined several 

meters and stream channels became 

incised along many southwestern 

streams, including the San Pedro River. 

Conservation of the remaining natural 

cienegas in Arizona will be contingent 

on their protection from severe flooding 

and from lowering of groundwater 

levels (Hendrickson and Minckley 1984, 

p. 169). 

Many sub-basins where cienegas have 

been severely modified or lost entirely 

overlap, wholly or partially, the 

historical distribution of the northern 

Mexican gartersnake, including the San 

Simon, Sulphur Springs, San Pedro, and 

Santa Cruz valleys of southeastern and 

south-central Arizona. The San Simon 

Valley possessed several natural 

cienegas with luxuriant vegetation prior 

to 1885, and was used as a watering stop 

for pioneers, military, and surveying 

expeditions (Hendrickson and Minckley 

1984, pp. 139–140). In the subsequent 

decades, the disappearance of grasses 

and commencement of severe erosion 

were the result of heavy grazing 

pressure by large herds of cattle as well 

as the effects from wagon trails that 

paralleled arroyos, occasionally crossed 

them, and often required stream bank 

modification (Hendrickson and 

Minckley 1984, p. 140). Today, only the 

artificially-maintained San Simon 

Cienega exists in this valley. Similar 

accounts of past conditions, adverse 

effects from historical anthropogenic 

activities, and subsequent reduction in 

the extent and quality of cienega 

habitats in the remaining valleys are 

also provided in Hendrickson and 

Minckley (1984, pp. 138–160). 

Urban and Rural Development in the 

United States. Development within and 

adjacent to riparian areas has proven to 

be a significant threat to riparian 

biological communities and their 

suitability for native species (Medina 

1990, p. 351). Riparian communities are 

sensitive to even low levels (less than 10 

percent) of urban development within a 

watershed (Wheeler et al. 2005, p. 142). 

Development along or within proximity 

to riparian zones can alter the nature of 

stream flow dramatically, changing once 

perennial streams into ephemeral 

streams, which has direct consequences 

on the riparian community (Medina 

1990, pp. 358–359). Obvious examples 

of the influence of urbanization and 

development can be observed within the 

areas of greater Tucson and Phoenix, 

Arizona, where impacts have modified 

riparian vegetation, structurally altered 

stream channels, facilitated nonnative 

species introductions, and dewatered 

large reaches of formerly perennial 

rivers where the northern Mexican 

gartersnake historically occurred (Santa 

Cruz, Gila, and Salt rivers, respectively). 

Urbanization and development of these 

areas, along with the introduction of 

nonnative species, are largely 

responsible for the extirpation of the 

northern Mexican gartersnake from 

these areas. 

Urbanization on smaller scales can 

also impact habitat suitability and the 

prey base for the northern Mexican 

gartersnake. Medina (1990, pp. 358–359) 

concluded that perennial streams had 

greater tree densities in all diameter size 

classes of Arizona alder and box elder 

(Acer negundo) as compared to 

ephemeral reaches where small 

diameter trees were absent. Small 

diameter trees assist the northern 

Mexican gartersnake by providing 

additional habitat complexity and cover 

needed to reduce predation risk and 

enhance the usefulness of areas for 

thermoregulation. Regional 

development and subsequent land use 

changes, spurred by increasing 

populations, along lower Tonto Creek 

and within the Verde Valley where 

northern Mexican gartersnakes are 

extant continue to threaten this snake’s 

habitat and affect the habitat’s 

suitability for the northern Mexican 

gartersnake and its prey species 

(Girmendock and Young 1997, pp. 45–

52; Voeltz 2002, pp. 58–59, 69–71; 

Paradzick et al. 2006, pp. 89–90). 

Holycross et al. (2006, pp. 53, 56) 

recently documented adverse effects to 

northern Mexican gartersnake habitat in 

the vicinity of Rock Springs along the 

Agua Fria River and also throughout the 

Verde Valley along the Verde River. 

The effects of urban and rural 

development are expected to increase as 

populations increase. Consumer interest 

in second home and/or retirement real 

estate investments has increased 

significantly in recent times within the 

southwestern United States. Medina 

(1990, p. 351) points out that many real 

estate investors are looking for 

aesthetically scenic, mild climes to 

enjoy seasonally or year-round and 

hence choose to develop pre- or postretirement properties that are within or 

adjacent to riparian areas due to their 

aesthetic appeal and available water. 

Arizona increased its population by 394 

percent from 1960 to 2000, and is 

second only to Nevada as the fastest 

growing State in terms of human 

population (SSDAR 2000). Over the 

same time period, population growth 

rates in Arizona counties where the 

northern Mexican gartersnake 

historically occurred or may still be 

extant have varied by county but are no 

less remarkable: Maricopa (463 percent); 

Pima (318 percent); Santa Cruz (355 

percent); Cochise (214 percent); Yavapai 

(579 percent); Gila (199 percent); 

Graham (238 percent); Apache (228 

percent); Navajo (257 percent); Yuma 

(346 percent); LaPaz (142 percent); and 

Mohave (2004 percent) (SSDAR 2000). 

Population growth trends in Arizona, 


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and Maricopa County in particular, are 

expected to continue into the future. 

The Phoenix metropolitan area, founded 

in part due to its location at the junction 

of the Salt and Gila rivers, is a 

population center of 3.63 million 

people. The Phoenix metropolitan area 

is the sixth largest in the United States 

and resides in the fastest growing 

county in the United States since the 

2000 census (Arizona Republic 2006). 

Development growth predictions have 

also been made for traditionally rural 

portions of Arizona. The populations of 

developing cities and towns of the 

Verde watershed are expected to more 

than double in the next 50 years, which 

may pose exceptional threats to riparian 

and aquatic communities of the Verde 

Valley where northern Mexican 

gartersnakes occur (Girmendock and 

Young 1993, p. 47; American Rivers 

2006; Paradzick et al. 2006, p. 89). 

Communities in Yavapai and Gila 

counties such as the Prescott-Chino 

Valley, Strawberry, Pine, and Payson 

have all seen rapid population growth 

in recent years. For example, the 

population in the town of Chino Valley, 

at the headwaters of the Verde River, 

has grown by 22 percent between 2000 

and 2004; Gila County, which includes 

reaches of the Salt, White, and Black 

rivers and Tonto Creek, grew by 20 

percent between 2000 and 2003 

(http://www.census.gov). The upper San 

Pedro River is also the location of rapid 

population growth in the Sierra VistaHuachuca City-Tombstone area (http:// 

www.census.gov). All of these 

communities are near or within the 

vicinity of historical or extant northern 

Mexican gartersnake populations. 

Road Construction, Use, and 

Maintenance in the United States. 

Roads cover approximately one percent 

of the land area in the United States, but 

negatively affect 20 percent of the 

habitat and biota in the United States 

(Angermeier et al. 2004, p. 19). Roads 

pose unique threats to herpetofauna 

(reptiles and amphibians) and 

specifically to species like the northern 

Mexican gartersnake, its prey base, and 

the habitat where it occurs through: (1) 

Fragmentation, modification, and 

destruction of habitat; (2) an increase in 

genetic isolation; (3) alteration of 

movement patterns and behaviors; (4) 

facilitation of the spread of nonnative 

species via human vectors; (5) an 

increase in recreational access and the 

likelihood of subsequent, decentralized 

urbanization; (6) interference with and/ 

or inhibition of reproduction; (7) 

contributions of pollutants to riparian 

and aquatic communities; and (8) 

population sinks through direct 

mortality (Rosen and Lowe 1994, pp. 

146–148; Waters 1995, p. 42; Carr and 

Fahrig 2001, pp. 1074–1076; Hels and 

Buchwald 2001, p. 331; Smith and Dodd 

2003, pp. 134–138; Angermeier et al. 

2004, pp. 19–24; Shine et al. 2004, pp. 

9, 17–19; Andrews and Gibbons 2005, 

pp. 777–781; Wheeler et al. 2005, pp. 

145, 148–149; Roe et al. 2006, p. 161). 

Construction and maintenance of 

roads and highways near riparian areas 

can be a source of sediment and 

pollutants (Waters 1995, p. 42; Wheeler 

et al. 2005, pp. 145, 148–149). Sediment 

can adversely affect fish populations 

used as prey by the northern Mexican 

gartersnake by (1) interfering with 

respiration; (2) reducing the 

effectiveness of visually-based hunting 

behaviors; and (3) filling in interstitial 

spaces of the substrate which reduces 

reproduction and foraging success of 

fish interfering with respiration, and 

restricting reproduction and foraging of 

fish. Excessive sediment also fills in 

intermittent pools required for 

amphibian prey reproduction and 

foraging. Fine sediment pollution in 

streams impacted by highway 

construction without the use of 

sediment control structures was 5 to 12 

times greater than control streams. 

Sediment can lead to several effects in 

resident fish species used by northern 

Mexican gartersnakes as prey species, 

which can ultimately cause the northern 

Mexican gartersnake’s increased direct 

mortality, reduced reproductive success, 

lower overall abundance, lower species 

diversity, and reductions in food base as 

documented by Wheeler et al. (2005, p. 

145). The underwater foraging ability of 

northern Mexican gartersnakes can also 

be directly compromised by excessive 

turbidity caused by sedimentation of 

water bodies. Metal contaminants, 

including iron, zinc, lead, cadmium, 

nickel, copper, and chromium, are 

bioaccumulative) and are associated 

with highway construction and use 

(Foreman and Alexander 1998, p. 220; 

Hopkins et al. 1999, p. 1260; Campbell 

et al. 2005, p. 241; Wheeler et al. 2005, 

pp. 146–149). A bioaccumulative 

substance increases in concentration in 

an organism or in the food chain over 

time. A mid- to higher order predator, 

such as a gartersnake, may therefore 

accumulate these types of contaminants 

over time in their fatty tissues and lead 

to adverse health affects. 

Several studies have addressed the 

effects of bioaccumulative substances on 

watersnakes. We find these studies 

relevant because watersnakes and 

gartersnakes have very similar life 

histories and prey bases and therefore, 

the effects from contamination of their 

habitat from bioaccumulative agents are 

expected to have similar effects. 

Campbell et al. (2005, pp. 241–243) 

found that metal concentrations 

accumulated in the northern watersnake 

(Nerodia sipedon) at levels six times 

that of their primary food item, the 

central stoneroller (fish) (Campostoma 

anomalum). Metals, in trace amounts, 

affect the structure and function of the 

liver and kidneys of vertebrates and may 

also act as neurotoxins, affecting 

nervous system function (Rainwater et 

al. 2005, p. 670). Metals may also be 

sequestered in the skin of reptiles, but 

this effect is tempered somewhat by 

ecdysis (the regular shedding or molting 

of the skin) (Burger 1999, p. 212). 

Hopkins et al. (1999, p. 1261) found that 

metals may even interfere with 

metabolic rates of banded watersnakes 

(Nerodia fasciata), altering the 

allocation of energy between 

maintenance and reproduction, 

reducing the efficiency of energy stores, 

and forcing individuals to forage more 

often, which increases activity costs (the 

energy expended in hunting which 

effects the net nutritional intake of an 

organism) and predation risk. 

Snakes of all species are particularly 

vulnerable to mortality when they 

attempt to cross roads. There are several 

reasons for this phenomenon. First, all 

snakes are thigmotherms (animals that 

derive heat from warm surfaces), which 

often compels them to slow down or 

even stop and rest on road surfaces that 

have been warmed by the sun as they 

attempt to cross (Rosen and Lowe 1994, 

p. 143). Additionally, many species of 

snakes are active when traffic densities 

are greatest, as is the case with 

gartersnakes, which are generally 

diurnal (active during daylight hours) 

(Rosen and Lowe 1994, p. 147). Van 

Devender and Lowe (1977, p. 47), 

however, observed several northern 

Mexican gartersnakes crossing the road 

at night after the commencement of the 

summer monsoon, which highlights the 

seasonal variability in surface activity of 

this snake, and many other species of 

reptiles. Perhaps the most common 

factor in road mortality of snakes is the 

propensity for drivers to intentionally 

run over snakes, which generally make 

easy targets because they usually cross 

roads at a perpendicular angle (Klauber 

1956, p. 1026; Langley et al. 1989, p. 47; 

Shine et al. 2004, p. 11). This driving 

behavior is exacerbated by the general 

animosity that humans have toward 

snakes in general in modern-day society 

(Ernst and Zug 1996, p. 75; Green 1997 

pp. 285–286). In fact, Langley et al. 

(1989, p. 47) conducted an experiment 

on the propensity for drivers to hit 

reptiles on the road using turtle and 

snake models and found that many 


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people have a greater desire to hit a 

snake on the road than any other 

animal; several drivers actually stopped 

and backed-over the snake mimic to 

ensure it was dead. Roe et al. (2006, p. 

161) conclude that mortality rates due to 

roads are higher in vagile (mobile) 

species, such as gartersnakes (active 

hunters), than those of more sedentary 

species, such as the North American pit 

vipers in the genera Agkistrodon, 

Sistrurus, and Crotalus, which more 

commonly employ sit-and-wait foraging 

strategies. Roads that bisect wetland 

communities also act as mortality sinks 

in the dispersal or migratory movements 

of snakes (Roe et al. 2006, p. 161). The 

effect of road mortality of snakes 

becomes most significant in the case of 

small, highly fragmented populations 

where the chance removal of mature 

females from the population may 

appreciably degrade the viability of a 

population. 

Roads create easy access to areas 

previously infrequently visited or 

inaccessible to humans, increasing the 

frequency and significance of 

anthropogenic threats to riparian areas 

and fragmenting the landscape, which 

may genetically isolate herpetofaunal 

populations (Rosen and Lowe 1994, pp. 

146–148; Andrews and Gibbons 2005, 

p. 772). 

While snakes of all species may suffer 

direct mortality from attempting to cross 

roads, Andrews and Gibbons (2005, pp. 

777–779) found that many individuals 

of small, diurnal snake species avoid 

open areas (e.g., roads) instinctively in 

order to lower predation rates, which 

represents a different type of threat from 

roads. Shine et al. (2004, p. 9) found 

that the common gartersnake typically 

changed direction when encountering a 

road. These avoidance behaviors by 

individuals aversive to crossing roads 

affect movement patterns and may 

ultimately affect reproductive output 

within populations (Shine et al. 2004, 

pp. 9, 17–19). This avoidance behavior 

has been observed in the common 

gartersnake (Thamnophis sirtalis), a 

sister taxon to the Mexican gartersnake 

with similar life histories and behavior 

(Shine et al. 2004, p. 9). In our 

discussion and as evidenced by the 

literature we reviewed on the effect of 

roads on snake movements, we 

acknowledge the individuality of snakes 

in their behaviors towards road 

crossings in that roads may affect a 

snake’s movement behavior by a variety 

of means and that generalizing these 

resultant behaviors does not adequately 

address this variability. 

In addition to altering the movement 

patterns of some snakes, roads can 

interfere with the male gartersnake’s 

olfactory-driven ability to follow the 

pheromone trails left by receptive 

females (Shine et al. 2004, pp. 17–18). 

This effect to the male’s ability to trail 

females may exacerbate the effects of 

low population density and 

fragmentation that affect several species 

of snakes, including the northern 

Mexican gartersnake. Furthermore, 

roads can facilitate an increase in the 

distance traveled by male snakes 

seeking receptive females, which 

increases exposure to predation and 

subsequently increases mortality rates 

(Shine et al. 2004, pp. 18–19). Although 

the northern Mexican gartersnake was 

not the subject of the 2004 Shine et al. 

study, similar responses can be 

expected in the northern Mexican 

gartersnake because its life history is 

similar to the, study’s subject species 

(i.e., the common gartersnake). 

Roads tend to adversely affect aquatic 

breeding anuran (frog and/or toad) 

populations more so than other species 

due to their activity patterns, population 

structures, and preferred habitats (Hels 

and Buchwald 2001, p. 331). Carr and 

Fahrig (2001, pp. 1074–1076) found that 

populations of highly mobile anuran 

species such as leopard frogs (Rana 

pipiens) were affected more 

significantly than more sedentary 

species and that population persistence 

can be at risk depending on traffic 

densities, which may adversely affect 

the prey base for northern Mexican 

gartersnakes because leopard frogs are a 

primary prey species. 

Recreation in the United States. As 

discussed above, population growth 

trends are expected to continue into the 

future. Expanding population growth 

leads to higher recreational use of 

riparian areas. Riparian areas located 

near urban areas are vulnerable to the 

effects of increased recreation with 

predictable changes in the type and 

intensity of land use following 

residential development. An example of 

such an area within the existing 

distribution of the northern Mexican 

gartersnake is the Verde Valley. The 

reach of the Verde River that winds 

through the Verde Valley receives a high 

amount of recreational use from people 

living in central Arizona (Paradzick et 

al. 2006, pp. 107–108). Increased human 

use results in the trampling of nearshore vegetation, which reduces cover 

for gartersnakes, especially neonates. 

Increased human visitation of occupied 

habitat also increases the potential for 

human-gartersnake interactions, which 

frequently does not bode well for 

snakes, as it often leads to their capture, 

injury, or death of the snake due to the 

lay person’s fear of snakes (Rosen and 

Schwalbe 1988, p. 43; Ernst and Zug 

1996, p. 75; Green 1997, pp. 285–286; 

Nowak and Santana-Bendix 2002, 

p. 39). 

Groundwater Pumping, Surface Water 

Diversions, and Drought in the United 

States. Increased urbanization and 

population growth results in an increase 

in the demand for water and, therefore, 

water development projects. Collier et 

al. (1996, p. 16) mention that water 

development projects are one of two 

main causes of decline of native fish in 

the Salt and Gila rivers of Arizona. 

Municipal water use in central Arizona 

has increased by 39 percent in the last 

8 years (American Rivers 2006). Water 

for development and urbanization is 

often supplied by groundwater pumping 

and surface water diversions from 

sources that include reservoirs and 

Central Arizona Project’s allocations 

from the Colorado River. The hydrologic 

connection between groundwater and 

surface flow of intermittent and 

perennial streams is becoming better 

understood. Groundwater pumping 

creates a cone of depression within the 

affected aquifer that slowly radiates 

outward from the well site. When the 

cone of depression intersects the 

hyporheic zone of a stream (the active 

transition zone between two adjacent 

ecological communities under or beside 

a stream channel or floodplain between 

the surface water and groundwater that 

contributes water to the stream itself), 

the surface water flow may decrease, 

and the subsequent desiccation of 

riparian and wetland vegetative 

communities can follow. Continued 

groundwater pumping at such levels 

draws down the aquifer sufficiently to 

create a water-level gradient away from 

the stream and floodplain (Webb and 

Leake 2005, p. 309). Finally, complete 

disconnection of the aquifer and the 

stream results in strong negative effects 

to riparian vegetation (Webb and Leake 

2005, p. 309). If complete disconnection 

occurs, the hyporheic zone could be 

adversely affected. The hyporheic zone 

can promote ‘‘hot spots’’ of productivity 

where groundwater upwelling occurs by 

producing nitrates that can enhance the 

growth of vegetation, but its significance 

is contingent upon its activity and 

extent of connection with the 

groundwater (Boulton et al. 1998, p. 67; 

Boulton and Hancock 2006, pp. 135, 

138). Changes to the duration and 

timing of upwelling can potentially lead 

to localized extinctions in biota 

(Boulton and Hancock 2006, p. 139). 

To varying degrees, the effects of 

groundwater pumping on surface water 

flow and riparian communities have 

been observed in the Santa Cruz, San 

Pedro, and Verde rivers as a result of 

groundwater demands of Tucson, Sierra 


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Vista, and the rapidly growing Prescott 

Valley, respectively (Stromberg et al. 

1996, pp. 113, 124–128; Rinne et al. 

1998, p. 9; Voeltz 2002, pp. 45–47, 69–

71). Along the upper San Pedro River, 

Stromberg et al. (1996, pp. 124–127) 

found that wetland herbaceous species 

(important as cover for northern 

Mexican gartersnakes) are the most 

sensitive to the effects of a declining 

groundwater level. Webb and Leake 

(2005, pp. 302, 318–320) described a 

correlative trend regarding vegetation 

along southwestern streams from 

historically being dominated by marshy 

grasslands (preferable to northern 

Mexican gartersnakes) to being currently 

dominated by woody species more 

tolerant of declining water tables due to 

their associated deeper rooting depths. 

The full effects of largescale 

groundwater pumping associated with 

the proposed Big Chino Water Ranch 

Project and its associated 30-mile (48 

km), 36-in (91-cm) diameter pipeline 

have yet to be realized in the Verde 

River (McKinnon 2006c). This 

groundwater pumping and inter-basin 

transfer project is projected to deliver 

2.8 billion gallons of groundwater 

annually from the Big Chino sub-basin 

aquifer to the rapidly growing area of 

Prescott Valley for municipal use 

(McKinnon 2006c). The Big Chino subbasin provides 86 percent of the 

baseflow to the upper Verde River 

(American Rivers 2006; McKinnon 

2006a). The potential for this project to 

obtain funding and approval for 

implementation has placed the Verde 

River on American River’s ‘‘Ten Most 

Endangered Rivers List (of 2006)’’

(American Rivers 2006). This potential 

reduction or loss of baseflow in the 

Verde River could seasonally dry up 

large reaches and/or adversely affect the 

riparian community and the suitability 

of the habitat for extant populations of 

the northern Mexican gartersnake and 

its prey species in that area. 

Within the Verde River watershed, 

and particularly within the Verde Valley 

where the northern Mexican gartersnake 

remains extant, several other activities 

continue to threaten surface flows 

(Rinne et al. 1998, p. 9; Paradzick et al. 

2006, pp. 104–110). The demands for 

surface water allocations from rapidly 

growing communities and agricultural 

and mining interests have altered flows 

or dewatered significant reaches during 

the spring and summer months in some 

of the Verde River’s larger, formerly 

perennial tributaries such as Wet Beaver 

Creek, West Clear Creek, and the East 

Verde River, which may have supported 

the northern Mexican gartersnake 

(Girmendock and Young 1993, pp. 45–

47; Sullivan and Richardson 1993, pp. 

38–39; Paradzick et al. 2006, pp. 104–

110). Groundwater pumping in Tonto 

Creek regularly eliminates surface flows 

during parts of the year (Abarca and 

Weedman 1993, p. 2). The upper Gila 

River is also threatened by diversions 

and water allocations. In New Mexico, 

a proposed water project that resulted 

from a landmark Gila River water 

settlement in 2004 allows New Mexico 

the right to withhold 4.5 billion gallons 

of surface water every year (McKinnon 

2006d). If this proposed water diversion 

project is implemented, in dry years, 

currently perennial reaches of the upper 

Gila River will dry completely which 

removes all suitability of this habitat for 

the northern Mexican gartersnakes and 

a host of other riparian and aquatic 

species (McKinnon 2006d). 

Further evidence of the threat of 

groundwater depletion can be found in 

the management activities of the 

Arizona Department of Water Resources 

(ADWR). ADWR manages water 

supplies in Arizona and has established 

five Active Management Areas (AMA) 

across the state (ADWR 2006). An AMA 

is established by ADWR when an area’s 

water demand has exceeded the 

groundwater supply and an overdraft 

has occurred. Geographically, all five 

AMAs overlap the historical 

distribution of the northern Mexican 

gartersnake in Arizona and provide 

further evidence of the role groundwater 

pumping has had and continues to have 

on historical and occupied northern 

Mexican gartersnake habitat. Such 

overdrafts are capable of adversely 

impacting surface water flow of streams 

that are hydrologically connected to the 

aquifer under stress and are often 

exacerbated by the ever-growing number 

of surface water diversions for various 

purposes. 

In order to accommodate the needs of 

rapidly growing rural and urban 

populations, surface water is commonly 

diverted to serve many industrial and 

municipal uses. These diversions have 

dewatered large reaches of once 

perennial or intermittent streams, 

adversely affecting northern Mexican 

gartersnake habitat throughout its range 

in Arizona and New Mexico. Many 

tributaries of the Verde River are 

permanently or seasonally dewatered by 

diversions for agriculture (Paradzick et 

al. 2006, pp. 104–110). 

The effects of the water withdrawals 

discussed above may be exacerbated by 

the current, long-term drought facing 

the arid southwestern United States. 

Philips and Thomas (2005) provided 

streamflow records that indicate that the 

drought Arizona experienced between 

1999 and 2004 was the worst drought 

since the early 1940s and possibly 

earlier. Ongoing drought conditions 

have depleted recharge of aquifers and 

decreased baseflows in the region. 

While drought periods have been 

relatively numerous in the arid 

Southwest according to recorded history 

from the mid-1800s to the present, the 

effects of anthropogenic threats on 

riparian and aquatic communities have 

compromised the ability of these 

communities to function under the 

additional stress of prolonged drought 

conditions. Holycross et al. (2006, pp. 

52–53) recently documented the effects 

of drought on northern Mexican 

gartersnake habitat in the vicinity of 

Arcosante along the Agua Fria River and 

at Big Bug Creek where the streams were 

completely dry and therefore unsuitable 

northern Mexican gartersnake habitats. 

Improper Livestock Grazing in the 

United States. Poorly managed livestock 

grazing has damaged approximately 80 

percent of stream, cienega, and riparian 

ecosystems in the western United States 

(Kauffman and Krueger 1984, pp. 433–

435; Weltz and Wood 1986, pp. 367–

368; Waters 1995, pp. 22–24; Pearce et 

al. 1998, p. 307; Belsky et al. 1999, p. 

1). Livestock grazing, as a resource use 

on public and private lands, has more 

than doubled quantitatively in 50 years; 

the number of cattle being grazed in the 

western United States increased from 

25.5 million head in 1940, to 54.4 

million head in 1990 (Belsky et al. 1999, 

p. 3). 

Effects of improper livestock 

management on riparian and aquatic 

communities have spanned from early 

settlement to modern day. Some 

historical accounts of riparian area 

conditions in Arizona elucidate early 

effects of poor livestock management. 

Cheney et al. (1990, pp. 5, 10) provide 

historical accounts of the early adverse 

effects of improper livestock 

management in the riparian zones and 

adjacent uplands of the Tonto National 

Forest and in south-central Arizona. 

These accounts describe the removal of 

riparian trees for preparation of 

livestock use and substantial changes to 

flow regimes accentuated by observed 

increases in runoff and erosion rates. 

Such accounts of riparian conditions 

within the historical distribution of the 

northern Mexican gartersnake in 

Arizona contribute to the understanding 

of when declines in abundance and 

distribution may have occurred and the 

causes for subsequent fragmentation of 

populations and widespread 

extirpations. 

In the recent past, riparian and 

aquatic communities have been 

negatively impacted by poor livestock 

management (e.g., overgrazing, 

uncontrolled access to riparian areas, 


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improper pasture rotation, no 

monitoring of use, etc.) within several 

watersheds that the northern Mexican 

gartersnake historically occupied, and 

in some cases, poor livestock 

management may constitute the greatest 

impact to riparian vegetation. The 

specific ways in which improper 

livestock grazing can adversely affect 

northern Mexican gartersnakes and 

contribute to their decline is discussed 

below. Watersheds where improper 

grazing has been documented as a 

contributing factor of northern Mexican 

gartersnake declines include the Verde, 

Salt, Agua Fria, San Pedro, Gila, and 

Santa Cruz (Hendrickson and Minckley 

1984, pp. 140, 152, 160–162; Rosen and 

Schwalbe 1988, pp. 32–33; Girmendock 

and Young 1997, p. 47; Voeltz 2002, pp. 

45–81; Krueper et al. 2003, pp. 607, 

613–614; Holycross et al. 2006, pp. 52–

61; McKinnon 2006d, 2006e; Paradzick 

et al. 2006, pp. 90–92). Holycross et al. 

(2006, pp. 53–55, 58) recently 

documented adverse effects from 

improper livestock grazing on northern 

Mexican gartersnake habitat along the 

Agua Fria from EZ Ranch to Bloody 

Basin Road, along Dry Creek from Dugas 

Road to Little Ash Creek, along Little 

Ash Creek from Brown Spring to Dry 

Creek, along Sycamore Creek in the 

vicinity of its confluence with the Verde 

River, and on potential northern 

Mexican gartersnake habitat along Pinto 

Creek at the confluence with the West 

Fork of Pinto Creek. In southeastern 

Arizona, there have been observations of 

effects to the vegetative community 

suggesting that livestock grazing 

activities continue to adversely affect 

extant populations of northern Mexican 

gartersnakes by reducing or eliminating 

cover required by the northern Mexican 

gartersnake for thermoregulation, 

protection from predation, and foraging 

(Hale 2001, pp. 32–34, 50, 56). 

Poor livestock management causes a 

decline in diversity, abundance, and 

species composition of riparian 

herpetofauna communities from direct 

or indirect threats to the prey base, the 

habitat, or to the northern Mexican 

gartersnake itself from: (1) Declines in 

the structural richness of the vegetative 

community; (2) losses or reductions of 

the prey base; (3) increased aridity of 

habitat; (4) loss of thermal cover and 

protection from predators; and (5) a rise 

in water temperatures to levels lethal to 

larval stages of amphibian and fish 

development (Szaro et al. 1985, p. 362; 

Schulz and Leininger 1990, p. 295; 

Belsky et al. 1999, pp. 8–11). Improper 

livestock grazing may also lead to 

desertification (the process of becoming 

arid land or desert as a result of land 

mismanagement or climate change) due 

to a loss in soil fertility from erosion 

and gaseous emissions spurred by a 

reduction in vegetative ground cover 

(Schlesinger et al. 1990, p. 1043). Stock 

tanks may facilitate the spread of 

nonnative species when nonnative 

species of fish, amphibians, and crayfish 

are intentionally or unintentionally 

stocked by anglers and private 

landowners (Rosen et al. 2001, p. 24). 

Specific attributes of ecosystems, such 

as composition, function, and structure, 

have been documented as being altered 

by improper livestock management 

through a variety of means including: 

(1) Decreasing the density and biomass 

of individual species, reducing species 

richness, and changing biological 

community organization; (2) interfering 

with nutrient cycling and ecological 

succession; and (3) changing vegetation 

stratification, contributing to soil 

erosion, and decreasing availability of 

water to biotic communities (Fleischner 

1994, p. 631). 

The management of stock tanks is an 

important consideration for northern 

Mexican gartersnakes. Stock tanks can 

be intermediary ‘‘stepping stones’’ in 

the dispersal of nonnative species from 

larger source populations to new areas 

(Rosen et al. 2001, p. 24). Additionally, 

dense bank and aquatic vegetation is an 

important habitat characteristic for the 

northern Mexican gartersnake that can 

be affected if the impoundment is 

poorly managed, which may lead to 

trampling or overgrazing of the bankside 

vegetation. Poor management may also 

favor nonnative predators of the 

northern Mexican gartersnake (Rosen 

and Schwalbe 1988, pp. 47, 32). 

Alternatively, well-managed stock tanks 

can provide habitat suitable for northern 

Mexican gartersnakes both structurally 

and in terms of prey base, especially 

when the tank remains devoid of 

nonnative species while supporting 

native prey species; provides adequate 

vegetation cover; and provides reliable 

water sources in periods of prolonged 

drought. Given these benefits of wellmanaged stock tanks, we believe wellmanaged stock tanks may be an 

important component to northern 

Mexican gartersnake conservation. 

A key to proper livestock management 

appears to be increasing the distribution 

of cattle across the entire grazing space. 

Fleischner (1994, p. 629) found that 

‘‘Because livestock congregate in 

riparian ecosystems, which are among 

the most biologically rich habitats in 

arid and semiarid regions, the ecological 

costs of grazing are magnified at these 

sites.’’ Stromberg and Chew (2002, p. 

198) and Trimble and Mendel (1995, p. 

243) also discussed the propensity for 

poorly managed cattle to remain within 

or adjacent to riparian communities. 

Trimble and Mendel (1995, p. 243) 

stated that ‘‘Cows, unlike sheep, appear 

to love water and spend an inordinate 

amount of time together lounging in 

streams and ponds, especially in 

summer (surface-active season for 

reptiles and amphibians), sometimes 

going in and coming out several times 

in the course of a day.’’ Expectedly, this 

behavior is more pronounced in more 

arid regions (Trimble and Mendel 1995, 

p. 243). In one rangeland study, it was 

concluded that 81 percent of the 

vegetation that was removed by cattle 

was from a riparian area which 

amounted to only two percent of the 

total grazing space (Trimble and Mendel 

1995, p. 243). Another study reported 

that grazing rates were 5 to 30 times 

higher in riparian areas than on the 

uplands which may be due in part to 

several factors: (1) Higher forage volume 

and palatability of species in riparian 

areas; (2) water availability; (3) the close 

proximity of riparian areas to the best 

upland grazing sites; and (4) 

microclimatic features such as cooler 

temperatures and shade (Trimble and 

Mendel 1995, p. 244). 

The northern Mexican gartersnake 

uses riparian herbaceous vegetation for 

cover, thermoregulation, and foraging. 

Clary and Webster (1989, p. 1) noted 

that excessive grazing and trampling 

from poor livestock management can 

affect riparian and stream communities 

by reducing or eliminating this 

vegetation, causing channel aggradation 

or degradation, causing widening or 

incisement of stream channels, and 

changing streambank morphology, with 

the cumulative result of lowering 

corresponding water tables. In support 

of findings made by Fleischner (1994, 

pp. 631–632), these effects can largely 

be attributed to the tendency of 

livestock in the arid Southwest to spend 

a disproportionately longer time in 

riparian areas than in upland range 

pasture (5–30 times longer, 

comparatively), which leads to 

overgrazing of the riparian vegetation 

(Clary and Medin 1990, p. 1). However, 

even when livestock’s access to riparian 

areas is restricted, poor livestock 

management in the uplands leads to soil 

compaction and decreased filtering 

capacity of vegetation. These effects 

increase the speed and amount of runoff 

from the uplands, which contributes 

heightened, unnatural amounts of 

sediment in aquatic habitat. This 

damages the suitability of that habitat 

and fills in pools, which affects their 

permanency and suitability for many 

prey species of the northern Mexican 


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gartersnake (Sartz and Tolsted 1974, p. 

354; Weltz and Wood 1986, pp. 367–

368; Orodho et al. 1990, p. 9; Trimble 

and Mendel 1995, pp. 235–236; Pearce 

et al. 1998, p. 302). The response of 

riparian herbaceous vegetation after the 

removal of cattle was documented as 

dramatic, with a four to six fold increase 

in density, as observed in the upper San 

Pedro River (Krueper et al. 2003, pp. 

607, 613–614). Schulz and Leininger 

(1990, p. 295) also remarked that 

riparian ecosystems can improve 

quickly when livestock are removed. 

As stated previously, dense vegetative 

cover is an essential component to 

habitat suitable for the northern 

Mexican gartersnake for several reasons 

(Szaro et al. 1985, p. 364; Rosen and 

Schwalbe 1988, p. 47). The removal or 

severe alteration of this habitat 

component significantly affects the 

foraging success and heightens the 

predation risk of the northern Mexican 

gartersnake. Small, isolated populations 

of northern Mexican gartersnakes that 

use stock tanks as refugia may be 

extirpated within 1 year of vegetation 

removal (Rosen and Schwalbe 1988, p. 

33). Northern Mexican gartersnake 

populations that occur in isolated 

wetlands or stock tanks are not likely to 

recolonize naturally (i.e. without 

reestablishment efforts) once extirpated 

due to the species’ tendency to avoid 

long overland movements (Rosen and 

Schwalbe 1988. p. 33). 

Szaro et al. (1985, p. 360) assessed the 

effects of improper livestock 

management on the same stream on a 

sister taxon. They found that western 

(terrestrial) gartersnake (Thamnophis 

elegans vagrans) populations were 

significantly higher (versus controls) in 

terms of abundance and biomass in 

areas that were excluded from grazing, 

where the streamside vegetation 

remained lush, than where uncontrolled 

access to grazing was permitted. This 

effect was complemented by higher 

amounts of cover from organic debris 

from ungrazed shrubs that accumulates 

as the debris moves downstream during 

flood events. Specifically, results 

indicated that snake abundance and 

biomass were significantly higher in 

ungrazed habitat, with a five-fold 

difference in number of snakes 

captured, despite the difficulty of 

making observations in areas of 

increased habitat complexity (Szaro et 

al. 1985, p. 360). Szaro et al. (1985, p. 

362) also noted the importance of 

riparian vegetation for the maintenance 

of an adequate prey base and as cover 

in thermoregulation and predation 

avoidance behaviors, as well as for 

foraging success. 

Direct mortality of amphibian species, 

in all life stages, from being trampled by 

livestock has been documented in the 

literature (Bartelt 1998, p. 96; Ross et al. 

1999, p. 163). The resultant extirpation 

risk of amphibian populations as a prey 

base for northern Mexican gartersnakes 

by direct mortality is governed by the 

relative isolation of the amphibian 

population, the viability of that 

population, and the propensity for 

stochastic events such as wildfires. 

Livestock grazing within habitat 

occupied by northern Mexican 

gartersnakes can result in direct 

mortality of individual gartersnakes as 

observed in a closely related taxon on 

the Apache-Sitgreaves National Forest. 

In that instance, a black-necked 

gartersnake (Thamnophis cyrtopsis 

cyrtopsis) had apparently been killed by 

trampling hoof action of cattle along the 

shore of a stock tank within an actively 

grazed allotment (Chapman 2005). This 

event was not observed first-hand, but 

was supported by postmortem photo 

documentation of the physical injuries 

to the specimen and the location of the 

carcass among a dense cluster of hoof 

tracks along the shoreline of the stock 

tank. It is also unlikely that a predator 

would kill the snake and leave it 

uneaten. While this type of direct 

mortality of gartersnakes has long been 

suspected by agency biologists and 

academia, this may be the first recorded 

observation of direct mortality of a 

gartersnake due to livestock trampling. 

We expect this type of direct mortality 

to be uncommon but significant in the 

instance of a fragmented population 

with a skewed age-class distribution and 

low to no recruitment as currently 

observed in many northern Mexican 

gartersnake populations in the United 

States. In these circumstances, the loss 

of one or more adults, most notably 

reproductive females, may lead directly 

to extirpation of the species from a 

given site with no expectation of 

recolonization. 

Our analysis of the best available 

scientific and commercial information 

available indicates that adverse effects 

from improper livestock management on 

the northern Mexican gartersnake, its 

habitat, and its prey base can be 

significant. However, we recognize that 

well-managed grazing can occur with 

limited effects to this species when 

management emphasis is directed to 

moderated access restrictions for 

occupied habitat combined with the use 

of remote drinkers (containerized water 

sources supplied by water pumped from 

a nearby source) as well as other 

livestock management protocols that 

lessen the effect of vegetation 

disturbance and removal adjacent to 

occupied habitat by increasing the 

distribution of cattle across an 

allotment. Lastly, as previously stated, 

we also recognize the value of wellmanaged stock tanks in the conservation 

of northern Mexican gartersnakes. 

Catastrophic Wildfires in the United 

States. Low-intensity fire has been a 

natural disturbance factor in forested 

landscapes for centuries, and lowintensity fires were common in 

southwestern forests prior to European 

settlement (Rinne and Neary 1996, pp. 

135–136). Rinne and Neary (1996, p. 

143) discuss the current effects of fire 

management policies on aquatic 

communities in Madrean-type 

ecosystems in the southwestern United 

States. They concluded that existing 

wildfire suppression policies intended 

to protect the expanding number of 

human structures on forested public 

lands have altered the fuel loads in 

these ecosystems and increased the 

probability of devastating wildfires. The 

effects of these catastrophic wildfires 

include the removal of vegetation, the 

degradation of watershed condition, 

altered stream hydrographs, and 

increased sedimentation of streams. 

These effects can harm fish 

communities, as observed in the 1990 

Dude Fire, in which corresponding ash 

flows decimated some fish populations 

in Dude Creek and the East Verde River 

(Voeltz 2002, p. 77). These effects can 

significantly lessen the prey base for 

northern Mexican gartersnakes and 

could lead to direct mortality in the case 

of fires that are within occupied habitat. 

Fire has also become an increasingly 

significant threat in lower elevation 

communities as well. Esque and 

Schwalbe (2002, pp. 180–190) discuss 

the effect of wildfires in the upper and 

lower subdivisions of Sonoran 

desertscrub where the northern Mexican 

gartersnake historically occurred. The 

widespread invasion of nonnative 

annual grasses, such as brome species 

(Bromus sp.) and Mediterranean grasses 

(Schismus sp.), appear to be largely 

responsible for altered fire regimes that 

have been observed in these 

communities, which are not adapted to 

fire (Esque and Schwalbe 2002, p. 165). 

In areas comprised entirely of native 

species, ground vegetation density is 

mediated by barren spaces that do not 

allow fire to carry itself across the 

landscape. However, in areas where 

nonnative grasses have become 

established, the fine fuel load is 

continuous, and fire is capable of 

spreading quickly and efficiently (Esque 

and Schwalbe 2002, p. 175). After 

disturbances such as fire, brome grasses 

may exhibit dramatic population 


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explosions, which hasten their effect on 

native vegetative communities. 

Additionally, with increased fire 

frequency, these population explosions 

ultimately lead to a type-conversion of 

the vegetative community from 

desertscrub to grassland (Esque and 

Schwalbe 2002, pp. 175–176). Fires 

carried by the fine fuel loads created by 

nonnative grasses often burn at 

unnaturally high temperatures, which 

may result in soils becoming 

hydrophobic (water repelling), 

exacerbate sheet erosion, and contribute 

large amounts of sediment to receiving 

water bodies, thereby affecting the 

health of the riparian community (Esque 

and Schwalbe 2002, pp. 177–178). The 

siltation of isolated, remnant pools in 

intermittent streams has significant 

effects on lower-elevation species, as 

observed in lowland leopard frogs and 

native fish, important prey species for 

northern Mexican gartersnakes (Esque 

and Schwalbe 2002, p. 190). 

Undocumented Immigration and 

International Border Enforcement and 

Management in the United States. 

Undocumented immigrants attempt to 

cross the International border from 

Mexico into the United States in areas 

historically or currently occupied by the 

northern Mexican gartersnake. This 

method of immigration and the 

corresponding efforts to enforce U.S. 

border laws and policies have been 

occurring for many decades with 

increasing intensity and have resulted 

in unintended adverse effects to biotic 

communities in the border region. 

During the warmest months of the year, 

many attempted border crossings occur 

in riparian areas that serve to provide 

shade, water, and cover. Increased U.S. 

border enforcement efforts that began in 

the early 1990s in California and Texas 

have resulted in concentrated levels of 

attempted undocumented immigrant 

crossings into Arizona (Segee and 

Neeley 2006, p. 6). 

Riparian habitats that historically 

supported or may currently support 

northern Mexican gartersnakes in the 

San Bernardino National Wildlife 

Refuge, the San Pedro River corridor, 

the Santa Cruz River corridor, the lower 

Colorado River corridor, and along 

many smaller streamside and canyon 

bottom areas within Cochise, Santa 

Cruz, and Pima counties have high 

levels of undocumented immigrant 

traffic (Segee and Neeley 2006, 

Executive Summary, pp. 10–12, 21–23). 

Use of new roads and trails from 

immigration and enforcement activities, 

as well as the construction, use, and 

maintenance of enforcement 

infrastructure (i.e., fences, walls, and 

lighting systems), leads to compaction 

of streamside soils, and the destruction 

and removal of riparian vegetation 

necessary as cover for the northern 

Mexican gartersnake. These activities 

also serve as a source of additional 

sediment to streams that affect their 

suitability as habitat for prey species of 

the northern Mexican gartersnake and 

affect the suitability and availability of 

pool habitats by filling them in with 

sediment. Riparian areas along the 

upper San Pedro River have been 

impacted by out of control fires that 

undocumented immigrants likely 

started to keep warm and/or prepare 

food (Segee and Neeley 2006, p. 23). 

There also remains the threat of pursuit, 

capture, and death of northern Mexican 

gartersnakes when they are encountered 

by undocumented immigrants and 

border enforcement personnel in high 

use areas due to the snake’s stigma in 

society (Rosen and Schwalbe 1988, p. 

43; Ernst and Zug 1996, p. 75; Green 

1997, pp. 285–286; Nowak and Santana 

Bendix 2002, p. 39). 

The wetland habitat within the San 

Bernardino National Wildlife Refuge has 

been adversely affected by 

undocumented immigration. It is 

estimated that approximately 1,000 

undocumented immigrants per month 

use these important wetlands for 

bathing, drinking, and other uses during 

their journey northward. These 

activities can contaminate the water 

quality of the wetlands and lead to 

reductions in the prey base for the 

northern Mexican gartersnake, as well 

as increase exposure of the snake to 

humans, and thereby increase direct 

mortality rates (Rosen and Schwalbe 

1988, p. 43; Ernst and Zug 1996, p. 75; 

Green 1997, pp. 285–286; Nowak and 

Santana-Bendix 2002, p. 39; Segee and 

Neeley 2006, pp. 21–22). In addition, 

numerous observations of littering and 

destruction of vegetation and wildlife 

occur annually throughout the San 

Bernardino National Wildlife Refuge, 

which adversely affect the quality and 

quantity of vegetation as habitat for the 

northern Mexican gartersnake (USFWS 

2006, p. 95). 

There remains the possibility that 

adverse effects to riparian communities 

may increase in the future as land 

access and infrastructure restrictions in 

sensitive wildlife areas may be relaxed 

according to proposed policy changes 

that aim to boost border enforcement 

activities in these currently roadless 

areas and as concentrated enforcement 

efforts in urban locations funnel more 

undocumented immigrant traffic to 

remote wilderness areas (Segee and 

Neeley 2006, pp. 15–16). 

Habitat Threats in Mexico. Threats to 

northern Mexican gartersnake habitat in 

Mexico include the intentional and 

unintentional introductions of 

nonnative species, improper livestock 

grazing, urbanization and development, 

water diversions and groundwater 

pumping, loss of vegetation cover and 

deforestation, erosion, and pollution, as 

well as impoundments and dams that 

have modified or destroyed riparian and 

aquatic communities within Mexico in 

areas where the species occurred 

historically (Conant 1974, p. 471; 

Contreras Balderas and Lozano 1994, p. 

384; va Landa et al. 1997, p. 316; Miller 

et al. 2005, pp. 60–61; Abarca 2006). We 

experienced difficulty finding specific 

information documenting that 

populations of northern Mexican 

gartersnakes in Mexico are directly 

affected by these threats which is 

problematic in a rangewide analysis 

given that approximately 70 to 80 

percent of the historic distribution of 

the northern Mexican gartersnake 

occurs in Mexico. We did, however, 

find enough information to provide 

some refined discussion of smaller 

geographic areas within Mexico, and 

acknowledge that many of the threats 

that affect the northern Mexican 

gartersnake in the United States also 

occur in Mexico and could affect the 

northern Mexican gartersnake in similar 

ways but at potentially varying 

intensities. 

Conant (2003, p. 4) noted 

anthropogenic threats to seven 

fragmented, endemic subspecies of 

Mexican gartersnake in the 

Transvolcanic Belt Region of southern 

Mexico, which extends from southern 

Jalisco eastward through the state of 

Me ́xico to central Veracruz which 

comprises a small proportion of the 

subspecies’ range. Although Conant 

(2003) addresses threats to a small 

percentage of the historic distribution, 

many of these rural land uses are 

regionally ubiquitous and therefore 

these threats can be extrapolated to the 

surrounding vicinity of the distribution 

of these seven recently described 

subspecies of the Mexican gartersnake 

in Mexico. Some of these threats 

included water diversions, pollution 

(e.g., discharge of raw sewage), 

sedimentation of aquatic habitats, and 

eutrophication (increase of dissolved 

nutrients and decrease of dissolved 

oxygen) of lentic (still water) habitats. 

Conant (2003, p. 4) expressed great 

concern that while many of these threats 

were evident during his field work in 

the 1960s, they are ‘‘continuing with 

increased velocity.’’

Water pollution, dams, groundwater 

pumping, and impoundments were 

identified by Miller et al. (2005, pp. 60–

61) as significant threats to aquatic 


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biota. Miller et al. (2005, p. 60) stated 

that ‘‘During the time we have 

collectively studied fishes in Me ́xico 

and southwestern United States, the 

entire biotas of long reaches of major 

streams [where the northern Mexican 

gartersnake is distributed] such as the 

Rı ́o Grande de Santiago below 

Guadalajara (Jalisco) and Rı ́o Colorado 

downstream of Hoover (Boulder) Dam, 

have simply been destroyed by 

pollution and river alteration.’’ Near 

Torreo ́n, Coahuila, where the northern 

Mexican gartersnake was historically 

distributed, groundwater pumping has 

resulted in flow reversal, which has 

driedup many local springs, drawn 

arsenicladen water, further 

contaminated the area, and resulted in 

adverse human health effects in that 

area. Severe water pollution from 

untreated domestic waste is evident 

downstream of large Mexican cities, and 

inorganic pollution from nearby 

industrialized areas and agricultural 

irrigation return flow has dramatically 

affected aquatic communities (Miller et 

al. 2005, p. 60). Miller et al. (2005, p. 61) 

provides an excerpt from Soto Galera et 

al. (1999) addressing the threats to the 

Rı ́o Lerma (Mexico’s longest river) 

where the northern Mexican gartersnake 

was historically distributed: ‘‘The basin 

has experienced a staggering amount of 

degradation during the 20th Century. By 

1985–1993, over half of our study sites 

had disappeared or become so polluted 

that they could no longer support fishes. 

Only 15 percent of the sites were still 

capable of supporting sensitive species. 

Forty percent (17 different species) of 

the native fishes of the basin had 

suffered major declines in distribution, 

and three species may be extinct. The 

extent and magnitude of degradation in 

the Rı ́o Lerma basin matches or exceeds 

the worst cases reported for comparably 

sized basins elsewhere in the world.’’

Several rivers within the historic 

distribution of the northern Mexican 

gartersnake have been impounded and 

dammed throughout Mexico, resulting 

in habitat modification and the 

dispersal and establishment of 

nonnative species. The damming and 

modification of the Rı ́o Colorado, where 

the northern Mexican gartersnake was 

distributed, has facilitated the 

replacement of the entire native fishery 

with nonnative species (Miller et al. 

2005, p. 61). Nonnative species continue 

to pose significant threats in the decline 

of native, often endemic, prey species of 

the northern Mexican gartersnake in 

several regions of Mexico, as discussed 

further in Factor C below (Miller et al. 

2005, p. 60). 

Miller et al. (2005) does provide some 

locality specific information on the 

status and threats of freshwater fishes 

and riparian and aquatic communities 

in specific waterbodies throughout 

Mexico that historically overlapped, or 

are adjacent to, the historic distribution 

of the northern Mexican gartersnake: the 

Rı ́o Grande (dam construction, p. 78); 

the Rı ́o Bravo (extirpations, pp. 82, 112); 

headwaters of the Rı ́o Lerma 

(extinction/rediscovery, nonnatives, 

pollution, dewatering, pp. 60, 105, 197); 

Lago de Chapala and its outlet to the Rı ́o 

Grande de Santiago (major declines, p. 

106); medium-sized streams throughout 

the Sierra Madre Occidental (localized 

extirpations, logging, dewatering, pp. 

109, 177, 247); the Rı ́o Conchos 

(extirpations, p. 112); the rı ́os Casas 

Grandes, Santa Marı ́a, del Carmen, and 

Laguna Bustillos (diversions, 

groundwater pumping, channelization, 

flood control practices, pollution, and 

introduction of nonnative species, pp. 

124, 197); the Rı ́o Santa Cruz 

(extirpations, p. 140); the Rı ́o Yaqui 

(nonnatives, pp. 148, Plate 61); the Rı ́o 

Colorado (nonnatives, p. 153); the rı ́os 

Fuerte and Culiaca ́n (logging, p. 177); 

canals, ponds, lakes in the endorheic 

(closed) Valle de Me ́xico (nonnatives, 

extirpations, pollution, pp. 197, 281); 

the Rı ́o Verde Basin (dewatering, 

nonnatives, extirpations, Plate 88); the 

Rı ́o Mayo (dewatering, nonnatives, p. 

247); the Rı ́o Papaloapan (pollution, p. 

252); lagos de Zacapu and Yuriria 

(habitat destruction, p. 282); and the Rı ́o 

Pa ́nuco Basin (nonnatives, p. 295). 

Conant (1974, pp. 486–489) described 

significant threats to northern Mexican 

gartersnake habitat within its historical 

distribution in various locations in 

western Chihuahua, Mexico, and within 

the Rio Concho system where it is 

known to occur. These threats 

specifically included impoundments, 

diversions, and purposeful 

introductions of largemouth bass, 

common carp, and bullfrogs. We discuss 

the threats from nonnative species 

introductions below in our discussion of 

Factor C. McCranie and Wilson (1987, 

p. 2) discuss threats to the pine-oak 

communities of higher elevation 

habitats in the Sierra Madre Occidental, 

specifically noting that ‘‘ * * * the 

relative pristine character of the pine 

oak woodlands is threatened * * * 

every time a new road is bulldozed up 

the slopes in search of new madera or 

pasturage. Once the road is built, further 

development follows; pueblos begin to 

pop up along its length, especially if the 

road is paved as has been the case with 

(Mexican) Highway 40 through southern 

Durango. We feel fortunate to have 

worked in an area of this country of 

rapid population growth that is all too 

fast disappearing.’’ In Mexico, as 

compared to the United States, there is 

believed to be a delay in the magnitude 

and significance of adverse effects to 

riparian communities, but it is believed 

that threats to riparian and aquatic 

communities that have been observed in 

Arizona as described below are 

currently occurring with increasing 

significance in several regions across 

Mexico within the historic distribution 

of the northern Mexican gartersnake 

(Conant 1974, pp. 471, 487–489; 

Contreras Balderas and Lozano 1994, 

pp. 379–381; va Landa et al. 1997, 

p. 316; Miller et al. 2005, p. 60–61; 

Abarca 2006; Rosen 2006). 

Collectively, the impacts described 

above are expected to continue as a 

result of Mexico’s expanding role as an 

economical labor force for international 

manufacturing under the North 

American Free Trade Agreement 

(NAFTA) and the subsequent increase 

in population size, economic growth 

and development, and infrastructure. 

Mexico’s human population grew 700 

percent from 1910 to 2000 (Miller et al. 

2005, p. 60). More recently, Mexico’s 

population increased by 245 percent 

from 1950 to 2002, and is projected to 

grow by another 28 percent by 2025 

(EarthTrends 2005). As of 1992, Mexico 

had the second highest gross domestic 

product in Latin America at 5.8 percent, 

following Brazil (DeGregorio 1992, 

p. 60). As a result of NAFTA, the 

number of maquiladoras (export 

assembly plants) is expected to increase 

by as many as 3,000 to 4,000 (Contreras 

Balderas and Lozano 1994, p. 384). To 

accommodate Mexico’s increasing 

population, rural areas are largely 

devoted to food production based on 

traditional methods, which has led to 

serious losses in vegetative cover and 

soil erosion (va Landa et al. 1997, 

p. 316). To increase forage and stocking 

rates for livestock production in the arid 

lowlands of northern Mexico, African 

buffelgrass (Pennisetum ciliare) was 

widely introduced in Mexico and has 

spread on its own (Bu ́rquez-Montijo et 

al. 2002, p. 131). Buffelgrass invasions 

pose a serious threat to native arid 

ecosystems because buffelgrass prevents 

germination of native species, competes 

for water, crowds out native vegetation, 

and creates fine fuels in vegetation 

communities not adapted to fire; in such 

native arid ecosystems, buffelgrass has 

caused many changes, including severe 

soil erosion (Bu ́rquez-Montijo et al. 

2002, pp. 135, 138). Erosion affects the 

suitability of habitat for northern 

Mexican gartersnakes and their prey 

species. Recent estimates indicate that 

80 percent of Mexico is affected by soil 


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erosion with the most serious erosion 

occurring in the states of Guanajuato (43 

percent of the state’s land area), Jalisco 

(25 percent of the state’s land area), and 

Me ́xico (25 percent of the state’s land 

area) (va Landa et al. 1997, p. 317), the 

states in which the northern Mexican 

gartersnake historically occurred. 

The threats to riparian and aquatic 

communities in Mexico (such as the 

intentional and unintentional 

introductions of nonnative species, 

improper livestock grazing, urbanization 

and development, water diversions and 

groundwater pumping, loss of 

vegetation cover and deforestation, 

erosion, pollution, impoundments, and 

dams) vary in their significance both 

geographically and ecologically, based 

on geographical distribution of land 

management activities and urban 

centers, but are expected to continue 

into the future. Threats that affect the 

amount of water within an occupied 

area directly affect its suitability to 

northern Mexican gartersnakes. Threats 

that alter the vegetation of occupied 

habitat reduce the habitat’s suitability as 

cover for protection from predators, as 

a foraging area, and as an effective 

thermoregulatory site. Nonnative 

species, explained further in our Factor 

C discussion, compete with the northern 

Mexican gartersnake for prey as well as 

prey on juvenile and sub-adult northern 

Mexican gartersnakes, which hampers 

the recruitment of young snakes into the 

population and lessens the viability of 

that population over time. However, 

because specific and direct survey 

information is significantly limited 

concerning the presence and potential 

effect of these threats to the subspecies 

in Mexico, this discussion is based on 

extrapolation of how we understand 

these threats to affect the subspecies in 

the United States. Furthermore, the 

subspecies was historically distributed 

in several regions within Mexico that 

have remained roadless and isolated, 

thus suggesting that the severity of 

threats may be less than that found 

within the range in United States where 

lands have greater past and current 

economic pressures such as grazing and 

development. As such we can not 

conclude that the subspecies is likely to 

become endangered throughout its range 

in Mexico. Although we acknowledge 

that these threats are affecting the 

subpecies in the United States, we have 

determined that the portion of the 

subspecies’ range in the United States 

does not constitute a significant portion 

of the range of the subspecies or a DPS. 

Therefore, on the basis of the best 

available information, we determine 

that it is not likely that the northern 

Mexican gartersnake will become an 

endangered species within the 

foreseeable future based on threats 

under this factor. 

B. Overutilization for commercial, 

recreational, scientific, or educational 

purposes 

The northern Mexican gartersnake 

may not be collected in the United 

States without special authorization by 

the Arizona Game and Fish Department 

or the New Mexico Department of Game 

and Fish. We have found no evidence 

that current or historical levels of lawful 

or unlawful field collecting of northern 

Mexican gartersnakes has played a 

significant role in the decline of this 

species. The Arizona Game and Fish 

Department recently produced field 

identification cards for distribution that 

provide information to assist with the 

field identification of each of Arizona’s 

five native gartersnake species as well as 

guidance on submitting photo vouchers 

for university museum collections. 

Additionally, universities such as 

Arizona State University and the 

University of Arizona recently began to 

accept photo voucher record, versus 

physical specimens, in their respective 

museum collections. We believe these 

measures further reduce the necessity 

for field biologists to collect physical 

specimens (unless discovered 

postmortem) for locality voucher 

purposes and therefore further reduce 

impacts to vulnerable populations from 

formal biological field investigations 

and field specimen collections. We were 

unable to obtain any information about 

the effect of overutilization for 

commercial, recreational, scientific, or 

educational purposes in Mexico. 

Specific discussion of the regulatory 

protections for the northern Mexican 

gartersnake is provided under Factor D 

‘‘Inadequacy of Existing Regulatory 

Mechanisms’’ below. 

C. Disease or Predation 

Disease 

Disease in northern Mexican 

gartersnakes has not yet been 

documented as a specific threat in the 

United States or Mexico. However, 

because little is known about disease in 

wild snakes, it is premature to conclude 

that there is no disease threat that could 

directly affect remaining northern 

Mexican gartersnake populations (Rosen 

2006). 

Disease and nonnative parasites have 

been implicated in the decline in the 

prey base of the northern Mexican 

gartersnake. The outbreak of chytrid 

fungus (of the genus Batrachochytrium) 

has been identified as a chief causative 

agent in the significant declines of many 

of the native ranid frogs and other 

amphibian species, and regional 

concerns exist for the native fish 

community due to nonnative parasites 

such as the Asian tapeworm 

(Bothriocephalus achelognathi) in 

southeastern Arizona (Rosen and 

Schwalbe 1997, pp. 14–15; 2002c, pp. 

1–19; Morell 1999, pp. 728–732; Sredl 

and Caldwell 2000, p. 1; Hale 2001, pp. 

32–37; Bradley et al. 2002, p. 206). The 

chytrid fungus has been implicated in 

both large-scale declines and local 

extirpations of many amphibians, 

chiefly anuran species, around the 

world (Johnson 2006, p. 3011). Lips et 

al. (2006, pp. 3166–3169) suggest that 

the high virulence and large number of 

potential hosts make the chytrid fungus 

a serious threat to amphibian diversity. 

In Arizona, chytrid infections have been 

reported in several northern Mexican 

gartersnake native prey species (Morell 

1999, pp. 731–732; Sredl and Caldwell 

2000, p. 1; Hale 2001, pp. 32–37; 

Bradley et al. 2002, p. 207; USFWS 

2002a, pp. 40802–40804). Declines of 

native prey species of the northern 

Mexican gartersnake from chytrid 

infections have contributed to the 

decline of this species in the United 

States. However, we do not have 

specific information regarding potential 

impacts of chytrid infections on 

northern Mexican gartersnake native 

prey species in Mexico. 

We also note that in a pure culture 

(uncontaminated growth medium), the 

fungus Batrachochytrium can grow on 

boiled snakeskin (keratin), which 

indicates the potential for the fungus to 

live saprobically (obtaining nutrients 

from non-living organic matter, 

commonly dead and decaying plant or 

animal matter, by absorbing soluble 

organic compounds) on gartersnake skin 

in the wild if other components of the 

ecosystem limit the growth of 

competing bacteria and oomycetes (a 

taxonomic group of fungi that produce 

oospores such as the genera Pythium, 

Phytophthora, and Aphanomyces) 

(Longcore et al. 1999, p. 227). While the 

genus Batrachochytrium has been 

grown on snakeskin in the laboratory, 

no reports of the organism on reptilian 

hosts in the wild have been 

documented. We anticipate diligence in 

monitoring the status of incidence of 

this disease in this species in the wild 

for early detection purposes should this 

potential threat come to fruition in wild 

populations of northern Mexican 

gartersnakes. 

Nonnative Species Interactions 

A host of native predators prey upon 

northern Mexican gartersnakes 


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including birds of prey, other snakes 

[kingsnakes (Lampropeltis sp.), 

whipsnakes (Masticophis sp.), etc.], 

wading birds, raccoons (Procyon lotor), 

skunks (Mephitis sp.), and coyotes 

(Canis latrans) (Rosen and Schwalbe 

1988, p. 18). However, nonnative 

species, such as the bullfrog, the 

northern (virile) (Orconectes virilis) and 

red swamp (Procambarus clarki) 

crayfish, and numerous species of exotic 

sport and bait fish species continue to 

be the most prominent threat to the 

northern Mexican gartersnake and to its 

prey base from direct predation, 

competition, and modification of habitat 

in the United States and potentially in 

Mexico (Conant 1974, pp. 471, 487–489; 

Meffe 1985, pp. 179–185; Rosen and 

Schwalbe 1988, pp. 28, 32; 1997, p. 1; 

Bestgen and Propst 1989, pp. 409–410; 

Clarkson and Rorabaugh 1989, pp. 531, 

535; Marsh and Minckley 1990, p. 265; 

Stefferud and Stefferud 1994, p. 364; 

Rosen et al. 1995, pp. 257–258; 1996b, 

pp. 2, 11–13; 2001, p. 2; Degenhardt et 

al. 1996, p. 319; Fernandez and Rosen 

1996, pp. 8, 23–27; Weedman and 

Young 1997, pp. 1, Appendices B, C; 

Inman et al. 1998, p. 17; Rinne et al. 

1998, pp. 4–6; Fagan et al. 2005, pp. 34, 

34–41; Olden and Poff 2005, pp. 82–87; 

Unmack and Fagan 2004, p. 233; Miller 

et al. 2005, pp. 60–61; Abarca 2006; 

Brennan and Holycross 2006, p. 123; 

Holycross et al. 2006, pp. 13–15; Rosen 

and Melendez 2006, p. 54). 

Nonnative Species Interactions in the 

United States. Nonnative species 

represent serious threats to the northern 

Mexican gartersnake through 

competition for prey, direct predation, 

and alteration of habitat. Riparian and 

aquatic communities have been 

dramatically impacted by a shift in 

species’ composition. Specifically, 

riparian and wetland communities have 

experienced a shift from being 

historically dominated by native fauna 

to being increasingly occupied by an 

expanding assemblage of nonnative 

plant and animal species that have been 

intentionally or accidentally introduced, 

or have colonized new areas from 

neighboring occupied localities. For 

example, nonnative shrub species in the 

genus Tamarix have been widely 

introduced throughout the western 

States and appear to thrive in regulated 

river systems (Stromberg and Chew 

2002, pp. 210–213). Tamarix invasions 

may result in habitat alteration from 

potential effects to water tables, changes 

to canopy and ground vegetation 

structures, and increased fire risk, 

which hasten the demise of native 

cottonwood and willow communities 

and affect the suitability of the 

vegetation component to northern 

Mexican gartersnake habitat (Stromberg 

and Chew 2002, pp. 211–212; USFWS 

2002b, p. H–9). 

Declines in the Northern Mexican 

Gartersnake Anuran Prey Base in the 

United States. The decline of the 

northern Mexican gartersnake within its 

historical and extant distribution was 

subsequent to the declines in its prey 

base (native amphibian and fish 

populations) from introductions of 

nonnative bullfrogs, crayfish, and 

numerous species of exotic sport and 

bait fish as documented in an extensive 

body of literature (Nickerson and Mays 

1970, p. 495; Hulse 1973, p. 278; Vitt 

and Ohmart 1978, p. 44; Meffe 1985, pp. 

179–185; Ohmart et al. 1988, pp. 143–

147; Rosen and Schwalbe 1988, pp. 28–

31; 1997, pp. 8–16; Bestgen and Propst 

1989, pp. 409–410; Clarkson and 

Rorabaugh 1989, pp. 531–538; Marsh 

and Minckley 1990, p. 265; Sublette et 

al. 1990, pp. 112, 243, 246, 304, 313, 

318; Stefferud and Stefferud 1994, p. 

364; Holm and Lowe 1995, p. 5; Rosen 

et al. 1995, pp. 251, 257–258; 1996a, pp. 

2–3; 1996b, p. 2; 2001, p. 2; Sredl et al. 

1995a, pp. 7–8; 1995b, pp. 8–9; 1995c, 

pp. 7–8; 2000, p. 10; Degenhardt et al. 

1996, p. 319; Fernandez and Rosen 

1996, pp. 8–27; Drost and Nowak 1997, 

p. 11; Weedman and Young 1997, pp. 1, 

Appendices B, C; Inman et al. 1998, p. 

17; Rinne et al. 1998, pp. 4–6; Turner et 

al. 1999, p. 11; Nowak and Spille 2001, 

p. 11; Bonar et al. 2004, p. 3; Fagan et 

al. 2005, pp. 34, 34–41; Olden and Poff 

2005, pp. 82–87; Holycross et al. 2006, 

pp. 13–15, 52–61; Brennan and 

Holycross 2006, p. 123). The northern 

Mexican gartersnake is particularly 

vulnerable to a loss in native prey 

species (Rosen and Schwalbe 1988, p. 

20). Rosen et al. (2001, pp. 10, 13, 19) 

examined this issue in detail and 

proposed a hypothesis involving two 

reasons for the decline in northern 

Mexican gartersnakes following the loss 

or decline in the native prey base: (1) 

The northern Mexican gartersnake is 

unlikely to increase foraging efforts at 

the risk of increased predation; and (2) 

the species needs substantial food 

regularly to maintain its weight and 

health. If forced to forage more often for 

smaller prey items, a reduction in 

growth and reproductive rates will 

result (Rosen et al. 2001, pp. 10, 13). 

Native ranid frog species such as 

lowland leopard frogs, northern leopard 

frogs, and federally threatened 

Chiricahua leopard frogs have all 

experienced significant declines 

throughout their distribution in the 

Southwest, partially due to predation 

and competition with nonnative species 

(Clarkson and Rorabaugh 1989, pp. 531, 

535; Hayes and Jennings 1986, p. 490). 

Rosen et al. (1995, pp. 257–258) found 

that Chiricahua leopard frog distribution 

in the Chiricahua Mountain region of 

Arizona was inversely related to 

nonnative species distribution and 

without corrective action, predicted that 

the Chiricahua leopard frog will be 

extirpated from this region. Along the 

Mogollon Rim, Holycross et al. (2006, p. 

13) found that only 8 sites of 57 

surveyed (15 percent) consisted of an 

entirely native anuran community and 

that native frog populations in another 

19 sites (33 percent) had been 

completely displaced by invading 

bullfrogs. 

Declines in the native leopard frog 

populations in Arizona have 

significantly contributed to declines in 

the northern Mexican gartersnake, as a 

primary native predator. Scotia Canyon 

in the Huachuca Mountains of 

southeastern Arizona is a location 

where corresponding declines between 

leopard frog and northern Mexican 

gartersnake populations has been 

documented through repeated survey 

efforts over time (Holm and Lowe 1995, 

p. 33). Surveys of Scotia Canyon 

occurred during the early 1980s and 

again during the early 1990s. Leopard 

frogs in Scotia Canyon were 

infrequently observed during the early 

1980s and were apparently extirpated 

by the early 1990s (Holm and Lowe 

1995, pp. 45–46). Northern Mexican 

gartersnakes in low numbers were 

observed in decline during the early 

1980s with low capture rates remaining 

through the early 1990s (Holm and 

Lowe 1995, pp. 27–35). Surveys 

documented further decline in 2000 

(Rosen et al. 2001, pp. 15–16). A former 

stronghold for the northern Mexican 

gartersnake, the San Bernardino 

National Wildlife Refuge has also been 

affected by correlative declines between 

leopard frog and northern Mexican 

gartersnake populations (Rosen and 

Schwalbe 1988, p. 28; 1995, p. 452; 

1996, pp. 1–3; 1997, p. 1; 2002b, pp. 

223–227; 2002c, pp. 31, 70; Rosen et al. 

1996b, pp. 8–9; 2001, pp. 6–10). 

Declines of leopard frog populations, 

often correlated with nonnative species 

introductions (but also with the spread 

of chytridiomycosis, symptomatic 

disease caused by the chytrid fungus, 

and habitat modification and 

destruction), has not just occurred 

throughout southeastern Arizona, but 

throughout much of the U.S. 

distribution of the northern Mexican 

gartersnake based on survey data 

(Nickerson and Mays 1970, p. 495; Vitt 

and Ohmart 1978, p. 44; Ohmart et al. 

1988, p. 150; Rosen and Schwalbe 1988, 


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Appendix I; 1995, p. 452; 1996, pp. 1–

3; 1997, p. 1; 2002b, pp. 232–238; 

2002c, pp. 1, 31; Clarkson and 

Rorabaugh 1989, pp. 531–538; Sredl et 

al. 1995a, pp. 7–8; 1995b, pp. 8–9, 

1995c, pp. 7–8; 2000, p. 10; Holm and 

Lowe 1995, pp. 45–46; Rosen et al. 

1996b, p. 2; 2001, pp. 2, 22; Degenhardt 

et al. 1996, p. 319; Fernandez and Rosen 

1996, pp. 6–20; Drost and Nowak 1997, 

p. 11; Turner et al. 1999, p. 11; Nowak 

and Spille 2001, p. 32; Holycross et al. 

2006, pp. 13–14, 52–61). Specifically, 

Holycross et al. (2006, pp. 53–57, 59) 

recently documented extirpations of the 

northern Mexican gartersnake’s native 

leopard frog prey base at several 

currently historically, or potentially 

occupied locations including the Agua 

Fria River in the vicinity of Table Mesa 

Road and Little Grand Canyon Ranch 

and at Rock Springs, Dry Creek from 

Dugas Road to Little Ash Creek, Little 

Ash Creek from Brown Spring to Dry 

Creek, Sycamore Creek (Agua Fria 

watershed) in the vicinity of the Forest 

Service Cabin, at the Page Springs and 

Bubbling Ponds fish hatchery along Oak 

Creek, Sycamore Creek (Verde River 

watershed) in the vicinity of the 

confluence with the Verde River north 

of Clarkdale, along several reaches of 

the Verde River mainstem, Cherry Creek 

on the east side of the Sierra Ancha 

Mountains, and Tonto Creek from Gisela 

to ‘‘the Box.’’

Rosen et al. (2001, p. 22) concluded 

that the presence and expansion of 

nonnative predators (mainly bullfrogs, 

crayfish, and green sunfish) are the 

primary causes of decline in northern 

Mexican gartersnakes in southeastern 

Arizona. Specifically, the authors 

identified the expansion of bullfrogs 

into the Sonoita grasslands (the 

threshold to the Canelo Hills) and the 

introduction of crayfish into Lewis 

Springs as being of particular concern in 

terms of future recovery efforts for the 

northern Mexican gartersnake. It should 

also be noted that Rosen et al. (2001, 

Appendix I) documented the decline of 

several native fish species in several 

locations visited, further affecting the 

prey base of northern Mexican 

gartersnakes. Rosen et al. (1995, pp. 

252–253) sampled 103 sites in the 

Chiricahua Mountains region which 

included the Chiricahua, Dragoon, and 

Peloncillo mountains, and the Sulphur 

Springs, San Bernardino, and San 

Simon valleys. They found that 43 

percent of all ectothermic aquatic and 

semi-aquatic vertebrate species detected 

were nonnative. The most commonly 

encountered nonnative species was the 

bullfrog (Rosen et al. 1995, p. 254). 

Declines in the Northern Mexican 

Gartersnake Native Fish Prey Base in 

the United States. Native fish species 

such as the federally endangered Gila 

chub, petitioned roundtail chub, and 

federally endangered Gila topminnow 

are among the primary prey species for 

the northern Mexican gartersnake 

(Rosen and Schwalbe 1988, p. 18). 

Similar to bullfrogs, predatory 

nonnative fish species such as 

largemouth bass also prey upon juvenile 

northern Mexican gartersnakes. 

Additionally both nonnative sport and 

bait compete with the northern Mexican 

gartersnake in terms of its native fish 

and native anuran prey base. Collier et 

al. (1996, p. 16) note that interactions 

between native and nonnative fish have 

significantly contributed to the decline 

of many native fish species from direct 

predation and indirectly from 

competition (which has adversely 

affected the prey base for northern 

Mexican gartersnakes). Holycross et al. 

(2006, pp. 53–55) recently documented 

significantly depressed or extirpated 

native fish prey bases for the northern 

Mexican gartersnake along the Agua 

Fria in the vicinity of Table Mesa Road 

and the Little Grand Canyon Ranch, 

along Dry Creek from Dugas Road to 

Little Ash Creek, along Little Ash Creek 

from Brown Spring to Dry Creek, along 

Sycamore Creek (Agua Fria watershed) 

in the vicinity of the Forest Service 

Cabin, and along Sycamore Creek 

(Verde River watershed) in the vicinity 

of its confluence with the Verde River 

north of Clarkdale. 

The widespread decline of native fish 

species from the arid southwestern 

United States and Mexico has resulted 

largely from interactions with nonnative 

species and has been captured in the 

listing rules of 13 native species listed 

under the Act whose historical ranges 

overlap with the historical distribution 

of the northern Mexican gartersnake. 

These native fish species were likely 

prey species for the northern Mexican 

gartersnake, including: bonytail chub 

(Gila elegans, 45 FR 27710, April 23, 

1980), Yaqui catfish (Ictalurus pricei, 49 

FR 34490, August 31, 1984), Yaqui chub 

(Gila purpurea, 49 FR 34490, August 31, 

1984), Yaqui topminnow (Poeciliopsis 

occidentalis sonoriensis, 32 FR 4001, 

March 11, 1967), beautiful shiner 

(Cyprinella formosa, 49 FR 34490, 

August 31, 1984), humpback chub (Gila 

cypha, 32 FR 4001, March 11, 1967), 

Gila chub (Gila intermedia, 70 FR 

66663, November 2, 2005), Colorado 

pikeminnow (Ptychocheilus lucius, 32 

FR 4001, March 11, 1967), spikedace 

(Meda fulgida, 51 FR 23769, July 1, 

1986), loach minnow (Tiaroga cobitis, 

51 FR 39468, October 28, 1986), 

razorback sucker (Xyrauchen texanus, 

56 FR 54957, October 23, 1991), desert 

pupfish (Cyprinodon macularius, 51 FR 

10842, March 31, 1986), and Gila 

topminnow (Poeciliopsis occidentalis 

occidentalis, 32 FR 4001, March 11, 

1967)]. In total within Arizona, 19 of 31 

(61 percent) of native fish species are 

listed under the Act. Arizona ranks the 

highest of all 50 States in the percentage 

of native fish species at risk (85.7 

percent, Stein 2002, p. 21). 

Fragmentation of extant listed native 

fish populations is exacerbating the 

decline of these species and may 

preclude their recovery as well as 

continue to affect their role in the prey 

base of northern Mexican gartersnakes. 

Fagan et al. (2005, pp. 34–41) examined 

the correlation between fragmentation of 

extant distributions and the relative risk 

of extinction of any given species. They 

found the strongest correlation to risk of 

extinction due to fragmentation of fish 

populations occurred at the 

intermediate to large spatial scales, 

which geographically correspond to 

tributaries and river basins (Fagan et al. 

2005, p. 38). At this range in spatial 

scale, the effects of dam building, water 

diversions, and introduced nonnatives 

appear to be significant factors 

exacerbating the fragmentation by acting 

as barriers to the exchange of genetic 

material among listed fish populations 

(Fagan et al. 2005, pp. 38–39). 

Olden and Poff (2005, p. 75) stated 

that environmental degradation and the 

proliferation of nonnative fish species 

threaten the endemic and unique fish 

faunas of the American Southwest. The 

fastest expanding nonnative species are 

red shiner (Cyprinella lutrensis), fathead 

minnow (Pimephales promelas), green 

sunfish (Lepomis cyanellus), largemouth 

bass (Micropterus salmoides), western 

mosquitofish, and channel catfish 

(Ictalurus punctatus). These species are 

considered to be the most invasive in 

terms of their negative impacts on 

native fish communities (Olden and Poff 

2005, p. 75). Many nonnative fishes in 

addition to those listed immediately 

above, including yellow and black 

bullheads (Ameiurus sp.), flathead 

catfish (Pylodictis olivaris), and 

smallmouth bass (Micropterus 

dolomieue), have been introduced into 

formerly and currently occupied 

northern Mexican gartersnake habitat 

(Bestgen and Propst 1989, pp. 409–410; 

Marsh and Minckley 1990, p. 265; 

Sublette et al. 1990, pp. 112, 243, 246, 

304, 313, 318; Abarca and Weedman 

1993, pp. 6–12; Stefferud and Stefferud 

1994, p. 364; Weedman and Young 

1997, pp. 1, Appendices B, C; Voeltz 

2002, p. 88; Bonar et al. 2004, pp. 1–

108). 


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Several authors have identified both 

the presence of nonnative fish species as 

well as their deleterious effects on 

native species within Arizona. Abarca 

and Weedman (1993, pp. 6–12) found 

that the number of nonnative fish 

species was twice the number of native 

fish species in Tonto Creek in the early 

1990s, with a stronger nonnative 

influence in the lower reaches where 

the northern Mexican gartersnake is 

considered extant. At the Gisela 

sampling point, four of six sampling 

attempts resulted in no fish captured; of 

the 41 fish captured in the remaining 

two attempts, 90 percent were 

nonnative, including 28 fathead 

minnows, 5 green sunfish, 3 red shiner, 

and 1 yellow bullhead. Surveys in the 

Salt River above Lake Roosevelt indicate 

a decline of roundtail chub and other 

natives with an increase in flathead and 

channel catfish numbers (Voeltz 2002, 

p. 49). In New Mexico, nonnative fish 

have been identified as the main cause 

for declines observed in roundtail chub 

populations (Voeltz 2002, p. 40). 

A report provided by Bonar et al. 

(2004, pp. 1–108) is the most current 

and perhaps one of the most complete 

assessments of native and nonnative 

fish species interactions in the Verde 

River mainstem. Overall, Bonar et al. 

(2004, p. 57) found that nonnative fishes 

were approximately 2.6 times more 

dense per unit volume of river than 

native fishes, and their standing crop 

was approximately 2.8 times that of 

native fishes per unit volume of river. 

Bonar et al. (2004, p. 79) verified the 

findings of Voeltz (2002, pp. 71, 88), in 

stating that red shiner were the most 

commonly encountered nonnative fish 

species in the Verde River by almost 

four-fold; they found the species to be 

present throughout the Verde River 

year-around, but noted the highest 

numbers in the reach between Beasley 

Flat to Sheep Bridge above Horseshoe 

Reservoir in riffle habitats. River reaches 

above Horseshoe Reservoir have 

resident self-sustaining populations of 

bass, green sunfish, catfish, and carp, 

with a low, unstable native fish 

community, which results in fewer 

native fish predation observations in 

sampling results for this reach (Bonar et 

al. 2004, pp. 80, 87). Reaches below 

Bartlett Reservoir had both high native 

and nonnative fish abundance, which 

resulted in more frequent observations 

of nonnative predation on native fish 

according to Bonar et al. (2004, p. 87). 

Lastly, Bonar et al. (2004, p. 6) found 

that channel and flathead catfish, green 

sunfish, largemouth and smallmouth 

bass, and yellow bullhead had the 

highest rates of piscivory (fish 

predation) on native and nonnative fish 

species in all river reaches; of these 

species, largemouth bass were 

documented as the most significant 

predator on native fish. 

Northern Mexican gartersnakes can 

successfully use some nonnative 

species, such as mosquitofish and red 

shiner, as prey species. However, all 

other nonnative species, most notably 

the spiny-rayed fish, are not considered 

prey species for the northern Mexican 

gartersnake. These nonnative species 

can be difficult to swallow due to their 

body shape and spiny dorsal fins, are 

predatory on juvenile gartersnakes, and 

reduce the abundance of or completely 

eliminate native fish populations. This 

is particularly important in the wake of 

a stochastic event such as flooding, 

extreme water temperatures, or 

excessive turbidity. Native fish are 

adapted to the dramatic fluctuations in 

water conditions and flow regimes and 

persist in the wake of stochastic events 

as a prey base for the northern Mexican 

gartersnake. Nonnative fish, even 

species that may be used as prey by the 

northern Mexican gartersnake, generally 

are ill-adapted to these conditions and 

may be removed from the area 

temporarily or permanently, depending 

on the hydrologic connectivity to extant 

populations. If an area is solely 

comprised of nonnative fish, the 

northern Mexican gartersnake may be 

faced with nutritional stress or 

starvation. The most conclusive 

evidence for the northern Mexican 

gartersnake’s intolerance for nonnative 

fish remains in the fact that, in most 

incidences, nonnative fish species 

generally do not occur in the same 

locations as the northern Mexican 

gartersnake and its native prey species. 

Bullfrog Diet and Distribution in the 

United States. Bullfrogs are widely 

considered one of the most serious 

threats to the northern Mexican 

gartersnake throughout its range (Conant 

1974, pp. 471, 487–489; Rosen and 

Schwalbe 1988, pp. 28–30; Rosen et al. 

2001, pp. 21–22). Bullfrogs adversely 

affect northern Mexican gartersnakes 

through direct predation of juvenile and 

sub-adults and from competition with 

native prey species. Bullfrogs first 

appeared in Arizona in 1926, as a result 

of a systematic introduction effort by the 

State Game Department (now, the 

Arizona Game and Fish Department) for 

the purposes of sport hunting and as a 

food source. (Tellman 2002, p. 43). By 

1982, the Arizona Game and Fish 

Department had systematically 

introduced some 682,000 bullfrog 

tadpoles into streams throughout the 

State (Tellman 2002, p. 43). Bullfrogs 

are extremely prolific, adept at 

colonizing new areas, and may disperse 

to distances of 6.8 miles (10.9 km) and 

likely further within drainages (Bautista 

2002, p. 131; Rosen and Schwalbe 

2002a, p. 7; Casper and Hendricks 2005, 

p. 582). Batista (2002, p. 131) confirmed 

‘‘the strong colonizing skills of the 

bullfrog and that the introduction of this 

exotic species can disturb local anuran 

communities.’’

Bullfrogs are voracious, opportunistic, 

even cannibalistic predators that readily 

attempt to consume any animal smaller 

than themselves, including conspecifics 

(other species within the same genus) 

which can encompass 80 percent of 

their diet (Casper and Hendricks 2005, 

p. 543). Bullfrogs have demonstrated 

astonishing variability in their diet, 

which has been documented to include 

vegetation, earthworms, leeches, insects, 

centipedes, millipedes, spiders, 

scorpions, crayfish, snails, numerous 

species of larval and metamorphosed 

amphibians, fish, small alligators, 

turtles, lizards, numerous species of 

snakes [seven genera; including six 

different species of gartersnakes, two 

species of rattlesnakes, and Sonoran 

gophersnakes (Pituophis catenifer 

affinis)], small mammals (e.g., 

chipmunks, cotton rats, shrews, mice, 

and voles), numerous species of birds, 

bats, muskrats, and even juvenile mink 

(Bury and Whelan 1984, p. 5; Clarkson 

and DeVos 1986, p. 45; Holm and Lowe 

1995, pp. 37–38; Carpenter et al. 2002, 

p. 130; King et al. 2002; Hovey and 

Bergen 2003, pp. 360–361; Casper and 

Hendricks 2005, p. 544; Combs et al. 

2005, p. 439; Wilcox 2005, p. 306). 

Bullfrogs have been documented 

throughout the State of Arizona. 

Holycross et al. (2006, pp. 13–14, 52–61) 

found bullfrogs at 55 percent of sample 

sites in the Agua Fria watershed, 62 

percent of sites in the Verde River 

watershed, 25 percent of sites in the Salt 

River watershed, and 22 percent of sites 

in the Gila River watershed. In total, 

bullfrogs were observed at 22 of the 57 

sites surveyed (39 percent) across the 

Mogollon Rim (Holycross et al. 2006, p. 

13). 

A number of authors have 

documented the presence of bullfrogs 

through their survey efforts Statewide in 

specific regional areas, drainages, and 

disassociated wetlands that include the 

Kaibab National Forest (Sredl et al. 

1995a, p. 7); the Coconino National 

Forest (Sredl et al. 1995c, p. 7); the 

White Mountain Apache Reservation 

(Hulse 1973, p. 278); Beaver Creek 

(tributary to the Verde River) (Drost and 

Nowak 1997, p. 11); the Watson Woods 

Riparian Preserve near Prescott (Nowak 

and Spille 2001, p. 11); the Tonto 

National Forest (Sredl et al. 1995b, p. 9); 


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the Lower Colorado River (Vitt and 

Ohmart 1978, p. 44; Clarkson and DeVos 

1986, pp. 42–49; Ohmart et al. 1988, p. 

143); the Huachuca Mountains (Rosen 

and Schwalbe 1988, Appendix I; Holm 

and Lowe 1995, pp. 27–35; Sredl et al. 

2000, p. 10; Rosen et al. 2001, Appendix 

I); the Pinaleno Mountains region 

(Nickerson and Mays 1970, p. 495); the 

San Bernardino National Wildlife 

Refuge (Rosen and Schwalbe 1988, 

Appendix I; 1995, p. 452; 1996, pp. 1–

3; 1997, p. 1; 2002b, pp. 223–227; 

2002c, pp. 31, 70; Rosen et al. 1995, p. 

254; 1996b, pp. 8–9; 2001, Appendix I); 

the Buenos Aires National Wildlife 

Refuge (Rosen and Schwalbe 1988, 

Appendix I); the Arivaca Area (Rosen 

and Schwalbe 1988, Appendix I; Rosen 

et al. 2001, Appendix I); Cienega Creek 

drainage (Rosen et al. 2001, Appendix 

I); Babocamari River drainage (Rosen et 

al. 2001, Appendix I); Turkey Creek 

drainage (Rosen et al. 2001, Appendix 

I); O’Donnell Creek drainage (Rosen et 

al. 2001, Appendix I); Audubon 

Research Ranch near Elgin (Rosen et al. 

2001, Appendix I); Santa Cruz River 

drainage (Rosen and Schwalbe 1988, 

Appendix I; Rosen et al. 2001, 

Appendix I); San Rafael Valley (Rosen et 

al. 2001, Appendix I); San Pedro River 

drainage (Rosen and Schwalbe 1988, 

Appendix I; Rosen et al. 2001, 

Appendix I); Bingham Cienega (Rosen et 

al. 2001, Appendix I); Sulfur Springs 

Valley (Rosen et al. 1996a, pp. 16–17); 

Whetstone Mountains region (Turner et 

al. 1999, p. 11); Aqua Fria River 

drainage (Rosen and Schwalbe 1988, 

Appendix I; Holycross et al. 2006, pp. 

13, 15–18, 52–53); Verde River drainage 

(Rosen and Schwalbe 1988, Appendix I; 

Holycross et al. 2006, pp. 13, 26–28, 55–

56); greater metropolitan Phoenix area 

(Rosen and Schwalbe 1988, Appendix 

I); greater metropolitan Tucson area 

(Rosen and Schwalbe 1988, Appendix 

I); Sonoita Creek drainage (Rosen and 

Schwalbe 1988, Appendix I); Sonoita 

Grasslands (Rosen and Schwalbe 1988, 

Appendix I); Canelo Hills (Rosen and 

Schwalbe 1988, Appendix I); Pajarito 

Mountains (pers. observation, J. Servoss, 

Fish and Wildlife Biologist, U.S. Fish 

and Wildlife Service); Picacho Reservoir 

(Rosen and Schwalbe 1988, Appendix 

I); Dry Creek drainage (Holycross et al. 

2006, pp. 19, 53); Little Ash Creek 

drainage (Holycross et al. 2006, pp. 19, 

54); Oak Creek drainage (Holycross et al. 

2006, pp. 23, 54); Sycamore Creek 

drainages (Holycross et al. 2006, pp. 20, 

25, 54–55); Rye Creek drainage 

(Holycross et al. 2006, pp. 37, 58); 

Spring Creek drainage (Holycross et al. 

2006, pp. 25, 59); Tonto Creek drainage 

(Holycross et al. 2006, pp. 40–44, 59); 

San Francisco River drainage (Holycross 

et al. 2006, pp. 49–50, 61); and the 

upper Gila River drainage (Holycross et 

al. 2006, pp. 45–50, 60–61). 

Perhaps one of the most serious 

consequences of bullfrog introductions 

is their persistence in an area once they 

have become established, and the 

subsequent difficulty in eliminating 

bullfrog populations. Rosen and 

Schwalbe (1995, p. 452) experimented 

with bullfrog removal at various sites on 

the San Bernardino National Wildlife 

Refuge in addition to a control site with 

no bullfrog removal in similar habitat on 

the Buenos Aires National Wildlife 

Refuge. Removal of adult bullfrogs 

resulted in a substantial increase in 

younger age-class bullfrogs where 

removal efforts were the most intensive 

(Rosen and Schwalbe 1997, p. 6). 

Evidence from dissection samples from 

young adult and sub-adult bullfrogs 

indicated these age-classes readily prey 

upon juvenile bullfrogs (up to the 

average adult leopard frog size) as well 

as juvenile gartersnakes, which suggests 

that the selective removal of only the 

large adult bullfrogs (favoring the young 

adult and sub-adult age classes) could 

indirectly lead to increased predation of 

leopard frogs and juvenile gartersnakes 

(Rosen and Schwalbe 1997, p. 6). 

Consequently, this strategy was viewed 

as being potentially ‘‘self-defeating’’ and 

‘‘counter-productive’’ but required 

further investigation (Rosen and 

Schwalbe 1997, p. 6). 

Bullfrog Effects on the Native Anuran 

Prey Base for the Northern Mexican 

Gartersnake in the United States. 

Bullfrog introductions in the United 

States and Mexico have adversely 

affected the native leopard frog prey 

base for northern Mexican gartersnakes 

(Conant 1974, pp. 471, 487–489; Hayes 

and Jennings 1986, pp. 491–492; Rosen 

and Schwalbe 1988, p. 28–30; 2002b, 

pp. 232–238; Rosen et al. 1995, pp. 257–

258; 2001, pp. 2, Appendix I). Different 

age classes of bullfrogs within a 

community can affect native ranid 

populations via different mechanisms. 

Juvenile bullfrogs may affect native 

ranids by competition, male bullfrogs 

may affect native ranids by predation, 

and female bullfrogs may affect native 

ranids by both mechanisms depending 

on body size and microhabitat (Wu et al. 

2005, p. 668). Pearl et al. (2004, p. 18) 

also suggested that the effect of bullfrog 

introductions on native ranids may be 

different based on microhabitat use, but 

also suggested that an individual ranid 

frog species’ physical ability to escape 

influences the effect of bullfrogs on each 

native ranid community. 

Kupferberg (1994, p. 95) found that 

where bullfrogs were present in 

California, native anurans were rare or 

absent. Effects of larval bullfrogs on 

native ranid frogs have also been 

described in the literature. Survivorship 

of larval threatened California redlegged frogs (Rana aurora) was 700 

percent greater in the absence of 

bullfrog larvae (Lawler et al. 1999). Bury 

and Whelan (1986, pp. 9–10) implicated 

bullfrog introductions in the decline of 

several native ranid frogs in several 

States within the western United States 

including Nevada, California, Montana, 

Colorado, Oregon, and Washington. 

Hayes and Jennings (1986, pp. 500–501) 

conclude that while bullfrog 

introductions have affected the status of 

native ranid frogs throughout the 

western United States, the synergistic 

effect of other factors, such as habitat 

alteration and destruction, introduced 

nonnative fishes, commercial 

exploitation, toxicants, pathogens and 

parasites, and acid rain, likely also 

played significant roles. 

Bullfrog Predation on Northern 

Mexican Gartersnakes in the United 

States. Sub-adult and adult bullfrogs not 

only compete with the northern 

Mexican gartersnake for prey items, but 

directly prey upon juvenile and 

occasionally sub-adult northern 

Mexican gartersnakes (Rosen and 

Schwalbe 1988, pp. 28–31; 1995, p. 452; 

2002b, pp. 223–227; Holm and Lowe 

1995, pp. 29–29; Rossman et al. 1996, p. 

177; AGFD In Prep, p. 12; 2001, p. 3; 

Rosen et al. 2001, pp. 10, 21–22; 

Carpenter et al. 2002, p. 130; Wallace 

2002, p. 116). A well-circulated 

photograph of an adult bullfrog in the 

process of consuming a northern 

Mexican gartersnake at Parker Canyon 

Lake, Cochise County, Arizona, taken by 

John Carr of the Arizona Game and Fish 

Department in 1964, provides 

photographic documentation of bullfrog 

predation (Rosen and Schwalbe 1988, p. 

29; 1995, p. 452). A common 

observation in northern Mexican 

gartersnake populations that co-occur 

with bullfrogs is a preponderance of 

large, mature adult snakes with 

conspicuously low numbers of 

individuals in the neonate (newborn) 

and juvenile age size classes due to 

bullfrogs preying on young small 

snakes, which ultimately leads to low 

recruitment levels (reproduction and 

survival of young) (Rosen and Schwalbe 

1988, p. 18; Holm and Lowe 1995, p. 

34). 

The tails of gartersnakes are easily 

broken-off through predation attempts 

(tails of gartersnakes do not regenerate), 

which may assist in escaping an 

individual predation attempt but may 

also lead to infection or compromise an 

individual’s physical ability to escape 


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future predation attempts or 

successfully forage. The incidence of 

tail breaks in gartersnakes can often be 

used to assess predation pressures 

within gartersnake populations. Rosen 

and Schwalbe (1988, p. 22) found the 

incidence of tail breaks to be 

statistically higher in females than in 

males. Fitch (2003, p. 212) also found 

that tail breaks in the common 

gartersnake occurred more frequently in 

females than males and in adults more 

than in juveniles. Fitch (2003, p. 212) 

also commented that, while tail 

breakage in gartersnakes can save the 

life of an individual snake, it also leads 

to permanent handicapping of the 

snake, resulting in slower swimming 

and crawling speeds, which could leave 

the snake more vulnerable to predation 

or affect its foraging ability. 

Furthermore, Mushinsky and Miller 

(1993, pp. 662–664) found that the 

incidence of tail injury in water snakes 

in the genera Nerodia and Regina 

(which have similar life histories to 

northern Mexican gartersnakes) was 

higher in females than in males and in 

adults more than juveniles. We believe 

this could be explained by higher 

basking rates associated with gravid 

(pregnant) females that increased their 

visibility to predators and that predation 

on juvenile snakes generally results in 

complete consumption of the animal, 

which would limit observations of tail 

injury in the juvenile age class. Rosen 

and Schwalbe (1988, p. 22) suggested 

that the indication that female northern 

Mexican gartersnakes bear more injuries 

is consistent with the inference that 

they employ a riskier foraging strategy. 

Willis et al. (1982, p. 98) discussed the 

incidence of tail injury in three species 

in the genus Thamnophis [common 

gartersnake, Butler’s gartersnake (T. 

butleri), and the eastern ribbon snake (T. 

sauritus)] and concluded that 

individuals that suffered nonfatal 

injuries prior to reaching a length of 12 

in (30 cm) are not likely to survive and 

that physiological stress during postinjury hibernation may play an 

important role in subsequent mortality. 

Ecologically significant observations 

on tail injuries were made by Rosen and 

Schwalbe (1988, pp. 28–31) from the 

once-extant population of northern 

Mexican gartersnakes on the San 

Bernardino National Wildlife Refuge 

where 78 percent of specimens had 

broken tails with a ‘‘soft and club-like’’

terminus, which suggests repeated 

injury from multiple predation attempts. 

While palpating (medically examining 

by touch) gravid female northern 

Mexican gartersnakes, Rosen and 

Schwalbe (1988, p. 28) noted bleeding 

from this region which suggested the 

snakes suffered from ‘‘squeeze-type’’

injuries inflicted by adult bullfrogs. 

While a sub-adult or adult northern 

Mexican gartersnake may survive an 

individual predation attempt from a 

bullfrog while only incurring tail 

damage, secondary effects from 

infection of the wound can significantly 

contribute to mortality of individuals. 

Research on the effects of attempted 

predation performed by Mushinsky and 

Miller (1993, pp. 661–664) and Willis et 

al. (1982, pp. 100–101) supports the 

observations made by Holm and Lowe 

(1995, p. 34) on the northern Mexican 

gartersnake population age class 

structure in Scotia Canyon in the 

Huachuca Mountains of southeastern 

Arizona in the early 1990s. Specifically, 

Holm and Lowe (1995, pp. 33–34) 

observed a conspicuously greater 

number of adult snakes, in that 

population than sub-adult snakes as 

well as a higher incidence of tail injury 

(89 percent) in all snakes captured. 

Bullfrogs have been identified as the 

primary cause for both the collapse of 

the native leopard frog (prey base for the 

northern Mexican gartersnake) and 

northern Mexican gartersnake 

populations on the San Bernardino 

National Wildlife Refuge (Rosen and 

Schwalbe 1988, p. 28; 1995, p. 452; 

1996, pp. 1–3; 1997, p. 1; 2002b, pp. 

223–227; 2002c, pp. 31, 70; Rosen et al. 

1996b, pp. 8–9). Rosen and Schwalbe 

(1988, p. 18) stated that the low 

survivorship of neonates, and possibly 

yearlings, due to bullfrog predation is an 

important proximate cause of 

population declines of this snake at the 

San Bernardino National Wildlife 

Refuge and throughout its distribution 

in Arizona. 

Effects of Crayfish on Northern 

Mexican Gartersnakes in the United 

States. Crayfish represent another 

category of nonnative species threat as 

they are a primary threat to many prey 

species of the northern Mexican 

gartersnake and may also prey upon 

juvenile gartersnakes (Fernandez and 

Rosen 1996, p. 25; Voeltz 2002, pp. 87–

88). Fernandez and Rosen (1996, p. 3) 

studied the effects of crayfish 

introductions on two stream 

communities in Arizona, a lowelevation semi-desert stream and a high 

mountain stream, and concluded that 

crayfish can noticeably reduce species 

diversity and destabilize trophic 

structures (food chains) in riparian and 

aquatic ecosystems through their effect 

on vegetative structure, stream substrate 

composition, and predation on eggs, 

larval, and adult forms of native 

invertebrate and vertebrate species. 

Crayfish fed on embryos, tadpoles, 

newly metamorphosed frogs, and adult 

leopard frogs, but they did not feed on 

egg masses (Fernandez and Rosen 1996, 

p. 25). However, Gamradt and Kats 

(1996, p. 1155) found that crayfish 

readily consumed the egg masses of 

California newts (Taricha torosa). 

Fernandez and Rosen (1996, pp. 6–19, 

52–56) and Rosen (1987, p. 5) discussed 

observations of inverse relationships 

between crayfish abundance and native 

herpetofauna including narrow-headed 

gartersnakes (Thamnophis 

rufipunctatus rufipunctatus), northern 

leopard frogs, and Chiricahua leopard 

frogs. Crayfish may also affect native 

fish populations. Carpenter (2005, pp. 

338–340) documented that crayfish may 

reduce the growth rates of native fish 

through competition for food and noted 

that the significance of this impact may 

vary between species. Crayfish also prey 

on fish eggs and larvae (Inman et al. 

1998, p. 17). 

Crayfish alter the abundance and 

structure of aquatic vegetation by 

grazing on aquatic and semiaquatic 

vegetation, which reduces the cover 

needed for frogs and gartersnakes as 

well as the food supply for prey species 

such as tadpoles (Fernandez and Rosen 

1996, pp. 10–12). Fernandez and Rosen 

(1996, pp. 10–12) also found that 

crayfish frequently burrow into stream 

banks, which leads to increased bank 

erosion, stream turbidity, and siltation 

of substrates. Creed (1994, p. 2098) 

found that filamentous alga (Cladophora 

glomerata) was at least 10-fold greater in 

aquatic habitat absent crayfish. 

Filamentous alga is an important 

component of aquatic vegetation that 

provides cover for foraging gartersnakes 

as well as microhabitat for prey species. 

Inman et al. (1998, p. 3) documented 

nonnative crayfish as widely distributed 

and locally abundant in a broad array of 

natural and artificial lotic (free-flowing) 

and lentic (still water) habitats 

throughout Arizona, many of which 

overlapped the historical and extant 

distribution of the northern Mexican 

gartersnake. Hyatt (undated, p. 71) 

concluded that the majority of waters in 

Arizona contained at least one species 

of crayfish. Holycross et al. (2006, p. 14) 

found crayfish in 64 percent of the 

sample sites in the Agua Fria watershed; 

in 85 percent of the sites in the Verde 

River watershed; in 46 percent of the 

sites in the Salt River watershed; and in 

67 percent of the sites in the Gila River 

watershed. In total, crayfish were 

recently observed at 35 (61 percent) of 

the 57 sites surveyed across the 

Mogollon Rim (Holycross et al. 2006, p. 

14). 

Several other authors have 

specifically documented the presence of 


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crayfish in many areas and drainages 

throughout Arizona, which is testament 

to their ubiquitous distribution in 

Arizona and their strong colonizing 

abilities. These areas included the 

Kaibab National Forest (Sredl et al. 

1995a, p. 7); the Coconino National 

Forest (Sredl et al. 1995c, p. 7); the 

Watson Woods Riparian Preserve near 

Prescott (Nowak and Spille 2001, p. 33); 

the Tonto National Forest (Sredl et al. 

1995b, p. 9); the Lower Colorado River 

(Ohmart et al. 1988, p. 150; Inman et al. 

1998, Appendix B); the Huachuca 

Mountains (Sredl et al. 2000, p. 10); the 

Arivaca Area (Rosen et al. 2001, 

Appendix I); Babocamari River drainage 

(Rosen et al. 2001, Appendix I); 

O’Donnell Creek drainage (Rosen et al. 

2001, Appendix I); Santa Cruz River 

drainage (Rosen and Schwalbe 1988, 

Appendix I; Rosen et al. 2001, 

Appendix I); San Pedro River drainage 

(Inman et al. 1998, Appendix B; Rosen 

et al. 2001, Appendix I); Aqua Fria River 

drainage (Inman et al. 1998, Appendix 

B; Holycross et al. 2006, pp. 14, 15–18, 

52–54); Verde River drainage (Inman et 

al. 1998, Appendix B; Holycross et al. 

2006, pp. 14, 20–28, 54–56); Salt River 

drainage (Inman et al. 1998, Appendix 

B; Holycross et al. 2006, pp. 15, 29–44, 

56–60); Black River drainage (Inman et 

al. 1998, Appendix B); San Francisco 

River drainage (Inman et al. 1998, 

Appendix B; Holycross et al. 2006, pp. 

14, 49–50, 61); Nutrioso Creek drainage 

(Inman et al. 1998, Appendix B); Little 

Colorado River drainage (Inman et al. 

1998, Appendix B); Leonard Canyon 

Drainage (Inman et al. 1998, Appendix 

B); East Clear Creek drainage (Inman et 

al. 1998, Appendix B); Chevelon Creek 

drainage (Inman et al. 1998, Appendix 

B); Eagle Creek drainage (Inman et al. 

1998, Appendix B; Holycross et al. 

2006, pp. 47–48, 60); Bill Williams 

drainage (Inman et al. 1998, Appendix 

B); Sabino Canyon drainage (Inman et 

al. 1998, Appendix B); Dry Creek 

drainage (Holycross et al. 2006, pp. 19, 

53); Little Ash Creek drainage 

(Holycross et al. 2006, pp. 19, 54); 

Sycamore Creek drainage (Holycross et 

al. 2006, pp. 25, 54–55); East Verde 

River drainage (Holycross et al. 2006, 

pp. 21–22, 54); Oak Creek drainage 

(Holycross et al. 2006, pp. 23, 54); Pine 

Creek drainage (Holycross et al. 2006, 

pp. 24, 55); Spring Creek drainage 

(Holycross et al. 2006, pp. 25, 55); Big 

Bonito Creek drainage (Holycross et al. 

2006, pp. 29, 56); Cherry Creek drainage 

(Holycross et al. 2006, pp. 33, 57); East 

Fork Black River drainage (Holycross et 

al. 2006, pp. 34, 57); Haigler Creek 

drainage (Holycross et al. 2006, pp. 35, 

58); Houston Creek drainage (Holycross 

et al. 2006, pp. 35–36, 58); Rye Creek 

drainage (Holycross et al. 2006, pp. 37, 

58); Tonto Creek drainage (Holycross et 

al. 2006, pp. 40–44, 59); Blue River 

drainage (Holycross et al. 2006, pp. 45, 

60); Campbell Blue River drainage 

(Holycross et al. 2006, pp. 46, 60); and 

the Gila River drainage (Inman et al. 

1998, Appendix B; Holycross et al. 

2006, pp. 45–50, 61). 

Bullfrog and Crayfish Eradication in 

the United States. As previously noted, 

nonnative species such as bullfrogs and 

crayfish have proven difficult, if not 

impossible, to eradicate once 

established in certain environments. 

Bullfrogs, for example, are particularly 

damaging to, and persistent in, riparian 

communities. A population of adult 

bullfrogs can sustain itself even when 

the native vertebrate prey base has been 

severely reduced or extirpated because 

adult bullfrogs are cannibalistic and 

larval bullfrogs can be sustained by 

grazing on aquatic vegetation (Rosen 

and Schwalbe 1995, p. 452). Effective 

removal of semi-aquatic nonnative 

species is possible in simple, 

geographically isolated systems that can 

be manipulated (e.g., stock tanks); 

however, it can be expensive, and 

specially designed fencing is likely 

needed to prevent reinvasion until 

entire landscapes (e.g., an entire valley) 

have been cleared of nonnative species 

(Rosen and Schwalbe 2002a, p. 7; Hyatt 

undated). No single method is available 

to effectively remove bullfrogs or 

crayfish from lotic, or complex interconnected systems (Rosen and 

Schwalbe 1996a, pp. 5–8; 2002a, p. 7; 

Hyatt Undated, pp. 63–71). The inability 

of land managers to effectively address 

the invasion of nonnative species in 

such communities highlights the serious 

nature of nonnative species invasions. 

Hyatt (undated, p. 71) concluded that 

successful eradication of crayfish in 

Arizona is highly unlikely. While 

potential threats to physical habitat 

from human land use activities can 

usually be lessened or removed 

completely with adjustments to land 

management practices, the concern for 

the apparent irreversibility of nonnative 

species invasions becomes paramount 

which leaves us to conclude that 

nonnative species are the greatest threat 

to the northern Mexican gartersnake due 

to the long-term implications. 

Nonnative Fish distribution and 

Community Interactions in the United 

States. Rosen et al. (2001, Appendix I) 

and Holycross et al. (2006, pp. 15–51) 

conducted large-scale surveys for 

northern Mexican gartersnakes in 

southeastern and central Arizona and 

narrow-headed gartersnakes in central 

and east-central Arizona and 

documented the presence of nonnative 

fish at many locations. Rosen et al. 

(2001, Appendix I) found nonnative fish 

in the following survey locations: the 

Arivaca Area; Babocamari River 

drainage; O’Donnell Creek drainage; 

Audubon Research Ranch (Post Canyon) 

near Elgin; Santa Cruz River drainage; 

Agua Caliente Canyon; Santa Catalina 

Mountains; and the San Pedro River 

drainage. Holycross et al. (2006, pp. 14–

15, 52–61) found nonnative fish in the 

Aqua Fria River drainage; the Verde 

River drainage; the Dry Creek drainage; 

the Little Ash Creek drainage; the 

Sycamore Creek drainage; the East 

Verde River drainage; the Oak Creek 

drainage; the Pine Creek drainage; the 

Big Bonito Creek drainage; the Black 

River drainage; the Canyon Creek 

drainage; the Cherry Creek drainage; the 

Christopher Creek drainage; the East 

Fork Black River drainage; the Haigler 

Creek drainage; the Houston Creek 

drainage; the Rye Creek drainage; the 

Salt River drainage; the Spring Creek 

drainage; the Tonto Creek drainage; the 

Blue River drainage; the Campbell Blue 

River drainage; the Eagle Creek 

drainage; and the San Francisco River 

drainage. Other authors have 

documented the presence of nonnative 

fish through their survey efforts in 

specific regions that include the Tonto 

National Forest (Sredl et al. 1995b, p. 8) 

and the Huachuca Mountains (Sredl et 

al. 2000, p. 10). 

Holycross et al. (2006, pp. 14–15) 

found nonnative fish species while 

surveying for narrow-headed and 

Mexican gartersnakes in 64 percent of 

the sample sites in the Agua Fria 

watershed, 85 percent of the sample 

sites in the Verde River watershed, 75 

percent of the sample sites in the Salt 

River watershed, and 56 percent of the 

sample sites in the Gila River 

watershed. In total, nonnative fish were 

observed at 41 of the 57 sites surveyed 

(72 percent) across the Mogollon Rim 

(Holycross et al. 2006, p. 14). Entirely 

native fish communities were detected 

in only 8 of 57 sites surveyed (14 

percent) (Holycross et al. 2006, p. 14). 

While the locations and drainages 

identified above that are known to 

support populations of nonnative fish 

do not provide a thorough 

representation of the status of nonnative 

fish distribution Statewide in Arizona, it 

is well documented that nonnative fish 

have infiltrated the majority of aquatic 

communities in Arizona. 

Rinne et al. (1998, p. 3) documented 

over a dozen species of nonnative fish 

that have been stocked within the 

historical distribution of the northern 

Mexican gartersnake in the Verde Basin 

with over 850 stocking events occurring 


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in Horseshoe and/or Bartlett reservoirs 

and almost 4,500 in streams (mostly 

tributaries to the Verde) over the past 60 

years. Rinne et al. (1998, pp. 4–6) found 

that in all but the uppermost reach, 

nonnatives predominated the sampling 

results in the Verde River. Voeltz (2002, 

p. 88) documented an ‘‘alarming trend’’

in the Verde River with the reduction of 

native fish abundance corresponding 

with an explosion in red shiner 

populations. 

Nonnative fish can also affect native 

amphibian populations. Matthews et al. 

(2002, p. 16) examined the relationship 

of gartersnake distributions, amphibian 

population declines, and nonnative fish 

introductions in high elevation aquatic 

ecosystems in California. Matthews et 

al. (2002, p. 16) specifically examined 

the effect of nonnative trout 

introductions on populations of 

amphibians and mountain gartersnakes 

(Thamnophis elegans elegans). Their 

results indicated the probability of 

observing gartersnakes was 30 times 

greater in lakes containing amphibians 

than in lakes where amphibians have 

been extirpated by nonnative fish. These 

results supported prediction by Jennings 

et al. (1992, p. 503) that native 

amphibian declines will lead directly to 

gartersnake declines. Matthews et al. 

(2002, p. 20) noted that in addition to 

nonnative fish species adversely 

impacting amphibian populations that 

are part of the gartersnake’s prey base, 

direct predation on gartersnakes by 

nonnative fish also occurs. Inversely, 

gartersnake predation on nonnative 

species, such as centrarchids, may 

physically harm the snake. Choking 

injuries to northern Mexican 

gartersnakes may occur from attempting 

to ingest nonnative spiny-rayed fish 

species (such as green sunfish and bass) 

because the spines located in the dorsal 

fins of these species can become lodged, 

or cut into the gut tissue of the snake, 

as observed in narrow-headed 

gartersnakes (Nowak and SantanaBendix 2002, p. 25). 

Nonnative fish invasions can 

indirectly affect the health, 

maintenance, and reproduction of the 

northern Mexican gartersnake by 

altering its foraging strategy and 

foraging success. Observations made by 

Dr. Phil Rosen at Finley Tank on the 

Audubon Research Ranch near Elgin, 

Arizona, of northern Mexican 

gartersnake populations and individual 

growth trends prior to the arrival of the 

nonnative bullfrog, provides 

information on the effects of nonnative 

fish invasions and the likely nutritional 

ramifications of a fish-only diet in a 

species that normally has a varied diet 

largely supported by amphibian prey 

items (Rosen et al. 2001, p. 19). The 

more energy expended in foraging, 

coupled by the reduced number of small 

to medium-sized fish available in lower 

densities, may lead to deficiencies in 

nutrition affecting growth and 

reproduction because energy is instead 

allocated to maintenance and the 

increased energy costs of intense 

foraging activity (Rosen et al. 2001, p. 

19). In contrast, a northern Mexican 

gartersnake diet that includes both fish 

and amphibians such as leopard frogs 

provides larger prey items which reduce 

the necessity to forage at a higher 

frequency allowing metabolic energy 

gained from larger prey items to be 

allocated instead to growth and 

reproductive development. Myer and 

Kowell (1973, p. 225) experimented 

with food deprivation in common 

gartersnakes and found significant 

reductions in lengths and weights in 

juvenile snakes that were deprived of 

regular feedings versus the control 

group that were fed regularly at natural 

frequencies. Reduced foraging success 

may therefore increase mortality rates in 

the juvenile size class and consequently 

affect recruitment of northern Mexican 

gartersnakes where their prey base has 

been compromised by nonnative 

species. 

Nonnative fish species also facilitate 

the invasion of other aquatic nonnative 

species such as bullfrogs. Adams et al. 

(2003, pp. 343, 349) found that the 

invasion of nonnative fish species 

indirectly facilitates the invasion of 

bullfrogs. Survivorship of tadpoles is 

increased when nonnative fish prey 

upon predatory macroinvertebrates, 

which reduces the densities of 

predatory macroinvertebrates and 

relaxes their predation rate on bullfrog 

tadpoles. These findings support the 

‘‘invasional meltdown’’ hypothesis, 

which suggests that when positive 

interactions among nonnatives are 

prevalent, that community of nonnative 

species can increase the probability of 

further invasions (Simberloff and Von 

Holle 1999, p. 21; Adams et al. 2003, pp. 

343, 348–350). While mutually 

facilitative interactions among 

introduced species have not been 

thoroughly examined, it has been 

concluded that nonnatives can and do 

facilitate the expansion of other 

nonnative species (Simberloff and Van 

Holle 1999, p. 21). 

Nonnative Species in Mexico. The 

native fish prey base for northern 

Mexican gartersnakes has been 

dramatically affected by the 

introduction of nonnative species in 

several regions of Mexico (Conant 1974, 

pp. 471, 487–489; Miller et al. 2005, pp. 

60–61; Abarca 2006). In the lower 

elevations of Mexico where northern 

Mexican gartersnakes occurred 

historically and may still be extant, 

there are approximately 200 species of 

native freshwater fish documented with 

120 native species under some form of 

threat and an additional 15 that have 

become extinct due to human activities 

(Contreras Balderas and Lozano 1994, 

pp. 383–384). In 1979, The American 

Fisheries Society listed 69 species of 

native fish in Mexico as threatened or in 

danger of becoming extinct. Ten years 

later that number rose to 123 species, an 

increase of 78 percent (Contreras 

Balderas and Lozano 1994, pp. 383–

384). Miller et al. (2005, p. 60) 

concludes that some 20 percent of 

Mexico’s native fish are threatened or in 

danger of becoming extinct. Nonnative 

species are increasing everywhere 

throughout Mexico and the outlook for 

this trend looks ‘‘bleak’’ for native fish 

according to Miller et al. (2005, p. 61). 

A number of freshwater fish populations 

have been adversely affected by 

nonnative species in many documented 

localities, several of which were 

previously noted in the discussion 

under Factor A. 

Bullfrogs were purposefully 

introduced nationwide in a concerted 

effort to establish the species in all lakes 

and canal systems throughout Mexico as 

a potential food source for humans 

although frog legs ultimately never 

gained popularity in Mexican culinary 

culture (Conant 1974, pp. 487–489). 

Rosen and Melendez (2006, p. 54) report 

bullfrog invasions to be prevalent in 

northwestern Chihuahua and 

northeastern Sonora where the northern 

Mexican gartersnake is thought to occur. 

In many areas, native leopard frogs were 

completely displaced (extirpated) where 

bullfrogs were observed. Rosen and 

Melendez (2006, p. 54) also 

demonstrated the relationship between 

fish and amphibian communities in 

Sonora and western Chihuahua in that 

native leopard frogs, a primary prey 

item for the northern Mexican 

gartersnake, only occurred in the 

absence of nonnative fish and were 

absent from waters containing 

nonnative species, which included 

several major waters. In addition to 

bullfrog invasions, the first record in 

Mexico for the nonnative Rio Grande 

leopard frog was recently documented 

in northwestern Sonora, Mexico where 

the northern Mexican gartersnake is 

considered likely extirpated (Rorabaugh 

and Servoss 2006, p. 102). 

Unmack and Fagan (2004, p. 233) 

compared historical museum collections 

of nonnative fish species from the Gila 

River basin in Arizona and the 

geographically small Yaqui River basin 


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in Sonora, Mexico, to gain insight into 

the trends in distribution, diversity, and 

abundance of nonnative fishes in each 

basin over time. They found that 

nonnative species are slowly but 

steadily increasing in distribution, 

diversity, and abundance in the Yaqui 

Basin (Unmack and Fagan 2004, p. 233). 

Unmack and Fagan (2004, p. 233) 

predicted that, in the absence of 

aggressive management intervention, 

significant extirpations and/or range 

reductions of native fish species are 

expected to occur in the Yaqui Basin of 

Sonora, Mexico which may have extant 

populations of northern Mexican 

gartersnake, as did much of the Gila 

Basin before the introduction of 

nonnative species. The implications of 

these declines in native fish to northern 

Mexican gartersnakes indicate a 

potentially serious threat to the 

gartersnake’s persistence in these areas. 

However, because specific and direct 

survey information is significantly 

limited concerning the presence and 

potential effect of nonnative species on 

the northern Mexican gartersnake in 

Mexico, this discussion is based on 

extrapolation of how we understand 

these threats to affect the subspecies in 

the United States. Furthermore, based 

on the information available concerning 

the threats in Mexico we can not 

conclude that the subspecies is likely to 

become endangered throughout its range 

in Mexico. Although we acknowledge 

that these threats are affecting the 

subpecies in the United States, we have 

determined that the portion of the 

subspecies’ range in the United States 

does not constitute a significant portion 

of the range of the subspecies or a DPS. 

Therefore, on the basis of the best 

available information, we determine 

that it is not likely that the northern 

Mexican gartersnake will become an 

endangered species within the 

foreseeable future based on threats 

under this factor. 

D. The Inadequacy of Existing 

Regulatory Mechanisms 

Currently, the northern Mexican 

gartersnake is considered ‘‘State 

Endangered’’ in New Mexico. In the 

State of New Mexico, an ‘‘Endangered 

Species’’ is defined as ‘‘any species of 

fish or wildlife whose prospects of 

survival or recruitment within the state 

are in jeopardy due to any of the 

following factors: (1) The present or 

threatened destruction, modification or 

curtailment of its habitat; (2) 

overutilization for scientific, 

commercial or sporting purposes; (3) the 

effect of disease or predation; (4) other 

natural or man-made factors affecting its 

prospects of survival or recruitment 

within the state; or (5) any combination 

of the foregoing factors’’ as per New 

Mexico Statutory Authority (NMSA) 17–

2–38.D. ‘‘Take’’, defined as ‘‘means to 

harass, hunt, capture or kill any wildlife 

or attempt to do so’’ by NMSA 17–2–

38.L., is prohibited without a scientific 

collecting permit issued by the New 

Mexico Department of Game and Fish as 

per NMSA 17–2–41.C and New Mexico 

Administrative Code (NMAC) 19.33.6. 

However, while the New Mexico 

Department of Game and Fish can issue 

monetary penalties for illegal take of 

northern Mexican gartersnakes, only 

recommendations are afforded with 

respect to actions that result in 

destruction or modification of habitat 

(NMSA 17–2–41.C and NMAC 19.33.6) 

(Painter 2005). 

Prior to 2005, the Arizona Game and 

Fish Department allowed for take of up 

to four northern Mexican gartersnakes 

per person per year as specified in 

Commission Order Number 43. The 

Arizona Game and Fish Department 

defines ‘‘take’’ as ‘‘pursuing, shooting, 

hunting, fishing, trapping, killing, 

capturing, snaring, or netting wildlife or 

the placing or using any net or other 

device or trap in a manner that may 

result in the capturing or killing of 

wildlife.’’ The Arizona Game and Fish 

Department has subsequently amended 

Commission Order Number 43, which 

closed the season on northern Mexican 

gartersnakes, effective January 2005. 

Take of northern Mexican gartersnakes 

is no longer permitted in Arizona 

without issuance of a scientific 

collecting permit as per Arizona 

Administrative Code R12–4–401 et seq. 

While the Arizona Game and Fish 

Department can seek criminal or civil 

penalties for illegal take of northern 

Mexican gartersnakes, only 

recommendations are afforded with 

respect to actions that result in 

destruction or modification of northern 

Mexican gartersnake habitat. 

As previously mentioned, humans 

encounter gartersnake species somewhat 

regularly in riparian areas used for 

recreational purposes or for other 

reasons. This is partially due to 

gartersnakes having an active foraging 

strategy as well as diurnal behavior. 

Many such encounters result in the 

capture, injury, or death of the 

gartersnake due to the lay person’s fear 

or dislike of snakes (Rosen and 

Schwalbe 1988, p. 43; Ernst and Zug 

1996, p. 75; Green 1997, pp. 285–286; 

Nowak and Santana-Bendix 2002, p. 

39). It would be very difficult for the 

Arizona Game and Fish Department or 

the New Mexico Department of Fish and 

Game to cite lay people (who are not 

reptile hobbyists or amateur 

herpetologists in specific pursuit of 

herpetofauna) for such forms of take. 

Consequently, while the pursuit and 

intentional collection of reptiles, 

including the northern Mexican 

gartersnake, is regulated by these 

agencies, unregulated capture, 

collection, or killing likely occurs 

regularly. 

We are reasonably certain that the 

level of illegal field collecting by the 

hobbyist community is low because 

gartersnakes are relatively undesirable 

in amateur herpetological collections. 

However, there remains the possibility 

that small, isolated, and/or low-density 

populations could be negatively affected 

by the collection of reproductive 

females (Painter 2000, p. 39; Painter 

2005; Holycross 2006). 

The northern Mexican gartersnake is 

considered a ‘‘Candidate Species’’ in the 

Arizona Game and Fish Department 

draft document, Wildlife of Special 

Concern (WSCA) (AGFD In Prep., p. 12). 

A ‘‘Candidate Species’’ is one ‘‘whose 

threats are known or suspected but for 

which substantial population declines 

from historical levels have not been 

documented (though they appear to 

have occurred)’’ (AGFD In Prep., p. 12). 

The purpose of the WSCA list is to 

provide guidance in habitat 

management implemented by landmanagement agencies. 

Neither the New Mexico Department 

of Game and Fish nor the Arizona Game 

and Fish Department have specified or 

mandated recovery goals for the 

northern Mexican gartersnake, nor has 

either State developed a conservation 

agreement or plan for this species. 

The U.S. Bureau of Land Management 

considers the northern Mexican 

gartersnake as a ‘‘Special Status 

Species,’’ and agency biologists actively 

attempt to identify gartersnakes 

observed incidentally during fieldwork 

for their records (Young 2005). 

Otherwise, no specific protection or 

land-management consideration is 

afforded to the species on Bureau of 

Land Management lands. 

The presence of water is a primary 

habitat constituent for the northern 

Mexican gartersnake. Public concern 

over the inadequacy of Arizona surface 

water laws to ensure that flow is 

maintained perennial streams was 

discussed by Arizona Republic 

columnist Shaun McKinnon (2006b). 

McKinnon (2006b) highlighted the fact 

that because the existing water laws are 

so old, they reflect a legislative 

interpretation of the resource that is not 

consistent with what we know today; 

yet the laws have never been updated or 

amended to account for this 

discrepancy. For example, over 100 


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years ago when Arizona’s water laws 

were written, the important connection 

between groundwater and surface water 

was not known (McKinnon 2006b). 

Furthermore, meaningful changes to 

these regulations that account for the 

relative scarcity of surface water are 

unlikely to come about because Arizona 

is so ‘‘entrenched in tradition and in 

property ownership’’ and because the 

threat of litigation over proposed 

changes precludes such efforts 

(McKinnon 2006b). McKinnon (2006b) 

specifically, mentions the Gila, Salt, 

Verde, Santa Cruz, and San Pedro rivers 

as having habitat attributes that have 

directly suffered from inadequate 

surface water regulations. 

The U.S. Forest Service does not 

include northern Mexican gartersnake 

on their ‘‘Management Indicator Species 

List,’’ but it is included on the 

‘‘Regional Forester’s Sensitive Species 

List.’’ This means that northern Mexican 

gartersnakes are ‘‘considered’’ in land 

management decisions. Individual U.S. 

Forest Service biologists may 

opportunistically gather data on the 

gartersnakes observed incidentally in 

the field for their records, although it is 

not required. 

Activities that could adversely affect 

northern Mexican gartersnakes and their 

habitat continue to occur throughout 

their extant distribution on U.S. Forest 

Service lands. Clary and Webster (1989, 

p. 1) stated that ‘‘* * * most riparian 

grazing results suggest that the specific 

grazing system used is not of dominant 

importance, but good management is—

with control of use in the riparian area 

a key item.’’ Due to ongoing constraints 

in funding, staff levels, and time, and 

regulatory compliance pertaining to 

monitoring and reporting duties tied to 

land management, proactive measures 

continue to be limited. These factors 

affect a land manager’s ability to employ 

adaptive management procedures when 

effects to sensitive species or their 

habitat could be occurring at levels 

greater than accounted for in regulatory 

compliance mechanisms, such as in 

section 7 consultation under the Act for 

other listed species that may co-occur 

with the northern Mexican gartersnake 

in an area. 

The majority of extant populations of 

northern Mexican gartersnake in the 

United States occur on lands managed 

by the U.S. Bureau of Land Management 

and U.S. Forest Service. Although both 

agencies have riparian protection goals, 

neither agency has specific management 

plans for the northern Mexican 

gartersnake. 

Riparian communities are complex 

and recognized as unique in the 

southwestern United States but are 

highly sensitive to many anthropogenic 

land uses, as evidenced by the 

comparatively high number of federally 

listed riparian or aquatic species. Four 

primary prey species for the northern 

Mexican gartersnake, the Chiricahua 

leopard frog, Gila topminnow, Gila 

chub, and roundtail chub, are federally 

listed or were petitioned for listing. 

Other listed or proposed riparian 

species or their proposed or designated 

critical habitat overlap the current or 

historical distribution of the northern 

Mexican gartersnake. Despite secondary 

protections that may be afforded to the 

northern Mexican gartersnake from 

federally listed species and/or their 

critical habitat, riparian and aquatic 

communities continue to be adversely 

impacted for reasons previously 

discussed, contributing to the declining 

status of the northern Mexican 

gartersnake throughout its range in the 

United States. 

Throughout Mexico, the Mexican 

gartersnake is federally listed at the 

species level of its taxonomy as 

‘‘Amenazadas,’’ or Threatened, by the 

Secretaria de Medio Ambiente y 

Recursos Naturales (SEMARNAT) 

(SEDESOL 2001). Threatened species 

are ‘‘those species, or populations of the 

same, likely to be in danger of 

disappearing in a short or medium time 

frame, if the factors that impact 

negatively their viability, cause the 

deterioration or modification of their 

habitat or directly diminish directly the 

size of their populations continue to 

operate’’ (SEDESOL 2001 [NOM–059–

ECOL–2001], p. 4). This designation 

prohibits taking of the species, unless 

specifically permitted, as well as 

prohibits any activity that intentionally 

destroys or adversely modifies its 

habitat [SEDESOL 2000 (LGVS) and 

2001 (NOM–059–ECOL–2001)]. 

Additionally, in 1988, the Mexican 

Government passed a regulation that is 

similar to the National Environmental 

Policy Act of the United States (42 

U.S.C. 4321 et seq.). This Mexican 

regulation requires an environmental 

assessment of private or government 

actions that may affect wildlife and/or 

their habitat (SEDESOL 1988 

[LGEEPA]). 

The Mexican Federal agency known 

as the Instituto Nacional de Ecologı ́a 

(INE) is responsible for the analysis of 

the status and threats that pertain to 

species that are proposed for listing in 

the Norma Oficial Mexicana NOM–059, 

and if appropriate, the nomination of 

species to the list. INE is generally 

considered the Mexican counterpart to 

the United States’ Fish and Wildlife 

Service. INE recently developed the 

Method of Evaluation of the Risk of 

Extinction of the Wild Species in 

Mexico (MER) which unifies the criteria 

of decision on the categories of risk and 

permits the use of specific information 

fundamental to listing decisions. The 

MER is based on four independent, 

quantitative criteria: (1) Size of the 

distribution of the taxon in Mexico; (2) 

state of the habitat with respect to 

natural development of the taxon; (3) 

intrinsic biological vulnerability of the 

taxon; and (4) impacts of human activity 

on the taxon. INE began to use the MER 

in 2006; therefore, all species previously 

listed in the NOM–059 were based 

solely on expert review and opinion in 

many cases. Specifically, until 2006, the 

listing process under INE consisted of a 

panel of scientific experts who 

convened as necessary for the purpose 

of defining and assessing the status and 

threats that affect Mexico’s native 

species that are considered to be at risk 

and applying those factors to the 

definitions of the various listing 

categories. In 1994, the Mexican 

gartersnake was placed on the NOM–

059 [SEDESOL 1994 (NOM–059–ECOL–

1994), p. 46] as a threatened species as 

determined by a panel of scientific 

experts. However, we are uncertain of 

the specific information that was used 

as the basis for the listing in Mexico and 

were unable to obtain any information 

that was used to validate the Federal 

listing of the Mexican gartersnake in 

Mexico. 

Our review of the existing 

governmental regulatory mechanisms 

that pertain to the management of the 

northern Mexican gartersnake or its 

habitat in the United States leads us to 

conclude that the protections afforded 

by existing regulations may be 

insufficient to adequately address the 

declining status of the subspecies in the 

United States. However, the Mexican 

gartersnake (inclusive of the northern 

Mexican gartersnake) is considered a 

Federally-threatened species in Mexico. 

Although we do not have sufficient 

information to analyze the efficacy of 

existing regulatory mechanisms in 

Mexico, the best available data does not 

support the conclusion that the species 

is likely to become in danger of 

extinction within the foreseeable future 

due to the threats posed by the other 

factors. Therefore, uncertainty with 

respect to the efficacy of existing 

regulatory mechanisms is not 

dispositive as to the listing status of the 

subspecies, and it is not a threatened 

species on the basis of the lack of 

existing regulatory mechanisms. 


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Federal Register / Vol. 71, No. 186 / Tuesday, September 26, 2006 / Proposed Rules 56255 

E. Other Natural or Manmade Factors 

Affecting Its Continued Existence in the 

United States 

Marcy’s checkered gartersnake 

(Thamnophis marcianus marcianus) 

may have ecological implications in the 

decline and future conservation of the 

northern Mexican gartersnake in 

southern Arizona. Marcy’s checkered 

gartersnake is a semi-terrestrial species 

that is able to co-exist to some degree 

with riparian and aquatic nonnative 

predators. This is largely due to its 

ability to forage in more terrestrial 

habitats, specifically in the juvenile size 

classes (Rosen and Schwalbe 1988, p. 

31; Rosen et al. 2001, pp. 9–10). In every 

age class, the northern Mexican 

gartersnake forages in aquatic habitats 

where bullfrogs, nonnative sportfish, 

and crayfish also occur, which increases 

not only the encounter rate between the 

species but also the juvenile mortality 

rate of the northern Mexican 

gartersnake. Marcy’s checkered 

gartersnake is a potential benefactor of 

this scenario. As northern Mexican 

gartersnake numbers decline within a 

population, space becomes available for 

occupation by checkered gartersnakes. 

Marcy’s checkered gartersnake 

subsequently maintains pressure on the 

carrying capacity (the maximum 

number of a given species that an area 

can maintain based upon available 

resources) for an area and could 

potentially accelerate the decline of the 

northern Mexican gartersnake (Rosen 

and Schwalbe 1988, p. 31). 

Rosen et al. (2001, pp. 9–10) 

documented the occurrence of Marcy’s 

checkered gartersnakes out-competing 

and replacing northern Mexican 

gartersnakes at the San Bernardino 

National Refuge and surrounding 

habitats of the Black Draw. They 

suspected that the drought from the late 

1980s through the late 1990s played a 

role in the degree of competition for 

aquatic resources, provided an 

advantage to the more versatile Marcy’s 

checkered gartersnake, and expedited 

the decline of the northern Mexican 

gartersnake. The ecological relationship 

between these two species, in 

combination with other factors 

described above that have adversely 

affected the northern Mexican 

gartersnake prey base and the suitability 

of occupied and formerly occupied 

habitat, may be contributing to the 

decline of this species. 

We were unable to obtain any 

information on other natural or 

manmade factors affecting the 

continued existence of the northern 

Mexican gartersnake in Mexico. 

Finding 

We have carefully examined the best 

scientific and commercial information 

available regarding the past, present, 

and future threats faced by the northern 

Mexican gartersnake. We reviewed the 

petition, information available in our 

files, other published and unpublished 

information submitted to us during the 

public comment period following our 

90-day petition finding and consulted 

with recognized northern Mexican 

gartersnake experts and other Federal, 

State, and Mexican resource agencies. 

Because specific and direct survey 

information is significantly limited 

concerning the presence and potential 

effect of the threats discussed in this 

finding to the subspecies in Mexico, 

much of our discussion is based on 

extrapolation of how we understand 

these threats to affect the subspecies in 

the United States. Furthermore, based 

on the information available concerning 

the threats in Mexico we can not 

conclude that the subspecies is likely to 

become endangered throughout its range 

in Mexico. Although we acknowledge 

that several threats are affecting the 

subpecies in the United States, we have 

determined that the portion of the 

subspecies’ range in the United States 

does not constitute a significant portion 

of the range of the subspecies or a DPS. 

On the basis of the best scientific and 

commercial information available, we 

determine that it is not likely that the 

northern Mexican gartersnake is likely 

to become an endangered species within 

the foreseeable future and that listing of 

the northern Mexican gartersnake 

throughout its range in the United States 

and Mexico based on its rangewide 

status is not warranted. 

In making this finding, we 

respectfully acknowledge that the 

Mexican government has found 

Thamnophis eques to be in danger of 

disappearance in the short-or mediumterm future in their country from the 

destruction and modification of its 

habitat and/or from the effects of 

shrinking population sizes and has 

therefore listed the species as 

Threatened, under the listing authority 

of SEMARNAT (SEDESOL 2001). 

However, as discussed at length in 

Factor D above, we also note that the 

level of information required to list a 

species in Mexico may not be as 

rigorous as that required to list a species 

in the United States under the 

Endangered Species Act. Our 

conclusion that listing is not warranted 

under the Act is based on: (1) The 

apparent differences in listing protocols; 

(2) the significantly limited amount of 

information available on the status of 

and threats to the northern Mexican 

gartersnake in Mexico in comparison to 

our knowledge of the same in the 

United States; and most importantly (3) 

the relatively large percentage (70 to 80 

percent) of the subspecies’ historic 

distribution in Mexico for which we 

have little to no information about with 

respect to status and threats. 

In making this Finding, we also 

recognize there have been declines in 

the distribution and abundance of the 

northern Mexican gartersnake within its 

distribution in the United States which 

are primarily attributed to individual 

and community interactions with 

nonnative species that occur in every 

locality where northern Mexican 

gartersnakes have been documented in 

the United States. As discussed in 

Factor C above, the documented 

mechanisms for which nonnative 

interactions occur include: (1) Direct 

predation on northern Mexican 

gartersnakes by nonnative species; and 

(2) the effects of a diminished prey base 

via nonnative species preying upon and 

competing with native prey species 

(Meffe 1985, pp. 179–185; Rosen and 

Schwalbe 1988, pp. 28–31; 1995, p. 452; 

2002b, pp. 223–227; Bestgen and Propst 

1989, pp. 409–410; Clarkson and 

Rorabaugh 1989, pp. 531, 535; Marsh 

and Minckley 1990, p. 265; Stefferud 

and Stefferud 1994, p. 364; Rosen et al. 

1995, pp. 257–258; 1996, pp. 2, 11–12; 

2001, pp. 2, 21–22; Degenhardt et al. 

1996, p. 319; Fernandez and Rosen 

1996, pp. 21–33; Weedman and Young 

1997, pp. 1, Appendices B, C; Inman et 

al. 1998, p. 17; Rinne et al. 1998, pp. 4–

6; Fagan et al. 2005, pp. 38–39; Olden 

and Poff 2005, pp. 82–87; Holycross et 

al.2006, pp. 12–15; Brennan and 

Holycross 2006, p. 123). However, we 

again note that the portion of the 

historic distribution of the northern 

Mexican gartersnake in the United 

States represents approximately 20 to 30 

percent of its rangewide distribution. 

Furthermore, we were unable to obtain 

substantial information regarding the 

status of the northern Mexican 

gartersnake in Mexico (representing 

approximately 70 to 80 percent of its 

rangewide distribution). 

Throughout the range of the northern 

Mexican gartersnake, but most 

accurately within its distribution in the 

United States, literature documents the 

cause and effect relationship of 

disturbances to the trophic structure 

(food chain) of native riparian and 

aquatic communities. The substantial 

decline of primary native prey species, 

such as leopard frogs and native fish, 

has contributed significantly to the 

decline of a primary predator, the 

northern Mexican gartersnake. In this 


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56256 Federal Register / Vol. 71, No. 186 / Tuesday, September 26, 2006 / Proposed Rules 

respect, the northern Mexican 

gartersnake is considered an indicator 

species, or a species that can be used to 

gauge the condition of a particular 

habitat, community, or ecosystem. The 

synergistic effect of nonnative species 

both reducing the prey base of, and 

directly preying upon, northern 

Mexican gartersnakes has placed 

significant pressure upon the viability 

and sustainability of extant northern 

Mexican gartersnake populations and 

has led to significant fragmentation and 

risks to the continued viability of extant 

populations. The evolutionary biology 

of the northern Mexican gartersnake, 

much like that of native fish and 

leopard frogs, has left the species 

without adaptation to and defenseless 

against the effect of nonnative species 

invasions. 

We further recognize that in addition 

to the deleterious effects of nonnative 

species invasions, the decline of the 

northern Mexican gartersnake has been 

exacerbated by historical and ongoing 

threats to its habitat in the United 

States. The threats identified and 

discussed above in detail in Factor A, 

‘‘The Present or Threatened Destruction, 

Modification, or Curtailment of its 

Habitat or Range,’’ effectively 

summarize our knowledge of the current 

and future status of its riparian and 

aquatic habitat in the United States. 

Chiefly, these threats include: (1) The 

modification and loss of ecologically 

valuable cienegas (Hendrickson and 

Minckley 1984, p. 161; Stromberg et al. 

1996, p. 113); (2) urban and rural 

development (Medina 1990, p. 351; 

Girmendock and Young 1997, pp. 45–

47; Voeltz 2002, p. 88; Wheeler et al. 

2005, pp. 153–154); (3) road 

construction, use, and maintenance 

(Rosen and Lowe 1994, pp. 143, 146–

148; Waters 1995, p. 42; Carr and Fahrig 

2001, pp. 1074–1076; Hels and 

Buchwald 2001, p. 331; Smith and Dodd 

2003, pp. 134–138; Angermeier et al. 

2004, p. 19; Shine et al. 2004, pp. 9, 17–

19; Andrews and Gibbons 2005, p. 772; 

Wheeler et al. 2005, pp. 145, 148–149; 

Roe et al. 2006, pp. 163–166); (4) human 

population growth (Girmendock and 

Young 1993, p. 47; American Rivers 

2006; Arizona Republic, March 16, 

2006); (5) groundwater pumping, 

surface water diversions, and drought 

(Abarca and Weedman 1993, p. 2; 

Girmendock and Young 1993, pp. 45–

52; Sullivan and Richardson 1993, pp. 

35–42; Stromberg et al. 1996, pp. 124–

127; Boulton et al. 1998, pp. 60–62; 

Rinne et al. 1998, pp. 7–11; Voeltz 2002, 

p. 88; Philips and Thomas 2005; Webb 

and Leake 2005, pp. 307–308; American 

Rivers 2006; Boulton and Hancock 2006, 

p. 139); (6) improper livestock grazing 

(Sartz and Tolsted 1974, p. 354; 

Kauffman and Krueger 1984, pp. 433–

434; Szaro et al. 1985, pp. 361–363; 

Weltz and Wood 1986, p. 367–368; 

Clary and Webster 1989, pp. 1–3; Clary 

and Medin 1990, pp. 1–6; Orodho et al. 

1990, p. 9; Fleischner 1994; pp. 631–

632; Trimble and Mendel 1995, p. 233; 

Waters 1995, pp. 22–24; Girmendock 

and Young 1997, p. 47; Pearce et al. 

1998, p. 302; Belsky et al. 1999, p. 1; 

Voeltz 2002, p. 88; Krueper et al. 2003, 

pp. 607, 613–614); (7) catastrophic 

wildfire and wildfire in non-fire 

adapted communities (Rinne and Neary 

1996, p. 135; Esque and Schwalbe 2002, 

pp. 165, 190); and (8) undocumented 

immigration and international border 

enforcement and management activities 

(Segee and Neeley 2006, pp. 5–7; 

USFWS 2006, pp. 91–105). 

In our discussion under Factors A 

through E above, we have provided a 

comprehensive, in-depth analysis of all 

known threats that have or continue to 

affect the status of the northern Mexican 

gartersnake in the United States, 

including those which have not yet been 

documented but where potential effects 

exist. As a result of our assessment, we 

note that certain land use activities such 

as road construction and use, direct 

mortality from livestock grazing, 

undocumented immigration and 

international border enforcement and 

management activities, and some types 

of development, pose a more significant 

risk to highly fragmented, low density 

populations of northern Mexican 

gartersnakes. As noted on several 

occasions above, in these types of 

situations where the viability of a 

known northern Mexican gartersnake 

population is clearly at risk, the loss of 

a single reproductive female due to 

these threats is of concern. However, 

these types of threats are less significant 

to the northern Mexican gartersnake 

when the status of these at-risk 

populations improves through the 

implementation of conservation 

activities. We also remain optimistic 

that our local, State, and Federal 

partners in wildlife conservation will be 

proactive in monitoring populations and 

implementing conservation measures to 

ensure that apparent declines of the 

northern Mexican gartersnake in the 

United States are reversed and that this 

species remains a member of our native 

riparian and aquatic communities. But 

we do not rely upon any future 

conservation actions in making this 

finding. 

Notwithstanding our extensive 

discussion of the past and ongoing 

threats affecting this species, and the 

evidence of range contraction within the 

United States, neither the existence of 

the threats nor past range contraction 

means that a species meets the 

definition of a threatened or endangered 

species under the Act. Based on our 

evaluation of the best available data, we 

conclude that the northern Mexican 

gartersnake is not likely to become an 

endangered species in all or a 

significant portion of its range in the 

foreseeable future. 

References Cited 

A complete list of all references cited 

in this document is available upon 

request from the Field Supervisor at the 

Arizona Ecological Services Office (see 

ADDRESSES section). 

Author 

The primary author of this document 

is the Arizona Ecological Services Office 

(see ADDRESSES section). 

Authority: The authority for this action is 

the Endangered Species Act of 1973, as 

amended (16 U.S.C. 1531 et seq.). 

Dated: September 14, 2006. 

H. Dale Hall, 

Director, Fish and Wildlife Service. 

[FR Doc. 06–7784 Filed 9–25–06; 8:45 am] 

BILLING CODE 4310–55–P 


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