78094 Federal Register / Vol. 75, No. 239 / Tuesday, December 14, 2010 / Proposed Rules 

DEPARTMENT OF THE INTERIOR 

Fish and Wildlife Service 

50 CFR Part 17 

[Docket No. FWS–R2–ES–2009–0032; MO 

92210–0–008] 

Endangered and Threatened Wildlife 

and Plants; 12-Month Finding on a 

Petition To List the Sonoran 

Population of the Desert Tortoise as 

Endangered or Threatened 

AGENCY: Fish and Wildlife Service, 

Interior. 

ACTION: Notice of 12-month petition 

finding. 

SUMMARY: We, the U.S. Fish and 

Wildlife Service, announce a 12-month 

finding on a petition to list the Sonoran 

population of the desert tortoise 

(Gopherus agassizii) as endangered or 

threatened and to designate critical 

habitat under the Endangered Species 

Act of 1973, as amended (Act). After 

review of all available scientific and 

commercial information, we find that 

listing the Sonoran population of the 

desert tortoise is warranted. Currently, 

however, listing the Sonoran population 

of the desert tortoise is precluded by 

higher priority actions to amend the 

Lists of Endangered and Threatened 

Wildlife and Plants. Upon publication 

of this 12-month petition finding, we 

will add the Sonoran population of the 

desert tortoise to our candidate species 

list. We will develop a proposed rule to 

list the Sonoran population of the desert 

tortoise as our priorities allow. We will 

make any determination on critical 

habitat during development of the 

proposed listing rule. In any interim 

period we will address the status of the 

candidate taxon through our annual 

Candidate Notice of Review (CNOR). 

DATES: The finding announced in this 

document was made on December 14, 

2010. 

ADDRESSES: This finding is available on 

the Internet at http:// 

www.regulations.gov at Docket Number 

FWS–R2–ES–2009–0032. Supporting 

documentation we used in preparing 

this finding is available for public 

inspection, by appointment, during 

normal business hours at the U.S. Fish 

and Wildlife Service, Arizona Ecological 

Services Office, 2321 West Royal Palm 

Road, Suite 103, Phoenix, Arizona 

85021. Please submit any new 

information, materials, comments, or 

questions concerning this finding to the 

above address. 

FOR FURTHER INFORMATION CONTACT: 

Steven L. Spangle, Field Supervisor 

Arizona Ecological Services Office (see 

ADDRESSES); by telephone at (602) 242– 

0210; or by facsimile at (602) 242–2513. 

If you use a telecommunications device 

for the deaf (TDD), please call the 

Federal Information Relay Service 

(FIRS) at 800–877–8339. 

SUPPLEMENTARY INFORMATION: 

Background 

Section 4(b)(3)(B) of the Endangered 

Species Act of 1973, as amended (Act) 

(16 U.S.C. 1531 et seq.), requires that, 

for any petition to revise the Federal 

Lists of Endangered and Threatened 

Wildlife and Plants that contains 

substantial scientific or commercial 

information that listing the species may 

be warranted, we make a finding within 

12 months of the date of receipt of the 

petition. In this finding, we determine 

that the petitioned action is: (a) Not 

warranted, (b) warranted, or (c) 

warranted, but the immediate proposal 

of a regulation implementing the 

petitioned action is precluded by other 

pending proposals to determine whether 

species are threatened or endangered, 

and expeditious progress is being made 

to add or remove qualified species from 

the Federal Lists of Endangered and 

Threatened Wildlife and Plants. Section 

4(b)(3)(C) of the Act requires that we 

treat a petition for which the requested 

action is found to be warranted but 

precluded as though resubmitted on the 

date of such finding, that is, requiring a 

subsequent finding to be made within 

12 months. We must publish these 12- 

month findings in the Federal Register. 

Previous Federal Actions 

On October 15, 2008, we received a 

petition dated October 9, 2008, from 

WildEarth Guardians and Western 

Watersheds Project (petitioners) 

requesting that the Sonoran population 

of the desert tortoise be listed under the 

Act as a distinct population segment 

(DPS), as threatened or endangered 

rangewide (in the United States and 

Mexico), and critical habitat be 

designated. The petition contained 

detailed information on the natural 

history, biology, current status, and 

distribution of the Sonoran population 

of the desert tortoise. It also contained 

information on what the petitioners 

reported as potential threats to the 

Sonoran population of the desert 

tortoise, such as livestock grazing, 

urbanization and development, mining, 

international border patrol activities, 

illegal collection, inadequacy of existing 

regulations, altered fire regimes, offhighway vehicle use, drought, and 

climate change. We acknowledged the 

receipt of the petition in a letter to the 

WildEarth Guardians and Western 

Watersheds Project, dated November 26, 

2008. In that letter we also stated that 

we had reviewed the information 

presented in the petition and 

determined that issuing an emergency 

regulation temporarily listing the 

species as per section 4(b)(7) of the Act 

was not warranted. We also stated that 

we intended to make our finding on 

whether the petition presented 

substantial information that the 

requested action may be warranted, to 

the maximum extent practicable within 

90 days of receipt of the petition, 

according to the provisions of section 

4(b)(3) of the Act. 

On August 28, 2009, we made our 90- 

day finding that the petition presented 

substantial scientific information 

indicating that listing the Sonoran 

population of the desert tortoise 

(Gopherus agassizii) may be warranted. 

The finding and notice of our initiation 

of a status review was published in the 

Federal Register on August 28, 2009 (74 

FR 44335). 

On April 10, 2010, a stipulated 

settlement agreement (WildEarth 

Guardians and Western Watersheds 

Project v. Salazar, 10–cv–86–ACT–RHS 

(D. NM)) was filed. In this agreement, 

we agreed to submit a 12-month finding 

to the Federal Register on or before 

December 5, 2010. The stipulated 

settlement agreement was signed and 

adopted by the District Court of New 

Mexico on April 15, 2010. 

This notice constitutes our 12-month 

finding for the petition to list the 

Sonoran population of the desert 

tortoise as threatened or endangered. 

Other Federal Actions 

Throughout this finding, we use 

‘‘Mojave’’ to describe desert tortoise 

populations north and west of the 

Colorado River, as well as any reference 

to the biotic community known as the 

‘‘Mojave Desert’’ or ‘‘Mojave 

desertscrub.’’ These uses are consistent 

with the previous and current spelling 

of the common name in Federal actions 

that have addressed this population. We 

use ‘‘Mohave’’ in the geographic context 

to remain consistent with its reference 

by the U.S. Board of Geographic Names 

(e.g., Mohave County). In addition, 

while the Sonoran population of the 

desert tortoise is not currently formally 

recognized as a unique taxonomic 

entity, for ease of reference, we refer to 

the Sonoran population of the desert 

tortoise as the ‘‘Sonoran desert tortoise’’ 

in this finding. 

On December 30, 1982, we published 

a notice of review which determined the 

desert tortoise throughout its range in 

the United States and Mexico to be a 

Category 2 Candidate species (47 FR 


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58454); this was reaffirmed on 

September 18, 1985 (50 FR 37958). 

Category-2 status was granted to species 

for which information in our possession 

indicated that a proposed listing as 

threatened or endangered was possibly 

appropriate, but for which sufficient 

data were not available to make a 

determination of listing status under the 

Act. 

On April 2, 1990, we issued a final 

rule designating the Mojave population 

of the desert tortoise (occurring north 

and west of the Colorado River) as a 

threatened species under the Act (55 FR 

12178; see final rule for a summary of 

previous actions regarding the Mojave 

population of the desert tortoise). 

Currently, the Mojave population of the 

desert tortoise is recognized as a distinct 

population segment under the Act. As 

part of that rulemaking, we designated 

any desert tortoise from the Sonoran 

population as threatened when observed 

outside of its known range, due to 

similarity of appearance under section 

4(a) of the Act. 

On December 5, 1996, we published 

a rule that discontinued the practice of 

keeping a list of Category 2 Candidate 

species (61 FR 64481). Since that time, 

the Sonoran desert tortoise observed 

inside its known range has had no 

Federal Endangered Species Act status. 

For a detailed account of previous 

Federal actions that pertained to the 

desert tortoise in the United States, 

please review the following Federal 

Register documents: ‘‘Proposed 

Endangered Status and Critical Habitat 

for the Beaver Dam Slope Population of 

the Desert Tortoise’’ (43 FR 37662, 

August 23, 1978); ‘‘Requirement to 

withdraw or supplement proposals to 

determine various U.S. taxa of plants 

and wildlife as Endangered or 

Threatened or to determine Critical 

Habitat for such species’’ (44 FR 12382, 

March 6, 1979); ‘‘Reproposal of Critical 

Habitat for the Illinois mud turtle and 

Beaver Dam Slope population of the 

desert tortoise’’ (44 FR 70680, December 

7, 1979); ‘‘Listing as Threatened With 

Critical Habitat for the Beaver Dam 

Slope Population of the Desert Tortoise 

in Utah’’ (45 FR 55654, August 20, 

1980); ‘‘Review of Vertebrate Wildlife for 

Listing as Endangered or Threatened 

Species’’ (47 FR 58454, December 30, 

1982); ‘‘Notice of Findings on Four 

Petitions, and Review of One Species’’ 

(50 FR 13054, April 2, 1985); ‘‘Review of 

Vertebrate Wildlife’’ (50 FR 37958, 

September 15, 1985); ‘‘Finding on Desert 

Tortoise Petition’’ (50 FR 49868, 

December 5, 1985); ‘‘Findings on 

Pending Petitions and Description of 

Progress of Listing Actions’’ (53 FR 

25511, July 7, 1988); ‘‘Findings on 

Pending Petitions and Description of 

Progress of Listing Actions’’ (53 FR 

52746, December 29, 1988); ‘‘Emergency 

Determination of Endangered Status for 

the Mojave Population of the Desert 

Tortoise’’ (54 FR 32326, August 4, 1989); 

‘‘Desert Tortoise’’ (54 FR 42270, October 

13, 1989); ‘‘Determination of Threatened 

Status for the Mojave Population of the 

Desert Tortoise’’ (55 FR 12178, April 2, 

1990); ‘‘Finding on a Petition to List the 

Sonoran Desert Tortoise as Threatened 

or Endangered’’ (56 FR 29453, June 27, 

1991); ‘‘Proposed Determination of 

Critical Habitat for the Mojave 

Population of the Desert Tortoise’’ (58 

FR 45748, August 30, 1993); 

‘‘Determination of Critical Habitat for 

the Mojave Population of the Desert 

Tortoise’’ (59 FR 5820, February 8, 

1994); ‘‘Determination of Critical Habitat 

for the Mojave Population of the Desert 

Tortoise’’ (59 FR 9032, February 24, 

1994); ‘‘Notice of Final Decision on 

Identification of Candidates for Listing 

as Endangered or Threatened’’ (61 FR 

64481, December 5, 1996); and ‘‘90–Day 

Finding on a Petition To List the 

Sonoran Population of the Desert 

Tortoise (Gopherus agassizii) as a 

Distinct Population Segment (DPS) with 

Critical Habitat’’ (74 FR 44335, August 

28, 2009). 

Species Information 

Taxonomy 

The desert tortoise is in the genus 

Gopherus (Rafinesque 1832), or gopher 

tortoises, and is a member of the 

Testudinidae family, or terrestrial 

tortoises. The North American tortoises 

formerly comprised two genera, 

Gopherus and Xerobates, with the latter 

including X. agassizii, the desert 

tortoise (Crumly 1994, pp. 7–8). 

Scientific nomenclature assigned to the 

desert tortoise has undergone a series of 

changes since its initial description by 

Cooper (1863) as X. agassizii (Barrett 

and Johnson 1990, p. 5); the currently 

recognized scientific name for the desert 

tortoise is Gopherus agassizii. Further 

information is available in Barrett and 

Johnson (1990, p. 5) or in the detailed 

account of desert tortoise phylogeny 

(evolutionary development) and 

systematics (taxonomic classification) 

by Crumly (1994, pp. 7–32). The desert 

tortoise is known in Mexico with the 

common names of ‘‘tortuga del monte,’’ 

‘‘Gala ́pago de desierto,’’ or the 

‘‘xtamo ́osni’’ (Rorabaugh 2008, p. 35). 

Physical Description of Sonoran Desert 

Tortoises 

Adult Sonoran desert tortoises range 

in total carapace (straight-line top shell) 

length from 8 to 15 inches (in) (20 to 38 

centimeters (cm)), with a relatively high 

domed shell (AGFD 2001, p. 1; Brennan 

and Holycross 2006, p. 54). The record 

length for a Sonoran desert tortoise is 

19.4 in (49 cm) total carapace length 

(Jackson and Wilkinson-Trotter 1980, p. 

430). The carapace is usually brownish 

with a definite pattern and prominent 

growth lines (AGFD 2001, p. 1). The 

plastron (bottom shell) is yellowish and 

is not hinged (AGFD 2001, p. 1; Brennan 

and Holycross 2006, p. 54). The hind 

limbs are very stocky and elephantine; 

forelimbs are flattened for digging and 

covered with large conical scales (AGFD 

2001, p. 1; Brennan and Holycross 2006, 

p. 54). Male Sonoran desert tortoises are 

differentiated from females by having 

elongated gular (throat) shields, chin 

glands visible on each side of the lower 

jaw (most evident during the breeding 

season), a concave plastron, and larger 

overall size (AGFD 2001, p. 1). 

Distribution 

The desert tortoise includes portions 

of southern California, southern Nevada, 

southwestern Utah, and the western, 

northwestern, and southern portions of 

Arizona in the United States, and also 

includes the Mexican State of Sonora 

into the northern portion of Sinaloa. 

One-third of the geographic range of the 

desert tortoise occurs in northwestern 

Mexico (Bury et al. 2002, p. 86). The 

specific distribution of desert tortoise is 

influenced by habitat and climatic 

characteristics (vegetation community 

for food), soil and substrate 

characteristics (for shelter), and 

precipitation pattern (for water 

availability) within the appropriate 

elevation range. 

The distribution of the Sonoran desert 

tortoise in the United States is 

considered to be entirely within Arizona 

and comprises approximately 26.8 

million acres (ac) (10.8 million hectares 

(ha)); east and south of the Colorado 

River (Barrett and Johnson 1990, pp. 4– 

5; Lamb et al. 1989, p. 84). Sonoran 

desert tortoise distribution in Arizona is 

limited to the northeast by the limits of 

the Sonoran Desert. The Arizona portion 

of their range constitutes approximately 

52 percent of their total distribution. In 

Arizona, the Sonoran desert tortoise 

occurs primarily on Federal land but 

also occurs on a variety of non-federal 

lands as well as on ten Native American 

reservations: (1) Fort Mojave Indian 

Tribe; (2) Colorado River Indian Tribe; 

(3) Hualapai Tribe; (4) Fort McDowell 

Yavapai Nation; (5) Salt River PimaMaricopa Indian Community; (6) Gila 

River Indian Community; (7) Ak Chin; 

(8) Tohono O’odham Nation; (9) Pasqua 

Yaqui Tribe; and, (10) San Carlos 

Apache Tribe (AIDTT 2000, p. 40). 


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In Mexico, where 48 percent of their 

range occurs, the distribution of the 

Sonoran desert tortoise extends from the 

international border of Sonora and 

Arizona, south to the vicinity of 

Guaymas, and north of the Rı ́o Yaqui 

(the southern and southeastern-most 

border of their distribution), in southern 

Sonora (Germano et al. 1994, p. 77; 

Fritts and Jennings 1994, p. 51; Bury et 

al. 2002, p. 88; Van Devender 2002a, p. 

5; Edwards et al. 2009, pp. 7–8). This 

includes approximately the western half 

of the State of Sonora from the Gulf of 

California coast east roughly to the 

transition to unsuitable woodland and 

conifer forest areas in the higher 

elevations of the Sierra Madre 

Occidental. In 30 timed searches 

conducted August to September 1983, 

and beyond the known distribution of 

Sonoran desert tortoises in Sonora, 

Mexico, Fritts and Jennings (1994, p. 52) 

found several patterns in Sonoran desert 

tortoise distribution. First, most 

Sonoran desert tortoises in the eastern 

and northern extent of their distribution 

in Mexico occur below the 2,600 foot (ft) 

(790 meters (m)) elevation contour 

(Fritts and Jennings 1994, p. 52). 

Second, populations may be the densest 

and the least patchy between elevations 

of 900 and 1,600 ft (270 and 490 m) 

(Fritts and Jennings 1994, p. 52). They 

were also not found in habitat in Mexico 

that received an average of 3.9 in (10 

cm) or less of rain annually (Fritts and 

Jennings 1994, p. 53). 

One question about the distribution of 

the Sonoran desert tortoise concerns the 

origin of a small number of tortoises that 

have been found in far southeastern 

Cochise County, Arizona, an area 

generally considered well east of the 

known distribution. There is some 

evidence that these tortoises may 

represent a naturally occurring 

population based on the presence of 

suitable habitat (Rorabaugh 2009, pers. 

comm.), similar animal communities 

(Rosen 2009, pers. comm.), and historic 

and current observations of tortoises in 

the area (Hulse and Middendorf 1979, p. 

546; Radke 2009, pers. comm.; Van 

Devender et al. 1976, pp. 300–303). 

However, these observations have 

traditionally been discounted as 

released pets rather than a natural 

population (AIDTT 2000, p. 3; Germano 

et al. 1994, p. 81). Also, recent genetic 

analysis of a Sonoran desert tortoise 

collected from this area in 2009 

indicated it was most closely related to 

tortoises in the Phoenix, Arizona, area 

and is likely, therefore, a ‘‘released or 

escaped captive’’ tortoise (Edwards 

2010, pers. comm.). We recognize there 

is a fair amount of uncertainty regarding 

the origin of this population. However, 

because Sonoran desert tortoises are 

infrequently documented from this area 

and recent genetic testing indicated that 

observations represent released 

captives, we conclude that desert 

tortoises from this area do not represent 

a naturally-occurring, disjunct 

population. Consequently, we will not 

evaluate potential threats to the tortoises 

in this area of Cochise County in this 

finding. 

Habitat 

Sonoran desert tortoises are most 

closely associated with the Arizona 

Upland and Lower Colorado River 

subdivisions of Sonoran desertscrub and 

Mojave desertscrub vegetation types. 

They occur most commonly on rocky 

(predominantly granitic rock), steep 

slopes and bajadas (lower mountain 

slopes often formed by the coalescing of 

several alluvial fans (fan-shaped 

deposits at the ends of canyons formed 

when fast flowing streams slow and 

widen)) and in paloverde-mixed cacti 

associations (Ortenburger and 

Ortenburger 1927, p. 120; Burge 1979, p. 

49; 1980, p. 48). Sonoran desert tortoise 

density has been observed to be higher 

in the Arizona Upland subdivision of 

the Sonoran desertscrub than in the 

Lower Colorado subdivision of the 

Sonoran desertscrub or in Mojave 

desertscrub (Berry 1984, p. 434; AIDTT 

2000, p. 4; Boarman and Kristan 2008, 

p. 19). In addition to the use of 

vegetation to meet energy and 

nutritional needs, the Sonoran desert 

tortoise uses vegetation for predator 

avoidance, thermal protection, and in 

social behaviors (Avery and Neibergs 

1997, p. 13; Grandmaison et al. in press, 

p. 3). An important attribute of Sonoran 

desert tortoise habitat is the presence of 

cryptogamic crusts (soil crusts with 

unique, microscopic association of flora 

and fauna) (Bowker et al. 2008, p. 2309). 

These occur on the surface of Sonoran 

Desert soils and assist with nitrogenfixing to enhance soil fertility, improve 

water infiltration into soils, and prevent 

or lessen effects from wind and water 

erosion, all of which help to sustain 

vegetation vital to the Sonoran desert 

tortoise (DeFalco 1995, p. 22; DeFalco et 

al. 2001, pp. 1, 9). 

Sonoran desert tortoises rarely occur 

in oak woodland habitat. However, one 

such population occurs at 

approximately 5,000-ft (1,500-m) 

elevation in Chiminea Canyon in the 

Rincon Mountains of Pima County, 

Arizona (Van Devender 2002a, p. 23), 

and they are also known from similar 

elevation in the Atascosa and Pajarito 

Mountains in south-central Arizona. 

Zylstra and Steidl (2008, p. 747) found 

that habitat selection by Sonoran desert 

tortoises was most closely associated 

with topographic (degree of steepness of 

slope) and geomorphologic (rock type 

and structure) influences rather than by 

vegetation type. Specifically, Zylstra 

and Steidl (2008, p. 747) found that the 

likelihood of observing Sonoran desert 

tortoises increased with increasing 

slope, with a strong association to 

aspect (the direction to which a slope 

faces), with east-facing slopes preferred 

over north-facing slopes. However, the 

season of use may affect which slopeaspects (the direction a particular slope 

faces) Sonoran desert tortoises are likely 

to use based on their needs at that time 

(Zylstra and Steidl 2008, p. 752). 

Specifically, Sonoran desert tortoises 

have different thermoregulatory and 

physiological needs based upon their 

seasonal behaviors, such as hibernation 

or seeking temporary shelter during the 

tortoise’s surface-active seasons. 

In addition to steep, rocky slopes and 

bajadas, Sonoran desert tortoises also 

use inter-mountain valleys as part of 

their home ranges and for dispersal at 

all age classes (Averill-Murray and 

Averill-Murray 2002, p. 16). In the 

Ironwood National Forest, AverillMurray and Averill-Murray (2005, p. 65) 

found tortoises or their signs (such as 

scat (droppings) and burrows) on 92 

percent of transects in boulder habitat, 

on 71 percent of transects that included 

incised washes (dry stream beds that 

flow in response to precipitation), and 

on 25 percent of transects that had 

neither boulder habitat nor incised 

washes. Sonoran desert tortoises were 

found up to one mile (mi) (1.6 

kilometers (km)) away from the nearest 

slope, indicating that they occur in low 

densities in inter-mountain valleys. 

Averill-Murray and Averill-Murray 

(2005, p. 65) stated that maintaining 

these areas ‘‘may be important for longterm population viability.’’ Washes 

might also be selectively chosen by 

reproductive female Sonoran desert 

tortoises as all eggs and hatchling desert 

tortoises observed by Barrett (1990, p. 

205) occurred there. Sonoran desert 

tortoises on the 40-square-mile (sq mi) 

(64-square-kilometer (sq km)) Florence 

Military Reservation in Pinal County, 

Arizona, primarily use xeroriparian 

habitat (a habitat association with plant 

species tolerant to hyper-arid 

conditions) along washes, with caliche 

caves (caves formed along steep banks 

of washes within cemented, 

sedimentary rock formations of calcium 

carbonate) within washes being an 

important component to occupied 

habitat (Lutz et al. 2005, p. 22; Riedle 

et al. 2008, p. 418). Another frequently 


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used habitat type on the Florence 

Military Reservation included gently 

rolling alluvial fans dominated by 

creosote bush (Larrea tridentata) and 

white bursage (Ambrosia dumosa) 

during all periods of the year; somewhat 

atypical for Sonoran desert tortoises in 

other portions of its range (Lutz et al. 

2005; p. 22; Grandmaison et al. in press, 

p. 4). In this habitat, Sonoran desert 

tortoises often used packrat middens 

(organic debris piles constructed for 

nesting purposes which often are 

comprised of wood material, cactus 

pads, etc.) as shelter sites, especially 

those with suitable canopy cover, an 

absence of cattle activity, and proximity 

to roads and washes (Lutz et al. 2005, 

p. 22; Grandmaison et al. in press, p. 2). 

Sonoran desert tortoises in Arizona 

generally occur within elevations from 

510 to 5,300 ft (155 to 1,615 m) (Barrett 

and Johnson 1990, p. 7; AGFD 2001, p. 

4). According to the AGFD’s Heritage 

Data Management system, 95 percent of 

Sonoran desert tortoise observations in 

Arizona have occurred at an elevation of 

904 to 4,198 ft (275 to 1279 m) (Zylstra 

and Steidl 2009, p. 8). However, one 

example of an extreme exception was a 

Sonoran desert tortoise observed at 

7,808 ft (2,379 m) in a ponderosa pinedominated coniferous community in the 

Rincon Mountain District of Saguaro 

National Park in Pima County, Arizona 

(Aslan et al. 2003, p. 57). The nearest 

road was 8.6 mi (13.9 km) away by trail 

and nearly 2,000 ft (610 m) lower in 

elevation from the observed location of 

the tortoise, which strongly dismisses 

any notion that human activity was 

responsible for its location at such a 

high elevation (Aslan et al. 2003, p. 57). 

Sonoran desert tortoises in Mexico are 

generally found at lower elevations, 

ranging from approximately 1,000 to 

1,640 ft (305 to 500 m) in elevation in 

rocky outcrops in desertscrub and 

foothills thornscrub habitat (Bury et al. 

2002, p. 89). As in Sonoran desertscrub 

habitat in Arizona, Sonoran desert 

tortoises in Mexico often use shrubs as 

temporary shelter sites, and species 

such as mesquite (Prosopis spp.) and 

ironwood (Olneya tesota) may play 

important roles in the natural history of 

Sonoran desert tortoises in Mexico 

(Bury et al. 2002, p. 100). Sonoran 

desert tortoises in Mexico have not been 

documented in flatter areas between 

mountain ranges (Bury et al. 2002, p. 

89), although we presume they use these 

areas to some extent for dispersal much 

like they do in similar inter-mountain 

basins of Arizona. With the exception of 

the El Pinacate Desert Bioreserve in 

northwestern Sonora, Sonoran desert 

tortoises have not been documented 

using the extremely arid Lower 

Colorado subdivision of the Sonoran 

Desert in Mexico (Bury et al. 2002, p. 

89). However, based on their presence 

in El Pinacate and the general lack of 

surveys in Mexico, the Sonoran desert 

tortoise may potentially be found in this 

habitat in northwestern Sonora in low 

densities. The extent of Sonoran desert 

tortoise distribution in northeastern 

Sonora, an area characterized as a 

transitional zone of foothills thornscrub, 

tropical deciduous forest, and Madrean 

oak woodland, is poorly understood 

(Bury et al. 2002, p. 89). 

Burrow Use 

Adequate shelter, often in the form of 

constructed burrows, is one of the most 

important habitat features for the 

Sonoran desert tortoise. Burrows are 

constructed under rocks and boulders, 

beneath vegetation, on semi-open 

slopes, within the sidewalls of washes, 

or by using rocky crevices which may or 

may not be altered by the tortoise (Burge 

1979, p. 44; 1980, pp. 44–45; Barrett 

1990, p. 205; Averill-Murray et al. 

2002a, pp. 136–137, Grandmaison et al. 

in press, p. 14). Sonoran desert tortoises 

construct burrows in a variety of soil 

types including silt, silt with loose 

gravel, diatomite (a light-colored porous 

rock composed of the shells of diatoms) 

and diatomaceous marl (a crumbly 

mixture of clays, calcium and 

magnesium carbonates, with remnants 

of shells), and well-lithified (process 

whereby loose particles are converted 

into rock) volcanic ash, as observed in 

the lower San Pedro River Valley of 

Arizona (Bailey et al. 1995, pp. 363– 

364). Burrows are used for 

thermoregulation, nesting, and 

protection from predators, and the lack 

of suitable conditions for constructing 

burrows may be a limiting factor in 

Sonoran desert tortoise populations 

(Barrett and Humphrey 1986, p. 262; 

Bailey et al. 1995, p. 366; Zylstra and 

Steidl 2008, p. 752). In fact, Sonoran 

desert tortoise population densities 

appear to be highly correlated with 

available burrows, or potential burrow 

sites (Averill-Murray and Klug 2000, p. 

69; Averill-Murray et al. 2002b, p. 126). 

Sonoran desert tortoises often use a 

group of relatively closely-located 

burrows as focal areas of activity in their 

home range. In doing so, they establish 

circular or slightly linear movement 

patterns, and may temporarily move on 

to another such cluster of burrows 

within the same active season (Bulova 

1994, p. 140; Averill-Murray and Klug 

2000, p. 62; Lutz et al. 2005, p. 21). 

Burrows influence a variety of 

Sonoran desert tortoise behaviors and 

physiological characteristics. During the 

winter dormancy period (colder, winter 

months of inactivity), female Sonoran 

desert tortoises typically use more 

shallow burrows that are more 

susceptible to variation in ambient 

temperatures and consequently females 

emerge earlier in the spring (as early as 

late February) than do males, who often 

remain dormant until the 

commencement of the summer monsoon 

(AIDTT 2000, p. 7; Ernst and Lovich 

2009, p. 547). Averill-Murray and Klug 

(2000, p. 66) and Bailey et al. (1995, p. 

367) suggest that shallow burrows may 

account for responsiveness of females to 

warming periods in early spring for 

additional foraging opportunities to 

increase energy reserves for egg 

development, as shallower burrows are 

more reflective of ground-surface 

temperatures. Alternatively, cool, less 

variable temperatures in deeper burrows 

selected by male Sonoran desert 

tortoises may enhance sperm 

development and viability, as cooler 

temperatures allow more sperm 

production (Bailey et al. 1995, p. 367). 

The season may influence the 

locations and dimensions of burrows 

used by Sonoran desert tortoises in 

order to meet their behavioral and 

physiological needs (Barrett 1990, p. 

205; Bailey et al. 1995, pp. 363, 366). 

Finally, particularly in hatchling and 

juvenile size classes, the burrow 

microclimate can affect the rate of water 

loss in desert tortoises, which results in 

behaviors (drinking pooled rain, 

withdrawing into their shell, seeking 

long, deep burrows) to avoid lethal 

dehydration in relatively hot, dry 

seasons (Wilson et al. 2001, p. 158; 

Bulova 2002, pp. 184–186). 

Other forms of shelter used by 

Sonoran desert tortoise include packrat 

middens, which are often shared with 

other native reptiles, including other 

tortoises (Averill-Murray et al. 2002a, 

pp. 136–137; Lutz et al. 2005, p. 22; 

Grandmaison et al. in press, p. 2). These 

shelter types provide less insulation 

than earthen burrows and are therefore 

used for shorter duration, especially 

during the months with extremely hot 

or cold temperatures. This was the most 

commonly used shelter site at Florence 

Military Reservation. 

Seasonal Behavior and Long-Distance 

Movements 

The Sonoran desert tortoise is diurnal 

(active during daylight hours) but 

sometimes emerge at night in response 

to rainfall (Ernst and Lovich 2009, p. 

544). Sonoran desert tortoises may be 

surface-active every month of the year, 

but in the winter, surface activity is 

likely a response to thermoregulatory 

needs or movements between burrows 

(Averill-Murray and Klug 2000, p. 66). 


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Temperature and precipitation are 

important predictors of Sonoran desert 

tortoise activity (Meyer et al. 2010, p. 

11). Precipitation amounts and timing 

vary among the populations of desert 

tortoise. The lowest amount of rainfall 

(usually during the winter) occurs in the 

northwestern-most portion of the 

species’ range, and gradually increases 

and becomes seasonally bimodal pattern 

(rains in winter and summer) to the 

south into the southern-most extent of 

the species range in northern Sinaloa, 

Mexico (Germano et al. 1994, p. 76). 

Sonoran desert tortoise surface activity 

largely mimics the warm-season 

precipitation pattern (Averill-Murray et 

al. 2002a, p. 139; Van Devender 2002a, 

p. 7). Like the Arizona populations, 

Sonoran desert tortoises in Mexico seem 

to be most active in late summer (Ernst 

and Lovich 2009, p. 544). Sonoran 

desert tortoises are approximately half 

as active during the spring as they are 

in the summer, with females typically 

becoming surface active to forage in late 

March, while males typically emerge 

(but are not necessarily active) in late 

April (Averill-Murray et al. 2002a, p. 

138). 

The summer monsoon (occurring 

typically from late June through 

September), characterized by both 

excessive heat and frequent 

thunderstorms, is the peak activity 

season for the Sonoran desert tortoise 

(Averill-Murray et al. 2002a, pp. 139– 

140). During this period, new growth of 

perennial plants is initiated and annual 

plants germinate, providing forage for 

tortoises (Averill-Murray et al. 2002a, p. 

140). The onset of the summer monsoon 

triggers Sonoran desert tortoises to 

drink, flush their bladders, and 

rehydrate, establishing a positive water 

and energy balance, and spurring 

reproductive behaviors (AIDTT 2000, p. 

7). Sonoran desert tortoises have been 

observed to seek out rocks with surface 

depressions during summer months to 

drink puddled water from monsoon 

storm events (Oftedal 2007, p. 23). 

Surface activity in Sonoran desert 

tortoises begins to wane as early as late 

September and ends by mid-December 

as they prepare for hibernation. 

Temperature and photoperiod (the 

duration of daylight) are likely the cues 

used by Sonoran desert tortoises to 

commence hibernation (Bailey et al. 

1995, p. 367; Averill-Murray et al. 

2002a, p. 147). Periods of hibernation 

(typically from mid-November through 

mid-February) in Sonoran desert 

tortoises appear to vary greatly among 

populations and among years but appear 

to correlate with seasonal temperatures 

(Bailey et al. 1995, p. 367; AverillMurray and Klug 2000, p. 66). 

The behavior and ecology of hatchling 

Sonoran desert tortoises is poorly 

understood because their small size 

makes them very difficult to observe in 

the wild. Their scat is small, 

inconspicuous, and ephemeral, and 

burrows used by individuals in this size 

class resemble those of other terrestrial 

vertebrates in Sonoran desert tortoise 

habitat (Germano et al. 2002, pp. 271– 

272). This size class is thought to be the 

most vulnerable, experiencing the 

highest mortality rates (Morafka 1994, p. 

161). 

Home range sizes of Sonoran desert 

tortoises vary with precipitation levels, 

contracting during wet years and 

expanding during dry years in response 

to the availability of forage plants 

(Averill-Murray and Klug 2000, p. 67). 

The home range of Sonoran desert 

tortoises may be as small as 6.4 ac (2.6 

ha) but can vary widely, with males 

having larger home ranges than females 

(Barrett 1990, p. 203; Averill-Murray 

and Klug 2000, pp. 55–61; AverillMurray et al. 2002a, pp. 150–151). In the 

lower San Pedro River Valley, Meyer 

(1993, p. 99) found Sonoran desert 

tortoise home ranges varied between 45 

and 640 ac (18 and 258 ha) in size. 

Sonoran desert tortoises are known to 

exhibit high fidelity to their home 

ranges, with exception to dispersal 

movements when they move to new 

areas (Zylstra and Swann 2009, p. vi). 

They likely habituate to specific 

attributes of their home range, including 

the location of mates, water catchments, 

mineral licks, and burrow sites (Berry 

1986a, p. 113). 

Sonoran desert tortoises are known to 

make long-distance movements between 

populations in adjacent mountain 

ranges. In an extreme example, Edwards 

et al. (2004, p. 494) tracked an adult 

female Sonoran desert tortoise moving 

20 mi (32 km) between the Rincon and 

Santa Rita mountains of southern 

Arizona (also see Zylstra and Swann 

2009, p. 10). During this long-distance 

movement, this tortoise encountered 

several barriers to movement that 

required human intervention to 

overcome such as fence lines, railroad 

tracks, an interstate highway, and 

several captures (including a temporary 

adoption) by humans (Edwards et al. 

2004, p. 494). In another example, in the 

San Pedro Valley of southern Arizona, 

a sub-adult Sonoran desert tortoise was 

captured and marked in 1992. It was 

recaptured in 2005 approximately 14 mi 

(23 km) from its original point of 

capture (Meyer et al. 2010, p. 18). 

Dispersal distances of hatchling 

Sonoran desert tortoises are not well 

understood, but are likely shorter than 

those of adults because of the complex 

habitat of boulders and vegetation 

(where they occur) may inhibit longdistance movements (Van Devender 

2002a, p. 14). 

Gibbons (1986, p. 104) suspected that 

long-distance movements by turtles can 

be explained by: (1) Nest site selection; 

(2) seasonal migration; (3) departure 

from unfavorable habitat conditions; or 

(4) movement by males in search of 

females. Averill-Murray and Klug (2000, 

p. 68) suggested that long-distance 

movements may be interpreted as 

random wanderings, infrequent travels 

to known sources of biological needs, 

explorations, adaptations for genetic 

exchange, or for dispersal to other 

suitable areas. Precipitation may 

influence the likelihood of long-distance 

movements, especially in individuals 

approaching reproductive age in 

populations that experience aboveaverage precipitation for a 2- to 3-year 

period (AIDTT 2000, p. 8). AverillMurray and Klug (2000, p. ii) stated, ‘‘A 

large cohort of young tortoises that 

experiences a relatively wet and 

productive environment, with high 

survival, may provide the stock for 

dispersal between populations as they 

approach sexual maturity, in addition to 

replacing aging adults within the local 

population.’’ Long-distance movements 

by Sonoran desert tortoises observed by 

Averill-Murray and Klug (2000, p. 69) 

suggest the potential for metapopulation 

(interrelated population dynamics 

between regionally proximal 

populations) relationships between 

local populations inhabiting regional 

hillsides. Habitat features may also 

influence the Sonoran desert tortoises’ 

ability to make long-distance 

movements. Dispersal of Sonoran desert 

tortoises between populations might be 

less likely through sparse desertscrub in 

very hot, dry river valleys in the Lower 

Colorado River subdivision of Sonoran 

desertscrub. Van Devender (2002a, p. 

16) suggested that populations occurring 

in the Eagletail, Maricopa, Sand Tank, 

and similarly situated mountain ranges 

might have existed in isolation for 

decades, if not centuries. 

There are no data to evaluate longdistance movements in populations that 

occur in Mexico. Although Sonoran 

desert tortoises in Mexico are known to 

occupy slopes, arroyos, and bajadas, 

they are infrequently observed using 

valley bottoms (Fritts and Jennings 

1994, p. 52). Sonoran desert tortoise 

populations in Mexico have been poorly 

studied, but we presume individuals 

make similar long-distance movements 

between populations. 


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Longevity 

Estimates of longevity in wild 

Sonoran desert tortoises vary 

considerably from 30 years to over 100 

years (Germano 1992, pp. 369–370; 

1994, p. 176; Zylstra and Swann 2009, 

p. vii). Using a growth equation to 

extrapolate longevity in Sonoran desert 

tortoises, Germano et al. (2002, p. 271) 

estimated that the average oldest ages 

attained for Sonoran desert tortoises is 

62.2 years in females and 64.4 years in 

males; however, they admitted that 

correlating age with size is problematic 

in turtles. Zylstra and Swann (2009, p. 

vii) suspected that Sonoran desert 

tortoises may reach 80 to 100 years of 

age in the wild. Sonoran desert tortoises 

have been shown to live longer in the 

wild than those from the Mojave 

population. 

Bladder Physiology 

The bladder in the Sonoran desert 

tortoise is unique and serves an 

important function in its survival. 

Sonoran desert tortoises are capable of 

drinking large amounts of water when it 

is available, and may even construct 

water catchments by digging earthen 

depressions, likely as an adaptation to 

the infrequent and unpredictable nature 

of rainfall events throughout their range 

(Ernst and Lovich 2009, p. 546). The 

bladder of Sonoran desert tortoises is a 

large and bilobed (divided into two 

lobes) organ critical for withstanding the 

effects of seasonal and short-term 

drought because of its ability to store 

water, dilute excess dietary salts and 

metabolic wastes, and reabsorb water 

into the bloodstream (Averill-Murray et 

al. 2002a, p. 146; Ernst and Lovich 

2009, p. 545). In seasonal or short-term 

drought conditions, the concentration of 

urine in Sonoran desert tortoises allows 

them to forage on dried vegetation by 

reducing the dehydration effects of such 

forage types (Averill-Murray et al. 

2002a, p. 146; Ernst and Lovich 2009, p. 

545). Water serves an important role in 

flushing salts from the body of Sonoran 

desert tortoises and resetting the 

electrolytic balance, preparing the 

Sonoran desert tortoise for the next dry 

period (Averill-Murray et al. 2002a, pp. 

140, 146). 

Diet, Foraging Behavior, and Potassium 

Excretion Potential 

The Sonoran desert tortoise is an 

herbivore, and has been documented to 

eat 199 different species of plants, 

including herbs (55.3 percent), grasses 

(17.6 percent), woody plants (22.1 

percent), and succulents (5 percent) 

(Ogden 1993, pp. 1–8; Van Devender et 

al. 2002; pp. 175–176; Brennan and 

Holycross 2006, p. 54; Oftedal 2007, p. 

21; Ernst and Lovich 2009, p. 562; 

Meyer et al. 2010, pp. 28–29, 44–48). Of 

the numerous nonnative plant species 

that have become established 

throughout the range of the Sonoran 

desert tortoise, only red brome (Bromus 

rubens) and redstem filaree (Erodium 

cicutarium) are frequently eaten and 

considered relatively important 

nonnative species in the diets of 

Sonoran desert tortoises (Van Devender 

et al. 2002, p. 183). However, physical 

injury to Mojave desert tortoises 

resulting from consuming nonnative 

grass species (i.e., red brome and 

cheatgrass (Bromus tectorum)) has been 

documented, and sharp seeds have been 

found lodged between the tortoises’ 

upper and lower jaw. This injury may 

adversely affect their foraging ability or 

become a source for infection (Medica 

and Eckert 2007, p. 447). Though this 

study focused on Mojave desert 

tortoises, this may affect all desert 

tortoises wherever these plant species 

occur (i.e., within the Sonoran Desert in 

Arizona). 

Significant differences in the 

nutritional quality of native versus 

nonnative forage for desert tortoises 

were not found by Hazard et al. (2010, 

pp. 139–145). Nagy et al. (1998, pp. 260, 

263) compared the nutritional values of 

native and nonnative grasses (native: 

Indian ricegrass (Achnatherum 

(Oryzopsis) hymenoides); nonnative: 

Mediterrean grass (Schismus barbatus)) 

and forbs (native: desert dandilion 

(Malacothrix glabrata); nonnative: 

redstem filaree), finding that the two 

grasses possessed similar nutritional 

value. The dry matter and energy 

digestibility of the two grasses were 

much lower than those of the forbs, 

providing little nitrogen, and tortoises 

lost more water than they gained while 

processing grasses. The native forb was 

more readily digestible than the 

nonnative forb as dried mass, but the 

inverse was true as fresh mass (Nagy et 

al. 1998, p. 263). However, the native 

forbs provide significantly more 

nitrogen and water than the nonnative 

forbs, which is important in maintaining 

a positive water balance. Results of 

these feeding trials suggest that the 

proliferation of nonnative grasses 

leading to the exclusion of forbs places 

desert tortoises at a nutritional 

disadvantage. If, instead of eating to 

obtain a given volume of food, tortoises 

consume just enough food to satisfy 

their energy needs (as commonly noted 

in other vertebrate groups), then the 

native forbs provide the best nutrition. 

Nagy et al. (1998, p. 260) concluded that 

the life stage of the plant and the plant 

type (forb or grass) were important 

predictors of nutritional quality versus a 

plant being native or nonnative to a 

particular region. In summary, research 

has shown that forbs are more valuable 

to Sonoran desert tortoise nutrition than 

grasses, and that native forbs are more 

valuable than nonnative forbs in a dried 

state, which may be important in 

periods of drought. 

Diets of Sonoran desert tortoises vary 

among populations in response to 

seasonal availability of plant species 

and in response to precipitation 

amounts (Martin and Van Devender 

2002, p. 31). In years of low winter 

rainfall, Sonoran desert tortoises are less 

selective in plant species consumed 

because there are fewer options, but in 

years of high winter rainfall, Sonoran 

desert tortoises have exhibited highly 

selective foraging habits (Oftedal 2002, 

pp. 205–206). During years when 

monsoon rains are light or irregular, 

Sonoran desert tortoises consume dried 

plant material (Averill-Murray et al. 

2002a, p. 140). Within Saguaro National 

Park in southern Arizona, Sonoran 

desert tortoises frequently ate annual 

legumes in the spring (high in water 

content, low in potassium), and annual 

and perennial grasses (supplemented by 

prickly pear fruit (Opuntia 

engelmannii)) during the monsoon 

when ponding water can replenish 

water reserves (Oftedal 2007, p. 17). In 

most years, Sonoran desert tortoises 

consume enough calories during the 

summer monsoon to fuel growth and 

store fat for the next year (Van Devender 

2002a, p. 10). 

Desert tortoises are uniquely 

vulnerable to changes in their potassium 

levels (Oftedal 2002, p. 208). Because 

potassium cannot be easily stored in the 

body, excess potassium must be 

excreted to avoid toxicological effects 

(Oftedal 2002, p. 208). Therefore, 

Sonoran desert tortoises that must 

forage on plants with high potassium 

content must also flush their bladders 

more frequently and therefore risk a net 

loss in metabolic water levels and 

subsequent dehydration (Oftedal 2002, 

p. 209). 

The potassium excretion potential 

(PEP) is an index of water, nitrogen, and 

potassium levels in a plant that relates 

to a desert tortoise’s ability to efficiently 

excrete potassium. PEP is a critical 

consideration for determining the value 

or risk of particular forage species 

during times of drought or major 

perturbations to habitat, and for 

comparing potential effects of forage 

competition between tortoises and 

livestock. A positive PEP value for a 

plant species (preferred by tortoises) 

means there is more water and nitrogen 


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in the food than is needed to excrete 

potassium, and vice-versa for a negative 

PEP value (Oftedal 2002, p. 215; Ernst 

and Lovich 2009, p. 545). Sonoran 

desert tortoises have been documented 

to selectively forage on high PEP plant 

species to minimize water loss 

associated with metabolizing potassium 

(Oftedal 2002, p. 214; Ernst and Lovich 

2009, p. 545). High PEP values can be 

found in certain species of primroses, 

filaree, legumes, mustards, and spurges 

(Ernst and Lovich 2009, p. 545). 

Sonoran desert tortoises have been 

found to be seasonally selective for high 

PEP forage species, based on the 

abundance and diversity of plants and 

precipitation (Oftedal 2002, p. 223; 

2007, pp. 3, 22). 

In addition to herbivory, Sonoran 

desert tortoises are also geophagous; in 

other words, they consume bones, 

stones, and soil for additional nutrient 

and mineral supplements, for 

mechanical assistance in grinding plant 

matter in the stomach, or to expel 

parasites in the intestinal tract (Sokol 

1971, p. 70; Marlow and Tollestrup 

1982, p. 475; Esque and Peters 1994, pp. 

108–109; Stitt and Davis 2003, p. 57; 

Walde et al. 2007b, p. 148). Sonoran 

desert tortoises are highly attracted to 

sites with exposed calcium carbonate 

and have been observed congregating at 

these sites year after year eating these 

soils (Meyer et al. 2010, p. 11). Soil 

condition and quality are important to 

the Sonoran desert tortoise, not only for 

nutrients derived from eating soil, but 

also production and maintenance of 

vegetation that is consumed by tortoises 

(Avery and Neibergs 1997, p. 13). 

Desert tortoises have been observed 

eating scat from black-tailed jack 

rabbits, wood rats, collared peccaries, 

and even desert tortoises. This behavior 

could possibly aid in the transfer of gut 

microflora such as bacteria or fungi or 

it could be used as a source of 

additional nutrients (Walde et al. 2005, 

p. 77–78). Bostick (1990, p. 149) 

asserted that desert tortoises feed 

‘‘primarily on dung’’ although this claim 

was refuted in the literature (Boarman 

2002, pp. 27, 35, 38). Infrequent 

observations of sand, bird feathers, 

arthropod parts, and snake and lizard 

skins have also been made during fecal 

analyses of desert tortoises (Ernst and 

Lovich 2009, p. 560). 

Reproduction 

The Sonoran desert tortoise breeding 

season begins with the summer 

monsoon when male-male combat over 

receptive females can be observed, such 

as at sites with exposed calcium 

carbonate soils, where tortoise densities 

may be higher (discussed above) (Meyer 

et al. 2010, p. 11). Sexual maturity and 

first reproduction in female Sonoran 

desert tortoises occurs from 12 to 22 

years of age, or at 8.7 in (22 cm) in 

midline carapace length, and 

reproductive activity is highly 

influenced by winter and spring 

precipitation (Averill-Murray and Klug 

2000, p. 69; Averill-Murray et al. 2002b, 

p. 119; Bury et al. 2002, p. 100; 

Germano et al. 2002, p. 265). Females 

may store sperm for up to two years, 

meaning that one season’s mating 

produces the following season’s clutch 

of eggs (Palmer et al. 1998, pp. 704–705; 

Averill-Murray et al. 2002a, p. 141). 

Female Sonoran desert tortoises may lay 

one clutch of 1–12 eggs per year, usually 

around the onset of the summer rainy 

season, although they may not produce 

a clutch every year (Averill-Murray 

2002b, p. 295). Eggs hatch in September 

and October (Van Devender 2002a, pp. 

10–11; Averill-Murray 2002b, p. 295). 

The average clutch size is 3.8 to 5.7 

eggs, and in contrast to Mojave Desert 

tortoises, clutch size is not positively 

correlated with female body size 

(Mueller et al. 1998, p. 313; AverillMurray 2002b, p. 299; Averill-Murray et 

al. 2002b, p. 119). Late oviposition 

(deposition of eggs) dates recorded on 

the Sugarloaf study site in central 

Arizona in 1998 and 1999 suggest that 

eggs and hatchlings may occasionally 

overwinter in nests (Averill-Murray 

2002b, p. 299). Female desert tortoises 

have been known to urinate on their 

nest sites before and after nesting; this 

may be to aid in digging the nest, and 

might make it more difficult to dig up 

the nest after the soil dries, or possibly 

to hydrate soils in contact with eggs as 

the rigid-shelled eggs of desert tortoises 

have been shown to uptake moisture 

from the soil faster than it evaporates 

from the shell exposed to air (Patterson 

1971, p. 199; Spotila et al. 1994, p. 112). 

Female Sonoran desert tortoises that 

survive to reproductive age are believed 

to produce as many as 85 eggs over the 

course of their lives, with perhaps two 

or three of those hatchlings surviving to 

reproductive age (Van Devender 2002a, 

p. 11). 

Desert tortoises exhibit environmental 

sex determination, which means that 

incubation temperatures during 

embryonic development determine the 

sex of the tortoises. Higher incubation 

temperatures produce more females and 

lower temperatures produce more males 

(Spotila et al. 1994, pp. 109–111; Rostal 

et al. 2002, p. 313). Incubation 

temperatures at or below 86.9 degrees 

Fahrenheit (° F) (30.5 degrees Celsius 

(° C)) result in the production of all male 

desert tortoises, whereas temperatures 

of 90.5 °F (32.5 °C) result in all females, 

and eggs incubated at the ‘‘pivotal’’ 

temperature of 88.3 °F (31.3 ° C) develop 

a 1:1 sex ratio (Rostal et al. 2002, p. 

313). 

Predation 

As adults, Sonoran desert tortoises are 

relatively protected from natural 

predation because of their hard shells. 

Mountain lions (Felis concolor) appear 

to be the only natural predator in the 

Sonoran Desert with the jaw strength 

required to puncture or crack the shells 

of adult Sonoran desert tortoises. 

However, mountain lion predation is 

not known to contribute to elevated 

mortality rates within monitored 

Sonoran desert tortoise populations 

(AIDTT 2000, p. 8; Meyer et al. 2010, p. 

18; Riedle et al. 2010, p. 165). 

Dickenson et al. (2001, p. 254) recorded 

14 Sonoran desert tortoise mortalities in 

the Little Shipp Wash and Harcuvar 

monitoring plots from 1990–1994, five 

of which were attributed to mountain 

lion predation. Javelina (Tayassu tajacu) 

predation on Sonoran desert tortoises 

was suspected in the San Pedro Valley 

of southern Arizona (Meyer et al. 2010, 

p. 18). Other mammalian predators may 

include badgers (Taxidea taxus), ringtailed cats (Bassiriscus astutus), bobcats 

(Felis rufus), skunks (Spilogale gracilis, 

Mephitis mephitis, M. macroura, 

Conepatus mesoleucus), kit foxes 

(Vulpes macrotis), gray foxes (Urocyon 

cinereoargenteus), coyotes (Canis 

latrans), and domestic dogs (Canis 

familiaris) (Boarman 2002, p. 17; Ernst 

and Lovich 2009, p. 563). 

Both golden eagles (Aquila 

chrysaetos) and common ravens (Corvus 

corvax) have been documented to prey 

upon all size classes of Mojave desert 

tortoises in California (Berry 1985, pp. 

1, 6–10). Such predation might also 

occur on Sonoran desert tortoises. The 

greater roadrunner (Geococcyx 

californianus) is also a suspected 

predator on juvenile Mojave desert 

tortoises, based upon one field 

observation of roadrunner tracks next to 

a freshly killed individual (Berry 1985, 

p. 11); such predation might also occur 

on Sonoran desert tortoises. However, 

because avian predators rely exclusively 

on their vision to detect prey, we expect 

lower rates of avian predation on 

Sonoran desert tortoises occupying 

Arizona upland Sonoran desertscrub 

because the dense, complex habitat 

structure likely limits birds’ ability to 

detect tortoises. Habitat-associated 

protection from avian predation may be 

less pronounced where Sonoran desert 

tortoises occur in the sparser vegetation 

of the Lower Colorado River subdivision 

of Sonoran desertscrub. 


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Sonoran desert tortoises are most 

vulnerable to predation while in their 

eggs or as hatchlings and juveniles 

predominantly because of their size and 

undeveloped, softened shells (which do 

not adequately harden until 

approximately 7 years of age) which 

provide little protection and are easily 

compromised. Higher mortality rates in 

the hatchling and juvenile age classes 

may also be partially due to their higher 

metabolic rates, which necessitates 

longer periods of surface activity to 

obtain suitable amounts of forage. 

Longer surface activity may cause 

greater risk of detection by predators 

(Morafka 1994, p. 163). Nest predation 

levels may be high in some populations. 

Seventy-five percent of Sonoran desert 

tortoise nests suffered predation over a 

two-year period at the Sugarloaf study 

plot in Maricopa County, Arizona 

(Averill-Murray 2002b, p. 298). Gila 

monsters (Heloderma suspectum) are a 

primary predator on tortoise eggs, and 

female Sonoran desert tortoises in the 

process of oviposition will actively 

defend the burrow and aggressively 

pursue Gila monsters in attempting to 

drive them away (Barrett and Humphrey 

1986, p. 262). Coachwhips (Coluber 

flagellum) and gophersnakes (Pituophis 

catenifer) have been reported 

consuming juvenile Sonoran desert 

tortoises (Amarello et al. 2004, p. 178; 

Ernst and Lovich 2009, p. 563). 

Presumably, other snake species such as 

common kingsnakes (Lampropeltis 

getula) with generalized prey 

preferences consume eggs or hatchling 

Sonoran desert tortoises, but we did not 

find other examples in the literature. 

For more detailed information on all 

aspects of Sonoran desert tortoise 

biology, see Barrett and Johnson (1990, 

pp. 1–95) and Bury and Germano (1994, 

pp. 1–212). 

Monitoring and Population Status 

Monitoring and Statistical Analyses 

We are unaware of any structured, 

long-term monitoring program for 

Sonoran desert tortoises in Mexico; 

therefore, we are unable to assess the 

current status or population trends in 

that part of the range. Therefore, we 

discuss only Arizona studies in this 

section. 

Twenty-eight individual Sonoran 

desert tortoise populations in Arizona 

have been studied since the mid-1970s 

but few populations have been studied 

for more than a few years (AverillMurray 2000, p. 1; Averill-Murray et al. 

2002b, p. 109). Monitoring plots (also 

referred to as ‘‘plots’’) have varied from 

0.2 to 1.5 sq mi (0.3 to 2.4 sq km) in size 

(Averill-Murray 2000, p. 4). Beginning 

in 1987, AGFD and the U.S. Bureau of 

Land Management (BLM) have 

established and maintained 17 plots in 

Arizona as long-term monitoring plots 

and have surveyed them in a somewhat 

irregular, but repeated fashion. Each 

plot has been surveyed between two and 

nine times during this timeframe, with 

11 to 86 person-days (cumulative days 

spent by researchers working on plots) 

spent during each survey (AGFD 2010, 

p. 1). These long-term monitoring plots 

are located in six counties within 

Arizona, and their locations were 

chosen to represent Sonoran desert 

tortoise distribution in the State. 

General monitoring objectives for the 

17 plots are to document abundance, 

density, and changes of Sonoran desert 

tortoise populations across the State 

using capture-recapture methods 

(Averill-Murray 2000, p. 3). Records of 

demographic characteristics of each 

population, including sex ratios and 

age/size structure as well as individual 

health and signs of disease within each 

population were also recorded during 

monitoring activities (Averill-Murray 

2000, p. 3). Monitoring protocols used 

from 1987 to 2000 are summarized in 

Averill-Murray (2000, pp. 3–7). 

The Sonoran desert tortoise is a 

difficult species to monitor in the wild 

because of its slow movement and 

camouflaged appearance, especially in 

the smaller hatchling and juvenile age 

classes. These factors can significantly 

hamper a surveyor’s ability to detect 

them in the field (Zylstra et al. 2010, p. 

1311). In addition, Arizona Upland 

subdivision of Sonoran desertscrub 

(where Sonoran desert tortoise 

population densities are the highest) is 

complex, often with many large 

boulders, somewhat dense vegetation, 

and challenging topographic relief. 

Drought and emigration also affect the 

reliability of data from Sonoran desert 

tortoise population monitoring because 

the tortoises may be inactive (in their 

burrows) or have left the population 

(dispersed). In these cases the absence 

of observations might be mistaken as 

mortality. Also, Sonoran desert tortoises 

can occur in low densities with little 

surface activity both seasonally and 

daily (Zylstra et al. 2010, p. 1311). 

Alone or in combination, these factors, 

in addition to a relatively short 

sampling period for such a long-lived 

species, make subtle population trends 

difficult to distinguish and overall 

population trend analysis problematic. 

Low detectability may have been 

responsible for long periods between 

recaptures of marked desert tortoises in 

an 18-year desert tortoise study from 

1980 to 1997 in the San Pedro Valley, 

Arizona. For example, a sub-adult 

Sonoran desert tortoise was captured 

and marked in 1992, and was not 

encountered again until 2005, when it 

was incidentally observed 

approximately 14 mi (22.5 km) from its 

original point of capture, 8 years after 

the conclusion of the study (Meyer et al. 

2010, p. 18). Within the entire duration 

of this study, approximately 30 percent 

of 577 marked Sonoran desert tortoises 

were never recaptured, with only 15 

total carcasses found, indicating 

potential emigration, long-term burrow 

use, or difficulties in detecting 

individuals in complex landscapes 

(Meyer et al. 2010, p. 20). The amount 

of time between recaptures of Sonoran 

desert tortoises can be significant; 

durations between recaptures of some 

individuals in the San Pedro Valley 

study were as high as 18 years (Meyer 

et al. 2010, p. 20). 

Several authors have investigated how 

detectability may bias results of Mojave 

desert tortoise monitoring. For example, 

Anderson et al. (2001, p. 583) studied 

the degree to which field observers can 

meet the assumptions underlying linetransect sampling to monitor 

populations of desert tortoises in 

Mojave desertscrub. They found that 

when all Mojave desert tortoises are not 

detected along the centerline of the 

transect route (which routinely occurs), 

biases in sampling data result 

(Anderson et al. 2001, p. 583). Anderson 

et al. (2001, p. 593–596) noted that 

surveyor numbers and level of 

experience contribute to the reliability 

of line transect methods. Freilich and 

LaRue (1998, p. 594) experimentally 

tested the effect of personnel experience 

on Mojave desert tortoise survey 

outcomes in Mojave desertscrub. They 

found that observers consistently 

overestimated the number of desert 

tortoise burrows (falsely assigning other 

animal burrows as those made by desert 

tortoises), and found fewer desert 

tortoises and scat than were actually 

placed on test plots. Their results 

indicated that experience played a 

relatively small role in detecting Mojave 

desert tortoises (Freilich and LaRue 

1998, pp. 593–594). In an effort to 

increase detections, some investigators 

have tested the use of tortoise detection 

dogs in Mojave desert tortoise 

monitoring projects (Cablk and Heaton 

2006, p. 1926; Heaton et al. 2008, pp. 

476–477; Nussear et al. 2008, pp. 109– 

111). Because Sonoran desertscrub is 

more dense and complex than Mojave 

desertscrub, detection is even more 

difficult in Sonoran desert tortoise 

monitoring. Zylstra and Steidl (2009, p. 

16) found that line transect methods are 


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not an efficient means with which to 

monitor Sonoran desert tortoises. 

The seasonal timing of surveys and 

fluctuating influence of precipitation on 

Sonoran desert tortoise surface activity 

also create problems with monitoring 

populations and interpreting results. 

Sonoran desert tortoises often become 

inactive, residing in their burrows, 

during periods of seasonal or short-term 

drought. For example, in a multi-year 

mark and recapture study of Mojave 

desert tortoises in Joshua Tree National 

Park, Freilich et al. (2000, pp. 1487– 

1488) found that in years of belownormal precipitation, desert tortoise 

home ranges decreased, individual 

captures decreased, and the effort 

required to find each tortoise nearly 

doubled; indicating the significant 

influence of precipitation on the 

possible discrepancy between the 

number of tortoises that can be observed 

versus the number of tortoises that 

actually occur within a monitoring plot. 

In an attempt to improve monitoring 

protocols to account for such 

complicating factors described above, 

Averill-Murray (2000, pp. 7–13) 

critiqued the original protocols used for 

long-term monitoring plots of Sonoran 

desert tortoise populations in Arizona. 

This work became the basis for several 

changes in monitoring protocols, 

beginning in 2000. Although line 

transect methods have not been 

implemented on Arizona’s Sonoran 

desert tortoise long-term monitoring 

plots, the capture-recapture methods 

currently used likely violate 

assumptions about equal detection 

probability (all animals having the same 

probability of being captured during 

every sampling occasion) (Zylstra and 

Steidl 2009, p. 9). 

While monitoring of Sonoran desert 

tortoise populations in Arizona has been 

ongoing for several decades, attempts to 

quantify temporal trends in abundance 

have been hampered by the data 

limitations discussed above (Zylstra and 

Steidl 2009, p. 5; Zylstra et al. 2010, pp. 

1311–1317). Effective monitoring is 

largely dictated by the objective of the 

monitoring, whether that objective is to 

detect changes in distribution, 

abundance, density, or survival. In 

addition, using existing plot data to 

establish rangewide trends in Sonoran 

desert tortoise populations is generally 

problematic because the current set of 

monitoring plots does not represent a 

random sample from the species’ entire 

range in Arizona (Averill-Murray and 

Klug 2000, p. 25). Despite the history 

and effort dedicated to monitoring 

Sonoran desert tortoise populations in 

Arizona since 1987, there are limitations 

of these data with respect to interpreting 

rangewide trends of the Sonoran desert 

tortoise. Averill-Murray (2000, pp. 12– 

13) identified problems with 

extrapolating the results of the plot 

monitoring data to making range-wide 

assessments outside of the plots. We 

elaborate on these problems in our 

assessment of Boarman and Kristan 

(2008) below. 

Boarman and Kristan (2008, pp. 3–12) 

analyzed mark and recapture data from 

the 17 Sonoran desert tortoise long-term 

monitoring plots throughout Arizona 

that were surveyed on the average of 

once every 4 years from 1987 to 2006. 

Boarman and Kristan (2008, p. ii) 

concluded that the Sonoran population 

of the desert tortoise in Arizona 

experienced statistically significant 

declines, at an annual rate of 3.52 

percent over the 20-year period; 

equating to a cumulative 51 percent 

decline in overall numbers during this 

timeframe. 

We received several comments from 

the public in response to our 90-day 

finding that addressed the Boarman and 

Kristan (2008) report (AGFD 2010, pp. 

4–6; Carothers et al. 2010, pp. 5, 8–12; 

Ogden 2009, pp. 3–12, Smith 2010, pp. 

4–5). Commenters criticized the method 

and manner with which Boarman and 

Kristan (2008) used statistical tests, as 

well as the conclusions they made. 

Significant concerns were noted with 

respect to the type of statistical tests 

used by Boarman and Kristan (2008) 

because data were extrapolated beyond 

the statistical tests’ ability to avoid 

inherent biases (AGFD 2010, p. 4). 

Problems associated with the statistical 

confidence intervals for monitoring plot 

data used by Boarman and Kristan 

(2008) were also identified (Ogden 2009, 

pp. 2–3). Also, monitoring plot data 

used in Boarman and Kristan (2008, p. 

20) were not designed to compare 

population trends among individual 

plots (Ogden 2009, p. 2). Carothers et al. 

(2010, pp. 8–12) identified numerous 

additional problems with the statistical 

analysis provided by Boarman and 

Kristan (2008). Collectively, based upon 

comments received from the public as 

well as our internal review, the number 

and magnitude of potential problems 

associated with Boarman and Kristan’s 

(2008) statistical analysis call into 

question the validity of their 

conclusions. After careful review of the 

report and the questions raised by 

reviewers of the report, we decided that 

the conclusions pertaining to overall 

Sonoran desert tortoise population 

trends do not represent the best 

available information and, therefore, we 

did not use the report in this finding. 

However, other information in the 

Boarman and Kristan (2008) report was 

used in our analysis of the status of and 

threats to the Sonoran desert tortoise 

and is cited in this finding. For a more 

detailed analysis of the Boarman and 

Kristan (2008) report, see our ‘‘Review of 

Boarman and Kristan (2008)’’ provided 

at http://www.regulations.gov (Docket 

Number FWS–R2–ES–2009–0032). 

Survivorship and Population Densities 

in Arizona 

Viable populations in turtles usually 

require that both juvenile and adult size 

classes have high survivorship (AverillMurray and Klug 2000, p. 70). Data on 

the recruitment of juveniles into 

Sonoran desert tortoise populations, and 

their survivorship, are generally lacking 

due to the difficulty detecting juveniles 

in the field (AGFD 2010, p. 3). Data on 

juvenile and adult survivorship in 

Sonoran desert tortoises require longterm, repeated population monitoring, 

which in turn, requires long-term, 

reliable funding sources. Consequently, 

these data are conspicuously rare or 

absent for most Sonoran desert tortoise 

monitoring plots making population 

viability estimates for Sonoran desert 

tortoise populations within Arizona 

problematic at best. As expected for a 

long-lived species, survivorship in 

Sonoran desert tortoises (using data 

generated from a few long-term 

monitoring plots in Arizona) is 

generally high for adults but potentially 

lower for juveniles and hatchlings 

(Zylstra and Steidl 2009, p. 7). Where 

enough data from long-term monitoring 

plots or independent studies exist, 

survivorship has been calculated for 

adults in the following plots or study 

areas: Sugarloaf Mountain (96–98 

percent), Florence Military Reservation 

(88–97 percent), Little Shipp Wash (94– 

97 percent), Granite Hills (94–97 

percent), and Eagletail Mountains (94– 

97 percent) (AGFD 2010, p. 2; Riedle et 

al. 2010, p. 165). 

Densities of Sonoran desert tortoises 

among populations vary considerably. 

In 2000, the density of Sonoran desert 

tortoises, as determined by surveys on 

long-term monitoring plots and other 

monitoring plots during the 1990s, 

varied from 15 to 150 individuals per 

square mile (2.6 sq km) (AIDTT 2000, 

pp. 5–6; Averill-Murray and Klug 2000, 

p. i). In the San Pedro Valley of 

southern Arizona, the average density of 

the Sonoran desert tortoise population 

was 38 individuals per square mile 

(Meyer et al. 2010, p. 17). Stager et al. 

(2010, p. 37) suspect that Sonoran desert 

tortoise populations in Mohave County, 

Arizona may be naturally lower due to 

limited burrowing habitat available to 

them to survive cold winters and hot 

summers. 


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Periodic, Localized Declines in Arizona 

Populations 

There are no records of actual 

extirpations of Sonoran desert tortoises 

from any of the monitored populations. 

However, periodic, localized, and 

sometimes substantial declines have 

been documented in at least five of 17 

monitored populations (Hart et al. 1992, 

p. 60; Averill-Murray et al. 2002b, p. 

124; AGFD 2010, p. 4). Because of their 

life history, Sonoran desert tortoise 

populations may be slow to rebound 

from declines (Howland and Rorabaugh 

2002, p. 340). The AGFD (2010, p. 4) 

suggested that observed declines in 

certain plots demonstrate localized, 

stochastic events and are not indicative 

of population trends as a whole across 

the distribution of the Sonoran desert 

tortoise. Sonoran desert tortoise 

populations are particularly vulnerable 

to elevated mortality of adults. 

Sustaining the adult, reproductive age 

class within Sonoran desert tortoise 

populations is important because 

mortality rates of juveniles are high and 

because it takes a long time for a 

Sonoran desert tortoise to reach sexual 

maturity (Howland and Rorabaugh 

2002, p. 339). The relatively higher 

visibility of adult Sonoran desert 

tortoises leaves them more vulnerable to 

human impacts like collecting or 

shooting, and their tendency to move 

longer distances make them more 

susceptible to road mortality (Howland 

and Rorabaugh 2002, p. 340). 

The largest population decline noted 

at any Sonoran desert tortoise 

monitoring plot was observed on the 

Maricopa Mountains plot, where 

substantially more tortoise carcasses 

were found than live tortoises in 

successive years from 1987 through 

1991 (Hart et al. 1992, p. 54; AverillMurray et al. 2002b, p. 124). Regional 

drought from 1984–1992 was a 

suspected cause of the die-off of 

Sonoran desert tortoises in the Maricopa 

Mountains (Hart et al. 1992, p. 60; 

Averill-Murray et al. 2002b, p. 124). 

However, in 1987, the estimated density 

of Sonoran desert tortoises on the 

Maricopa Mountains plot was 

uncharacteristically high at 146 tortoises 

per square mile (2.6 sq km), suggesting 

that the population may have been in 

the process of naturally correcting to 

carrying capacity (the state at which a 

population level is commensurate with 

available resources) (AGFD 2010, p. 3). 

Since 1991, the Sonoran desert tortoise 

population on the Maricopa Mountains 

plot has experienced relatively high 

survivorship and shown evidence of 

reproduction. No additional carcasses 

have been documented, indicating the 

population may be stable, if not 

returning to the previous 1987 level 

(AGFD 2010, p. 3). 

The AGFD (2010, p. 3) and Hart et al. 

(1992, p. 120) confirm Sonoran desert 

tortoise populations declined from 

initial population estimates (as 

demonstrated by density estimates and 

relative carcass numbers) on three 

additional plots (Hualapai Foothills, 

San Pedro Valley, and East Bajada), 

suspecting that drought conditions may 

have played a role in the observed 

declines on these plots (Ogden 2009, pp. 

12–13). An observed decline on the 

Tortilla Mountains plot in 2001 may 

have been an artifact of low surface 

activity in response to below-average 

precipitation, because an increase in 

carcasses was not detected (AGFD 2010, 

p. 3). 

For detailed information on 

monitoring and survey results from the 

previous three decades for the Sonoran 

desert tortoise in Arizona, see the 

following reports: Schneider (1981), 

Shields and Woodman (1987), Wirt 

(1988), Woodman and Shields (1988), 

Holm (1989), Shields et al. (1990), 

SWCA (1990a; 1990b; 1990c), Hart et al. 

(1992), Murray and Schwalbe (1993; 

1997), Woodman et al. (1993; 1994; 

1995; 1996; 1998; 1999a; 1999b; 2000; 

2001; 2002; 2003; 2004; 2005; 2006; 

2007; 2008; 2009), AIDTT (2000, pp. 5– 

6), Averill-Murray (2000, pp. 3–7), 

Averill-Murray and Klug (2000, pp. 3– 

25), Averill-Murray et al. (2002b, pp. 

110–112), Walker and Wood (2002), 

Young et al. (2002), and Zylstra and 

Swann (2009). 

It should be noted that an average 

generation time for a Sonoran desert 

tortoise is 12–15 years and that 

monitoring of Sonoran desert tortoise 

populations has only occurred for about 

30 years, representing approximately 

two generations. Many threats described 

below have been potentially acting on 

Sonoran desert tortoise populations for 

many decades, longer than populations 

have been studied. Below, we discuss 

the effects of various threats to 

individual Sonoran desert tortoises. 

However, due to limitations in 

monitoring data, we are unable to 

discern how Sonoran desert tortoise 

populations may have responded to 

these threats over time, or identify any 

long-term, historical trends in tortoise 

populations. We have not observed any 

extirpations among monitored 

populations. 

Distinct Population Segment 

We consider a species for listing 

under the Act if available information 

indicates such an action might be 

warranted. ‘‘Species’’ is defined by the 

Act as including any subspecies of fish 

or wildlife or plants, and any distinct 

population segment (DPS) of any 

species of vertebrate fish or wildlife that 

interbreeds when mature (16 U.S.C. 

1532(16)). We, along with the National 

Marine Fisheries Service (now the 

National Oceanic and Atmospheric 

Administration—Fisheries), developed 

the Policy Regarding the Recognition of 

Distinct Vertebrate Population Segments 

(61 FR 4722; February 7, 1996), to help 

us in determining what constitutes a 

DPS. The policy identifies three 

elements that are to be considered 

regarding the status of a possible DPS. 

These elements include: (1) The 

discreteness of the population segment 

in relation to the remainder of the taxon 

(group of similar biological organisms); 

(2) the significance of the population 

segment 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., 

whether the population segment, when 

treated as if it were a species, is 

endangered or threatened) (61 FR 4722, 

February 7, 1996). The first two 

elements are used to determine if a 

population segment constitutes a valid 

DPS. If it does, then the third element 

is used to consider whether such DPS 

warrants listing. In this section, we will 

consider the first two criteria 

(discreteness and significance) to 

determine if the Sonoran desert tortoise 

is a valid DPS (i.e., a valid listable 

entity). 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 4722). 

Discreteness 

Under the DPS policy, a population 

segment of a vertebrate species may be 

considered discrete if it satisfies either 

one of the following two conditions: 

(1) It is markedly separated from other 

populations of the same taxon as a 

consequence of physical, physiological, 

ecological, or behavioral factors. 

Quantitative measures of genetic or 

morphological discontinuity (separation 

based on genetic or morphological 

characters) may provide evidence of this 

separation. 

(2) It is delimited by international 

governmental boundaries within which 

significant differences in control of 

exploitation, management of habitat, 

conservation status, or regulatory 

mechanisms exist that are significant in 

light of section 4(a)(1)(D) of the Act. 

Based upon available information, the 

international boundary between Mexico 

and the United States is not considered 

for delineation of discreteness because 


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the edge of the DPS is not located at the 

International Border and, therefore, will 

not be addressed further. 

The Colorado River and Rı ́o Yaqui are 

two perennial rivers that form 

biogeographical barriers (a natural 

barrier that prevents the migration of 

species) to movement of tortoises 

between the Mojave and Sonoran desert 

tortoise populations, and between the 

Sonoran and Sinaloan desert tortoise 

populations, respectively. The Colorado 

River, separating California and 

Arizona, comprises the northern and 

western boundaries of the Sonoran 

desert tortoise population as identified 

in the April 2, 1990, final rule 

designating the Mojave population of 

the desert tortoise (occurring north and 

west of the Colorado River) as a 

threatened species under the Act (55 FR 

12178; see final rule for a summary of 

previous actions regarding the Mojave 

population of the desert tortoise). The 

eastern boundary is the extent of the 

range of the Sonoran desert tortoise 

where desert habitats end and grassland, 

chaparral, and mountain habitats begin, 

which are areas that do not contain 

desert tortoises. The southern boundary 

of the Sonoran desert tortoise DPS, as 

considered in this finding, is the Rı ́o 

Yaqui in southern Sonora, Mexico; 

south and east of there, desert tortoises 

are considered Sinaloan populations. 

Potential threats to the Sinoloan desert 

tortoise are not evaluated as part of this 

finding. 

In view of this biogeographical 

isolation, significant ecological 

divergence has occurred between the 

Mojave and Sonoran populations of 

desert tortoise, largely due to significant 

differences in geology, vegetation types, 

and precipitation cycles where the 

populations are distributed. Desert 

tortoises in the Mojave population are 

most dense in the intermountain valleys 

that have soil types favorable to the 

construction of large, deep burrows 

(Bury et al. 1994, pp. 66–70). However, 

Sonoran desert tortoises reach 

maximum densities in the rocky bajadas 

and hillsides of higher slopes, with 

reduced densities in the intermountain 

valleys (Berry 1984, p. 434; AIDTT 

2000; p. 4; Van Devender 2002a, p. 7; 

Brennan and Holycross 2006, p. 54; 

Zylstra and Steidl 2008, p. 747). At the 

southern end of the DPS, Edwards et al. 

(2009, pp. 7–8) suggested that Sinaloan 

population of desert tortoise uses 

Sinaloan thornscrub and tropical 

deciduous forest habitats. These 

different habitat types with differing 

soils and vegetation communities are 

created by higher precipitation levels. 

However, some level of gradation may 

occur in the vegetative transition zone 

between Plains of Sonora subdivision of 

Sonoran desertscrub and Sinaloan 

thornscrub habitats of central Sonora 

such as in the vicinity of the Rı ́o Yaqui 

(Edwards et al. 2009, p. 8). 

In addition to habitat differences, 

morphological differences have also 

been documented among the three 

populations of desert tortoise. Several 

morphological differences in carapace 

size and shape have been documented 

between the Mojave, Sonoran, and 

Sinaloan populations of desert tortoise: 

The carapace of the Mojave desert 

tortoise is the widest and tallest of the 

three, the Sinaloan desert tortoise 

carapace is the most narrow and least 

domed, and the carapace of the Sonoran 

desert tortoise is intermediate between 

the two in those dimensions (Germano 

1993, pp. 324–325; AGFD 2001, p. 1). 

Using eight independent shell 

measurements, Weinstein and Berry 

(1987, pp. 26–28) documented three 

distinct phenotypes (physical 

appearances) in desert tortoise 

populations within the United States 

based on morphometric (body 

measurement) analyses: The ‘‘California’’ 

phenotype (Mojave population), ‘‘Beaver 

Dam Slope’’ phenotype (Mojave form in 

Arizona), and the ‘‘Sonoran type’’ 

(Sonoran population). Desert tortoises 

from southern Sonora and northern 

Sinaloa in Mexico were not studied as 

part of this effort. 

Differences in reproduction strategies 

between the Sonoran and Mojave 

populations of desert tortoises also 

occur. Mojave desert tortoises lay up to 

three clutches of eggs per year with 

larger clutch sizes (more eggs), earlier in 

the year (April to mid-July) (Wallis et al. 

1999, p. 405) while those in the Sonoran 

population lay one clutch per year of 

smaller size, later in the year (June 

through August) (Averill-Murray et al. 

2002a, p. 141). These differences led 

Averill-Murray (2002b, pp. 119–122) to 

the conclusion that Sonoran desert 

tortoises invest all reproductive effort 

into a single clutch which hatches at the 

peak of forage and water availability and 

abundance owing to late-summer 

rainfall. Whereas desert tortoises in the 

Mojave population (maturing at smaller 

body sizes) (Berry et al. 2002a, p. 259) 

have higher clutch numbers to offset 

higher mortality from greater variability 

in environmental conditions. 

The Mojave, Sonoran, and Sinaloan 

populations of the desert tortoise have 

been found to have significantly 

differentiated genotypes (genetic 

characteristics) (Lamb and McLuckie 

2002, p. 74; Van Devender 2002a, p. 24). 

Genetic distances, expressed as percent 

sequence divergence (an estimate of 

percent difference in the genetic code), 

are substantial among the three 

populations of desert tortoise. 

Divergence is 5.1–5.6 percent between 

the Sonoran and Mojave populations, 

4.2 percent between the Sonoran and 

Sinaloan populations, and 5.1 percent 

between the Sinaloan and Mojave 

populations (Lamb and McLuckie 2002, 

pp. 74, 77). Considering geographic 

distribution, genealogical depth, and a 

suite of other characteristics, the 

Mojave, Sonoran, and Sinaloan 

populations of desert tortoise are 

considered to be ecologically significant 

units (populations or groups of 

populations historically isolated from 

one another, and thus representing deep 

phylogenetic (evolutionary development 

of species over time) subdivisions 

within species) (Lamb and McLuckie 

2002, pp. 81–82). According to 

mitochondrial DNA markers, the 

Sonoran and Mojave populations appear 

to have diverged some 5 million years 

ago (Lamb et al. 1989, p. 83; Lamb and 

McLuckie 2002, p. 76). 

McCord (2002, p. 62) presented three 

possible causes of the significant genetic 

differentiation between Sonoran and 

Mojave desert tortoises. First, genetic 

differentiation between Sonoran and 

Mojave desert tortoises may have been 

the result of differences in rainfall 

patterns between the winter-dominated 

rainfall pattern of the Mojave Desert and 

the summer-dominated rainfall pattern 

of the Sonoran desert. Second, genetic 

differentiation between Sonoran and 

Mojave desert tortoises may have 

occurred because the Sonoran desert 

tortoises may be represented as a relict 

population (remnant survivor from the 

past) of the tropical deciduous forestevolved population of the Sinaloan 

population (based upon their general 

absence in valley bottoms due to heavy 

flooding during summer rains, a 

phenomenon generally absent in the 

Mojave Desert). Last, genetic differences 

between Sonoran and Mojave desert 

tortoises may have resulted from their 

mutual competition with the Bolson 

tortoise (Gopherus flavomarginatus), 

another desert tortoise species which 

was widely distributed throughout 

Arizona in the Pleistocene, but which 

never occurred in California. The 

competing Bolson tortoise population 

may have acted as a wedge between the 

Sonoran and Mojave populations, 

driving them even farther apart, in a 

process known as competitive 

displacement. 

To explore the evolutionary track the 

three desert tortoise populations may 

have taken and the extent of their 

current genetic differentiation on the 

landscape, Edwards et al. (2009, p. 8) 

collected genetic samples from desert 


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tortoises within three regions of Sonora, 

Mexico: Twenty-two samples from near 

Alamos, Sonora (tropical deciduous 

forest in extreme southern Sonora), 19 

samples from near Ciudad Obrego ́n 

(foothill thornscrub in south-central 

Sonora, south of the Rı ́o Yaqui), and 14 

samples from two sites north of 

Hermosillo (Sonoran desertscrub in 

central Sonora). When they compared 

genetic data with previously collected 

samples from Arizona, they found a 

‘‘continuum of genetic similarity’’ in 

genetic samples taken from desert 

tortoises from the Hermosillo area of 

Sonora, Mexico, 528 mi (850 km) 

northwest to the Kingman, Arizona area 

when they compared genetic data with 

previously collected samples from 

Arizona (Edwards et al. 2009, p. 8). This 

confirms the similar genetic 

relationships of Sonoran desert tortoises 

throughout the DPS. Genetic samples 

from the Ciudad Obrego ́n region, 

southward, showed clear genetic 

distinction and supported prior 

evidence for a third distinct population 

of desert tortoise, referred to as the 

Sinaloan population (Edwards et al. 

2009, p. 8). The southern limits of desert 

tortoise distribution in northern Sinaloa 

are likely influenced by the growth of 

disease-causing bacteria and fungi 

present in the soil of burrows, 

exacerbated by the hot, humid, and wet 

conditions during tropical summer 

rainy seasons (Van Devender 2002b, p. 

43). 

Evaluation of Discreteness 

Some biological similarities do exist 

among the three populations of desert 

tortoise (Mojave, Sonoran, and 

Sinaloan). For example, some overlap in 

habitat use occurs. It is well known that 

Sonoran desert tortoises generally occur 

on steep, rocky slopes and bajadas in 

contrast to the Mojave desert tortoise, 

which occurs primarily along the valley 

bottoms. But to a lesser extent, Sonoran 

desert tortoises also use valley bottoms 

and Mojave desert tortoises also use 

steep slopes and mountain bajadas 

(Gardner and Brodie 2000, p. 51; 

Averill-Murray and Averill-Murray 

2002, p. 16; Lutz et al. 2005, p. 22; 

Grandmaison et al. in press, p. 4; Riedle 

et al. 2008, p. 418). However, there are 

many more numerous and convincing 

data in the scientific literature to 

support the discreteness of the three 

recognized populations of Gopherus 

agassizii, including differences in their 

ecology, behavior, morphology, 

physiology, and genetics (Weinstein and 

Berry 1987, pp. 26–28; Germano 1993, 

pp. 324–325; Germano et al. 1994, p. 82; 

AGFD 2001, p. 1; Averill-Murray 2002b, 

pp. 299–300; Berry et al. 2002a, p. 259; 

Lamb and McLuckie 2002, pp. 74, 77; 

McCord 2002, p. 62; Van Devender 

2002a, pp. 24–25; Van Devender 2002b, 

p. 45; Zylstra and Steidl 2008, p. 747; 

Edwards et al. 2009, p. 8). 

We have reviewed the best available 

commercial and scientific information 

and find that the Sonoran population of 

the desert tortoise as it occurs east and 

south of the Colorado River, south to the 

Rı ́o Yaqui, in Sonora, Mexico, is 

discrete, under the Service’s DPS policy, 

from the Mojave and Sinaloan desert 

tortoise populations. We base this 

conclusion on ecological (habitat use), 

physiological (reproductive 

characteristics), morphological (shell 

dimensions), and behavioral (seasonal 

activity patterns) differences that are 

further supported by analysis of genetic 

differences that concluded significant 

divergence has occurred among the 

three populations. 

Significance 

If a population segment is considered 

discrete under one or more of the 

conditions described in the Service’s 

DPS policy, its biological and ecological 

significance will be considered in light 

of Congressional guidance that the 

authority to list DPSs be used 

‘‘sparingly’’ while encouraging the 

conservation of genetic diversity. In 

making this determination, we consider 

available scientific evidence of the 

discrete population segment’s 

importance to the taxon to which it 

belongs. Since precise circumstances are 

likely to vary considerably from case to 

case, the DPS policy does not describe 

all the classes of information that might 

be used in determining the biological 

and ecological importance of a discrete 

population. However, the DPS policy 

describes four possible classes of 

information that provide evidence of a 

population segment’s biological and 

ecological importance to the taxon to 

which it belongs. As specified in the 

DPS policy (61 FR 4722), this 

consideration of the population 

segment’s significance may include, but 

is not limited to, the following: 

(1) Persistence of the discrete 

population segment in an ecological 

setting unusual or unique to the taxon; 

(2) Evidence that loss of the discrete 

population segment would result in a 

significant gap in the range of a taxon; 

(3) Evidence that the discrete 

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; or 

(4) Evidence that the discrete 

population segment differs markedly 

from other populations of the species in 

its genetic characteristics. 

A population segment needs to satisfy 

only one of these conditions to be 

considered significant. Furthermore, 

other information may be used as 

appropriate to provide evidence for 

significance. 

The current range of the Sonoran 

desert tortoise, as described in the 

discussion above pertaining to 

discreteness, represents several hundred 

miles of occupied habitat spanning 

across an international border. This 

population segment is confined by two 

large perennial rivers: The Colorado 

River in its northern periphery 

(separating the Mojave and Sonoran 

populations), and the Rı ́o Yaqui at its 

southern periphery (separating the 

Sonoran and Sinaloan populations). 

These two rivers represent significant 

biogeographical barriers to genetic 

exchange between adjacent population 

segments and, therefore, preclude 

recolonization of this expanse of habitat 

from adjacent populations, should the 

Sonoran population of the desert 

tortoise become extirpated. Thus, the 

loss of the Sonoran desert tortoise 

would constitute a significant gap of 

several hundred miles in the range 

between the Mojave and Sinaloan 

populations of desert tortoises, and may 

constitute as much as 40 percent of the 

total range occupied by desert tortoises 

as a whole, rangewide, which affirms its 

significance to the entire species. 

In addition, our evaluation of 

discreetness above found extensive 

scientific support concluding that the 

Sonoran desert tortoise differs 

significantly in its behavior 

(reproduction, seasonal activity), 

ecology (habitat use and burrow 

construction), morphology (physical 

characteristics), and genetics from either 

the Sinaloan or the Mojave populations. 

Because of these distinctions, the loss of 

the Sonoran desert tortoise population 

would result in the permanent loss of a 

unique biological entity and would 

diminish the natural variation within 

the species as a whole. 

Evaluation of Significance 

We have reviewed the best available 

commercial and scientific data, and 

based on that review, we find that the 

Sonoran desert tortoise is significant to 

the continued existence of the taxon. 

We base this conclusion on: (1) The 

large geographic range of the Sonoran 

population, which is significant 

(approximately 40 percent) to the taxon 

as a whole; (2) a gap of several hundred 

miles that would result from the loss of 

the Sonoran population, which would 

effectively bisect the species’ range; and 


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(3) the behavioral, ecological, physical, 

and genetic distinctions among the three 

desert tortoise populations. 

Determination of Distinct Population 

Segment 

Based on our review of the best 

commercial and scientific information 

available, the Sonoran population of 

desert tortoise is discrete from the 

Mojave and Sinaloan populations and 

significant to the species as a whole. As 

a result, we have determined that the 

Sonoran population of desert tortoise 

qualifies as a DPS and a listable entity 

under the Act. 

In the August 23, 2009, 90-day finding 

(74 FR 44335), we discussed a local 

population of Mojave-genotype 

(genotype: genetic code) desert tortoises 

(that also share Mojave phenotype (the 

physically-expressed genetic code) and 

habitat-use characteristics with the 

Mojave desert tortoise population) 

occurring within the delineated Sonoran 

population in the Black Mountains area 

of western Mohave County, Arizona. 

This population is isolated from the 

threatened Mojave DPS that occurs 

north and west of the Colorado River. 

The exact geographic extent of this 

Mojave-genotype in Arizona is currently 

undefined and we expect there is 

interbreeding between desert tortoises 

with the Mojave and Sonoran genotype 

along the periphery of this population 

in the Black Mountains. Therefore, we 

include this population of desert 

tortoises as part of our status assessment 

for the Sonoran desert tortoise in this 

finding. 

Distinct Population Segment FiveFactor Analysis 

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

and implementing regulations (50 CFR 

part 424) set forth procedures for adding 

species to, removing species from, or 

reclassifying species on the Federal 

Lists of Endangered and Threatened 

Wildlife and Plants. Under section 

4(a)(1) of the Act, a species may be 

determined to be endangered or 

threatened based on any of the 

following five factors: 

(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 

pertaining to the Sonoran desert tortoise 

in relation to the five factors provided 

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

below. 

In considering what factors might 

constitute threats to a species, we must 

look beyond the exposure of the species 

to a particular factor to evaluate whether 

the species may respond to that factor 

in a way that causes actual impacts to 

the species. If there is exposure to a 

factor and the species responds 

negatively, the factor may be a threat 

and, during the status review, we 

attempt to determine how significant a 

threat it is. The threat is significant if it 

drives, or contributes to, the risk of 

extinction of the species such that the 

species warrants listing as endangered 

or threatened as those terms are defined 

in the Act. However, the identification 

of factors that could impact a species 

negatively may not be sufficient to 

compel a finding that the species 

warrants listing. The information must 

include evidence sufficient to suggest 

that these factors are operative threats 

that act on the species to the point that 

the species may meet the definition of 

endangered or threatened under the Act. 

In our review of the best scientific and 

commercial data available, we found 

numerous threats are impacting Sonoran 

desert tortoises or their habitat 

throughout their range. Some of these 

threats occurred historically, some are 

current, and some will continue into the 

foreseeable future. As described in 

detail below, these threats include 

nonnative plant species and altered fire 

regimes, urban and agricultural 

development, barriers to dispersal and 

genetic exchange, off-highway vehicles, 

roads and highways, ironwood and 

mesquite tree harvest, improper 

livestock grazing, undocumented 

human immigration, illegal collection, 

effects from field research and 

manipulation, predation from feral dogs, 

human depredation and vandalism, 

drought, and climate change. The effect 

of habitat disturbances on Sonoran 

desert tortoises may differ among age 

classes, but may be most significant to 

hatchlings or juveniles (Tracy et al. 

2006b, pp. 271–272). 

Factor A. The Present or Threatened 

Destruction, Modification, or 

Curtailment of Its Habitat or Range. 

Nonnative Plant Species and Altered 

Fire Regimes 

The most significant modification to 

Sonoran desert tortoise habitat is 

associated with the ongoing invasion of 

nonnative plants in Mojave and Sonoran 

desertscrub habitats, permanently 

altering these ecosystems and causing a 

change in the frequency, duration, 

intensity, and magnitude of wildfires in 

a region that largely evolved in the 

absence of invasive nonnative plants. 

These ecosystem-level changes cause 

both direct and indirect effects on the 

Sonoran desert tortoise and its habitat. 

Much of the available research on the 

effects of nonnative plant species 

invasions and wildfire used in our 

analysis has focused on Mojave 

desertscrub habitats, largely due to the 

presence of the Mojave desert tortoise, 

which is already listed as endangered. 

However, Brooks and Matchett (2006, p. 

158) suggest that research from the 

Mojave Desert is applicable to the 

Sonoran Desert when stating, ‘‘Both 

(Mojave and Sonoran deserts) occur at 

elevations above the hyperarid 

shrublands, are often positioned on the 

lower slopes of mountain ranges, and 

possess moderate woody plant cover.’’ 

Therefore, we used the information 

available from research on Mojave 

Desert habitats in our assessment of the 

effects of nonnative plants in the 

Sonoran Desert. 

Nonnative perennial plants like 

buffelgrass, fountain grass, and 

Lehmann lovegrass were historically 

introduced to the Sonoran Desert of 

Arizona as livestock forage and to 

prevent soil erosion. For example, 

buffelgrass was included in the 

nonnative plant species recommended 

for release by the Tucson Plant 

Materials Center of the Soil 

Conservation Service until at least 1987 

(Bahr 1991, p. 156). These nonnative 

plant species subsequently became 

common and widespread in Sonoran 

desertscrub in Arizona (Brooks and 

Pyke 2001, p. 5). They have since 

colonized new areas, often taking 

advantage of disturbed soils, such as 

those resulting from construction 

associated with roadways, power lines, 

and railroad tracks (Bahre 1991, p. 155; 

D’Antonio and Vitousek 1992, p. 65). 

Construction and maintenance of roads 

and highways can also significantly 

enhance the likelihood of nonnative 

plant invasions by increasing nitrogen 

deposition in the soil, the dispersal 

potential of nonnative seeds, and 

adjacent soil moisture (Brooks 2007, pp. 

153–154). Roadside ditches along 

highways are particularly important 

dispersal corridors for nonnative plant 

species such as red brome and 

buffelgrass (Esque et al. 2002, p. 313). 

Mechanisms that allow the spread of 

nonnative species generally pertain to 

ground disturbance, but the plants may 

also be spread by other mechanisms. For 

example, Smith et al. (2000, pp. 79–80), 

and Brooks and Esque (2002, p. 337) 

both found that elevated atmospheric 

carbon dioxide levels, predicted as a 

result of climate change (discussed in 


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Factor E below), are likely to favor 

nonnative plant species, such as red 

brome, over native species in 

desertscrub habitats. Increases in 

atmospheric nitrogen deposition may 

also be advantageous to nonnative plant 

species. Brooks (2003, pp. 344–345) 

suspected that increasing human 

populations will lead to increased levels 

of atmospheric pollution and nitrogen 

deposition and stated, ‘‘Increased levels 

of soil nitrogen caused by atmospheric 

nitrogen deposition may increase the 

dominance of invasive alien plants and 

decrease the diversity of (native) plant 

communities in desert regions, as it has 

in other ecosystems.’’ Sonoran desert 

tortoise habitat may be particularly 

vulnerable to even minor increases in 

soil nitrogen levels, because the ratio of 

increased nitrogen to plant biomass is 

higher compared with that of most other 

ecosystems (Brooks 2003, p. 344). This 

suggests that even small changes in 

nitrogen levels could result in 

substantial changes in the plant 

community that supports Sonoran 

desert tortoise habitat. 

The prevalence of nonnative grasses 

in many areas of Sonoran desertscrub 

habitats has resulted in high amounts of 

flammable fuels in interspaces between 

native plants that would otherwise be 

free of vegetation. This situation serves 

to promote the ignition and carrying of 

wildfire (Brooks 1999, p. 13). In our 

review of the best scientific data 

available, red brome, splitgrass (or 

Mediterranean grass, Schismus spp.), 

and buffelgrass were considered the 

nonnative plant species that pose the 

greatest concern to the Sonoran desert 

tortoise and its habitat, because they are 

thoroughly integrated into some areas of 

the desertscrub communities, and serve 

to promote and carry wildfire (Bahre 

1991, p. 155; D’Antonio and Vitousek 

1992, pp. 65, 75; Brooks 1999, p. 13; 

Brooks and Pyke 2001, p. 5; Brooks and 

Esque 2002, p. 337; Esque et al. 2002, 

p. 313; Van Devender 2002a, p. 16; 

Brooks and Matchett 2006, p. 148; 

DeFalco 2007a, p. 1; Zouhar et al. 2008, 

p. 157; Abella 2010, p. 1249; AGFD 

2010, p. 13). Red brome is known to 

carry wildfire in Sonoran desertscrub 

habitat north of Tucson, natal grass is 

known to carry wildfire in desert 

grassland habitat south of Tucson to 

Nogales, Arizona, and buffelgrass is 

known to carry wildfire in Sonoran 

desertscrub and foothills thornscrub 

south of the international border to 

central Sonora (Esque et al. 2002, p. 

316). Other nonnative plant species 

identified in the literature as present in 

Sonoran and Mojave desertscrub 

communities include Saharan (or Asian) 

mustard (Brassica tournefortii), thistles 

(genera Centaurea and Cirsium), 

crimson fountaingrass (Pennisetum 

setaceum), natal grass (Melinus repens), 

and Lehmann lovegrass (Eragrostis 

lehmanniana) (Brooks 2001, p. 4; 

Brooks and Pyke 2001, pp. 3, 5). 

We are not aware of any good 

estimates of the number of acres of 

desertscrub that have been invaded by 

nonnative plant species, but Thomas 

and Guertin (2007, Appendices I and II) 

calculated the number of records by 

county for many known invasive, 

nonnative plants in Arizona that are 

harmful to Sonoran desert tortoise 

habitat. These data illustrate general 

locations where certain nonnative 

species are most common and describe 

which nonnative species are the most 

reported in each area. Thomas and 

Guertin (2007, Appendices I and II) 

reported the following for Arizona as of 

2007 (relative number of reports of 

densities being ‘‘extremely high,’’ ‘‘high,’’ 

‘‘moderate,’’ and ‘‘occurs,’’ all within the 

distribution of the Sonoran desert 

tortoise): 

(1) Buffelgrass is the most-reported 

nonnative plant species in Arizona, at 

16.3 percent of total reports with 6,287 

reports (p. 3); it reaches extremely high 

densities in Maricopa and Pima 

Counties, with high densities in Pinal 

and Yuma Counties and moderate 

densities in Santa Cruz and La Paz 

Counties, but it also occurs in Yavapai, 

Gila, and Cochise Counties (A–I, p. 60); 

(2) Schismus spp. is one of the top 20 

invasive plant species, at 2.4 percent of 

total reports, with 919 reports (p. 3); it 

reaches high densities in Maricopa, 

Pinal, and Pima Counties, with 

moderate densities in Mohave, Yavapai, 

Gila, La Paz, and Yuma Counties, but it 

also occurs in Santa Cruz County (A–I, 

p. 69); 

(3) Red brome is one of the top 20 

invasive plant species, at 3 percent of 

total reports, with 1,152 reports (p. 3); 

it reaches high densities in Yavapai, 

Gila, Pinal, and Pima Counties, with 

moderate densities in Mohave and 

Maricopa Counties, but it also occurs in 

La Paz and Yuma Counties (A–I, p. 24); 

(4) Saharan mustard is one of the top 

20 invasive plant species, at 3.3 percent 

of total reports, with 1,261 reports (p. 3); 

it reaches high densities in Maricopa, 

Pinal, Pima, La Paz, and Yuma 

Counties, with moderate densities in 

Mohave, Yavapai, and Gila Counties, 

but it also occurs in Cochise County (A– 

I, p. 21); 

(5) Centaurea spp. had a total of 3– 

318 reports (depending on species) (p. 

9) and reaches high densities in Pima 

County, with moderate densities in 

Mohave, Yavapai, Gila, Pinal, and 

Cochise Counties (A–I, pp. 15, 28–30); 

(6) Bull thistle (Cirsium vulgare) is 

one of the top 20 invasive plant species, 

at 3.1 percent of total reports, with 1,195 

reports (p. 3); it reaches moderate 

densities in Yavapai and Gila Counties 

(A–I, p. 35); 

(7) Crimson fountaingrass is one of 

the top 20 invasive plant species, at 2.6 

percent of total reports, with 999 reports 

(p. 3); it reaches high densities in Pima 

County, with moderate densities in 

Yavapai, Gila, La Paz, Santa Cruz, and 

Maricopa Counties (A–I, p. 61); and 

(8) Lehman lovegrass is one of the top 

20 invasive plant species, at 2.5 percent 

of total reports, with 980 reports (p. 3); 

it reaches high densities in Pima and 

Cochise Counties, with moderate 

densities in Yavapai, Gila, Santa Cruz, 

Maricopa, and Pinal Counties, but also 

occurs in La Paz County (A–I, p. 45). 

No spatial data were provided for 

natal grass, but there were 191 

observations (Thomas and Guertin 2007, 

p. 10). 

Buffelgrass has widely invaded 

Arizona and northern Mexico since its 

introduction in 1939 (Stevens and 

Fehmi 2009, p. 379). While buffelgrass 

invasions are occurring and are poised 

to seriously impact the southwestern 

United States, the species has already 

exacted significant tolls on Sonoran 

desertscrub communities in Sonora, 

Mexico, because its expansion 

continues to be facilitated through 

intentional plantings and cultivation. 

Consequently, the clearing of Sonoran 

desertscrub and Sinaloan thornscrub in 

Sonora to plant pastures of buffelgrass 

for livestock grazing creates a near 

monoculture (area covered by a single 

plant species) that is highly prone to 

wildfires, and therefore represents a 

substantial threat to the Sonoran desert 

tortoise in Mexico (Bury et al. 2002, p. 

104; Walker and Pavlakovich-Kochi 

2003, p. 14; Van Devender and Reina 

2005, pp. 160–161; University of 

Arizona 2010, p. 2). Buffelgrass has been 

planted in Sonora’s desertscrub lands 

since the 1950s and at least 5.5 million 

ac (2.2 million ha) of potential Sonoran 

desert tortoise habitat has already been 

converted into a near monoculture of 

buffelgrass (Stoleson et al. 2005, p. 62). 

Buffelgrass has become established in 

both the lower valley habitats and into 

the granite boulder-strewn areas of 

adjacent foothills, and has altered 

historical fire regimes, regionally 

converting large areas of Sonoran 

desertscrub into habitat resembling the 

African savannah (Bury et al. 2002, p. 

104). 

In Arizona, the Southern Arizona 

Buffelgrass Coordination Center 


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(SABCC, a coalition of non-profit 

organizations, Federal, State, and local 

governments, conservation 

organizations, private businesses, and 

individual citizens) reports dense stands 

of buffelgrass on public reserves, State 

and local lands, and private property, 

including Saguaro National Park, 

Coronado National Forest, Bureau of 

Land Management’s (BLM) Ironwood 

Forest National Monument, 

neighborhoods of Tucson, Sahuarita, 

Marana and Oro Valley, and along 

roadsides throughout this region of 

Arizona (SABCC 2010, p. 1) These areas 

are all within the distribution of the 

Sonoran desert tortoise in Arizona. 

Brooks and Minnich (2006, p. 9) 

stated that southwestern desert 

ecosystems likely evolved in a fire 

regime best described by ‘‘low intensity, 

patchy burns and long fire return 

intervals.’’ Wildfire capable of carrying 

itself in Sonoran desertscrub is a recent 

phenomenon in evolutionary and 

geological contexts and only became 

apparent recently in the Sonoran Desert 

(Brooks and Pyke 2001, p. 5; Esque et 

al. 2002, p. 312; Zouhar et al. 2008, pp. 

155, 160). From 1937 to 1986, only 1 

percent of all lightning-caused fires in 

the Rincon Mountains area of southern 

Arizona occurred in desertscrub habitat; 

5.6 percent occurred in desert grassland 

habitat (Bahre 1991, p. 126). While 

historical wildfires in desertscrub 

habitat were exceptionally rare, after 

successive years of above-average levels 

of precipitation, enough native fuels can 

develop to carry wildfire in desertscrub 

communities, such as happened south 

of Florence, Arizona in 1979 (Bahre 

1991, p. 141; Brooks and Esque 2002, p. 

336; Brooks and Minnich 2006, p. 9). 

While increased precipitation enhances 

plant growth and subsequently 

increases the likelihood for wildfire 

starts in desertscrub habitat, drought 

can have an inverse effect with respect 

to certain nonnative plant species. Red 

brome, for example, is sensitive to 

drought conditions and, therefore, might 

contribute to reduced fuel loads and 

decreased fire frequency during longterm drought (Brooks and Esque 2002, 

p. 337), which might help to minimize 

the likelihood of wildfires in areas 

where red brome has formed a 

monoculture. Smith et al. (2000, p. 79) 

noted, ‘‘This shift in species 

composition in favor of exotic annual 

grasses, driven by global [climate] 

change, has the potential to accelerate 

the fire cycle, reduce biodiversity and 

alter ecosystem function in the deserts 

of western North America.’’ 

Wildfire ignitions in the Sonoran 

Desert region historically resulted from 

lightning but ignitions are now more 

common from human sources such as 

burning trash, parking vehicles over dry 

vegetation, fireworks, discarded 

cigarettes, and accidental starts from 

backcountry recreationists (Esque et al. 

2002, p. 313). Human-caused wildfires 

in desertscrub habitat are most common 

near urban developments, major 

roadways, and in areas where offhighway vehicle use is unregulated, 

while lightning-caused wildfire in 

desertscrub is typically located in more 

remote wilderness areas (Brooks 1999, 

p. 13). In central Sonora, ranchers 

intentionally set fires to maintain the 

vigor of buffelgrass for livestock forage 

(Esque et al. 2002, p. 313). 

Numerous wildfires, varying in size, 

have occurred in recent times in many 

areas throughout the Sonoran Desert 

including the: (1) Pusch Ridge Fire of 

1987 on the southern slopes of the Santa 

Catalina Mountains; (2) Skyline (1992) 

and Rock Peak (1993) fires in the San 

Tan Mountains; (3) Mother’s Day Fire of 

1994 on the eastern slope of the Rincon 

Mountains (Esque et al. 2002, p. 323; 

2003, p. 104); and (4) Cave Creek 

Complex fire of 2005 northeast of Cave 

Creek, Arizona, which burned 248,310 

ac (100,487 ha) of Sonoran desert 

tortoise habitat; the largest wildfire ever 

recorded in Sonoran desertscrub in the 

United States. 

The BLM has kept records of wildfire 

in Sonoran desert tortoise habitat. From 

1990 to 2008, there have been 61 

wildfires, affecting 21,977 ac (8,894 ha) 

in Category I Sonoran desert tortoise 

habitat; 285 wildfires, affecting 33,364 

ac (13,502 ha) in Category II Sonoran 

desert tortoise habitat; and 508 

wildfires, affecting 109,460 ac (44, 297 

ha) in Category III Sonoran desert 

tortoise habitat (USBLM 2010, p. 9). In 

total, during the 1990–2008 period, 

164,801 ac (66,693 ha) of categorized 

and uncategorized Sonoran desert 

tortoise habitat has burned on BLM 

lands (USBLM 2010, p. 9). Combining 

the known area of habitat affected by 

fire on both BLM and other lands, an 

estimated 1.5 percent of habitat in 

Arizona has been adversely affected due 

to wildfire in recent years; rangewide 

this is estimated to be 0.8 percent, 

although total acreage data on wildfires 

in Mexico are unknown and the total 

percentage of affected habitat is likely 

higher because of the higher incidence 

of buffelgrass and lessened capacity to 

fight wildfire in Sonora, Mexico. The 

total area reported as burned is a 

relatively small proportion of BLM 

lands and has not likely been a 

significant impact to most Sonoran 

desert tortoise populations in Arizona 

so far. As the invasion of nonnative 

plants continues to expand, the high 

number of fire starts has a greater 

potential of creating larger and more 

destructive wildfires, especially where 

they occur in remote, inaccessible areas 

as a result of lightning strikes. 

Indirect effects of wildfires on 

Sonoran desert tortoises are variable and 

can be significant, including habitat 

changes such as altered nutrient 

availability and quality, loss of 

perennial plant species that are 

important as temporary cover from 

predators, loss of thermal refugia, 

altered tortoise behavior, shifts in biotic 

community, pronounced desert tortoise 

emigration from burned habitat, and 

lower growth and reproductive output 

(Esque et al. 2003, p. 107; DeFalco 2006, 

p. 5; McLuckie et al. 2007, p. 8). While 

a single fire in an area may or may not 

produce long-term reductions in plant 

cover or biomass, repeated wildfires in 

a given area are capable of ecosystem 

type-conversion from native desertscrub 

to nonnative annual grassland, and 

render the area unsuitable for desert 

tortoises (Brooks and Esque 2002, p. 

336). Increased frequency in wildfires 

caused by nonnative plant species 

invasion increases light intensity at 

ground level and soil nutrient 

availability, and reduces competition 

from native perennial plants. These 

changes further promote dominance by 

nonnative plant species (Brooks and 

D’Antonio 2003, p. 29). Wildfire in 

desertscrub habitats can reduce native 

and nonnative seed banks (Brooks and 

Draper 2006, p. 2). In Mojave 

desertscrub, the effects of fire are most 

pronounced under shrubs, where fire 

can kill seed banks and reduce annual 

grass diversity, due to higher burn 

intensity (Brooks 2002a, p. 1; 2002b, p. 

1088). Microhabitat associated with 

shrubs in Sonoran desert tortoise habitat 

is an important source of temporary 

shelter and provides foraging 

opportunities while tortoises are 

thermoregulating. 

Fires associated with nonnative plant 

species have already affected Sonoran 

desert tortoise populations in Arizona. 

The AGFD (2010, p. 13) reported results 

from an unpublished study after the 

Edge Complex Fire of 2005 in the Four 

Peaks area on the Tonto National Forest, 

which indicated higher numbers of 

Sonoran desert tortoises (or their scat 

were observed in unburned versus 

burned habitat), but they acknowledged 

that the study was preliminary and very 

limited in scope (AGFD 2010, p. 13). 

In Sonora, Mexico, 5.5 million ac (2.2 

million ha), representing an estimated 

22 percent of Sonoran desert habitat in 

Mexico, or 11 percent rangewide, has 

been planted to bufflegrass. This figure 

still does not account for the land area 


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where buffelgrass has become naturally 

established or the 11.9 million ac (4.8 

million ha) (or one-third of the land area 

of the state of Sonora) that are suitable 

for future natural establishment of 

buffelgrass (Stoleson et al. 2005, p. 62). 

Combining the current and predicted 

number of acres converted to buffelgrass 

in Mexico, 34 percent of the Sonoran 

desert tortoises’ habitat is lost or at risk 

across its range. In the area of El 

Batamote, 29 mi (47 km) north of 

Hermosillo, Sonora, buffelgrass has 

invaded Sonoran desert tortoise habitat 

in the adjacent foothills, which has led 

to wildfires that burned so hot that the 

soil was scorched and the bedrock 

cracked (Esque et al. 2002, p. 321). 

In addition to impacts from fire, 

Franklin and Molina-Freaner (in press, 

p. 1) found that these large-scale 

conversions from desertscrub to 

grasslands in Sonora have reduced plant 

species richness by half, and reduced 

tree and shrub cover by 78 percent, 

vastly affecting the ability of Sonoran 

desert habitat to meet the species’ 

thermoregulatory needs (that is, using 

vegetation as cover to regulate body 

temperature). These changes have 

resulted in substantial changes in 

primary productivity (creation of 

organic nutrients and the lowest level of 

the food chain, the plant community) 

and vegetation structure (the physical 

structure of plant sizes and shapes as a 

mosaic on the landscape) which can 

affect the forage base and habitat 

suitability for Sonoran desert tortoises, 

as well as lessened the feasibility of 

restoring native plant communities in 

Sonora without aggressive land 

management (Franklin and MolinaFreaner, in press, p. 1). Dense stands of 

buffelgrass have also been shown to 

physically disrupt tortoise movements 

in the closely related Texas tortoises 

(Gopherus berlandieri) (Fujii and 

Forstner 2010, p. 61), so this may also 

be true for Sonoran desert tortoises. The 

grass can become so thick that the 

tortoises cannot walk through it, and the 

grass may be too tall for the tortoises to 

walk on top of it. 

In addition to damaging Mojave and 

Sonoran desertscrub habitat, wildfires 

can directly injure and kill Sonoran 

desert tortoises. Wildfire may kill a 

desert tortoise by incineration, by 

elevating body temperature, by 

poisoning from smoke inhalation, or by 

asphyxiation (Brooks et al. 1999, p. 40; 

Brooks and Esque 2002, p. 335; 

McLuckie et al. 2007, p. 7). Survival 

rates of Sonoran desert tortoises may be 

contingent upon several factors, 

including soil type, substrate, 

vegetation, tortoise activity during fire, 

whether tortoises are active and above 

ground or in shelter during a fire, 

weather, fire behavior, and shelter depth 

(McLuckie et al. 2007, p. 8). The desert 

tortoise is most vulnerable to the direct 

effects of wildfire when they are surface 

active and away from primary cover 

sites such as burrows, caliche caves, and 

rock shelters, because these structures 

reduce direct exposure to heat and 

smoke (Brooks and Esque 2002, p. 335). 

Gravid (with fertilized eggs) female 

Sonoran desert tortoises may be more 

likely to perish from wildfire than other 

tortoises because peak wildfire season 

in Sonoran desertscrub occurs during 

the months of May and June. This is 

when reproductive females are actively 

foraging on spring growth to 

compensate for energy used in egg 

development; (Esque et al. 2002, pp. 

323–324; 2003, p. 106). 

Sonoran desert tortoises that survive 

the wildfire itself may struggle to 

survive in post-burned Sonoran 

desertscrub habitat due to: (1) A 

reduction in forage and shade structure, 

such as packrat (Neotoma sp.) middens 

and shrubs; and (2) increased visibility 

to predators (which may be further 

increased in intermountain valleys 

where temporary shade, predator 

avoidance, and available forage are 

particularly important in long-distance 

movements in these dispersal corridors) 

(Esque et al. 2002, pp. 325–326). 

The effects on Sonoran desert 

tortoises of one particular fire were 

studied in some detail. Within Saguaro 

National Park, the Mother’s Day Fire of 

1994 burned 340 ac (138 ha) of Arizona 

Upland Sonoran desertscrub habitat that 

was occupied by Sonoran desert 

tortoises, killing an estimated 11 percent 

of the tortoise population (Esque et al. 

2003, p. 105). To assess how Sonoran 

desert tortoises used burned versus 

unburned habitat following this fire, 

transmitters were attached to 12 

tortoises, 6 each in burned and 

unburned habitat within or adjacent to 

the Mother’s Day Fire footprint. 

Surprisingly, no differences were 

observed in movement or activity 

patterns between tortoises in burned 

and unburned areas, nor were long-term 

effects of the fire on surviving tortoises 

noted over the 6-year study period 

(Zylstra and Swann 2009, p. 7). These 

results indicate that different tortoise 

populations may respond differently to 

wildfires and that numerous variables 

and factors are at work. 

One of the principal reasons that 

nonnative plants pose a significant 

impact to Sonoran desert tortoise habitat 

is because few, if any, reasonable 

methods currently exist to control the 

ongoing invasion of these plants or to 

remediate areas where they have 

become established. Mechanical 

removal is one option that has been 

implemented on a small scale in some 

areas, but is extremely labor intensive 

and not practical for treating large areas. 

Prescribed fire has been proposed as an 

alternative means to control nonnative 

plant species invasions, but also carries 

obvious inherent risks to habitat and to 

Sonoran desert tortoises (Brooks 2006, 

p. 31). 

It is also important to note the 

limitations of Sonoran desert habitat 

with respect to post-disturbance (for 

example, after fires) regeneration (ability 

for native vegetation to recover). 

Desertscrub regions receive low annual 

precipitation totals, and the plant 

communities have correspondingly low 

growth rates. Based on the type of 

disturbance, recovery time estimates 

range from 40 years to centuries (Abella 

2010, pp. 1271, 1273). Combined, these 

factors result in slow, post-disturbance 

recovery periods and it may take a long 

time before any area becomes suitable 

for Sonoran desert tortoises to 

recolonize, if at all. The presence of 

nonnative species such as buffelgrass, 

cheatgrass, or red brome in disturbed 

Mojave or Sonoran desertscrub may 

further limit post-disturbance recovery, 

delay recovery, or prevent recovery 

altogether (Brown and Minnich 1986, p. 

411; Brooks 1999, p. 18). 

In our review of the best available 

information, we have documented that 

nonnative plant species pose a 

significant threat to the Sonoran desert 

tortoise and its habitat, both in Arizona 

and Sonora, by promoting and carrying 

wildfire in an ecosystem that evolved in 

its absence. Wildfires that are facilitated 

by nonnative plant species invasions 

may have direct and indirect adverse 

effects on tortoises and tortoise 

populations. The threat from nonnative 

plant species to the Sonoran desert 

tortoise occurs throughout the species’ 

range and is expected to increase over 

time with the expansion of nonnative 

plants. There is currently no viable 

solution to curbing this continued 

expansion across the landscape. This 

threat also acts synergistically with 

other threats discussed in this finding. 

Urban Development and Agriculture 

Human population growth results in 

the disturbance or loss of Sonoran 

desertscrub or the conversion of land for 

urban and agricultural development. 

Arizona increased its population by 394 

percent from 1960 to 2000, and was 

second only to Nevada as the fastest 

growing State during this timeframe 

(Social Science Data Analysis Network 

(SSDAN) 2000, p. 1). Since 1990, 

Arizona’s population has grown by 44 


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percent. From 1960 to 2000, population 

growth rates in Arizona counties where 

the Sonoran desert tortoise occurs have 

varied by county but are no less 

remarkable, and all are increasing: 

Maricopa (463 percent); Pima (318 

percent); Pinal (54 percent); Santa Cruz 

(355 percent); Cochise (214 percent); 

Yavapai (579 percent); Gila (199 

percent); Graham (238 percent); Yuma 

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

Mohave (2,004 percent) (see SSDAN 

2000). The population of Phoenix, 

Arizona, grew 67 percent from 1980 to 

2000 (Berry et al. 2006, p. 7). 

Urban expansion and human 

population growth trends in Arizona are 

expected to continue into the future. 

Maricopa-Pima-Pinal county areas of 

Arizona are expected to grow by as 

much as 71 percent in the next 15 years, 

creating rural-urban edge effects across 

millions of acres of public lands 

currently supporting Sonoran desert 

tortoise populations (AIDTT 2000, p. 10; 

BLM files—Lands Livability Initiative). 

In another projection, the population in 

Arizona is expected to more than double 

within the next 20 years compared to 

the 2000 population estimate (U.S. 

Census Bureau 2005, p. 1). Many cities 

and towns within the distribution of the 

Sonoran desert tortoise have already 

experienced substantial growth during 

the 8-year time span, 2000–2008: City of 

Avondale (118.3 percent); City of 

Buckeye (392.5 percent); Bullhead City 

(20.3 percent), Town of Carefree (30.5 

percent); Casa Grande (56 percent); 

Town of Cave Creek (44.2 percent); City 

of Chandler (37.5 percent); City of 

Coolidge (24.9 percent); City of El 

Mirage (195.6 percent); City of Eloy 

(22.3 percent); City of Florence (20.3 

percent); Town of Fountain Hills (23.2 

percent); City of Gilbert (84.5 percent); 

City of Goodyear (203 percent); City of 

Kingman (32.2 percent); Lake Havasu 

City (33.3 percent); City of Litchfield 

Park (34.2 percent); City of Mammoth 

(45 percent); Town of Marana (139.9 

percent); City of Maricopa (2,508 

percent); Town of Oro Valley (32.5 

percent); Town of Queen Creek (544.5 

percent); Town of Saguarita (507.3 

percent); City of San Luis (58.5 percent); 

City of Somerton (63.2 percent); City of 

Surprise (187.3 percent); City of 

Tolleson (43.2 percent); and, Town of 

Youngtown (62.2 percent) (U.S. Census 

Bureau 2008, pp. 1–4). 

This population growth has spurred a 

significant increase in urbanization and 

development in these areas. Regional 

development is predicted to be extreme 

in certain areas within the distribution 

of the Sonoran desert tortoise in 

Arizona. In particular, a wide swath 

from the international border in 

Nogales, through Tucson, Phoenix, and 

north into Yavapai County (called the 

Sun Corridor ‘‘Megapolitan’’) is 

predicted to have 8 million people by 

2030, an 82.5 percent increase from 

2000 (Gammage et al. 2008, pp. 15, 22– 

23). If build-out occurs as expected, it 

will encompass a significant proportion 

of the Sonoran desert tortoise 

distribution in Arizona, and will in 

effect permanently isolate Sonoran 

desert tortoise populations that occur on 

either side of the Interstate 19, Interstate 

10, and Interstate 17 corridors. 

The land area permanently altered by 

human activities from urban 

development and agriculture has grown 

to 13 percent of all land in the western 

United States, Lue et al. (2008, p. 1130). 

Lue et al. (2008, p. 1133) concluded that 

in low-productivity habitat, such as 

desertscrub habitats, slight human 

disturbances can have pronounced 

effects. Significant urban development 

occurs within intermountain valleys, 

within or adjacent to occupied Sonoran 

desert tortoise habitat, which increases 

the likelihood of effects along the ruralurban interface, and may also inhibit 

movement of individuals between 

populations on nearby hillsides or 

mountain ranges. Disturbances to 

Sonoran desert tortoise habitat on the 

landscape can take many forms and 

cover extreme distances. Roads, canals, 

pipelines, and railroad tracks are 

examples of linear habitat destruction. 

We discuss the potential effects of linear 

disturbances below in the section titled, 

‘‘Development as a Barrier.’’ 

Development pressure across Arizona 

has slowed due to the recent economic 

downturn and decline in the housing 

market. However, development will 

likely continue in the future, although 

perhaps at a slower pace than in the 

earlier part of this century. We also 

recognize that economic trends are 

difficult to predict into the future. The 

most recent draft Pinal County 

Comprehensive Plan (February 2009) 

acknowledges that the county is in the 

middle of the Sun Corridor Megapolitan 

and proposes four shorter-term growth 

areas in defining where development 

will likely occur, or be encouraged to 

develop, over the next decade, but does 

not discourage growth outside of these 

areas (Pinal County Comprehensive 

Plan 2009, p. 109). These four growth 

areas (Gateway/Superstition Vistas, 

West Pinal, Red Rock, and TriCommunities) fall completely within 

the range of the Sonoran desert tortoise. 

The Gateway/Superstition Vistas growth 

area alone encompasses 176,000 ac 

(71,225 ha), or 275 sq mi (712 sq km), 

of State Trust land, and it is anticipated 

that 800,000 to more than 1 million 

people will one day live in this 

development (Pinal County 

Comprehensive Plan 2009, p. 115). The 

loss of 176,000 ac (71,225 ha) 

constitutes a loss of 0.7 percent of 

Sonoran desert tortoise habitat in 

Arizona; rangewide, 0.34 percent. The 

Pinal County Comprehensive Plan 

(2009, p. 117) identifies many miles of 

new freeways and principal arterials in 

the analysis area at build-out, which the 

plan acknowledges may take over a half 

century to realize (Pinal County 

Comprehensive Plan 2009, p. 115). The 

effect of roads on Sonoran desert 

tortoises is discussed below. 

Additionally, the Maricopa County 

Comprehensive Plan calls for growth 

areas to the south and east of Chandler 

and Mesa, Arizona, which are within 

the range of the Sonoran desert tortoise 

(Maricopa County Comprehensive Plan 

2002 (revised), p. 92). City 

comprehensive plans within the range 

of the Sonoran desert tortoise also call 

for future growth areas. For example, 

the City of Eloy has designated six such 

areas encompassing 15,520 ac (6,281 

ha), mostly along the Interstate 10 

corridor (City of Eloy General Plan 2004, 

pp. 7–6 through 7–10). The loss of 

15,520 ac (6,281 ha) constitutes a loss of 

0.06 percent of their habitat in Arizona; 

rangewide, 0.03 percent. While much of 

this area has already been impacted by 

development or irrigated agriculture, 

any remaining dispersal habitat for the 

Sonoran desert tortoise will likely be 

negatively affected as development and 

its associated infrastructure progress 

into these areas. 

Much of the past and projected 

development within the range of the 

Sonoran desert tortoise in central and 

southwestern Arizona has occurred and 

is expected to continue as a conversion 

from agricultural uses to municipal 

uses. Land traditionally used for 

agriculture is not occupied by Sonoran 

desert tortoises, but has a comparatively 

minor effect on adjacent Sonoran desert 

tortoises. When these lands are 

converted to municipal uses, the effect 

to adjacent Sonoran desert tortoise 

populations increases human access, 

and use of adjacent undeveloped land 

increases as a result of development of 

these former agricultural areas. 

The human population of Sonora, 

Mexico, doubled in size from 1970 (1.1 

million) to 2000 (2.2 million) (Stoleson 

et al. 2005, p. 54). The population of 

Sonora is expected to increase by 23 

percent, to 2.7 million people, in 2020 

(Stoleson et al. 2005, p. 54). In 

discussing threats to Sonoran desert 

tortoise populations adjacent to, and 

stemming from, urbanization in Sonora, 

Mexico, Fritts and Jennings (1994, p. 53) 


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stated, ‘‘Tortoise populations adjacent to 

large population centers such as 

Hermosillo, Guaymas, and Caborca 

probably have experienced long-term 

harm, including direct human 

exploitation, habitat degradation, road 

kills, predation by domestic dogs, and 

use as pets. However, we found 

evidence of tortoise populations on 

hillsides and mountain slopes near each 

of these cities, which suggests that some 

tortoise populations have survived 

despite perturbations by humans.’’ 

Therefore, Sonoran desert tortoises may 

persist as depressed populations 

adjacent to urban development, but 

without long-term population trend data 

for these areas, we are unable to know 

for how long. 

Urban development has been 

identified as a concern for Sonoran 

desert tortoise conservation in several 

areas within Arizona because of the 

associated increase in human-based 

threats to populations in close 

proximity. Averill-Murray and Swann 

(2002, p. 1) stated that urban 

development adjacent to the Saguaro 

National Park in Pima County threatens 

the Sonoran desert tortoise via several 

mechanisms including harassment and 

predation by feral or off-leash domestic 

dogs, illegal releases of captive Sonoran 

desert tortoises and exotic species that 

may transmit diseases to wild Sonoran 

desert tortoises, elevated mortality on 

roads, and illegal collection for pets. 

Averill-Murray and Swann (2002, p. 7) 

stated that mid- to large-scale 

development projects on the bajadas 

and foothills of the Rincon, Santa Rita, 

Santa Catalina, Tortolita, and Tucson 

Mountains has likely led to area-wide 

decreases in Sonoran desert tortoise 

populations. However, no population 

estimates for Sonoran desert tortoises 

before development of these areas exist, 

and, therefore, population responses to 

development of these areas cannot be 

ascertained. 

In addition to the Tucson 

metropolitan area, urban encroachment 

on Sonoran desert tortoise habitat 

occurs adjacent to the greater Phoenix 

metropolitan area, in the area around 

South Mountain and adjacent to the 

Superstition Mountains (AGFD 2010, p. 

7). Sonoran desert tortoises are known 

or suspected to still occur in 12 of the 

16 Maricopa County and City of 

Phoenix urban mountain parks and 

reserves. The four parks where no 

tortoise sign has been found in recent 

years are completely surrounded by 

urban development (AGFD 2010, p. 7). 

Urban development has occurred 

adjacent to five monitoring plots, but 

only the Hualapai Foothills plot is 

completely surrounded by developed 

lands (AGFD 2010, p. 7). A development 

consisting of 48,000 single family 

homes, south of the Colorado River in 

western Mohave County, is also 

currently being planned (THS 2009, p. 

4; Mardian 2010, p. 1). 

Because less area is being used 

currently for agriculture in the United 

States, habitat loss due to agricultural 

development is more of a historical 

issue. However, impacts to Sonoran 

desert tortoise dispersal habitat within 

valley floors from historical agricultural 

use and wood harvesting are still 

evident. The vegetation and soils of 

many valleys in the Sonoran Desert 

were shaped by the periodic flooding of 

dynamic wash systems, which partially 

recharged a shallow, fluctuating 

groundwater table. Because of 

agricultural development, these valleys 

no longer experience these defining 

processes and there has been a 

permanent loss of meso- and 

xeroriparian habitat which are known to 

be corridors for movement by Sonoran 

desert tortoises (Jackson and Comus 

1999, pp. 233, 249; Lutz et al. 2005, p. 

22; Riedle et al. 2008, p. 418). 

Agriculture in Sonora, Mexico, has 

shifted from small-scale, local markets 

toward large-scale agro-industry, with 

Sonora producing 40 percent of the 

country’s total wheat crop (Stoleson et 

al. 2005, p. 59). While agriculture in 

Sonora is largely constrained to valleys 

(along the Rio Sonora), many types of 

habitat used by Sonoran desert tortoises 

have been cleared for agriculture, 

including Sonoran desertscrub, 

thornscrub, and tropical deciduous 

forest (Stoleson et al. 2005, p. 60). In 

1994, the total irrigated acreage in 

Sonora was 128,000 ac; in 2004 that 

figure rose to 530,509 ac (214,689 ha), 

an increase of 314 percent (AQUASTAT 

2007, p. 2). This constitutes an 

estimated loss of 2 percent of Sonoran 

desert tortoise habitat in Mexico; 

rangewide, 1 percent. 

The projected growth of the human 

population in Arizona and northern 

Mexico and subsequent urbanization 

discussed above is expected to place 

onerous demands on lands where the 

Sonoran desert tortoise occurs, 

increasing the need for infrastructure 

associated with development, such as 

power lines, power plants, pipelines, 

landfills, roads, sand and gravel mines, 

and removal of boulders for landscaping 

(AIDTT 2000, p. 10). In addition, these 

growth projections will increase human 

visitation to formerly remote Sonoran 

desert tortoise habitat as urban-rural 

interface expands, whereby increasing 

human-associated threats discussed in 

detail below (AIDTT 2000, p. 10). The 

AGFD (2010, p. 7) concluded that 

‘‘* * * as urbanization continues to 

expand, (Sonoran desert tortoise) habitat 

will continue to be lost.’’ In a Global 

Information System exercise, we 

calculated that currently, 75 percent of 

potentially occupied Sonoran desert 

tortoise habitat within Arizona occurs 

within 30 mi (48 km) (a reasonable 

distance a person might travel to 

recreate outdoors on public land) or less 

of a city or town with a population of 

1,000 or more. As the human population 

of Arizona grows and development 

expands as expected, we assume that 

100 percent of Sonoran desert tortoise 

populations will occur within 30 mi (48 

km) or less of a city or town with a 

population of 1,000 or more, in the 

foreseeable future. Tortoise populations 

are being increasingly exposed to 

humans and human activities, and 

therefore to numerous threats that 

would otherwise be minimized or 

nonexistent. We discuss these types of 

threats and how they affect Sonoran 

desert tortoises and their habitat below 

in Factors B, C, D, and E. 

Some forms of development are likely 

to increase. The interest in renewable 

energy projects is expected to increase 

significantly in the future. Solar 

radiation levels in the southwestern 

United States, including Arizona, are 

some of the highest in the world, and 

interest in tapping into this source of 

potential energy is growing. Potentially 

significant tracts of BLM lands in 

southwestern Arizona have been 

identified for possible solar energy 

development, encompassing large 

percentages of Arizona’s valley 

bottomland in La Paz and Yuma 

Counties and adjacent to or within the 

foothills of the Black Mountains of 

western Mohave County, which could 

isolate Sonoran desert tortoise 

populations and affect genetic exchange 

among regional populations in those 

areas (USDOE 2009, p. 1). Since most 

solar projects are in the early planning 

stages and have yet to be officially 

approved by the BLM, we are unable to 

ascertain the amount of Sonoran desert 

tortoise habitat likely to be impacted. 

However, we acknowledge that large 

areas within the distribution of the 

Sonoran desert tortoise in Arizona are 

being considered for solar projects. 

In one example, 12,100–15,100 ac 

(4,897–6,110 ha) of BLM, State, and 

private land containing Sonoran desert 

tortoise habitat along the southern 

bajada of the Black Mountains in 

western Mohave County, Arizona, has 

been identified for development of the 

Sterling Solar Generating Facility within 

the next 4 to 6 years (Needle Mountain 

Power, LLC 2010, pp. 4, 8, 11). At buildout, the Sterling Solar Generating 


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Facility will consist of solar fields, 

power blocks, buildings, retention 

ponds, rainwater catch basins, 

evaporation ponds, wastewater and 

water treatment facilities, water storage 

tanks, on-site housing, a substation, a 

visitors center, a substation and 

switching station interconnection with 

the Western Area Power Administration 

power lines, and septic tanks (Needle 

Mountain Power, LLC 2010, p. 11). We 

expect the construction of this facility to 

render at least 13,100 ac (5,300 ha) of 

Sonoran desert tortoise habitat as 

unusable because this type of 

construction requires the complete 

grading (removal of vegetation) of the 

project footprint. It could, therefore, 

significantly affect the Black Mountains 

desert tortoise population, especially in 

consideration of other effects acting in 

combination with those poised from the 

proposed housing development and 

highway construction in the immediate 

area (THS 2009, p. 4; ADOT 2010, p. 3; 

Mardian 2010, p. 1). The estimated loss 

of 13,100 ac (5,300 ha) constitutes an 

estimated loss of 0.05 percent of their 

habitat in Arizona; rangewide, 0.025 

percent. 

Other solar energy development and 

transmission corridors pose similar 

threats to the Sonoran desert tortoise as 

development and roadway projects (see 

discussion below). An average utilityscale solar facility to generate 250 

megawatts of electricity would occupy 

about 1,250 ac (500 ha) of land (BLM 

2009a, p. 1), and would involve removal 

of all vegetation within its footprint. 

Additionally, concentrating solar power 

facilities requires liquids such as oils or 

molten salts to create steam to power 

conventional turbines and generators, as 

well as various industrial fluids, such as 

hydraulic fluids, coolants, and 

lubricants, all of which may present a 

contaminant risk should these fluids 

leak onto the ground (Scott 2009, p. 12). 

New transmission lines would need to 

be built for these facilities, as well as 

roads to maintain the facilities, posing 

additional threats to the Sonoran desert 

tortoise through the destruction or 

contamination of remaining habitat and 

increased potential for road-kill 

mortality. 

In conclusion, the literature 

documents that urban development and 

population growth in Arizona and 

Sonora has been remarkable, and no 

information is available to suggest these 

trends will not continue into the 

foreseeable future. Sonoran desert 

tortoise habitat is permanently lost 

where urban development occurs. 

Sonoran desert tortoises and their 

habitats that occur adjacent to 

developed areas are also threatened by 

the increased incidence of an array of 

human activities or influences such as 

off-highway vehicle use, facilitation of 

the spread of nonnative plant species 

via soil disturbances, and increased 

wildfire ignitions. These threats act in 

combination with other threats 

discussed elsewhere in this finding, 

including ironwood and mesquite tree 

harvest, livestock grazing, nonnative 

plants and altered fire regimes, roads 

and highways, and undocumented 

human immigration and interdiction. 

Development as a Barrier 

Urban development, canals, and 

transportation infrastructure, such as 

roads and railroads, disrupt ecological 

processes, increase mortality in animals, 

promote the degradation, loss, and 

isolation of wildlife habitat, and cause 

fragmentation of populations (Spang et 

al. 1988, p. 9; Saunders et al. 1991, pp. 

23–24; Averill-Murray and Klug 2000, p. 

68; Seiler 2001, p. 3; Howland and 

Rorabaugh 2002, p. 335; Edwards et al. 

2004, p. 496). Sonoran desert tortoise 

populations are island-like in their 

distribution, meaning they are generally 

concentrated on the bajadas and 

hillsides of mountains, and lessdistributed within the valleys between 

these areas. As a result, they may be 

particularly vulnerable to large-scale 

disturbances that affect the suitability of 

intervening habitat (Spang et al. 1988, p. 

9). Factors that affect inter-population 

dynamics in Sonoran desert tortoises 

include distance between populations, 

physical size of habitat areas, sizes of 

source populations, and the ease of 

which intervening areas can be crossed 

by dispersing individuals (Howland and 

Rorabaugh 2002, p. 335). 

The effect of potential barriers to 

inter-population movements of Sonoran 

desert tortoises (discussed above in the 

Species Information section) is not 

equal across their range. The ability for 

the Sonoran desert tortoise to move 

among populations is also important for 

allowing shifts in their range in 

response to climate change, and to 

promote recolonization after fire or 

other regional disturbances (Beier and 

Majka 2006, p. 2). Dispersal of Sonoran 

desert tortoises between populations 

through sparse desertscrub is less likely 

in very hot, dry valleys in the Lower 

Colorado subdivision of Sonoran 

desertscrub and populations in 

mountain ranges, such as the Eagletails, 

Maricopas, and Sand Tanks, have likely 

been existing in isolation for a long time 

(Van Devender 2002a, p. 16). 

Genetic analysis of blood samples 

collected from Sonoran desert tortoises 

in Saguaro National Park in Pima 

County, Arizona, suggest that 

intermediate gene flow still occurs, or 

occurred recently, among isolated 

populations at the rate of at least 1 

migrant per generation (12–15 years) 

(Edwards et al. 2004, p. 485). However, 

thousands of acres of tortoise habitat 

have been recently lost to large 

residential developments in the foothills 

of the Santa Catalina, Tortolita, Rincon, 

and Tucson Mountains in the greater 

Tucson metropolitan area (Edwards et 

al. 2004, p. 485). 

The importance of allowing 

movement of individual tortoises 

between populations is observable by 

evaluating historical gene flow. Edwards 

et al. (2004, p. 485) used seven 

microsatellite DNA markers to examine 

the genetic relationships of tortoises in 

eight populations in southern and 

central Arizona, in the vicinity of 

Tucson and Phoenix. They also 

calculated migration rates among these 

populations to estimate historical rates 

of gene flow, and, therefore, the 

importance of individuals moving 

between populations (Edwards et al. 

2004, p. 485). Edwards et al. (2004, p. 

496) found no evidence of recent loss of 

genetic diversity that would indicate 

genetic bottlenecking that could occur 

from lack of mixing among Sonoran 

desert tortoise populations in southern 

Arizona. However, the authors 

acknowledged that a small sample size 

and small number of genetic markers 

(alleles) used in their analyses would 

likely not detect this genetic effect. 

Despite reduced mixing among 

populations, Sonoran desert tortoises 

may be capable of maintaining small 

effective population sizes (still viable 

populations, despite small size), even 

with a low degree of genetic diversity 

(Edwards et al. 2004, p. 496). However, 

Edwards et al. (2004, p. 496) also stated, 

‘‘Because effective population sizes of 

Sonoran desert tortoises are small, 

dispersal events probably play an 

important role in the long-term 

maintenance of these populations.’’ This 

suggests that while dispersal and 

movement of tortoises may be rare, they 

may be important events. Therefore, 

barriers that prevent this movement 

could result in significant genetic 

impacts, by preventing mixing of 

populations over the long term. 

The effect of urban barriers limits 

inter-population movements of Sonoran 

desert tortoises resulting in ‘‘closed’’ 

populations. Experts believe that an 

isolated population of Sonoran desert 

tortoises that experiences significant 

declines in population size could not 

overcome losses simply through an 

increase in reproduction, based on 

evidence of past gene flow (Edwards et 

al. 2004, p. 496). Therefore, if a 


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population were to experience a 

catastrophic decline as a result of a 

stochastic event such as drought, the 

immigration of new tortoises from 

adjacent populations would be 

necessary for population recovery 

(Edwards et al. 2004, p. 496). Urban 

barriers effectively prevent this 

immigration of new tortoises, resulting 

in closed, or isolated, Sonoran desert 

tortoise populations, which are now 

evident within the metropolitan areas of 

Phoenix and Tucson. Mountains and 

associated foothills with Sonoran 

desertscrub habitat occur in these urban 

areas, and although development within 

this habitat has been restricted by 

zoning laws, development is still 

allowed to virtually surround the bases 

of the mountains, isolating tortoise 

populations. Examples of this 

development include the Union Hills, 

White Tank Mountains, McDowell 

Mountains, Black Mountains, and South 

Mountain Park in the Phoenix 

metropolitan area and Tumamoc Hill, 

Tucson Mountains, and Saguaro 

National Park West in the Tucson 

metropolitan area (Edwards et al. 2004, 

p. 496). Zylstra and Swann (2009, pp. 

10–11) remarked that the increasing 

negative effect of human-made barriers 

on Sonoran desert tortoise movements 

between populations may require 

translocation (moving animals out of 

harm’s way into more secured areas of 

suitable habitat), or occasional 

augmentation of populations with 

tortoises from other populations, to 

remain viable. 

Translocation has been considered an 

option, and implemented to some 

degree for Mojave desert tortoise 

conservation and recovery. In assessing 

the viability of translocation as a 

recovery and conservation tool for the 

Mojave population, concern has been 

expressed for potentially moving 

tortoises into areas where threats to 

desert tortoise populations remain, 

which could negate any conservation 

value associated with the action. Our 

(Mojave) Desert Tortoise Recovery 

Office stresses that translocation of 

tortoises should not occur under such 

circumstances, emphasizing the need to 

address threats which impact all 

tortoises regardless of origin. 

Translocation of desert tortoises has 

received mixed reviews in the scientific 

literature and, as noted, may not be a 

viable option for the Sonoran desert 

tortoise. There are several factors that 

must be considered in deciding whether 

or not to translocate tortoises into new 

areas, including temporary or longerterm holding conditions of tortoises; the 

propensity for post-release, longdistance movements; drought; the status 

of receiving population; and disease 

screening, among other factors (Berry 

1986a, p. 113; Field et al. 2007, pp. 232, 

237, 240, 242; Martel et al. 2009, p. 218). 

Translocated Mojave desert tortoises 

have been shown to settle at release 

sites, travel in straight lines for 

substantial distances, or disperse up to 

approximately 4 miles (6.4 km) (Berry 

1986a, p. 113). Translocated desert 

tortoises may disrupt social hierarchies 

in receiving populations by displacing 

residents or they may be displaced 

themselves (Berry 1986a, p. 113). 

Howland and Rorabaugh (2002, p. 341) 

suggest that translocation of Sonoran 

desert tortoises may not be a viable tool 

for conservation because most intact 

Sonoran desert tortoise populations in 

Arizona are currently considered 

relatively healthy, and likely occur at or 

near carrying capacity. Mullen and Ross 

(1997, pp. 145–146) found that 

translocated Mojave desert tortoises 

have a lower survivorship than resident 

individuals (especially when moved 

during the summer versus during the 

spring), but that negative effects 

commonly associated with 

translocations are generally short-lived 

(1–2 years). 

A 2004 population viability analysis 

for the Mojave desert tortoise 

recommended that a minimum of 

50,000 individuals are required for a 50 

percent chance of persistence for 500 

years, yet extrapolation of Sonoran 

desert tortoise population data from 

southern Arizona suggest that most 

populations number less than 20,000 

individuals, with some as low as several 

hundred (Edwards et al. 2004, p. 496). 

Because the average generation time of 

a Sonoran desert tortoise is 

approximately 12–15 years and much of 

the urban development is relatively 

recent, the full effect of developments as 

barriers to genetic exchange among 

Sonoran desert tortoise populations 

cannot be fully assessed at this time 

(Edwards et al. 2004, p. 486). Edwards 

et al. (2004, p. 495) further cautioned 

that their estimates of gene flow are 

contingent on what occurred presettlement, and should not be taken as 

evidence that natural immigration or 

emigration still occurs. 

In conclusion, the literature 

documents that urban development and 

population growth, roads and highways, 

canals, railroad tracks, and other types 

of development threaten the Sonoran 

desert tortoise by creating barriers to 

movement in Arizona and, perhaps to a 

lesser extent, in Sonora, Mexico. The 

creation of barriers affects the tortoises’ 

genetic exchange capacity within and 

between populations, which in turn 

affects their ability to recolonize habitat 

in the event of population declines or 

extirpations, and may lead to isolation 

and eventual genetic bottlenecking. This 

threat acts synergistically with other 

factors as discussed above. 

Off-Highway Vehicles 

Off-highway vehicle use may pose a 

variety of threats to the suitability of 

habitat within the range of the Sonoran 

desert tortoise. Off-highway vehicle use 

in Sonoran desert tortoise habitat can 

result in damage to soil, riparian areas, 

wetlands, water quality, and air quality. 

This damage occurs due to reduced 

vegetation cover and growth rates, soil 

compaction, diminished water 

infiltration, diminished presence and 

impaired function of soil stabilizers 

(biotic and abiotic soil crusts), noise, 

wildlife habitat fragmentation, spread of 

invasive plant species, and accelerated 

erosion rates (Boarman 2002, pp. 43–51; 

Ouren et al. 2007, pp. 5, 11; USGAO 

2009, pp. 10, 13; Vega 2010, p. 3). Offhighway vehicle use in Sonoran desert 

tortoise habitat can also potentially 

affect Sonoran desert tortoises directly 

by crushing individuals or their burrows 

(Boarman 2002, pp. 43–51). 

Off-highway vehicle use has grown 

considerably in Arizona. Between 21 

and 56 percent of Arizona residents 

(depending on the county in Arizona) 

consider themselves off-highway 

vehicle users as of 1999, and projected 

increases in population growth are 

expected to increase recreation on 

public lands, in particular off-highway 

vehicle use (AIDTT 2000, p. 10). As of 

2007, 385,000 off-highway vehicles 

were registered in Arizona (a 350 

percent increase since 1998), and 1.7 

million people (29 percent of the 

Arizona’s public) engaged in off-road 

activity from 2005–2007 (Sacco, pers. 

comm., 2007). Over half of off-highway 

vehicle users reported that merely 

driving off-road was their primary 

activity, versus using the off-highway 

vehicle for the purpose of hunting, 

fishing, or hiking (Sacco, pers. comm., 

2007). The BLM (USBLM 2001, p. 1) 

stated that interest in off-highway 

vehicle use has increased substantially 

in recent years and cited several 

reasons, such as urban growth in the 

west, improved capabilities of offhighway vehicles in accessing 

previously inaccessible areas, and 

greater public interest in unconfined 

outdoor recreational opportunities. 

The Forest Service stated that ‘‘the 

number of off-highway vehicle users has 

climbed sevenfold in the past 28 years, 

from approximately 5 million in 1972 to 

36 million in 2000’’ (USFS 2009, p. 2). 

The Tonto National Forest, which 

encompasses a considerable amount of 


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Sonoran desert tortoise habitat, receives 

the highest off-highway vehicle use of 

any national forest nationwide, partially 

due to its close proximity to the Phoenix 

metropolitan area. The Arizona State 

Land Department recently closed to offhighway vehicle use many of their lands 

in Maricopa County (which includes 

Phoenix), to control dust pollution, 

which appears to have shifted offhighway vehicle access to the nearby 

Tonto National Forest (USFS 2009, p. 2; 

USGAO 2009, p. 11). The Tonto 

National Forest has indicated that soil 

erosion appears to be the most 

significant result from off-highway 

vehicle use on their lands and identified 

‘‘unmanaged recreation’’ (off-highway 

vehicle use) as one of four key threats 

to soil, water, and wildlife habitat 

(USFS 2009, p. 1; USGAO 2009, pp. 10, 

13). 

Off-highway vehicle use is 

widespread across Arizona, occurring 

on Forest Service, BLM, private, tribal, 

and State Trust lands, and has been 

documented on all 17 Sonoran desert 

tortoise monitoring plots. Pronounced 

effects are found on the Four Peaks and 

Wickenburg Mountains plots, which are 

near urbanized areas (greater Phoenix 

and Wickenburg, respectively) (AGFD 

2010, p. 13). 

The Tonto National Forest has 

proposed to designate approximately 

800 mi (1,287 km) of roads as open for 

use, and close 280 mi (451 km) of roads 

which are currently open (due to 

significant resource damage). This is a 

net increase of 520 mi (837 km) of offhighway vehicle trails and roads on the 

Tonto National Forest (USFS 2009, p. 3). 

In addition, the Tonto National Forest 

has proposed the designation of five 

more off-highway vehicle areas 

(representing 2,799 ac (1,132 ha) 

collectively, or 0.01 percent of its 

habitat in Arizona) within Sonoran 

desert tortoise habitat on the Mesa and 

Globe Ranger Districts (USFS 2009, p. 

3). All other motorized travel not 

specifically designated will be 

prohibited by the Tonto National Forest 

except as authorized for dispersed 

camping access and big game retrieval 

(USFS 2009, p. 4). Because of the 

increase in off-highway vehicle access 

and subsequent use anticipated to occur 

on the Tonto National Forest, associated 

threats to the Sonoran desert tortoise 

and its habitat on the Forest are 

expected to increase in scope and 

magnitude in the immediate future. 

BLM regulations require their lands 

be designated as open, limited, or closed 

to off-highway vehicle use (USGAO 

2009, p. 7). As of March 2009, the BLM 

has nationally designated approximately 

32 percent of its lands as open to offhighway vehicle use, 48 percent as 

limited-use, 4 percent as closed, and 16 

percent of lands have yet to be 

designated (USGAO 2009, p. 7). These 

figures indicate that at least 80 percent 

of BLM lands allow for off-highway 

vehicle use in some capacity. However, 

we do not have specific information for 

BLM off-highway vehicle use in 

Arizona. The BLM is taking actions to 

help manage off-highway vehicle use on 

their lands. 

Historically, competitive off-highway 

vehicle racing events have occurred on 

a comparatively infrequent basis in 

Arizona. On BLM lands in Arizona, 

these activities are generally restricted 

from March 31 to October 15, in 

consideration of potential surface 

activity of Sonoran desert tortoises 

(USBLM 2010, p. 4). However, similar 

considerations may not occur with 

respect to these events on lands 

managed by other agencies, thus making 

their lands more desirable for planning 

such events. For example, a Special 

Land Use Permit application was 

recently submitted to the Arizona State 

Land Department for the establishment 

of a semiannual competitive offhighway vehicle race within Sonoran 

desert tortoise habitat, slightly north of 

Tucson near Mammoth, Arizona (Vega 

2010, pp. 1–16). 

Competitive off-highway vehicle 

events can have a variety of detrimental 

effects on Sonoran desert tortoises or 

their habitat. Event courses have been 

found to create new destinations for 

increased, year-long use, and 

correspondingly greater impacts to local 

Sonoran desert tortoise habitats and 

higher incidence of illegal route 

proliferation (Vega 2010, p. 3). The high 

rates of speed associated with 

competitive off-highway vehicle events 

significantly increase the likelihood for 

damage to burrows or other habitat 

features (Vega 2010, p. 4). Lastly, event 

spectators seeking good views have been 

found to park their vehicles 

indiscriminately along the race course 

without regard to vegetation and may 

crush Sonoran desert tortoises and their 

burrows, or start wildfires if parked over 

dry vegetation (Vega 2010, p. 5). 

In his literature review, Boarman 

(2002a, p. 50) found that, as of 2002, 

most research on the effect of offhighway vehicles had been performed 

in areas of high off-highway vehicle use 

within the Mojave desert tortoise 

distribution. As a result, there are fewer 

available data for lightly-traveled areas 

(Boarman 2002, p. 50). 

On the Florence Military Reservation, 

Grandmaison et al. (in prep., p. 16) 

found that Sonoran desert tortoises use 

infrequently traveled gravel roads as 

movement corridors within their home 

ranges, placing individuals at greater 

risk of mortality from collisions with 

off-highway vehicles. Populations that 

occur in similar areas throughout their 

distribution may also be vulnerable to 

mortality associated with collisions, or 

previously discussed indirect effects to 

their habitat from off-highway vehicle 

use. 

Effects of off-highway vehicle use on 

Sonoran desert tortoises are likely to be 

more significant within washes that 

separate steep slopes and rocky bajadas 

used by Sonoran desert tortoises, where 

tortoises are known to frequent and offhighway vehicle use often occurs 

(AGFD 2010, p. 13). For example, ‘‘rock 

crawling’’ (technical off-roading usually 

with highly-modified, high clearance, 

four-wheel drive vehicles), generally 

occurs in boulder-strewn washes where 

Sonoran desert tortoises are most likely 

to inhabit. This activity may be 

uniquely destructive to Sonoran desert 

tortoise habitat because: (1) It occurs on 

steep slopes and rocky bajadas within 

Arizona Upland Sonoran desertscrub 

where populations reach their highest 

densities; and, (2) the intent of rock 

crawling is to aggressively challenge 

aspects of a given landscape that would 

otherwise clearly represent barriers to 

overland travel, which places habitat 

and tortoises at greater risk. However, 

rock crawling activity is presumed to be 

less popular an activity than more 

conventional off-highway vehicle use 

and, therefore, likely affects a much 

smaller percentage of Sonoran desert 

tortoise habitat. 

Bury (1987, p. 1) studied the effects of 

off-highway vehicle use on Mojave 

desert tortoises in Mojave desertscrub 

habitat. Some of his findings included a 

60 percent reduction in perennial plant 

cover, 1.3 desert tortoises per hectare 

(2.47 ac) in a control plot in which offhighway vehicles were excluded, versus 

0.3 desert tortoises in an area used by 

off-highway vehicles, and four times the 

number of active burrows in the control 

plot versus the off-highway vehicle area 

(Bury 1987, p. 1). Bury and Luckenbach 

(2002, p. 257) found that there were 1.3 

times more live plants, 3.9 times more 

plant cover, 3.9 times the number of 

Mojave desert tortoises, and four times 

the number of active burrows in 

undisturbed Mojave desertscrub as 

compared to areas where off-highway 

vehicles were used. We are not certain 

whether the areas studied by Bury 

(1987, p. 1) and Bury and Luckenbach 

(2002, p. 257) were unregulated, or 

regulated areas with designated routes, 

but similar effects to Sonoran desert 

tortoises and their habitat can be 

expected in areas of high off-road 


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vehicle use in Sonoran and Mojave 

desertscrub habitat within Arizona, 

particularly in areas of higher 

accessibility (such as valley bottoms and 

lower foothills), such as the Florence 

Military Reservation in Pinal County 

(AIDTT 2000, p. 34; Lutz et al. 2005; p. 

22; AGFD 2010, p. 7; Grandmaison et al. 

in press, p. 4). 

Brooks and Lair (2005, pp. 7–8) found 

that, in Mojave desertscrub, off-highway 

vehicle routes can cause a myriad of 

effects including: (1) Altering 

precipitation runoff patterns which 

promote increased erosion; (2) 

producing air-borne pollutants laden 

with heavy metals that affect habitat at 

distances ranging from 65 to 650 feet (20 

to 200 m) from the road; (3) increasing 

nitrogen deposition in soils, thereby 

favoring nonnative plant invasions; and 

(4) providing a pathway for nonnative 

plant species invasions. These impacts 

degrade Sonoran desert tortoise habitat 

as well as their forage base. 

Soil disturbance from off-highway 

vehicle use, development projects, and 

other activities can facilitate the 

invasion of nonnative plant species by 

eliminating competition and creating a 

rougher soil surface for seeds to lodge 

and germinate (Hobbs and Huenneke 

1992, pp. 329–330). Motorized and 

mechanical vehicles aid in the dispersal 

of plants by transporting seeds of both 

native and nonnative plant species. Rew 

and Pollnac (2010, p. 2) found that 

trucks and sport utility vehicles driven 

off road in dry conditions can pick up 

as many as 176 seeds from 50 mi (80 

km) of driving, and recreational offhighway vehicles can pick up as many 

as 200,000 seeds in 48 mi (77 km) of offroad driving. Off-highway vehicles are 

generally transported via trailer from 

site to site and may spread nonnative 

plant species in subsequent uses. Offhighway vehicle use has also been 

shown to create edge effects along trails 

that generate dust, blanketing adjacent 

vegetation, and inhibiting plant growth 

rates, size, and survivorship, all of 

which affect the forage base and 

available cover for Sonoran desert 

tortoises (Ouren et al. 2007, p. 11). 

We have documented that offhighway vehicle use poses a threat to 

the Sonoran desert tortoise and its 

habitat in Arizona because it damages 

soil, reduces vegetation cover and 

growth rates, leads to soil compaction, 

diminishes water infiltration, 

diminishes the presence and impairs the 

function of soil stabilizers (biotic and 

abiotic soil crusts), fragments habitat, 

facilitates the spread of nonnative plant 

species, ignites wildfire, accelerates soil 

erosion, enhances the potential for 

illegal collection (discussed below), and 

may crush or injure Sonoran desert 

tortoises (also discussed below). In 

addition, we have documented the 

tremendous growth in popularity of offhighway vehicle use in Arizona, as well 

as compliance deficiencies in offhighway vehicle licensing programs 

(and therefore deficient fees collected 

that are intended to fund enforcement 

and environmental mitigation) and 

enforcement programs (discussed above 

and below). This threat acts 

synergistically with other threats 

discussed herein. Considering the 

population growth estimates we have 

documented above for Arizona, we 

believe that the popularity of offhighway vehicle use will continue to 

grow, leading to an increase in severity 

and geographic extent of impacts across 

the distribution of the Sonoran desert 

tortoise in Arizona over time. 

Roads and Highways 

Foreman (2002, p. 35) estimated that 

at least 20 percent of land in the United 

States has been ecologically affected by 

roads. Roads and highways might also 

adversely affect Sonoran desert tortoises 

as they do Mojave desert tortoises. 

Studies of Mojave desert tortoises 

suggest that effects include providing 

human access to occupied habitat, 

facilitating the spread of nonnative 

plant species, altering movement 

patterns, enhancing the genetic 

fragmentation effect between 

populations of Sonoran desert tortoises 

by acting as barriers, and contaminating 

adjacent habitat (Boarman and Sazaki 

1996, p. 1; Forman and Alexander 1998, 

p. 207; Boarman 2002, pp. 54–55; 

Edwards et al. 2004, pp. 495, 497; 

Boarman and Sazaki 2006, p. 95; 

Andrews et al. 2008, pp. 127, 129–130; 

Rew and Pollnac 2010, p. 2). Roads that 

act as barriers to genetic exchange 

between Sonoran desert tortoise 

populations may increase the risk of 

inbreeding depression and population 

extirpation (Boarman and Sazaki 2006, 

p. 95). In one example, biological 

connectivity between Sonoran desert 

tortoise populations of the Harquahala 

and Wickenburg Mountains is 

significantly limited due to several 

barriers to tortoise movement including 

highways U.S. 60 and U.S. 93, the 

Burlington Northern Santa Fe Railroad, 

and urban development, and would be 

further limited by the proposed 

Wickenburg bypass highways which are 

in the planning phase (Beier et al. 

2006d, p. vi). 

The use of dirt or gravel roads by 

vehicles generates dust which may 

adversely affect physiological processes 

of adjacent plants and reduce overall 

primary productivity, whereby affecting 

the amount and quality of available 

forage vegetation for Sonoran desert 

tortoises (Sharifi et al. 1997, pp. 844– 

845). 

Construction of major highways 

planned in Arizona has the potential to 

greatly affect certain Sonoran desert 

tortoise populations. For example, the 

Arizona Department of Transportation 

(ADOT) has proposed rerouting State 

Route 95 through the southern and 

eastern bajada of the Black Mountains in 

Mohave County, Arizona (Jacobs 

Engineering Group, Inc. 2009, pp. 24, 

33; ADOT 2010, p. 3; Goodman 2010, 

pp. 3–4). The proposed realignment of 

State Route 95 is expected to pass 

directly through 30 mi (48 km) of a 

Sonoran desert tortoise population (THS 

2009, p. 4; Goodman 2010, pp. 3–4). We 

expect this new four-lane highway to 

eliminate considerable amounts of 

Sonoran desert tortoise habitat, become 

a significant source of mortality, and 

threaten the continued viability of the 

Black Mountains habitat to support the 

population of the Sonoran desert 

tortoise there, if appropriate mitigation 

measures are not enacted or are 

ineffective. 

Both the ADOT and the Federal 

Highways Administration participate in 

the BLM’s tortoise mitigation program 

and provide funding for the acquisition 

of Sonoran desert tortoise habitat using 

compensation rates prescribed for in the 

BLM’s mitigation policy (ADOT 2010, p. 

3). Compensation rates for disturbances 

in Category I or II habitat are 3–6:1 and 

2–5:1, respectively (USBLM 2009, p. 

18). To date, 584 ac (236 ha) of Sonoran 

desert tortoise habitat have been 

acquired through this program with 

ADOT and Federal Highways 

Administration. Another 98 ac (40 ha) 

are scheduled to be acquired as a result 

of the proposed rerouting of U.S. 

Highway 95 through the Black 

Mountains of Mohave County (ADOT 

2010, p. 3). 

Considerable planning efforts for 

future road and highway development 

in Arizona have been afforded to the 

preservation of wildlife corridors, or 

‘‘linkages.’’ Linkage design plans have 

been completed for several biological 

corridor areas in Arizona where 

Sonoran desert tortoises may be 

threatened by construction and 

development activities that could 

become barriers to movement between 

populations (Beier and Majka 2006, pp. 

1–81; Beier et al. 2006a, pp. 1–189; 

2006b, pp. 1–151; 2006c, pp. 1–88; 

2006d, pp. 1–97; 2006e, pp. 1–135). 

These linkage design plans are specific 

to both individual corridors that may be 

affected throughout Arizona, and to 

species (including the Sonoran desert 


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tortoise) chosen as representative ‘‘focal 

species’’ in each individual assessment 

(Beier and Majka 2006, pp. 1–81; Beier 

et al. 2006a, pp. 1–189; 2006b, pp. 1– 

151; 2006c, pp. 1–88; 2006d, pp. 1–97; 

2006e, pp. 1–135). 

In one example, a series of voluntary 

conservation recommendations were 

proposed in Beier et al. (2006c, pp. 15– 

16; 2006e, pp. 14–15) to mitigate effects 

of major roadways, such as U.S. 

Highway 60 which traverses Sonoran 

desert tortoise habitat in Pinal and Gila 

Counties, Arizona. However, the 

Sonoran desert tortoise was not afforded 

consideration in all projects. For 

example, Sonoran desert tortoise 

populations in Rincon and Santa Rita 

mountains in eastern Pima County, 

Arizona, are adversely affected by 

Interstate 10 and State Highway 83 

(known barriers to tortoise movement), 

yet were not addressed in the RinconSanta Rita-Whetstone linkage design 

plan (Beier et al. 2006a, pp. i–ii). In 

another example, the Sonoran desert 

tortoise was not afforded any 

consideration in the Santa RitaTumacacori linkage design plan, despite 

the likely adverse effects by Interstate 

19, a known barrier to movement 

between populations located in the 

Santa Rita and the Atascosa-PajaritoTumacacori mountains complex in 

southern Santa Cruz County, Arizona 

(Beier et al. 2006b, pp. i–ii). While some 

highways have associated structures 

that prevent or funnel tortoises to 

underground crossings, several 

populations are still affected by barriers 

to movement from major roads and 

highways that have no such structures. 

In our review of the literature, we 

have documented that roads and 

highways pose a threat to Sonoran 

desert tortoises in Arizona because they 

form barriers to movement, whether 

through direct mortality from vehicles 

or from avoidance of roads by tortoises. 

The effects associated with barriers are 

described in detail in the ‘‘Development 

as a Barrier’’ section above. While 

several roads or highways have 

associated tortoise fencing and or 

culverts to prevent road-kill of tortoises 

and facilitate safe movement, studies 

have shown that these devices are often 

not maintained and, therefore, become 

ineffective over time in achieving their 

desired goal. This threat also acts 

synergistically with other influences 

discussed herein. 

Ironwood and Mesquite Harvest 

The harvest of mesquite and 

ironwood trees for charcoal production 

and use in wood carvings adversely 

affects Sonoran desertscrub habitat in 

Mexico, both historically and more 

recently (Bahre 1991, pp. 143–146). The 

harvest of mature mesquites from 

Mexico’s Sonoran desertscrub habitat 

permanently alters desert ecosystems 

because these leguminous (bearing seed 

pods similar to pea or bean plants) trees 

are important anchors for these systems 

and their associated flora and fauna 

(Taylor 2006, p. 8). More than 200 plant 

and animal species depend on mesquite 

trees in northern Mexico for survival 

and reproduction (American University 

Database 2010, p. 1). Mesquite and 

ironwood trees are ecologically 

important to Sonoran desert habitat as 

they serve as nursery plants (i.e., aiding 

in dispersal, germination, seedling 

development, and survival) for other 

plant species used as forage for desert 

tortoises, and provide valuable shade for 

temporary shelter sites for Sonoran 

desert tortoises (American University 

Database 2010, p. 2). In areas where 

harvest has been concentrated, the loss 

of mesquite trees results in the loss of 

organic matter, fixed nitrogen, and 

sulfur and soluble salts, affecting overall 

habitat quality and quantity (Rodriguez 

Franco and Maldonado Aguirre 1996, p. 

47). 

The demand for mesquite wood, used 

for cooking, has increased in the 

Sonoran Desert region of northern 

Mexico; one million ac (400,000 ha) 

have been cleared of mesquite to meet 

these growing demands (American 

University Database 2010, p. 1). The 

modification of one million ac 

contributes to the degradation or 

possible loss of 4 percent of tortoise 

habitat in Mexico; rangewide, 2 percent. 

Ironwood trees are also being harvested 

in the Sonoran desert of northern 

Mexico, where it is cherished for its 

hardness and carving potential in Seri 

Indian artwork (American University 

Database 2010, p. 2). The accelerated 

rate of legume tree depletion for 

charcoal and carvings in Sonora has 

affected the health of ironwood 

populations and associated 

communities (Suzan et al. 1997, p. 955). 

This is evidenced by an increased 

number of damaged and dying trees, as 

well as generally small size classes for 

sampled areas (Suzan et al. 1997, pp. 

950–955). In the Sonoyta region of 

northern Sonora, more than 478,000 ac 

(193,000 ha) have been affected by 

deforestation related to charcoal 

production, brick foundries, tourist 

crafts, and pasture conversion (Nabhan 

and Suzan 1994, p. 64). The 

modification of 478,000 ac (193,000 ha) 

contributes to the degradation or 

possible loss of an estimated 2 percent 

of their habitat in Mexico; rangewide, 1 

percent. 

Pressure for fuel wood and crafts 

materials has been so intense in Mexico 

south of Organ Pipe Cactus National 

Monument that wood harvest, 

especially ironwood, has been detected 

more than a third of a mile inside the 

boundary of the Monument, as supplies 

have been decimated south of the border 

(Suzan et al. 1999, p. 1499). The 

structure of Sonoran desert tortoise 

habitat in both washes and upland 

habitats in the Monument boundary has 

been affected by this harvest (Suzan et 

al. 1999, p. 1499). 

In conclusion, the literature 

documents that harvest of ironwood and 

mesquite trees has degraded Sonoran 

desert tortoise habitat in Mexico, 

primarily, by the loss of organic matter, 

fixed nitrogen, and sulfur and soluble 

salts, affecting overall habitat quality 

and quantity, which collectively and 

indirectly affect the forage base and 

protective cover for Sonoran desert 

tortoises in as much as 4 percent of its 

range in Mexico. This threat acts in 

combination with other threats that 

affect Sonoran desert tortoise 

populations in Mexico discussed in this 

finding. 

Livestock Grazing 

Sonoran desert tortoises, livestock, 

and wild burros potentially share 

habitat throughout their distribution in 

Arizona, with the exception of lands 

managed by the U.S. Fish and Wildlife 

Service or National Park Service. Wild 

burro herds range across millions of 

acres of Sonoran desert tortoise habitat 

in Arizona, predominantly on BLM 

lands northwest of Phoenix, although 

the literature is generally lacking in 

analysis of potential effects of wild 

burros on Sonoran desert tortoise 

populations or habitat (AIDTT 2000, p. 

21). 

The Mexican government has 

designated over 5 million ac (2 million 

ha) of Sonoran desertscrub for 

conversion into grasslands for livestock 

production (American University 

Database 2010, p. 1). Sonoran desert 

tortoises are not found in grasslands, 

and this habitat type is not considered 

suitable for the species. The loss of 5 

million ac (2 million ha) would 

constitute an estimated loss of 20 

percent of their habitat in Mexico; 

rangewide, 10 percent. Livestock 

grazing began to expand and modernize 

in its extent and distribution in Sonora, 

Mexico, in 1950, when land considered 

unsuitable for agriculture was 

subsequently used for livestock grazing 

(Hawks 2003, p. 3). During this time, 

new bulls were introduced throughout 

ranching operations to improve herd 

genetics, and artificial seeding of 


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pastures also commenced at this time 

(Hawks 2003, p. 3). By 1970, buffelgrass 

was the chosen seed for artificial range 

supplementation for a growing rural 

livestock industry, and pastures were 

seeded with the species throughout 

Sonora, Mexico. In Sonora, buffelgrass 

has trended towards a monoculture in 

many areas, and changed the fire regime 

to the detriment of native vegetation 

(Hawks 2003, p. 4). We discuss the 

threat of nonnative plant species such as 

buffelgrass in the ‘‘Nonnative Plant 

Species and Altered Fire Regimes’’ 

section above. 

Livestock stocking rates in Sonora 

have been documented at 2–5 times the 

recommended rate for resource 

sustainability (Walker and PavlakovichKochi 2003, p. 14; University of Arizona 

2010, p. 2). Rorabaugh (2008, p. 25) 

found that livestock grazing ‘‘* * * is 

probably the most widespread human 

use of Sonora’s landscapes’’ and that 

rangelands in Sonora are often heavily 

grazed, with effects most apparent 

during periods of drought. Livestock 

production in Mexico is concentrated in 

the northern states, and the numbers of 

livestock have grown from 10 million in 

1940, to 37.5 million in 1983, largely 

due to the proximity to the United 

States, the major importer of Mexican 

cattle and beef (Stoleson et al. 2005, p. 

60). In Sonora, 79 percent of agricultural 

and rangelands are devoted to livestock 

production (Stoleson et al. 2005, p. 60). 

Effects of poorly-managed livestock 

grazing observed in Sonora include 

changes in plant species composition 

and vegetation cover and structure, soil 

compaction, erosion, altered fire 

regimes, and nonnative plant species 

introductions and invasions (Stoleson et 

al. 2005, pp. 61–62). 

In the United States, however, 

permitted levels of livestock grazing 

have been reduced to 10 percent of 

historical levels (Bostick 1990, p. 149). 

Potential effects of livestock grazing in 

desertscrub habitat received significant 

treatment in the literature, with varied 

scientific conclusions. Fleischner (1994, 

p. 631) listed specific attributes of 

ecosystems, such as composition, 

function, and structure, as vulnerable to 

the effects of livestock management 

through a variety of mechanisms 

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 (Waser 

and Price 1981, pp. 409–410). In Mojave 

desertscrub, livestock grazing can 

increase soil compaction and decrease 

water absorption, thereby reducing 

water availability to potential Sonoran 

desert tortoise forage species and 

subsequently reducing available forage 

(Boarman 2002, p. 30). Oldemeyer 

(1994, pp. 100–101) commented that 

there remains much uncertainty on the 

exact effects of livestock grazing on 

desert tortoises. Meyer et al. (2010, p. 

42) suggested that the effects of 

livestock grazing on Sonoran desert 

tortoises should be placed in the context 

of a grazing regime, effective 

precipitation, habitat type, topography, 

Sonoran desert tortoise behavior, and 

habitat requirements. Loeser et al. 

(2007, pp. 93–96) suggested that 

climatic variation is key in determining 

the ecological effects of grazing 

practices in arid rangelands. 

The effects of soil compaction on 

desertscrub vegetation have been 

analyzed. In Mojave desertscrub where 

Sonoran desert tortoises also occur, 

Adams et al. (1982, p. 167) found that 

soil strength of drying compacted soils 

increased at a greater rate than noncompacted soils, and that even minor 

compaction produced similar effects to 

soil strength. Soil strength was found to 

be inversely proportionate to production 

of summer annual grass species (Adams 

et al. 1982, p. 167). Plant species with 

taproots appeared more vulnerable to 

the effects of soil compaction whereas 

fibrous root systems common in 

nonnative species such as Schismus 

spp. appeared less vulnerable, which 

indicates that root structure affects the 

response of plant species and that plant 

species respond differently to soil 

compaction, potentially favoring 

nonnative species in compacted soils 

(Adams et al. 1982, p. 174). 

While the Mojave and Sonoran desert 

tortoises differ to some degree in their 

biology and behavior, research on 

livestock grazing effects on Mojave 

desert tortoises or their habitat does 

have applicability to Sonoran desert 

tortoises (especially where Sonoran 

desert tortoises occupy Mojave 

desertscrub habitat and by virtue of the 

arid-land commonality), representing 

the best scientific information available. 

However, because Mojave desert 

tortoises typically occur in flat or 

gently-sloped terrain and construct 

earthen burrows in soil, they may be 

more susceptible to direct effects from 

livestock grazing. In comparison, 

Sonoran desert tortoises typically occur 

on steeper slopes and often construct 

burrows that are reinforced by boulders 

and, consequently, less susceptible to 

direct effects from livestock grazing. 

Observed effects of livestock grazing 

within Mojave desert tortoise habitat 

include dietary overlap and competition 

for food resources, destruction of 

vegetation structure used as temporary 

shelter sites, trampling of tortoises, 

collapsing of tortoise burrows, altering 

plant species composition by facilitating 

the invasion of nonnative plant species, 

and compaction of soil which may 

inhibit the construction of burrows 

(Avery and Neibergs 1997, p. 13). 

Boarman (2002a, p. 32) as well as Hobbs 

and Huenneke (1992, p. 329) found that 

livestock grazing can import nonnative 

plant propagules (seeds and other plant 

parts that may propagate) into native 

vegetation and subsequent physical 

alterations in vegetation structure and 

soil disturbance, such as trampling by 

livestock hoof-action, may increase 

germination rates of seeds through 

burying and compaction and provide 

microsites for establishment of 

nonnative plant species. 

Avery and Neibergs (1997, p. 13) 

compared Mojave desert tortoise habitat 

in both grazed and ungrazed areas 

(where buffelgrass was not intentionally 

planted), and found no significant 

differences in annual plant cover, 

biomass, or density between study 

areas. The densities and individual 

volumes of big galleta (Hilaria rigida), a 

perennial grass species, were greater in 

grazed habitat than within the grazing 

exclosure (Avery and Neibergs 1997, p. 

13). There was no significant difference 

in total cover of perennial plant species 

within study plots (Avery and Neibergs 

1997, p. 13). Avery and Neibergs (1997, 

p. 13) documented livestock nudging 

and rubbing Mojave desert tortoises, 

collapsing (potentially occupied) desert 

tortoise burrows, and destroying 

vegetation shading actively used 

burrows. The number of damaged and 

undamaged burrows in grazed habitat 

was equal, whereas the number of 

undamaged burrows in ungrazed habitat 

was significantly higher (Avery and 

Neibergs 1997, p. 18). Winter grazing 

appears to affect a higher proportion of 

actively used Mojave desert tortoise 

burrows. Indirect effects from burrow 

damage include increased risk of 

tortoise mortality, increased energy 

costs, and altered activity time budgets 

as a result of the need to construct new 

burrows (Avery and Neibergs 1997, p. 

19). The potential for livestock to 

damage Sonoran desert tortoise burrows 

on lower slopes not reinforced with 

granite boulders may be similar to the 

findings of Avery and Neibergs (1997, p. 

18), as almost 200 Sonoran desert 

tortoise burrows were recorded as 

trampled during a survey of the East 

Bajada plot in the Black Mountains of 


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Arizona in 1997 (Woodman et al. 1998, 

pp. 74–75). 

Some degree of overlap was observed 

in the forage plant preferences between 

Mojave desert tortoises and livestock, 

with both preferring green annual 

species when available, and most 

overlap occurring during the spring 

(Avery and Neibergs 1997, pp. 18–19). 

However, preferences began to diverge 

as spring and summer ensued, with 

Mojave desert tortoises preferring dried 

annuals, beavertail cactus (Opuntia 

basilaris), and stems and dried flowers 

of silver cholla (Opuntia echinocarpa), 

and livestock preferring California 

jointfir (Ephedra californica) and big 

galleta grass (Avery and Neibergs 1997, 

p. 18). We presume similar relationships 

between preferred forage species of 

livestock and Sonoran desert tortoises 

exist, because of their highly varied, and 

often opportunistic, foraging behavior as 

they take advantage of both summer and 

winter rainy seasons characteristic of 

the Sonoran desert. This precipitation 

pattern affords Sonoran desert tortoises 

greater access to standing water and, 

therefore, the ability to forage on a more 

varied forage base, compared to the 

Mojave desert tortoise. 

Studies have shown that livestock 

grazing may result in varying effects on 

plant species richness, composition, and 

density of the Sonoran desert tortoise 

forage base. Blydenstein et al. (1957, pp. 

523, 525) found that vegetation density 

in some perennial species can be 

affected by livestock grazing in Sonoran 

desertscrub, while species composition 

and annual plant species density were 

unaffected. Sixteen years of rest from 

livestock grazing in the desert grassland 

and oak woodlands in southeastern 

Pima County in Arizona (at the extreme 

periphery of the Sonoran desert tortoise 

range) showed increases in plant species 

richness and significant increases in 

canopy cover for midgrass, shortgrass, 

shrubs, and forbs (Brady et al. 1989, pp. 

285–287). However, there was no 

statistical difference in total vegetation 

cover between grazed land and rested 

land (Brady et al. 1989, pp. 285–287). 

Features that attract livestock to 

certain locations within an allotment 

may have pronounced effects on desert 

tortoises and their habitat. Livestock 

watering, supplemental feeding, or saltlick sites in desertscrub attract higher 

use by greater densities of livestock in 

arid environments. Effects to 

desertscrub habitat are commensurate 

with livestock use of these areas and 

decrease with increasing distance from 

these sources (Avery and Neibergs 1997, 

p. 19; Boarman 2002, p. 34). The density 

of certain nonnative plant species, such 

as Schismus spp., has also been 

positively correlated to distance to 

watering sites, while others, such as red 

brome, are negatively correlated (Brooks 

et al. 2006, p. 139). Native plant species 

cover and richness has been shown to 

decrease with increasing proximity to 

livestock waters (Brooks et al. 2006, pp. 

140–141). Brooks et al. (2006, p. 138) 

state that these effects can be 

anticipated from 164 to 656 ft (50 to 200 

m) from the edge of the watering site. 

Juvenile and adult Sonoran desert 

tortoises were frequently observed by 

Meyer (1993, pp. 101–102) using salt 

licks provided for livestock. Frequenting 

salt licks may benefit desert tortoises 

(especially hatchlings and small 

juveniles), but likely increases risk of 

being trampled by livestock because the 

salt licks can attract higher 

concentrations of both livestock and 

tortoises in actively grazed pastures. 

Based on the results of a study 

conducted by Balph and Malecheck 

(1985, p. 227), cattle avoid stepping on 

uneven surfaces. Desert tortoises will 

likely be perceived as an uneven ground 

surface, therefore, cattle may 

intentionally avoid stepping on them. 

Neff et al. (2005, p. 87) compared the 

effects to soil geology, geomorphology, 

and geochemical characteristics of 

biological soil crusts that had been 

disturbed, and the subsequent wind 

erosion due to livestock grazing, to an 

ungrazed area in arid lands of 

southeastern Utah. They found that 

‘‘* * * despite almost 30 years without 

livestock grazing, surface soils in the 

historically grazed sites have 38–43 

percent less silt, as well as 14–51 

percent less total elemental soil 

magnesium, sodium, phosphorus, and 

magnesium content relative to soils 

never exposed to livestock disturbances’’ 

and 60–70 percent declines in surface 

soil carbon and nitrogen reserves (Neff 

et al. 2005, p. 87). We are not certain to 

what extent the loss of these surface soil 

nutrients may affect the forage quality or 

quantity for Sonoran desert tortoises in 

arid habitat. Approximately 46 livestock 

grazing allotments on the Tonto 

National Forest partially or wholly 

overlap the potential range of the 

Sonoran desert tortoise, with several 

rated as having impaired or 

unsatisfactory soil conditions (AIDTT 

2000, p. 37). 

We observed several instances in the 

literature that discussed an inherent 

partitioning of land used by livestock 

and that used by Sonoran desert 

tortoises. Livestock often take the paths 

of least resistance and are unlikely to 

venture great distances from water. 

These behavioral traits of domestic 

livestock limit, to some degree, the 

potential effects from livestock grazing 

in Sonoran desert habitat, as livestock 

are less likely to travel into rough, steep 

terrain, instead favoring valley bottoms 

and water sources (AIDTT 2000, pp. 9, 

21). Effects from livestock grazing are 

expected to be attenuated due to the 

relatively steep slopes and rugged 

terrain often preferred by Sonoran 

desert tortoises, but quantitative studies 

have not been conducted to confirm this 

assumption (AIDTT 2000, p. 9; Oftedal 

2007, p. 26). Because of the generalized 

differences in habitat usage by livestock 

(flats, ridge tops, and drainage bottoms) 

and Sonoran desert tortoises (steep 

slopes and rocky bajadas), ecological 

and dietary overlap is uncommon, but 

does occur to some degree (AGFD 2010, 

p. 6). Where such overlap is significant, 

in particular in periods of drought, the 

effect of livestock use on Sonoran desert 

tortoise habitat may be considerable 

(AGFD 2010, p. 7). Sonoran desert 

tortoises may also selectively avoid 

grazed areas. While Sonoran desert 

tortoises are generally known to use 

steep rocky slopes and bajadas as their 

primary habitat areas, they occasionally 

occur in more flat terrain, such as the 

Florence Military Reservation, where 

they are 35 percent less likely to use 

habitat where livestock grazing occurs 

(AGFD 2010, p. 7). Grandmaison et al. 

(in press, p. 2) examined microhabitat 

selection by the Sonoran desert tortoise 

on the Florence Military Reservation in 

south-central Arizona, and found that 

tortoises most strongly selected for 

canopy cover, followed by an absence of 

cattle activity and proximity to roads 

and washes. 

Of the 17 long-term monitoring plots, 

evidence of some degree of habitat usage 

overlap with livestock has been 

observed on 12 plots. On several plots 

(Arrastra Mountains, Bonanza Wash, 

West Silverbell Mountain, and Tortilla 

Mountains) extensive overlap with 

livestock use has been documented in 

each year they were surveyed (AGFD 

2010, p. 7). Heavy trampling and 

destruction of Sonoran desert tortoise 

burrows has been documented on the 

Bonanza Wash plot. One Sonoran desert 

tortoise was crushed by livestock 

trampling on the West Silverbell 

Mountain plot, although such extreme 

reports of livestock-related direct effects 

on Sonoran desert tortoises are 

uncommon in the literature (AGFD 

2010, p. 7). 

Sonoran desert tortoises might 

compete with livestock for high-PEP 

plants (for review, see discussion of diet 

in the Species Information section 

above) and therefore may place unique 

competitive pressure on Sonoran desert 

tortoise populations (Oftedal 2002, pp. 

235–236). Many high-PEP plant species 


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are found primarily in the transition 

zone between areas where livestock and 

Sonoran desert tortoises compete 

directly for these plant species, as noted 

in several Arizona long-term monitoring 

plots (East Bajada of the Black 

Mountains, Hualapai Foothills, Little 

Shipp Wash, New Water Mountains, 

San Pedro Valley), in addition to similar 

observations from studies performed at 

Ragged Top, Saguaro National Park, and 

Sugarloaf Mountain (Oftedal 2007, p. 

26). However, Oftedal (2007, p. 25) 

hypothesized that in situations where 

winter precipitation is modest, high-PEP 

plant species are in low abundance, and 

nonnative annual grass species are in 

high abundance, ‘‘the immediate effect 

of grazing (forage competition with 

Sonoran desert tortoise) would be [a] 

reduction of overall forage biomass, not 

[a] change in the quality of tortoise 

diets. This suggests that cattle grazing 

may be less damaging to tortoises in 

years of modest rainfall.’’ In conclusion, 

Oftedal (2007, p. 26) found that ‘‘the 

high degree of diet selection that occurs 

during spring leaves (Sonoran) desert 

tortoises susceptible to influences that 

may alter the abundance of the 

somewhat scarce high-PEP plants, and 

thus that may reduce the overall quality 

of the diet. Tortoises foraging in summer 

appear less susceptible to the impacts of 

livestock grazing.’’ Thus, seasonality and 

precipitation levels appear to affect the 

likelihood of grazing to adversely affect 

the forage base of Sonoran desert 

tortoises, with spring being a period of 

elevated sensitivity of Sonoran desert 

tortoises to livestock grazing where 

tortoises and livestock spatially overlap. 

Livestock grazing can influence the 

microclimate at the ground surface. 

Grazing may positively affect soil 

temperature and, therefore, benefit 

desert tortoise burrow temperatures 

where burrows are not associated with 

boulders, but instead constructed in 

more open habitat such as underneath 

shrubs (Boarman 2002, p. 31). Field 

research in Mojave desertscrub indicates 

that when the undergrowth beneath 

shrubs is grazed, and the shrub itself is 

minimally browsed or unaffected by 

grazing, underlying soils may cool from 

effects from wind and shade. Heavily 

vegetated undergrowth traps heat and 

increases soil temperature (Boarman 

2002, p. 31). Alternately, heavily 

browsed shrubs can increase soil 

temperatures (Boarman 2002, p. 31). 

Lower vegetative ground cover in 

northern Sonora, as a response to 

livestock overgrazing, was found to 

increase soil and air temperatures above 

the levels found in adjacent grazed 

lands within the United States (Bryant 

et al. 1990, p. 243). Increased soil 

temperatures may impact the Sonoran 

desert tortoise in a variety of ways, such 

as influencing changes in behavior, 

lowering survivorship, and skewing the 

sex ratios of hatchlings (which are 

determined by incubation temperatures; 

see Species Information, above). 

Bostick (1990, pp. 150–151) suggested 

that high desert tortoise densities are 

correlated with high livestock use, 

citing health examinations of Mojave 

desert tortoises that existed in grazing 

exclosures in northwestern Arizona. 

Bostick (1990, p. 149) also asserted that 

desert tortoises feed ‘‘primarily on 

dung,’’ inferring that with more 

livestock, there would be an abundance 

of available tortoise forage. Bostick 

(1990, p. 151) summarized his 

conclusions on the relationship between 

livestock grazing and desert tortoises 

with the following: (1) Desert tortoises 

have coexisted with cattle for 300 years 

in California and Mexico and at least 

100 years elsewhere; (2) the highest 

tortoise densities known occurred at a 

time when overgrazing by livestock was 

the most severe ever known; (3) the 

fewer the cattle on a range, the fewer the 

number of tortoises; and, (4) excluding 

cattle for many years endangers the 

tortoise population. Boarman (2002, pp. 

27, 35, 38) refuted the conclusions made 

by Bostick (1990, pp. 149–151) that 

grazing benefits the desert tortoise. In 

addition, we found no information in 

the scientific literature that supported 

the findings of Bostick (1990, pp. 149– 

151). 

Some research has examined the 

effects of various livestock grazing 

regimes to Sonoran desert tortoise 

populations. Meyer et al. (2010, pp. 20– 

26) compared the number and density of 

Sonoran desert tortoises in study plots 

exposed to four different livestock 

grazing regimes: Yearlong light grazing 

(plot size 2,279 ac (922 ha)), yearlong 

moderate grazing (plot size 3,254 ac 

(1,317 ha)), yearlong heavy grazing (plot 

size 4,634 ac (1,875 ha)), and restrotation (plot size 4,758 ac (1,925 ha)). 

They found that the highest number and 

density of Sonoran desert tortoises (266 

total individuals; 36.89 individuals per 

square mile) was observed in the 

pastures with yearlong heavy grazing as 

compared to rest-rotation (215 total 

individuals; 28.94 individuals per 

square mile), yearlong light grazing (52 

total individuals; 14.61 individuals per 

square mile), and yearlong moderate 

grazing (47 total individuals; 9.23 

individuals per square mile) (Meyer et 

al. 2010, p. 23). The study plots used for 

this comparison between the number 

and density of Sonoran desert tortoises 

and various livestock grazing regimes 

were of unequal size, with the yearlong 

light and moderate plots being the 

smallest. This could affect the number 

of tortoises observed but not likely the 

density of tortoises. Other variables that 

likely affected the analysis of Sonoran 

desert tortoise densities were 

differences in vegetation, topography, 

soil types, and the location of tortoise 

populations among study plots (Meyer 

et al. 2010, p. 38). In addition, the 

ability to detect Sonoran desert tortoises 

is likely to increase with intensity of 

livestock use and a subsequent decrease 

in ground cover, which could have 

further biased the number of 

observations in the yearlong moderate 

and heavy grazing study plots. Given 

the results of these analyses, Meyer et 

al. (2010, p. 42) surmised that ‘‘tortoise 

densities were affected by soil, 

topography and vegetation and had little 

or no relationship to livestock grazing or 

grazing systems.’’ 

Additional research examined effects 

of grazing regimes on fire behavior and 

wildlife and vegetation communities, 

citing beneficial effects. Bahre (1991, p. 

141) compared the relative frequency of 

wildfires that occurred in the mid-1900s 

(carried by nonnative plants), to fires in 

more recent times, and suggested that 

mechanical fuel reduction by livestock 

grazing might assist in reducing the 

propensity of wildfires in Sonoran 

desertscrub habitat. Loeser et al. (2007, 

p. 97) found that in Arizona grasslands 

‘‘* * * some intermediate level of cattle 

grazing may maintain greater levels of 

native plant diversity than the 

alternatives of cattle removal or highdensity, short-duration grazing 

practices.’’ 

In an unpublished review of livestock 

grazing literature, Holecheck (undated, 

p. 2) found that ‘‘* * * controlled 

livestock grazing may enhance 

rangeland vegetation by accelerating 

plant succession, increasing plant 

diversity, increasing plant productivity, 

and reducing plant mortality during 

drought. These positive impacts of 

livestock grazing are most likely to 

occur when grazing intensities are light 

to conservative.’’ Holecheck (undated, p. 

2) countered the unanimous findings of 

over 30 independent livestock grazing 

impact studies that documented that 

controlled grazing increases 

compaction, reduces infiltration, and 

increases erosion by claiming that ‘‘these 

impacts are generally of small 

magnitude and are ameliorated by 

natural processes that cause soil 

formation, soil deposition, and soil 

loosening.’’ 

Some local land management 

organizations are currently working on 

proactive conservations efforts to reduce 


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potential impacts of ranching and other 

activities on the Sonoran desert tortoise. 

For example, the Winkelman Natural 

Resource Conservation District 

(WNRCD, a coalition of local livestock 

ranchers and grazing lease permittees in 

the Winkelman area of the lower San 

Pedro River in Arizona) has prepared a 

draft conservation plan for the desert 

tortoise within their area (WNRCD 2010, 

pp. 1–13). This draft plan proposes 

conservation and land management 

prescriptions for land managers in their 

area as recommended by the Arizona 

Interagency Desert Tortoise Team. 

However, presently the draft plan has 

not secured specific agreements with 

land managers responsible for Sonoran 

desert tortoise habitat, and it lacks 

financial commitments to carry out the 

recommended conservation actions. For 

example, Pinal County was identified as 

having responsibilities for conservation 

actions but has since indicated that they 

are unable to participate in the draft 

plan (Pinal County 2010, p. 1). While 

this draft conservation plan could 

further Sonoran desert tortoise 

conservation in this area once all the 

necessary management and financial 

agreements are in place and the plan is 

finalized, it currently provides limited 

conservation benefit to the Sonoran 

desert tortoise. 

In consideration of the literature 

presented above, we conclude that 

grazing effects to the Sonoran desert 

tortoise may occur but are likely limited 

in severity and scope in Arizona, 

because habitat shared by livestock and 

Sonoran desert tortoises is not a 

significant proportion in most areas in 

Arizona, and because livestock grazing 

in Arizona is actively managed by land 

management agencies (see Factor D). We 

also acknowledge that data generated 

from research on grazing effects to 

tortoises and their habitat are variable, 

making it difficult to accurately assess 

the risk of livestock grazing to the 

Sonoran desert tortoise. However, due 

to limited regulations affecting livestock 

management in Mexico, and the 

information we have examined on its 

extent in Sonora, we conclude that 

livestock grazing likely poses a threat to 

the Sonoran desert tortoise in Mexico. 

We also acknowledge the potential for 

livestock grazing effects to act 

synergistically with other influences 

discussed herein. 

Undocumented Human Immigration 

United States border-enforcement 

efforts have significantly increased 

along the United States-Mexico 

international border in Arizona in 

recent years. Sonoran desert tortoise 

habitat occurs along approximately 140 

mi (225 km) of the border, from 

approximately Nogales west to the 

California State line. International 

border fencing structures and barriers 

(especially the impenetrable pedestrian 

fencing) along the Arizona-Sonoran 

border pose population-connectivity 

problems for the Sonoran desert 

tortoise, which depends on emigration 

and immigration for genetic fitness of 

regional populations. However, along 

most of the border, just vehicle barriers 

occur, which allow tortoises to pass 

through them, and do not pose a barrier 

to movement (Cohn 2007, p. 96; Flesch 

et al. 2010, p. 179; Audsley 2010, p. 5; 

Sferra 2010, pers. comm.). The two 

primary types of barrier devices that 

have been constructed, or are planned 

for construction, are vehicle barriers and 

pedestrian fences, the latter of which 

may be impenetrable to Sonoran desert 

tortoises where the fence is buried into 

the ground (Audsley 2010, p. 5; Sferra 

2010, pers. comm.). Where pedestrian 

fences are not buried completely and 

bollard fences (barriers formed by a 

series of vertical posts) are installed, 

Sonoran desert tortoises less than 4 in 

(10 cm) in width may be able to get 

through (Audsley 2010, p. 5; Sferra 

2010, pers. comm.). 

Undocumented immigrants affect 

Sonoran desert tortoise habitat by 

trampling vegetation along well-used 

routes and cutting wood for campfires, 

which affects the quality and amount of 

forage and also reduces the number of 

temporary shelter sites for Sonoran 

desert tortoises (Averill-Murray and 

Averill-Murray 2002, p. 29). Other 

human activities along the international 

border (off-road driving, high-speed 

driving, accidentally setting fires from 

cooking or purposefully for distraction 

of law enforcement personnel, and 

interdiction activities by the U.S. Border 

Patrol, U.S. Immigration and Customs 

Enforcement, and other enforcement 

agencies) also impact Sonoran desert 

tortoises and their habitat (AIDTT 2000, 

p. 27; Marris 2006, pp. 338–339; Sayre 

and Knight 2010, p. 347). 

Historically, border enforcement 

policies and associated structures have 

indirectly channeled undocumented 

immigration pressure onto the Cabeza 

Prieta National Wildlife Refuge (Marris 

2006, pp. 338–339; Cohn 2007, p. 96). 

Analysis has shown there are about 

8,000 mi (12,875 km) of unauthorized 

routes on the approximate 1,000 sq mi 

(2,600 sq km) refuge, mostly in 

designated wilderness (McCasland 

2010, pers. comm.). These routes are 

most likely attributable to illegal crossborder traffic and associated law 

enforcement response by Border Patrol 

(McCasland 2010, pers. comm.). 

Recently, 33.5 mi (54 km) of permanent 

vehicle barriers were installed along the 

international border within the Cabeza 

Prieta National Wildlife Refuge, which 

has likely reduced illegal vehicular 

access to the Refuge (SBBI Incorporated 

2010, p. 1). 

Along the entire southern boundary of 

the Buenos Aires National Wildlife 

Refuge, a 7-mi- (11.3-km-) long 

pedestrian barrier has been constructed 

(USDHS 2007, pp. 4, Figure 2–1). 

Because pedestrian barriers on the 

border are generally well-fortified, 

complete barriers to terrestrial 

movement, we assume that Sonoran 

desert tortoises in the larger juvenile 

and adult size classes are now 

prevented from making trans-border 

dispersal movements as a result of the 

barrier construction in this area. 

The border region associated with the 

Tohono O’odham Nation in Pima 

County, Arizona, was recently 

considered to have one of the highest 

rates of attempted crossings, because it 

is relatively remote (Sferra 2010, pers. 

comm.). Currently, all but 3 mi (4.8 km) 

of the 70-mi (113-km) section of border 

between the Tohono O’odham Nation 

and Mexico is reinforced with a vehicle 

barrier (Lackner 2010b, pers. comm.). 

Vehicle barriers are not constructed 

where terrain is too steep or rocky, or 

where vehicular access is considered 

impossible (Lackner 2010b, pers. 

comm.). The lands of the Tohono 

O’odham Nation are predominantly 

classified as Arizona Upland Sonoran 

desertscrub. The lands presumably have 

significant numbers of Sonoran desert 

tortoises, although survey data are 

generally scarce from that area. 

Along the Organ Pipe Cactus National 

Monument border with Mexico, vehicle 

barriers exist across most of the 

monument, and a potentially 

impenetrable pedestrian fence has been 

erected in Arizona Upland Sonoran 

desertscrub on Monument Hill and 

along 4 mi (6.4 km) of the border at the 

Lukeville Port of Entry (Sferra 2010, 

pers. comm.). 

The comparison of 2009 and 2010 

apprehension rates of undocumented 

immigrants reflects both the number of 

attempted illegal crossings and the 

intensity of enforcement activities 

within various regions of the ArizonaMexico border, as well as areas north of 

the border (Lackner 2010a, pers. 

comm.). Within Sonoran desert tortoise 

habitat, significant increases in 

apprehension rates have occurred in the 

following areas (percentage denotes 

change from 2009 to June 2010): Tohono 

O’odham Nation (18.37 percent); Organ 

Pipe Cactus National Monument (63.8 

percent), and the Sonoran Desert 


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National Monument (70.69 percent) 

(U.S. Border Patrol 2010, pers. comm.). 

In other areas, the apprehension rates 

have substantially decreased over the 

same time period: Ironwood Forest 

National Monument (¥47.18 percent), 

Barry M. Goldwater Air Force Range 

(¥32.02 percent), and the Cabeza Prieta 

National Wildlife Refuge (¥13.19 

percent) (U.S. Border Patrol 2010, pers. 

comm.). Over the same time period, and 

in total, there have been 79,307 

apprehensions made, compared to 

71,775 apprehensions in 2009, which 

represents a 10 percent increase 

(Lackner 2010a, pers. comm.). 

New border- and access-road 

construction has connected previously 

remote and undisturbed habitat to the 

existing network of Arizona roads, 

providing vehicular access to areas 

previously only accessible by foot or on 

horseback (Sayre and Knight 2010, pp. 

346–347; Sferra 2010, pers. comm.). An 

unintended consequence of these new 

roads is that they are used not only by 

U.S. Border Patrol, but by the public 

and illegal traffic, increasing the risk of 

wildfires, invasions of nonnative plant 

species, alteration of erosion and water 

movement patterns (affecting infiltration 

and soil stability), and mechanical 

damage to vegetation (Sayre and Knight 

2010, p. 347; Sferra 2010, pers. comm.). 

Many new roads along the border have 

included cattle guards built with 

enclosed concrete pits that have the 

unintended consequence of acting as 

lethal pit-fall traps for reptiles, such as 

smaller size class Sonoran desert 

tortoises (Sayre and Knight 2010, p. 

347). 

Based on our review of the literature 

and communications with resource 

experts and enforcement personnel, we 

conclude that Sonoran desert tortoises 

and their habitat, both near the 

international border and within 

corridors of heavy undocumented 

immigrant travel and enforcement 

interdiction, are threatened by these 

activities. Specifically, off-road route 

proliferation, high-speed driving, road 

construction (providing new access to 

formerly inaccessible areas), human 

depredation of tortoises as food sources, 

and barriers to tortoise movement 

created by pedestrian fencing are 

recognized as having serious impacts to 

Sonoran desert tortoise habitat. The 

geographic scope of these threats is 

relatively small on the landscape, 

restricted to the immediate border 

region, and to undocumented immigrant 

migration corridors, such as that 

recognized through the Tohono 

O’odham Nation, extending through 

Ironwood Forest National Monument. 

However, these impacts are significant 

where they occur. 

Summary of Factor A 

Our analysis under Factor A 

identified an array of threats to Sonoran 

desert tortoise habitat. The documented 

invasion and purposeful cultivation of 

nonnative plant species within the 

distribution of the Sonoran desert 

tortoise in the United States and Mexico 

significantly increases the threat of 

wildfire in an ecosystem that evolved in 

the absence of wildfire. This threat is 

widespread and, although currently and 

comparatively less significant in 

Arizona, is substantial in Mexico, and is 

expected to increase in the future. When 

including the total land area adversely 

modified by ironwood and mesquite 

harvesting, an estimated 98 percent of 

the Sonoran desert tortoises’ habitat will 

be lost or adversely modified in Mexico 

in the near future, or 47 percent of the 

Sonoran desert tortoise’s habitat 

rangewide. It is important to recognize 

that while nonnative plant species are 

expanding their distribution on the 

landscape, Sonoran desert tortoise 

populations have persisted in affected 

areas that remain unburned, for 

decades. The effect of nonnative plants 

on Sonoran desert tortoise populations 

is most significant after a wildfire has 

occurred; effectively giving nonnative 

species a distinct competitive advantage 

over native vegetation, and threatening 

a type-conversion in habitat. While we 

have found evidence of numerous 

wildfires in occupied desertscrub, the 

majority of occupied habitat that has 

been invaded by nonnative plants has 

not yet burned and remains suitable 

habitat for the tortoise. 

In addition, projections for human 

population growth and urban 

development throughout the species’ 

range are likely to both pose significant 

problems for genetic exchange among 

Sonoran desert tortoise populations as 

well as increase the degree and scope of 

human interactions with tortoises and 

occupied habitat, which threatens the 

tortoise in a variety of ways. Currently 

in Arizona, 75 percent of potentially 

occupied Sonoran desert tortoise habitat 

occurs within 30 mi or less of a city or 

town with a population of 1,000 or 

more, and considering future growth 

projections, it is likely that 100 percent 

of occupied tortoise habitat will be 

affected in the future. Livestock grazing 

in Mexico poses significant threats to 

the Sonoran desert tortoise habitat there 

due to ineffective livestock management 

and continued overgrazing. Lastly, 

desertscrub habitat that has been 

disturbed takes a very long time to 

recover, on the order of decades or 

centuries, which hinders remediation 

projects with respect to their ability to 

prevent population declines in Sonoran 

desert tortoises in the short- or mediumterm. Each of these impacts results in 

significant cumulative threats to the 

species’ habitat and, based upon our 

review of the best commercial and 

scientific data available, we conclude 

that the present or threatened 

destruction, modification, or 

curtailment of its habitat or range is an 

immediate threat of high magnitude to 

the Sonoran desert tortoise, both now 

and in the foreseeable future. 

Factor B. Overutilization for 

Commercial, Recreational, Scientific, or 

Educational Purposes 

Illegal Collection 

In urban areas of Sonora, Mexico, 

such as Hermosillo, desert tortoises 

have become increasingly common as 

household pets. They have been mostly 

obtained from the wild in adjacent areas 

(Bury et al. 2002, p. 103). The sale of 

desert tortoises in Mexican pet stores 

ended when the tortoise was listed as 

threatened in that country in 1994 (Bury 

et al. 2002, p. 103). 

Sonoran desert tortoises are a closed 

season species in Arizona (Commission 

Order 43), and therefore cannot be 

legally taken from the wild or possessed 

without special license. In Arizona, the 

current possession limit for Sonoran 

desert tortoises legally held in captivity, 

i.e., either obtained prior to season 

closure or obtained through the tortoise 

adoption program, is one per person per 

household (AGFD 2010, p. 12). The 

AGFD allows for disposition of lawfully 

possessed tortoises by gift to another 

person in Arizona, or as directed by the 

AGFD (AIDTT 2000, p. 14). Despite 

collection prohibitions in Arizona, the 

Sonoran desert tortoise is a very 

common reptile pet in Arizona 

households and has been so for decades. 

The actual number of Sonoran desert 

tortoises in captivity is unclear because 

there are no special licenses or permits 

required to possess Sonoran desert 

tortoises, or laws that prohibit their 

propagation in captivity (Jarchow et al. 

2002, p. 289; Jones 2008, p. 69). Jarchow 

et al. (2002, p. 289) state that the 

number of captive Sonoran desert 

tortoises in Arizona is so large that an 

outright prohibition of their possession 

is both impossible and impractical. 

The popularity of Sonoran desert 

tortoises in captivity, as well as the 

various adoption programs around the 

State, may unintentionally mislead the 

public into thinking that Sonoran desert 

tortoises are not protected, and may, 

therefore, be collected from the wild 


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(Grandmaison in press, p. 6). For 

example, the area surrounding the 

Hualapai Foothills plot experienced 

increased development in 2001, which 

may have increased human-tortoise 

interactions and possibly illegal 

collection. Declines in tortoise 

encounters at this plot in 2001 and 2005 

may have, in part, resulted from illegal 

collection due to that plot’s proximity to 

developed land (AGFD 2010, p. 7). 

Arizona’s regulations have no 

provisions requiring permits for 

possession of Sonoran desert tortoises, 

which would aid in identification of 

those tortoises that were in lawful 

possession before January 1, 1988. In 

addition, there may be incentive created 

for the illegal release of captive tortoises 

into the wild because of the number of 

tortoises breeding in captivity, and the 

difficulty associated with finding 

recipients of offspring within the legal 

24-month window (under Arizona’s 

Commission Order 42). This could 

result in a higher number of illegal and 

indiscriminant releases into the wild 

(AIDTT 2000, p. 14). Edwards et al. 

(2010, pp. 801–807) conducted genetic 

testing of 180 captive tortoises from 

Arizona to discern their genetic origin 

(as Sonoran, Mojave, or a hybrid). They 

found that 45 percent of sampled 

captive tortoises were not of strictly 

Sonoran origin, but rather either pure 

Mojave, Sonoran-Mojave cross, or Texas 

tortoise (Gopherus berlandieri)— 

Sonoran desert tortoise hybrids 

(Edwards et al. 2010, p. 804). These data 

indicate there may be a risk of genetic 

contamination of wild populations 

when captives are released. Genetic 

contamination can weaken the genetic 

fitness of a population and render it 

vulnerable to extirpation. In addition, as 

documented in Factor C below, captive 

Sonoran desert tortoises have been 

shown to have a higher incidence of 

disease, and their release can place wild 

populations at risk. 

Opportunities to collect Sonoran 

desert tortoises often result from 

incidental observations by motorists 

while using dirt, gravel, or paved roads. 

In a recent study, out of a total of 561 

opportunities for motorist-Sonoran 

desert tortoise interaction, 1.43 percent 

resulted in attempted collection of a live 

decoy, and 7.4 percent attempted the 

collection of an artificial Sonoran desert 

tortoise decoy (Grandmaison in press, 

pp. 8–9). Combining the data, 

Grandmaison (in press, p. 11–12) found 

that collection attempts varied with 

road type and approximately 1 in 12 (8 

percent) motorists that detect a Sonoran 

desert tortoise in the wild may attempt 

to illegally collect it. Adult tortoises are 

the most conspicuous and are likely the 

most-frequently collected age class, 

which could be detrimental to 

populations, especially when 

reproductive females are collected. 

Grandmaison (2010a, pers. comm.) 

stated, ‘‘Illegal collection of desert 

tortoises is a form of additive mortality 

resulting from the impacts of roadways 

in tortoise habitat. Given that adult 

tortoises are the most likely 

demographic to be collected (i.e., they 

are easier to detect than juveniles or 

hatchlings), and the sensitivity of 

tortoise population growth rates to even 

small increases in adult mortality, 

illegal collection really needs to be 

considered when discussing the 

cumulative impacts of roads.’’ 

While the actual collection of Sonoran 

desert tortoises detected on roadways is 

one form of interaction, a higher 

percentage of motorists attempt to move 

Sonoran desert tortoises off the roadway 

when they are detected. Grandmaison 

(2010a, pers. comm.) found that 28 

percent of all motorists passing a desert 

tortoise will move the tortoise off of the 

road. While moving a Sonoran desert 

tortoise off the roadway may be 

considered well-intended, the stress to a 

Sonoran desert tortoise that is created 

when it is handled may result in 

intestinal torsion (which can cause 

intestinal obstructions), or lead to the 

tortoise voiding its bladder. As 

discussed below, bladder voiding has 

serious implications, potentially 

resulting in decreased survival, 

especially during late spring and early 

summer in the Sonoran Desert, when 

precipitation is usually rare or nonexistent (Grandmaison 2010a, pers. 

comm.; in press, p. 11). 

Although removal of Sonoran desert 

tortoises from the wild has clear 

negative effects on wild populations, 

their popularity as household pets may 

provide some educational benefits to the 

public. Jarchow et al. (2002, p. 310) 

provided evidence for potential 

conservation benefits from Sonoran 

desert tortoises that are already in 

captivity by stating, ‘‘The captive 

population of desert tortoises provides 

not only enjoyment to their custodians 

but, more importantly, opportunities for 

education of the public and increased 

awareness of the species among those 

who may never see a desert tortoise in 

nature. Thus, the captive population 

may play an important role in mustering 

public support for conservation of their 

wild relatives.’’ 

In conclusion, research suggests that 

about 1 in 12 motorists in Arizona who 

detect a Sonoran desert tortoise will 

attempt to collect it, and that the highest 

incidence of collection is within the 

adult age class. The removal of an adult 

Sonoran desert tortoise from a 

population poses a higher threat to that 

population, because the survivorship of 

tortoises in this size class is the highest, 

and the odds of a given Sonoran desert 

tortoise reaching this size class is 

believed to be comparatively low, 

further adding importance to the 

maintenance of adults within a 

population. The removal of an adult 

female from a population also removes 

the opportunity for numerous clutches 

of eggs. In addition, nearly one-third of 

all motorists who encounter a Sonoran 

desert tortoise will attempt to move it 

off the roadway, which increases the 

risk of bladder-voiding, which may 

place additional physiological stress on 

moved tortoises and may decrease their 

survivorship. We also found data on 

collection and sale of Sonoran desert 

tortoises in Mexico, which is likely less 

of a threat in current times, due to the 

prohibition of commercial sale and to 

the demographic trend associated with 

more people moving to urban areas, 

reducing the number of wild encounters 

with tortoises in Mexico. 

Field Research and Physical 

Manipulation 

Field research and monitoring of wild 

Sonoran desert tortoise populations has 

been ongoing since the 1970s, 

producing invaluable information for 

wildlife and habitat managers to make 

reasoned decisions with respect to 

conservation planning. However, some 

level of harassment or potential harm 

from disease transmission or 

dehydration is inherent to hands-on 

manipulation (such as collecting blood 

samples, affixing radio transmitters, and 

conducting health assessments). 

One of the more significant risks to 

Sonoran desert tortoises from the 

handling of wild tortoises by researchers 

is the increased potential for them to 

void water reserves stored in their 

bladder. As a defense mechanism when 

threatened, Sonoran desert tortoises 

may occasionally evacuate their 

bladders, releasing valuable water stores 

important for survival in their arid 

habitat, especially during drought years. 

Averill-Murray (2002a, p. 430) noted, 

‘‘This water loss could result in serious 

health threats or compromise normal 

behavior or physiology, especially 

during hot, dry summer months.’’ Water 

loss in Sonoran desert tortoises can also 

result in reductions of reproductive 

output and survivorship (AverillMurray 2002a, pp. 430, 433–434). 

Averill-Murray (2002a, pp. 430, 434) 

found that Sonoran desert tortoises that 

urinated during field research handling 

had a 5–13 percent lower survival rate. 


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Any kind of handling of tortoises 

during field research or monitoring of 

Sonoran desert tortoise populations 

during periods of excessive drought may 

be stressful to the tortoises (Berry et al. 

2002b, p. 436). Berry et al. (2006b, p. 

436) recommended that scientists 

working with wild desert tortoises 

recognize abnormalities in behavior and 

laboratory data as early warning signs of 

stress to modify, delay, or terminate 

specific field protocols on stressed 

populations. 

Use of radio telemetry technology on 

desert tortoises may affect their 

behavior, survival, and reproductive 

success, but available literature is 

largely inconclusive (Boarman et al. 

1998, p. 26). There is little doubt that 

radio telemetry studies have provided 

many insightful data on the biology and 

behavior of Sonoran desert tortoises, 

and are therefore more of a benefit than 

a potential threat. 

Jacobson et al. (1992, pp. 238–239) 

reviewed the recommended procedures 

for obtaining blood samples from desert 

tortoises, including collection from the 

heart, jugular vein, brachial vein, 

ventral coccygeal vein, orbital sinus, 

and trimmed toenails, and assessed the 

potential risks associated with each 

collection site. At a minimum, the 

collection of blood samples from desert 

tortoises is considered relatively 

invasive and is likely a source of 

temporary stress to the animal, 

potentially leading to bladder voiding 

and subsequent dehydration if fluid 

levels are not replenished before release. 

However, we believe the majority of 

field researchers exercise appropriate 

caution when collecting blood samples 

from Sonoran desert tortoises, and the 

literature does not indicate these 

procedures are an appreciable source of 

mortality for wild Sonoran desert 

tortoises. 

Over the years, field protocols have 

been developed and standardized to 

minimize risks to Sonoran desert 

tortoises while they are being physically 

handled. These protocols are outlined in 

Averill-Murray (2000, p. 17) and Berry 

and Christopher (2001a, pp. 433–434). 

We believe these field protocols have 

minimized potential risks to individual 

tortoises posed by researchers during 

their field work. 

Summary of Factor B 

We identified two possible 

mechanisms for which the potential 

overutilization of Sonoran desert 

tortoises for commercial, recreational, 

scientific, or educational purposes 

could occur: Illegal collection and field 

research. Many desert tortoises exist in 

captivity, and are generally available to 

those who want one as a household pet, 

through several channels within the 

captive population (discussed further in 

Factor D). In addition, efforts are being 

made to educate the public about the 

Sonoran desert tortoise, with an 

emphasis on leaving Sonoran desert 

tortoises in the wild when they are 

observed. We believe these factors may 

reduce the likelihood of illegal 

collection. However, a recent scientific 

study found that one in 12 tortoises that 

is detected by a motorist (mostly adult 

tortoises) is illegally collected. We 

expect that in the foreseeable future, 

incidence of collection will likely 

increase as the human population grows 

and more people will use off-road trails, 

with higher frequency, within occupied 

tortoise habitat. Scientists who conduct 

field research on and monitoring of wild 

Sonoran desert populations have 

identified the potential risk for bladder 

voiding and disease transmission during 

field manipulation of tortoises, and have 

now built appropriate protocols in their 

field methodology to minimize these 

risks. Based on this information, we find 

that overutilization for commercial, 

recreational, scientific, or educational 

purposes, in the form of illegal 

collection, is likely to threaten the 

Sonoran desert tortoise now or in the 

foreseeable future. 

Factor C. Disease or Predation 

Natural predation of Sonoran desert 

tortoises occurs as discussed previously 

in the Species Information section 

above. Unnatural sources of predation, 

such as from feral, or off-leash dogs, 

human depredation for recreation or as 

food, and as an indirect result of human 

land uses (referred to as subsidized 

predation) also occur. A subsidized 

predator is one whose survival in a 

particular area is facilitated by the 

availability of food, water, or other 

potentially limiting resources made 

available by the presence of human 

activities in that area (Boarman 1993, p. 

192). Common examples of subsidized 

predators are coyotes and ravens. 

Human activity-related resources that 

provide basic biological needs for 

subsidized predators include such 

things as roads, landfills, sewage and 

septic ponds, open dumpsters, 

agricultural fields, feedlots, parks, 

picnic areas, livestock waters, utility 

poles, building sites, and overpasses 

(Boarman 1993, p. 193; Rosentstock et 

al. 2004, p. 3; Boarman et al. 2006, p. 

259; Webb et al. 2009, p. 72). 

For example, Averill-Murray and 

Swann (2002, p. 1) stated that urban 

development adjacent to the Saguaro 

National Park in Pima County threatens 

the Sonoran desert tortoise via several 

mechanisms, including harassment and 

predation by feral or off-leash domestic 

dogs, and illegal releases of captive 

Sonoran desert tortoises and exotic 

species that may transmit diseases to 

wild Sonoran desert tortoises. 

Predation by Ravens 

Ravens and coyotes are known 

predators on Mojave desert tortoises, 

and possibly on Sonoran desert 

tortoises, and are most likely to benefit 

from anthropogenic subsidization 

(Boarman 1993, p. 192; Boarman et al. 

2006, p. 259). Ravens turn over 

hatchling desert tortoises and pierce 

through their soft plastrons, or pierce 

directly through their carapace, to 

access their meat and organs. Ravens are 

often less likely to emigrate long 

distances to colonize would-be suitable 

areas, but subsidization from human 

activities on the landscape create 

opportunities for rapid population 

growth of ravens where they formerly 

did not occur (Boarman et al. 1995, 

p. 1; Fleischner et al. 2008, p. 472). 

Ravens, in particular, have been 

identified as subsidized predators on 

juvenile Mojave desert tortoises, and 

possibly on juvenile Sonoran desert 

tortoises (Boarman 1993, p. 192). Roads 

and power line rights of way attract 

potential avian predators of Sonoran 

desert tortoises, such as ravens and redtailed hawks that use power lines as 

nesting and perching sites, and roads 

can serve as sources of carrion (Knight 

and Kawashima 1993, p. 266). Raven 

populations, and potential risk of 

predation of Sonoran desert tortoises by 

ravens, are both higher with increasing 

proximity to human development 

(Kristan and Boarman 2003, p. 2432). 

Documented reports of raven 

predation on Sonoran desert tortoises 

are rare in the literature, however. One 

local rancher in southeastern Mohave 

County, Arizona, reported an 

observation of raven predation on a 

Sonoran desert tortoise (Dieringer 2010, 

p. 1). Ravens have also been observed 

on the Four Peak monitoring plot on 

several occasions, but their predation on 

Sonoran desert tortoises within this plot 

has never been documented (Murray 

and Schwalbe 1997, p. 33). Mojave 

desert tortoises are most commonly 

associated with valley bottomlands 

characterized by relatively open, sparse 

vegetation communities which may be 

advantageous to a purely visual-based 

predator such as the raven. In Arizona 

Upland Sonoran desertscrub, where 

Sonoran desert tortoises reach their 

peak population densities, habitat is a 

more complex mosaic of boulders and 

denser vegetation, which would hamper 

the ability of such predators to locate 


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prey, in particular, small hatchlings. 

Some exceptions include habitat within 

sparsely vegetated valley bottoms that 

are used for dispersal between 

populations on adjacent mountains or 

foothills, or similar, uncharacteristic 

areas that maintain Sonoran desert 

tortoise populations, such as the 

Florence Military Reservation. The best 

scientific and commercial data available 

indicates that predation by ravens is 

significantly less of a concern for 

Sonoran desert tortoises than it is for 

Mojave desert tortoises. 

In conclusion, although raven 

predation has been identified as a 

substantial threat to the Mojave desert 

tortoise, largely because of the relatively 

open, valley bottomland where they 

occur, the risk to Sonoran desert tortoise 

populations is relatively low. Very few 

observations of raven predation of 

Sonoran desert tortoises in Arizona or 

Sonora have been documented in the 

literature, leading us to conclude that 

raven predation on the Sonoran desert 

tortoise is not a concern. 

Predation From Feral or Off-Leash Dogs 

Feral dogs are known to interact with 

numerous species of animals, including 

desert tortoises and related species, and 

they may force Sonoran desert tortoises 

to use their habitat in an unnatural 

manner (Causey and Cude 1978, pp. 94– 

95; Lenth et al. 2008, pp. 222–223). The 

risk of feral or off-leash dog predation 

on Sonoran desert tortoises is expected 

to be highest within the urban-rural 

interface (a likely source of 

domesticated, feral dogs). 

Jones (2008, p. 66) documented 35 

separate incidences of harassment by 

wild or domestic dogs in surveys 

conducted in high-use public lands 

adjacent to the urban centers of Tucson, 

Phoenix, and Kingman, Arizona (Pima, 

Maricopa, and Mohave Counties, 

respectively), based upon observed shell 

damage. These incidences were 

positively correlated with increasing 

proximity to urban centers. Also, three 

to five packs of presumably feral dogs 

were observed in both the East Bajada 

monitoring plot in Mohave County and 

in Saguaro National Park West in Pima 

County (Jones 2008, p. 66). Researchers 

of Sonoran desert tortoises within the 

Tucson Mountain District of Saguaro 

National Park noted a high number of 

tortoises with injuries consistent with 

dog attacks, attributing these 

observations to the close proximity of 

this district to urban development 

(Zylstra and Swann 2009, pp. 14–15). 

The AGFD (2010, pp. 11–12) reported 

that domestic dogs, their scat, and chew 

marks on, or trauma to, Sonoran desert 

tortoises have been reported in 47 

percent of the monitoring plots. Three 

such plots occur within 1 mi (1.6 km) 

of developed areas. Domestic dogs have 

been observed attacking and chewing on 

Sonoran desert tortoises in the Hualapai 

Foothills and Bonanza Wash plots 

(AGFD 2010, p. 12). Domestic dogs 

appear to be a significant problem, 

which may be worsening, in the East 

Bajada plot, where in 1997, 53 percent 

of live tortoises, and in 2002, 78 percent 

of live tortoises, exhibited injuries 

associated with domestic dogs (AGFD 

2010, p. 12). One citizen commented 

that in 1997 a purebred Rottweiler was 

observed roaming freely on the 

Ironwood Forest National Monument 

with an adult Sonoran desert tortoise in 

its jaws. The tortoise was mortally 

wounded from a punctured carapace, 

suggesting that large, powerful domestic 

dog breeds may be able to penetrate the 

carapace of adult tortoises and kill them 

(Coping 2009, p. 7). 

Numerous signs of attempted 

predation (consistent with those from 

feral dogs), ranging from mild to severe, 

were observed in wild Sonoran desert 

tortoises examined in Sonora, Mexico 

(Brown et al. 2006, p. 6). We are 

unaware of the locations where these 

wild Sonoran desert tortoises were 

captured by Brown et al., but the 

proximity to human settlements, and 

free-ranging domestic dogs (a common 

sight in Mexico) may have been 

responsible. 

In conclusion, the threat of feral dog 

predation exists in both Arizona and 

Sonora, Mexico, and has been shown to 

be strongly correlated with distance to 

urbanized areas in most cases. We found 

numerous reports of observed or 

suspected feral dog predation in the 

literature, most in immediate proximity 

to urban areas. Feral dog predation has 

been documented in approximately half 

of the long-term monitoring plots in 

Arizona, and may be a significant cause 

of population decline in one plot. As 

urbanization and human population 

growth continues into the future, as 

described in Factor A, the incidence of 

feral dog predation of Sonoran desert 

tortoises is expected to also increase. 

Human Depredation and Vandalism 

Human depredation (intentional 

killing) of Sonoran desert tortoises has 

been documented to occur either as a 

result of vandalism (most commonly via 

gunshot) or as a source of food. The 

intentional shooting of Mojave desert 

tortoises in southern California was 

reported to be relatively common, at 

least before the Mojave population was 

Federally listed. Berry (1986b, p. 127) 

found that 14 percent of 635 carcasses 

taken from 11 sites in the Mojave Desert 

over a 6-year time period exhibited 

signs of gunshots. Many of these 

observations occurred before the listing 

of the Mojave desert tortoise, indicating 

that tortoises may have been shot 

simply for misdirected recreational 

sport or entertainment, not from 

politically-driven motives (people 

disliking the protections of the Act). 

Bury and Marlow (1973, p. 11) 

described examples of Mojave desert 

tortoise mortalities in California as a 

result of shooting, including eight 

independent observations of shot 

Mojave desert tortoises along two miles 

(3.2 km) of dirt road; an individual’s 

confession of using juvenile desert 

tortoises as skeet (aerial shotgun) 

targets; and a report of an individual 

lining up a total of 47 desert tortoises 

and shooting each of them with a 

shotgun. 

Recreational firearms target practice 

occurs in dispersed fashion throughout 

Federal and State lands in Arizona 

within the distribution of Sonoran 

desert tortoises. Some reports of gunshot 

deaths of Sonoran desert tortoises on 

these lands have been made (Hart et al. 

1992, p. 120; AGFD 2010, p. 9; Jones 

2010, pers. comm.). In some locations, 

recreational firearms target practice is 

highly conspicuous (as evidenced by 

large amounts of debris used as targets 

and left behind) in densely occupied 

Sonoran desert tortoise habitat, most 

notably in areas near urban population 

centers, such as at Sugarloaf Mountain 

on the Cave Creek Ranger District of the 

Tonto National Forest. In this location, 

two incidences of shot Sonoran desert 

tortoises have been reported, although it 

could not be determined whether these 

wounds occurred pre- or post-mortem 

(Jones 2010, pers. comm.). Another 

incidence of shooting was reported in 

the Hualapai Foothills monitoring plot 

(Hart et al. 1992, p. 120). The AGFD 

(2010, p. 9) reported 13 separate 

incidences of vandalism on Sonoran 

desert tortoises on or adjacent to 7 

different monitoring plots; several of the 

Sonoran desert tortoises appeared to 

have been killed by gunshot. 

When studying Mojave desert 

tortoises, Berry (1986b, p. 129) found 

that the incidence of gunshot deaths is 

likely to be higher in areas of greater 

vehicular access and in proximity to 

urban areas. The potential effect of 

gunshot deaths on Sonoran desert 

tortoise populations is not entirely 

known, but is likely most significant on 

the adult size class, which is the most 

conspicuous, and this effect may act 

synergistically with other threats we 

have identified. Combined with the 

relatively low recruitment rate of 

juvenile desert tortoises into adult size 


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classes, adverse effects to survivorship 

of populations adjacent to urban areas 

might be expected (Berry 1986b, p. 130). 

Sonoran desert tortoises are 

sometimes used as a food source in 

Sonora, and likely experience 

population declines where they occur 

adjacent to moderately sized settlements 

(Fritts and Jennings 1994, p. 52). Bury 

et al. (2002, p. 102) reported several 

historical incidences of Sonoran desert 

tortoises being used as a source of food 

by native peoples in Sonora, but less 

frequently in current times. According 

to 12 interviews at 6 ranches in central 

Sonora, 67 percent of local people 

described Sonoran desert tortoises as 

declining. All but one interviewee 

stated they have eaten Sonoran desert 

tortoise meat at some point in their lives 

(Bury et al. 2002, p. 102). However, 

demographic trends in Sonora indicate 

the number of people living on ranches 

and ejidos (commonly owned lands 

used for agriculture and livestock 

grazing) have declined, while city 

populations have increased, potentially 

reducing the likelihood of Sonoran 

desert tortoises being used for food 

(Bury et al. 2002, pp. 102–103). 

Sonoran desert tortoises have also 

been documented as a food source for 

undocumented immigrants on their 

journey through the Sonoran Desert of 

Arizona, specifically in the Ironwood 

Forest National Monument. Coping 

(2009, p. 4) claims that by the time 

undocumented immigrants reach the 

Ironwood Forest National Monument, 

many have been abandoned by their 

guides and left without food, water, or 

a sense of direction, leaving them in 

intense desperation (Coping 2009, p. 4). 

In one instance on June 2, 1997, a small 

group of undocumented immigrants 

approached a resident living within the 

Ironwood Forest National Monument. 

The immigrants had a live Sonoran 

desert tortoise they had captured along 

the way that had a rope tethered to its 

front leg. They told this resident that if 

they did not receive food from him, they 

planned to eat the tortoise (Coping 2009, 

p. 5). In another reported observation, a 

livestock grazing permittee on the 

Ironwood Forest National Monument 

stated that he had seen immigrants 

carrying tortoises, ‘‘presumably with the 

intent to consume’’ (Averill-Murray and 

Averill-Murray 2002, p. 29). Indigenous 

communities of the Sonoran Desert 

historically used Sonoran desert 

tortoises for food and medicine, and 

their shells for ladles, dippers, bowls, 

and shovels (Nabhan 2002, p. 356). 

However, we have no information to 

suggest these uses have continued into 

modern times. 

In conclusion, direct human 

depredation on Sonoran desert tortoises 

is most likely to occur via vandalism 

(i.e., shooting) and utilization as a 

source of food. While the deliberate 

shooting of Sonoran desert tortoises has 

been documented in Arizona, reports 

are comparatively rare, especially 

considering the amount of monitoring 

and survey effort that has been afforded 

to wild populations over the past 

several decades. However, as the human 

population continues to grow and 

urbanization expands, we expect the 

incidence of human depredation to 

increase. Sonoran desert tortoises have 

been used for food in Mexico 

historically, but these occurrences are 

suspected to be comparatively rare in 

current times. Sonoran desert tortoises 

may also be captured by undocumented 

immigrants as they pass through remote 

areas of Arizona, but increasing borderenforcement activities are expected to 

reduce the number of undocumented 

immigrants entering Arizona in the 

foreseeable future, reducing this risk. 

Upper Respiratory Tract Disease 

The threats of mycoplasmosis (or 

upper respiratory tract disease (URTD)), 

and cutaneous dyskeratosis (shell 

disease) were major factors in the listing 

of the Mojave desert tortoise (Berry 

1997, p. 91). Genetic analyses were 

performed by Brown et al. (1994, p. 

4580) on seven Mycoplasma organisms 

that were recovered from the upper 

respiratory tract of clinically ill desert 

tortoises. These laboratory tests led to 

the discovery and subsequent species 

description of Mycoplasma agassizii, 

the species of bacteria that causes upper 

respiratory tract disease in infected 

tortoises (Berry and Christopher 2001b, 

p. 413). Although M. agassizii has been 

studied in Mojave and Sonoran desert 

tortoises, as well as gopher tortoises (G. 

polyphemus), since the 1980s, its 

origins are unknown. It may be a 

naturally occurring or an exotic 

pathogen. There are several potential 

routes of inoculation of vertebrates by 

microbiota such as Mycoplasma spp., 

including horizontal (transmission 

between individuals), vertical (passed 

down from parent to offspring), and 

environmental (passed from 

environment to individual) (Belden and 

Harris 2007, p. 536). Brown (2002, p. 

1340) states that direct contact with 

infected individuals is the most likely 

route of transmission. Brown (2003, p. 

1) stated that M. agassizii is not known 

to be transferred through the eggshell. 

Disease may be spread to wild 

populations as a result of the release of 

captive native or nonnative tortoise 

species, which can be carriers of 

diseases that could affect wild Sonoran 

desert tortoises (Howland and 

Rorabaugh 2002, p. 343). The release of 

any captive reptile or amphibian is 

strictly prohibited by the AGFD. In a 

study investigating the relationship 

between exposure to M. agassizii and an 

urban gradient of Greater Tucson, 

Arizona, Jones (2008, p. 36–37) found 

evidence to suggest a positive 

correlation between the likelihood of 

testing seropositive for antibodies to M. 

agassizii (meaning a tortoise has been 

exposed to URTD), and proximity to 

urban centers. These results suggest that 

there may be a relationship between 

urbanization and this pathogen. 

Tortoises from suburban sites are 2.3 

times more likely to test seropositive for 

antibodies to M. agassizii than tortoises 

from other sites in the greater Tucson 

area. In fact, Sonoran desert tortoise 

populations in the Rincon Mountains 

(adjacent to Tucson, Arizona) had the 

highest prevalence of exposure to URTD 

of any sites tested in Arizona, with 72.7 

percent of sampled Sonoran desert 

tortoises identified as seropositive 

(Jones 2008, p. 93). 

Jones (2008, p. 60) also explored the 

relationship between URTD and captive 

and wild desert tortoises from high-use 

public lands in Maricopa, Mohave and 

Pima counties, and found that captive 

desert tortoises are 1.8 times more likely 

to test seropositive for exposure to M. 

agassizii than wild tortoises (p. 65). 

Sonoran desert tortoises from Pima 

County (wild and captive) had the 

highest incidence of exposure to URTD 

and were 5.4 times more likely to be 

seropositive for antibodies to M. 

agassizii than those from Mohave or 

Maricopa Counties (Jones 2008, p. 65). 

While clinical signs of URTD are 

infrequently observed in wild Sonoran 

desert tortoises in Arizona, Jones (2008, 

pp. 37, 74) found that M. agassizii is 

widespread among captive desert 

tortoises in Arizona, suggesting that the 

captive population may be an important 

reservoir of URTD-infected tortoises that 

can spread the disease to wild 

populations if unlawfully released or 

allowed to escape. 

Even though URTD appears to occur 

widely and has been documented in 

Sonoran desert tortoise populations, no 

die-offs have been attributed to URTD in 

Arizona. Currently, URTD does not 

appear to be a source of mortality for 

Sonoran desert tortoise populations 

(Hart et al. 1992, p. 120; AIDTT 2000, 

p. 9; Averill-Murray and Klug 2000, p. 

69; Dickinson et al. 2002, p. 256; 

Howland and Rorabaugh 2002, p. 343; 

Jones 2008, p. 22; AGFD 2010, p. 9). 

Howland and Rorabaugh (2002, p. 343) 

hypothesized that if disease does 


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become a significant threat to Sonoran 

desert tortoise populations in the future, 

their patchy distribution may limit the 

spread of disease. However, because the 

captive population of desert tortoises 

may serve as a reservoir of disease and 

because captives are unlawfully 

released into the wild by the public, 

monitoring wild tortoise populations 

that occur near urban areas will 

continue to be important (Howland and 

Rorabaugh 2002, p. 343; Jones 2008, pp. 

6–7, 41, and 72–73). 

An indirect effect of disease is that it 

may also subject individuals to 

increased predation. Sonoran desert 

tortoises that are exhibiting clinical 

signs of URTD may be more active 

during winter months, in order to 

increase their metabolism and elevate 

their body temperatures. This increase 

in surface activity might result in a 

greater chance of predation or human 

detection (Jones 2008; p. 105). Jones 

(2008, p.103) found that periods of 

surface activity may increase in 

clinically ill Sonoran desert tortoises; 

however, home range size did not differ 

between seropostive and seronegative 

tortoises (p. 103), so seropositive 

tortoises which are more active in 

winter months do not appear to be 

increasing the areas over which they 

move. 

Wild Sonoran desert tortoises in 

Sonora, Mexico, were tested for the 

presence of antibodies to two 

Mycoplasma species, M. agassizii and 

M. testudineum, and were found to be 

generally unexposed (Brown et al. 2006, 

p. 5). Twenty-seven of 28 wild Sonoran 

desert tortoises were found to be 

seronegative, indicating they had not 

been exposed to Mycoplasma spp.; and 

one individual was serosuspect (a result 

indicating that the antibody level is 

intermediate between positive and 

negative, and is considered 

inconclusive) for M. testudineum 

(Brown et al. 2006, p. 5). However, 11 

of 21 captive Sonoran desert tortoises in 

Sonora, Mexico, tested seropositive for 

antibodies, indicating exposure to M. 

agassizii; and four were serosuspect for 

exposure to M. testudineum. Ten 

captive desert tortoises had M. agassizii 

isolated from nasal flushes, indicating a 

current infection, suggesting that 

disease may be more prevalent in the 

Sonora captive population (Brown et al. 

2006, pp. 5–6). Nearly all of the captive 

desert tortoises exhibited mild to severe 

clinical signs of URTD. Of the captive 

tortoises, six had swollen or draining 

chin glands and four had evidence of 

nasal discharge (Brown et al. 2006, p. 5– 

6). Once infected by URTD, tortoises 

may ultimately die from the disease. 

Cutaneous Dyskeratosis 

Cutaneous dyskeratosis, a shell 

disease, was also a major factor 

considered in the listing of Mojave 

desert tortoises. In populations of 

Mojave desert tortoises exhibiting 

clinical signs of this disease, significant 

die-offs have been documented, some as 

high as 70 percent mortality rate 

(Jacobson et al. 1994, p. 69). Cutaneous 

dyskeratosis may appear on the 

carapace, plastron, and thickened scales 

of the forelimbs, but is most often 

apparent on the plastron (Jacobson et al. 

1994, pp. 70–74). Potential causes of 

cutaneous dyskeratosis have not been 

confirmed, but may be related to 

deficiency diseases and environmental 

contamination (Berry 1997, p. 91). 

Cutaneous dyskeratosis has been 

reported as more prevalent than URTD 

within Sonoran desert tortoise 

populations across Arizona. As of 2000, 

Sonoran desert tortoises infected with 

cutaneous dyskeratosis had been 

observed in every monitored 

population, with the exception of the 

Wickenburg Mountains plot (AIDTT 

2000, p. 9; Averill-Murray and Klug 

2000, p. 69). However, noticeable 

population-level effects have not been 

reported in any of the monitoring plots 

(AIDTT 2000, p. 9; Averill-Murray and 

Klug 2000, p. 69; AGFD 2010, p. 9). Of 

the 36 individual Sonoran desert 

tortoises sampled from the Little Shipp 

Wash and the Harcuvar Mountains from 

1990 to 1994, only 5 (all females 

presumed to be at least 30 years old) 

had signs of cutaneous dyskeratosis, and 

all lived through the end of the field 

study. This prompted Dickinson et al. 

(2002, p. 258) to suspect that Sonoran 

desert tortoises might not be affected by 

this disease, although they 

acknowledged that more research was 

necessary. As of 2000, the highest 

incidence of cutaneous dyskeratosis (62 

percent of individuals) was reported 

from the East Bajada plot (AIDTT 2000, 

p. 9). In Sonora, Mexico, 14 of the 28 

wild Sonoran desert tortoises examined 

exhibited clinical signs of cutaneous 

dyskeratosis (Brown et al. 2006, p. 6). 

In conclusion, disease has been 

documented as a serious threat to the 

Mojave desert tortoise, and was a 

primary cause for its listing under the 

Act. The two most prevalent diseases 

that could affect Sonoran desert tortoise 

populations are URTD and cutaneous 

dyskeratosis. Researchers have 

speculated that Sonoran desert tortoises 

may be able to clear infections of M. 

agassizii, and no wild Sonoran desert 

tortoises have been found to have died 

from URTD in Arizona, although it is 

nearly impossible to document the 

precise cause of death in many 

situations. The literature documents 

that Sonoran desert tortoise populations 

in proximity to urbanized areas are most 

at risk of disease (as a result of released 

captives), because the captive 

population (both in Arizona and 

Mexico) has a significantly higher 

percentage of seropositive tortoises and 

tortoises that have acquired URTD. 

Cutaneous dyskeratosis has been 

documented in virtually all Sonoran 

desert tortoise long-term monitoring 

plots in Arizona, although no Sonoran 

desert tortoises have been documented 

to have succumbed to this disease, and 

we conclude that cutaneous 

dyskeratosis is not a substantial threat to 

populations. Disease screening has been 

a regular component to field research 

and monitoring of wild Sonoran desert 

tortoise populations throughout their 

range for many years, and has not 

indicated that either URTD or cutaneous 

dyskeratosis pose a current threat to the 

Sonoran desert tortoise. 

For additional information on disease 

in desert tortoises, or specific disease 

data from monitored Sonoran desert 

tortoise populations, see Hart et al. 

(1992, p. 120); Berry (1997, p. 91); 

Brown et al. (1994, p. 4580; 1995, p. 

350; 2002, p. 497; 2006, pp. 5–6); 

Jacobson et al. (1994, pp. 69, 70–74); 

Schumacher et al. (1999, pp. 829–830); 

AIDTT (2000, p. 9); Averill-Murray and 

Klug (2000, p. 69); Berry and 

Christopher (2001b, p. 413); AverillMurray and Averill-Murray (2002, pp. 

16, 19, 26); Brown (2002, pp. 1340, 

1343; 2003, p. 1); Dickinson et al. (2001, 

pp. 254–256; 2002, pp. 256, 258, 260– 

261; 2005, p. 841); Howland and 

Rorabaugh (2002, p. 343); Tracy et al. 

(2006a, p. 1191); Belden and Harris 

(2007, pp. 536, 538); Wendland et al. 

(2007, p. 1190); Jones et al. (2005, p. 1); 

Boarman and Kristan (2008, p. 19); 

Jones (2008, pp. 6–7, 70, 93, 103, 105); 

Zylstra and Swann (2009, pp. ix–x); and 

AGFD (2010, p. 9). 

Summary of Factor C 

In review of the information 

presented above, we conclude that 

predation from feral domestic dogs and, 

to a lesser extent, human depredation 

and vandalism, in combination with 

other threats, threaten Sonoran desert 

tortoise populations, most notably as a 

result of the expansion of urbanization 

and associated increases in human 

activity in remote areas. We conclude 

this threat to be of moderate magnitude. 

Based upon our review of the available 

literature, disease does not appear to be 

significantly affecting the status of wild 

Sonoran desert tortoise populations. 

Therefore, we conclude that disease 


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does not pose a significant threat to the 

Sonoran desert tortoise now or in the 

foreseeable future. 

Factor D. The Inadequacy of Existing 

Regulatory Mechanisms 

Within its distribution in the United 

States, the Sonoran desert tortoise 

occurs on lands managed by a myriad of 

Federal and State agencies and Native 

American tribes, and on private lands. 

State agencies, such as the Arizona 

Game and Fish Department (AGFD) or 

the Arizona Department of 

Transportation (ADOT), have either 

direct management authority over the 

Sonoran desert tortoise, or could 

potentially impact Sonoran desert 

tortoise populations or habitat directly 

or indirectly in carrying out their 

intended missions. Internationally, the 

Sonoran desert tortoise is listed in 

Appendix I of the Convention on 

International Trade in Endangered 

Species of Wild Fauna and Flora 

(commonly referred as to CITES), which 

requires permits to transport individuals 

between member nations (Bury et al. 

2002, p. 86; Howland and Rorabaugh 

2002, p. 348). Under the International 

Union for Conservation of Nature’s ‘‘Red 

List,’’ the desert tortoise (rangewide) is 

considered ‘‘vulnerable’’—meaning it 

faces a high risk of extinction in the 

medium-term (Rorabaugh 2008, p. 27). 

In our review, we found that the 

Sonoran desert tortoise is commonly 

considered in conservation planning 

where it occurs on public or tribal lands 

in Arizona. Below we discuss how each 

agency or entity manages their land, or 

otherwise considers the Sonoran desert 

tortoise in their planning activities. 

U.S. Bureau of Land Management 

BLM is very proactive in their 

conservation management, directly and 

indirectly, through three main 

mechanisms: (1) Sonoran desert tortoise 

habitat categorization and compensation 

(monies derived from adverse effects to 

Sonoran desert tortoise habitat for the 

acquisition of new habitat, funding 

research, etc.); (2) resource management 

planning; and (3) land designation. The 

BLM has developed numerous 

documents that outline how Sonoran 

desert tortoise habitat management goals 

and objectives are to be achieved and 

accounted for in their land use 

planning. 

The BLM developed the document 

titled ‘‘Desert Tortoise Management on 

the Public Lands: A Rangewide Plan’’ 

(authored by Spang et al. 1988), and 

created the designation of three 

categories of desert tortoise habitat 

throughout the species’ range, using 

four main criteria to indicate the 

importance of the habitat: (1) 

Maintaining viable populations, (2) 

resolvability of conflicts, (3) desert 

tortoise density, and (4) population 

status (stable, increasing, or decreasing) 

(AIDTT 2000, p. 16; USBLM 2010, p. 1). 

The BLM categorized habitat based 

upon its suitability for the desert 

tortoise, with Category I being the most 

suited, and Category III the least, with 

the goals of maintaining viable desert 

tortoise populations in Category I and II 

habitat, and limiting population 

declines in Category III habitat to the 

extent possible (AIDTT 2000, p. 16). 

However, not all Sonoran desert tortoise 

habitat was included in this 

categorization process. 

AIDTT (2000, p. 19) depicts the 

distribution of the categorized habitat 

included in Arizona. In Arizona, there 

are 723,769 ac (292,899 ha) of Category 

I Sonoran desert tortoise habitat, 2.6 

million ac (1.1 million ha) of Category 

II habitat, and 3.8 million ac (1.5 million 

ha) of Category III habitat, totaling 7.1 

million ac (2.9 million ha) of 

categorized habitat (AIDTT 2000, p. 18). 

The 1988 Rangewide Plan also 

indentified 14 different management 

objectives the BLM has defined 

specifically for desert tortoise 

management, each with its own 

itemized management action plan. 

These management objectives include 

the following categories: (1) Increased 

awareness; (2) inventory and 

monitoring; (3) cumulative impacts; (4) 

identification of endangered 

populations; (5) coordination and 

cooperation; (6) research and studies; (7) 

management of tortoise habitat; (8) 

regulation of lands and realty actions; 

(9) regulation of off-highway vehicles; 

(10) regulation of livestock use; (11) 

regulation of wild horses and burros; 

(12) wildlife habitat management; (13) 

predator control; and (14) management 

of energy and minerals research and 

extraction (Spang et al. 1988, pp. 14–23; 

AIDTT 2000, p. 18). 

In 1990, BLM’s Arizona State Office 

issued the policy titled Strategy for 

Desert Tortoise Habitat Management on 

Public Lands in Arizona, Instruction 

Memorandum No. AZ–91–16. It 

outlined objectives and management 

actions to be implemented, and also 

established the BLM Desert Tortoise 

Mitigation Policy, which was later 

reissued in 1999 (USBLM 2010, p. 2). In 

2009, the BLM finalized the Desert 

Tortoise Mitigation Policy, in order ‘‘to 

articulate mitigation policy including 

off-site compensation for the Sonoran 

desert tortoise and its habitat on public 

lands managed by (BLM) in Arizona, in 

a consistent manner between District 

and Field Offices’’ (USBLM 2009b, p. 1). 

The BLM’s Desert Tortoise Mitigation 

Policy ‘‘establishes policy to mitigate for 

impacts to desert tortoises and their 

habitats including compensation for 

residual impacts that cannot otherwise 

be mitigated. Mitigation, including 

compensation must be designed to meet 

the purposes of the Rangewide Plan, 

including maintaining viable 

populations as well as maintaining the 

quantity and quality of Category I and 

II desert tortoise habitat’’ (USBLM 

2009b, p. 1). Compensatory funds 

derived from BLM’s compensation 

policy are then used for a variety of 

conservation activities to lessen impacts 

to Sonoran desert tortoises including 

protective tortoise fencing, culverts for 

crossing, land acquisition, and research 

(AIDTT 2000, p. 19). Details of this 

policy can be found in USBLM (2009b, 

pp. 1–45). 

The BLM implements various 

objectives and management actions 

through resource management plans 

unique to certain geographic regions of 

BLM-managed lands (USBLM 2010, p. 

3). Currently, there are eight individual 

resource management plans, some 

recently issued and others up to 22 

years old, representing the areas with 

potential Sonoran desert tortoise habitat 

(USBLM 2010, p. 3). The Phoenix 

Resource Management Plan, which 

directs the management of 

approximately 440,000 ac (178,000 ha) 

of Sonoran desert tortoise habitat, does 

not contain district-specific 

management actions, but incorporates 

management actions described in the 

Strategy for Desert Tortoise Habitat 

Management on Public Lands in 

Arizona (USBLM 2010, p. 3). 

Approximately 1.1 million ac (455,000 

ha) in the Yuma, Lake Havasu, 

Bradshaw-Harquahala, and Kingman 

resource management planning areas 

that were considered Sonoran desert 

tortoise habitat have been designated as 

‘‘priority habitats,’’ meaning that the 

BLM prioritizes management of wildlife 

habitat over other multiple-use activities 

(USBLM 2010, p. 3). 

The BLM can directly or indirectly 

manage for the Sonoran desert tortoise 

through the process of land designation, 

such as Areas of Critical Environmental 

Concern (ACEC) and Wilderness Areas. 

In the case of ACECs, those values may 

pertain to specific species or habitats, or 

cultural or scenic values (AIDTT 2000, 

p. 22). Sonoran desert tortoises were the 

impetus for the Poachie and McCracken 

ACECs, while other ACECs benefit the 

Sonoran desert tortoise through broad 

protections, such as in the Agua Fria 

and Ironwood Forest National 

Monuments (AIDTT 2000, p. 22). 

Sixteen Arizona ACECs contain Sonoran 


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desert tortoise habitat (AIDTT 2000, p. 

22). ACEC designations facilitate the 

minimization of surface-disturbing 

activities, such as vehicular travel, 

camping, fire use, mineral extraction 

activities, and grazing (AIDTT 2000, p. 

22). There are also 48 wilderness areas 

managed by the BLM in Arizona, 

including approximately 850,000 ac 

(344,000 ha) of Sonoran desert tortoise 

habitat, through ‘‘reclaiming damaged 

areas, reclaiming old vehicle ways and 

routes, establishing campfire and 

camping policies to avoid resource 

impacts, establishing livestock grazing 

use objectives with respect to desired 

vegetation, setting objectives for wildlife 

habitat including the desert tortoise, and 

setting prescriptions for wildfire’’ 

(AIDTT 2000, pp. 22–23). In addition, 

the BLM manages Sonoran desert 

tortoise habitat in Wilderness Areas and 

National Monuments with an emphasis 

on maintaining natural conditions and 

biological function of these areas 

(USBLM 2010, p. 10). Approximately 22 

percent of categorized Sonoran desert 

tortoise habitat falls under these 

management prescriptions on BLM 

lands in Arizona (USBLM 2010, p. 10). 

Livestock grazing is the most 

widespread land-use activity permitted 

on BLM lands, with 273 individual 

allotments covering approximately 6 

million ac (2.4 million ha), and 74 

percent of Sonoran desert tortoise 

habitat in the U.S. on their lands 

(Rosmarino and Connor 2008, p. 49). A 

policy was developed by the BLM’s 

Arizona State Office in 1994, addressing 

livestock use of upland vegetation 

growth in response to significant winter 

precipitation, ensuring adequate 

amounts of forage remained for the 

Sonoran desert tortoise (and other 

species) before and after livestock use. 

These ‘‘ephemeral’’ pastures or 

allotments are permitted for 30 days of 

livestock grazing, with additional 30- 

day extensions if monitoring concludes 

adequate forage capacity exists (AIDTT 

2000, p. 22). AIDTT (2000, p. 22) 

viewed this grazing policy as a 

‘‘significant protective change that 

ensured forage for other animals, such 

as desert tortoises, and also ensured that 

perennial plants would not be damaged 

due to insufficient ephemeral growth.’’ 

In 1997, the BLM (USBLM 1997, pp. 1– 

18) further developed standards and 

guidelines for livestock grazing and 

rangeland health. In upland sites, the 

BLM standard is ‘‘Upland soils exhibit 

infiltration, permeability, and erosion 

rates that are appropriate to soil type, 

climate and landform (ecological site)’’ 

(USBLM 1997, p. 5). To assess whether 

an allotment is meeting this standard, 

the BLM uses descriptive criteria that 

pertain to soil conditions, ground cover, 

and erosion rates (USBLM 1997, p. 5). 

The BLM generally prohibits mineral 

material sales (mining activities) in 

Category I and II Sonoran desert tortoise 

habitat, but requests are evaluated on a 

case-by-case basis (USBLM 2010, p. 3). 

For example, in the Phoenix District, the 

BLM has denied 11 such mineral 

material sales, while others have been 

denied in the Tucson District, to prevent 

potential impacts to Sonoran desert 

tortoises and their habitat (USBLM 

2010, p. 4). 

In summary, the BLM considers the 

Sonoran desert tortoise in its land 

management planning and has denied 

or altered projects which could 

adversely affect the Sonoran desert 

tortoise or its habitat, specifically with 

respect to mining and livestock-grazing 

activities. However, we are not aware of 

specific actions the BLM is taking with 

respect to invading nonnative plant 

species and subsequent wildfire 

concerns, vandalism of tortoises, feral 

dog predation, or management to 

counter anticipated climate change. In 

addition and as discussed below, BLM 

management of off-highway vehicle use 

on their lands is not protective of 

Sonoran desert tortoise populations. 

Therefore, we conclude that BLM 

management of the Sonoran desert 

tortoise and its habitat is currently 

inadequate. 

U.S. Forest Service 

The Sonoran desert tortoise is 

included on the U.S. Forest Service’s 

Regional Forester’s Sensitive Species 

List, which means it is evaluated in all 

biological evaluations for activities and 

projects proposed within its habitat 

(AIDTT 2000, p. 35). Sonoran desert 

tortoises occur on the Prescott 

(Bradshaw Ranger District), Coronado 

(Santa Catalina and Nogales Ranger 

Districts), and Tonto National Forests in 

Arizona (Murray and Schwalbe 1993, p. 

39). The Tonto National Forest manages 

the most Sonoran desert tortoise habitat 

of the three National Forests in Arizona, 

where they occur in the Cave Creek, 

Mesa, Globe, and Tonto Basin Ranger 

Districts. 

Multiple land uses occur on these 

National Forests, including recreation, 

camping, livestock grazing, and offhighway vehicle use. Approximately 46 

livestock grazing allotments on the 

Tonto National Forest partially or 

wholly overlap the potential range of 

the Sonoran desert tortoise, with several 

rated as having impaired or 

unsatisfactory soil conditions (AIDTT 

2000, p. 37). We are not aware of the 

exact number of livestock grazing 

allotments that overlap Sonoran desert 

tortoise habitat on the Coronado or 

Prescott National Forests. With the 

exception of livestock grazing, the 

majority of land uses that have the 

highest potential to affect the Sonoran 

desert tortoise occur in districts adjacent 

to urbanized areas, such as the Santa 

Catalina Ranger District on the 

Coronado National Forest (adjacent to 

the Tucson metropolitan area) and the 

Cave Creek and Mesa Ranger Districts 

on the Tonto National Forest (adjacent 

to the Phoenix metropolitan area). 

While the Coronado National Forest 

does not have specific management 

policies for the Sonoran desert tortoise, 

two policies may serve its benefit: (1) 

‘‘Provide habitat for wildlife populations 

consistent with the goals outlined in the 

Arizona and New Mexico Department of 

Game and Fish Comprehensive Plans 

and consistent with other resource 

values;’’ and, (2) ‘‘Provide for ecosystem 

diversity by at least maintaining viable 

populations of all native and desirable 

nonnative wildlife, fish, and plant 

species through improved habitat 

management’’ (AIDTT 2000, p. 36). 

In September 2005, Region 3 of the 

U.S. Forest Service adopted a new 

policy for rangeland adaptive 

management (USFS 2007, pp. 1–34), 

called the Chapter 90 policy. Under this 

policy, limits on timing, intensity, 

frequency, and duration of livestock 

grazing are set in Allotment 

Management Plans. Monitoring and 

adaptive management are key attributes 

of the Chapter 90 policy and are 

intended to ensure livestock grazing 

outcomes meet desired resource 

conditions which include the needs of 

wildlife such as the Sonoran desert 

tortoise. The term ‘‘conservative use’’ in 

this policy is defined as forage 

utilization on key forage species 

between 30 and 40 percent or less of 

annual forage production by weight for 

herbaceous perennials, and 50 percent 

or less on woody browse species (USFS 

2007, pp. 26, 30). It is inherent in the 

term ‘‘conservative use’’ that watershed 

conditions and vegetative ground cover 

will be optimized as appropriate to 

various range sites. At no time is 

excessive use considered acceptable. 

The goal is to achieve conservative use 

in the uplands over successive years. 

This strategy recognizes the importance 

of adaptive management, and may 

include adjustments of timing, intensity, 

frequency, and duration of grazing to 

reach resource objectives (USFS 2007, 

pp. 13–14). 

Implementation monitoring of 

livestock grazing under conservative use 

practices can be done using a variety of 

methods, and is designed to provide 


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information that will enable decisionmakers to practice adaptive 

management by making necessary 

changes needed for plant development 

and recovery, and to assess physical 

improvements to allotments (USFS 

2007, pp. 16–17). Effectiveness 

monitoring of conservative use practices 

documents whether management 

actions are having the expected progress 

toward achieving resource-management 

objectives, and is used to track upland 

vegetative and soil condition over the 

long term (USFS 2007, pp. 16–17). From 

a short-term (within-year) perspective, 

wildlife habitat and watershed 

conditions are gauged by monitoring 

seasonal utilization on key forage 

species during the grazing period. Due 

to a warmer climate, variable 

precipitation, and mild winters, 

seasonal-utilization monitoring is 

important because the end of a 

particular growing season is not welldefined for all plant communities in 

Sonoran desert tortoise habitat on Forest 

Service lands. In review of this policy, 

we conclude that implementation of the 

Forest Service’s rangeland management 

strategy is likely to retain physical 

characteristics necessary to provide for 

the necessary forage and shelter 

requirements for Sonoran desert 

tortoise. 

In summary, the USFS considers the 

Sonoran desert tortoise in all biological 

evaluations for activities and projects 

proposed within its habitat. The USFS 

has developed a system of adaptive 

management for livestock grazing on 

their lands, using resource monitoring 

to indicate when changes in land 

conditions occur or prescribed use 

levels are unsustainable, preventing 

excessive harm to sensitive Sonoran 

desert tortoise habitat. However, we are 

not aware of specific actions the USFS 

is taking with respect to management of 

invasive, nonnative plant species and 

subsequent wildfire concerns, 

vandalism of tortoises, feral dog 

predation, or efforts to counter 

anticipated climate change. In addition, 

and as discussed below, USFS 

management of off-highway vehicle on 

their lands is not protective of Sonoran 

desert tortoise populations. Therefore, 

we conclude that USFS management of 

the Sonoran desert tortoise and its 

habitat is currently inadequate. 

Off-Highway Vehicle Management and 

Enforcement on Public Lands 

While both the USFS and BLM have 

developed broad, strategic plans to 

manage off-highway vehicle use, these 

plans have been found to be missing 

some key elements that could improve 

off-highway vehicle management, such 

as results-oriented goals, strategies to 

achieve the goals, timeframes for 

implementing strategies, or performance 

measures to monitor incremental 

progress (USGAO 2009, p. 16). 

Limitations of the USFS’s strategic plan 

have resulted from a general failure to 

address motorized travel designations 

on the ground, communicate with the 

public, monitor off-highway vehicle 

trail systems, or enforce off-highway 

vehicle regulations (USGAO 2009, p. 

16). 

In response to public concerns, the 

BLM developed the ‘‘National 

Management Strategy for Motorized OffHighway Vehicle Use on Public Lands’’ 

(USBLM 2001, p. 9). This strategy 

outlines action items that are to be 

implemented ‘‘as soon as practical’’ 

(USBLM 2001, pp. 10–21). However, the 

U.S. Government Accountability Office 

(2009, pp. 17–18) found that ‘‘[d]espite 

identifying numerous goals and 

strategies to achieve the goals, BLM’s 

recreation plan does not identify any 

timeframes for implementing the 

strategies or any performance measures 

for monitoring incremental progress 

* * *. Without performance measures 

and timeframes, the BLM cannot ensure 

that it is making progress on achieving 

its goals in a timely manner.’’ 

The BLM generally prohibits 

competitive off-highway vehicle events 

that could adversely affect Sonoran 

desert tortoises, from March 31 through 

October 15, but noncompetitive offhighway vehicle activities are evaluated 

on a case-by-case basis, and mitigation 

measures are implemented to reduce 

potential impacts to Sonoran desert 

tortoises (USBLM 2010, p. 4). Although 

requests to permit rock crawling events 

(defined in Factor A, above) have been 

denied where they were proposed in 

Sonoran desert tortoise habitat (USBLM 

2010, p. 4), this activity still occurs 

outside of organized ‘‘events.’’ Rock 

crawling is allowed where it might 

adversely affect the Sonoran desert 

tortoise or its habitat (USBLM 2010, p. 

4). 

Both the USFS and BLM acknowledge 

limited staff and financial resources for 

off-highway vehicle management 

(USGAO 2009, p. 37). Off-highway 

vehicles that pass over undisturbed 

desertscrub habitat may leave tracks 

which are then noticed by others and 

subsequently used until the trail is 

mistakenly recognized as a designated 

route; this process is known as ‘‘route 

proliferation’’ (Brooks and Lair 2005, p. 

5). Illegal proliferation of roads and 

unauthorized use of off-highway 

vehicles has left persistent scars in the 

Sonoran Desert (Abella 2010, p. 1249). 

In the Kingman area, between 1994 to 

1999, the BLM tracked an increase of 

greater than 20 percent of off-highway 

vehicle use within the range of the 

Sonoran desert tortoise, and reported 

124 and 123 violations of improper 

vehicle use Statewide in 1998 and 1999, 

respectively (AIDTT 2000, p. 10). The 

BLM has only 195 law enforcement 

officers nation-wide, which means that 

on average, each officer is responsible 

for overseeing approximately 1.2 

million ac (490,000 ha) of land, or 1,875 

sq mi (4,856 sq km) (USGAO 2009, p. 

38). Law enforcement of off-highway 

vehicle use in the Arizona-Mexico 

border region is further complicated by 

increasing demands to address drug 

smuggling and other border-related 

issues (USGAO 2009, p. 39). To address 

an inadequate law enforcement 

presence, the BLM’s Phoenix District 

has initiated an ‘‘ambassador program’’ 

which recruits volunteers to ‘‘educate 

users and promote safe, sustainable offhighway vehicle use in the area’’ 

(USGAO 2009, p. 38). The use of signs 

is a common method to enforce offhighway vehicle regulations on Federal 

lands, but signs are often vandalized 

(sometimes within 48 hours of their 

installation), and must be frequently 

replaced (USGAO 2009, p. 40). 

In addition to wildlife management 

(described below), the AGFD also 

licenses, promulgates rules for, and 

assists with regulatory enforcement of 

off-highway vehicles use on public 

lands. In January 2009, the AGFD 

created an off-highway vehicle decal 

program, designed to increase revenues 

for off-highway vehicle enforcement, 

education, and signage on public lands 

(AGFD 2009, p. 1). However, as of 

November 2009, only 21 percent of all 

eligible off-highway vehicles and offhighway vehicle owners in Arizona 

were participating in the off-highway 

vehicle decal program (AGFD 2009, p. 

1). 

In review of off-highway vehicle 

management on USFS and BLM lands in 

Arizona, we conclude that the current 

status of law enforcement is inadequate 

to protect Sonoran desert tortoises and 

their habitat. We considered the 

following in making this conclusion: (1) 

The documented adverse effects of offhighway vehicle use on Sonoran desert 

tortoise habitat (see Factor A); (2) the 

propensity for off-highway vehicle users 

to illegally collect Sonoran desert 

tortoises in the wild (discussed in 

Factor B); (3) the significant, and 

growing, use of off-highway vehicles in 

Arizona (discussed above in Factor A); 

and (4) the deficient level of law 

enforcement staff responsible for 

regulating the use of off-highway 

vehicles on these lands discussed above. 


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In addition, we accept the U.S. 

Government Accountability Office 

finding that the USFS and BLM goals 

and objectives, intended to protect trust 

resources from damage associated with 

off-highway vehicle use, miss some key 

elements that could improve offhighway vehicle management. 

Ironwood and Mesquite Harvest 

To address ecological problems 

stemming from wide-ranging mesquite 

and ironwood harvesting in northern 

Mexico (discussed above in Factor A), 

the Arizona-Mexico Commission, and 

state government in Sonora, Mexico, 

made it illegal to cut and export these 

species (American University Database 

2010, p. 4). Additionally, Mexico’s 

Federal government has protected the 

ironwood tree, adding additional 

monitoring and enforcement to protect 

remaining ironwood trees (American 

University Database 2010, p. 4). Finally, 

non-profit, bi-national groups are raising 

awareness and funds to help stop these 

practices in Mexico (American 

University Database 2010, p. 4). We 

consider these regulations effective in 

reducing the harvest of ironwood and 

mesquite in the future, but the land area 

already adversely modified by ironwood 

and mesquite harvesting, as discussed 

in Factor A above, constitutes a current 

threat to Sonoran desert tortoise habitat. 

U.S. Department of Defense 

Three prominent Department of 

Defense-administered lands maintain 

populations of Sonoran desert tortoise: 

The Yuma Proving Ground, Barry M. 

Goldwater Range, and Florence Military 

Reservation. The Yuma Proving Ground, 

administered by the Department of the 

Army, encompasses 840,000 ac (340,000 

ha) in LaPaz and Yuma Counties of 

southwestern Arizona (AIDTT 2000, p. 

32). The majority of land on the Yuma 

Proving Ground is closed to public 

access year-round with the exception of 

133,000 ac (54,000 ha) that are open to 

hunting access for 6 months per year. 

The relative inaccessibility of these 

lands results in little disturbance to the 

Sonoran desert tortoise and its habitat 

(AIDTT 2000, p. 33). In addition, the 

Yuma Proving Ground developed a 

management plan for the Sonoran desert 

tortoise in 1996 (AIDTT 2000, pp. 33– 

34). We are uncertain whether or not 

this management plan is effective in 

Sonoran desert tortoise conservation on 

the Yuma Proving Ground. 

The Barry M. Goldwater Range, used 

for aerial training exercises, is the 

largest contiguous portion of 

Department of Defense lands in Arizona 

(1.7 million ac, 690,000 ha), and is 

jointly administered by the Luke Air 

Force Base and Marine Corps Air 

Station—Yuma, and is located in 

portions of Maricopa, Yuma, and Pima 

Counties (AIDTT 2000, pp. 32–33). The 

majority of military training exercises 

occur over the valleys where Sonoran 

desert tortoise densities are low, leaving 

the majority of Sonoran desert tortoise 

populations unexposed to potential 

threats from these exercises (AIDTT 

2000, p. 34). Outside of training 

exercises, the public may access the 

Barry M. Goldwater Range with a 

permit, via designated routes (AIDTT 

2000, p. 34). 

The Florence Military Reservation 

encompasses 25,752 ac (10,421 ha), and 

is jointly administered by the Arizona 

Army National Guard, the Arizona State 

Land Department, and the BLM (AIDTT 

2000, p. 34). As stated previously, the 

Sonoran desert tortoise population on 

the Florence Military Reservation is 

unique among other populations across 

their range, because of the conspicuous 

absence of boulder outcrops and use by 

tortoises of broad alluvial fans and 

incised washes (Riedle et al. 2008, p. 

418; Grandmaison et al. in press, p. 4). 

There is significant public access and 

multiple land uses allowed on the 

Florence Military Reservation, with no 

specific protections afforded to the 

Sonoran desert tortoise (AIDTT 2000, p. 

34). Sonoran desert tortoise home ranges 

overlap with concentrated military 

training areas on the Florence Military 

Reservation (Grandmaison et al. in 

press, p. 1). When not used for military 

training, these areas serve as 

recreational areas for camping, hunting, 

and off-highway vehicle use, which 

cumulatively have degraded Sonoran 

desert tortoise habitat by removing 

vegetative cover, which in turn may 

have led to reduced use of these areas 

by Sonoran desert tortoises 

(Grandmaison et al. in press, p. 4). 

There are few data on the potential 

effects of military operations to Sonoran 

desert tortoises on U.S. Department of 

Defense lands, specifically with respect 

to aircraft operations. However, Bowles 

et al. (1999, pp. 19–26) tested the 

response of Mojave desert tortoises to 

simulated aircraft sound and to sonic 

booms associated with aircraft, in an 

attempt to ascertain potential effects to 

wild desert tortoises that are exposed to 

such auditory stimuli within and 

adjacent to aircraft flight paths and 

military training areas. They found that 

Mojave desert tortoises could detect 

these sounds and had somewhat 

subdued reactions ranging from 

‘‘freezing’’ all movements, to bladder 

voiding (Bowles et al. 1999, pp. xxiixxiv). We are not certain whether 

Sonoran desert tortoise populations on 

U.S. Department of Defense lands are 

subjected to aircraft noise at similar 

sound pressure levels, but we presume 

they are, because aircraft training occurs 

on these lands in Arizona. 

In summary, the Barry M. Goldwater 

Range and Yuma Proving Ground 

provide for considerable protection of 

Sonoran desert tortoise habitat on their 

installations as a result of access 

restrictions or through a permitting 

program. The Barry M. Goldwater Range 

also created a management plan 

specifically for the Sonoran desert 

tortoise in 1996. In addition, since these 

lands are unlikely to be developed in 

the future, these areas will likely be 

important in future Sonoran desert 

tortoise conservation planning. 

However, the literature has documented 

that current management on the 

Florence Military Reservation is not 

adequate for protecting Sonoran desert 

tortoises or their habitat. In discussion 

under Factors A and B, we discussed 

several activities that occur in this area 

which adversely affect the Sonoran 

desert tortoise and its habitat. 

U.S. Fish and Wildlife Service National 

Wildlife Refuges 

Sonoran desert tortoises occur on 

several National Wildlife Refuges in 

Arizona. Sonoran desert tortoise 

populations are highest on the Kofa, 

Buenos Aires, and Cabeza Prieta 

National Wildlife Refuges, although 

they also may occur in low densities 

within the Cibola, Imperial, and Lake 

Havasu National Wildlife Refuges along 

the Colorado River (AIDTT 2000, p. 31). 

The mission of the National Wildlife 

Refuge System is ‘‘ * * * to administer 

a national network of lands and waters 

for the conservation, management, and 

where appropriate, restoration of the 

fish, wildlife, and plant resources and 

their habitats within the United States 

for the benefit of present and future 

generations of Americans’’ (AIDTT 2000, 

p. 31). Management on these National 

Wildlife Refuges is largely protective of 

Sonoran desert tortoises, as multiple use 

activities such as livestock grazing and 

off-highway vehicle use are prohibited 

(AIDTT 2000, p. 31). However, the U.S. 

Border Patrol uses administrative roads, 

which are closed to public use in these 

areas, along the border region of the 

Buenos Aires and Cabeza Prieta 

National Wildlife Refuges, which may 

affect Sonoran desert tortoises or their 

habitat in these areas. For further 

discussion of the effect of U.S. Border 

Patrol operations on Sonoran desert 

tortoises or their habitat, see the section 

on Undocumented Immigration in 

Factor A of this finding. 


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In summary, we conclude that the 

regulations establishing the mission and 

management of the National Wildlife 

Refuge system are consistent with 

Sonoran desert tortoise habitat 

management, and are therefore adequate 

to protect the tortoise where it occurs on 

our lands. 

National Park Service 

Sonoran desert tortoise habitat occurs 

on Organ Pipe Cactus National 

Monument, Saguaro National Park, and 

the Lake Mead National Recreation Area 

(AIDTT 2000, p. 27). The National Park 

Service is mandated by law to ‘‘conserve 

the scenery and the natural and historic 

objects and the wildlife therein and to 

provide for the enjoyment of the same 

in such manner and by such means as 

will leave them unimpaired for the 

enjoyment of future generations’’ 

(AIDTT 2000, p. 26). The resourcemanagement goals on National Park 

Service lands are broad in scope, and 

include reducing ground disturbance, 

developing and implementing inventory 

and monitoring programs, assessing and 

mitigating resource disturbance, and 

developing environmental restoration 

and research programs (AIDTT 2000, p. 

26). Livestock grazing and off-highway 

vehicle use are not permitted on 

National Park Service lands. While the 

National Park Service has no specific 

provision for Sonoran desert tortoise 

conservation on their lands, all wildlife 

inhabiting National Park Service lands 

in Arizona, including the Sonoran 

desert tortoise, are protected, and 

possession or removal of wildlife is 

prohibited (AIDTT 2000, p. 26). 

However, where National Park 

Service lands are adjacent to urban 

areas, such as Saguaro National Park 

outside of the Tucson metropolitan area, 

threats to Sonoran desert tortoises have 

been documented. Averill-Murray and 

Swann (2002, p. 1) and Jones (2008, p. 

66) documented threats such as 

harassment and predation by feral 

domestic dogs, releases of captive 

Sonoran desert tortoises and exotic 

species (that may transmit diseases), 

road mortality, and illegal collection of 

tortoises, as affecting the Sonoran desert 

tortoise population on Saguaro National 

Park land. 

In summary, we acknowledge that the 

mission and management of the 

National Park Service and their lands is 

consistent with Sonoran desert tortoise 

habitat management, but where Park 

Service land is affected by adjacent 

urbanized areas, adequate regulatory 

protections for the tortoise have not 

been realized. 

Arizona State Land Department 

Arizona State Trust Land, managed to 

derive revenues for trust beneficiaries 

including educational, health, and penal 

institutions, comprises 13 percent of all 

land in Arizona, much of which 

contains Sonoran desert tortoise habitat 

(AIDTT 2000, p. 15). In general, the 

mission of the Arizona State Land 

Department is to maximize economic 

return (AIDTT 2000, p. 16). The Arizona 

State Land Department has no broad 

management practices, policies, or 

directives that pertain to Sonoran desert 

tortoise management, but does 

coordinate with the AGFD on some 

projects to reduce potential impacts to 

the Sonoran desert tortoise (AIDTT 

2000, p. 16). Four Sonoran desert 

tortoise monitoring sites occur partially 

or fully on Arizona State Trust Lands: 

Granite Hills, Little Shipp Wash, 

Tortolita Mountains, and Picacho 

Mountains; two of these sites, Granite 

Hills (Pinal County) and Little Shipp 

Wash (Yavapai County) are long-term 

monitoring plots (AIDTT 2000, pp. 5–6, 

15). Other blocks of Sonoran desert 

tortoise habitat on Arizona State Trust 

Lands occur west of the Upper Burro 

Creek, Arrastra Mountain, and Tres 

Alamos wilderness areas in Yavapai 

County and from the Tortolita to the 

Tortilla Mountains in Pinal County 

(AIDTT 2000, p. 15). Recreation on State 

Trust Lands is generally not monitored 

and therefore may not be protective of 

Sonoran desert tortoises or their habitat. 

The Arizona State Land Department is 

considering restricting access to its 

lands for purposes of conducting 

wildlife studies. These access 

restrictions may prohibit further 

research due to numerous permit 

requirements. These new policies are 

not yet in place and could be changed 

prior to final issuance (Jody Latimer, 

ASLD, 2010, pers. comm.). If 

implemented as described by Latimer 

(ASLD, 2010, pers. comm.), these 

proposed procedures and fees have the 

potential to limit Sonoran desert tortoise 

monitoring and research on Arizona 

State Trust lands in the future through 

new monetary and procedural 

requirements. While these new policies 

and regulations are not yet in effect, 

even if they are implemented it appears 

they will not address conservation and 

management of the Sonoran desert 

tortoise and its habitat, and further, may 

have a negative effect by potentially 

restricting important research needed 

for conservation of the tortoise. 

Furthermore, we are not aware of 

specific actions the Arizona State Land 

Department is taking with respect to 

management of invasive, nonnative 

plant species and subsequent wildfire 

concerns, vandalism of tortoises, feral 

dog predation, or efforts to counter 

anticipated climate change. Therefore, 

we conclude that Arizona State Land 

Department management of the Sonoran 

desert tortoise and its habitat is 

currently inadequate. 

Arizona Game and Fish Department 

The Arizona Game and Fish 

Department (AGFD) currently classifies 

the Sonoran desert tortoise as a Tier 1b 

‘‘Species of Greatest Conservation Need’’ 

AGFD (2006, p. 485). A Tier 1b species 

is one that requires immediate 

conservation actions aimed at 

improving conditions through 

intervention at the population or habitat 

level. Before April 28, 1989, the AGFD 

allowed the collection and possession of 

one lawfully captured Sonoran desert 

tortoise per person (AIDTT 2000, p. 14). 

After this date, under Commission 

Order 43, the AGFD closed the season 

on Sonoran desert tortoises, which 

prohibited the take of desert tortoises 

from the wild, except under special 

permit (for example, scientific or 

educational) (AIDTT 2000, p. 14). 

Unless otherwise prescribed in title 17, 

it is unlawful to [t]ake, possess, 

transport, buy, sell or offer or expose for 

sale wildlife except as expressly 

permitted by this title’’ (ARS 17–309). It 

is also unlawful to release wildlife into 

the wild except as authorized by the 

Arizona Game and Fish Commission or 

as defined in title 3 (see ARS 17–306). 

As a closed-season species, the desert 

tortoise cannot be taken from the wild 

or possessed without special permit 

(Commission Order 43). As restricted 

live wildlife (R12–4–406), they cannot 

be imported, exported, or possessed 

without special license or lawful 

exemption. 

Enforcement of the State closure on 

collection of Sonoran desert tortoises 

occurs when directly observed by law 

enforcement personnel, but the 

remoteness of many Sonoran desert 

tortoise populations makes enforcement 

strategies and techniques problematic 

(AIDTT 2000, p. 14). Furthermore, 

regulations regarding the collection or 

possession of Sonoran desert tortoises 

are poorly known to the public, 

emphasizing the importance of 

education efforts (AIDTT 2000, p. 14). 

The effect of illegal collection of 

Sonoran desert tortoises on wild 

populations in Mexico is largely 

unknown (see Factor B). 

The AGFD has led Sonoran desert 

tortoise conservation in Arizona through 

research, guidance provided to the 

public and other agencies, and 

cooperative conservation management 


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on public lands. For example, the AGFD 

(2007a, p. 1) provides construction and 

development contractors with guidance, 

should a Sonoran desert tortoise be 

encountered within an area of a 

development. In addition, the AGFD 

(2007b, p. 1) also provides 

environmental consultants guidance on 

proper survey techniques and 

considerations when surveying for 

Sonoran desert tortoises. AGFD (2006, 

pp. 485–487) described numerous 

management priorities with respect to 

mitigating potential threats facing the 

tortoise in Arizona. The 

recommendations outlined in these 

documents are recommended guidance, 

voluntary in nature, and no reporting 

requirements are mandated. Therefore, 

we are uncertain whether project 

proponents implement these 

recommendations. 

Arizona Interagency Desert Tortoise 

Team 

As part of a multi-agency 

collaborative project, the Arizona 

Interagency Desert Tortoise Team 

(AIDTT) was formed in 1985 to 

coordinate research and management of 

Sonoran desert tortoise populations in 

Arizona. Participating agencies in the 

AIDTT manage habitat, manage the 

species, or conduct research, and 

include the AGFD, Arizona State Lands 

Department, U.S. Forest Service, BLM, 

U.S. Bureau of Reclamation, U.S. 

Bureau of Indian Affairs, U.S. Fish and 

Wildlife Service, National Park Service, 

U.S. Geological Survey, and several U.S. 

Department of Defense military 

reservations (AIDTT 1996, Preface; 

AIDTT 2000, p. 2). The AIDTT is cochaired by representatives from the U.S. 

Fish and Wildlife Service (Arizona 

Ecological Services Office) and the 

AGFD. Since its inception, the AIDTT 

has collaborated in the development of 

numerous documents addressing 

conservation of the Sonoran desert 

tortoise including ‘‘Survey Protocol for 

Sonoran Desert Tortoise Monitoring 

Plots: Reviewed and Revised’’ (AverillMurray 2000a), ‘‘Status of the Sonoran 

Population of the Desert Tortoise in 

Arizona: An Update’’ (Averill-Murray 

2000b), ‘‘Guidelines for Handling 

Sonoran Desert Tortoises Encountered 

on Development Projects’’ (AGFD 

2007a), ‘‘Desert Tortoise Survey 

Guidelines for Environmental 

Consultants’’ (AGFD 2007b), and 

‘‘Recommended Standard Mitigation 

Measures for Projects in Sonoran Desert 

Tortoise Habitat’’ (AIDTT 2008). 

Available online, the AIDTT (2008, pp. 

1–7) offers guidance on standard types 

of mitigation for projects that may affect 

Sonoran desert tortoises; these measures 

are voluntary. 

The AIDTT’s Memorandum of 

Understanding, signed in 1995, 

established specific objectives for the 

team including: (1) Ensuring the 

survival of the species; (2) preventing 

loss of the species; and (3) improving 

the quality of Sonoran desert tortoise 

habitat in Arizona, with the team to 

function as an advocate for the Sonoran 

desert tortoise (AIDTT 1996, Preface; 

AIDTT 2000, p. 2). A management plan 

for the Sonoran desert tortoise 

completed in 1996 called for improved 

monitoring protocols, the 

implementation of threat-minimization 

activities, and the creation of Sonoran 

Desert Management Areas (AIDTT 1996, 

pp. 20–26). However, common 

criticisms of the 1996 plan include: (1) 

Lack of meaningful goals and objectives; 

(2) lack of political willpower without 

legal protection for the Sonoran desert 

tortoise; (3) failure to designate Sonoran 

Desert Management Areas; and (4) poor 

funding (AIDTT 2000, p. 2). 

Collectively, these recognized 

shortcomings hampered the 

implementation of threat-minimization 

activities. In recognition of these 

shortcomings, the AIDTT is currently in 

the process of developing a State 

Conservation Agreement, Assessment 

and Strategy with the goal of identifying 

reasonable, obtainable conservation 

goals and objectives that will contribute 

to Sonoran desert tortoise conservation 

on public lands in a meaningful 

capacity. 

Mexican Government (Secretaria de 

Medio Ambiente y Recursos Naturales) 

Throughout Mexico, the desert 

tortoise is listed as ‘‘Amenazadas,’’ or 

Threatened, by the Secretaria de Medio 

Ambiente y Recursos Naturales 

(SEMARNAT) (Bury et al. 2002, p. 86; 

Howland and Rorabaugh 2002, p. 348; 

SEDESOL 2008, p. 99). Threatened 

species are ‘‘those species, or 

populations of the same, likely to be in 

danger of disappearing in a short or 

medium timeframe, if the factors that 

negatively impact their viability, cause 

the deterioration or modification of their 

habitat or directly diminish the size of 

their populations continue to operate’’ 

(SEDESOL 2008 (NOM–059–ECOL– 

2008), p. 5). This designation prohibits 

taking of the species, unless specifically 

permitted, and also prohibits any 

activity that intentionally destroys or 

adversely modifies its habitat (SEDESOL 

2000 and 2001 (NOM–059–ECOL–2001). 

However, activities that unintentionally 

destroy or adversely modify their 

habitat do not appear to be specifically 

prohibited (e.g., cultivation of 

buffelgrass for livestock grazing). 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 or their habitat (SEDESOL 1988 

(LGEEPA)). 

The Mexican Federal agency known 

as the Instituto Nacional de Ecologı ́a 

(INE) is generally considered the 

Mexican counterpart to the U.S. Fish 

and Wildlife Service. 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 (the Mexican 

equivalent to a threatened and 

endangered species list), and if 

appropriate, the nomination of species 

to the list. INE developed the Method of 

Evaluation of the Risk of Extinction of 

the Wild Species in Mexico (MER), 

which unifies the criteria of decisions 

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 (quality) 

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 implemented use of the MER 

in 2006; therefore, all species previously 

listed in the NOM–059 were, in many 

cases, based solely on expert review and 

opinion. 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 for applying those factors to the 

definitions of the various listing 

categories. 

In summary, while the desert tortoise 

is federally listed in Mexico, we have 

documented significant threats to its 

persistence in that country (see Factors 

A and C) that are not controlled by the 

listing, and therefore conclude that 

regulations establishing management of 

the Sonoran desert tortoise in Mexico do 

not provide adequate assurances of its 

continued existence in that country. 

Summary of Factor D 

Numerous State and Federal entities 

have regulations or policies which 

implement management of either the 

Sonoran desert tortoise or its habitat 

throughout the species’ range in 

Arizona. In Mexico, the species is 

currently listed as threatened. In our 


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review of the available information on 

each entity’s management policies and 

regulations, we found numerous 

examples where the Sonoran desert 

tortoise is considered in management 

actions and tortoise-specific mitigation 

measures are mandated, or where land 

activities that could appreciably 

threaten Sonoran desert tortoise 

populations are prohibited. While 

several land managers and agencies in 

Arizona actively consider the Sonoran 

desert tortoise in their resource 

planning, we found deficiencies in 

management of off-highway vehicle use, 

policies and procedures inconsistent 

with Sonoran desert tortoise 

conservation, and some threats such as 

invasive, nonnative plant species and 

subsequent wildfire concerns, 

vandalism of tortoises, feral dog 

predation, or efforts to counter 

anticipated climate change were not 

addressed by land management control. 

Lastly, significant threats we discuss 

above in Factors A and C are not being 

adequately addressed by land managers, 

including invasive, nonnative plant 

species and associated wildfire 

concerns, vandalism of tortoises, feral 

dog predation, and management to 

counter anticipated climate change. 

Although the Sonoran desert tortoise 

is considered a threatened species in 

Mexico, we are not aware of 

conservation planning or enforcement of 

regulations that has occurred because of 

this status. Based upon our review of 

the information pertaining to threats in 

Mexico, it is unlikely that protections 

afforded to the Sonoran desert tortoise 

are adequate to ensure conservation for 

the foreseeable future in Mexico. As a 

result, we conclude that the Sonoran 

desert tortoise is threatened due the 

inadequacy of existing regulatory 

mechanisms, in combination with the 

other threats identified in this finding, 

both now and in the foreseeable future. 

Factor E. Other Natural or Manmade 

Factors Affecting Its Continued 

Existence 

Environmental Contaminants 

Many sources of potential 

contamination presently occur 

throughout the distribution of the 

Sonoran desert tortoise. Copper mining 

in the Sonoran Desert has occurred in 

Arizona and adjacent Mexico for 

centuries, and many of these sites have 

smelters (now decommissioned), which 

are former sources of airborne 

contaminants. In Arizona, historical or 

current large-scale copper mining 

operations exist in Pima, Pinal, Yavapai, 

Gila, and Mohave Counties, which are 

sources of low-level, persistent 

contaminants in surrounding areas as a 

result of fugitive dust, contaminated 

surface runoff, and other mechanisms 

consistent with contaminant fate and 

transport. Soil contamination within 

ephemeral washes from leaching 

operations associated with mining 

activities has occurred throughout the 

Sonoran Desert, and will likely continue 

to occur where these activities take 

place. Sonoran desert tortoises that 

forage in contaminated ephemeral 

washes may ingest toxic constituents 

through soil or contaminated plant 

matter, but we are not aware of any 

specific reports of tortoises that became 

sick or deceased from this risk. The 

mining industry in Mexico is largely 

concentrated in the northern tier of that 

country, with Sonora as the leader for 

generating copper, gold, graphite, 

molybdenum, and wollastonite, as well 

as the leader among Mexican States 

with the most surface area dedicated to 

mining (Stoleson et al. 2005, p. 56). The 

three largest mines (all copper) are 

found in Sonora (Stoleson et al. 2005, p. 

57). The sizes of mines in Sonora vary 

considerably, as do the known 

environmental effects from miningrelated activities (from exploration to 

long after closure), which include 

contamination and drawdown of 

groundwater aquifers, erosion, acid 

mine drainage, fugitive dust, pollution 

from smelter emissions, and landscape 

clearing (Stoleson et al. 2005, p. 57). 

Rowe (2008, p. 623) investigated 

potential effects of persistent, low-level 

contaminants (e.g., heavy metals, 

polychlorinated biphenols, 

organochlorides) on long-lived 

vertebrates (such as the Sonoran desert 

tortoise). Cadmium and lead are of 

special concern due to their toxicity, 

and because they are persistent, 

common environmental contaminants 

(Martı ́nez-Lo ́pez et al. 2010, p. 671). 

Cadmium may affect turtle gonadal 

development, and lead may affect an 

individual tortoise’s susceptibility to 

infections and disease, because it may 

suppress its immune capacity. The latter 

can potentially affect the spread of 

known diseases such as herpesvirus, 

cutaneous dyskeratosis, and URTD 

within and among affected populations 

(Martı ́nez-Lo ́pez et al. 2010, p. 671). As 

stated previously, cutaneous 

dyskeratosis is prevalent within most 

populations of Sonoran desert tortoise 

throughout their distribution in 

Arizona, but this disease has not been 

determined to currently be a significant 

threat to Sonoran desert tortoise 

populations. Another common 

environmental contaminant is the heavy 

metal arsenic, which is carcinogenic 

(cancer-causing) and may also already 

occur in naturally-high levels in some 

areas of the American Southwest 

(Seltzer and Berry 2005, p. 263). 

Because the Sonoran desert tortoise is 

characterized as having a delayed sexual 

maturation and a long generation time, 

potential effects from persistent, lowlevel contaminants in the environment 

include: (1) Mortality before 

reproduction, (2) chronic accumulation 

of contaminants that may be transferred 

to offspring upon maturation, (3) 

reduced size at maturity reducing 

offspring quantity or quality, (4) delayed 

expression of fitness effects at the 

population level, and (5) delayed 

recovery of populations following 

abatement of fitness effects (Rowe 2008, 

p. 626). In several areas of the Sonoran 

Desert in Arizona and Sonora, Mexico, 

mining operations and other humanrelated activities can result in 

remobilization and concentration of 

elemental toxicants in the air, on the 

soil surface, and on the surfaces of 

forage plants, both from ground 

disturbance and from long-range 

atmospheric deposition associated with 

old copper smelter sites, coal-fired 

power plants, and fugitive dust from 

abandoned and active mining sites 

(Seltzer and Berry 2005, p. 263; Rowe 

2008, p. 628). The most likely routes for 

exposure of Sonoran desert tortoises to 

these types of contaminants are through 

ingestion of contaminated soil or plant 

matter, or through inhalation of 

contaminated dust or particles, 

especially when a tortoise constructs or 

modifies a burrow (Seltzer and Berry 

2005, p. 263; Hinck et al. 2010, p. 287). 

We have no specific records of Sonoran 

desert tortoises becoming sick or dying 

from this type of contamination; effects 

from these contaminants can be 

significantly delayed and slow to 

manifest. Also, few field researchers are 

sampling wild tortoises to test for 

contaminant exposure. 

Conversion of habitat to large-scale 

agriculture has been concentrated in 

Sonora, Mexico, which has provided 

sources of surface and groundwater 

pollution such as salt intrusion due to 

agricultural water use extraction; 

municipal and agricultural discharges; 

and solid waste, including cast-off 

agrochemical containers, winery 

residues, and hog farm muck (Nauman 

2007, p. 1). The extent to which 

Sonoran desert tortoises drink freely 

from perennial or intermittent streams is 

not known, but since tortoises are 

opportunistic drinkers, we presume 

they use streams as a source of water in 

addition to ephemeral pools generated 

by precipitation events, and that they 

may subsequently ingest such toxins. 


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In conclusion and based upon our 

review of the best available scientific or 

commercial data, little is known of the 

potential effect of low-level 

environmental contamination on 

Sonoran desert tortoises. We did 

ascertain that the risk of environmental 

contaminants affecting Sonoran desert 

tortoise populations is most likely from 

the presence of persistent, low-level 

toxicants such as heavy metals, 

polychlorinated biphenols, and 

organochlorides. However, potential 

effects of this type of environmental 

contamination are often delayed and 

difficult to observe in long-lived species 

such as the Sonoran desert tortoise, 

largely because of delayed sexual 

maturation and long generation times. 

We did not find documentation of 

population-level effects in Sonoran 

desert tortoises as a result of 

environmental contamination. 

Therefore, we conclude that 

environmental contamination of 

Sonoran desert tortoise habitat is not 

currently threatening populations; 

however, we acknowledge that further 

study is warranted to identify whether 

there is a risk for population-level 

impacts, and we recommend that land 

managers consider collecting baseline 

soil data in areas that may be 

vulnerable. 

Vehicle Strike Mortalities 

We expect that the increased use of 

off-highway vehicles within Sonoran 

desert tortoise habitat will increase the 

likelihood of encounters with Sonoran 

desert tortoises which can result in a 

variety of potential outcomes for 

tortoises. According to the Arizona 

Interagency Desert Tortoise Team 

(AIDTT 2000, p. 10), ‘‘[a]n abundance of 

anecdotal knowledge indicates that 

contacts between people and wild 

tortoises usually end to the detriment of 

tortoises (e.g., collection, handling, 

vandalism, crushing under vehicle tires, 

and shooting).’’ 

Averill-Murray and Swann (2002, p. 

1) stated that urban development 

adjacent to the Saguaro National Park in 

Pima County threatens the Sonoran 

desert tortoise via several mechanisms, 

including elevated mortality on roads. 

The high rates of speed associated with 

competitive off-highway vehicle events 

significantly increase the risk of direct 

mortality of Sonoran desert tortoises 

from vehicle collisions (Vega 2010, p. 

4). 

Reptiles, including the Sonoran desert 

tortoise, may be particularly vulnerable 

to roads due to the higher risk of 

mortality as a result of vehicle strikes 

(Boarman and Sazaki 1996, p. 1; 

Boarman et al. 1997, p. 57; Forman and 

Alexander 1998, p. 213; Boarman 2002, 

pp. 54–55; Boarman and Sazaki 2006, p. 

98; Dieringer 2010, p. 1). Anticipated 

adverse effects of roads on Sonoran 

desert tortoise populations are likely 

related to the level of their use. For 

example, Hoff and Marlow (2002, pp. 

451–454) found that the impact of roads 

on the prevalence of Mojave desert 

tortoise signs (tracks, scat, etc.) was 

commensurate with traffic volume— 

with the impacts more significant 

adjacent to heavily traveled roads. 

Mojave desert tortoise populations 

showed depressed numbers within 

1,300 feet (400 m) of highways in the 

Mojave Desert (Boarman and Sazaki 

2006, p. 98). Similar effects to Sonoran 

desert tortoise populations might be 

expected when heavily used roads are 

adjacent to, or are routed through, core 

Sonoran desert tortoise habitat such as 

steep, boulder-strewn slopes within 

Arizona Upland Sonoran desertscrub 

(Dieringer 2010, p. 1; Grandmaison 

2010b, p. 3). 

Sonoran desert tortoises move slowly 

and take a relatively long time to cross 

roads and highways, which may place 

them at elevated risk (Andrews et al. 

2008, p. 124). However, we suspect that, 

due to their size and shape (particularly 

in the sub-adult and adult size classes), 

drivers may instinctively avoid striking 

a crossing tortoise because of their 

similarity to rocks, and the subsequent 

damage that hitting a ‘‘rock’’ could do to 

a vehicle. However, intentional vehicle 

strikes of Mojave desert tortoises have 

been reported (Bury and Marlow 1973, 

p. 11). While unpaved roads traverse 16 

of the 17 Sonoran desert tortoise 

monitoring plots, the AGFD is only 

aware of one instance of direct mortality 

of a Sonoran desert tortoise from a 

vehicle on a long-term monitoring plot, 

on the East Bajada Plot (AGFD 2010, p. 

14). 

Increased vegetation adjacent to 

paved or heavily compacted roads 

resulting from increased water runoff 

may be beneficial to Sonoran desert 

tortoises, serving as a means to 

rehydrate them, but it may also attract 

them to these areas, indirectly 

increasing the likelihood of adverse 

interactions from: (1) Tortoises 

wandering onto the road, (2) vehicles 

pulling onto the vegetated shoulder of 

the road and crushing tortoises, (3) 

injury from grading or mowing 

activities, (4) exposure to herbicides 

applied to control growth of weeds 

along the road shoulder, and (5) 

increased potential for observation and 

collection by passers-by (Boarman 2002, 

p. 55). As stated previously, Sonoran 

desert tortoises may use infrequently 

traveled gravel roads as travel routes 

within their home ranges (Grandmaison 

et al. in press, p. 16). This suggests that 

low density Sonoran desert tortoise 

populations observed adjacent to 

heavily traveled roads may be the result 

of mortality from vehicle collisions and 

illegal collection rather than road 

avoidance behavior (Grandmaison et al. 

in press, p. 16). 

There appears to be a concerted effort 

to mitigate the potential effect of several 

roads and highways on Sonoran desert 

tortoise populations and their habitat. 

Barrier fencing (or tortoise fencing) and 

culverts along roads and highways are 

recognized methods employed 

throughout Arizona to reduce potential 

mortality through vehicle strikes of 

Sonoran desert tortoises. Installing 

tortoise fencing along roads and 

highways minimizes the risk of road 

mortality of tortoises but may also 

enhance the barrier effect between 

populations by restricting long-distance 

movements (Boarman and Sazaki 1996, 

p. 3). Culverts that pass under roads and 

highways may provide opportunities for 

Sonoran desert tortoises to safely cross 

roads and highways (Boarman and 

Sazaki 1996, pp. 3–4). 

The ADOT constructs and maintains 

roads and highways that comprise 

Arizona’s transportation system. It 

routinely implements varied 

conservation and mitigation actions 

with respect to Sonoran desert tortoise 

populations that may be affected by 

these activities. The ADOT (ADOT 

2010, pp. 2–5) listed numerous 

conservation measures including those 

which address standard (voluntary and 

involuntary) mitigation measures, 

education, new construction design, 

habitat acquisition, native plant 

restoration, nonnative plant control, 

establishment of wildlife corridors, and 

research that have been integrated into 

their road system planning, 

construction, and improvement 

activities. Tortoise-proof fencing 

adjacent to highways has been installed 

along numerous routes throughout 

Arizona including 27.6 mi (44.4 km) 

along U.S. Highway 93 and 10.8 mi 

(17.4 km) along State Route 85 (ADOT 

2010, p. 3). Numerous, additional 

structures that assist Sonoran desert 

tortoises to cross roads safely, such as 

pathways, ramps, and culverts, have 

been installed along the U.S. Highway 

93 corridor and along a segment of the 

U.S. Highway 60 through the Tonto 

National Forest (ADOT 2010, p. 3). 

The ability of tortoise fencing to 

prevent road mortality of Sonoran desert 

tortoises is highly contingent on 

inspections and maintenance. Sonoran 

desert tortoise fencing along 10 mi (16 

km) of U.S. Highway 93 in Mohave and 


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Yavapai Counties in Arizona, between 

mile markers 144 and 155, was shown 

to have major deficiencies, including 

567 individual fencing breaches and 

instances of culvert undercutting, which 

diminish the effectiveness of these 

mitigation techniques (Grandmaison 

2010b, p. 3). Five Sonoran desert 

tortoise road-mortalities were 

documented in 2008 in this stretch of 

highway, though none was documented 

in 2009 (Grandmaison 2010b, p. 5). A 

rancher in southeastern Mohave County, 

Arizona, reported observations of 

Sonoran desert tortoises being killed on 

U.S. Highway 93, particularly after 

heavy rains, when adjacent tortoise 

barrier fencing along the highway gets 

washed out, allowing access of tortoises 

to the highway surface (Dieringer 2010, 

p. 1). Using radio-telemetry, 

Grandmaison (2010b, p. 6) found that 

Sonoran desert tortoises with home 

ranges within 0.62 mi (1 km) adjacent to 

this stretch of Highway 93 did not cross 

the highway. However, additional 

instances of Sonoran desert tortoise 

mortality on this and other major routes 

within the distribution of Sonoran 

desert tortoises undoubtedly occurs but 

is rarely reported. 

Many activities undertaken by the 

ADOT minimize the effect of roads and 

highways on tortoise populations. 

However, we have concern regarding 

the lack of ongoing maintenance of 

protection structures such as tortoise 

barrier fencing. Therefore, we conclude 

that maintenance of tortoise protection 

structures is not adequate to meet the 

desired objective of these structures in 

many areas, or to protect Sonoran desert 

tortoise populations affected by heavily 

used roads and highways in Arizona. 

Balloons and Trash 

Helium-filled balloons are capable of 

dispersing great distances (greater than 

164 mi (264 km)) from their release 

points, and have been shown to make 

up the largest percentage of litter types 

encountered in desert tortoise field 

studies (Walde et al. 2007a, p. 148). 

Desert tortoises are known to eat trash, 

such as balloons, plastic, and other 

garbage, which may kill them by 

becoming lodged in the gastrointestinal 

tract or by entangling the tortoise 

(Averill-Murray and Averill-Murray 

2002, p. 27; Walde et al. 2007a, p. 148). 

Balloons and balloon string can also 

entangle the tortoise, sometimes leading 

to induced amputation of an appendage 

(Burge 1989, p. 7). Averill-Murray and 

Averill-Murray (2002, p. 27) reported 36 

balloons found on Ironwood Forest 

National Monument in Pima County, 

Arizona, indicating that opportunities 

for a Sonoran desert tortoise to 

consume, or become entangled with 

balloons, exist. However, AverillMurray and Averill-Murray (2002, p. 29) 

posited that while balloons may affect 

individuals, they are unlikely to cause 

population-level impacts to Sonoran 

desert tortoises. 

Illegal dumping in Arizona is 

ubiquitous throughout the Sonoran 

Desert, but most concentrated in areas 

adjacent to human settlements. These 

relatively small but widely dispersed 

piles of solid and potentially hazardous 

waste may also serve as sources of 

toxicological contamination of Sonoran 

desert tortoises in areas where ingestion 

of contaminated soils or plant matter 

can occur. 

In conclusion, balloons and trash 

occur throughout the range of the desert 

tortoise. Trash piles are most 

concentrated adjacent to human 

settlements but helium-filled balloons 

can travel many miles away from cities 

or towns and be deposited in remote 

habitat as they fall from the sky. We 

have documented that balloons in 

particular may pose health risks to 

Sonoran desert tortoises and are 

encountered in monitoring plots 

although specific reports of tortoises 

directly affected by balloons are rare in 

the literature. While effects can occur to 

individual tortoises, the literature did 

not indicate that population-level effects 

can be expected from such exposure. 

Climate Change 

There is unequivocal evidence that 

the earth’s climate is warming based on 

observations of increases in average 

global air and ocean temperatures, 

widespread melting of glaciers and 

polar ice caps, and rising sea levels 

(IPCC 2007, p. 4). Furthermore, the 

Intergovernmental Panel on Climate 

Change (IPCC 2007, p. 7) summarized 

the likelihood of general future trends in 

several climatic variables, predicting: 

(1) Warmer and fewer cold days and 

nights over most land areas, (2) warmer 

and more frequent hot days and nights 

over most land areas, (3) more frequent 

warm spells/heat waves over most land 

areas, (4) changes in precipitation 

patterns favoring an increased frequency 

of heavy precipitation events (or 

proportion of total rainfall from heavy 

falls) over most areas, and (5) an 

increase in the area affected by 

droughts. All of these changes are 

caused by alterations in the energy 

balance within the atmosphere and the 

Earth’s surface. The primary factors that 

affect this balance are concentrations of 

greenhouse gases (mainly carbon 

dioxide), aerosols, land surface 

properties, and solar radiation. These 

global climate changes will influence 

climatic patterns at regional and local 

scales. 

At a regional scale, there is a broad 

consensus among climate models that 

the area encompassing the southwestern 

United States and northern Mexico will 

get drier in the twenty-first century and 

that the trend towards a more arid 

climate is already under way (Seager et 

al. 2007). Evidence to support such 

changes in temperature and rainfall in 

the southwest deserts is abundant. For 

example, maximum summer 

temperatures in the southwestern 

United States are expected to increase 

over time in response to changes in the 

climate system (Christensen et al. 2007, 

p. 887). Weiss and Overpeck (2005, p. 

2075) examined low-temperature data 

over a 40-year timeframe from 

numerous weather stations in the 

Sonoran Desert ecoregion of Arizona 

and California, as well as the Mexican 

States of Baja California, Baja California 

Sur, and Sonora. They found: 

(1) Widespread warming trends in 

winter and spring, (2) decreased 

frequency of freezing temperatures, (3) 

lengthening of the freeze-free season, 

and (4) increased minimum 

temperatures per winter year. Such 

changes are likely to have widespread 

impacts on Southwestern ecosystems. 

While temperatures in the Southwest 

are predicted to increase, rainfall 

patterns will also be affected. The 

current, multi-year drought in the 

western United States, including most 

of the Southwest, is the most severe 

drought recorded since 1900 (Overpeck 

and Udall, 2010, p. 1642). Numerous 

models predict a decrease in annual 

precipitation in the southwestern 

United States and northern Mexico. 

Solomon et al. (2009, p. 1707) predict 

precipitation amounts in the 

southwestern United States and 

northern Mexico will decrease by as 

much as 9 to 12 percent (measured as 

percentage of change in precipitation 

per degree of warming, relative to 1900 

to 1950 as the baseline period). 

Christensen et al. (2007, p. 888) state, 

‘‘The projection of smaller warming over 

the Pacific Ocean than over the 

continent, * * * is likely to induce a 

decrease in annual precipitation in the 

southwestern USA and northern 

Mexico.’’ In addition, Seager et al. (2007, 

pp. 1181–1184) analyzed 19 models of 

differing variables to estimate the future 

climate of the southwestern United 

States and northern Mexico in response 

to predictions of changing climatic 

patterns. All but one of the 19 models 

predicted a drying trend within the 

southwest (Seager et al. 2007, p. 1181). 

A total of 49 projections were created 

using the 19 models and all but 3 


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predicted a shift to increasing aridity 

(dryness) in the southwest as early as 

2021 to 2040 (Seager et al. 2007, p. 

1181). While most climate change 

models predict less precipitation in the 

southwestern United States, a model 

produced by the Hadley Centre for 

Climate Prediction and Research 

(HadCM2) predicted increased 

precipitation throughout most of the 

United States, and particularly in the 

southwest (Weltzin et al. 2003, p. 942). 

While there may be some uncertainty 

associated with the predictions of 

decreased rainfall in the arid deserts, 

there is broad agreement that the overall 

trend will be reduced precipitation. 

In addition to increasing trends in 

aridity, the timing of precipitation may 

also be altered as a result of climate 

change, which would result in 

important changes in the vegetation 

community within habitat of the 

Sonoran desert tortoise. The IPCC (2007, 

p. 20) found that winter precipitation in 

the southwestern United States is 

predicted to decline by as much as 20 

percent as a result of climate change, 

while summer precipitation may 

increase slightly. Precipitation in 

Mojave desertscrub occurs 

predominantly during the cool-season 

(winter) months but, depending on 

location, it may also occur during the 

warm-summer months (Hereford et al. 

2006, p. 29). Perennial plant species in 

Mojave desertscrub are most affected by 

changes in winter precipitation, as 

increases in winter precipitation 

increases germination and the 

establishment of new plants (Hereford et 

al. 2006, p. 25). In contrast, decreases in 

winter precipitation substantially 

increase mortality in perennial plants, 

most notably in short-lived species 

(Hereford et al. 2006, p. 25). In addition, 

decreasing winter precipitation has been 

linked with a high mortality of droughtresistant shrubs in parts of the Sonoran 

and Mojave deserts (McAuliffe and 

Hamerlynck 2010, p. 885). A reduction 

in winter precipitation could 

significantly alter the plant 

communities of the Sonoran and Mojave 

deserts. 

Arid environments are especially 

sensitive to climate change, because the 

plants and animals that inhabit these 

areas are near their physical tolerances 

for temperature and water stress. Slight 

changes in temperature and rainfall, 

along with increases in the magnitude 

and frequency of extreme climatic 

events, can significantly alter species 

distributions and abundance (Archer 

and Predick 2008, p. 23). In fact, 

warming effects may be particularly 

severe for reptiles and amphibians. For 

instance, Walther et al. (2002, pp. 393– 

394) found that because of their 

physiology, reptiles and amphibians are 

sensitive to climatic changes, which 

may result in effects to their 

development, spatial distribution, and 

interactions with other species. 

Specifically, egg development, sperm 

development, and sex ratios may be 

affected by climatic changes in 

temperatures. Increased temperatures 

may influence sex ratios within clutches 

to favor females over males, which may 

benefit populations as one male can 

fertilize several females. However, if 

temperatures rise too much, the effect 

could strongly select for female-only 

clutches, significantly skewing the sex 

ratio within populations, and posing 

long-term problems for reptiles such as 

Sonoran desert tortoise populations 

(Walther et al. 2002, pp. 393–394). But 

as stated earlier, Sonoran desert 

tortoises build their nests in burrows 

underground, thereby tempering the 

effects of rising surface temperatures. 

Sonoran desert tortoises may be 

affected directly by regional climate 

change. For example, increasing 

temperatures may cause desert tortoises 

to overheat (Ernst and Lovich 2009, p. 

544). Sonoran desert tortoises are 

vulnerable to overheating because they 

heat up 10 times faster than they can 

cool down, making them potentially 

sensitive to temperature extremes 

associated with anticipated climate 

change (Ernst and Lovich 2009, p. 544). 

While climate change may directly 

affect the Sonoran desert tortoise, most 

of the impacts of climate change are 

anticipated to be indirect effects to the 

tortoise caused by other changes in the 

ecosystem that supports them. The 

following discussion describes 

anticipated indirect effects to the 

tortoise in response to predicted climate 

change effects. 

Changes in atmospheric carbon 

dioxide and soil nitrogen levels are 

anticipated to affect the Sonoran desert 

tortoise through responses observed in 

their forage base. The desert ecosystems 

inhabited by the Sonoran desert tortoise 

are also expected to be sensitive to 

increased levels of carbon dioxide in the 

atmosphere. Desert shrub cover may 

increase with increasing carbon dioxide, 

but nonnative species may also respond 

positively, out-competing native 

vegetation (Smith et al. 2000, p. 79; 

Loubimsteva and Adams 2004, p. 401), 

thereby increasing the risk of fire. In 

addition, water and nitrogen are the 

biggest constraints that influence 

biological productivity in desert 

ecosystems (Ramanujan 2009, p. 1). 

Predicted higher temperatures are 

expected to cause higher levels of 

nitrogen to escape as a gas from desert 

soils, leading to a decrease in soil 

fertility (Ramanujan 2009, p. 1). Murphy 

et al. (in prep., p. ii) expect these 

responses in the vegetation community 

to adversely affect the quality of forage 

for Sonoran desert tortoises, leading to 

dietary nitrogen deficiencies. 

Desert tortoises are likely to be 

affected by decreases in precipitation 

due to climate change. Rain is the single 

most important climatic factor that 

drives desert ecosystems because it 

ultimately determines recruitment rates, 

growth and reproduction rates, nutrient 

cycling, and net ecosystem productivity, 

resulting in these ecosystems being the 

most vulnerable to changes in 

precipitation levels (Weltzin et al. 2003, 

p. 944; Huxman et al. 2004, p. 254; 

Hereford et al. 2006, p. 25). Peterson 

(1996a, p. 1831) highlights the 

importance of rain for desert tortoises: 

‘‘Energy acquisition and expenditure in 

desert tortoises are strongly constrained 

by the contingencies of rainfall, both 

indirectly through effects on availability 

and quality of food, and directly 

through reliance on freestanding water 

for drinking, which is apparently 

necessary for achieving a net annual 

energy profit.’’ Desert tortoises evolved 

in arid conditions, and possess 

numerous physiological and behavioral 

adaptations to survive some degree of 

drought (Schmidt-Nelson and Bently 

1966, p. 911; Peterson 1996b, p. 1325; 

Christopher 1999, p. 365; Duda et al. 

1999, p. 1188; AIDTT 2000, p. 4; Berry 

et al. 2002b, pp. 443–446; Dickinson et 

al. 2002, pp. 251–252). Peterson (1996a, 

p. 1831) found desert tortoises have a 

very low field metabolic rate when 

compared to other desert reptiles, which 

may provide them an advantage in 

drought conditions. However, a 

decrease in winter precipitation may 

disproportionately affect reproductive 

females because they are highly 

dependent upon springtime forage. A 

decrease in winter precipitation is 

expected to adversely affect the quantity 

and quality of their forage. This, in turn, 

is likely to directly affect reproductive 

output of Sonoran desert tortoise 

populations (Hereford et al. 2006, p. 25). 

Persistent drought, and subsequent 

changes in the tortoise forage base, can 

affect blood chemistry and water 

metabolism, reduce or eliminate the 

thymus and fat stores, and result in 

skeletal muscle and liver atrophy in 

desert tortoises (Berry et al. 2002b, pp. 

443–446; Dickinson et al. 2002, pp. 

251–252). 

Seasonal changes in rainfall may 

contribute to the spread of invasive 

species, such as Sahara mustard and 

exotic grasses, which are capable of 

explosive growth, and able to quickly 


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out-compete native species (Barrows et 

al. 2009, p. 673). As explained in Factor 

A, invasive species displace the native 

vegetation, reducing forage for tortoises, 

and increasing the threat of wildfires in 

desert ecosystems, resulting in further 

reduction of forage plants for the 

tortoise. 

Droughts, which are likely to be more 

frequent and severe as a result of 

climate change, have been suggested to 

have caused in declines in local 

Sonoran desert tortoise populations. 

Periodic times of drought are not 

uncommon in the Southwest, and 

tortoises have evolved with drought. 

However, future drought conditions 

may be more severe and long-lasting 

than previously recorded droughts 

(Cook et al. 2004, p. 1016). The effects 

of drought have been shown to have 

significant population-level impacts on 

Mojave desert tortoises, as exhibited by 

the observed declines in their 

populations during years of droughtinduced reductions in annual plants 

(Longshore et al. 2003, p. 169). As stated 

previously, Sonoran desert tortoises 

strongly benefit from the bimodal 

precipitation pattern characteristic of 

the Sonoran Desert region, specifically 

from precipitation received during the 

summer monsoon. However, the 

monsoon is characterized by highlylocalized rainfall events of short 

duration and high magnitude, and can 

be spatially unpredictable. Therefore, 

while some Sonoran desert tortoise 

populations may receive satisfactory 

amounts of summer precipitation, 

others may be exposed to reduced 

monsoon precipitation totals, and 

potentially zero precipitation in a given 

year. This seems to have been the case 

during the late 1980s in the Maricopa 

Mountains near Phoenix, Arizona. The 

precipitous loss of 226 Sonoran desert 

tortoises in the Maricopa Mountains 

plot, which occurred between 1987 and 

1990, is believed to have resulted from 

severe, localized drought, when no 

measurable rainfall occurred in that area 

in 1989. This indicates that even 

Sonoran desert tortoises may succumb 

to excessive drought conditions 

(Schwalbe 2010, p. 2). Subsequent 

surveys have shown that survivorship 

within this population has improved, 

and there is evidence that reproduction 

has resumed in this population. Also, a 

lack of additional carcasses found on 

the plot indicates that population 

declines have stabilized, and the 

population might be rebounding (AGFD 

2010, p. 4). Drought conditions also 

apparently played a significant role in a 

decline of new Sonoran desert tortoise 

captures between 1988 and 1990 in the 

San Pedro Valley (Meyer et al. 2010, p. 

11). Localized cases of population 

declines as a result of drought could be 

more common in the future, due to 

decreasing rainfall caused by climate 

change. 

Another way to evaluate the threats to 

a species is the use of vulnerability 

assessments. The results of one 

assessment, conducted by Galbraith and 

Price (2009, p. ii) concluded that the 

desert tortoise within the United States 

was ‘‘highly vulnerable’’ to extinction as 

a result of climate change. The 

framework used by Galbraith and Price 

(2009, pp. 80–82) considered numerous 

factors including: (1) Current population 

size and trends, (2) range trends, 

(3) likely future stressor trends, (4) 

individual replacement time, (5) likely 

future vulnerability to stochastic events, 

(6) future vulnerability to policy/ 

management change, (7) likely future 

vulnerability to natural stressors, (7) 

physiological sensitivity to temperature 

and precipitation change and to extreme 

weather events, (8) dispersive capability 

and potential rate of increase, (9) habitat 

specialization, (10) likely event of future 

habitat loss due to climate change, (11) 

ability of habitats to shift in response to 

climate change, and (12) dependence on 

temporal inter-relations and other 

species. They summarized: ‘‘Over the 

last three or four decades, these 

populations (Mohave and Sonoran) have 

come under high degrees of stress due 

largely to human activity (particularly 

urbanization and recreational intrusion) 

* * * Climate change may be a 

significant new stressor, causing even 

more habitat loss and exacerbating an 

already difficult situation. Together, 

existing stressors and the direct and 

indirect effects of climate change could 

result in desert tortoises being put at 

even greater risk of population 

reduction and extinction in their U.S. 

range.’’ 

Galbraith and Price (2009, pp. 79–80) 

estimate that at least 20 to 50 percent of 

habitat today will not be available to 

desert tortoises by 2020 as a result of 

climate change and, to a much lesser 

extent, anticipated development. 

However, in their analysis, Galbraith 

and Price (2009, pp. 74–84) largely 

disregarded the fact that the Sonoran 

desert tortoise ranges into Mexico 

(which represents approximately half of 

its total distribution), which should be 

factored into their vulnerability 

analysis. They also often misapplied or 

gave disproportionate influence to 

specific research on the Mojave desert 

tortoise in addressing the desert tortoise 

in the U.S. as a whole. While we found 

certain attributes of Galbraith and Price 

(2009, pp. 74–84) to be accurate, these 

identified shortcomings provide an 

incomplete picture of the status of the 

desert tortoise and its vulnerability to 

the effects of climate change. 

Weiss and Overpeck (2005, p. 2074) 

disagreed with Galbraith and Price 

(2009, pp. 74–84). Accelerated increases 

in temperature projected as a result of 

climate change will potentially result in 

changes to the current geographical 

boundaries of the Sonoran Desert, as 

well as the distribution of associated 

plant species (Weiss and Overpeck 

2005, p. 2074). Specifically, Weiss and 

Overpeck (2005, p. 2074) predicted that 

the current geographic boundary of the 

Sonoran Desert will contract in its 

southeast portion and expand in 

distribution and rise in elevation in the 

eastern and northern portions, thus 

potentially expanding areas of suitable 

habitat for the Sonoran desert tortoise. 

Weiss and Overpeck (2005, p. 2075) and 

Galbraith and Price (2009, p. 80) agreed 

that observed changes to the fire regime 

of the Sonoran Desert favor nonnative 

plant species, and may impede the 

trajectory or degree of potential 

expansion of the Sonoran Desert. 

With the differences in predicted 

climate change under different 

scenarios, and the uncertainty of those 

effects on the tortoise, it is difficult to 

come to a definitive conclusion as to the 

potential impacts of climate change on 

the Sonoran desert tortoise. All, none, or 

a combination of these predictions may 

actually be realized in the future within 

the distribution of the Sonoran desert 

tortoise, which adds uncertainty to how 

the tortoise may respond to any given 

combination of these predictions. For 

example, warmer average temperatures 

may affect the Sonoran desert tortoise 

positively by lengthening annual 

surface-activity periods which may 

enhance reproduction potential and 

survivorship. Increased frequencies in 

heavy precipitation may provide more 

opportunities for rehydration of 

Sonoran desert tortoises and promote 

the production of forage species, 

whereby reducing daily foraging periods 

to both avoid excessive high 

temperatures and, as a consequence, 

lessen predation risks. However, higher 

temperatures coupled with drought 

conditions could also negatively affect 

the Sonoran desert tortoise by 

increasing metabolism rates, foraging 

needs, and associated foraging time, 

therefore increasing predation risk. 

Higher temperatures coupled with 

drought conditions could also reduce 

forage availability of plant species that 

depend on higher frequencies of 

precipitation events for growth (annual 

plant species that respond to monsoon 

storms). 


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The temporal aspect of anticipated 

changes in climate and their effects on 

the Sonoran desert tortoise and its 

habitat must be considered in context 

with the rate of evolutionary adaptation 

of the species. Skelly et al. (2007, 

pp. 1353–1355) examined preferred 

temperature ranges and thermal 

maxima, and suggested that some 

species with short generation times 

might evolve to meet the demands of a 

changing climate. The Sonoran desert 

tortoise has much longer generation 

times (approximately 12 to 15 years) 

and may therefore be more vulnerable to 

the effects of climate change, because 

they are unlikely to be able to rapidly 

adapt to environmental changes. 

Specifically, we do not expect their 

evolutionary processes to keep pace 

with the relatively fast-paced changes 

predicted as a result of climate change 

in the near- or mid-term. 

Perhaps the most important aspect of 

projected changes in climate is the 

relative irreversibility of these changes 

into the future. Solomon et al. (2009, 

p. 1704) state that the effects of climate 

change will be irreversible for 

approximately 1,000 years, even if 

carbon emissions dropped to zero in 

current times, as a result of the 

longevity of atmospheric carbon dioxide 

and feedbacks associated with ocean 

warming (Solomon et al. 2009, p. 1709). 

Summary of Factor E 

Our review of the best scientific and 

commercial data available indicated that 

Sonoran desert tortoises may be 

vulnerable to the effects of 

environmental contamination: Ingestion 

of trash, including balloons; and 

substances from illegal solid waste 

dumps. However the literature did not 

indicate these threats were currently 

affecting populations and specific 

reports of affected individual tortoises 

were rare. Vehicle strike mortalities 

have been documented, and may have 

some local sub-population effects in 

close proximity to more heavily traveled 

roads and highways, but again, these 

effects are more localized and not 

rangewide, and thus do not appear to 

have overall population-level effects. 

Further, while management and 

mitigation actions are being 

implemented, such as the construction 

of barrier fences and culverts, these 

devices are generally not maintained 

and appear to be ineffective in helping 

to reduce these individual mortalities. 

Climate change may also affect 

Sonoran desert tortoises. The combined 

effects of global and regional climate 

change, along with the effects of longterm drought, will play a role in the 

long-term persistence of the species. 

However, we are not able to quantify, 

with certainty, how the direct and 

indirect effects of climate change will 

affect Sonoran desert tortoise 

populations. Tortoise habitat may shift, 

native vegetation may change 

depending on rainfall patterns, 

increasing temperatures may affect the 

growth of native vegetation, the quality 

and quantity of desert tortoise forage 

may be affected, precipitation patterns 

will likely affect desert vegetation, and 

tortoises may experience physiological 

effects that could result in changes in 

reproduction and overall survival. We 

conclude that climate change may be a 

significant stressor that exacerbates 

current threats, both directly 

(physiological effects to the tortoise) and 

indirectly (habitat loss and 

fragmentation). As such, climate change, 

in and of itself, may affect Sonoran 

desert tortoise populations, but the 

magnitude of the impacts to the Sonoran 

desert tortoise remains uncertain. 

Climate change is not currently a threat 

to the Sonoran desert tortoise, but it has 

the potential to be a threat in the 

foreseeable future. Impacts from climate 

change in the future will likely 

exacerbate the current and ongoing 

threat of habitat loss caused by other 

factors, as discussed above. 

Finding 

As required by the Act, we conducted 

a review of the status of the Sonoran 

desert tortoise DPS and considered the 

five factors in assessing whether the 

DPS is threatened or endangered 

throughout all or a significant portion of 

its range. We examined the best 

scientific and commercial information 

available regarding the past, present, 

and future threats faced by the Sonoran 

desert tortoise. We reviewed the 

petition, information available in our 

files, and other available published and 

unpublished information, and we 

consulted with species experts, land 

managers, and numerous stakeholders 

including Federal, State, and Tribal 

agencies. 

In considering what factors might 

constitute threats, we must look beyond 

the mere exposure of the species to the 

factor to determine whether the species 

responds to the factor in a way that 

causes actual impacts to the species. If 

there is exposure to a factor, but no 

response, or only a positive response, 

that factor is not a threat. If there is 

exposure and the species responds 

negatively, the factor may be a threat 

and we then attempt to determine how 

significant a threat it is. If the threat is 

significant, it may drive or contribute to 

the risk of extinction of the species such 

that the species warrants listing as 

threatened or endangered as those terms 

are defined by the Act. This does not 

necessarily require empirical proof of a 

threat; however, reasonably strong databased inferences are the minimum 

standard for considering a threat 

significant. The mere identification of 

factors that could impact a species 

negatively is not sufficient to compel a 

finding that listing is appropriate; we 

require evidence that these factors are 

operative threats that act on the species 

to the point that the species meets the 

definition of threatened or endangered 

under the Act. 

Despite the history of conservation 

and management efforts afforded the 

Sonoran desert tortoise in Arizona, our 

review of the literature identified threats 

to the Sonoran desert tortoise 

attributable to all Threat Factors (A–E). 

The primary threats to the Sonoran 

desert tortoise from habitat modification 

and destruction (Factor A) include the: 

(1) Current and ongoing invasion of 

nonnative plant species resulting in an 

unnatural, destructive wildfire regime 

in portions of the species’ distribution; 

(2) cumulative, anticipated indirect 

effects to habitat and individual 

tortoises from increased human activity 

tied to urbanization and human 

population growth; (3) current and 

anticipated creation of barriers to 

genetic exchange among populations 

from urbanization and associated 

infrastructure; (4) high and growing use 

and popularity of OHV use in Sonoran 

desert tortoise habitat; (5) mesquite and 

ironwood tree harvest in Mexico; (6) 

improper livestock grazing in Mexico; 

and (7) undocumented human 

immigration and interdiction activities. 

The primary threat to the Sonoran 

desert tortoise from overutilization for 

commercial, recreational, scientific, or 

educational purposes (Factor B) is 

illegal collection. The primary threat to 

the Sonoran desert tortoise from 

predation (Factor C) is the increase in 

feral or off-leash domestic dog predation 

and human depredation associated with 

anticipated increases in urbanization 

and human population growth. The 

Sonoran desert tortoise is also 

threatened by the inadequacy of 

regulatory mechanisms (Factor D). In 

our review of the available information, 

we found numerous examples where the 

Sonoran desert tortoise is considered in 

management actions and tortoisespecific mitigation measures are 

mandated, or where land activities that 

could appreciably threaten Sonoran 

desert tortoise populations are 

prohibited. However, significant threats 

we have identified in Factors A, C, and 

E (primarily invading nonnative plant 


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species and subsequent wildfire 

concerns, vandalism of tortoises, feral 

dog predation, and climate change) are 

not being adequately addressed by land 

managers or other regulatory 

mechanisms. The primary threats to the 

Sonoran desert tortoise from other 

natural or manmade factors affecting its 

continued existence (Factor E) include 

the threats from vehicle strike mortality 

due to unmaintained structures 

intended to prevent tortoise mortality 

along heavily traveled routes through 

core Sonoran desert tortoise 

populations. In addition, anticipated 

effects from climate change are likely to 

exacerbate the ongoing threat of habitat 

loss and degradation by other factors, 

but we were unable to conclude that 

climate change, by itself, currently 

threatens the Sonoran desert tortoise. 

We have documented adverse effects of 

many of these threats on existing 

Sonoran desert tortoise populations, 

both historically and currently, and note 

that many threats act in synergistic 

combination in their effects to the 

tortoise. The factors that are the primary 

drivers of these threats, such as 

urbanization, human population 

growth, and drought, are predicted to 

increase in the foreseeable future. 

As a result of the numerous threats to 

the Sonoran desert tortoise identified 

above—which have occurred 

historically, continue today, and are 

predicted to continue in the foreseeable 

future—the tortoise has lost appreciable 

amounts of habitat to the collective 

footprint of urban development, 

agriculture, and infrastructure on the 

landscape. Collectively, these land 

changes have not only destroyed former 

Sonoran desert tortoise habitat, but have 

fragmented remaining populations, 

threatening long-term genetic fitness of 

the tortoise and precluding their 

recolonization ability in the event of 

population extirpations. In Mexico, 

significant areas of former Sonoran 

desert habitat have been significantly 

altered by the cultivation and natural 

colonization of invasive, nonnative 

plant species, and in combination with 

other threats, have likely greatly affected 

the viability of the Sonoran desert 

tortoise in that country. 

Available monitoring data are not 

adequate to accurately determine how 

the Sonoran desert tortoise historically 

responded to the loss of habitat or how 

populations have individually 

responded to threats, but we are 

reasonably certain that there are fewer 

Sonoran desert tortoises currently than 

historically, and that populations have 

become significantly fragmented over 

time. Currently within Arizona, 

approximately 75 percent of potential 

Sonoran desert tortoise habitat is within 

30 mi (48 km) or less of human 

populations of 1,000 people or more. 

The factors that have resulted in the loss 

or degradation of habitat, or threaten the 

tortoise directly, are predicted to worsen 

in the foreseeable future as the footprint 

of development and infrastructure 

grows and human population growth 

ensues. Some populations may 

disappear altogether, while others 

become smaller and more contracted; 

each of these scenarios exacerbates 

isolation and genetic and demographic 

exchange. Therefore, we reasonably 

anticipate that the Sonoran desert 

tortoise DPS is in danger of extinction 

in the foreseeable future throughout all 

or a significant portion of its range. 

On the basis of the best scientific and 

commercial information available, we 

find that the petitioned action, to list the 

Sonoran desert tortoise is warranted. In 

making this finding, we gave significant 

deference to the irreversible effect of 

threats as they are anticipated to occur 

in the foreseeable future. We will make 

a determination on the status of the 

species as threatened or endangered 

when we do a proposed listing 

determination. However, as explained 

in more detail below, an immediate 

proposal of a regulation implementing 

this action is precluded by higher 

priority listing actions, and progress is 

being made to add or remove qualified 

species from the Lists of Endangered 

and Threatened Wildlife and Plants. 

We reviewed the available 

information to determine if the existing 

and foreseeable threats render the 

species at risk of extinction at this time 

such that issuing an emergency 

regulation temporarily listing the DPS 

under section 4(b)(7) of the Act is 

warranted. We determined that issuing 

an emergency regulation temporarily 

listing the species is not warranted for 

this species at this time because we 

have not documented any significant 

population extirpations. However, if at 

any time we determine that issuing an 

emergency regulation temporarily 

listing the Sonoran desert tortoise is 

warranted, we will initiate this action at 

that time. 

Listing Priority Number 

The Service adopted guidelines on 

September 21, 1983 (48 FR 43098) to 

establish a rational system for utilizing 

available resources for the highest 

priority species when adding species to 

the Lists of Endangered and Threatened 

Wildlife and Plants or reclassifying 

species listed as threatened to 

endangered status. These guidelines, 

titled ‘‘Endangered and Threatened 

Species Listing and Recovery Priority 

Guidelines’’ address the immediacy and 

magnitude of threats, and the level of 

taxonomic distinctiveness. The system 

places greatest importance on the 

immediacy and magnitude of threats, 

but also factors in the level of taxonomic 

distinctiveness by assigning priority in 

descending order to monotypic genera 

(genus with one species), full species, 

and subspecies (or equivalently, distinct 

population segments of vertebrates). As 

a result of our analysis of the best 

available scientific and commercial 

information, we assigned the Sonoran 

desert tortoise a Listing Priority Number 

of 6, based on the high magnitude and 

non-imminence of threats. One or more 

of the threats discussed above are 

occurring in virtually every known 

population throughout its range. These 

threats are ongoing, and will continue to 

occur into the foreseeable future and, in 

some cases (such as nonnative plant 

species invasions and climate change 

effects), are considered irreversible. Our 

rationale for assigning the Sonoran 

desert tortoise an LPN of 6 is outlined 

below. 

Under the Service’s LPN Guidance, 

the magnitude of threat is the first 

criterion we look at when establishing a 

listing priority. The guidance indicates 

that species with the highest magnitude 

of threat are those species facing the 

greatest threats to their continued 

existence. These species receive the 

highest listing priority. Threats to the 

Sonoran desert tortoise vary in their 

magnitude. We found the most 

significant threats to the Sonoran desert 

tortoise to be the expansion of range and 

increase in number of nonnative plant 

species, urban development and 

associated human population growth in 

Arizona, and the highly popular and 

growing use of OHVs in Arizona. These 

threats have both direct and indirect 

effects to the Sonoran desert tortoise 

and its habitat. The area of land affected 

by nonnative species is widespread and, 

although currently and comparatively 

less significant in Arizona, it is 

substantial in Mexico. It is also expected 

to increase in the foreseeable future in 

both countries. When including the total 

land area adversely modified by 

ironwood and mesquite harvesting, it is 

projected that an estimated 98 percent 

of the Sonoran desert tortoises’ habitat 

in Mexico (47 percent of habitat 

rangewide) will be lost or adversely 

modified in the foreseeable future. 

Additionally, there is currently no 

viable solution to the threat posed by 

the increase in nonnative plants on this 

landscape. The projected human 

population growth and urban 

development throughout this DPS are 


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likely to both pose significant problems 

for genetic exchange among Sonoran 

desert tortoise populations. This will 

increase the degree and scope of human 

interactions with tortoises and occupied 

habitat, which threatens the tortoise in 

a variety of ways that we discuss in 

detail above. Currently in Arizona, 75 

percent of potentially occupied Sonoran 

desert tortoise habitat occurs within 30 

mi (48 km) or less of a city or town with 

a human population of 1,000 or more, 

and, considering future growth 

projections, it is likely that 100 percent 

of occupied tortoise habitat will be 

affected in the future. The everexpanding human population in 

Arizona is also likely to lead to 

commensurate increases in OHV use. As 

of 2007, 385,000 off-highway vehicles 

were registered in Arizona (a 350 

percent increase since 1998), and 1.7 

million people (29 percent of the 

Arizona’s public) engaged in off-road 

activity from 2005 to 2007. We 

identified significant threats from OHV 

use in Sonoran desert tortoise habitat, 

including habitat destruction, increased 

illegal collection of tortoises, and 

significant problems with law 

enforcement of OHV users. Despite 

problems associated with OHV 

management, several land management 

agencies responsible for Sonoran desert 

tortoise habitat have plans to expand 

OHV use on their lands. These three 

major threats operate in combination 

with other threats which, by themselves, 

might not be as serious, but acting 

together, cause a more serious 

cumulative impact. These threats 

include improper livestock management 

in Mexico, illegal collection and release 

of tortoises, undocumented human 

immigration and associated interdiction 

activities, predation from feral or offleash dogs, vehicle strike mortality from 

unmaintained, roadside mitigation 

devices, and anticipated possible effects 

from climate change. In their totality, 

these threats are high in magnitude 

because of the amount of habitat that is 

likely to be affected and the irreversible 

nature of the effect of these threats in 

sensitive habitats that are slow to 

rebound. 

Under our LPN Guidance, the second 

criterion we consider in assigning a 

listing priority is the immediacy of 

threats. This criterion is intended to 

ensure that the species that face actual, 

identifiable threats are given priority 

over those species for which threats are 

only potential or that are intrinsically 

vulnerable but are not known to be 

presently facing such threats. The 

threats are non-imminent because they 

are not ubiquitous throughout the range 

of the Sonoran desert tortoise where 

they occur. Some are acting currently in 

some areas, but not the whole DPS; 

some threats are likely to expand 

geographically over time; some are 

stabilized or even reducing in impact. 

Although we reviewed and discussed 

the numerous ways that individual 

Sonoran desert tortoises are affected by 

various threats, there is currently no 

evidence that any existing population is 

threatened with extirpation in the near 

future. So while some of the threats are 

happening now, impacts to tortoise 

populations are not likely to be evident 

in the immediate future. 

For example, we have documented 

that the invasion of nonnative plants is 

most significant in Sonora, Mexico, 

because of active planting for livestock 

grazing purposes. However, while there 

were historic practices of planting 

nonnative plant species as forage for 

livestock in the United States, these 

activities have ceased, leaving only 

slower, natural mechanisms to facilitate 

the expansion of nonnative plant 

species in this country. Thus, 

comparatively less habitat area is 

significantly altered by nonnative plant 

distribution and abundance in Arizona, 

representing approximately half of the 

Sonoran desert tortoises’ range. 

Additionally, monitoring data indicate 

that Sonoran desert tortoise populations 

persist in habitat that is unburned, but 

where nonnative species have become 

established. As stated in Factor A, 

wildfire is an important trigger, capable 

of making nonnative-invaded habitat 

unsuitable for Sonoran desert tortoises. 

The majority of nonnative-invaded 

Sonoran desert tortoise habitat remains 

unburned in the United States; however 

we are less certain about the occurrence 

of wildfire in nonnative-invaded habitat 

in Mexico. In both cases in Arizona and 

Mexico the ongoing conversion of 

habitats to nonnative grasses are not 

expected to impact tortoise populations 

in the very immediate future. Therefore, 

the actual impacts on tortoise 

populations from these and similar 

threats, such as climate change, are 

more likely to occur in the mid- to longterm future and are not considered 

imminent. 

Also, many of the threats we discuss 

above are linked to urbanization and 

human population growth. In Arizona, 

we have observed significant 

development and human population 

growth over the past several decades, 

but a weakened economy has slowed 

growth in recent years. We documented 

that the Sun Corridor Megapolitan is 

expected to nearly double the human 

population of southern and central 

Arizona by 2030. However, much of the 

urbanization that has already occurred 

replaced agricultural land that was not 

usable Sonoran desert tortoise habitat. 

Additionally, our evaluation of Sonoran 

desert tortoise population monitoring 

data has not indicated that any 

monitored population has been 

extirpated and less than one-third of 

monitored populations have shown 

declines, indicating that impacts on 

Sonoran desert tortoise populations are 

not currently imminent. These actual, 

identifiable threats are covered in detail 

under the discussion of Factors A 

through E of this finding and currently 

include habitat destruction, 

modification, and fragmentation, 

overutilization, predation from 

unnatural sources, inadequate 

regulatory mechanisms, and other 

natural and manmade factors. 

The third criterion in our LPN 

guidance is intended to devote 

resources to those species representing 

highly distinctive or isolated gene pools 

as reflected by taxonomy. The Sonoran 

desert tortoise is a valid taxon at the 

DPS level, and therefore receives a 

lower priority than species in a 

monotypic genus. The Sonoran desert 

tortoise faces high magnitude, nonimminent threats, and is a valid taxon 

at the DPS level. Thus, in accordance 

with our LPN guidance, we have 

assigned the Sonoran desert tortoise an 

LPN of 6. 

We will continue to monitor the 

threats to the Sonoran desert tortoise, 

and the species’ status on an annual 

basis, and should the magnitude or the 

imminence of the threats change, we 

will revisit our assessment of the LPN. 

Work on a proposed listing 

determination for the Sonoran desert 

tortoise is precluded by work on higher 

priority listing actions with absolute 

statutory, court-ordered, or courtapproved deadlines and final listing 

determinations for those species that 

were proposed for listing with funds 

from Fiscal Year 2011. This work 

includes all the actions listed in the 

tables below under expeditious 

progress. 

Preclusion and Expeditious Progress 

Preclusion is a function of the listing 

priority of a species in relation to the 

resources that are available and the cost 

and relative priority of competing 

demands for those resources. Thus, in 

any given fiscal year (FY), multiple 

factors dictate whether it will be 

possible to undertake work on a listing 

proposal regulation or whether 

promulgation of such a proposal is 

precluded by higher-priority listing 

actions. 


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The resources available for listing 

actions are determined through the 

annual Congressional appropriations 

process. The appropriation for the 

Listing Program is available to support 

work involving the following listing 

actions: Proposed and final listing rules; 

90-day and 12-month findings on 

petitions to add species to the Lists of 

Endangered and Threatened Wildlife 

and Plants (Lists) or to change the status 

of a species from threatened to 

endangered; annual ‘‘resubmitted’’ 

petition findings on prior warrantedbut-precluded petition findings as 

required under section 4(b)(3)(C)(i) of 

the Act; critical habitat petition 

findings; proposed and final rules 

designating critical habitat; and 

litigation-related, administrative, and 

program-management functions 

(including preparing and allocating 

budgets, responding to Congressional 

and public inquiries, and conducting 

public outreach regarding listing and 

critical habitat). The work involved in 

preparing various listing documents can 

be extensive and may include, but is not 

limited to: gathering and assessing the 

best scientific and commercial data 

available and conducting analyses used 

as the basis for our decisions; writing 

and publishing documents; and 

obtaining, reviewing, and evaluating 

public comments and peer review 

comments on proposed rules and 

incorporating relevant information into 

final rules. The number of listing 

actions that we can undertake in a given 

year also is influenced by the 

complexity of those listing actions; that 

is, more complex actions generally are 

more costly. The median cost for 

preparing and publishing a 90-day 

finding is $39,276; for a 12-month 

finding, $100,690; for a proposed rule 

with critical habitat, $345,000; and for 

a final listing rule with critical habitat, 

the median cost is $305,000. 

We cannot spend more than is 

appropriated for the Listing Program 

without violating the Anti-Deficiency 

Act (see 31 U.S.C. 1341(a)(1)(A)). In 

addition, in FY 1998 and for each fiscal 

year since then, Congress has placed a 

statutory cap on funds which may be 

expended for the Listing Program, equal 

to the amount expressly appropriated 

for that purpose in that fiscal year. This 

cap was designed to prevent funds 

appropriated for other functions under 

the Act (for example, recovery funds for 

removing species from the Lists), or for 

other Service programs, from being used 

for Listing Program actions (see House 

Report 105–163, 105th Congress, 1st 

Session, July 1, 1997). 

Since FY 2002, the Service’s budget 

has included a critical habitat subcap to 

ensure that some funds are available for 

other work in the Listing Program (‘‘The 

critical habitat designation subcap will 

ensure that some funding is available to 

address other listing activities’’ (House 

Report No. 107–103, 107th Congress, 1st 

Session, June 19, 2001)). In FY 2002 and 

each year until FY 2006, the Service has 

had to use virtually the entire critical 

habitat subcap to address courtmandated designations of critical 

habitat, and consequently none of the 

critical habitat subcap funds have been 

available for other listing activities. In 

some FYs since 2006, we have been able 

to use some of the critical habitat 

subcap funds to fund proposed listing 

determinations for high-priority 

candidate species. In other FYs, while 

we were unable to use any of the critical 

habitat subcap funds to fund proposed 

listing determinations, we did use some 

of this money to fund the critical habitat 

portion of some proposed listing 

determinations so that the proposed 

listing determination and proposed 

critical habitat designation could be 

combined into one rule, thereby being 

more efficient in our work. In FY 2011 

we anticipate that we will be unable to 

use any of the critical habitat subcap 

funds to fund proposed listing 

determinations. 

We make our determinations of 

preclusion on a nationwide basis to 

ensure that the species most in need of 

listing will be addressed first and also 

because we allocate our listing budget 

on a nationwide basis. Through the 

listing cap, the critical habitat subcap, 

and the amount of funds needed to 

address court-mandated critical habitat 

designations, Congress and the courts 

have, in effect, determined the amount 

of money available for other listing 

activities nationwide. Therefore, the 

funds in the listing cap, other than those 

needed to address court-mandated 

critical habitat for already listed species, 

set the limits on our determinations of 

preclusion and expeditious progress. 

Congress identified the availability of 

resources as the only basis for deferring 

the initiation of a rulemaking that is 

warranted. The Conference Report 

accompanying P.L. 97–304, which 

established the current statutory 

deadlines and the warranted-butprecluded finding, states that the 

amendments were ‘‘not intended to 

allow the Secretary to delay 

commencing the rulemaking process for 

any reason other than that the existence 

of pending or imminent proposals to list 

species subject to a greater degree of 

threat would make allocation of 

resources to such a petition [that is, for 

a lower-ranking species] unwise.’’ 

Although that statement appeared to 

refer specifically to the ‘‘to the 

maximum extent practicable’’ limitation 

on the 90-day deadline for making a 

‘‘substantial information’’ finding, that 

finding is made at the point when the 

Service is deciding whether or not to 

commence a status review that will 

determine the degree of threats facing 

the species, and therefore the analysis 

underlying the statement is more 

relevant to the use of the warranted-butprecluded finding, which is made when 

the Service has already determined the 

degree of threats facing the species and 

is deciding whether or not to commence 

a rulemaking. 

In FY 2010, $10,471,000 is the 

amount of money that Congress 

appropriated for the Listing Program 

(that is, the portion of the Listing 

Program funding not related to critical 

habitat designations for species that are 

already listed). Therefore, a proposed 

listing is precluded if pending proposals 

with higher priority will require 

expenditure of at least $10,471,000, and 

expeditious progress is the amount of 

work that can be achieved with 

$10,471,000. Since court orders 

requiring critical habitat work will not 

require use of all of the funds within the 

critical habitat subcap, we used 

$1,114,417 of our critical habitat subcap 

funds in order to work on as many of 

our required petition findings and 

listing determinations as possible. This 

brings the total amount of funds we had 

for listing actions in FY 2010 to 

$11,585,417. 

The $11,585,417 was used to fund 

work in the following categories: 

compliance with court orders and courtapproved settlement agreements 

requiring that petition findings or listing 

determinations be completed by a 

specific date; section 4 (of the Act) 

listing actions with absolute statutory 

deadlines; essential litigation-related, 

administrative, and listing programmanagement functions; and highpriority listing actions for some of our 

candidate species. For FY 2011, on 

September 29, 2010, Congress passed a 

continuing resolution which provides 

funding at the FY 2010 enacted level. In 

2009, the responsibility for listing 

foreign species under the Act was 

transferred from the Division of 

Scientific Authority, International 

Affairs Program, to the Endangered 

Species Program. Therefore, starting in 

FY 2010, we use a portion of our 

funding to work on the actions 

described above as they apply to listing 

actions for foreign species. This has the 

potential to further reduce funding 

available for domestic listing actions. 

Although there are currently no foreign 

species issues included in our highVerDate Mar<15>2010 19:59 Dec 13, 2010 Jkt 223001 PO 00000 Frm 00049 Fmt 4701 Sfmt 4702 E:\FR\FM\14DEP3.SGM 14DEP3

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priority listing actions at this time, 

many actions have statutory or courtapproved settlement deadlines, thus 

increasing their priority. The budget 

allocations for each specific listing 

action are identified in the Service’s FY 

2011 Allocation Table (part of our 

administrative record). 

Based on our September 21, 1983, 

guidance for assigning an LPN for each 

candidate species (48 FR 43098), we 

have a significant number of species 

with an LPN of 2. Using this guidance, 

we assign each candidate an LPN of 1 

to 12, depending on the magnitude of 

threats (high or moderate to low), 

immediacy of threats (imminent or 

nonimminent), and taxonomic status of 

the species (in order of priority: 

monotypic genus (a species that is the 

sole member of a genus); species, or part 

of a species (subspecies, distinct 

population segment, or significant 

portion of the range)). The lower the 

listing priority number, the higher the 

listing priority (that is, a species with an 

LPN of 1 would have the highest listing 

priority). 

Because of the large number of highpriority species, we have further ranked 

the candidate species with an LPN of 2 

by using the following extinction-risk 

type criteria: International Union for the 

Conservation of Nature and Natural 

Resources (IUCN) Red list status/rank, 

Heritage rank (provided by 

NatureServe), Heritage threat rank 

(provided by NatureServe), and species 

currently with fewer than 50 

individuals, or 4 or fewer populations. 

Those species with the highest IUCN 

rank (critically endangered), the highest 

Heritage rank (G1), the highest Heritage 

threat rank (substantial, imminent 

threats), and currently with fewer than 

50 individuals, or fewer than 4 

populations, originally comprised a 

group of approximately 40 candidate 

species (‘‘Top 40’’). These 40 candidate 

species have had the highest priority to 

receive funding to work on a proposed 

listing determination. As we work on 

proposed and final listing rules for those 

40 candidates, we apply the ranking 

criteria to the next group of candidates 

with an LPN of 2 and 3 to determine the 

next set of highest-priority candidate 

species. Finally, proposed rules for 

reclassification of threatened species to 

endangered are lower priority, since as 

listed species, they are already afforded 

the protection of the Act and 

implementing regulations. However, for 

efficiency reasons, we may choose to 

work on a proposed rule to reclassify a 

species to endangered if we can 

combine this with work that is subject 

to a court-determined deadline. 

With our workload so much bigger 

than the amount of funds we have to 

accomplish it, it is important that we be 

as efficient as possible in our listing 

process. Therefore, as we work on 

proposed rules for the highest priority 

species in the next several years, we are 

preparing multi-species proposals when 

appropriate, and these may include 

species with lower priority if they 

overlap geographically or have the same 

threats as a species with an LPN of 2. 

In addition, we take into consideration 

the availability of staff resources when 

we determine which high-priority 

species will receive funding to 

minimize the amount of time and 

resources required to complete each 

listing action. 

As explained above, a determination 

that listing is warranted but precluded 

must also demonstrate that expeditious 

progress is being made to add and 

remove qualified species to and from 

the Lists of Endangered and Threatened 

Wildlife and Plants. As with our 

‘‘precluded’’ finding, the evaluation of 

whether progress in adding qualified 

species to the Lists has been expeditious 

is a function of the resources available 

for listing and the competing demands 

for those funds. Although we do not 

discuss it in detail here, we are also 

making expeditious progress in 

removing species from the list under the 

Recovery program in light of the 

resource available for delisting, which is 

funded by a separate line item in the 

budget of the Endangered Species 

Program. During FY 2010, we have 

completed two proposed delisting rules 

and two final delisting rules. Given the 

limited resources available for listing, 

we find that we made expeditious 

progress in FY 2010 in the Listing 

Program. This progress included 

preparing and publishing the following 

determinations: 

FY 2010 AND FY 2011 COMPLETED LISTING ACTIONS

Publication date Title Actions FR pages 

10/08/2009 ........ Listing Lepidium papilliferum (Slickspot Peppergrass) as a Threatened Species Throughout Its Range.Final Listing Threatened ............. 74 FR 52013–52064. 

10/27/2009 ........ 90-day Finding on a Petition To List the American Dipper in the 

Black Hills of South Dakota as Threatened or Endangered.

Notice of 90-day Petition Finding, Not substantial.74 FR 55177–55180. 

10/28/2009 ........ Status Review of Arctic Grayling (Thymallus arcticus) in the 

Upper Missouri River System.

Notice of Intent to Conduct Status Review for Listing Decision.74 FR 55524–55525. 

11/03/2009 ........ Listing the British Columbia Distinct Population Segment of the 

Queen Charlotte Goshawk Under the Endangered Species Act: 

Proposed rule.

Proposed Listing Threatened ..... 74 FR 56757–56770. 

11/03/2009 ........ Listing the Salmon-Crested Cockatoo as Threatened Throughout 

Its Range with Special Rule.

Proposed Listing Threatened ..... 74 FR 56770–56791. 

11/23/2009 ........ Status Review of Gunnison sage-grouse (Centrocercus minimus) Notice of Intent to Conduct Status Review for Listing Decision.74 FR 61100–61102. 

12/03/2009 ........ 12-Month Finding on a Petition to List the Black-tailed Prairie Dog 

as Threatened or Endangered.

Notice of 12-month petition finding, Not warranted.74 FR 63343–63366. 

12/03/2009 ........ 90-Day Finding on a Petition to List Sprague’s Pipit as Threatened or Endangered.Notice of 90-day Petition Finding, Substantial.74 FR 63337–63343. 

12/15/2009 ........ 90-Day Finding on Petitions To List Nine Species of Mussels 

From Texas as Threatened or Endangered With Critical Habitat.

Notice of 90-day Petition Finding, Substantial.74 FR 66260–66271. 

12/16/2009 ........ Partial 90-Day Finding on a Petition to List 475 Species in the 

Southwestern United States as Threatened or Endangered 

With Critical Habitat.

Notice of 90-day Petition Finding, Not substantial and Substantial.74 FR 66865–66905. 

12/17/2009 ........ 12-month Finding on a Petition To Change the Final Listing of the 

Distinct Population Segment of the Canada Lynx To Include 

New Mexico.

Notice of 12-month petition finding, Warranted but precluded.74 FR 66937–66950. 


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FY 2010 AND FY 2011 COMPLETED LISTING ACTIONS—Continued 

Publication date Title Actions FR pages 

1/05/2010 .......... Listing Foreign Bird Species in Peru and Bolivia as Endangered 

Throughout Their Range.

Proposed Listing Endangered .... 75 FR 605–649. 

1/05/2010 .......... Listing Six Foreign Birds as Endangered Throughout Their Range Proposed Listing Endangered .... 75 FR 286–310.

1/05/2010 .......... Withdrawal of Proposed Rule to List Cook’s Petrel ........................ Proposed rule, withdrawal .......... 75 FR 310–316. 

1/05/2010 .......... Final Rule to List the Galapagos Petrel and Heinroth’s 

Shearwater as Threatened Throughout Their Ranges.

Final Listing Threatened ............. 75 FR 235–250. 

1/20/2010 .......... Initiation of Status Review for Agave eggersiana and Solanum 

conocarpum.

Notice of Intent to Conduct Status Review for Listing Decision.75 FR 3190–3191. 

2/09/2010 .......... 12-month Finding on a Petition to List the American Pika as 

Threatened or Endangered.

Notice of 12-month petition finding, Not warranted.75 FR 6437–6471. 

2/25/2010 .......... 12-Month Finding on a Petition To List the Sonoran Desert Population of the Bald Eagle as a Threatened or Endangered Distinct Population Segment.Notice of 12-month petition finding, Not warranted.75 FR 8601–8621. 

2/25/2010 .......... Withdrawal of Proposed Rule To List the Southwestern Washington/Columbia River Distinct Population Segment of Coastal 

Cutthroat Trout (Oncorhynchus clarki clarki) as Threatened.

Withdrawal of Proposed Rule to 

List.

75 FR 8621–8644. 

3/18/2010 .......... 90-Day Finding on a Petition to List the Berry Cave salamander 

as Endangered.

Notice of 90-day Petition Finding, Substantial.75 FR 13068–13071. 

3/23/2010 .......... 90-Day Finding on a Petition to List the Southern Hickorynut 

Mussel (Obovaria jacksoniana) as Endangered or Threatened.

Notice of 90-day Petition Finding, Not substantial.75 FR 13717–13720. 

3/23/2010 .......... 90-Day Finding on a Petition to List the Striped Newt as Threatened.Notice of 90-day Petition Finding, Substantial.75 FR 13720–13726. 

3/23/2010 .......... 12-Month Findings for Petitions to List the Greater Sage-Grouse 

(Centrocercus urophasianus) as Threatened or Endangered.

Notice of 12-month petition finding, Warranted but precluded.75 FR 13910–14014. 

3/31/2010 .......... 12-Month Finding on a Petition to List the Tucson Shovel-Nosed 

Snake (Chionactis occipitalis klauberi) as Threatened or Endangered with Critical Habitat.

Notice of 12-month petition finding, Warranted but precluded.75 FR 16050–16065. 

4/5/2010 ............ 90-Day Finding on a Petition To List Thorne’s Hairstreak Butterfly 

as Endangered.

Notice of 90-day Petition Finding, Substantial.75 FR 17062–17070. 

4/6/2010 ............ 12-month Finding on a Petition To List the Mountain Whitefish in 

the Big Lost River, Idaho, as Endangered or Threatened.

Notice of 12-month petition finding, Not warranted.75 FR 17352–17363. 

4/6/2010 ............ 90-Day Finding on a Petition to List a Stonefly (Isoperla jewetti) 

and a Mayfly (Fallceon eatoni) as Threatened or Endangered 

with Critical Habitat.

Notice of 90-day Petition Finding, Not substantial.75 FR 17363–17367. 

4/7/2010 ............ 12-Month Finding on a Petition to Reclassify the Delta Smelt 

From Threatened to Endangered Throughout Its Range.

Notice of 12-month petition finding, Warranted but precluded.75 FR 17667–17680. 

4/13/2010 .......... Determination of Endangered Status for 48 Species on Kauai and 

Designation of Critical Habitat.

Final Listing Endangered ........... 75 FR 18959–19165. 

4/15/2010 .......... Initiation of Status Review of the North American Wolverine in the 

Contiguous United States.

Notice of Initiation of Status Review for Listing Decision.75 FR 19591–19592. 

4/15/2010 .......... 12-Month Finding on a Petition to List the Wyoming Pocket Gopher as Endangered or Threatened with Critical Habitat.Notice of 12-month petition finding, Not warranted.75 FR 19592–19607. 

4/16/2010 .......... 90-Day Finding on a Petition to List a Distinct Population Segment of the Fisher in Its United States Northern Rocky Mountain Range as Endangered or Threatened with Critical Habitat.Notice of 90-day Petition Finding, Substantial.75 FR 19925–19935. 

4/20/2010 .......... Initiation of Status Review for Sacramento splittail (Pogonichthys 

macrolepidotus).

Notice of Initiation of Status Review for Listing Decision.75 FR 20547–20548. 

4/26/2010 .......... 90-Day Finding on a Petition to List the Harlequin Butterfly as Endangered.Notice of 90-day Petition Finding, Substantial.75 FR 21568–21571. 

4/27/2010 .......... 12-Month Finding on a Petition to List Susan’s Purse-making 

Caddisfly (Ochrotrichia susanae) as Threatened or Endangered.

Notice of 12-month petition finding, Not warranted.75 FR 22012–22025. 

4/27/2010 .......... 90-day Finding on a Petition to List the Mohave Ground Squirrel 

as Endangered with Critical Habitat.

Notice of 90-day Petition Finding, Substantial.75 FR 22063–22070. 

5/4/2010 ............ 90-Day Finding on a Petition to List Hermes Copper Butterfly as 

Threatened or Endangered.

Notice of 90-day Petition Finding, Substantial.75 FR 23654–23663. 

6/1/2010 ............ 90-Day Finding on a Petition To List Castanea pumila var. 

ozarkensis.

Notice of 90-day Petition Finding, Substantial.75 FR 30313–30318. 

6/1/2010 ............ 12-month Finding on a Petition to List the White-tailed Prairie 

Dog as Endangered or Threatened.

Notice of 12-month petition finding, Not warranted.75 FR 30338–30363. 

6/9/2010 ............ 90-Day Finding on a Petition To List van Rossem’s Gull-billed 

Tern as Endangered or Threatened.

Notice of 90-day Petition Finding, Substantial.75 FR 32728–32734. 

6/16/2010 .......... 90-Day Finding on Five Petitions to List Seven Species of Hawaiian Yellow-faced Bees as Endangered.Notice of 90-day Petition Finding, Substantial.75 FR 34077–34088. 

6/22/2010 .......... 12-Month Finding on a Petition to List the Least Chub as Threatened or Endangered.Notice of 12-month petition finding, Warranted but precluded.75 FR 35398–35424. 

6/23/2010 .......... 90-Day Finding on a Petition to List the Honduran Emerald Hummingbird as Endangered.Notice of 90-day Petition Finding, Substantial.75 FR 35746–35751. 

6/23/2010 .......... Listing Ipomopsis polyantha (Pagosa Skyrocket) as Endangered 

Throughout Its Range, and Listing Penstemon debilis (Parachute Beardtongue) and Phacelia submutica (DeBeque 

Phacelia) as Threatened Throughout Their Range.

Proposed Listing Endangered 

Proposed Listing Threatened.

75 FR 35721–35746. 


emcdonald on DSK2BSOYB1PROD with PROPOSALS3
78144 Federal Register / Vol. 75, No. 239 / Tuesday, December 14, 2010 / Proposed Rules 

FY 2010 AND FY 2011 COMPLETED LISTING ACTIONS—Continued 

Publication date Title Actions FR pages 

6/24/2010 .......... Listing the Flying Earwig Hawaiian Damselfly and Pacific Hawaiian Damselfly As Endangered Throughout Their Ranges.Final Listing Endangered ........... 75 FR 35990–36012. 

6/24/2010 .......... Listing the Cumberland Darter, Rush Darter, Yellowcheek Darter, 

Chucky Madtom, and Laurel Dace as Endangered Throughout 

Their Ranges.

Proposed Listing Endangered .... 75 FR 36035–36057. 

6/29/2010 .......... Listing the Mountain Plover as Threatened .................................... Reinstatement of Proposed Listing Threatened.75 FR 37353–37358. 

7/20/2010 .......... 90-Day Finding on a Petition to List Pinus albicaulis (Whitebark 

Pine) as Endangered or Threatened with Critical Habitat.

Notice of 90-day Petition Finding, Substantial.75 FR 42033–42040. 

7/20/2010 .......... 12-Month Finding on a Petition to List the Amargosa Toad as 

Threatened or Endangered.

Notice of 12-month petition finding, Not warranted.75 FR 42040–42054. 

7/20/2010 .......... 90-Day Finding on a Petition to List the Giant Palouse Earthworm 

(Driloleirus americanus) as Threatened or Endangered.

Notice of 90-day Petition Finding, Substantial.75 FR 42059–42066. 

7/27/2010 .......... Determination on Listing the Black-Breasted Puffleg as Endangered Throughout its Range; Final Rule.Final Listing Endangered ........... 75 FR 43844–43853. 

7/27/2010 .......... Final Rule to List the Medium Tree-Finch (Camarhynchus pauper) 

as Endangered Throughout Its Range.

Final Listing Endangered ........... 75 FR 43853–43864. 

8/3/2010 ............ Determination of Threatened Status for Five Penguin Species ..... Final Listing Threatened ............. 75 FR 45497–45527. 

8/4/2010 ............ 90-Day Finding on a Petition To List the Mexican Gray Wolf as 

an Endangered Subspecies With Critical Habitat.

Notice of 90-day Petition Finding, Substantial.75 FR 46894–46898. 

8/10/2010 .......... 90-Day Finding on a Petition to List Arctostaphylos franciscana 

as Endangered with Critical Habitat.

Notice of 90-day Petition Finding, Substantial.75 FR 48294–48298. 

8/17/2010 .......... Listing Three Foreign Bird Species from Latin America and the 

Caribbean as Endangered Throughout Their Range.

Final Listing Endangered ........... 75 FR 50813–50842. 

8/17/2010 .......... 90-Day Finding on a Petition to List Brian Head Mountainsnail as 

Endangered or Threatened with Critical Habitat.

Notice of 90-day Petition Finding, Not substantial.75 FR 50739–50742. 

8/24/2010 .......... 90-Day Finding on a Petition to List the Oklahoma Grass Pink Orchid as Endangered or Threatened.Notice of 90-day Petition Finding, Substantial.75 FR 51969–51974. 

9/1/2010 ............ 12-Month Finding on a Petition to List the White-Sided Jackrabbit 

as Threatened or Endangered.

Notice of 12-month petition finding, Not warranted.75 FR 53615–53629. 

9/8/2010 ............ Proposed Rule To List the Ozark Hellbender Salamander as Endangered.Proposed Listing Endangered .... 75 FR 54561–54579. 

9/8/2010 ............ Revised 12–Month Finding to List the Upper Missouri River Distinct Population Segment of Arctic Grayling as Endangered or 

Threatened.

Notice of 12-month petition finding, Warranted but precluded.75 FR 54707–54753. 

9/9/2010 ............ 12-Month Finding on a Petition to List the Jemez Mountains Salamander (Plethodon neomexicanus) as Endangered or Threatened with Critical Habitat.Notice of 12-month petition finding, Warranted but precluded.75 FR 54822–54845. 

9/15/2010 .......... 12-Month Finding on a Petition to List Sprague’s Pipit as Endangered or Threatened Throughout Its Range.Notice of 12-month petition finding, Warranted but precluded.75 FR 56028–56050. 

9/22/2010 .......... 12-Month Finding on a Petition to List Agave eggersiana (no 

common name) as Endangered.

Notice of 12-month petition finding, Warranted but precluded.75 FR 57720–57734. 

9/28/2010 .......... Determination of Endangered Status for the African Penguin ....... Final Listing Endangered ........... 75 FR 59645–59656. 

9/28/2010 .......... Determination for the Gunnison Sage-grouse as a Threatened or 

Endangered Species.

Notice of 12-month petition finding, Warranted but precluded.75 FR 59803–59863. 

9/30/2010 .......... 12-Month Finding on a Petition to List the Pygmy Rabbit as Endangered or Threatened.Notice of 12-month petition finding, Not warranted.75 FR 60515–60561. 

10/6/2010 .......... Endangered Status for the Altamaha Spinymussel and Designation of Critical Habitat.Proposed Listing Endangered .... 75 FR 61664–61690. 

10/7/2010 .......... 12-month Finding on a Petition to list the Sacramento Splittail as 

Endangered or Threatened.

Notice of 12-month petition finding, Not warranted.75 FR 62070–62095. 

Our expeditious progress also 

includes work on listing actions that we 

funded in FY 2010 and FY 2011, but 

have not yet been completed to date. 

These actions are listed below. Actions 

in the top section of the table are being 

conducted under a deadline set by a 

court. Actions in the middle section of 

the table are being conducted to meet 

statutory timelines, that is, timelines 

required under the Act. Actions in the 

bottom section of the table are highpriority listing actions. These actions 

include work primarily on species with 

an LPN of 2, and, as discussed above, 

selection of these species is partially 

based on available staff resources, and 

when appropriate, include species with 

a lower priority if they overlap 

geographically or have the same threats 

as the species with the high priority. 

Including these species together in the 

same proposed rule results in 

considerable savings in time and 

funding compared to preparing separate 

proposed rules for each of them in the 

future. 

ACTIONS FUNDED IN FY 2010 AND FY 2011 BUT NOT YET COMPLETED

Species Action 

Actions Subject to Court Order/Settlement Agreement 

6 Birds from Eurasia .............................................................................................................................................. Final listing determination. 


emcdonald on DSK2BSOYB1PROD with PROPOSALS3
Federal Register / Vol. 75, No. 239 / Tuesday, December 14, 2010 / Proposed Rules 78145 

ACTIONS FUNDED IN FY 2010 AND FY 2011 BUT NOT YET COMPLETED—Continued 

Species Action 

Flat-tailed horned lizard ......................................................................................................................................... Final listing determination. 

Mountain plover 4 ................................................................................................................................................... Final listing determination. 

6 Birds from Peru .................................................................................................................................................. Proposed listing determination. 

Pacific walrus ......................................................................................................................................................... 12-month petition finding. 

Wolverine ............................................................................................................................................................... 12-month petition finding. 

Solanum conocarpum ............................................................................................................................................ 12-month petition finding. 

Desert tortoise—Sonoran population .................................................................................................................... 12-month petition finding. 

Thorne’s Hairstreak butterfly 3 ............................................................................................................................... 12-month petition finding. 

Hermes copper butterfly 3 ...................................................................................................................................... 12-month petition finding. 

Actions With Statutory Deadlines 

Casey’s june beetle ............................................................................................................................................... Final listing determination. 

Georgia pigtoe, interrupted rocksnail, and rough hornsnail .................................................................................. Final listing determination. 

7 Bird species from Brazil ..................................................................................................................................... Final listing determination. 

Southern rockhopper penguin—Campbell Plateau population ............................................................................. Final listing determination. 

5 Bird species from Colombia and Ecuador ......................................................................................................... Final listing determination. 

Queen Charlotte goshawk ..................................................................................................................................... Final listing determination. 

5 species southeast fish (Cumberland darter, rush darter, yellowcheek darter, chucky madtom, and laurel 

dace) 4.

Final listing determination. 

Altamaha spinymussel ........................................................................................................................................... Final listing determination. 

Salmon crested cockatoo ...................................................................................................................................... Proposed listing determination. 

CA golden trout ...................................................................................................................................................... 12-month petition finding. 

Black-footed albatross ........................................................................................................................................... 12-month petition finding. 

Mount Charleston blue butterfly ............................................................................................................................ 12-month petition finding. 

Mojave fringe-toed lizard 1 ..................................................................................................................................... 12-month petition finding. 

Kokanee—Lake Sammamish population 1 ............................................................................................................ 12-month petition finding. 

Cactus ferruginous pygmy-owl 1 ............................................................................................................................ 12-month petition finding. 

Northern leopard frog ............................................................................................................................................ 12-month petition finding. 

Tehachapi slender salamander ............................................................................................................................. 12-month petition finding. 

Coqui Llanero ........................................................................................................................................................ 12-month petition finding. 

Dusky tree vole ...................................................................................................................................................... 12-month petition finding. 

3 MT invertebrates (mist forestfly(Lednia tumana), Oreohelix sp.3, Oreohelix sp. 31) from 206 species petition.12-month petition finding. 

5 UT plants (Astragalus hamiltonii, Eriogonum soredium, Lepidium ostleri, Penstemon flowersii, Trifolium 

friscanum) from 206 species petition.

12-month petition finding. 

2 CO plants (Astragalus microcymbus, Astragalus schmolliae) from 206 species petition ................................. 12-month petition finding. 

5 WY plants (Abronia ammophila, Agrostis rossiae, Astragalus proimanthus, Boechere (Arabis) pusilla, 

Penstemon gibbensii) from 206 species petition.

12-month petition finding. 

Leatherside chub (from 206 species petition) ....................................................................................................... 12-month petition finding. 

Frigid ambersnail (from 206 species petition) ....................................................................................................... 12-month petition finding. 

Gopher tortoise—eastern population ..................................................................................................................... 12-month petition finding. 

Wrights marsh thistle ............................................................................................................................................. 12-month petition finding. 

67 of 475 southwest species ................................................................................................................................. 12-month petition finding. 

Grand Canyon scorpion (from 475 species petition) ............................................................................................ 12-month petition finding. 

Anacroneuria wipukupa (a stonefly from 475 species petition) ............................................................................ 12-month petition finding. 

Rattlesnake-master borer moth (from 475 species petition) ................................................................................. 12-month petition finding. 

3 Texas moths (Ursia furtiva, Sphingicampa blanchardi, Agapema galbina) (from 475 species petition) ........... 12-month petition finding. 

2 Texas shiners (Cyprinella sp., Cyprinella lepida) (from 475 species petition) .................................................. 12-month petition finding. 

3 South Arizona plants (Erigeron piscaticus, Astragalus hypoxylus, Amoreuxia gonzalezii) (from 475 species 

petition).

12-month petition finding. 

5 Central Texas mussel species (3 from 475 species petition) ............................................................................ 12-month petition finding. 

14 parrots (foreign species) ................................................................................................................................... 12-month petition finding. 

Berry Cave salamander 1 ....................................................................................................................................... 12-month petition finding. 

Striped Newt 1 ........................................................................................................................................................ 12-month petition finding. 

Fisher—Northern Rocky Mountain Range 1 .......................................................................................................... 12-month petition finding. 

Mohave Ground Squirrel 1 ..................................................................................................................................... 12-month petition finding. 

Puerto Rico Harlequin Butterfly ............................................................................................................................. 12-month petition finding. 

Western gull-billed tern .......................................................................................................................................... 12-month petition finding. 

Ozark chinquapin (Castanea pumila var. ozarkensis) .......................................................................................... 12-month petition finding. 

HI yellow-faced bees ............................................................................................................................................. 12-month petition finding. 

Giant Palouse earthworm ...................................................................................................................................... 12-month petition finding. 

Whitebark pine ....................................................................................................................................................... 12-month petition finding. 

OK grass pink (Calopogon oklahomensis) 1 .......................................................................................................... 12-month petition finding. 

Southeastern pop snowy plover & wintering pop. of piping plover 1 .................................................................... 90-day petition finding. 

Eagle Lake trout 1 .................................................................................................................................................. 90-day petition finding. 

Smooth-billed ani 1 ................................................................................................................................................. 90-day petition finding. 

Bay Springs salamander 1 ..................................................................................................................................... 90-day petition finding. 

32 species of snails and slugs 1 ............................................................................................................................ 90-day petition finding. 

42 snail species (Nevada & Utah) ......................................................................................................................... 90-day petition finding. 

Red knot roselaari subspecies .............................................................................................................................. 90-day petition finding. 

Peary caribou ......................................................................................................................................................... 90-day petition finding. 


emcdonald on DSK2BSOYB1PROD with PROPOSALS3
78146 Federal Register / Vol. 75, No. 239 / Tuesday, December 14, 2010 / Proposed Rules 

ACTIONS FUNDED IN FY 2010 AND FY 2011 BUT NOT YET COMPLETED—Continued 

Species Action 

Plains bison ........................................................................................................................................................... 90-day petition finding. 

Spring Mountains checkerspot butterfly ................................................................................................................ 90-day petition finding. 

Spring pygmy sunfish ............................................................................................................................................ 90-day petition finding. 

Bay skipper ............................................................................................................................................................ 90-day petition finding. 

Unsilvered fritillary ................................................................................................................................................. 90-day petition finding. 

Texas kangaroo rat ................................................................................................................................................ 90-day petition finding. 

Spot-tailed earless lizard ....................................................................................................................................... 90-day petition finding. 

Eastern small-footed bat ........................................................................................................................................ 90-day petition finding. 

Northern long-eared bat ........................................................................................................................................ 90-day petition finding. 

Prairie chub ............................................................................................................................................................ 90-day petition finding. 

10 species of Great Basin butterfly ....................................................................................................................... 90-day petition finding. 

6 sand dune (scarab) beetles ................................................................................................................................ 90-day petition finding. 

Golden-winged warbler 4 ........................................................................................................................................ 90-day petition finding. 

Sand-verbena moth ............................................................................................................................................... 90-day petition finding. 

404 Southeast species .......................................................................................................................................... 90-day petition finding. 

Franklin’s bumble bee 4 ......................................................................................................................................... 90-day petition finding. 

2 Idaho snowflies (straight snowfly & Idaho snowfly) 4 ......................................................................................... 90-day petition finding. 

American eel 4 ........................................................................................................................................................ 90-day petition finding. 

Gila monster (Utah population) 4 ........................................................................................................................... 90-day petition finding. 

Arapahoe snowfly 4 ................................................................................................................................................ 90-day petition finding. 

Leona’s little blue 4 ................................................................................................................................................. 90-day petition finding. 

High-Priority Listing Actions 3

19 Oahu candidate species 2 (16 plants, 3 damselflies) (15 with LPN = 2, 3 with LPN = 3, 1 with LPN = 9) .... Proposed listing. 

19 Maui-Nui candidate species 2 (16 plants, 3 tree snails) (14 with LPN = 2, 2 with LPN = 3, 3 with LPN = 8) Proposed listing. 

Dune sagebrush lizard (formerly Sand dune lizard) 3 (LPN = 2) .......................................................................... Proposed listing. 

2 Arizona springsnails 2 (Pyrgulopsis bernadina (LPN = 2), Pyrgulopsis trivialis (LPN = 2)) ............................... Proposed listing. 

New Mexico springsnail 2 (Pyrgulopsis chupaderae (LPN = 2) ............................................................................ Proposed listing. 

2 mussels 2 (rayed bean (LPN = 2), snuffbox No LPN) ........................................................................................ Proposed listing. 

2 mussels 2 (sheepnose (LPN = 2), spectaclecase (LPN = 4),) ........................................................................... Proposed listing. 

8 Gulf Coast mussels (southern kidneyshell (LPN = 2), round ebonyshell (LPN = 2), Alabama pearlshell (LPN 

= 2), southern sandshell (LPN = 5), fuzzy pigtoe (LPN = 5), Choctaw bean (LPN = 5), narrow pigtoe (LPN 

= 5), and tapered pigtoe (LPN = 11)).

Proposed listing. 

Umtanum buckwheat (LPN = 2) 4 .......................................................................................................................... Proposed listing. 

Grotto sculpin (LPN = 2) 4 ..................................................................................................................................... Proposed listing. 

2 Arkansas mussels (Neosho mucket (LPN =2) & Rabbitsfoot (LPN = 9)) 4 ....................................................... Proposed listing. 

Diamond darter (LPN = 2) 4 ................................................................................................................................... Proposed listing. 

Gunnison sage-grouse (LPN = 2) 4 ....................................................................................................................... Proposed listing. 

1Funds for listing actions for these species were provided in previous FYs. 

2Although funds for these high-priority listing actions were provided in FY 2008 or 2009, due to the complexity of these actions and competing 

priorities, these actions are still being developed. 3Partially funded with FY 2010 funds and FY 2011 funds. 

4Funded with FY 2010 funds. 

5Funded with FY 2011 funds. 

We have endeavored to make our 

listing actions as efficient and timely as 

possible, given the requirements of the 

relevant law and regulations, and 

constraints relating to workload and 

personnel. We are continually 

considering ways to streamline 

processes or achieve economies of scale, 

such as by batching related actions 

together. Given our limited budget for 

implementing section 4 of the Act, these 

actions described above collectively 

constitute expeditious progress. 

The Sonoran desert tortoise will be 

added to the list of candidate species 

upon publication of this 12-month 

finding. We will continue to monitor the 

status of this DPS as new information 

becomes available. This review will 

determine if a change in status is 

warranted, including the need to make 

prompt use of emergency listing 

procedures. 

We intend that any proposed listing 

determination for the Sonoran desert 

tortoise will be as accurate as possible. 

Therefore, we will continue to accept 

additional information and comments 

from all concerned governmental 

agencies, the scientific community, 

industry, or any other interested party 

concerning this finding. 

References Cited 

A complete list of references cited is 

available on the Internet at http:// 

www.regulations.gov and upon request 

from the Arizona Ecological Services 

Office (see ADDRESSES section). 

Author(s) 

The primary authors of this notice are 

the staff members of the Arizona 

Ecological Services Office. 

Authority 

The authority for this section is 

section 4 of the Endangered Species Act 

of 1973, as amended (16 U.S.C. 1531 et 

seq.). 

Dated: November 23, 2010. 

Rowan W. Gould, 

Acting Director, Fish and Wildlife Service. 

[FR Doc. 2010–31000 Filed 12–13–10; 8:45 am] 

BILLING CODE 4310–55–P 


emcdonald on DSK2BSOYB1PROD with PROPOSALS3