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, 78094-78146 [2010-31000]

Agencies

• Fish and Wildlife Service
• DEPARTMENT OF THE INTERIOR

[Federal Register Volume 75, Number 239 (Tuesday, December 14, 2010)]
[Proposed Rules]
[Pages 78094-78146]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2010-31000]



[[Page 78093]]

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Part V





Department of the Interior





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Fish and Wildlife Service



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50 CFR Part 17



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; Proposed Rule

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

[[Page 78094]]


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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.

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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 https://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, off-highway 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,'' ``Gal[aacute]pago de desierto,'' or the 
``xtam[oacute]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 Pima-
Maricopa 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 
R[iacute]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 nitrogen-fixing 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 slope-
aspects (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, Averill-Murray 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 long-term 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 pine-dominated 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).

[[Page 78098]]

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; Averill-Murray 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; Averill-Murray 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 long-distance 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 above-average 
precipitation for a 2- to 3-year period (AIDTT 2000, p. 8). Averill-
Murray 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 long-distance 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.

[[Page 78099]]

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

[[Page 78100]]

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; Averill-Murray 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 ([deg] F) (30.5 
degrees Celsius ([deg] C)) result in the production of all male desert 
tortoises, whereas temperatures of 90.5 [deg]F (32.5 [deg]C) result in 
all females, and eggs incubated at the ``pivotal'' temperature of 88.3 
[deg]F (31.3 [deg] 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), ring-tailed 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.

[[Page 78101]]

    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 (Averill-Murray 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 line-transect 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

[[Page 78102]]

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 below-normal 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 de