14210 Federal Register / Vol. 76, No. 50 / Tuesday, March 15, 2011 / Proposed Rules 

DEPARTMENT OF THE INTERIOR 

Fish and Wildlife Service 

50 CFR Part 17 

[Docket No. FWS–R8–ES–2010–0008; MO 

92210–0–0008] 

RIN 1018–AX07 

Endangered and Threatened Wildlife 

and Plants; Withdrawal of Proposed 

Rule To List the Flat-Tailed Horned 

Lizard as Threatened 

AGENCY: Fish and Wildlife Service, 

Interior. 

ACTION: Proposed rule; withdrawal. 

SUMMARY: We, the U.S. Fish and 

Wildlife Service (Service), determine 

that the listing of the flat-tailed horned 

lizard (Phrynosoma mcallii) as a 

threatened species under the 

Endangered Species Act of 1973, as 

amended (Act), is not warranted, and 

we therefore withdraw our November 

29, 1993, proposed rule to list it under 

the Act. We made this determination in 

this withdrawal because threats to the 

species as identified in the 1993 

proposed rule are not as significant as 

earlier believed, and available data do 

not indicate that the threats to the 

species and its habitat, as analyzed 

under the five listing factors described 

in section 4(a)(1) of the Act, are likely 

to endanger the species in the 

foreseeable future throughout all or a 

significant portion of its range. 

DATES: The November 29, 1993 (58 FR 

62624), proposal to list the flat-tailed 

horned lizard as a threatened species is 

withdrawn as of March 15, 2011. 

ADDRESSES: This withdrawal of the 

proposed rule is available on the 

Internet at http://www.regulations.gov. 

Comments and materials received, as 

well as supporting documentation for 

this rulemaking is available for public 

inspection, by appointment, during 

normal business hours at the U.S. Fish 

and Wildlife Service, Carlsbad Fish and 

Wildlife Office, 6010 Hidden Valley 

Road, Suite 101, Carlsbad, CA 92011; 

telephone 760–431–9440; facsimile 

760–431–9624. 

FOR FURTHER INFORMATION CONTACT: Jim 

Bartel, Field Supervisor, Carlsbad Fish 

and Wildlife Office (see ADDRESSES

section). If you use a 

telecommunications device for the deaf 

(TDD), call the Federal Information 

Relay Service (FIRS) at 800–877–8339. 

SUPPLEMENTARY INFORMATION: 

Background 

The flat-tailed horned lizard 

(Phrynosoma mcallii) is a small, spiny 

lizard found in the Sonoran Desert of 

the southwestern United States and 

northwestern Mexico. All of the species 

of lizards in the genus Phrynosoma—the 

horned lizards—have dorso-ventrally 

flattened, ‘‘pancake-like’’ bodies; spiny 

scales; head spines or ‘‘horns’’; cryptic 

coloration; and certain similar 

behavioral traits (Sherbrooke 2003, pp. 

4–17; Stebbins 2003, p. 299; Leache ́ and 

McGuire 2006, p. 629). 

Among horned lizard species, the flattailed horned lizard has particularly 

long and sharp horns (Funk 1981, p. 

281.1; Sherbrooke 2003, p. 40; Young et 

al. 2004a, p. 65). Other characteristics 

that help distinguish flat-tailed horned 

lizards from other members of the genus 

include a dark line down the middle of 

the back (vertebral stripe), lack of 

external ear openings, two rows of 

fringe scales, an unspotted vent, and— 

as indicated by its common name—a 

long, broad, flattened tail (Funk 1981, p. 

281.1; Sherbrooke 2003, p. 40). The flattailed horned lizard is average in size 

when compared to other horned lizard 

species. Flat-tailed horned lizards 

become adults when about 60 to 64 

millimeters (mm) (2.4 to 2.5 inches (in)) 

long, not including the tail (snout-tovent length), and may grow to be about 

87 mm (3.4 in) long (Young and Young 

2000, p. 34; Rorabaugh and Young 2009, 

p. 182). The dorsal coloration of flattailed horned lizards varies and closely 

matches the colors of the desert soils on 

which they live, ranging from pale gray 

to light rust-brown, while their ventral 

coloration is white or cream-colored 

(Funk 1981, p. 281.1; Flat-tailed Horned 

Lizard Interagency Coordinating 

Committee [FTHLICC] 2003, p. 1; 

Stebbins 2003, p. 304). First described 

by Hallowell in 1852, no subspecies 

have been described or are recognized 

for the flat-tailed horned lizard (Crother 

et al. 2008, p. 35). 

The flat-tailed horned lizard occurs 

within the range of the desert horned 

lizard (Phrynosoma platyrhinos). 

Additionally, Goode’s horned lizard (P. 

[platyrhinos] goodie), which Klauber 

(1935, p. 179) considered to be a 

subspecies of the desert horned lizard 

(Klauber 1935, p. 179), also occurs 

within the range of the flat-tailed 

horned lizard in the portion southeast of 

the confluence of the Gila and Colorado 

Rivers (Mulcahy et al. 2006, p. 1823). 

Recent genetic analyses support Goode’s 

horned lizard as a differentiable 

evolutionary species (Mulcahy et al. 

2006, pp. 1807–1826). Hybrids between 

flat-tailed and Goode’s horned lizards, 

exhibiting a mix of morphological and 

genetic characters, have been observed 

southeast of Yuma, Arizona (Mulcahy et 

al. 2006, p. 1810), while apparent 

hybrids between flat-tailed and desert 

horned lizards have been observed in 

the vicinity of Ocotillo, California 

(Stebbins 2003, p. 302). Additionally, 

the regal horned lizard (P. solare) also 

occurs in northwestern Sonora, Mexico 

(Rorabaugh 2008, p. 39); we are not 

aware of hybridization with this species. 

Life History 

Flat-tailed horned lizards are 

oviparous (egg-laying), are early 

maturing, and may produce multiple 

clutches within a breeding season 

(Howard 1974, p. 111; Turner and 

Medica 1982, p. 819), which, when it 

occurs, results in two groups of 

individuals in a single year that are all 

generally the same age (that is, two 

cohorts). However, some authors 

question whether the observed two 

cohorts is the result of individual 

females producing two clutches in a 

year or whether different groups of 

females lay eggs at different times (Muth 

and Fisher 1992, p. 46; Young and 

Young 2000, p. 11). Flat-tailed horned 

lizards produce relatively small clutches 

of eggs (mean clutch size = 4.7; range = 

3 to 7) (Howard 1974, p. 111) compared 

to most other horned lizards (Sherbrook 

2003, p. 139). The first cohort hatches 

in July to August (Muth and Fisher 

1992, p. 19; Young and Young 2000, p. 

13), and when it occurs, the second 

cohort may be produced in September 

(Howard 1974, p. 111; Muth and Fisher 

1992, p. 19). Hatchlings from the first 

cohort may reach sexual maturity after 

their first winter season, whereas 

individuals that hatch later may require 

an additional growing season to mature 

(Howard 1974, p. 111). Flat-tailed 

horned lizards typically live for 4 years, 

or rarely even 6 years, in the wild 

(FTHLICC 2003a, p. 10). 

A home range is the area in which an 

animal (as an individual) typically lives. 

Flat-tailed horned lizards can have 

relatively large home ranges compared 

to other species of lizards of similar size 

(FTHLICC 2003a, p. 9). Muth and Fisher 

(1992, p. 34) found the mean home 

range size was 2.7 hectares (ha) (6.7 

acres (ac)) on the West Mesa, California. 

In the Yuma Desert of Arizona, Young 

and Young (2000, p. 54) found mean 

home ranges for males differed between 

drought and wet years, while those of 

females did not. The mean home range 

size for males was 2.5 ha (6.2 ac) during 

a dry year versus 10.3 ha (25.5 ac) 

during a wet year. Female mean home 

ranges were smaller at 1.3 ha (3.2 ac) 

and 1.9 ha (4.7 ac) in dry and wet years, 

respectively (Young and Young 2000, p. 

54). Young and Young (2000, p. 55) 

noted a wide variation in movement 


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patterns, with a few home ranges 

estimated at greater than 34.4 ha (85 ac). 

Flat-tailed horned lizards are not 

known to drink standing water 

(FTHLICC 2003a, p. 8), but they 

apparently do rain-harvest (Grant 2005, 

pp. 66–67), which is a behavior that 

some horned lizard species use to 

channel precipitation or condensation 

collected on the lizard’s body to its 

mouth for consumption (Sherbrook 

2003, p. 104). Thus, nearly all of the 

water consumed by flat-tailed horned 

lizards is from the food they eat 

(preformed water) (FTHLICC 2003a, p. 

8; Grant 2005, pp. 66–67). Most horned 

lizard species, including the flat-tailed 

horned lizard, are ant-foraging 

specialists (Pianka and Parker 1975, pp. 

141–162; Sherbrooke and Schwenk 

2008, pp. 447–459). More than 95 

percent of the diet of flat-tailed horned 

lizards is composed of ants, with 

species of harvester ants (genera Messor 

and Pogonomyrmex) predominating in 

most areas of the lizard’s range, but 

species of Dorymyrmex, Pheidole, and 

Myrmecocystus are also consumed 

(Pianka and Parker 1975, p. 148; Turner 

and Medica 1982, p. 820; Young and 

Young 2000, p. 38; FTHLICC 2003a, p. 

8). 

Flat-tailed horned lizards, typical of 

reptiles, obtain their body heat from the 

surrounding environment (ectothermic) 

(Mayhew 1965, p. 104; Sherbrooke 

2003, pp. 75–81). To gain body heat, 

they bask in the sun, often on rocks or 

other substrates that are warmed by 

insolation. During the heat of the day, 

to escape extreme surface temperatures, 

flat-tailed horned lizards may bury 

themselves just below the surface 

(Norris 1949, pp. 178–179) or retreat to 

a burrow made by other organisms 

(Young and Young 2000, p. 12). Adult 

flat-tailed horned lizards are reported to 

be obligatory hibernators (i.e., an 

organism that must enter a dormant 

period regardless of environmental 

conditions) (Mayhew 1965, p. 103). 

Hibernation may begin as early as 

October and end as late as March (Muth 

and Fisher 1992, p. 33), although 

individuals have been noted on the 

surface during January and February 

(FTHLICC 2003a, p. 9). Hibernation 

burrows appear to be self-constructed 

(as opposed to using burrows 

constructed by other animals) and are 

typically within 10 centimeters (cm) 

(3.9 in) of the surface (Muth and Fisher 

1992, p. 33). Mayhew (1965, p. 115) 

found that the majority of lizards 

hibernated within 5 cm (2 in) of the 

surface, with one as deep as 20 cm (8 

in) below the surface. 

Flat-tailed horned lizards generally lie 

close to the ground and remain 

motionless when approached (Wone 

and Beauchamp 1995, p. 132); however, 

they may occasionally bury themselves 

in loose sand if it is available (Norris 

1949, p. 176), and even more rarely, flee 

(Young and Young 2000, p. 12). Their 

propensity to remain motionless and 

bury in the sand, along with their 

cryptic coloration and flattened body, 

make them difficult to detect visually, 

which serves as a way to evade 

predators but also makes them difficult 

for surveyors to find in the field 

(FTHLICC 2003a, pp. 9, 65; Grant and 

Doherty 2007, p. 1050) (see also 

‘‘Population Dynamics’’ section, below). 

Additional life-history information is 

available in the Flat-tailed Horned 

Lizard Rangewide Management Strategy 

(FTHLICC 2003a, pp. 6–11). 

Setting and Habitat 

The flat-tailed horned lizard is 

endemic (restricted) to the Salton 

Trough and the region north of the Gulf 

of California in northwest Sonora, 

Mexico, both of which lie within the 

Lower Colorado Subdivision of the 

Sonoran Desert (Shreve and Wiggins 

1964, p. 6). The climatic conditions over 

the range of the flat-tailed horned lizard 

are characterized by hot summer 

temperatures, mild winter temperatures, 

and little rainfall. Winter rainfall 

predominates in the western portion of 

the species’ range while summer rainfall 

predominates in the eastern portion of 

the species’ range (Shreve and Wiggins 

1964, pp. 17–20, 49, 50; Johnson and 

Spicer 1985, p. 14). Periods of drought 

are not uncommon (Shreve and Wiggins 

1964, p. 18). 

Although the region in northwest 

Sonora, Mexico, represents roughly half 

of the current range of the flat-tailed 

horned lizard, its distribution within the 

Salton Trough has been more dynamic. 

As discussed below, the geologic and 

land use changes in the Salton Trough 

have substantially shaped the status of 

the species today. 

To better understand population 

trends of the flat-tailed horned lizard 

relative to the geologic setting and its 

current distribution within sandy 

habitat, we are providing a summary of 

the recent geologic history of the area in 

the following paragraphs (summarized 

from Parish 1914, pp. 85–114; Sykes 

1914, pp. 13–20; Durham and Alison 

1960, pp. 47–91; van de Kamp 1973, pp. 

827–848; Waters 1983, pp. 373–387; 

Blount and Lancaster 1990, pp. 724– 

728; Blount et al. 1990, pp. 15,463– 

15,482; Stokes et al. 1997, pp. 63–75; 

Patten et al. 2003, pp. 1–6; Li et al. 

2008, pp. 182–197). 

The Salton Trough (Trough) is a lowelevation valley that represents the 

northwestward continuation of the Gulf 

of California. During the period starting 

at least several million years ago, as sea 

levels rose and fell, the Gulf of 

California filled the present-day Salton 

Trough, often extending the Gulf 

northward into the present-day San 

Gorgonio Pass, east of Cabazon, 

California. The Colorado River flowed 

into the Gulf at roughly the same 

geographical area as today, but with the 

Gulf extending to a more northerly 

point, the river flowed into the Gulf 

mid-way along its length. 

The Colorado River, which originates 

in the Rocky Mountains and flows 

through the Grand Canyon, historically 

transported large quantities of finegrained sediment. Where the river 

joined the Gulf, sediments were 

deposited forming a broad delta. These 

sediments continued to increase and 

created a barrier that divided the Gulf 

into a land-locked northern portion (the 

Trough) and a marine-linked southern 

portion (the Gulf). The northern portion, 

which remains below sea level but 

without a direct connection with the 

ocean, eventually dried out. However, 

the Colorado River continued to 

meander across its delta and seasonal 

flooding promoted avulsion (i.e., 

abandonment of an old river channel 

and the creation of a new one). Thus, 

the river would sometimes flow into the 

Gulf and sometimes into the Trough, the 

lowest point of which—referred to as 

the Salton Basin—is about minus 84 

meters (m) elevation (277 feet (ft) below 

sea level). 

Water from the meandering Colorado 

River periodically filled the Salton 

Basin to varying depths (and areal 

extent), depositing sediments in the 

process. The lake that periodically 

formed, especially in its recent but 

prehistoric incarnations, is referred to 

by most authors as Lake Cahuilla. Its 

maximum depth depended on elevation 

of the delta, which is now about 12 m 

elevation (39 ft above sea level). The 

Lake was full as recently as the early 

1600s, but smaller, shallower 

manifestations were present at various 

times since then (including the modern 

Salton Sea, discussed below). When 

Lake Cahuilla was full, the Colorado 

River water flowed into the Basin from 

the southeast, marked today by the 

Alamo River and New River channels, 

and exited the Basin farther west along 

a southerly route, marked today by the 

Rı ́o Hardy channel, ultimately emptying 

into the Gulf of California. Floodwaters 

and sediments also periodically flowed 

into Laguna Salada, in northwestern 

Baja California, Mexico. Thus, even 

areas of the present-day Imperial, 

Mexicali, and San Luis Valleys that 


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were never or were less-frequently 

inundated by Lake Cahuilla, were 

regularly influenced by hydrologic 

forces associated with the Colorado 

River. Despite being in the middle of 

one of the driest deserts in North 

America, some of these areas were, at 

least periodically, part of an intricate 

water distribution system of channels, 

sloughs, and lagoons. 

Water also flowed into the Trough 

from surrounding highlands, bringing 

locally derived sediments with it. One 

notable inflow is marked by the presentday Whitewater River that flows into the 

Basin from the north. Water from the 

local sources would occasionally result 

in standing water in the Basin, but these 

sources could not compete with the 

sheer volume the Colorado River 

periodically provided. 

After flowing into the Trough for a 

period of time, the Colorado River 

would eventually meander back and 

once again flow into the Gulf. Over 

time, Lake Cahuilla would then become 

dry and the transported sediments 

would become exposed, with local 

sediment sources predominating the 

north end of the Trough, and Colorado 

River-derived sediments predominating 

the south end of the Trough. During dry 

periods, the fine-grained sediments in 

the Trough would be transported and 

sorted by prevailing winds. Thus, much 

of the Trough outside of those areas that 

were regularly influenced by the 

flooding and meandering of the 

Colorado River was ultimately 

blanketed with soft, friable (crumbly) or 

arenaceous (sandy) soils. Similarly, 

sediments deposited in the Colorado 

River delta and along the northeast 

shore of the Gulf of California were 

transported by winds where they 

formed areas of soft, friable (crumbly) or 

arenaceous (sandy) soils, including the 

‘‘sand sea’’ of the Gran Desierto de Altar. 

As a result, typical flat-tailed horned 

lizard habitat today includes areas of 

these sandy flats as well as the 

associated valleys created by these 

geologic events. Turner et al. (1980, p. 

14) stated the best habitats are generally 

low-relief areas with surface soils of 

packed, fine sand or low-relief areas of 

pavement (hardpan) overlain with loose, 

fine sand. However, the available 

scientific information indicates that flattailed horned lizards may occur in areas 

with soil substrates and plant 

associations that differ from these 

generalizations, as described below. 

Flat-tailed horned lizards are also 

known to occur at the edges of vegetated 

sand dunes, on barren clay soils, and 

within sparse Atriplex spp. (saltbush) 

plant communities. Although Turner et 

al. (1980, p. 15) suspected that these 

recorded occurrences were actually 

individuals that had dispersed from 

more suitable habitats, Wone et al. 

(1991, p. 16) questioned this conclusion 

(see also Wone and Beauchamp 1995, p. 

132; Beauchamp et al. 1998, p. 213), 

suggesting instead that flat-tailed 

horned lizards regularly occupy at least 

some of these areas. 

Within a creosote plant community in 

the West Mesa area, Muth and Fisher 

(1992, p. 61) found that flat-tailed 

horned lizards preferred sandy 

substrates with white bursage and 

Psorothamnus emoryi (Emory dalea), 

and avoided areas with creosote and 

Tiquilia plicata (fanleaf crinklemat). In 

Arizona, Rorabaugh et al. (1987, p.103) 

found flat-tailed horned lizard 

abundance correlated with Pleuraphis 

rigida (big galleta grass) and sandy 

substrates, but they suggested that the 

presence of sandy substrates was more 

important than grass. 

Several researchers have investigated 

the relationship between density of 

perennial plants and flat-tailed horned 

lizard abundance. The observed 

relationships varied among studies. For 

example, Altman et al. (1980, p. ii) and 

Turner and Medica (1982, p. 815) found 

the relative abundance of flat-tailed 

horned lizards was significantly and 

positively correlated with perennial 

plant density in creosote-white bursage 

plant communities (that is, horned 

lizard abundance increased as perennial 

plant density increased). In contrast, 

Beauchamp et al. (1998, p. 210) found 

flat-tailed horned lizards to be present 

in higher densities in sparsely vegetated 

areas with large patches of concretions 

(i.e., a volume of sedimentary rock in 

which a mineral cement fills the spaces 

between the sediment grains), gravel, 

and silt, than in areas that were sandy 

or densely vegetated. Altman et al. 

(1980, p. 7) also reported finding flattailed horned lizards in desert pavement 

areas. Foley (2002, p. 54) found little 

correlation in substrate texture and 

distribution of flat-tailed horned lizards, 

when using three experimental 

treatments consisting of sandy, rocky 

and mixed substrates. However, Wright 

and Grant (2003, p. 3) found flat-tailed 

horned lizard abundance was positively 

correlated with percentage of sand 

cover. Thus, flat-tailed horned lizard 

habitat includes a variety of soils and 

other plant associations, but the habitat 

is best characterized as sandy flats and 

valleys in a creosote-white bursage plant 

association. 

Plants and harvester ants are 

important components to flat-tailed 

horned lizard habitat because they 

comprise its primary food chain. Seeds 

make up the primary food of harvester 

ants (Johnson 2000, p. 92). The ants 

often collect seeds from annual plants, 

including some nonnative species 

(Rissing 1988, p. 362), but they also 

gather seeds from perennial plants 

(Gordon 1980, p. 72). Thus, a simplified 

food chain for the flat-tailed horned 

lizard may be described as follows: 

Plants produce seeds, harvester ants eat 

the seeds, and flat-tailed horned lizards 

eat harvester ants. 

Range and Distribution 

A species’ range is the region over 

which it is distributed. The range of the 

flat-tailed horned lizard includes the 

Salton Trough and the region north of 

the Gulf of California. In general, this 

range includes portions of southeastern 

California (eastern San Diego County, 

central Riverside County, and 

southwestern Imperial County) and 

southwestern Arizona (southwestern 

Yuma County) in the United States, and 

northeastern Baja California and 

northwestern Sonora in Mexico (Turner 

and Medica 1982, p. 815) (Figure 1). 

Within its range, the flat-tailed horned 

lizard is limited to areas below an upper 

elevation. Although the species has 

been recorded as high as 520 m (1,706 

ft) above sea level (Turner et al. 1980, 

p. 13), flat-tailed horned lizards are 

more commonly found below about 230 

m (about 750 ft) in elevation (FTHLICC 

2003a, p. 3). 

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Extensive manmade changes, chiefly 

for agriculture, have occurred over a 

large portion of the land within the 

Salton Trough. Below we present a 

summary of the history of agricultural 

development in the Salton Trough 

(summarized from Furnish and Ladman 

1975, pp. 83–107; Woerner 1989, pp. 

109–112; Imperial Irrigation District 

[IID] 2002, pp. 3.1–66 to 3.1–77; Patten 

et al. 2003, pp. 1–6). 

Near the start of the 20th century, a 

canal was built to import water to the 

Salton Trough from the Colorado River. 

The Salton Basin is below sea level and 

much of the rest of the Salton Trough is 

at a lower elevation than where the head 

of the canal was located. Thus, with the 

regionally abundant sunshine and riversediment soils, the importation of water 

by a gravity-fed system allowed 

agriculture to proliferate. For example, 

by 1904 approximately 60,700 ha 

(150,000 ac) were in cultivation. 

Unlike the current canal, the original 

canal was poorly designed because it 

had no headgate to regulate flows into 

the canal. Prior to extensive dams on the 

Colorado River, the river was prone to 

flooding. The high waters of one such 

flood during the winter of 1904–05 

flowed into the canal. Soon, nearly the 

entire Colorado River flowed through 

the canal, releasing water into the 

Salton Basin. Part of the flow followed 

the two historical riverbeds (the Alamo 

River and the New River) that were 

deepened and widened by the torrent. 

Despite heroic efforts, the flow 

continued until 1907. The Salton Basin 

filled to a depth of about 22 m (72 ft) 

(at its deepest point) and covered about 

121,400 ha (300,000 ac), thus creating 

the modern Salton Sea. 

Although the ‘‘creation’’ of the Salton 

Sea is often times described as an 

accident, the inundation of the Salton 

Basin by water flowing from the 

Colorado River from 1905 to 1907 was 

merely the most recent of many such 

inundations over historical and 

prehistorical times (see ‘‘Setting and 

Habitat’’ section above). Even without 

the canal, the flood of 1905 may have 

naturally flowed into the Basin. 

Since the formation of the modern 

Salton Sea, agricultural practices in the 

region have maintained the water levels 

of the Salton Sea. If too much irrigation 

water is allowed to evaporate in the 

fields, salt levels, which are high in 

Colorado River water, build up in the 

soil, making it inhospitable for crops. To 

prevent this hypersalinization of the 

soils, a surplus of water is used for 

irrigation. The excess water drains by 

gravity from the fields through a 

network of ditches into the Salton Sea. 

Even with the high evaporation rates in 

the desert climate, inflow rates of 

drainage water have been high enough 

to maintain, and, for a time, even 

increase, the surface water elevation of 

the Salton Sea. 

Efforts to bring irrigation water to the 

region continued through the 1900s, and 

the system of irrigation canals was 

eventually improved and expanded. In 

addition to the Imperial Valley, the 

Coachella Canal was constructed to 

bring water to the southern Coachella 

Valley, allowing irrigated agriculture to 

develop north of the Salton Sea. Similar 

canal systems were built in Mexico, 

allowing agriculture to develop and 

expand in the Mexicali and San Luis 

Valleys. Because these systems were 

gravity fed, the distribution canals 

within the region were dictated by 

elevation, which in turn, determined 

where irrigated agricultural 

development occurred. Thus, the 

majority of agricultural development 

was confined within the outer-most 

(highest elevation) canals. Moreover, 

croplands (and associated urbanization 

and infrastructure) were contiguous in 

the Salton Trough region, with little to 

no intervening undeveloped natural 

areas. Additionally, smaller amounts of 

agricultural development using pumped 

groundwater have occurred on a smaller 

scale outside these areas. 

The geographically confined 

agricultural growth in the region is 

currently limited by the amount of 

water available from the Colorado River, 

which is dependent on annual 

precipitation in the Upper and Lower 

Colorado River Basins. The amount of 

irrigation water that can be delivered to 

the Salton Trough from the Colorado 

River is limited by interstate and 

international agreements (Furnish and 

Ladman 1975, pp. 83–107). Water 

conservation and transfer agreements 

completed in 2003 with the San Diego 

County Water Authority, Imperial 

Irrigation District, Metropolitan Water 

District of Southern California, and 

Coachella Valley Water District has 

reduced the amount of water available 

in the Imperial Valley and some fields 

have been fallowed, resulting in a 

decrease in the amount of irrigated 

agriculture in this region (IID 2006, p. 

1). 

Aerial and satellite imagery (Carlsbad 

Fish and Wildlife Office geographic 

information system (GIS) files) 

illustrates the development of active 

cultivation and associated urbanization 

and infrastructure extending from the 

present-day delta of the Colorado River, 

with a longer fork extending northnorthwest through the Mexicali and 

Imperial Valleys to the Coachella Valley 

(punctuated by the Salton Sea), and a 

smaller fork extending northeast 

through the eastern Mexicali Valley and 

the San Luis Valley (Lower Colorado 

River Valley) to Yuma. Although there 

are specimens of flat-tailed horned 

lizards collected historically from 

within the now-altered region (Funk 

1981, p. 281.1; Johnson and Spicer 

1985, pp. 14–24), areas of agricultural 

and urban development do not 

constitute habitat for the flat-tailed 

horned lizard, and this continuous 

swath of altered land use is no longer 

occupied by flat-tailed horned lizards. 

The current distribution of the flattailed horned lizard is often described 

within four, geographically descriptive 

‘‘populations.’’ We use the term 

population in this document to refer to 

a loosely bounded, regionally 

distributed collection of individuals of 

the same species. These four 

populations are defined as: 

(1) The Coachella Valley Population, 

including those individuals northwest 

of the Salton Sea, California; 

(2) The Western Population, including 

those individuals in the areas west of 

the Salton Sea and the Imperial Valley, 

California, and west of the Mexicali 

Valley, Baja California, Mexico; 

(3) The Eastern Population, including 

those individuals in the areas east of the 

Salton Sea and the Imperial Valley but 

west of the Colorado River; and 

(4) The Southeastern Population, 

including those individuals in the areas 

east of the Colorado River, extending 

from Yuma south into Mexico and east 

to the Gulf of California. 

These current designations closely 

follow the description of populations 

discussed in our January 3, 2003, 

analysis (68 FR 331), although in that 

document we used the United StatesMexico border to further divide the 

populations (see Figure 1 above). 

Additionally, these populations roughly 

correspond to those used by Mulcahy et 

al. (2006, pp. 1807–1826) in their 

analysis of flat-tailed horned lizard 

genetic data (see below for details). At 

the end of the Background section, 

below, we summarize these four 

populations in greater detail. We also 

use these four population names to 

identify the geographical habitat they 

occupy. 

Populations and Genetics 

The separation of the four populations 

of flat-tailed horned lizards described 

above in the ‘‘Range and Distribution’’ 

section is supported by genetic data, to 

varying degrees. Analyses of 

mitochondrial DNA data (Mulcahy et al. 

2006, pp. 1807–1826; see also 

Mendelson et al. 2004, pp. 1–42) and 

nuclear microsatellite data (Culver and 


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Dee 2008, pp. 1–14) revealed significant 

differences in the prevalence of certain 

alleles in flat-tailed horned lizard 

populations on either side of the 

Colorado River; that is, the Southeastern 

Population differs from the other three 

populations. These analyses also 

showed that more gene flow has 

occurred near the Colorado River delta, 

suggesting the shifting course of the 

river over time in this area posed less of 

a barrier than the more stable portions 

of the river channel farther north 

(Mulcahy et al. 2006, p. 1822; Culver 

and Dee 2008, p. 11). Although Culver 

and Dee (2008, p. 10) noted genetic 

variation in some individuals across the 

Southeastern Population, they found 

that flat-tailed horned lizards in Arizona 

are ‘‘not genetically isolated from 

neighboring populations in Mexico.’’ 

Thus, the flat-tailed horned lizards east 

of the Colorado River (i.e., the 

Southeastern Population) may be 

considered one population that is 

significantly and genetically distinct 

from the populations west of the river 

(i.e., the Coachella Valley, Western, and 

Eastern Populations). 

The three populations west of the 

Colorado River also showed varying 

levels of genetic differentiation. 

Mulcahy et al. (2006, p. 1821) noted the 

Eastern Population ‘‘was significantly 

differentiated from [the Western and 

Coachella Valley Populations], 

suggesting that there has not been 

substantial gene flow across the 

Imperial Valley since the drying of Lake 

Cahuilla.’’ However, the difference 

between the Coachella Valley and 

Western Populations was less 

pronounced. Although their difference 

was supported by the presence of 

haplotypes unique to the Coachella 

Valley Population (Mulcahy et al. 2006, 

Table 1 on p. 1811, and p. 1817), the 

difference between the Western and 

Coachella Valley Populations was not 

statistically significant (the other 

populations had unique haplotypes, 

too). This lack of significant difference 

suggested to the authors that the 

Coachella Valley Population ‘‘had more 

recent gene flow’’ with the Western 

Population (Mulcahy et al. 2006, p. 

1821). Thus, genetic data readily 

support three of the four geographic 

populations described above, but the 

distinction between the Western and 

Coachella Valley Populations is weak or 

equivocal. This suggests that the 

Coachella Valley Population was not a 

separate population historically, but is 

one now because it was ‘‘created’’ by an 

artificial barrier resulting from past 

agricultural and urban development. 

Management and Populations 

Three notable management 

mechanisms are in place within the U.S. 

portion of the flat-tailed horned lizard 

range: the Interagency Conservation 

Agreement, which includes the Flattailed Horned Lizard Rangewide 

Management Strategy (Rangewide 

Management Strategy); the Coachella 

Valley Multiple Species Habitat 

Conservation Plan (Coachella Valley 

MSHCP); and the Lower Colorado River 

Multi-Species Conservation Plan (Lower 

Colorado MSCP). Implementation of the 

Interagency Conservation Agreement 

has recently positively affected and is 

anticipated to continue to positively 

affect the status of flat-tailed horned 

lizard populations in the United States 

and, to a lesser extent, in Mexico. The 

recently permitted Coachella Valley 

MSHCP is also worth noting because it 

is a regional habitat conservation plan 

(HCP) developed under section 10 of the 

Act that covers the flat-tailed horned 

lizard in the Coachella Valley, an area 

addressed at length in our previous 

withdrawals. Additionally, the Lower 

Colorado MSCP is also an HCP that 

addresses the flat-tailed horned lizard. 

Interagency Conservation Agreement 

and Flat-tailed Horned Lizard 

Rangewide Management Strategy 

In June of 1997, the Service, Bureau 

of Land Management (BLM), Bureau of 

Reclamation (BOR), U.S. Marine Corps, 

U.S. Navy, Arizona Game and Fish 

Department, California Department of 

Fish and Game (CDFG), and California 

Department of Parks and Recreation 

(CDPR) entered into an Interagency 

Conservation Agreement. All signatories 

agreed to: 

(1) Further develop and implement 

the objectives, strategies, and tasks of 

the Flat-tailed Horned Lizard 

Rangewide Management Strategy 

[original, FTHLICC 1997, pp. 1–106; 

revised: FTHLICC 2003a, p. 104; see 

below]; 

(2) As needed for the conservation 

effort, and as available, provide program 

personnel with facilities, equipment, 

logistical support, and access to lands 

under their control; 

(3) Participate regularly in Interagency 

Coordinating Committee and 

Management Oversight Group meetings 

to enhance communication and 

cooperation, and to help develop annual 

or other work plans and reports; 

(4) Develop and distribute public 

information and educational materials 

on the conservation effort; 

(5) Provide ongoing review of, and 

feedback on, the conservation effort; 

(6) Cooperate in development of major 

media releases and media projects; 

(7) Keep local governments, 

communities, the conservation 

community, citizens, and other 

interested and affected parties informed 

on the status of the conservation effort, 

and solicit their input on issues and 

actions of concern or interest to them; 

(8) Whenever possible, develop 

voluntary opportunities and incentives 

for local communities and private 

landowners to participate in the 

conservation effort; and 

(9) Assist in generating the funds 

necessary to implement the 

conservation effort. 

The purpose of the Rangewide 

Management Strategy is to provide a 

framework for conserving sufficient 

habitat to maintain several viable 

populations of the flat-tailed horned 

lizard throughout the range of the 

species in the United States. The 

Rangewide Management Strategy was 

developed by an interagency working 

group over a 2-year period. Despite 

being a voluntary agreement, many of 

the measures to conserve flat-tailed 

horned lizards are formally incorporated 

into planning documents of 

participating agencies, such as the 

Bureau of Land Management’s 

California Desert Conservation Area 

Plan. 

As part of the Interagency 

Conservation Agreement, agencies 

delineated specific areas under their 

jurisdiction as Management Areas. As of 

2009, approximately 185,653 ha 

(458,759 ac) of the flat-tailed horned 

lizard habitat managed by signatories of 

the Interagency Conservation Agreement 

exists within five Management Areas 

(see Table 1 below) (FTHLICC 2009, p. 

10). These Management Areas include 

the Borrego Badlands, West Mesa, and 

Yuha Desert (also referred to as the 

Yuha Basin) in the Western Population, 

the East Mesa in the Eastern Population, 

and the Yuma Desert in the 

Southeastern Population (Figure 2). 

Additionally, the Ocotillo Wells State 

Vehicular Recreation Area (SVRA) was 

designated as a research area. 


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The five Management Areas were 

designed to include large areas of public 

land in the United States where flattailed horned lizards have been found, 

and to include most flat-tailed horned 

lizard habitat identified by the FTHLICC 

(1997, p. 35) as ‘‘key’’ areas for survival 

as determined in previous studies 

(Turner et al. 1980, pp. 1–47; Turner 

and Medica 1982, pp. 815–823; 

Rorabaugh et al. 1987, pp. 103–109). 

Management Areas were proposed 

based on standard principles of preserve 

design, utilizing the best information 

available at the time (FTHLICC 2003a, p. 

47). 

The Management Areas were 

delineated to include areas as large as 

possible, while avoiding extensive, 

existing and predicted management 

conflicts (such as off-highway vehicle 

(OHV) open areas). The Management 

Areas are meant to be the core areas for 

maintaining self-sustaining populations 

of flat-tailed horned lizards in the 

United States (FTHLICC 2003a, p. 24). 

The Management Areas constitute 

roughly 42 percent of the U.S. current 

distribution. Although the majority of 

lands within each Management Area are 

State or federally owned, some private 

inholdings occur within Management 

Area boundaries. 

The 2003 Rangewide Management 

Strategy includes measures to avoid, 

minimize, and compensate impacts to 

the flat-tailed horned lizard and its 

habitat from construction projects and 

other development activities permitted 

by signatory agencies. As described in 

detail in the Rangewide Management 

Strategy (FTHLICC 2003a, pp. 58–60), 

the avoidance and minimization 

measures include (in part) avoidance of 

flat-tailed horned lizard Management 

Areas and the Research Area, project 

oversight and compliance measures, 

minimized project footprint, use of 

existing roads rather than creating new 

roads, use of barrier fencing, and 

project-specific habitat restoration. The 

Rangewide Management Strategy 

outlines avoidance, minimization, and 

mitigation measures intended to limit 

the impacts from permitted projects 

within the Management Areas to a 

maximum of 1 percent of the total area 

of each Management Area (FTHLICC 

2003a, pp. 24–43). Additionally, the 

Rangewide Management Strategy 

(FTHLICC 2003a, pp. 60–62) describes 

compensation measures for projects 

within and outside the Management 

Areas where residual effects would 

occur after all reasonable on-site 

mitigation has been applied. The goal of 

compensation under the Rangewide 

Management Strategy is to ‘‘prevent the 

net loss of [flat-tailed horned lizard] 

habitat and make the net effect of a 

project neutral or positive to [flat-tailed 

horned lizards] by maintaining a habitat 

[baseline]’’ (FTHLICC 2003a, p. 61). 

Compensation funds may be used ‘‘to 

acquire, protect, or restore [flat-tailed 

horned lizard] habitat both within and 

contiguous with [Management Areas]’’ 

(FTHLICC 2003a, p. 60). Compensation 

ratios range from one-to-one to six-toone (meaning, in latter ratio for 

instance, that six acres-worth of 

compensation will be required for every 

one acre of impact), depending on the 

location and nature of the impacts 

(FTHLICC 2003a, p. 61). Funds obtained 

through compensation associated with 

implementation of the Rangewide 

Management Strategy are being used to 

consolidate land ownership within the 

Management Areas or to enhance flattailed horned lizard habitat (FTHLICC 

2003a, p. 25; FTHLICC 2010, p. 8). The 

original and current acreages of each 

Management Area are listed in Table 1. 


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TABLE 1—AREA (HECTARES AND ACRES) OF FLAT-TAILED HORNED LIZARD MANAGEMENT AREAS OWNED BY SIGNATORIES

TO THE INTERAGENCY CONSERVATION AGREEMENT IMPLEMENTING THE FLAT-TAILED HORNED LIZARD RANGEWIDE

MANAGEMENT STRATEGY AND AREA OWNED BY NON-SIGNATORIES (PREDOMINANTLY PRIVATE) IN 1997 AND

THROUGH 2009, PLUS AREA AND PERCENTAGE OF PROJECT-RELATED IMPACTS PERMITTED BY SIGNATORIES WITHIN

EACH MANAGEMENT AREA (SOURCES: FTHLICC 1997, P. 74; FTHLICC 2003A, P. 48; FTHLICC 2009, P. 10; 

FTHLICC 2010, P. 8) 

Management 

area 

Area of 

signatory lands 

in 1997 

Area of nonsignatory lands 

in 1997 

Area of nonsignatory lands 

added to 

signatory lands 

since 1997 

Total area of 

signatory lands 

in 2009 

Total area of 

management 

area 

Total area 

permitted for 

impact as of 

2009 

Percent of total 

area of 

management 

area permitted 

for impact as of 

2009 (percent) 

Borrego Badlands.14,771 ha 

(36,500 ac).

2,388 ha (5,900 

ac).

592 ha *(1,464 

ac).

15,363 ha 

(37,964 ac).

17,159 ha 

(42,400 ac).

0 ha (0 ac) ...... 0.0 

West Mesa ........ 46,256 ha 

(114,300 ac).

8,822 ha 

(21,800 ac).

2,624 ha (6,483 

ac).

48,880 ha 

(120,785 ac).

55,078 ha 

(136,100 ac).

86.77 ha 

(214.42 ac).

0.16 

Yuha Desert ...... 23,148 ha 

(57,200 ac).

1,214 ha (3,000 

ac).

0 ha (0 ac) ...... 23,148 ha 

(57,200 ac).

24,362 ha 

(60,200 ac).

35.90 ha (88.70 

ac).

0.15 

East Mesa ......... 43,868 ha 

(108,400 ac).

2,792 ha (6,900 

ac).

1,380 ha (3,410 

ac).

45,248 ha 

(111,810 ac).

46,660 ha 

(115,300 ac).

38.40 ha (94.90 

ac).

0.08 

Yuma Desert ..... 46,741 ha 

(115,500 ac).

6,273 ha 

(15,500 ac).

6,273 ha 

(15,500 ac).

53,014 ha 

(131,000 ac).

53,014 ha 

(131,000 ac).

10.50 ha (25.95 

ac).

0.02 

Total ........... 174,784 ha 

(431,900 ac).

21,489 ha 

(53,100 ac).

10,869 ha 

(26,857 ac).

185,653 ha 

(458,759 ac).

196,273 ha 

(485,000 ac).

171.57 ha 

(423.97 ac).

0.09 

* Includes 350 ha (864 ac) owned by the Anza-Borrego Foundation. 

Representatives from the agencies 

participating on the Rangewide 

Management Strategy (also known as 

the Interagency Coordinating 

Committee) meet several times a year to 

coordinate and implement management 

actions (FTHLICC 2003a, pp. 1–104). 

The Interagency Coordinating 

Committee regularly documents 

progress made to conserve the flat-tailed 

horned lizard collectively or by 

participating agencies (FTHLICC 1998, 

pp. 1–11; FTHLICC 1999, pp. 1–13; 

FTHLICC 2001, pp. 1–24; FTHLICC 

2003b, pp. 1–32; FTHLICC 2004, pp. 1– 

33; FTHLICC 2005, pp. 1–37; FTHLICC 

2006, pp. 1–34; FTHLICC 2007, pp. 1– 

33; FTHLICC 2008a, pp. 1–35; FTHLICC 

2009, pp. 1–38; FTHLICC 2010, pp. 1– 

33). These reports document and 

summarize the progress member 

agencies have made towards 

implementation of the Planning Actions 

identified in Rangewide Management 

Strategy (FTHLICC 2003a, pp. 25–32). 

The reports indicate that progress by 

signatory agencies has been made in the 

following areas: (1) Designation of the 

five Management Areas and the one 

Research Area; (2) requiring actions by 

permittees to follow the avoidance, 

minimization, and mitigation measures 

outlined in the Rangewide Management 

Strategy; (3) rehabilitating damaged and 

degraded habitat within the 

Management Areas; and (4) purchase of 

lands for flat-tailed horned lizard 

conservation from willing sellers. 

Although some lower priority actions 

(tasks), such as research on natural 

barriers, remain outstanding, the 

committee reports that nearly all tasks, 

many of which are ongoing or multiyear actions, are on schedule (FTHLICC 

2010, pp. 21–25). Thus, despite being a 

voluntary agreement, the signatory 

agencies generally have been 

implementing the Interagency 

Conservation Agreement and associated 

Rangewide Management Strategy by 

meeting regularly, working to 

implement the measures of the 

Rangewide Management Strategy 

including providing personnel, 

developing and distributing public 

information, and providing ongoing 

review and feedback. 

Coachella Valley Multiple Species 

Habitat Conservation Plan (Coachella 

Valley MSHCP) 

Our past assessments of the status of 

the flat-tailed horned lizard, particularly 

the 2003 withdrawal (68 FR 331), 

addressed the Coachella Valley in 

detail; thus, for consistency we again 

address the Coachella Valley here and 

elsewhere in this document. Since the 

2003 withdrawal, and even since our 

June 28, 2006, withdrawal (71 FR 

36745), we have issued an incidental 

take permit for a large, regional HCP in 

the Coachella Valley. The Coachella 

Valley MSHCP is a large-scale, multijurisdictional habitat conservation plan 

encompassing about 445,156 ha (1.1 

million ac) in the Coachella Valley of 

central Riverside County. An additional 

27,923 ha (69,000 ac) of Tribal 

reservation lands distributed within the 

plan area boundary are not included in 

the Coachella Valley MSHCP. The 

Coachella Valley MSHCP addresses 27 

listed and unlisted ‘‘covered species,’’ 

including the flat-tailed horned lizard. 

On October 1, 2008, the Service issued 

a single incidental take permit (TE– 

104604–0) under section 10(a)(1)(B) of 

the Act to 19 permittees under the 

Coachella Valley MSHCP for a period of 

75 years. Participants in the Coachella 

Valley MSHCP include eight cities 

(Cathedral City, Coachella, Indian 

Wells, Indio, La Quinta, Palm Desert, 

Palm Springs, and Rancho Mirage); the 

County of Riverside, including the 

Riverside County Flood Control and 

Water Conservation District, Riverside 

County Parks and Open Space District, 

and Riverside County Waste 

Management District; the Coachella 

Valley Association of Governments; 

Coachella Valley Water District; 

Imperial Irrigation District; California 

Department of Transportation; 

California State Parks; Coachella Valley 

Mountains Conservancy; and the 

Coachella Valley Conservation 

Commission (the created joint powers 

regional authority). The Coachella 

Valley MSHCP was designed to 

establish a multiple species habitat 

conservation program that minimizes 

and mitigates the expected loss of 

habitat and incidental take of covered 

species, including flat-tailed horned 

lizard (USFWS 2008, pp. 1–207, and 

Appendix A, pp. 298–328). The 

Coachella Valley MSHCP is also a 

‘‘Subregional Plan’’ under the State of 


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California’s Natural Community 

Conservation Planning (NCCP) Act, as 

amended. 

The permit covers incidental take 

resulting from habitat loss and 

disturbance associated with urban 

development and other proposed 

covered activities. These activities 

include public and private development 

within the plan area that require 

discretionary and ministerial actions by 

permittees subject to consistency with 

the Coachella Valley MSHCP policies. 

An associated Management and 

Monitoring Program is also included in 

the Coachella Valley MSHCP and 

identifies specific management actions 

for the conservation of the flat-tailed 

horned lizard and its habitat. 

The Coachella Valley MSHCP 

identifies a reserve system that, upon 

full implementation, will establish 21 

conservation areas that are either 

adjacent to each other or are linked by 

biological corridors. The acquisition 

program for the plan’s reserve system is 

designed to conserve 52,484 ha (129,690 

ac) during the first 30 years. This 

program is to be implemented such that 

acquisitions occur commensurate (in 

‘‘rough step’’) with impacts from urban 

development that is covered under the 

plan. 

The flat-tailed horned lizard is now 

known to occur only at two locations 

within the Coachella Valley MSHCP 

area, the Thousand Palms and Dos 

Palmas conservation areas (CVCC 2010, 

p. 13) (see also Description of Specific 

‘‘Populations’’ section below). Table 2 

describes the amount of flat-tailed 

horned lizard habitat conserved and 

identified to be conserved through 

implementation of the Coachella Valley 

MSHCP. Additionally, plan 

implementation is expected to limit 

impacts of development and other 

covered activities on lands within 

conservation areas but that have not yet 

been acquired for conservation as part of 

the Coachella Valley MSHCP reserve 

system. The plan also designates one 

core habitat area (as used in that plan, 

this refers to an area that is large enough 

to maintain a self-sustaining 

population)—the Thousand Palms 

conservation area—and commits to 

establishing two more self-sustaining 

populations in other parts of the reserve 

system, if feasible, to benefit the flattailed horned lizard. Because of the 

distances separating appropriate parts of 

the reserve system, relocation of flattailed horned lizards will be required to 

re-establish or enhance populations in 

suitable habitat areas that have the 

potential to, but currently do not, 

support self-sustaining populations. 

Additionally, the plan calls for 

Management and Monitoring Programs 

that are expected to conserve this 

species in the plan area. Required 

management activities include limiting 

activities that degrade flat-tailed horned 

lizard habitat, evaluation and 

management of edge effects and other 

impacts through adaptive management, 

control of invasive species where 

necessary, and restoration and 

enhancement of degraded habitat as 

necessary according to monitoring 

results (CVAG 2007, p. 9–123). In our 

evaluation of the potential impacts of 

the plan’s implementation on the flattailed horned lizard (USFWS 2008, p. 

178), we concluded: ‘‘After reviewing 

the current status of this species, 

environmental baseline for the action 

area, effects of the proposed action, and 

cumulative effects, it is the Service’s 

biological opinion that the action, as 

proposed, is not likely to jeopardize the 

continued existence of the flat-tailed 

horned lizard. Loss of the Coachella 

Valley population would have a 

negligible [effect] on the status of the 

species as a whole, since it makes up 

approximately 1 percent of the current 

range of the flat-tailed horned lizard. 

Persistence of the species in the Plan 

area is likely only with effective Plan 

implementation.’’ 

TABLE 2—AREA OF FLAT-TAILED HORNED LIZARD HABITAT CONSERVED, ANTICIPATED TO BE CONSERVED, IMPACTED, 

AND ANTICIPATED TO BE IMPACTED THROUGH IMPLEMENTATION OF THE COACHELLA VALLEY MSHCP 

Criterion (source) Thousand Palms Dos Palmas 

Flat-tailed horned lizard habitat area conserved at permit issuance in 

2008 (CVAG 2007, p. 9–115).

1,318 ha (3,256 ac) 608 ha (1,503 ac) 

Additional flat-tailed horned lizard habitat area conserved in 2008 

(CVCC 2009, p. 79).

274 ha (678 ac) 107 ha (265 ac) 

Additional flat-tailed horned lizard habitat area conserved in 2009 

(CVCC 2010, pp. 39 & 51).

8 ha (20 ac) 0 ha (0 ac) 

Total flat-tailed horned lizard habitat area under conservation through 

2009 (calculated).

1,600 ha (3,954 ac) 715 ha (1,768 ac) 

Total flat-tailed horned lizard habitat area expected to be conserved by 

MSHCP implementation (CVAG 2007, p. 9–115).

1,707 ha (4,219 ac) 2,078 ha (5,134 ac) 

Percent flat-tailed horned lizard habitat area conserved through 2009 

compared to amount required upon full implementation of the plan 

(calculated).

94% 34% 

Area of flat-tailed horned lizard habitat impacted by permitted activities 

through 2009 (CVCC 2009, p. 79; CVCC 2010, pp. 39 & 51).

0 ha (0 ac) 0 ha (0 ac) 

Area of flat-tailed horned lizard habitat anticipated to be impacted by 

permitted activities (CVAG 2007, p. 9–115).

44 ha (108 ac) 163 ha (403 ac) 

Percent flat-tailed horned lizard habitat area anticipated to be impacted 

compared to total area of flat-tailed horned lizard habitat in conservation area (calculated).

2% 7% 

Lower Colorado River Multi-Species 

Conservation Plan (Lower Colorado 

River MSCP) 

The Lower Colorado River MSCP is a 

joint effort by Federal and non-Federal 

(State, local, and private) entities with 

management authority for storage, 

delivery, and diversion of water; 

hydropower generation, marketing, and 

delivery; and land management or 

Native American Trust responsibilities 

along the Lower Colorado River, to 

address regulatory requirements under 

sections 7, 9, and 10 of the Act for their 

activities. We issued the 50-year permit 

(TE–086834) on April 4, 2005. Most of 

the activities addressed by the Lower 

Colorado MSCP are outside the range of 

the flat-tailed horned lizard. The flattailed horned lizard habitat contained 

within the Lower Colorado River MSCP 


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planning area is under control of 

agencies, especially the Bureau of 

Reclamation, that have agreed to 

implement the Rangewide Management 

Strategy (USFWS 2005, p. 202). 

Implementation of the Lower 

Colorado River MSCP is expected to 

provide for the acquisition and longterm protection of 230 acres of existing 

flat-tailed horned lizard habitat that is 

currently unprotected. This action is 

compensation for anticipated impacts to 

approximately 128 acres of flat-tailed 

horned lizard habitat (USFWS 2005, pp. 

201–202). Purchase of protected habitat, 

potentially near the Dos Palmas reserve 

area, is scheduled to start in 2011 (BOR 

2010, p. 274). Additionally, activities 

covered under the permit will be 

designed to avoid or minimize effects to 

the species and its habitat in accordance 

the conservation needs identified in the 

Rangewide Management Strategy 

(USFWS 2005, pp. 201–202). 

We found that implementation of the 

Lower Colorado River MSCP was ‘‘Not 

Likely to Jeopardize the Continued 

Existence of the Species’’ (USFWS 2005, 

p. 202), noting ‘‘The habitat area that 

would be included [under the plan] is 

not a significant amount of the available 

habitat for the species. * * * Research 

and monitoring of the species within the 

[Lower Colorado River MSCP] area will 

contribute to understanding the species, 

its distribution, and habitat needs. 

* * * [and] There are not likely to be 

any adverse effects to the species’ 

conservation elsewhere in the range 

from the issuance of an incidental take 

permit for the [Lower Colorado River 

MSCP]’’ (USFWS 2005, p. 202). 

Population Dynamics 

Flat-tailed horned lizards are difficult 

to detect, which limits the effectiveness 

of surveys for the species (FTHLICC 

2003a, pp. 9, 65; Grant and Doherty 

2007, p. 1050). As a result, not only is 

presence and especially absence 

difficult to determine, but determining 

the size, trend, and demography of 

populations is problematic as well. The 

history of flat-tailed horned lizard 

monitoring and the shortcomings of the 

techniques used are described in the 

Rangewide Management Strategy 

(FTHLICC 2003a, p. 64) and our 2003 

withdrawal document (68 FR 332–333). 

Monitoring using more rigorous data 

collection and analytical methodologies 

has been conducted as part of the 

implementation of the Rangewide 

Management Strategy (FTHLICC 2003a, 

pp. 64–66; FTHLICC 2008b, pp. 1–38). 

The results from this monitoring effort 

are described below. 

As detailed in the Flat-tailed Horned 

Lizard Monitoring Plan (FTHLICC 

2008b, pp. 1–38), flat-tailed horned 

lizard monitoring consists of two 

surveys used in tandem: (1) Occupancy 

estimation surveys and (2) demographic 

plot surveys. Occupancy estimation was 

designed to determine whether the 

distribution (but not numbers of 

individuals or densities) of flat-tailed 

horned lizards in the management and 

research areas is stable, increasing, or 

decreasing. This component of the 

monitoring was meant to detect largescale changes in the status of flat-tailed 

horned lizard distribution in the 

Management Areas. The monitoring of 

demographic plots was designed to 

delineate flat-tailed horned lizard 

population dynamics and trends by 

estimating abundance each summer and 

yearly survival, recruitment, and 

population growth rate between years. 

This component was meant to gather 

more in-depth information on a smaller 

number of plots. However, the 

demographic plots were non-randomly 

established within areas known or 

suspected to support greater densities of 

flat-tailed horned lizards. The 

Management Areas overall were 

selected because they provided 

generally high-quality flat-tailed horned 

lizard habitat. However, the use of the 

two complementary survey types, one 

dispersed and coarse and the other 

focused and narrow, allows managers to 

draw, with caution, more detailed 

conclusions about an entire 

Management Area than they could have 

otherwise done by interpreting just one 

of the survey types alone. Below we 

summarize the information available 

from these monitoring efforts (source: 

USFWS 2010a, pp. 1–76). 

Occupancy surveys were conducted at 

West Mesa (2005 and 2009), East Mesa 

(2006), Yuha Desert (2008), and Ocotillo 

Wells State Vehicular Recreation Area 

(SVRA) (2006–2009). Separate 

occupancy analyses of these areas were 

conducted based on three survey 

methodologies: visual observations of 

flat-tailed horned lizards, lizard scat 

observations, and a combination of 

visual and scat observations. Multi-year 

analyses also were conducted for a 

subset of 53 plots in Ocotillo Wells 

SVRA that were surveyed annually from 

2006 to 2009. Our analysis indicates the 

combined visual-and-scat surveys were 

the most likely to correctly yield a 

statistically significant result (i.e., this 

survey methodology had the greatest 

statistical power). Although there are no 

comparable historical data with which 

to provide context, our analysis suggests 

that the level of occupancy of flat-tailed 

horned lizards within the surveyed 

areas seemed relatively high at all sites. 

For example, visual-and-scat survey 

results show that flat-tailed horned 

lizards occupied at least 80 percent of 

the Management Areas in the years 

surveyed, except in the West Mesa 

Management Area in 2005, which had a 

low level of survey effort that year. 

Additionally, results from the 53-plot 

subset with multi-year data from 2006 to 

2009 suggested that the level of flattailed horned lizard occupancy stayed 

about the same or may have even 

increased slightly over time. Moreover, 

our analysis showed considerable 

support to conclude that there was no 

linear decline in the proportion of 

survey plots occupied by flat-tailed 

horned lizards. These results only 

reflect the occupancy of flat-tailed 

horned lizards within the areas 

surveyed and do not necessarily reflect 

the level of occupancy throughout the 

range of the species; nevertheless, we 

conclude from the above results that the 

level of occupancy within the survey 

areas is not low, and that there is no 

indication of a decline. 

Data from the demographic plots were 

gathered from six 9-hectare (22.2-acre) 

plots at the following flat-tailed horned 

lizard Management Areas: East Mesa (1 

plot, 2007–2009), West Mesa (1 plot, 

2007–2009; 1 plot, 2008–2009), Yuha 

Desert (1 plot, 2007–2009), and Yuma 

Desert (2 plots, 2008–2009). Hatchlings 

were captured at all Management Areas 

except East Mesa (which was surveyed 

prior to the time that flat-tailed horned 

lizards eggs would have been likely to 

have hatched), indicating that flat-tailed 

horned lizards were reproducing. The 

presence of hatchlings during 2008, and 

especially 2009, suggested that 

reproductive conditions were favorable 

in those years. 

Because of the complexities of 

analyzing a cryptic species, we used two 

methodologies to calculate flat-tailed 

horned lizard abundance. Because the 

surveyed plots were not closed 

(meaning flat-tailed horned lizards 

could move in and out of the areas being 

surveyed), we used two different 

methods (calculations) to estimate the 

‘‘effective survey area’’ so that we could 

translate abundance (number of 

individuals) into densities (number of 

individuals per unit area). Using the 

first method (using a mean maximum 

distance moved buffer strip to estimate 

effective survey area), the density of 

adult flat-tailed horned lizards ranged 

from 0.3 to 3.3 individuals per ha (0.1 

to 1.3 individuals per ac), while the 

second method (using a hierarchical, 

spatially indexed capture-recapture 

model to estimate effective survey area) 

yielded a range from 0.7 to 4.4 

individuals per ha (0.3 to 1.8 


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individuals per ac). The results from the 

second method are likely to be more 

realistic because they incorporated 

additional spatial information. 

Other estimates of density of flattailed horned lizards are available in the 

scientific literature, but comparisons 

between and among the different studies 

(including the recent monitoring) are 

confounded by differing survey and 

analysis methodologies. Nevertheless, 

the above densities at the three 

California Management Areas were 

generally within the range of estimates 

reported by Grant (2005, pp. 39–40) 

during 2002–2004. Similarly, the 

densities of adult flat-tailed horned 

lizards at the Yuma Desert Management 

Area reported above were generally 

similar to the ranges of estimates 

presented by Young and Young (2000, 

p. 28) during 1997–1998, Young et al. 

(2004b, p. i) during 2003, and Young 

and Royle (2006, p. 9) in 2005. 

Comparisons to even earlier estimations 

of flat-tailed horned lizard densities, 

although even more tenuous because of 

differing methodologies, are also within 

similar ranges. Despite similar ranges in 

densities reported from the various 

studies through time, the increased 

statistical and methodological rigor of 

recent efforts has reduced the level of 

uncertainty in the results. Thus, these 

recent density estimates are an 

improvement over older estimates. 

The available data indicate that flattailed horned lizard abundances and 

densities have remained relatively 

stable from 2007 to 2009; however, with 

only 3 years of standardized monitoring, 

these data cannot yet provide 

meaningful inferences about long-term 

trends. Additionally, no abundance or 

density information is available for the 

lower-quality habitat areas outside the 

demographic plots. However, the 

complementary coarse-scale occupancy 

survey data mentioned above suggests 

flat-tailed horned lizards are widely 

distributed spatially and, in at least at 

one Management Area, temporally 

consistent. This conclusion suggests 

that flat-tailed horned lizard population 

trends in the surveyed lower-quality 

habitat areas are not dissimilar to those 

of the surveyed higher-quality habitat 

areas. Moreover, because the recent 

(2007–2009) and older (1997–2005) 

density estimates are all generally 

within similar ranges, this suggests the 

overall density of flat-tailed horned 

lizards within the surveyed 

Management Areas has not markedly 

decreased over the past decade or so. 

Thus, with the previously mentioned 

caveats in mind, we conclude that flattailed horned lizard populations in the 

Management Areas are not low and have 

not declined since 2007, and probably 

not declined since 1997. 

Description of Specific ‘‘Populations’’ 

As stated earlier, we have divided the 

current range of the flat-tailed horned 

lizard into four populations based on 

geographic locales. The 2003 Rangewide 

Management Strategy includes a GISbased map (FTHLICC 2003a, p. 5) of the 

‘‘current distribution’’ of the flat-tailed 

horned lizard. Except for the Coachella 

Valley Population, where the flat-tailed 

horned lizard is now limited to two 

occurrences, we used the GIS data as a 

basis for our assessment of the 

distribution of flat-tailed horned lizard 

populations. A summary of these 

populations is presented below. 

Coachella Valley Population 

(California)—The ‘‘current distribution’’ 

within the Coachella Valley as defined 

by the Rangewide Management Strategy 

(FTHLICC 2003a, pp. 3–5) does not 

represent the best scientific distribution 

information available for this region. 

Urban and agricultural development has 

continued in the Coachella Valley, and 

there are many areas of unsuitable or 

degraded habitat. In addition to areas of 

unsuitable habitat, many of which serve 

as a barrier to flat-tailed horned lizard 

movement, other potential manmade 

barriers exist, including several major 

highways, a railway, and canals. The 

only area within the Coachella Valley 

proper that is now known to be 

occupied by flat-tailed horned lizards is 

in the Thousand Palms reserve (CVCC 

2010, p. 13). Other areas of potentially 

suitable habitat occur in the region, 

including areas that were formerly 

known to be occupied (Barrows et al. 

2008, p. 1891), although recent surveys 

have not detected any flat-tailed horned 

lizards (CVCC 2010, p. 13). Thus, the 

‘‘current distribution’’ as defined by the 

Rangewide Management Strategy 

(FTHLICC 2003a, pp. 3–5) does not 

accurately reflect the area occupied by 

flat-tailed horned lizards in the 

Coachella Valley; as such, we do not use 

a GIS-based assessment for the 

Coachella Valley as we do for the other 

geographical ‘‘populations.’’ 

The Coachella Valley MSHCP is the 

primary driver of monitoring and 

management activities for the Coachella 

Valley Population of the flat-tailed 

horned lizard because the Rangewide 

Management Strategy does not include 

any Management Areas in this region. 

The Coachella Valley Population area is 

the smallest of the four geographic 

‘‘populations,’’ and we primarily 

identify it as a separate population to be 

consistent with our past analyses. Flattailed horned lizards also occur in the 

vicinity of the Dos Palmas Preserve near 

the northeast shore of the Salton Sea 

(Turner and Medica 1982, p. 817; 

FTHLICC 2003a, pp. 2–6; CVCC 2010, p. 

13). The Dos Palmas population is small 

and likely isolated from other 

populations because of the presence of 

the Salton Sea to the west; canals, roads 

and urban and agricultural development 

to the northwest; and canals, roads and 

urban and agricultural development to 

the southeast. However, not all of these 

barriers are likely to completely restrict 

flat-tailed horned lizard movement (see 

the Factor E discussion, below). The 

genetic affinities of the Dos Palmas 

population are not known. 

Geographically, the flat-tailed horned 

lizards at Dos Palmas Preserve could 

arguably be considered part of either the 

Western Population or Eastern 

Population (see below); however, 

because the true affinities of this 

population are not known, and because 

the Dos Palmas reserve area is covered 

under the Coachella Valley MSHCP and 

its associated monitoring and 

management, herein we consider the 

Dos Palmas flat-tailed horned lizards to 

be part of the Coachella Valley 

Population. The area of flat-tailed 

horned lizard habitat in the Coachella 

Valley Population is about 3,785 ha 

(9,353 ac) (see Table 2). 

Western Population (California and 

Baja California)—This population 

includes flat-tailed horned lizards in the 

areas west of the Salton Sea, the 

Imperial Valley, and the Mexicali 

Valley. Using a GIS-based assessment to 

estimate the area of this portion of the 

‘‘current distribution’’ as defined by the 

Rangewide Management Strategy 

(FTHLICC 2003a, pp. 3–5), we estimated 

that the Western Population occupies 

341,989 ha (845,073 ac). Of this acreage, 

approximately 253,020 ha (625,226 ac) 

is within the United States. Within the 

U.S. portion of the Western Population, 

approximately 48,262 ha (119,258 ac), 

or about 19 percent, is non-Federal or 

non-State owned, or is more likely to be 

developed. The habitat within this area 

is mostly intact except for a few 

developed areas, but as discussed in the 

‘‘Barriers and Small Populations’’ 

section under Factor E, potential 

manmade barriers to flat-tailed horned 

lizard movement (in addition to areas of 

urban and agricultural development) 

include Interstate 8; State Routes 78, 86, 

and 98; two railways; the fence and 

other activities along the international 

border in the United States, and Mexico 

Federal Highway 2 in Mexico. The 

Rangewide Management Strategy 

designates three Management Areas in 

this population area, including Borrego 

Badlands, West Mesa, and Yuha Desert 


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(see Table 1), and a research area at the 

Ocotillo Wells SVRA. Much of the 

westernmost portion of this population 

is within Anza-Borrego Desert State 

Park. Additionally, private lands are 

scattered throughout the U.S. portion, 

with large aggregations in the Borrego 

Springs area and in the vicinity of (but 

outside of) Ocotillo Wells SVRA. The 

range of the flat-tailed horned lizard in 

this population also extends southward 

into Mexico, crossing the international 

border at the Yuha Desert and 

continuing south along the east side of 

the Peninsular Ranges and west of 

Laguna Salada in Baja California 

(FTHLICC 2003a, pp. 2–5). The status of 

the population in this portion of the 

range in Mexico is poorly known, but 

there have been few substantive changes 

to the landscape in this area. 

Additionally, flat-tailed horned lizards 

were observed recently near Cerro 

Prieto, Baja California, which is east of 

the Sierra de Los Cucapahs (Sierra 

Cucapa ́) and west of the agricultural 

areas of the Mexicali Valley (A. Calvo 

Fonseca, Pronatura Noroeste, in litt. 

2010). This recent detection is outside 

of the current distribution as depicted in 

the Rangewide Management Strategy 

(FTHLICC 2003a, p. 5). 

Eastern Population (California and 

Baja California)—This population 

includes flat-tailed horned lizards in the 

areas east of the Salton Sea and the 

Imperial Valley but west of the Colorado 

River. While the isolated population at 

Dos Palmas Preserve could be included 

as part of either the Eastern Population 

or the Coachella Valley Population 

based on its geographic location, for the 

purposes of our analysis of threats to the 

species we consider the Dos Palmas 

Preserve population to be part of the 

Coachella Valley Population because of 

the similarity of potential threats when 

compared to the populations in the 

Coachella Valley, and its inclusion 

within the Coachella Valley MSHCP 

plan area. Using a GIS-based assessment 

to estimate the area of the Eastern 

Population portion of the ‘‘current 

distribution’’ (as defined by the 

Rangewide Management Strategy 

(FTHLICC 2003a, pp. 3–5)), we 

estimated that the Eastern Population 

occupies 169,617 ha (419,133 ac). Of 

this acreage, approximately 146,121 ha 

(361,073 ac) is within the United States. 

Within the U.S. portion of the Eastern 

Population, approximately 5,844 ha 

(14,441 ac), or about 4 percent, is nonFederal or non-State owned, or is more 

likely to be developed. The area 

occupied by the Eastern Population is 

mostly intact except for a few developed 

areas, but potential manmade barriers to 

flat-tailed horned lizard movement (in 

addition to areas of urban and 

agricultural development) include 

Interstate 8, State Routes 78 and 98, the 

All-American Canal and the Coachella 

Canal, and the international border 

fence in the United States (see ‘‘Barriers 

and Small Populations’’ section under 

Factor E, below). The Rangewide 

Management Strategy designated the 

East Mesa Management Area within the 

area occupied by the Eastern Population 

(see Table 1). The geographic extent of 

the Eastern Population also includes the 

Algodones Dunes (also known as the 

Imperial Sand Dunes or Glamis Sand 

Dunes), a portion of which is designated 

Wilderness, and a narrow strip of 

habitat south of the international border 

at the southern edge of the Algodones 

Dunes (FTHLICC 2003a, pp. 2–5). The 

portion of the Eastern Population area in 

Mexico is bound by agricultural 

development (unsuitable habitat) on the 

west, south, and east. The status of the 

portion of the Eastern Population in 

Mexico is poorly known, but flat-tailed 

horned lizards were observed recently 

in this area (A. Calvo Fonseca, in litt. 

2010). 

Southeastern Population (Arizona and 

Sonora)—This population includes flattailed horned lizards in the areas east of 

the Colorado River, extending from 

Yuma, Arizona, south and east to the 

Gulf of California in northwestern 

Mexico. In Arizona, the flat-tailed 

horned lizard occurs in Yuma County, 

ranging over the Yuma Desert south of 

the Gila River and west of the Gila and 

Butler Mountains (Rorabaugh et al. 

1987, p. 104; FTHLICC 2003a, pp. 2–6). 

The Rangewide Management Strategy 

designated the Yuma Desert 

Management Area within the area 

occupied by the Southeastern 

Population (see Table 1). In Mexico, the 

flat-tailed horned lizard ranges from the 

international border in the Yuma Desert 

south and east through the Pinacate 

Region to the sandy plains around 

Puerto Pen ̃ asco and Bahia de San Jorge 

along the Gulf of California (Johnson 

and Spicer 1985, p. 13; Gonza ́lesRomero and Alvarez-Cardenas 1989, p. 

519; FTHLICC 2003a, pp. 2–5). About 60 

percent of the flat-tailed horned lizard 

habitat in Sonora lies within two 

Mexican Federal natural protected 

areas: the Upper Gulf of California and 

Colorado Delta Biosphere Reserve, and 

the Pinacate and Gran Desierto de Altar 

Biosphere Reserve (CEDO 2001, p. 3). 

Using a GIS-based assessment to 

estimate the area of this portion of the 

‘‘current distribution’’ as defined by the 

Rangewide Management Strategy 

(FTHLICC 2003a, pp. 3–5), we estimated 

that the area occupied by the 

Southeastern Population is 1,073,551 ha 

(2,652,802 ac), by far the largest of the 

four population areas. Of this acreage, 

approximately 67,922 ha (167,839 ac) is 

within the United States. Within the 

U.S. portion of the Southeastern 

Population, approximately 5,158 ha 

(12,746 ac), or about 8 percent, is 

privately owned; an additional 5,832 ha 

(14,411 ac), or about 9 percent, is State 

of Arizona-owned lands. The habitat 

within the Southeastern Population area 

is mostly intact except for a few 

developed areas, but potential barriers 

to flat-tailed horned lizard movement 

(in addition to areas of urban and 

agricultural development) include 

Interstate 8 and the Yuma Areas Service 

Highway in the United States; the 

international border (combined with 

Mexico Federal Highway 2); Mexico 

Federal Highway 8; and a railway in 

Mexico (see ‘‘Barriers and Small 

Populations’’ section under Factor E, 

below). 

In summary, using a GIS-based 

assessment to estimate the size of the 

current distribution of the flat-tailed 

horned lizard as defined by the 

Rangewide Management Strategy 

(FTHLICC 2003a, p. 5), we estimated 

that the three population areas 

(excluding the Coachella Valley 

Population) comprise roughly 1,585,000 

ha (3,916,600 ac), of which 

approximately 467,000 ha (1,154,000 ac) 

(less than 30 percent) is within the 

United States and approximately 

1,100,000 ha (2,718,000 ac) (more than 

70 percent) is within Mexico. The area 

of flat-tailed horned lizard habitat 

occupied or likely to be occupied that 

already is or is expected to be conserved 

in the Coachella Valley Population is 

about 3,785 ha (9,353 ac) (see Table 2). 

Previous Federal Actions 

In 1982, we first identified the flattailed horned lizard as a category 2 

candidate species for listing under the 

Act (47 FR 58454; December 30, 1982). 

Category 2 candidate species were ‘‘taxa 

for which information now in 

possession of the Service indicates that 

proposing to list the species as 

Endangered or Threatened is possibly 

appropriate, but for which sufficient 

data on are not currently available to 

biologically support a proposed rule’’ 

(47 FR 58454). We again identified the 

flat-tailed horned lizard as a category 2 

candidate species in our 1985 notice of 

review (50 FR 37958; September 18, 

1985). In 1989, we elevated the species 

to category 1 status (54 FR 554; January 

6, 1989). Category 1 included species 

‘‘for which the Service currently has 

substantial information on hand to 

support the biological appropriateness 


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of proposing to list as endangered or 

threatened’’ (54 FR 554). We maintained 

the category 1 status for the flat-tailed 

horned lizard in our 1991 notice of 

review (56 FR 58804; November 21, 

1991). 

On November 29, 1993, we published 

in the Federal Register a proposed rule 

to list the flat-tailed horned lizard as a 

threatened species under the Act (58 FR 

62624). On February 22, 1994 (59 FR 

8450), we published a notice reopening 

the public comment period and 

announcing that we had scheduled a 

public hearing on March 22, 1994, in 

Imperial, California, in response to a 

request from the public. Our November 

15, 1994, candidate notice of review 

stated that we had proposed to list the 

species as threatened (59 FR 58982). 

Subsequently, the passage of Public 

Law 104–6, 109 Stat. 73 on April 10, 

1995, resulted in a delay in our final 

listing determination for the flat-tailed 

horned lizard. Although the statute’s 

primary purpose was to provide 

additional funds for overseas military 

operations, it also included a rider that 

withdrew funding for listing 

determinations. Through a series of 

moratoria, funding restrictions, and 

continuing resolutions, this restriction 

in use of funds remained in effect until 

April 26, 1996, when the Omnibus 

Appropriations Act was enacted (Pub. L. 

104–134, 110 Stat. 1321, (1996)), which 

contained a moratorium on certain 

listing activities but allowed the 

President to waive the moratorium. On 

April 26, 1996, President Clinton 

suspended the provision limiting 

implementation of Section 4 of the Act 

(61 FR 24667; May 16, 1996). Earlier in 

1996, our notice of review had indicated 

that we had proposed to list the species 

as threatened (61 FR 7596; February 28, 

1996). 

On January 21, 1997, the Bureau of 

Land Management (BLM) announced in 

the Federal Register that the draft Flattailed Horned Lizard Rangewide 

Management Strategy was available for 

public comment (62 FR 3052). On May 

16, 1997, in response to a lawsuit filed 

by the Defenders of Wildlife and other 

plaintiffs to compel us to make a final 

listing determination on the flat-tailed 

horned lizard, the District Court in 

Arizona ordered us to issue a final 

listing decision within 60 days. In June 

1997, several State and Federal 

agencies, including the Service, signed 

an Interagency Conservation Agreement 

committing to implement the recently 

finalized Flat-tailed Horned Lizard 

Rangewide Management Strategy 

(FTHLICC 1997, pp. 1–106). Pursuant to 

the Interagency Conservation 

Agreement, cooperating parties agreed 

to take voluntary steps aimed at 

‘‘reducing threats to the species, 

stabilizing the species’ populations, and 

maintaining its ecosystem’’ (see 

FTHLICC 2003a, p. 80). 

On July 15, 1997, we issued a final 

decision to withdraw the proposed rule 

to list the flat-tailed horned lizard as a 

threatened species (62 FR 37852). We 

based the withdrawal on three factors: 

(1) Population trend data did not 

conclusively demonstrate significant 

population declines; (2) Some of the 

threats to the flat-tailed horned lizard 

habitat had abated since the proposed 

rule was issued; and (3) Our conclusion 

that the recently approved Interagency 

Conservation Agreement would ensure 

further reductions in threats (62 FR 

37852). 

On December 30, 1997, the Defenders 

of Wildlife and others filed a complaint 

in the U.S. District Court for the 

Southern District of California 

challenging our 1997 withdrawal of the 

proposed rule. On June 16, 1999, the 

District Court upheld our decision to 

withdraw the proposed listing rule. The 

District Court’s decision was appealed 

and on July 31, 2001, the Ninth Circuit 

Court of Appeals vacated the previous 

ruling of the District Court. The case 

was remanded back to the Secretary 

because: (1) The withdrawal of the 

proposed rule did not expressly 

consider whether the flat-tailed horned 

lizard is likely to become an endangered 

species within the foreseeable future in 

a significant portion of its range; and (2) 

The withdrawal of the proposed rule 

did not ‘‘address the lizard’s viability in 

a site-specific manner with regard to the 

putative benefits of the Interagency 

Conservation Agreement.’’ In 

accordance with the Appeals Court’s 

ruling, we published a document in the 

Federal Register on December 26, 2001, 

reinstating the 1993 proposed rule and 

opening a 120-day public comment 

period (66 FR 66384). 

On May 30, 2002, we published a 

document in the Federal Register 

reopening the public comment period 

for an additional 60 days (67 FR 37752) 

and announced that we would be 

holding public hearings in El Centro, 

California, on June 19, 2002. On 

September 24, 2002, we published in 

the Federal Register another document 

(67 FR 59809) announcing the 

reopening of the public comment period 

for an additional 15 days to allow for 

peer review, additional public comment 

on the proposed rule, and submittal of 

information that became available since 

our 1997 withdrawal. 

On January 3, 2003, we again 

published in the Federal Register a 

decision to withdraw the November 29, 

1993, proposed rule to list the flat-tailed 

horned lizard as a threatened species 

(68 FR 331). The Service found the 

lizard to be ‘‘in danger of extirpation in 

the Coachella Valley’’ (68 FR 348); 

however, we determined that the 

Coachella Valley is not a significant 

portion of the species’ range. We 

concluded in the January 3, 2003, 

withdrawal that the flat-tailed horned 

lizard populations on either side of the 

Imperial Valley-Salton Sea and in 

Arizona were not likely to become 

endangered in the foreseeable future 

and that listing the species was not 

warranted. 

The Tucson Herpetological Society 

and others filed a complaint with the 

District Court for the District of Arizona 

challenging the January 3, 2003, 

withdrawal of the proposed rule. In a 

ruling issued on August 30, 2005, the 

District Court for the District of Arizona 

issued an order granting plaintiffs’ 

motion for summary judgment, citing 

our failure to specifically evaluate the 

lost habitat of the flat-tailed horned 

lizard, and whether the amount of lost 

habitat represented a significant portion 

of the species’ range. On December 7, 

2005, we published a document in the 

Federal Register reinstating the 1993 

proposed rule (70 FR 72776). On March 

2, 2006, we announced in the Federal 

Register that we were reopening the 

public comment period on the 1993 

proposed rule for 14 days for the 

purpose of soliciting comments and 

information relevant to the specific 

issue identified in the District Court’s 

November 2005 ruling (i.e., whether the 

flat-tailed horned lizard’s lost historical 

habitat rendered the species likely to 

become in danger of extinction in the 

foreseeable future throughout all or a 

significant portion of its range) (71 FR 

10631). On April 21, 2006, we 

announced in the Federal Register an 

additional public comment period on 

the 1993 proposed rule from April 21, 

2006, to May 8, 2006 (71 FR 20637). 

After re-examining the lost historical 

habitat of the flat-tailed horned lizard in 

relation to our January 3, 2003, 

withdrawal, we determined that the lost 

historical habitat is not a significant 

portion of the species’ range, and its loss 

does not result in the species likely 

becoming endangered in the foreseeable 

future throughout all or a significant 

portion of its range. We published our 

decision in the Federal Register on June 

28, 2006, to once again withdraw the 

November 29, 1993, proposed rule to 

list the flat-tailed horned lizard as a 

threatened species (71 FR 36745). 

Following a supplemental complaint 

from Tucson Herpetological Society and 

others challenging the 2006 withdrawal 


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of the proposed rule to list the flat-tailed 

horned lizard under the Act, the United 

States District Court for the District of 

Arizona (the District Court) granted 

summary judgment in favor of the 

Secretary of the Interior (Tuscon 

Herpetological Society v. Kempthorne, 

04–CV–00075–PHX–NVW); however, 

this ruling was appealed to the Court of 

Appeals for the Ninth Circuit. In a 

ruling issued on May 18, 2009, the 

Court of Appeals for the Ninth Circuit 

reversed the District Court’s ruling 

when it determined that in the context 

of the analysis of whether the lizard’s 

lost historical range constituted a 

significant portion of the species’ range, 

the administrative record did not 

support what the Court of Appeals for 

the Ninth Circuit viewed as the 

Service’s conclusion that flat-tailed 

horned lizard populations were stable 

and viable throughout most of its 

current range. 

On November 3, 2009, the District 

Court remanded the 2006 withdrawal to 

the Service for further consideration and 

reinstated the 1993 proposal to list the 

species. The District Court ordered the 

Service to complete this reconsideration 

in accordance with the deadlines set 

forth in 16 U.S.C. 1533(b). On March 2, 

2010, we published a notice in the 

Federal Register announcing the 

reinstatement of the 1993 proposed rule, 

the reopening of the public comment 

period for 60 days, and the scheduling 

of public hearings (75 FR 9377). Public 

hearings were held in Palm Desert, 

California, on March 23, 2010, and 

Yuma, Arizona, on March 24, 2010. 

Summary of Factors Affecting the 

Species 

Section 4 of the Act (16 U.S.C. 1531 

et seq.) and the regulations that 

implement the listing provisions of the 

Act (50 CFR part 424) set forth the 

procedures for adding species to the 

Federal Lists of Endangered and 

Threatened Wildlife and Plants. A 

species may be determined to be an 

endangered or threatened species due to 

one or more of the five factors described 

in section 4(a)(1) of the Act (Factors A 

through E). 

We evaluated threats to the flat-tailed 

horned lizard under the five listing 

factors in the 1993 proposed rule to list 

the flat-tailed horned lizard as 

threatened under the Act (58 FR 62624). 

Subsequent documents in 1997 and 

2003 withdrawing the proposed rule to 

list the species included additional 

evaluations (62 FR 37852; 68 FR 331). 

The 2003 document withdrawing the 

proposed rule was the most 

comprehensive and the most recent fivefactor analysis. The 2006 document 

withdrawing the proposed rule (71 FR 

36745) did not address the five factors 

in detail because its scope was limited 

by a court order (see Previous Federal 

Actions section). In this document, we 

use the best scientific and commercial 

data available to evaluate current 

potential threats to flat-tailed horned 

lizard and its habitat rangewide per the 

five listing factors, and we provide brief 

summaries of the 1993 and 2003 

evaluations for context. 

A. The Present or Threatened 

Destruction, Modification, or 

Curtailment of Its Habitat or Range 

For this factor, we evaluated the 

present (current) or threatened 

(anticipated) impacts that may be 

affecting the habitat or range of the flattailed horned lizard. This factor does 

not address historical or past actions 

that resulted in destruction, 

modification, or curtailment of the 

species’ habitat or range. Past actions 

that destroyed, modified, or curtailed 

the species’ habitat or range are not 

threats in and of themselves. Any 

persisting ramifications of such past 

actions that may be threats to the 

species are addressed under Factor E 

(other natural or manmade threats), 

below. However, for Factor A, we do 

look to past actions to inform our 

evaluation of potential future threats 

affecting the species’ habitat or range in 

that the history of past actions allows us 

to predict the likelihood of such actions 

continuing into the foreseeable future. 

In the 1993 proposed rule (58 FR 

62625–62626), we identified historical 

flat-tailed horned lizard habitat losses 

that resulted in the curtailment of the 

species’ range under Factor A. We noted 

threats that were current or anticipated 

at that time, including agricultural and 

urban development, off-highway vehicle 

(OHV) use, geothermal energy 

development, sand and gravel extraction 

operations, military training activities, 

and construction of roads and utility 

corridors. We also mentioned that flattailed horned lizard habitat had been 

fragmented, causing isolation of 

populations (curtailment of the species’ 

range) (see below for additional 

discussion on fragmentation). 

Additionally, the 1993 proposed rule 

also mentioned gold mining as a 

potential threat. There are currently no 

gold mines in flat-tailed horned lizard 

habitat, and we are not aware of any 

proposals for new gold mines; therefore, 

we do not expect gold mines to become 

a threat in the foreseeable future. 

In the 2003 withdrawal document (68 

FR 341–345), we found that current and 

anticipated urban and agricultural 

development was limited to a few, small 

areas and did not constitute a significant 

threat to the species. However, we did 

state that past agricultural, urban, and 

associated infrastructural development 

(such as canals and roads) had 

fragmented the species’ range, which we 

discuss below as a separate threat under 

Factor A. 

Fragmentation and Past Habitat Loss 

Because of our past treatment of 

fragmentation in our previous rules, we 

are providing a discussion of 

fragmentation as a term and its 

application to the five-factor analysis for 

the flat-tailed horned lizard. This 

discussion should: (1) Provide a clear 

definition of the term that we use in this 

document, and (2) acknowledge that our 

lack of clarity for this term in past 

documents may have resulted in 

unanswered questions as to how the 

flat-tailed horned lizard may have been 

affected by historical development in 

the Salton Trough. Because of the 

connection between fragmentation and 

historical habitat loss, we also describe 

how historical habitat loss was 

addressed in past assessments. 

In the 2003 withdrawal document, we 

defined fragmentation as the ‘‘breaking 

up of a habitat or ecosystem into smaller 

parcels’’ (68 FR 341). This definition is 

similar to the more detailed version 

used by Wilcove et al. (1986, p. 237) 

who defined habitat fragmentation as 

occurring ‘‘when a large expanse of 

habitat is transformed into a number of 

smaller patches of smaller total area, 

isolated from each other by a matrix of 

habitats unlike the original.’’ Thus, 

fragmentation is a process, one that 

inextricably involves habitat loss 

(Fahrig 1999, p. 87). However, in 

addition to the effects associated with 

habitat loss, fragmentation also includes 

the effects associated with the fractured 

nature of that habitat after its 

transformation (Fahrig 2003, p. 487). 

The implication is that the biological 

properties of the remaining, small, 

isolated patches of habitat have changed 

during or as a result of the 

fragmentation of the habitat (van den 

Berg et al. 2001, p. 225). In other words, 

after some portion of the habitat of a 

species has been destroyed, that species 

may be impacted by one or more 

secondary effects (threats) associated 

with reduction in the size of remaining 

habitat patches (or the populations of 

the species therein) and the isolation of 

those patches (and populations) from 

each other (Andre ́n 1994, p. 355). Thus, 

the effects of fragmentation include: (1) 

The effects associated with the ongoing 

loss of habitat; and (2) the subsequent, 

secondary effects that are the current 

ramifications of past habitat loss. 


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Because multiple secondary effects may 

be related or correlated to each other 

(Fahrig 2003, pp. 491–492), the term 

fragmentation, as it has been used in the 

scientific literature and by the Service 

in past assessments of this species, is 

ambiguous (Haila 2002, p. 321). Because 

of this ambiguity, in applying the Act’s 

five listing factors to the flat-tailed 

horned lizard, we will address current 

and anticipated habitat loss under 

Factor A, and the relevant, identifiable 

secondary effects (including threats 

associated with fragmentation) to the 

species under Factor E. 

Our past assessments describe in 

detail and attempted to quantify the 

historical development in the Salton 

Trough (58 FR 62626; 62 FR 37857; 68 

FR 341–345; 71 FR 36751), as did the 

scientific literature (such as Johnson 

and Spicer 1985, p. 38, 45–48; 

Rorabaugh et al. 1987, p. 106; Hodges 

1995, pp. 1–18; Hodges 1997, pp. 1–16; 

Piest and Knowels 2002, pp. 1–4; 

FTHLICC 2003a, pp. 2–3; Piest and 

Knowels 2006, pp. 1–4). These 

documents have, to a greater or lesser 

extent, estimated the areal extent of 

current and historical flat-tailed horned 

lizard habitat in all or certain portions 

of its range. One of the more detailed of 

such analyses was Hodges (1997, pp. 

15–16), who concluded that 503,161 ha 

(1,243,341 ac) out of 979,016 ha 

(2,419,200 ac), or about 51 percent, of 

flat-tailed horned lizard habitat in the 

United States had been destroyed by 

past development. 

However, such calculations, no matter 

how carefully crafted, are necessarily 

based on assumptions of what areas 

constituted historical habitat for the 

species (such as Hodges 1997, p. 10). 

Because much of the area within the 

range of the flat-tailed horned lizard was 

converted to agricultural and urban 

development during the early half of the 

20th century (see Background section, 

above) prior to any systematic surveys 

for the flat-tailed horned lizard, little 

reliable information exists on the 

historical distribution of the species 

(Barrows et al. 2008, p. 1886). 

We questioned the validity of such 

assumptions in our past assessments. 

For example, Hodges (1997, pp. 5, 7, 

and 16) included the area now 

inundated by Salton Sea as historical 

habitat, but we stated in our 2003 

withdrawal that the Salton Sea area 

could arguably be considered ephemeral 

historical habitat. In our 2006 

withdrawal, we concluded that the 

former lakebed of historical Lake 

Cahuilla (including and beyond the 

present-day Salton Sea) likely was not 

habitat important to the flat-tailed 

horned lizard (71 FR 36750–36751). The 

information on the genetics of flat-tailed 

horned lizard populations raises further 

doubts about the validity of the 

assumptions made in earlier 

assessments, both by us and by others, 

of historical flat-tailed horned lizard 

habitat. 

As discussed above (see Background 

section), genetic data readily support 

three of the four geographic populations 

as distinct, indicating that these 

populations generally had little genetic 

interchange among each other (Mulcahy 

et al. 2006, pp. 1807–1826; Culver and 

Dee 2008, pp. 1–14). This lack of genetic 

exchange suggests a barrier separated, 

and likely still separates, these 

populations. As discussed in the 

Background section, the areas within 

the present-day Imperial Valley, 

Mexicali Valley, and San Luis Valley 

were historically interlaced by a 

network of Colorado River-influenced 

water courses, including the Alamo 

River, the New River, and the Rı ́o Hardy 

(or their precursors or equivalents). 

Historically, these ‘‘rivers’’ were 

dependent upon the Colorado River for 

water and only transported water 

periodically. Prior to the increase of 

agricultural development and prior to 

the digging of the irrigation canal and 

subsequent flood that created the Salton 

Sea early in the 20th century (see 

Background section), some areas along 

these river channels were characterized 

by Parish (1914, p. 88) as having 

‘‘channels, sloughs, and lagoons.’’ These 

hydrologically influenced areas likely 

did not contain flat-tailed horned lizard 

habitat, as defined in the Background 

section. As such, not all of the area 

between the present-day Salton Sea and 

the Gulf of California, including areas 

outside the lakebed of historical Lake 

Cahuilla, historically supported flattailed horned lizard habitat. This 

information further supports our 

conclusion presented in our 2006 

withdrawal that the ‘‘area of the 

historical range periodically inundated 

by Lake Cahuilla was not important to 

the long-term viability of the flat-tailed 

horned lizard because this area was 

frequently unavailable and likely 

contained little quality habitat’’ (71 FR 

36750). 

Because of the extensive manmade 

changes to the landscape, we cannot 

precisely determine with any degree of 

specificity how much of the area was 

historically flat-tailed horned lizard 

habitat. Moreover, we maintain that 

much uncertainty exists with any 

attempt to precisely quantify the 

amount of flat-tailed horned lizard 

habitat that has been destroyed by 

historical agricultural development, as 

has been attempted in the past. We 

agree with the conclusions of previous 

assessments, both by us and by others, 

that portions of the Coachella, Imperial, 

Mexicali, Yuma, and San Luis Valleys 

once provided suitable areas of flattailed horned lizard habitat. We also 

agree that historical agricultural 

development (and, to a lesser extent, 

urban development) destroyed large 

areas with flat-tailed horned lizard 

habitat, thus curtailing the size of the 

Coachella Valley, Western, Eastern, and 

Southeastern flat-tailed horned lizard 

populations in both the United States 

and Mexico. However, the effects of past 

actions are better addressed under 

Factor E. 

In the sections below, we address the 

present or threatened destruction, 

modification, or curtailment of the 

habitat or range of the flat-tailed horned 

lizard. We evaluate the current and 

anticipated effects associated with 

several types of land development, the 

invasion of nonnative plants, OHV 

activity, and military training. We first 

describe the respective threats in 

general terms and then assess those 

threats to the habitat or range of the flattailed horned lizard, focusing on 

subareas (such as identified populations 

or Management Areas) within the 

species’ range, where appropriate. 

Development 

We define development as 

commercial and residential 

development (i.e., urban development), 

and the conversion of land for any 

agricultural purpose. Such development 

not only includes the obvious associated 

infrastructure (e.g., roads, pipelines, 

canals, and power lines), but also 

reservoirs, power generation facilities, 

and resource extraction operations such 

as drilling and mining. 

For the purpose of evaluating the 

threats to a species and its habitat, we 

focus on the developmental activities 

that threaten to convert land from a 

natural or undeveloped state to land no 

longer suitable as habitat for the species. 

We consider both the direct and, where 

appropriate (within the context of 

Factor A), the indirect effects of such 

developmental activities. While land 

development typically has a similar 

effect, that is the destruction or 

modification of habitat, differing land 

uses resulting from development 

activities can lead to different indirect 

effects. We therefore distinguish among 

the types of development when 

evaluating the effects of such 

development on a species or its habitat. 

For this evaluation of flat-tailed 

horned lizard under Factor A, we 

determine whether development is a 

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tailed horned lizard habitat. Below, we 

address agricultural and urban 

development, as well as development 

associated with energy generation 

projects. 

Agricultural Development 

Within the dry Colorado Desert, 

agricultural activity is substantially 

dependent upon irrigation water 

imported from the Colorado River. As 

discussed in the Background section, 

most of the agricultural development 

within the range of the flat-tailed 

horned lizard occurred early in the 20th 

century. Because Colorado River water 

is a finite resource, agricultural 

development is no longer expanding 

into new areas and destroying flat-tailed 

horned lizard habitat to any substantial 

degree. Information available from the 

Coachella Valley Water District (CVWD 

2002, p. 1; 2003, p. 1; 2004, p. 1; 2005, 

p. 25; 2006, p. 27; 2007, p. 25; 2008, p. 

25; 2009, p. 25) indicates a slight 

decline in the amount of irrigable acres 

and a fairly steady though variable 

amount of water delivered from 2001 to 

2008, indicating that new agricultural 

development has not occurred in the 

Coachella Valley within the past decade 

or so. Also, fields are being fallowed in 

the Imperial Valley because less water is 

available for irrigation in this area (IID 

2006, p. 1). Thus, conversion of land for 

agriculture is no longer considered a 

threat to flat-tailed horned lizard habitat 

in the Coachella Valley and in the 

Imperial Valley portions of the Western 

and Eastern Populations, and is not 

considered to be a threat in the 

foreseeable future. 

In contrast, recent agricultural 

development has destroyed flat-tailed 

horned lizard habitat in other areas. 

Between 2002 and 2006, an unreported 

but minority fraction of 1,534 ha (3,790 

ac) of flat-tailed horned lizard habitat 

was developed for agricultural use in 

Arizona (Piest and Knowles 2006, p. 1). 

Rodriguez (2002, p. 21) also recorded 

recent agricultural development in 

Mexico; however, the majority of the 

agricultural development in the 

Mexicali and San Luis Valleys occurred 

in the early to mid-20th century, closely 

following the historical agricultural 

development north of the border 

(Furnish and Ladman 1975, pp. 84–88). 

Additionally, about 60 percent of the 

flat-tailed horned lizard habitat in 

Mexico lies within two Mexican Federal 

natural protected areas, the Upper Gulf 

of California and Colorado Delta 

Biosphere Reserve (la Reserva de la 

Biosfera del Alto Golfo de California y 

Delta del Rı ́o Colorado), and the 

Pinacate and Gran Desierto de Altar 

Biosphere Reserve (la Reserva de la 

Biosfera El Pinacate y Gran Desierto de 

Altar) (CEDO 2001, p. 3), where 

agricultural development is limited by 

Mexican law. 

Agricultural activities outside of the 

areas receiving Colorado River water are 

severely restricted by the climate of the 

Salton Trough region, including in 

Mexico. Thus, while recent agricultural 

development destroyed areas of flattailed horned lizard habitat in the 

Southeastern Population, the overall 

acreages were small, especially 

compared to the amount of habitat 

available in the Southeastern 

Population. 

Agricultural development, most of 

which occurred between 1945 and the 

1980s (Mills 2009, p. 28), occurred in 

the Borrego Springs area of the habitat 

occupied by the Western Population. 

The Borrego Springs area uses a local 

aquifer for irrigation, and the area does 

not receive Colorado River water; 

however, the aquifer is overdrawn 

(County of San Diego 2008, p. 8; Mills 

2009, p. 4). We do not anticipate 

substantial amounts of agriculture to 

expand into adjoining natural lands in 

this area (see Mills 2009, pp. 40–42). 

Moreover, the area of private lands in 

the Borrego Valley is constrained within 

Anza-Borrego Desert State Park. As a 

result, we believe that agricultural 

development no longer threatens flattailed horned lizard habitat in the 

Borrego Springs portion of the Western 

Population, nor will it in the foreseeable 

future. 

In conclusion, the available 

information indicates that the vast 

majority of the agricultural development 

within the range of the flat-tailed 

horned lizard took place in the 

historical past and only a small amount 

of development has been documented in 

recent times. Because conversion of 

land to agriculture in the region is 

limited by the availability of irrigation 

water and that water is limited, we do 

not expect agriculture to expand 

significantly into adjoining flat-tailed 

horned lizard habitat in the future. 

Moreover, increased demand for water 

outside the region has resulted in a 

decreased amount of Colorado River 

water available for agriculture in the 

Imperial Valley, which has resulted in 

the fallowing of fields in this area. 

Therefore, we conclude that agricultural 

development is not a substantial threat 

to the flat-tailed horned lizard 

throughout its range, nor is it 

anticipated to be in the foreseeable 

future. 

Urban Development 

Like agricultural development, urban 

development largely occurred in the 

historic past. Many of the urban centers 

in the region that serve agricultural 

communities are contained within 

agricultural areas. While urbanization 

has continued as the human population 

within the region has grown (FTHLICC 

2003a, p. 12; Indrelunas 2010, pp. 1–3), 

most of this urban development 

associated with these urban centers has 

come at the expense of former 

croplands. As such, this development is 

not currently destroying substantial 

amounts of available flat-tailed horned 

lizard habitat (FTHLICC 2003a, p. 12). 

However, certain areas of urban 

development not associated with active 

or past agriculture have resulted in the 

destruction of flat-tailed horned lizard 

habitat. This impact is most evident in 

the Coachella Valley where urban 

development not associated with 

agricultural communities continues 

today (Indrelunas 2010, pp. 1–3). This 

growth is corroborated by the number of 

domestic water meter services, which 

grew by over 25 percent from 2001 to 

2008 (CVWD 2002, p. 1; 2003, p. 1; 

2004, p. 1; 2005, p. 25; 2006, p. 27; 

2007, p. 25; 2008, p. 25; 2009, p. 25). 

This urban growth is occurring in the 

surrounding desert areas, which likely 

include flat-tailed horned lizard habitat. 

Our interpretation of past and recent 

aerial imagery supports this trend. 

The flat-tailed horned lizard now 

appears to be restricted to two 

occurrences within the Coachella Valley 

MSHCP plan area, the Thousand Palms 

conservation area and the Dos Palmas 

conservation area (CVCC 2010, p. 13). 

The Coachella Valley MSHCP includes 

numerous measures to minimize and 

mitigate impacts of urban development 

on the flat-tailed horned lizard (see 

Coachella Valley Multiple Species 

Habitat Conservation Plan (Coachella 

Valley MSHCP) section above for a 

detailed discussion). Approximately 94 

percent of the potential habitat where 

flat-tailed horned lizards are known to 

occur in the Thousand Palms 

conservation area is land that is already 

protected (Table 2), including about 62 

percent that is part of the Coachella 

Valley National Wildlife Refuge. 

Similarly, approximately 34 percent of 

the habitat at Dos Palmas is protected 

(Table 2). The high level of protection 

of flat-tailed horned lizard habitat at the 

Thousand Palms conservation area 

translates into a low magnitude of threat 

from urban development at this 

location. In contrast, because only about 

one-third of the flat-tailed horned lizard 

habitat at the Dos Palmas conservation 

area is currently in protected status, the 

potential magnitude of urban 

development at the latter location is 


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greater. However, because this area of 

habitat is farther away from existing 

urban areas, the immediacy of the threat 

of urban development is likely lower, 

even without the protections for flattailed horned lizard included in the 

Coachella Valley MSHCP plan (which 

requires the protection of the Dos 

Palmas conservation area). Therefore, 

the overall threat from urban 

development of flat-tailed horned lizard 

habitat in the Coachella Valley 

Population is low. 

Most of the area occupied by the U.S. 

portion of the Western Population of 

flat-tailed horned lizards is owned by 

the State of California (more than 27 

percent) or by the Federal government 

(more than 52 percent), and the vast 

majority of the U.S. portion of the 

Eastern Population is federally owned 

(more than 95 percent). Much of the 

State of California land in the Western 

Population is administered by California 

State Parks, including Anza-Borrego 

Desert State Park and Ocotillo Wells 

State Vehicular Recreation Area. We do 

not expect any substantive urban 

development activities on State Parkadministered lands. However, such 

development, should it occur, would 

likely follow the avoidance, 

minimization, and compensation 

measures of the Rangewide Management 

Strategy because California State Parks 

is a signatory agency to the Interagency 

Conservation Agreement. 

Additionally, much of the Federal 

land is administered by the BLM, which 

is a signatory to the Interagency 

Conservation Agreement. Moreover, the 

BLM has incorporated the Rangewide 

Management Strategy into the California 

Desert Conservation Area (CDCA) Plan 

under the Federal Land Policy and 

Management Act of 1976 (43 U.S.C. 

1701 et seq.) (FLPMA). The CDCA Plan 

directs BLM’s permitting of 

development projects on the lands the 

plan covers, including the U.S. portions 

of the Western and Eastern Populations. 

Thus, the avoidance, minimization, and 

compensation measures in the 

Rangewide Management Strategy are 

implemented by BLM on these lands, 

which reduces the impact such 

development to flat-tailed horned lizard 

habitat. Other federally owned lands in 

these areas are lands owned by the 

Navy, which is also a signatory to the 

Interagency Conservation Agreement. 

Not only do we anticipate that the 

Navy’s participation in the Rangewide 

Management Strategy will continue, 

which will limit the amount of impact 

to flat-tailed horned lizard habitat, but 

the Navy’s use of these lands, largely as 

bombing ranges, will result in little 

urban development on these lands. As 

such, we expect the amount of impact 

from urban development on areas of 

flat-tailed horned lizard habitat owned 

by the State of California and the 

Federal government in the Western and 

Eastern Population to be small now and 

within the foreseeable future because 

little urban development is likely on 

State Park lands and most military 

lands, and what development that may 

occur on Federal lands will be 

minimized through implementation of 

the Rangewide Management Strategy, 

including through implementation of 

the CDCA Plan on BLM lands. 

Moreover, the designation of the 

Borrego Badlands, West Mesa, and Yuha 

Desert Management Areas offer 

protective mechanisms for 96,599 ha 

(238,700 ac) (Table 1) of flat-tailed 

horned lizard habitat within this 

population. Impacts from permittee 

actions are limited to 1 percent of the 

area within Management Areas 

(FTHLICC 2003a, p. 33). As described 

above, we expect minimal or no urban 

development on Federal and California 

State lands within the area occupied by 

the Western Population and Eastern 

Population, but urban development may 

occur within private lands. Although 

private inholdings are scattered 

throughout the Federal and State lands 

in the region, few concentrations of 

private land exist. The largest 

concentration of private inholdings 

within the areas occupied by the 

Western Population occurs in and 

around the community of Borrego 

Springs, California. Urban development 

in this area is limited to a finite area 

within the Borrego Springs area, which 

is an area of private lands completely 

surrounded by Anza-Borrego Desert 

State Park. Additionally, development 

in this area may be further restricted by 

a limitation in the amount of available 

groundwater (Mills 2009, p. 4). As we 

concluded in 2003 (68 FR 342), even if 

urban development continues, this area 

is small enough that it is unlikely that 

the combined urban or agricultural 

development in or around this 

geographically limited area poses a 

significant threat to the flat-tailed 

horned lizard throughout its range. 

Moreover, limited water and isolation of 

the remaining private lands scattered 

within the public lands likely will 

prevent any large-scale urban 

development in the region, further 

reducing the effects that urbanization 

may have on the Western Population of 

the flat-tailed horned lizard. Because the 

Mexican portion of the Western 

Population is isolated from other 

inhabited areas by the Sierra de Los 

Cucapahs and the dry lakebed of Laguna 

Salada, we believe urban development 

in this area is likely similarly limited by 

available resources and isolation. Thus, 

we conclude that urban development is 

not a threat to the species in the 

Western Population, nor is it likely to 

become a threat in the foreseeable 

future. 

As discussed above, we expect 

impacts from urban development on 

Federal lands in the Eastern Population 

to be limited. Moreover, the designation 

of the East Mesa Management Area 

offers protective mechanisms for 45,248 

ha (111,810 ac) (Table 1), or about 27 

percent of the Eastern Population, of 

flat-tailed horned lizard habitat within 

this population. Impacts from permittee 

actions are limited to 1 percent of the 

area within each Management Area 

(FTHLICC 2003a, p. 33). Additionally, 

10,654 ha (26,327 ac), or about 6 percent 

of the Eastern Population, is designated 

as a Wilderness Area where urban 

development is prohibited. Most urban 

development occurs on private 

property, and less than 5 percent of the 

U.S. portion of the Eastern Population 

area occurs on private property. Limited 

water and isolation of the private lands 

likely prevent any substantive urban 

development in the region, including 

the small amount of habitat in Mexico. 

Thus, we conclude that urban 

development is not a threat to the 

species in the Eastern Population, nor is 

it likely to become a threat in the 

foreseeable future. 

Urban development has occurred 

recently in the Southeastern Population 

of flat-tailed horned lizards. Areas of 

recent urbanization include 

development near the communities of 

Yuma, Arizona (Piest and Knowles 

2006, p. 1); San Luis Rı ́o Colorado, 

Sonora, Mexico (Rodriguez 2002, p. 23); 

and Puerto Pen ̃ asco, Sonora, Mexico 

(Rodriguez 2002, p. 23). Most (about 84 

percent) of the flat-tailed horned lizard 

habitat in Arizona is federally owned, 

where urban development is less likely, 

and most of the U.S. Federal land in the 

Southeastern Population is within the 

53,014-ha (131,000-ac) Yuma Desert 

Management Area (Table 1), where 

impacts from permittee actions are 

limited to 1 percent of the area 

(FTHLICC 2003a, p. 26). Additionally, 

avoidance and minimization measures 

are in place within the Barry M. 

Goldwater Range, Arizona, to prevent or 

limit impact to the flat-tailed horned 

lizard and its habitat from military 

development (USFWS 1996, pp. 18 and 

58). Nevertheless, development impacts 

may occur. For example, construction 

by Marine Corps Airs Station, Yuma, of 

a new aircraft landing field and 

associated infrastructure for the F–35B 


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Joint Strike Fighter at the Barry M. 

Goldwater Range is expected to 

permanently remove 33.5 ha (82.7 ac) of 

flat-tailed horned lizard habitat, plus 

have additional long-term adverse 

effects on a 17.8 ha (44 ac) (USFWS 

2010b, p. 46). Even so, this project 

includes minimization measures called 

for by Rangewide Management Strategy, 

thereby reducing the impact of this 

development to the species and its 

habitat (USFWS 2010b, pp. 10–12, 45). 

Thus, we conclude that urban 

development in Arizona is not a 

significant threat to the species, nor is 

it likely to become a threat in the 

foreseeable future. 

In Mexico, urban development is 

likely within the foreseeable future 

around San Luis Rı ́o Colorado, Puerto 

Pen ̃ asco, and elsewhere along the Gulf 

of California coast. Despite an increase 

in accessibility to remote areas (Bu ́rquez 

and Martı ́nez-Yrı ́zar 1997, p. 390), the 

vast majority of the habitat for the 

Southeastern Population in Mexico 

remains isolated with respect to urban 

development, because urban 

development requires access to other 

resources, which are not necessarily 

available with mere physical access. 

Moreover, compared to the 1,005,630 ha 

(2,484,966 ac) of flat-tailed horned 

lizard habitat in the Mexican portion of 

the Southeastern Population, roughly 60 

percent of which lies within two 

Mexican Federal natural protected areas 

where development is limited (CEDO 

2001, p. 3), we expect the amount of 

urban development to be relatively 

small. Thus, we conclude that urban 

development is not a significant threat 

to the species in the Mexican portion of 

the Southeastern Population, nor is it 

likely to become a threat in the 

foreseeable future. 

Therefore, despite some urban 

development occurring in the 

Southeastern Population, we believe 

that this development is small relative 

to the overall amount of flat-tailed 

horned lizard habitat in the 

Southeastern Population and is unlikely 

to significantly increase in the 

foreseeable future; thus, this 

development does not pose a substantial 

threat to the species in the Southeastern 

Population, nor is it likely to become a 

threat in the foreseeable future. 

In conclusion, flat-tailed horned 

lizard habitat has been lost to urban 

development in the Coachella Valley, 

and we expect urbanization to continue 

there. The available information 

indicates the distribution of the species 

in the Coachella Valley is now limited 

to two occurrences that are within two 

Coachella Valley MSHCP conservation 

areas (CVCC 2010, p. 8); although nearly 

all of the flat-tailed horned lizard 

habitat in the Thousand Palms reserve 

is already protected, most of the Dos 

Palmas reserve is not (see Table 2). 

Implementation of the Coachella Valley 

MSHCP is expected to limit the impacts 

to the flat-tailed horned lizard and its 

habitat (USFWS 2008, Appendix A, p. 

317). Furthermore, in our evaluation of 

the potential impacts of the plan’s 

implementation on the flat-tailed 

horned lizard (USFWS 2008, p. 178), we 

concluded: ‘‘After reviewing the current 

status of this species, environmental 

baseline for the action area, effects of 

the proposed action, and cumulative 

effects, it is the Service’s biological 

opinion that the action, as proposed, is 

not likely to jeopardize the continued 

existence of the flat-tailed horned lizard. 

Loss of the Coachella Valley population 

would have a negligible [effect] on the 

status of the species as a whole, since 

it makes up approximately 1 percent of 

the current range of the flat-tailed 

horned lizard. Persistence of the species 

in the Plan area is likely only with 

effective Plan implementation.’’ Because 

of the limited amount of private land, 

urban development is also only likely to 

destroy relatively small amounts of flattailed horned lizard habitat in the 

Western, Southeastern, and Eastern 

Populations. Additionally, in areas of 

flat-tailed horned lizard habitat in the 

United States and Mexico where 

urbanization has the potential to occur, 

it is likely that the amount of urban 

development will be limited by the 

availability of water and the isolated 

nature of many of these areas. The 

implementation of the Rangewide 

Management Strategy further restricts 

development in the United States, 

limiting impacts inside designated flattailed horned lizard Management Areas 

to 1 percent of the area. In Mexico, 

urban development is likely to be 

limited within the Federal natural 

protected areas (Rodriguez 2002, p. 25). 

Therefore, we conclude that urban 

development is not a significant threat 

to flat-tailed horned lizard habitat 

throughout its range, nor is it 

anticipated to become a significant 

threat in the foreseeable future. 

Energy Generation Facility Development 

The analyses in the 1993 proposed 

rule and 2003 withdrawal document 

both identified development of 

geothermal energy facilities as a 

potential threat to flat-tailed horned 

lizard habitat. Since then, increased 

interest in renewable forms of electrical 

generation has resulted in a greater 

number of proposed energy 

development facilities and their 

associated infrastructure. Recent 

proposals not only include geothermal 

facilities, but also projects harnessing 

solar radiation and wind. Examples of 

recent proposals that may affect flattailed horned lizard habitat include the 

following: geothermal projects near the 

Superstition Mountains (Navy 2008, pp. 

1–40) and the Truckhaven area west of 

Salton City (BLM 2007a, pp. 1–3), solar 

projects near Plaster City (BLM and CEC 

2010, p. ES–1), and a wind project west 

of the community of Ocotillo (Ocotillo 

Express 2009, p. 1). Because the 

development of energy generation 

facilities occurs within the range of the 

flat-tailed horned lizard habitat, we 

assess the magnitude of this 

development to the species below. 

Similar to other forms of 

development, energy generation projects 

may result in destruction or 

modification of flat-tailed horned lizard 

habitat. These projects can include 

buildings, roads, power lines, and 

pipelines, although they differ in the 

details. For example, geothermal plants 

typically include wells and pipelines 

(often aboveground), solar plants 

typically include solar collecting arrays 

(using various technologies to convert 

solar energy to electrical energy), and 

wind farms have lines or arrays of wind 

turbines. 

The total acreage of potential 

development for renewable energy 

facilities is small compared to the 

overall range of the species. For 

example, in California, the BLM 

maintains a GIS database of rights-ofway applications for energy generation 

facilities. Additional permits are needed 

before the potential facilities listed in 

the database can be built, and even if 

they obtain all of the necessary permits, 

it is not guaranteed that all of them will 

be built. Moreover, some of these rightof-way applications have been rejected, 

denied, or withdrawn. However, 

assuming that the facilities in the BLM 

database are built, the total area of 

development on BLM land for all of the 

applications on file as of December 2010 

would be about 2,585 ha (6,387 ac) in 

the Eastern Population, and 18,841 ha 

(46,556 ac) in the Western Population. 

The BLM data only include areas of 

BLM (Federal) land and do not include 

what, if any, nearby private land that 

may also be developed as part of these 

energy projects. We do not have data for 

the potential impacts to private lands 

adjacent to these areas, but we made a 

rough assessment of the adjacent private 

land that may potentially be included in 

these projects which may add about 260 

ha (about 640 ac) to the impacts in the 

Eastern Population and about 10,600 ha 

(about 26,000 ac) to the impacts in the 

Western Population. Using these values, 


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the energy development in the Eastern 

Population may impact roughly 2,845 

ha (7,030 ac) of BLM and private lands, 

which is about 1.7 percent of the 

Eastern Population area, and the energy 

development in the Western Population 

may impact roughly 29,441 ha (72,750 

ac) of BLM and private lands, which is 

about 8.6 percent of the Western 

Population area. Combined, these 

projects—assuming that they are all 

built, which is not likely—would 

impact a total of about 2 percent of the 

nearly 1.6 million ha (3.9 million ac) of 

the total range of the species (using 2003 

‘‘current distribution’’). 

Although we expect additional energy 

development facilities may be 

constructed elsewhere within the range 

of the species, including in Arizona and 

Mexico, we are not aware of any specific 

proposals that are as large as those 

proposed in California. Therefore, we 

conclude that the total acreage of 

potential development for renewable 

energy facilities is small compared to 

the overall range of the species. 

Additionally, on lands managed by 

signatory agencies to the Interagency 

Conservation Agreement, we expect the 

impacts to flat-tailed horned lizard 

habitat (whether inside or outside of 

designated Management Areas) will be 

further reduced because of the 

avoidance, minimization, and 

compensation measures of the 

Rangewide Management Strategy. 

Moreover, because of the avoidance 

and minimization measures, including 

the 1-percent impact limit in flat-tailed 

horned lizard Management Areas, most 

of the energy generation facilities have 

been proposed outside of the 

Management Areas, although some 

impacts to Management Areas are 

anticipated resulting from related 

infrastructure development (FTHLICC/ 

MOG 2010, p. 2). For example, the 

2,454–ha (6,063–ac) Imperial Valley 

Solar project site is proposed outside of 

the flat-tailed horned lizard 

Management Areas called for by the 

Rangewide Management Strategy, but an 

associated transmission line is expected 

to run for about 12 kilometers (km) (7.5 

miles (mi)) within the Yuha Desert 

Management Area. However, this 

proposed transmission line was routed 

along an existing powerline corridor to 

minimize effects to flat-tailed horned 

lizard habitat in the Management Area 

(BLM and CEC 2010, pp. B.1–18, C.2– 

9, and C.2–42). 

While project sites may be proposed 

within flat-tailed horned lizard 

Management Areas, the Rangewide 

Management Strategy limits the total 

acreage of impacts for a given 

Management Area to no more than 1 

percent. As of 2009, signatory agencies 

control approximately 196,273 ha 

(485,000 ac) of flat-tailed horned lizard 

habitat in the designated Management 

Areas and have collectively permitted 

activities on 171.57 ha (423.97 ac), or 

0.09 percent (Table 1). Thus far, 

signatory agencies have consistently 

implemented the Rangewide 

Management Strategy, even in 

permitting development electrical 

generation facilities. Moreover, the 

implementation of the Rangewide 

Management Strategy is not completely 

voluntary at this point; aspects of the 

Rangewide Management Strategy have 

been incorporated into documents that 

implement regulatory mechanisms, 

including the Federal Land Policy and 

Management Act (43 U.S.C.1701 et seq.) 

(FLPMA), which affects development on 

BLM lands (see Factor D). Many of the 

anticipated energy development 

facilities are on BLM lands or otherwise 

would require easements or access 

across BLM lands; thus, the 

development of these energy generation 

facilities would be subject to the 

provisions of the Rangewide 

Management Strategy through 

implementation of FLPMA. 

In sum, the overall acreage of 

potential impacts from development of 

energy facilities is likely to be small 

compared to the total range of the 

species, including private lands likely to 

be developed. Moreover, because of the 

prevalence of Federal and State lands in 

the U.S. portions of the range of the flattailed horned lizard and because most of 

this land is managed by signatories to 

the Interagency Conservation Agreement 

implementing the Rangewide 

Management Strategy, we expect that 

the vast majority of proposed energy 

development projects that are likely to 

affect flat-tailed horned lizard habitat in 

the United States will be subject to the 

avoidance, minimization, and 

compensation measures incorporated 

into the Rangewide Management 

Strategy, including in areas outside of 

designated Management Areas. The 

signatories to the Interagency 

Conservation Agreement have been 

actively implementing the Rangewide 

Management Strategy since its 

inception, and have committed to its 

continued implementation. 

Additionally, the Rangewide 

Management Strategy has been 

incorporated into the CDCA Plan, which 

means it will be implemented as a 

regulatory mechanism (as opposed to a 

voluntary agreement). Although the 

Rangewide Management Strategy is not 

in effect in Mexico, the amount of 

habitat that is likely to be destroyed by 

energy development projects in that 

country is likely to be small relative to 

the total amount of habitat. Therefore, 

we anticipate the development of energy 

generation facilities does not now nor in 

the foreseeable future pose a significant 

threat to flat-tailed horned lizard and its 

habitat. 

Invasive, Nonnative Plants 

In our 2003 withdrawal document, we 

included the effects of invasive, 

nonnative plants as a potential threat to 

flat-tailed horned lizard habitat (68 FR 

345). However, we concluded that 

nonnative plants did not pose a 

substantial threat because of the limited 

extent to which such plants had 

established themselves in flat-tailed 

horned lizard habitat (68 FR 345). The 

available literature also suggests 

invasive, nonnative plants are a 

potential threat to flat-tailed horned 

lizard habitat (such as Hodges 1997, pp. 

4, 5, and 9; CEDO 2001, p. 2; FTHLICC 

2003a, pp. 18–19; Hammerson et al. 

2007, p. 4), but specifics on how 

nonnative species are impacting flattailed horned lizard habitat are 

generally lacking. 

The perennial nonnative tree, 

Tamarix aphylla (athel pine), has been 

planted as a windbreak in the Coachella 

Valley. This tree can reduce or prevent 

wind-transport of sand, thereby 

reducing available flat-tailed horned 

lizard habitat there (England 1983, p. 

152). Although T. aphylla typically 

spreads vegetatively by adventitious 

roots or submerged stems, the species 

can spread sexually by seed following 

flood events (Walker et al. 2006, pp. 

191–201). While perhaps not as invasive 

as other species of Tamarix (Cal–IPC 

2003, p. 4), T. aphylla trees have been 

removed in some Coachella Valley 

MSHCP reserve areas in the Coachella 

Valley as management to improve 

habitat (FTHLICC 1999, p. 4). Moreover, 

the population of flat-tailed horned 

lizards in the Coachella Valley proper is 

now found only in the Thousand Palms 

reserve area (CVCC 2010, p. 8), where 

the plan’s habitat management is 

focused. Therefore, we do not consider 

T. aphylla to be an invasive, nonnative 

species that is threatening flat-tailed 

horned lizard habitat. 

Nonnative annual plants, such as 

Brassica tournefortii (Saharan mustard), 

Schismus barbatus (common 

Mediterranean grass), and Salsola kali 

(Russian thistle), can blanket certain 

areas of the Colorado Desert in years 

with higher amounts of rainfall (Brown 

and Minnich 1986, pp. 411–422; Lovich 

and Bainbridge 1999, p. 318; FTHLICC 

2003a, p. 18; Yurkowsky 2005, in litt., 

Anza-Borrego Desert State Park; Barrows 


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et al. 2009, pp. 673–686). Such 

nonnative plants may adversely affect 

flat-tailed horned lizard habitat 

throughout its range by altering fire 

regimes (Brown and Minnich 1986, pp. 

418–421; Brooks and Esque 2002, pp. 

334–336); stabilizing Aeolian soils (i.e., 

soil that is transported from one place 

to another by wind; Barrows et al. 2009, 

p. 684); changing plant assemblages 

(Barrows et al. 2009, p. 683); and 

changing the availability of seeds for 

harvester ants, the primary food source 

for the flat-tailed horned lizard (Gordon 

1980, p. 70). Dense stands of plants, 

which are typical of invasive, nonnative 

plant species in years of higher amounts 

of rainfall, also may challenge the 

locomotor abilities of the wide-bodied 

flat-tailed horned lizard (Newbold 2005, 

p. 17). 

Plant growth will vary annually in the 

Colorado Desert because of the variable 

amount and timing of rainfall that the 

region receives. Moreover, annual plants 

die by the end of spring, and in the 

harsh desert climate the amount of 

standing biomass of the annual plants, 

once dead, quickly decreases (Barrows 

et al. 2009, p. 684). We expect the 

amount and timing of rainfall within the 

range of the species will continue to be 

variable into the foreseeable future, even 

with the potential effects of climate 

change (Field et al. 1999, pp. 8–10). As 

a result, the effects of invasive, 

nonnative plants are generally shortlived in areas of flat-tailed horned lizard 

habitat (Barrows et al. 2009, p. 673), and 

because of the likelihood of continued 

variability in precipitation, we expect 

the potential effects of invasive, 

nonnative plants to continue to be shortlived into the foreseeable future. With 

the potential exception of increased 

occurrence of wildland fires, we do not 

believe that the growth of invasive, 

nonnative plants poses a lasting, 

significant threat to flat-tailed horned 

lizard habitat now or in the foreseeable 

future. We examine the potential threat 

of wildland fire below. 

Fires typically are rare events in the 

western Sonoran Desert because of the 

natural ‘‘limited biomass, wide spacing 

between shrubs and sparse ground 

cover’’ (Brown and Minnich 1986, p. 

411). However, the periodic increase in 

the amount of available fuel from 

nonnative, annual plants in years of 

heavy precipitation has allowed the 

frequency, size, and intensity of fires in 

desert plant communities to increase 

(Brown and Minnich 1986, p. 411; 

Brooks and Berry 2006, pp. 117–118; 

Trader et al. 2006, p. 314; see also 

Rorabaugh 2010, p. 191). Moreover, 

many of the native perennial plants 

within the range of the flat-tailed 

horned lizard typically take a long time 

to recover after a fire (O’Leary and 

Minnich 1981, pp. 61–66; Brown and 

Minnich 1986, p. 411; Brooks and Esque 

2002, p. 330). Thus, fire can change the 

species composition of the perennial 

and annual plant communities. 

Moreover, provided enough water 

(rainfall) is available, annual plants, 

especially nonnative species, proliferate 

after a fire (Minnich 1994, p. 104), 

which may provide additional fuel and 

promote additional wildfires. Plant 

communities in areas with recurrent 

fires may convert from vegetation types 

dominated by native shrubs into types 

dominated by nonnative annual grasses 

and forbs (type conversion) (Brown and 

Minnich 1986, p. 411). Type conversion 

appears to be occurring near the highly 

urbanized areas, such as the Coachella 

Valley (Brown and Minnich 1986, p. 

411), where increased human activity 

offers higher numbers of ignition 

sources (Brooks and Esque 2002, p. 

337), but not in the more remote areas 

of flat-tailed horned lizard habitat. 

Moreover, the amount of rainfall is a 

critical factor in how much plant growth 

occurs (Barrows et al. 2009, p. 673). The 

amount of rainfall is unpredictable 

within the range of the flat-tailed 

horned lizard, and is likely to be so for 

the foreseeable future. It is not clear 

how the fire regime will be affected long 

term, but in the foreseeable future, 

wildland fire does not appear to be a 

threat. 

Additionally, it is unclear whether 

this localized change in vegetation 

affects the specific habitat components 

upon which flat-tailed horned lizards 

rely. For example, flat-tailed horned 

lizards take refuge under perennial 

shrubs for shade and to avoid predators 

(Muth and Fisher 1992, pp. 1–77; 

Sherbrooke 2002, pp. 109–120). Fire 

typically kills the existing desert shrubs, 

but shrubs do regrow after a fire, 

although the plant species composition 

is likely to have changed (Brown and 

Minnich 1986, pp. 411). Thus, during 

the period of time following fire while 

shrubs are regrowing, flat-tailed horned 

lizards will have fewer options for 

thermoregulation and predator 

avoidance. While this condition is not 

permanent, it remains unclear if the 

change in plant species composition 

will have a lasting effect on the flattailed horned lizard, especially if type 

conversion were to occur. Nonetheless, 

because this change in plant species 

composition is localized, we conclude 

any potential effects are low in 

magnitude at the species level, likely 

temporary, and thus not a significant 

threat to the species. 

Another potential threat to the flattailed horned lizard that may arise from 

a change in plant species composition 

after a fire is that harvester ants, the 

primary food of the flat-tailed horned 

lizard, could be affected. Fire likely kills 

individual harvester ants on the surface 

at the time of the fire, but evidence 

suggests the underground colonies 

survive (Zimmer and Parmenter 1998, p. 

282; Underwood and Christian 2009, p. 

325). As described in the Background 

section, harvester ants eat seeds of 

annual and perennial plant species. 

Although changes in plant composition 

may alter the type and quantities of 

available seeds consumed by ants, ant 

forage likely will not be eliminated, and 

may even increase because of the 

increase in annual plants (Zimmer and 

Parmenter 1998, p. 282; Underwood and 

Christian 2009, p. 325). For example, 

several of the species found by Gordon 

(1980, p. 72) to be important to 

harvester ants were also species of 

plants found by Brown and Minnich 

(1986, p. 416) to do well after a fire. 

Therefore, wildland fire does not appear 

to pose a threat to harvester ants. 

In conclusion, the spread of invasive, 

nonnative plants does not appear to be 

a significant threat to flat-tailed horned 

lizard habitat throughout its range at 

this time, nor is it likely to become a 

significant threat in the foreseeable 

future. 

Off-Highway Vehicles (OHVs) 

The analyses in the 1993 proposed 

rule and 2003 withdrawal document 

included OHV activity as a potential 

threat to the flat-tailed horned lizard. 

The Rangewide Management Strategy 

also describes off-highway (OHV) or offroad vehicle activity as a potential 

threat (FTHLICC 2003a, pp. 12–14). We 

consider OHVs to be all vehicles used 

off-road, including, but not limited to, 

automobiles, dune buggies, motorcycles, 

all-terrain-cycles, four-wheelers, and 

military vehicles. OHV activity 

includes, but is not limited to, 

recreational, military, law-enforcement 

(such as Border Patrol), and trans-border 

trafficking activities. As discussed in the 

Background section, flat-tailed horned 

lizard habitat typically consists of sandy 

flats and valleys occupied by plant 

species that are typical of the creosotewhite bursage plant association. The 

presence of ants as a food source is also 

important. 

OHV activity may modify flat-tailed 

horned lizard habitat because of impacts 

to vegetation (Luckenbach 1975, p. 4; 

Vollmer et al. 1976, p. 115; Bury et al. 

1977, p. 7; Lathrop 1983, p. 164; 

Luckenbach and Bury 1983, p. 280; 

Groom et al. 2007, p. 133), soil 


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disturbance (Luckenbach 1975, p. 4;, 

Bury et al. 1977, pp. 16–18;, Webb 1983, 

pp. 51–79), and introduction of 

nonnative plants (Brooks and Lair 2005, 

p. 8). Additionally, some but not all 

areas with high OHV activity have been 

shown to have fewer harvester ant 

colonies (McGrann et al. 2006, p. 77). 

Past studies of OHV impacts on 

lizards (Busack and Bury 1974, p. 182; 

Bury et al. 1977, p. 10; Luckenbach and 

Bury 1983, p. 273; Klinger et al. 1990, 

pp. 1–17; Beauchamp et al. 1998, p. 214; 

Gardner 2002, p. 14; Wright and Grant 

2003, p. 30) have been largely 

inconclusive or cannot be readily 

applied across the range of the flattailed horned lizard (that is, they have 

limited ‘‘inference space’’ (Ratti and 

Garton 1994, pp. 1–23)). Luckenbach 

and Bury (1983, p. 278) reported that a 

pronounced reduction in flat-tailed 

horned lizard abundance around the 

Algodones Dunes had been anecdotally 

noted by scientists. Marked declines in 

herbaceous and perennial plants, 

arthropods, lizards, and mammals in 

OHV-used areas compared with nearby 

control areas were also reported by 

Luckenbach and Bury (1983, p. 265). 

The declines, however, were for the 

Colorado Desert fringe-toed lizard (Uma 

notata) and beetles, and did not include 

flat-tailed horned lizards or ants. 

Additionally, research has been 

conducted in creosote-dominated 

habitats in the Mojave Desert. 

Researchers compared reptile metrics 

(measures) between sites used 

differentially by OHVs and control sites 

(Bury et al. 1977, pp. 1–23). Bury et al. 

(1977, p. 11) found a significant 

decrease in numbers of reptiles on OHVused areas compared to numbers on 

control sites in the Mojave Desert. 

However, the highest number of desert 

horned lizards on any one plot occurred 

on a moderately used OHV site (Bury et 

al. 1977, p. 10). In research conducted 

by both Busack and Bury (1974, p. 182) 

and Bury et al. (1977, p. 1), there 

appeared to be an inverse relationship 

between increased use of OHVs and the 

abundance of lizards; this means that, as 

OHV use increased, lizard abundance 

decreased. Additionally, McGrann et al. 

(2006, pp. 77–79) found that the density 

of flat-tailed horned lizards was lower 

in areas of high OHV activity, as was the 

average body mass of individual flattailed horned lizards, suggesting the 

habitat quality—including harvester ant 

abundance—in some high-use OHV 

areas was not as good; however, the 

authors also noted that small sample 

size may have allowed qualitative 

differences between sites sampled to 

affect their results. 

Research in the Ocotillo Wells SVRA 

found flat-tailed horned lizards at 

higher densities in non-sandy habitats 

than sandy habitats within the SVRA, 

which differed from most other research 

findings (Beauchamp et al. 1998, pp. 

213–214). However, it was unclear if 

flat-tailed horned lizards were found in 

these atypical habitat types because they 

are more variable in habitat use than 

previously thought, because these 

habitat types are more available in the 

Ocotillo Wells SVRA than other areas in 

which flat-tailed horned lizards have 

been studied, or as a response to OHV 

activity (Beauchamp et al. 1998, p. 214). 

OHV activity occurs in the Western, 

Eastern, and Southeastern Populations, 

but the amount (intensity, frequency) of 

OHV activity varies across the 

landscape, with greater amounts of 

activity in areas designated for OHV use 

and areas near existing roads, and lesser 

amounts in areas where OHV use is not 

permitted or areas that are away from 

easy access. In the Coachella Valley, 

OHV activity is expected to be 

controlled in protected habitat areas 

through implementation of the 

Coachella Valley MSHCP (CVAG 2007, 

pp. 9–117) and OHV activity is not 

identified as a conservation issue in the 

annual report for 2009 (CVCC 2010, p. 

14). In our evaluation of the potential 

impacts of the plan’s implementation on 

the flat-tailed horned lizard (USFWS 

2008, p. 178), we concluded: ‘‘After 

reviewing the current status of this 

species, environmental baseline for the 

action area, effects of the proposed 

action, and cumulative effects, it is the 

Service’s biological opinion that the 

action, as proposed, is not likely to 

jeopardize the continued existence of 

the flat-tailed horned lizard. Loss of the 

Coachella Valley population would 

have a negligible [effect] on the status of 

the species as a whole, since it makes 

up approximately 1 percent of the 

current range of the flat-tailed horned 

lizard. Persistence of the species in the 

Plan area is likely only with effective 

Plan implementation.’’ Additionally, 

approximately 94 percent of the 

potential habitat where flat-tailed 

horned lizards are known to occur in 

the Thousand Palms conservation area 

is land that is already protected (Table 

2), including about 62 percent that is 

part of the Coachella Valley National 

Wildlife Refuge. 

OHV activity along the United StatesMexico international boundary (border) 

was identified as a potential threat to 

the flat-tailed horned lizard and its 

habitat (FTHLICC 2003a, p. 12). The 

amount of impact to flat-tailed horned 

lizard habitat along the border is not 

clear. To put the potential impact in 

context of the range of the species, we 

assumed an area of high impact from 

border-related OHV activity to be within 

a 1-km (0.6-mi)-wide zone north of the 

border. We estimate that the total area 

of flat-tailed horned lizard habitat 

within that ‘‘zone’’ is about 12,662 ha 

(31,288 ac), or about 0.8 percent of the 

range of the species, comprising 2,318 

ha (5,728 ac), 5,012 ha (12,385 ac), and 

5,332 ha (13,176 ac) of the Western, 

Eastern, and Southeastern Populations, 

or about 0.7 percent, 3 percent, and 0.5 

percent of those populations, 

respectively. This zone of assumed high 

activity is a broad-brush assessment (for 

example, the All-American Canal runs 

along the border in the Eastern 

Population, likely confining any borderrelated OHV activities in certain areas to 

less than 1 km (0.6 mi)). Nevertheless, 

the zone is small compared to the range 

of the species and the three populations, 

individually. Moreover, since 2008, the 

U.S. Customs and Border Protection 

constructed the ‘‘border fence,’’ which is 

a vehicle and, in some areas, pedestrian 

barrier, plus associated infrastructure, in 

certain areas between the United States 

and Mexico. Although some areas of the 

border are not fenced, the areas of flattailed horned lizard habitat along the 

border are fenced (USCBP 2008a, p. 1– 

5; USCBP 2008b, p. 2–4; Rorabaugh 

2010, p. 181). Prior to construction of 

the border fence, the new fence and 

associated infrastructure was 

anticipated to result in reduction of the 

amount of illegal, cross-border traffic 

(USCBP 2008b, p. 3–18). Additionally, 

as part of the installation of the border 

fence, a stabilized patrol road on the 

U.S. side was constructed. The use of 

the road was also expected to result in 

an overall decrease in ground 

disturbance because Border Patrol 

agents would patrol from vehicles on 

the road rather than through OHV 

activity (USCBP 2008a, p. 2–7). Indeed, 

evidence suggests the border fence has 

reduced illegal cross-border traffic and 

associated OHV activity (Rorabaugh 

2010, p. 190), thereby reducing the 

amount of potential impact to flat-tailed 

horned lizard habitat along the border 

from illegal trans-border OHV activity 

and subsequent law-enforcement OHV 

activity by the Border Patrol. 

Moreover, the scientific literature is 

mixed and inconclusive with respect to 

the impact of OHV activity on the flattailed horned lizard and its habitat. 

Setser and Young (2000, p. 11) and 

Setser (2001, p. 12) found flat-tailed 

horned lizards avoided areas disturbed 

by OHVs. However, there was no 

difference in flat-tailed horned lizard 

habitat use between areas within 10 m 


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(33 ft) of OHV trails and sites farther 

away from OHV trails (Setser and Young 

2000, p. 11; Setser 2001, p. 12). Setser 

and Young (2000, p. 11) and Setser 

(2001, p. 12) concluded that: (1) OHV 

use might render sites less suitable to 

flat-tailed horned lizard use, because of 

the impacts of OHV activity on 

vegetation and soil characteristics; or (2) 

OHV trails occur on sites not preferred 

by flat-tailed horned lizards (e.g., barren 

ground with no plants or rocks). 

However, Gardner (2002) and Setser 

(2004, p. 54) suggested that OHV 

activity did not have an effect on flattailed horned lizards at different areas 

in the Ocotillo Wells SVRA, on the basis 

of observations. 

In summary, while there has been 

some research on the adverse effects of 

OHV activity on vegetation, soils, and 

flat-tailed horned lizards, its 

applicability to flat-tailed horned lizard 

populations is limited and unreliable 

because of the lack of scientific rigor 

associated with the research designs. 

Additionally, the effects of OHV activity 

on flat-tailed horned lizard populations 

were not the primary research 

questions. Nevertheless, these studies 

have utility in generating hypotheses 

concerning variation in degree of OHV 

use and flat-tailed horned lizard 

abundance. At this time, we conclude 

that the available studies do not 

collectively show that OHV activity 

causes declines in flat-tailed horned 

lizard populations throughout the range 

of the species or that adverse OHV 

impacts pose a significant threat to flattailed horned lizard habitat. 

Management activities, including efforts 

to reduce conflicts with actions that 

impact flat-tailed horned lizard habitats, 

would be enhanced by focused research. 

Impacts of OHV activity on flat-tailed 

horned lizard populations should be 

studied using rigorous research designs 

to yield conclusions with high degrees 

of certainty (Ratti and Garton 1994, pp. 

1–23) regarding the effects of OHV 

activity on flat-tailed horned lizard 

populations across the range of the 

species. In conclusion, OHV activity 

does not appear to be a significant threat 

to flat-tailed horned lizard habitat 

throughout its range at this time, nor is 

it likely to become a significant threat in 

the foreseeable future. 

Military Training Activities 

The Rangewide Management Strategy 

(FTHLICC 2003a, p.15) summarizes 

military activity within the range of the 

flat-tailed horned lizard. The species 

occurs on two military installations: (1) 

The western Barry M. Goldwater Range, 

administered by Marine Corps Air 

Station (MCAS) Yuma, and (2) Naval 

Air Facility (NAF) El Centro. MCAS 

Yuma manages approximately 46,458 ha 

(114,800 ac) within the 53,014–ha 

(131,000–ac) Yuma Desert Management 

Area, while NAF El Centro manages 

approximately 12,060 ha (29,800 ac) 

within the 55,078–ha (136,100–ac) West 

Mesa Management Area and 3,440 ha 

(8,500 ac) in the 46,660–ha (115,300–ac) 

East Mesa Management Area. The U.S. 

Marine Corps and U.S. Navy are 

signatories to the Interagency 

Conservation Agreement implementing 

the Rangewide Management Strategy. 

The training ranges are primarily used 

for aircraft-related training. Activities 

that have the potential to impact flattailed horned lizard habitat include 

non-exploding bombing practice, 

ground-based training, target 

maintenance, clean up of target sites, 

road maintenance, mobile target 

activity, and target and run-in-line 

grading. Most military activities are 

confined to previously disturbed areas, 

so the amount of destruction or 

modification of flat-tailed horned lizard 

habitat is limited (FTHLICC 2003a, 

p.15). Additionally, the military is 

committed to be good stewards of lands 

they control, and the two installations 

have incorporated measures to benefit 

the flat-tailed horned lizard and other 

wildlife resources into their planning, 

training, and management activities 

(Navy 2001, chapter 3; USAF and USMC 

2007, p. 1–8 and chapter 5). Therefore, 

we do not anticipate military training 

activities to substantially affect flattailed horned lizard habitat now or in 

the foreseeable future. 

Summary of Factor A Threats 

Flat-tailed horned lizard habitat could 

potentially be impacted by urban or 

agricultural development. However, due 

to the remote location and increasingly 

limited availability of water, 

urbanization and agricultural 

conversion of flat-tailed horned lizard 

habitat will likely be limited in the 

United States and Mexico over the 

foreseeable future. We note that 

development of energy facilities is 

increasing, especially in the 

southwestern United States; however, 

the overall acreage of impact from these 

projects, assuming all of the proposed 

right-of-way applications are 

constructed, is small compared to the 

range of the species. In the United 

States, we expect development impacts 

to occur outside of the existing 

Management Areas due to avoidance 

and minimization measures that result 

from implementation of the Rangewide 

Management Strategy. As of 2009, 

signatory agencies control 

approximately 185,653 ha (458,757 ac), 

or about 40 percent of flat-tailed horned 

lizard habitat in the United States, 

within the Management Areas, of which 

only 0.09 percent has been permitted for 

impacts. Furthermore, in the United 

States, most of the species’ habitat is 

federally or State (such as California 

State Park) owned, where impacts to 

habitat from development are 

anticipated to be minimal. In Mexico, 

the amount of development that may 

occur in flat-tailed horned lizard habitat 

is small relative to the large amount of 

habitat that is available, and thus the 

effects to the species are expected to be 

low in magnitude. Therefore, current or 

anticipated future urban, agricultural, or 

energy development throughout the 

species’ range is not currently a 

substantial threat to the flat-tailed 

horned lizard, nor do we expect it to 

become a substantial threat in the 

foreseeable future. 

Invasive, nonnative plants could 

increase the potential for wildland fire 

in a desert environment where wildland 

fire is naturally infrequent. Research 

suggests that invasive, nonnative plant 

conversion of flat-tailed horned lizard 

habitat is limited to urbanized and 

adjacent areas, and is not a substantive 

threat to the species’ habitat throughout 

its range. Also, frequent OHV activity 

has the potential to affect flat-tailed 

horned lizard habitat; however, the 

available studies do not collectively 

show that OHV activity causes declines 

in flat-tailed horned lizard populations 

throughout the range of the species or 

that adverse OHV impacts pose a 

significant threat to flat-tailed horned 

lizard habitat. Lastly, military training 

activities have limited impacts on the 

ground and are not expected to 

substantially affect flat-tailed horned 

lizard habitat. We do not consider the 

potential threats analyzed above to be 

substantial threats to the flat-tailed 

horned lizard, either individually or in 

combination. Therefore, based on our 

review of the best available scientific 

and commercial information, we find 

the flat-tailed horned lizard is not 

threatened by the present or threatened 

destruction, modification, or 

curtailment of its habitat or range, either 

now or in the foreseeable future. 

B. Overutilization for Commercial, 

Recreational, Scientific, or Educational 

Purposes 

Within the context of this listing 

factor, overutilization is the capture or 

collection of individuals of a species to 

an extent (at a high enough rate) to 

affect the status of the species. 

Historically, in the United States, flattailed horned lizards may have been 

among the species of horned lizard 


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collected for the curio trade (Bolster and 

Nicol 1989, pp. 2 and 7). Flat-tailed 

horned lizard were identified by Stewart 

(1971, p. 33) as utilized in the pet trade. 

This species was also collected for 

scientific and educational purposes 

(Bolster and Nicol 1989, p. 9). However, 

the collection of the flat-tailed horned 

lizard is now prohibited except by 

permit in California (California 

Administrative Code 40.10, Title 14) 

and Arizona (Arizona Game and Fish 

Regulation, Title 17, R12–4–443, 

Commission Order 43). The flat-tailed 

horned lizard is also listed in the 

Official Mexican Norm NOM–059– 

ECOL–2001, Mexico’s threatened 

species law, as a threatened species in 

Mexico (SEMARNAT 2002, p. 134), and 

collection is prohibited without a 

permit. Because of the difficulty in 

locating these cryptically colored 

lizards, we expect unauthorized 

recreational collection to be rare. In 

Mexico, Hammerson et al. (2007, p. 5) 

noted that the species may be utilized 

in the pet trade. As noted in Rodriguez 

(2002, p. 26), some people in Mexico 

have flat-tailed horned lizards in their 

yards, but it is unclear whether those 

lizards are prevented from moving out. 

We have no information on the 

magnitude of the pet trade, but horned 

lizards in general are known to be 

difficult to keep alive as captive pets 

(Stewart 1971, p. 34), including in 

Mexico (Rodriguez 2002, p. 26). This 

suggests that the pet trade is small. The 

information we have, although limited, 

does not suggest that the amount of 

utilization that has occurred recently, 

regardless of purpose, has significantly 

affected the status of the flat-tailed 

horned lizard. Therefore, based on our 

review of the best scientific and 

commercial information, we find that 

overutilization for any purpose is not a 

threat to the flat-tailed horned lizard, 

now or in the foreseeable future. 

C. Disease or Predation 

Disease occurs to some extent in 

nearly all wildlife populations, but it is 

only a threat if the disease is virulent to 

the extent that it significantly impacts 

the population. We are not aware of any 

reports of disease in flat-tailed horned 

lizards. Thus, we do not consider 

disease to be a threat to the flat-tailed 

horned lizard anywhere within its 

range, nor is there any evidence to 

suggest it is likely to become a threat in 

the foreseeable future. 

Predation occurs naturally, and nearly 

all populations of wildlife species are 

subject to some level of predation. 

Predation of flat-tailed horned lizards is 

known to occur. For example, 16 of 42 

radio-tagged flat-tailed horned lizards 

were depredated in a 2-year study 

(Muth and Fisher 1992, p. 33), although 

the rate of predation they observed may 

have been affected by the presence of 

the radio tags themselves by making the 

otherwise cryptically colored lizard 

more apparent to predators. For 

predation to be a significant threat to the 

flat-tailed horned lizard, predation rates 

must be high enough to affect the status 

of the species such that mortality from 

predation outpaces births resulting in an 

overall population decline. Predation 

has been identified as a potential threat 

to the flat-tailed horned lizard 

(FTHLICC 2003a, pp. 16–17). A 

summary from multiple sources in the 

scientific literature is presented in the 

Rangewide Management Strategy 

(FTHLICC 2003a, p. 16), which 

identifies known or likely predators to 

be six species of birds, five species of 

reptiles, two species of mammals, and 

one arthropod. Of these, the roundtailed ground squirrel (Spermophilus 

tereticaudus) and the loggerhead shrike 

(Lanius ludovicianus) were highlighted 

as major predators (FTHLICC 2003a, p. 

16; see also Young and Young 2000, p. 

60; Young et al. 2004a, p. 65). Most of 

these predators occur naturally 

(including historically) in areas 

occupied by flat-tailed horned lizards; 

thus, predation is not a threat that has 

emerged recently. 

However, information from the 

scientific literature suggests that the 

populations of some of these predators 

are now higher as a result of manmade 

changes to the landscape, resulting in 

increased predation of flat-tailed horned 

lizards in these areas (FTHLICC 2003a, 

pp. 16–17; Young and Young 2005, p. 

8). For example, Barrows et al. (2006, 

pp. 492–493) found evidence suggesting 

that loggerhead shrikes and other avian 

predators were responsible for reduced 

populations of flat-tailed horned lizards 

near wildland-urban interface, and 

Young and Young (2005, p. 8) suspected 

round-tailed ground squirrel 

populations are similarly augmented 

with manmade changes to landscape, 

resulting in similar declines in flattailed horned lizard populations in and 

around urban areas. Additionally, the 

cryptic coloration that allows flat-tailed 

horned lizards to blend in with desert 

soils may be of little use on paved roads, 

allowing increased levels of predation 

(Young and Young 2000, p. 62). 

However, much of the range of the flattailed horned lizard is remote, away 

from areas of manmade change. Thus, 

for the flat-tailed horned lizard, 

predation does not appear to be 

excessively high throughout its range 

but instead localized near developed 

areas. This suggests that the observed 

high level of predation of flat-tailed 

horned lizards is an ‘‘edge effect’’ 

associated with the interface between 

natural areas and areas of urban and 

agricultural development. Because the 

proportion of developed areas within 

the range of the species is small in 

comparison to the undeveloped areas, 

we do not consider increased predation 

associated with urbanization to be a 

significant threat to the species. We 

further consider predation as a 

secondary effect of development, which 

is discussed under Factor E, below. 

Summary of Factor C Threats 

Disease does not appear to be a threat 

at this time, nor is it likely to become 

a significant threat in the foreseeable 

future. Predation likely occurs in some 

human-altered areas at higher than 

typical rates; however, compared to the 

distribution of the species, relatively 

few flat-tailed horned lizards are likely 

subjected to increased predation. 

Therefore based on our review of the 

best scientific and commercial 

information, we find the flat-tailed 

horned lizard is not threatened by 

disease or predation, now or in the 

foreseeable future. 

D. The Inadequacy of Existing 

Regulatory Mechanisms 

In the 1993 proposed rule to list the 

species, we identified several State 

(Arizona and California), U.S. Federal, 

and Mexican Federal laws and other 

existing regulatory mechanisms that 

could provide benefits to the flat-tailed 

horned lizard (58 FR 62627), and we 

concluded that these regulatory 

mechanisms were inadequate to protect 

the species or its habitat (58 FR 62628). 

In 1997, we also noted several State 

(Arizona and California), U.S. Federal, 

and Mexican Federal laws, but 

particularly noted the benefits provided 

to the flat-tailed horned lizard by the 

Interagency Conservation Agreement 

implementing the Rangewide 

Management Strategy (62 FR 37858– 

37859). In 2003, we again noted several 

State (Arizona and California), U.S. 

Federal, and Mexican Federal laws and 

other existing regulatory mechanisms 

that could provide benefits to the flattailed horned lizard (68 FR 346). 

Because the Interagency Conservation 

Agreement implementing the 

Rangewide Management Strategy is 

voluntarily implemented on the part of 

the signatories, we do not consider it to 

be a regulatory mechanism per se. Some 

entities have incorporated the 

Interagency Conservation Agreement 

into other regulatory mechanisms; in 

such cases, the Interagency 


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Conservation Agreement is mentioned 

in the context of those regulatory 

mechanisms. Additionally, two habitat 

conservation plans (HCPs) within the 

range of the flat-tailed horned lizard 

cover the species and provide mitigation 

for and conservation of habitat. While 

implementation of these HCPs will 

provide localized benefits to the flattailed horned lizards populations within 

the HCP boundaries, these HCPs cover 

a very small portion of the flat-tailed 

horned lizard’s range and will not 

substantially influence the overall status 

of the species. The Interagency 

Conservation Agreement and the two 

HCPs are discussed in greater detail in 

the Background section above. 

In the preceding analyses of the 

threats to the flat-tailed horned lizard 

under Factors A, B, and C, and in our 

analysis of threats under Factor E, 

below, all of the threats presented are of 

low magnitude, are non-imminent, and/ 

or cover very small portions of the 

species’ range. In the sections that 

follow, we first discuss the existing 

regulatory mechanism(s) that would be 

removed as a result of the withdrawal of 

the proposed rule to list the species. 

Then we review the existing regulatory 

mechanisms that would remain in effect 

to address the potential threats 

discussed herein under the other listing 

factors. 

U.S. Federal Laws 

Section 7(a)(4) of the Act 

The Act contains provisions for 

Federal agencies to confer with the 

Secretary on any action that is likely to 

jeopardize the continued existence of 

any species proposed to be listed under 

the Act. Commonly called a 

‘‘conference,’’ this requirement would no 

longer apply to the flat-tailed horned 

lizard once the withdrawal of the 

proposed listing rule is finalized. A 

conference opinion is an advisory 

mechanism by which the Service 

recommends measures to avoid adverse 

effects or jeopardy to the species. There 

are no requirements to implement 

reasonable and prudent measures and 

terms and conditions or for adoption of 

reasonable and prudent alternatives to 

avoid impacts to species or habitat. In 

this regard, the conference opinion 

requirement under the Act provides 

little if any additional regulatory 

protection for this species; although it 

may provide some benefits to the flattailed horned lizard by informing 

Federal agencies of potential adverse 

effects to the species that may result 

from their activities. However, the 

survival of the flat-tailed horned lizard 

is not dependent on any protections 

afforded by the application of section 

7(a)(4) of the Act because the potential 

threats facing the flat-tailed horned 

lizard are not substantial (see the other 

listing factors). 

Incidental Protection Via Other Listed 

Species 

The withdrawal of the proposed rule 

to list the flat-tailed horned lizard will 

not affect the listing status of other 

listed species, and the flat-tailed horned 

lizard may receive some level of 

protection in the United States through 

implementation of the Act because of 

overlapping ranges or proximity to other 

federally listed species. These 

associated federally listed species 

include Coachella Valley fringe-toed 

lizard (Uma inornata), Astragalus 

lentiginosus var. coachellae (Coachella 

Valley milk-vetch), Astragalus 

magdalenae var. peirsonii (Peirson’s 

milk-vetch), bighorn sheep in the 

Peninsular Ranges (Ovis canadensis 

nelsoni), and desert tortoise (Gopherus 

agassizii). 

The federally threatened Coachella 

Valley fringe-toed lizard is restricted to 

the Coachella Valley, but its distribution 

overlaps with the northern portion of 

the flat-tailed horned lizard’s range in 

the Coachella Valley. However, the flattailed horned lizard may use additional 

habitat within the Coachella Valley in 

which the fringe-toed lizard does not 

occur. The Coachella Valley MSHCP 

addresses the Coachella Valley fringetoed lizard, Coachella Valley milkvetch, and the flat-tailed horned lizard. 

Federal actions not covered by the 

Coachella Valley MSHCP that may affect 

the Coachella Valley fringe-toed lizard, 

the Coachella Valley milk-vetch, or both 

are subject to consultation with the 

Service under section 7 of the Act. 

These consultations may include 

avoidance or minimization measures 

that benefit the listed species and, 

where they co-occur, the flat-tailed 

horned lizard. Similarly, consultations 

on the federally endangered bighorn 

sheep of the Peninsular Ranges may 

include measures that benefit flat-tailed 

horned lizards in the Western and 

Coachella Valley Populations where 

suitable habitat for both species is in 

close proximity at the toe of slope of the 

mountains; however, the amount of 

such overlap is likely to be minimal. 

Likewise, the flat-tailed horned lizard 

may marginally benefit from 

consultations addressing the federally 

threatened Astragalus magdalenae var. 

peirsonii and the federally threatened 

desert tortoise where they co-occur, but 

these areas of overlap are also likely 

minimal. When the flat-tailed horned 

lizard overlaps with other listed species, 

we anticipate impacts to the species and 

its habitat may be avoided or 

minimized. 

Approved Habitat Conservation Plans— 

Section 10(a)(1)(B) of the Act 

Under section 10(a)(1)(B) of the Act, 

the Service may issue ‘‘incidental take’’ 

(i.e., taking of endangered species that is 

incidental to, but not the purpose of, 

carrying out of an otherwise lawful 

activity, see 50 CFR 402.02) permits for 

listed animal species to non-Federal 

applicants, which provide exemptions 

to the take prohibitions under section 9 

of the Act. To qualify for an incidental 

take permit, applicants must develop, 

fund, and implement a Serviceapproved habitat conservation plan that, 

among other requirements, does not 

jeopardize the continued existence of 

covered species, and details measures to 

minimize and mitigate the impact of the 

approved incidental taking on covered 

species. As discussed in the Background 

section and under Factor A, there are 

two existing incidental take permits that 

include the flat-tailed horned lizard as 

a covered species: the Coachella Valley 

MSHCP and the Lower Colorado MSCP. 

Regardless of the withdrawal of the 

proposed rule to list the species, the 

existing HCPs, and the conservation 

they provide, would remain in effect. 

Additional U.S. Federal Mechanisms 

Federal Land Policy and Management 

Act 

The Federal Land Policy and 

Management Act of 1976 (43 U.S.C. 

1701 et seq.) (FLPMA), which provides 

overall direction to the BLM for 

conservation and management of public 

lands, allows the agency to participate 

in Interagency Conservation 

Agreements. Section 601 required the 

preparation of the California Desert 

Conservation Area (CDCA) Plan. The 

CDCA Plan was amended to formally 

incorporate the Rangewide Management 

Strategy into BLM’s land use planning, 

including formal adoption of the BLMcontrolled Management Areas 

comprising the East Mesa Flat-tailed 

Horned Lizard Management Area, West 

Mesa Flat-tailed Horned Lizard 

Management Area, and Yuha Desert 

Flat-tailed Horned Lizard Management 

Area (BLM 2004, p. 2). Additionally, 

section 103(a) of the FLPMA defines an 

Area of Critical Environmental Concern 

(ACEC), which allows creation of areas 

‘‘where special management attention is 

required * * * [for] fish and wildlife 

resources.’’ BLM lands comprise much 

of the U.S. range of the flat-tailed 

horned lizard, including the 

aforementioned Management Areas. 


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Additionally, the BLM has designated 

ACECs for wildlife resources within the 

range of the flat-tailed horned lizard. 

The BLM’s implementation of FLPMA, 

through land management plans that 

incorporate certain provisions of the 

Rangewide Management Strategy 

including the avoidance, minimization, 

mitigation (compensation), and 

management measures, helps to reduce 

the severity of existing potential threats 

to the flat-tailed horned lizard, 

especially development and OHV 

activity. We conclude FLPMA is an 

adequate regulatory mechanism within 

the confines of its applicability—that is, 

allowing BLM to better manage flattailed horned lizard habitat and 

implement the Rangewide Management 

Strategy on BLM lands. Because much 

of the U.S. portion of the range of the 

flat-tailed horned lizard is comprised of 

BLM land, FLPMA is an important 

regulatory mechanism that helps to 

reduce the already low-level threats to 

the species in these areas. 

Implementation of the CDCA Plan, as 

amended, and the incorporated 

provisions of the Rangewide 

Management Strategy will continue 

regardless of the withdrawal of the 

proposed listing rule for the species. 

National Environmental Policy Act 

The National Environmental Policy 

Act (NEPA) (42 U.S.C. 4321 et seq.) 

requires all Federal agencies to formally 

document, consider, and publicly 

disclose the environmental impacts of 

major Federal actions and management 

decisions that have significant effects on 

the human environment (including 

natural resources), but NEPA does not 

require that mitigation alternatives be 

implemented. Additionally, NEPA 

applies only to actions by Federal 

agencies, so private landowners are not 

required to comply with NEPA unless a 

Federal agency is involved through 

provision of Federal funding or a 

Federal permit. Although NEPA 

requires disclosure of the effects of 

proposed Federal actions, it does not 

afford direct protection to the flat-tailed 

horned lizard. 

Fish and Wildlife Coordination Act 

Through the Fish and Wildlife 

Coordination Act (16 U.S.C. 661 et seq.) 

(FWCA), we may recommend 

discretionary conservation measures to 

avoid, minimize, and offset impacts to 

fish and wildlife resources resulting 

from Federal projects and water 

development projects authorized by the 

U.S. Army Corps of Engineers. 

Therefore, FWCA may provide some 

protection for the species and its habitat 

through avoidance and minimization 

measures that may be incorporated into 

Federal projects. We conclude FWCA is 

an adequate regulatory mechanism 

within the confines of its applicability, 

but its applicability is limited. The 

minor benefits provided by FWCA will 

continue regardless of the withdrawal of 

the proposed rule to list the flat-tailed 

horned lizard. 

Sikes Act 

In 1997, section 101 of the Sikes Act 

(16 U.S.C. 670a) was revised by the 

Sikes Act Improvement Act to authorize 

the Secretary of Defense to implement a 

program to provide for the conservation 

and rehabilitation of natural resources 

on military installations. To do so, the 

Department of Defense was required to 

work with Federal and State fish and 

wildlife agencies to prepare an 

Integrated Natural Resources 

Management Plan (INRMP) for each 

facility with significant natural 

resources. The INRMPs provide a 

planning tool for future improvements; 

provide for sustainable multipurpose 

use of the resources, including activities 

such as hunting, fishing, trapping, and 

non-consumptive uses; and allow some 

public access to military installations to 

facilitate their use. Implementation of 

the measures included in these plans is 

subject to funding availability. The 

primary purpose for military lands, 

including most areas of flat-tailed 

horned lizard habitat, is to provide for 

military support and training. 

Two major military installations are 

within the U.S. range of the flat-tailed 

horned lizard, the MCAS Yuma (within 

the Barry M. Goldwater Range) and the 

NAF El Centro, both are signatories to 

the Interagency Conservation Agreement 

and are implementing the Rangewide 

Management Strategy. Both installations 

have incorporated aspects of the 

Rangewide Management Strategy into 

their respective INRMPs, including 

avoidance and minimization measures, 

plus monitoring and management 

activities (Navy 2001, pp. 3–14 to 3–16; 

USAF and USMC 2007, pp. 6–2 and 6– 

8; see also USAF et al. 2006 entire). 

Additionally, areas designated as Flattailed Horned Lizard Management Areas 

under the Rangewide Management 

Strategy include military-owned areas 

(FTHLICC 2003a, pp. 51–53). Regardless 

of the withdrawal of the proposed rule 

to list the species, the application of the 

Sikes Act would continue and the 

benefits to the flat-tailed horned lizard 

would continue within the confines of 

its applicability—that is, providing 

benefits to the flat-tailed horned lizard 

and its habitat on military facilities and 

implementing the Rangewide 

Management Strategy on military lands. 

California State Laws 

California Endangered Species Act 

The flat-tailed horned lizard is not 

listed under the California Endangered 

Species Act (CESA), the State’s primary 

regulatory mechanism to protect 

species. Therefore, CESA provides no 

benefit to the flat-tailed horned lizard. 

California Environmental Quality Act 

The California Environmental Quality 

Act (CEQA) (chapter 2, section 21050 et 

seq. of the California Public Resources 

Code) requires State and local 

government agencies to consider and 

disclose environmental impacts of 

projects and to avoid or mitigate them 

where possible. Under CEQA, public 

agencies must prepare environmental 

documents to disclose environmental 

impacts of a project and to identify 

conservation measures and project 

alternatives. Section 15380 of the CEQA 

Guidelines indicates that species 

designated as ‘‘species of special 

concern’’ (see below) should be included 

in an analysis of project impacts if they 

can be shown to meet the criteria of 

sensitivity outlined therein (Comrack et 

al. 2008, p. 2). However, CEQA itself 

does not guarantee that conservation 

measures will be implemented; the lead 

agency may either require mitigation 

through changes to a project, or 

determine that overriding 

considerations make mitigation 

infeasible (CEQA Sec. 21002). In the 

latter case, projects may be approved 

that cause significant environmental 

damage, such as impacts to species or 

their habitat. Therefore, whether CEQA 

is an adequate regulatory mechanism 

within the confines of its applicability 

depends on the law’s application and 

the determination of the lead agency 

involved. The minor benefits provided 

by CEQA will continue regardless of the 

withdrawal of the proposed rule to list 

the species. 

Natural Community Conservation 

Planning Act 

The NCCP program is a cooperative 

effort involving the State of California 

and numerous private and public 

partners to protect regional habitats and 

species. The primary objective of NCCPs 

is to conserve natural communities at 

the ecosystem scale while 

accommodating compatible land use, 

including urban development (http:// 

www.dfg.ca.gov/habcon/). Natural 

Community Conservation Plans help 

identify and provide for the regional or 

area-wide protection of plants, animals, 

and their habitats, while allowing 

compatible and appropriate economic 

activity. Many NCCPs are developed in 


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conjunction with habitat conservation 

plans prepared under the Act, including 

the Coachella Valley MSHCP. 

Regardless of the withdrawal of the 

proposed rule to list the flat-tailed 

horned lizard, the existing NCCPs, and 

the protections they provide, would 

remain in effect. 

California Administrative Code 

California Administrative Code 40.10, 

Title 14, prohibits the collection of flattailed horned lizards without a permit. 

Therefore, we conclude the California 

Administrative Code is an adequate 

regulatory mechanism within the 

confines of its applicability—that is, 

limiting or preventing overutilization of 

the flat-tailed horned lizard in 

California. The benefits provided by 

California Administrative Code 40.10, 

Title 14, will continue regardless of the 

withdrawal of the proposed rule to list 

the flat-tailed horned lizard. 

Species of Special Concern 

The State’s Species of Special 

Concern (SSC) designation is an 

administrative designation that carries 

no formal legal status. According to 

Comrack et al. (2008, pp. 1–4), its intent 

is to focus attention on animals deemed 

to be at conservation risk, stimulate 

research, and achieve conservation and 

recovery of these animals before they 

meet California Endangered Species Act 

criteria for listing as a State endangered 

or threatened species. The flat-tailed 

horned lizard is on the list of reptile and 

amphibian species of special concern in 

California (Jennings and Hays 1994, pp. 

134–141). 

As stated in Comrack et al. (2008, p. 

2), sections 15063 and 15065 of the 

CEQA Guidelines, which address how 

an impact is identified as significant, are 

particularly relevant to SSCs. Projectlevel impacts to listed (endangered, 

threatened, or rare species) species are 

generally considered significant, thus 

requiring lead agencies to prepare an 

Environmental Impact Report to fully 

analyze and evaluate the impacts. 

Moreover, section 15380 of the CEQA 

Guidelines indicates that SSCs should 

be included in an analysis of project 

impacts if they can be shown to meet 

the criteria of sensitivity outlined 

therein (Comrack et al. 2008, p. 2). In 

assigning ‘‘impact significance’’ to 

populations of non-listed species, 

analysts usually consider factors such as 

population-level effects, proportion of 

the taxon’s range affected by a project, 

regional effects, and impacts to habitat 

features. 

Therefore, we conclude the State’s 

Species of Special Concern designation 

is an adequate regulatory mechanism 

within the confines of its applicability— 

that is, an administrative designation 

that increases the level of awareness and 

analysis (such as under CEQA) for flattailed horned lizard in California. The 

benefits provided by the Species of 

Special Concern designation will 

continue regardless of the withdrawal of 

the proposed rule to list the flat-tailed 

horned lizard. 

Arizona State Laws 

Arizona Game and Fish Regulation 

Arizona Game and Fish Regulation, 

Title 17, R12–4–443, Commission Order 

43 prohibits the collection of flat-tailed 

horned lizards without a permit by 

indicating that there is no ‘‘open season’’ 

to collect the species (AGFD 2009, p. 8). 

Additionally, the Arizona Game and 

Fish Department has included the flattailed horned lizard on the draft List of 

Wildlife of Special Concern in Arizona, 

which the State uses to prioritize 

species for planning and funding 

purposes, although State regulations do 

not exist in Arizona to protect this 

species’ habitat at this time. We 

conclude Arizona Game and Fish 

Regulation is an adequate regulatory 

mechanism within the confines of its 

applicability—that is, limiting or 

preventing overutilization of the flattailed horned lizard in Arizona. The 

benefits provided by the Arizona Game 

and Fish Regulation, Title 17, R12–4– 

443, Commission Order 43 will 

continue regardless of the withdrawal of 

the proposed rule to list the flat-tailed 

horned lizard. 

Mexican Federal Law 

Official Mexican Norm 

The Official Mexican Norm NOM– 

059–ECOL–2001, Mexico’s threatened 

species law, lists the flat-tailed horned 

lizard as a threatened species 

(SEMARNAT 2002, p. 134). The 

Mexican law may be implemented to 

modify development projects or support 

creation of Natural Protected Areas, but 

successful implementation occurs by 

individuals or groups outside of the 

Mexican government. We conclude 

Official Mexican Norm may be an 

adequate regulatory mechanism within 

the confines of its applicability—that is, 

reducing threats to the species in 

Mexico. The benefits provided by the 

Official Mexican Norm NOM–059– 

ECOL–2001 will continue regardless of 

the withdrawal of the proposed rule to 

list the flat-tailed horned lizard in the 

United States. 

Summary of Factor D 

With the withdrawal of the proposal 

to list the flat-tailed horned lizard, the 

only change in regulatory protections 

would be the removal of the conference 

requirement under section 7(a)(4) of the 

Act. Since a conference opinion is only 

advisory in nature, we do not expect 

this change to have any significant effect 

on the status of the species. The 

remainder of the existing regulatory 

mechanisms summarized above will 

remain in place and will continue to 

provide benefits to the species. The 

aforementioned existing regulatory 

mechanisms provide some level of 

protection for the species and its 

habitat. This includes several laws or 

mechanisms that reduce potential 

threats, such as State laws that restrict 

the collection of flat-tailed horned 

lizards, or planning documents 

developed under FLPMA or the Sikes 

Act that incorporate measures from the 

Rangewide Management Strategy. 

Therefore, we conclude the existing 

regulatory mechanisms are not 

inadequate and do not threaten the 

species throughout all or a significant 

portion of its range, now or in the 

foreseeable future. 

E. Other Natural or Manmade Factors 

Affecting Its Continued Existence 

For Factor E, we assess the natural or 

manmade threats to the species that 

were not addressed under the previous 

four factors. In the 1993 proposed rule 

to list the species as threatened and in 

the 2003 withdrawal document, we 

considered the potential effects of 

pesticide spraying and prolonged 

drought under this factor. Also in these 

two Federal Register publications, we 

addressed the effects of OHV use on the 

species and its habitat under Factor A. 

Similarly, in those earlier assessments, 

we addressed the potential effects 

associated with fragmentation on the 

species and its habitat under Factor A. 

Also, in our 2006 withdrawal document 

(71 FR 36750–36751), the scope of 

which was limited by court order, we 

addressed historical habitat loss as a 

component of Factor A on the grounds 

that Factor A addresses the curtailment 

of a species’ habitat or range as a threat 

to its continued existence, but this 

rationale was flawed because Factor A, 

as discussed here and under Factor A in 

the present document, is limited to 

current and anticipated losses of habitat, 

not past losses. Because of the 

confusion presented in previous 

analyses, we have emphasized in the 

current analysis the differences between 

present and future habitat loss from past 

habitat loss, including how 

‘‘fragmentation’’ as a concept interacts 

with the topic of habitat loss. 

To address explicitly the previously 

identified threat of ‘‘fragmentation,’’ we 


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need to address the specific threats 

encompassed by that ambiguous term. 

However, these threats include ones that 

are best addressed under separate listing 

factors under the Act. As mentioned 

previously, the term fragmentation 

includes habitat loss. Factor A addresses 

present (current) or threatened 

(anticipated) destruction, modification, 

or curtailment of a species’ habitat or 

range. Factor A does not address threats 

posed by past losses of habitat. How the 

species is affected by past habitat loss— 

or in other words, the present-day 

ramifications of those past actions of 

habitat destruction—is better addressed 

under Factor E. The effects of past 

habitat loss include in particular the 

effects of manmade barriers on 

populations and edge effects. Barriers 

may divide otherwise intact populations 

into smaller populations, and those 

smaller populations may be more 

susceptible to other effects (see below). 

Thus, below, we assess the effects of 

barriers and small populations and edge 

effects. We also assess the previously 

identified potential effects to the species 

from pesticide spraying, OHV use, and 

prolonged drought; we also address the 

potential effects associated with global 

climate change not previously 

identified. 

Barriers and Small Populations 

As mentioned previously, as used 

herein a ‘‘population’’ refers to a loosely 

bounded, regionally distributed 

collection of individuals of the same 

species. Thus, individuals of a given 

species when considered together 

within some boundary may be 

considered a population. For example, 

the group of individuals bounded 

within the entire range of the species 

may be considered a population, 

sometimes referred to as the ‘‘entire 

population’’ or ‘‘population as a whole.’’ 

Similarly, groups of individuals within 

the entire population may be considered 

to occur separately from each other, 

forming multiple populations. In typical 

usage, a separation is often a literal 

separation—that is a physical division, 

by a barrier for instance—but it may also 

be a figurative separation; for example, 

an arbitrary grouping of individuals for 

the purposes of discussion. Regardless 

of the criteria used to separate and 

group individuals, a species may be 

considered to comprise one or more 

populations, depending on how the 

term is used. Moreover, because the 

term is loosely defined, a given 

population could be considered to 

consist of other smaller populations, 

sometimes hierarchically referred to as 

‘‘subpopulations.’’ For the purposes of 

our discussion of barriers and small 

populations, below, we primarily refer 

to populations as being physically 

separated, or potentially so. 

Barriers prevent or severely limit 

contact (genetic interchange) between 

populations. Thus, an artificial barrier 

can split a population into two (or more) 

populations (Jackson 2000, p. 4). For 

animals that can move freely (vagile 

animals), like the flat-tailed horned 

lizard, barriers prevent individuals from 

moving from one area into another. 

Barriers not only include physical 

hindrances that prevent movement (e.g., 

a wall or a river), but may also include 

areas that a species may be disinclined 

to enter (e.g., unsuitable habitat) or areas 

of increased mortality (e.g., busy roads, 

or areas with an elevated number of 

predators) across which individuals 

would be unlikely to successfully 

traverse. 

The division of populations into 

other, smaller populations may or may 

not be deleterious; it largely depends on 

the size of the resulting populations, 

with small populations more likely to 

experience problems than large 

populations, as discussed below. 

Moreover, small populations may be 

disproportionately affected by other 

natural and manmade factors compared 

to large populations, such as edge 

effects, also discussed below. Thus, the 

creation of artificial barriers results in 

habitat loss (see Factor A) and may also 

affect the species through potential 

effects associated with the subsequent 

isolation, which largely depends upon 

the size of the resulting populations. 

Because the threats from barriers and 

small populations are connected, we 

discuss the potential threats faced by 

small populations generally and then 

discuss the potential effects of barriers 

and small population sizes on the flattailed horned lizard. 

The decline of a population is 

determined by a number of forces and 

factors that are often grouped into 

intrinsic and extrinsic. As described by 

Soule ́ and Simberloff (1986, pp. 27–28), 

‘‘extrinsic forces include deleterious 

interactions with other species 

(increases in predation, competition, 

parasitism, disease or decreases in 

mutualistic interactions) and deleterious 

events or changes to habitat or the 

physical environment. Intrinsic factors 

include random variation in genetically 

based traits of the species and 

interactions of these traits with the 

environment. These include: 

(1) Demographic stochasticity, which is 

random variation in sex ratio [and] in 

birth and death rates, * * * (2) social 

dysfunction or behaviors that become 

maladaptive at small population sizes; 

[and] (3) genetic deterioration brought 

on by inbreeding, genetic drift and other 

factors.’’ For a population to become 

extirpated (locally extinct), these 

extrinsic and intrinsic forces and factors 

must significantly affect the population. 

These forces and factors are more likely 

to be significant to small populations 

(Goodman 1987, pp. 11–34; Pimm et al. 

1988, pp. 757–785; Lande 1993, pp. 

911–927; Frankham 1996, pp. 1500– 

1508; Henle et al. 2004, pp. 207–251). 

Our 1993 and 2003 assessments of the 

flat-tailed horned lizard have described 

flat-tailed horned lizard populations as 

‘‘fragmented.’’ As discussed previously, 

fragmentation is an imprecise term, but 

one that clearly is associated with the 

breaking up of populations into smaller 

populations through the introduction of 

artificial barriers. As discussed in the 

Background section, historical 

agricultural development (and its 

associated urban development) has 

largely occurred in contiguous blocks. 

These large swaths of human-created 

non-habitat have, for the most part, 

exacerbated natural barriers separating 

the Western, Eastern, and Southeastern 

Populations, and severed the somewhat 

tenuous connection between the 

Coachella Valley Population and the 

Western Population. As a consequence 

of the past development, the 

geographical area occupied by these 

four populations became smaller. With 

the decrease in the amount of habitat 

area, we expect populations of flattailed horned lizards in those areas to 

also be smaller (a decrease in the 

abundance of individuals) (such as 

Hokit and Branch 2003, p. 261). 

The point at which a population 

becomes a ‘‘small population’’ is not 

clear and varies by species-specific or 

situational-specific factors. There is 

disagreement among scientists and 

considerable uncertainty as to the 

population size adequate for long-term 

persistence of wildlife populations; 

however, there is agreement that 

population viability over the long term 

is more likely to be ensured if 

population sizes are in the thousands of 

individuals rather than hundreds (Traill 

et al., 2010, p. 32, see also Reed et al. 

2003, p. 30, Table 3 therein). In 

vertebrates, a population of 5,000 is 

often used as a minimum number 

needed for high likelihood of viability 

over the long term (Traill et al., 2010, p. 

32), while Reed et al. (2003, p. 30) 

estimated that roughly 7,000 breedingage adults is the minimum number 

necessary for a vertebrate population to 

likely remain viable over the long term. 

However, as stated by Thomas (1990, p. 

324), ‘‘there is no ‘magic’ population 

size that guarantees the persistence of 

animal populations.’’ He went on to note 


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that populations of some vertebrates 

have survived for decades with 

population sizes of hundreds or even 

dozens of individuals, adding 

‘‘populations that occupy habitat 

fragments that are far too small to hold 

thousands of individuals may still 

possess great conservation potential’’ 

(Thomas 1990, p. 326). Moreover, the 

amount of time that most authors 

consider to be ‘‘long term’’ is many 

decades or even centuries (for example, 

see Shaffer 1981, p. 132; Soule and 

Simberloff 1986, p. 28; Traill et al. 2010, 

p. 31; see also Reed et al. 2003, p. 30, 

Table 3 therein). Although minimum 

population sizes for shorter time periods 

would be correspondingly smaller (see 

Figure 1 in Traill et al. 2010, p. 31), we 

use the long-term population size to be 

conservative. 

As discussed in the Background 

section, and discussed further in the 

present section, the distribution of the 

flat-tailed horned lizard is divided into 

discrete populations. Thus, to assess the 

threat implied by the term 

‘‘fragmentation,’’ it is more appropriate 

to consider the individual populations 

than to assess the population-as-awhole. Below we assess the four 

geographical Populations. We first 

examine the Western, Eastern, and 

Southeastern Populations, each as a 

whole. Then, looking at those three 

Populations further, we note that 

potential barriers within the larger 

Populations may divide each Population 

into smaller subpopulations. Lastly, we 

examine the Coachella Valley 

Population. We treat the Coachella 

Valley Population separately from the 

other three Populations because the 

current distribution of flat-tailed horned 

lizards in the Coachella Valley occurs in 

two widely isolated areas and are more 

like the subpopulations created by 

barriers within the Western, Eastern, 

and Southeastern Populations. Thus, we 

take advantage of the concepts 

developed in our discussion of barriercreated subpopulations to assess the 

Coachella Valley Population. 

Western, Eastern, and Southeastern 

Populations 

There are no direct, reliable estimates 

of flat-tailed horned lizard population 

size for the four geographically 

separated populations. The size of the 

Western Population, Eastern Population, 

and Southeastern Population areas are 

341,989 ha (845,073 ac), 169,617 ha 

(419,133 ac), and 1,073,551 ha 

(2,652,802 ac) respectively (Coachella 

Valley Population area is discussed 

separately, below). Even at the lowest 

(most conservative) estimated density of 

adult flat-tailed horned lizard of 0.3 

individuals per ha (0.1 individuals per 

ac) (see Background section) there are 

likely more than 50,000 adult flat-tailed 

horned lizards in the Western 

Population, 85,000 in the Eastern 

Population, and 322,000 in the 

Southeastern Population. We 

acknowledge that there are numerous 

assumptions in these calculations that 

limit accuracy of the extrapolated 

population sizes; however, even using 

the most conservative density value, 

these three populations are of sufficient 

size such that any threats associated 

with small populations would be 

unlikely. However, there are potential 

barriers that may subdivide the 

otherwise apparently continuous 

Western, Eastern, and Southeastern 

Populations. We examine subdivisions 

within these three populations, below. 

Subpopulations Within the Western, 

Eastern, and Southeastern Populations 

For the flat-tailed horned lizard, as a 

diminutive terrestrial animal, a number 

of manmade changes to the landscape 

may serve as barriers (see FTHLICC 

2003a, p. 14). These include: 

(1) Railways, canals, and certain types 

of roadways that are physical 

hindrances to the movement of flattailed horned lizards; (2) developed 

areas (unsuitable habitat) into which 

flat-tailed horned lizards may be 

disinclined to enter; and (3) busy 

roadways, powerline corridors, and 

areas adjacent to developed areas (that 

have artificial perches and nearby 

artificial food sources resulting in 

higher densities of predators) that are 

areas of increased mortality for flattailed horned lizards (FTHLICC 2003a, 

p. 14; see also Boarman et al. 1997, pp. 

54–58; Fagan et al. 1999, pp. 165–182; 

Jackson 2000, pp. 1–14; Germaine and 

Wakeling 2001, pp. 229–237; Young and 

Young 2005, pp. 1–11; Barrows et al. 

2006, pp. 486–494; Shepard et al. 2008, 

pp. 288–296). 

We expect these potential barriers 

will be variable in how thoroughly they 

prevent movement of flat-tailed horned 

lizards, and thus variable in the extent 

to which they prevent contact between 

individuals and separate populations. 

Canals generally extend for long 

distances without overcrossings, and 

flat-tailed horned lizards may be 

reluctant to use (go over) what few 

crossings exist (bridges); as such, canals 

are likely impermeable barriers in the 

same way the Colorado River has 

separated populations. However, as 

discussed below, roadways and 

railways, and the infrastructure 

associated with border security may or 

may not constitute complete barriers. 

Depending on how roads are 

constructed, they may serve as physical 

hindrances to the movement of flattailed horned lizards. For example, 

raised roadbeds, steep curbs, and 

roadway dividers may contribute to 

making a roadway a physically 

impassible barrier for flat-tailed horned 

lizards. Similarly, railways may serve as 

physical barriers. However, bridges and 

culverts, especially those with largersized openings, may allow flat-tailed 

horned lizards to cross under the 

physical impediments along roads and 

railways (Painter and Ingraldi 2007, p. 

17). Although it is not known whether 

the openings under such structures are 

used regularly by the species in the 

wild, it is likely that the undercrossings 

with natural substrates created by larger 

culverts, and especially bridges, are 

used to some extent. Additionally, 

blowing sand, which is not atypical for 

much of the range of the flat-tailed 

horned lizard, may build up along 

roadways and railways. Thus, it is 

possible that accumulated sand, at least 

until the sand is cleared by maintenance 

crews, may provide a ‘‘bridge’’ over the 

physical structures that prevent flattailed horned lizard movement. For 

example, the railway through the sandy 

Gran Desierto de Altar may be less of a 

barrier than railways in less sandy 

portions of the species’ range due to 

blowing and drifting sands that may 

provide passage over tracks. 

Additionally, roads that do not serve 

as physical hindrances may be barriers 

for other reasons. Flat-tailed horned 

lizards, particularly males (Young and 

Young 2000, p. 19), are often sighted on 

paved roads (Mayhew 1965, p. 104; 

Turner and Medica 1982, p. 822; 

Johnson and Spicer 1985, p. 40; 

Stebbins 2003, p. 304). This, combined 

with their propensity to not flee from 

oncoming traffic (Young and Young 

2000, p. 60), may make flat-tailed 

horned lizards particularly susceptible 

to traffic-related road mortality (Nicola 

and Lovich 2000, p. 211; Gardner et al. 

2001, p. 10). The stretches of multi-lane 

highways (Interstate 8 and State Route 

86) that cross areas within the current 

range of the flat-tailed horned lizard 

have, on average, over 25,000 vehicles 

pass over them daily, while the smaller, 

two-lane highways of State Routes 78 

and 98 within the species’ range have 

roughly 3,500 to 5,500 vehicles per day, 

on average (Caltrans 2008, electronic 

data). The increased level of vehicle 

traffic on the multi-lane highways along 

with the greater number of physical 

hindrances that may result from 

multiple lanes is more likely to serve as 

a barrier than the smaller, two-lane 


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highways. For example, the population 

of flat-tailed horned lizards occupying 

the small part of the Southeastern 

Population north of Interstate 8 (1,018 

ha (2,516 ac)) (see below) is small 

enough and isolated enough to exhibit 

some evidence of inbreeding or genetic 

drift (Culver and Dee 2008, p. 2), 

suggesting Interstate 8 in this area is an 

effective barrier preventing movement 

of flat-tailed horned lizards (see below). 

However, Interstate 8 likely poses less of 

a physical hindrance where it crosses 

the Eastern Population where blowing 

sand fills in gaps along the road edge, 

although the traffic volume remains 

high. Another way roadways may be 

barriers is that the cryptic coloration 

that allows flat-tailed horned lizards to 

blend in with desert soils may be of 

little use on paved roads, allowing 

increased levels of predation (Young 

and Young 2000, p. 62) (see Factor C, 

Disease and Predation). Thus, even 

though flat-tailed horned lizards may be 

able to physically cross two-lane roads 

(Barrows 2006, p. 119), these roads may 

be barriers to flat-tailed horned lizards 

for other reasons. 

However, it is not clear whether 

roadways or other potential barriers are 

complete barriers. They may instead be 

‘‘semipermeable’’ barriers, reducing 

contact between populations, but not 

stopping it. This may be especially true 

for small roads, especially gravel and 

unsurfaced roads and OHV ‘‘routes.’’ 

Although the amount of contact needed 

to maintain population connectivity of 

flat-tailed horned lizards is not known, 

Mills and Allendorf (1996, p. 1517) 

suggested that if 1 to 10 individuals per 

generation successfully cross a 

semipermeable barrier, that level of 

movement is likely sufficient to 

maintain the connection between 

populations, provided the overall 

population is of sufficient size. Thus, a 

potential barrier would have to severely 

limit flat-tailed horned lizard movement 

throughout its length and at all times for 

it to be a complete barrier; as such, only 

a few potential barriers are likely 

complete barriers. 

The ‘‘tactical infrastructure,’’ 

including fencing, lighting, and access 

and patrol roads (collectively, the 

‘‘border fence’’), along portions of the 

international border has the potential to 

serve as a barrier. The actual fencing in 

these areas includes vehicle and 

pedestrian fences that are constructed to 

allow movement of small animals 

(USCBP 2008a, pp. 1–4 to 1–6 and 

Appendix B; USCBP 2008b, pp. 2–5 and 

8–9). Although the shifting sand has 

meant some of the small slots that were 

incorporated into fine-mesh pedestrian 

fence to allow movement of flat-tailed 

horned lizards are no longer at ground 

level (FTHLICC 2010, p. 10), the shifting 

sand has also resulted in gaps under the 

fence that flat-tailed horned lizards may 

use to cross under the fence (Rorabaugh 

2010, p. 190). Thus, we do not 

anticipate the fence proper to be a 

complete physical hindrance to flattailed horned lizard movement. The 

additional infrastructure and activity 

may deter flat-tailed horned lizard 

movement or allow for increased 

mortality. However, in total, we do not 

believe the level of activity to be high 

enough to be a complete barrier to flattailed horned lizard movement (see also 

Rorabaugh 2010, p. 190). For example, 

genetic data from both sides of the 

border in the Southeastern Population 

suggests that populations of flat-tailed 

horned lizards in Arizona are not 

genetically isolated from neighboring 

populations in Mexico (Culver and Dee 

2008, p. 10). As such, the border fence 

is likely a semipermeable barrier, not a 

complete barrier, for the species. 

To assess the threat of barriers to the 

flat-tailed horned lizard, we examined 

maps of the region, including GIS data 

and aerial and satellite imagery. The 

areas in which flat-tailed horned lizards 

are currently distributed contain 

numerous potential manmade barriers. 

As mentioned above, the Coachella 

Valley Population area has numerous 

barriers, and the flat-tailed horned 

lizard is only known from two relatively 

small areas. Thus, as summarized 

below, we focused our attention on the 

three relatively contiguous Western, 

Eastern, and Southeastern Populations. 

For this analysis, we used GIS data of 

the species’ ‘‘current distribution’’ as 

delineated by the 2003 Rangewide 

Management Strategy to examine the 

size of the areas between those features 

we considered likely barriers. Barriers 

divide the areas of habitat into 

subareas—termed herein as ‘‘parts.’’ 

Similarly, barriers divide populations of 

flat-tailed horned lizards into smaller 

populations, or subpopulations. 

Features we considered potential likely 

barriers included: (1) The All-American 

Canal and the Coachella Canal, which 

are likely to be complete barriers 

throughout their lengths; and 

(2) Interstate 8; State Routes 78, 86, and 

98; Mexico Federal Highways 2 and 8; 

the (old) coastal highway (which is 

being upgraded to a multi-lane highway, 

but we do not have GIS data for the new 

route); the international border; and 

several railways, which are likely to be 

semipermeable barriers to varying 

degrees along their lengths. 

For the purposes of dividing the areas 

into ‘‘parts,’’ we assumed all potential 

barriers were complete barriers; 

however, in the analysis that follows we 

discuss the situations in which such 

barriers may be semipermeable. 

Additionally, for the purposes of the 

analysis, where two or more potential 

barriers are adjacent to each other (e.g., 

portions of Interstate 8 and the AllAmerican Canal), we mapped them as a 

single barrier. All of the area values 

(hectares and acres) are approximate 

and are not as precise as the values 

given; however, we believe they are 

sufficiently accurate for this coarse-scale 

analysis (especially because we used 

conservative estimates of flat-tailed 

horned lizard densities). 

We used the conservative estimated 

density of 0.3 adult flat-tailed horned 

lizards per ha (0.1 per ac) to determine 

whether potentially isolated parts 

between barriers were likely to contain 

more than 7,000 adults, in other words, 

to be large enough to avoid threats that 

may be associated with small 

population size (see above). Where 

populations were ‘‘small,’’ we also 

present other potential population sizes 

using higher densities, including the 

still-conservative, but perhaps more 

realistic (for certain ‘‘parts’’), value of 0.7 

individuals per ha (0.3 per ac) (see 

Population Dynamics section, above). 

As described in the Population 

Dynamics section in the Background, 

these density estimates were derived 

from data that were collected at sites in 

the northern portion of the species’ 

range. As a result, we are confident that 

the density estimates used are 

conservative. We do not have density 

estimates for the southern portion of the 

species’ range; thus, we do not know if 

0.3 or 0.7 individuals per ha (0.1 or 0.3 

per ac) are as conservative. 

Nevertheless, because these values are 

at the low end of a fairly wide range (0.3 

to 4.4 adults per ha (0.1 to 1.8 per acre)), 

we believe them to be within the 

density range even in the southern areas 

of the species’ distribution. 

Additionally, as discussed near the 

beginning of the ‘‘Barriers and Small 

Populations’’ section, above, the point at 

which a population becomes ‘‘small’’ 

varies from species to species and from 

situation to situation. Stated another 

way, the forces and factors that are more 

likely to be significant threats to a 

‘‘small’’ population of a given species are 

not guaranteed to be significant threats 

to a given population of a given size. We 

have limited information on the effects 

such forces and factors may have on the 

flat-tailed horned lizard. For example, 

even though information in the 

scientific literature suggests the 

previously mentioned population north 

Interstate 8 is exhibiting some evidence 

of inbreeding or genetic drift (Culver 


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and Dee 2008, p. 2), we do not have 

specific information as to whether or to 

what degree that population’s status is 

being affected; the information in the 

scientific literature (as discussed above) 

suggests that this population is likely 

facing a greater risk from threats 

associated with genetic deterioration, 

but we have no data (one way or the 

other) to assess that particular 

population’s status. Thus, for the 

purposes of evaluating the potential 

threats associated with the implied 

meaning of ‘‘fragmentation’’ to the flattailed horned lizard, we have assumed 

that the populations of flat-tailed 

horned lizards in areas that we 

identified as small, isolated parts are 

likely to experience adverse effects 

associated with small population size. 

Western Population 

The potential barriers listed above 

split the Western Population area into 

12 parts (Table 3, Figures 3 and 4), four 

of which are likely to support 

populations greater than 7,000 

individuals, even with the most 

conservative of the estimated densities. 

These include: (1) The area north of 

State Route 78 (77,566 ha (191,670 ac)) 

(Part W–1; Table 3), which includes the 

Borrego Badlands Management Area 

and Ocotillo Wells SVRA; (2) the area 

immediately south of State Route 78 

(89,105 ha (220,183 ac)) (Part W–3; 

Table 3), which includes the West Mesa 

Management Area; (3) the area in the 

vicinity of the southeastern corner of 

Anza-Borrego Desert State Park (42,443 

ha (104,879 ac)) (Part W–5; Table 3); and 

(4) the long, narrow area south of 

Mexico Federal Highway 2 in Baja 

California (74,254 ha (183,486 ac)) (Part 

W–12; Table 3). Although the long, 

narrow nature of the area in Baja 

California may make threats more 

pronounced (Faaborg et al. 1995, p. 

366), it remains a large habitat area. 

Thus, it is likely flat-tailed horned 

lizards in these four areas are not ‘‘small 

populations.’’ 

TABLE 3—THE SIZE (AREA) OF THE ‘‘PARTS’’ CREATED BY BARRIERS (SEE TEXT) WITHIN THE WESTERN POPULATION AND

OUR DETERMINATION AS TO WHETHER THE SPECIFIED PART IS UNLIKELY TO BE AT RISK OF DELETERIOUS EFFECTS

OF SMALL POPULATIONS AT THE CONSERVATIVE DENSITIES OF 0.3 OR 0.7 INDIVIDUALS PER HA (0.1 OR 0.3 PER AC) 

Part identifier (country) Area of part 1

Is this part large enough to avoid deleterious effects 

associated with small populations when the density is 

assumed to be: 

0.3 individuals per ha 

(0.1 per ac) 

0.7 individuals per ha 

(0.3 per ac) 

W–1 (U.S.) ................................ 77,566 ha (191,670 ac) .............................................. yes .................................... yes. 

W–2 (U.S.) ................................ 8,777 ha (21,688 ac) .................................................. no ..................................... no. 

W–3 (U.S.) ................................ 89,105 ha (220,183 ac) .............................................. yes .................................... yes. 

W–4 (U.S.) ................................ 539 ha (1,331 ac) ....................................................... no ..................................... no. 

W–5 (U.S.) ................................ 42,443 ha (104,879 ac) .............................................. yes .................................... yes. 

W–6 (U.S.) ................................ 4,081 ha (10,083 ac) .................................................. no ..................................... no. 

W–7 (U.S.) ................................ 19,527 ha (48,252 ac) ................................................ no ..................................... yes. 

W–8 (U.S.) ................................ 110 ha (272 ac) .......................................................... no ..................................... no. 

W–9 (U.S.) ................................ 10,873 ha (26,867 ac) ................................................ no ..................................... yes. 

W–10 (Mex.) ............................. 294 ha (726 ac) .......................................................... no ..................................... no. 

W–11 (Mex.) ............................. 14,420 ha (35,632 ac) ................................................ no ..................................... yes. 

W–12 (Mex.) ............................. 74,254 ha (183,486 ac) .............................................. yes .................................... yes 

1Area values are estimated through a GIS-based assessment. Despite the level of precision presented, area values are approximate; however, we believe they are accurate enough to draw the conclusions presented. 

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Of the remaining eight populations, 

three (Parts W–4, W–8, and W–10; Table 

3) were remnants of a few hundred 

hectares each, totaling less than 1,000 

ha (2,500 ac). If the flat-tailed horned 

lizards in these areas are isolated from 

other flat-tailed horned lizard 

populations, we expect they will be 

‘‘small populations’’ and that they, 

therefore, are more likely to be 

negatively impacted by the deleterious 

effects associated with small 

populations sizes. Although the 

populations in these parts may have 

some connection to their respective 

adjacent parts, Parts W–4, W–8, and W– 

10 are very small and on the periphery, 

and any such connection would likely 

be tenuous at best. 

Of the five remaining parts, three 

(located between Interstate 8 on the 

north and Mexico Federal Highway 2 to 

south) (Parts W–7, W–9, and W–11; 

Table 3) were large enough to likely 

support more than 7,000 flat-tailed 

horned lizards if the density of flattailed horned lizards was 0.7 

individuals per ha (0.3 per ac). Given 

that the two U.S. areas contain the Yuha 

Desert Management Area, an area that 

was selected to be a Management Area 

because it is likely to support higher 

densities of flat-tailed horned lizards 

and where one of the demographic plots 

from which the data for density 

estimates were gathered (see Population 

Dynamics section, above), and the one 

in Mexico is immediately adjacent to 

the Yuha Desert Management Area, we 

believe it is reasonable to conclude that 

the density of 0.7 individuals per ha (0.3 

per ac) is a realistic but still 

conservative density estimate to use. 

Moreover, as mentioned above, the 

border fence is likely a semipermeable 

barrier, allowing some connectivity 

between the Yuha Desert Management 

Area and the areas of habitat south of 

the international border. Thus, it is 

likely flat-tailed horned lizards in these 

areas are not ‘‘small populations.’’ 

One of the last two remaining parts is 

the area between Interstate 8 and the 

railway to the north (Part W–6; Table 3); 

it is over 4,000 ha (9,900 ac). This part 

should have some connectivity with the 

areas to the north because it is unlikely 

the railway is a complete barrier, and it 

may even have limited connection to 

the south across Interstate 8 because of 

culverts and bridges, especially the large 

bridge that allows Interstate 8 to span 

the typically dry South Fork Coyote 

Wash at the far west end of Part W–6 

(BLM and CEC 2010, p. C.2–22; USFWS 

2010c, p. 57). A 2,630-ha (6,500-ac) 

solar generation facility has been 

proposed in this area, which is likely to 

transform much of it into unsuitable 

habitat. However, requirements for the 

construction and operation of the solar 

generation facility include avoidance of 

impacts to the major washes that cross 

the site, which would allow the 

possibility of connectivity (USFWS 

2010c, p. 57). 

The last area, between State Route 86 

and the Salton Sea, is over 8,000 ha 

(19,800 ac) (Part W–2; Table 3, Figure 

3). The multi-lane State Route 86 is 

likely a substantial barrier, but our 

interpretation of aerial imagery suggests 

there are several bridges that may allow 

some connection. That connection, 

combined with the size of the area, may 

reduce the risk this population will 

suffer from threats associated with 

‘‘small populations.’’ 

In sum, for the Western Population, 

assuming the identified potential 

barriers are complete barriers (which is 

not likely, as explained above, although 

we do not know how permeable they 

may be), and assuming the most 

conservative density of 0.3 flat-tailed 


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horned lizards per ha (0.1 per ac), we 

calculate that nearly 83 percent of the 

area is in parts of sufficient size such 

that the populations of flat-tailed 

horned lizards therein are not likely to 

be substantially affected by the factors 

associated with small population size. If 

we assume a slightly less conservative 

density (though still at the low end of 

the reported range) of 0.7 individuals 

per ha (0.3 per ac), we calculate about 

96 percent of the area within the 

Western Population is in large enough 

blocks to not be substantially affected by 

small population size. Thus, the 

Western Population is not substantially 

composed of ‘‘small populations.’’ 

Therefore, we conclude the flat-tailed 

horned lizards in the Western 

Population are not substantially 

threatened by effects associated with 

barriers that subdivide populations or 

the deleterious effects that may follow, 

nor do we expect barriers to be a threat 

in the foreseeable future. 

Eastern Population 

The potential barriers listed above 

split the Eastern Population area into 

nine parts within three subareas (Table 

4). Two major canals, which we expect 

are complete barriers, divide the overall 

area. The east-to-west-flowing AllAmerican Canal isolates the southern 

roughly 20 percent (southern subarea) 

from the northern 80 percent, which in 

turn is divided by the southeast-tonorthwest-flowing Coachella Canal, 

essentially splitting the northern area in 

half (East Mesa subarea on the west and 

the Algodones Dunes subarea to the 

east). We discuss parts within these 

three subareas separately below. 

TABLE 4—THE SIZE (AREA) OF THE ‘‘PARTS’’ CREATED BY BARRIERS (SEE TEXT) WITHIN THE EASTERN POPULATION AND

OUR DETERMINATION AS WHETHER THE SPECIFIED PART IS UNLIKELY TO BE AT RISK FROM DELETERIOUS EFFECTS

OF SMALL POPULATIONS AT THE CONSERVATIVE DENSITIES OF 0.3 OR 0.7 INDIVIDUALS PER HA (0.1 OR 0.3 PER AC) 

Part identifier (country) Area of part 1

Is this part large enough to avoid deleterious effects 

associated with small populations when the density is 

assumed to be: 

0.3 individuals per ha 

(0.1 per ac) 

0.7 individuals per ha 

(0.3 per ac) 

E–1 (U.S.) ................................. 16,863 ha (41,669 ac) ................................................ no ..................................... yes. 

E–2 (U.S.) ................................. 156 ha (385 ac) .......................................................... no ..................................... no. 

E–3 (U.S.) ................................. 12,135 ha (29,986 ac) ................................................ no ..................................... yes. 

E–4 (U.S.) ................................. 50,270 ha (124,220 ac) .............................................. yes .................................... yes. 

E–5 (U.S.) ................................. 50,721 ha (125,334 ac) .............................................. yes .................................... yes. 

E–6 (U.S.) ................................. 8,968 ha (22,160 ac) .................................................. no ..................................... no. 

E–7 (U.S.) ................................. 2,867 ha (7,085 ac) .................................................... no ..................................... no. 

E–8 (U.S.) ................................. 4,140 ha (10,230 ac) .................................................. no ..................................... no. 

E–9 (Mex.) ................................ 23,496 ha (58,060 ac) ................................................ yes .................................... yes. 

1Area values are estimated through a GIS-based assessment. Despite the level of precision presented, area values are approximate; however, we believe they are accurate enough to draw the conclusions presented. 


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The southern subarea of the Eastern 

Population—that is, south of the AllAmerican Canal—is divided by the 

international border, with the part 

between the canal and border totaling 

8,968 ha (22,160 ac) (Part E–6; Table 4), 

and the part on the Mexico side of the 

international border totaling 23,496 ha 

(58,060 ac) (Part E–9; Table 4). However, 

as mentioned previously, the border 

fence is probably a semipermeable 

barrier. As such, we expect the area of 

flat-tailed horned lizard habitat to the 

south of the All-American Canal (Parts 

E–6 and E–9 combined) could be 

considered together. However, we 

estimate that roughly 6,400 ha (15,800 

ac) in the easternmost portions of these 

two parts contain areas of deep, actively 

shifting sands of the Algodones Dunes 

that are likely rarely used by flat-tailed 

horned lizards. Despite this, the area is 

large enough so as to likely not be 

affected by the deleterious effects 

associated with ‘‘small populations.’’ 

The East Mesa subarea (the western 

half of the northern 80 percent) is 

divided into four parts. The smallest 

part (Part E–2; Table 4) is a very small, 

isolated remnant of potential habitat 

(156 ha (385 ac)) at the far northern end 

of the Eastern Population area; it is a 

small population and may be at greater 

risk from the deleterious effects 

associated with small populations. The 

next smallest part is a triangle of flattailed horned lizard habitat between 

Interstate 8 and the All-American Canal 

(Part E–8; Table 4). It is 4,140 ha (10,230 

ac), likely too small of an area to 

support a ‘‘large population,’’ and the 

busy, multi-lane Interstate 8 probably 

has low ‘‘permeability’’ for flat-tailed 

horned lizard movement. The third part 

in the East Mesa subarea (Part E–5; 

Table 4), the area north of Interstate 8, 

south of State Route 78 and west of the 

Coachella Canal, is 50,721 ha (125,334 

ac) and includes the East Mesa 

Management Area, which is considered 

to be higher-quality flat-tailed horned 

lizard habitat. This part is large enough 

to support a large population; moreover, 

it is likely that the density in this area 

is at the higher end of the range of 

density estimates—thus, the population 

is likely much larger and not at risk of 

deleterious effects associated with small 

populations. The fourth part in the East 

Mesa subarea (Part E–3; Table 4), the 

area to the north of State Route 78 and 

west of the Coachella Canal, is 12,135 

ha (29,986 ac) and unlikely to support 

a ‘‘large population’’ of flat-tailed horned 

lizards at the most conservative density. 

However, because of this area’s 

proximity to the East Mesa Management 

Area, it likely supports higher densities 

of flat-tailed horned lizards such that at 

0.7 flat-tailed horned lizards per ha (0.3 

per ac), this part would support a 

population that would not be at risk 

from threats associated with small 

population size. Moreover, State Route 

78 in this area, because blowing sand 

has filled in any gaps along the road’s 

edge such that it is not a physical 

hindrance and it has a lower traffic 

volume (Caltrans 2008, electronic data), 

is likely a semipermeable barrier, 

allowing contact of flat-tailed horned 

lizards between the two areas (north and 

south of the highway). As such, we 

expect the area of flat-tailed horned 

lizard habitat north of Interstate 8 and 

west of the Coachella Canal (Parts E–3 

and E–5 combined) is large enough so 

as to not be affected now or in the 

foreseeable future by the deleterious 

effects associated with small 

populations. 

The Algodones Dunes subarea (the 

eastern half of the northern 80 percent) 

is divided into three parts. The part 

north of Interstate 8, south of State 

Route 78 and east of the Coachella 


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Canal, is 50,270 ha (124,220 ac) (Part E– 

4; Table 4), large enough to support a 

large population at the most 

conservative density estimate. However, 

this area is mainly composed of the 

Algodones Dunes, which is an area of 

deep, actively shifting sand that is likely 

rarely used by flat-tailed horned lizards 

(Turner et al. 1980, p. 14). Flat-tailed 

horned lizards in this area are likely 

(naturally) restricted to the peripheral 

portions of the dunes. Moreover, large 

portions of this region include areas of 

intense recreational OHV activity, 

including portions of the peripheral 

areas of the dunes, which may reduce 

the habitat quality in those areas (see 

Factor A). The third part of this subarea 

(Part E–1; Table 4), the area north of 

State Route 78, is 16,863 ha (41,669 ac), 

at the most conservative density 

estimates supporting a population that 

may be at risk from the deleterious 

effects of small population size. This 

part is also mainly composed of the 

deep, actively shifting sands of the 

Algodones Dunes, suggesting that higher 

densities of flat-tailed horned lizards are 

unlikely. However, unlike the areas to 

the south of State Route 78, most of the 

area is designated as Wilderness and, as 

such, OHV activity is prohibited. 

Moreover, as in the East Mesa subarea, 

State Route 78 is likely a semipermeable 

barrier, allowing contact of flat-tailed 

horned lizards between the two areas 

(north and south of the highway). Thus, 

the areas on the periphery of the 

Algodones Dunes are likely used by flattailed horned lizards within parts E–1 

and E–4, but the majority of these two 

parts, the areas of deep, shifting sands 

of the Algodones Dunes, likely 

contributes little to the Eastern 

Population, and likely contributed little 

even before the manmade barriers and 

OHV activity. The smallest part (Part E– 

7; Table 4), between the All-American 

Canal and Interstate 8, in the southeast 

corner of the Eastern Population area, is 

about 2,867 ha (7,085 ac). Using the 

conservative density estimate, the 

population of flat-tailed horned lizards 

in this part may be at risk of deleterious 

effects associated with small 

populations. This part, though sandy, is 

not dominated by the deep, actively 

shifting sands of the main dunes. 

In sum, for the Eastern Population, 

assuming the identified potential 

barriers are complete barriers (which is 

not likely, see above, although we do 

not know how permeable they may be), 

and assuming the most conservative 

density of 0.3 adult flat-tailed horned 

lizards per ha (0.1 per ac) for all the 

parts, we calculate that about 73 percent 

of the area is in large enough blocks that 

the populations of flat-tailed horned 

lizards therein are not likely to be 

affected by threats associated with small 

populations. However, the Eastern 

Population is divided by the AllAmerican Canal and the Coachella 

Canal, which we expect are complete 

barriers to flat-tailed horned lizards. As 

such, the Eastern Population area is 

divided into three subareas. The size of 

the population in the portion east of the 

Coachella Canal, the Algodones Dunes 

subarea, is not clear because much of 

the area includes the deep-sand areas of 

the Algodones Dunes, which is likely 

low-quality habitat for the flat-tailed 

horned lizard. As such, even using our 

conservative density estimate, this area 

likely supports—naturally, even prior to 

any manmade effects—fewer flat-tailed 

horned lizards compared to the other 

subareas in the Eastern Population than 

would be expected from its size. For the 

subarea south of the All-American 

Canal, the border fence between part E– 

6 and E–9 is likely permeable to some 

extent, but roughly 6,400 ha (15,800 ac) 

in the easternmost portions of these two 

parts contain areas of deep, actively 

shifting sands of the Algodones Dunes 

that are likely rarely used by flat-tailed 

horned lizards. Thus we expect the 

populations of flat-tailed horned lizards 

in parts E–6 and E–9 are connected, and 

even subtracting the area of deep sand 

in the east of these two parts, the 

subarea south of the All-American Canal 

is large enough to likely support a 

population of flat-tailed horned lizards 

that is unlikely to be substantially 

affected by the threats associated with 

small population size. For the subarea 

west of the Coachella Canal and north 

of the All-American Canal, the 

populations of flat-tailed horned lizards 

in parts E–3 and E–5 are likely 

connected because State Route 78 likely 

is a semipermeable barrier. Moreover, 

Part E–5 contains the East Mesa 

Management Area where the density of 

flat-tailed horned lizards is likely 

greater than the most conservative 0.3 

adults per ha (0.1 per ac) density 

estimate. Similarly, Part E–3 likely 

supports a population of flat-tailed 

horned lizards at a density greater than 

the most conservative 0.3 adults per ha 

(0.1 per ac). Thus, if we (1) exclude 

parts E–1, E–4, and the deep-sand areas 

at the east end of parts E–6 and E–9 

because these areas are naturally poorquality habitat and are likely rarely used 

by flat-tailed horned lizards; and (2) 

consider part E–3, E–5, and the nondeep-sand portions of E–6 and E–9 

(combined; see above) as likely 

supporting large populations of flattailed horned lizards, then about 93 

percent of the Eastern Population area 

likely supports populations of flat-tailed 

horned lizards that are large enough to 

be unlikely affected by threats 

associated with small populations. 

Therefore, we conclude that, overall, the 

flat-tailed horned lizards in the Eastern 

Population are not substantially 

threatened now or in the foreseeable 

future by effects associated with barriers 

that subdivide populations or the 

deleterious effects that may follow. 

Southeastern Population 

Identified potential barriers divide the 

Southeastern Population area into 13 

parts (Table 5). By far, the largest single 

part (Part SE–5; Table 5, Figures 6 and 

7) is in Mexico between the 

international border and the Mexicali to 

Puerto Pen ̃ asco railway, northwest of 

Mexico Federal Highway 8. It is over 

720,000 ha (1,779,000 ac) and includes 

the bulk of the Gran Desierto de Altar 

where the species occurs in the sandy 

flats and low, more-stabilized dunes 

within this region (Rorabaugh 2008, p. 

39; Rorabaugh and Young 2009, p. 183), 

but the deep, actively shifting sands of 

much of this area are likely rarely used 

by flat-tailed horned lizards (Rodriguez 

2002, p. 18; Rorabaugh and Young 2009, 

p. 182). Nevertheless, the sheer size and 

limited manmade alterations to the area 

suggests that this area likely supports a 

population large enough to avoid the 

deleterious effects associated with small 

populations, even if they are limited to 

the peripheral portions of the ‘‘sand 

sea.’’ This large part touches nearly all 

of the other parts in the Southeastern 

Population, and in our discussion of the 

other parts, we refer to this large, central 

part as the Gran Desierto part. 


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TABLE 5—THE SIZE (AREA) OF THE ‘‘PARTS’’ CREATED BY BARRIERS (SEE TEXT) WITHIN THE SOUTHEASTERN POPULATION AND OUR DETERMINATION AS WHETHER THE SPECIFIED PART IS UNLIKELY TO BE AT RISK OF DELETERIOUS

EFFECTS OF SMALL POPULATIONS AT THE CONSERVATIVE DENSITIES OF 0.3 OR 0.7 INDIVIDUALS PER HA (0.1 OR 0.3 

PER AC) 

Part identifier (country) Area of part 1

Is this part large enough to avoid deleterious effects 

associated with small populations when the density is 

assumed to be: 

0.3 Individuals per ha 

(0.1 per ac) 

0.7 Individuals per ha 

(0.3 per ac) 

SE–1 (U.S.) ............................... 56,736 ha (140,198 ac) .............................................. yes .................................... yes. 

SE–2 (U.S.) ............................... 1,018 ha (2,516 ac) .................................................... no ..................................... no. 

SE–3 (U.S.) ............................... 8,804 ha (21,755 ac) .................................................. no ..................................... no. 

SE–4 (U.S.) ............................... 1,364 ha (3,371 ac) .................................................... no ..................................... no. 

SE–5 (Mex.) .............................. 720,168 ha (1,779,573 ac) ......................................... yes .................................... yes. 

SE–6 (Mex.) .............................. 8,354 ha (20,643 ac) .................................................. no ..................................... no. 

SE–7 (Mex.) .............................. 496 ha (1,226 ac) ....................................................... no ..................................... no. 

SE–8 (Mex.) .............................. 110,242 ha (272,414 ac) ............................................ yes .................................... yes. 

SE–9 (Mex.) .............................. 110,857 ha (273,934 ac) ............................................ yes .................................... yes. 

SE–10 (Mex.) ............................ 5,175 ha (12,788 ac) .................................................. no ..................................... no. 

SE–11 (Mex.) ............................ 10,585 ha (26,156 ac) ................................................ no ..................................... yes. 

SE–12 (Mex.) ............................ 833 ha (2,058 ac) ....................................................... no ..................................... no. 

SE–13 (Mex.) ............................ 38,919 ha (96,171 ac) ................................................ yes .................................... yes. 

1Area values are estimated through a GIS-based assessment. Despite the level of precision presented, area values are approximate; however, we believe they are accurate enough to draw the conclusions presented. 

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The railway that runs from Mexicali 

to Puerto Pen ̃ asco and south, along with 

the ‘old’ coastal highway (see above), 

create four parts, three small and one 

large, along the coast of the Gulf of 

California northwest of Puerto Pen ̃ asco. 

The three small parts along the coast are 

8,354 ha (20,643 ac) (Part SE–6; Table 

5), 496 ha (1,226 ac) (Part SE–7; Table 

5), and 5,175 ha (12,788 ac) (Part SE–10; 

Table 5). These parts may be at risk from 

the deleterious effects associated with 

small populations; however, the road 

and railroad separating them from the 

Gran Desierto part are likely not 

complete barriers. We expect that 

blowing sand periodically covers the 

railway line and any gaps along the 

sides of the road, allowing some level of 

connectivity between flat-tailed horned 

lizard populations on the coast with 

those in the Gran Desierto part. 

Similarly, the one large coastal part 

northwest of Puerto Pen ̃ asco (110,242 ha 

(272,414 ac)) (Part SE–8; Table 5) is also 

likely connected with the Gran Desierto 

part; however, Part SE–8 is likely large 

enough by itself to support a population 

large enough that it would not be at risk 

from deleterious effects of small 

populations. Because we do not believe 

these four parts are completely isolated, 

the population of flat-tailed horned 

lizards along the coast of the Gulf of 

California northwest of Puerto Pen ̃ asco 

is likely not at risk from the deleterious 

effects associated with small 

populations. 

Mexico Federal Highway 8, the 

northeast to southwest-running highway 

from Sonoita (on the international 

border, outside of the range of the flattailed horned lizard) to Puerto Pen ̃ asco, 

separates the Gran Desierto part from 

the southeastern-most portion of the 

Southeastern Population. The 

southward continuation of the railway 

and the parallel-running coastal 

highway further divides this portion 

into a total of four parts. One of these 

parts is very small (833 ha (2,058 ac)) 

(Part SE–12; Table 5) and confined to a 

narrow strip along the coast. It may be 

at greater risk of deleterious effects 

associated with small populations. 

Another narrow coastal part is larger 

(10,585 ha (26,156 ac)) (Part SE–11; 

Table 5) and could support enough flattailed horned lizards to avoid 

deleterious effects of small populations 

if the densities were 0.7 individuals per 

ha (0.3 per ac). However, this area 

includes a portion of the urban 

development of Puerto Pen ̃ asco, and 

densities may be lower. The 

southernmost (coastal) part (Part SE–13; 

Table 5) is also separated by the 

railway-highway combination, but it is 

large (38,919 ha (96,171 ac)) and is 

likely to support a population large 

enough to avoid deleterious effects from 

small populations size even at the most 

conservative density. These three 

coastal parts are separated from the 

large interior part (Part SE–9; Table 5), 

which is 110,857 ha (273,934 ac) and 

large enough to support considerably 

more than 7,000 flat-tailed horned 

lizards. Additionally, if the railwayhighway combination separating the 

three coastal parts (Parts SE–11, SE–12, 

and SE–13) from the larger interior part 

is not a complete barrier, which is 

possible because of blowing sand, then 

the two larger coastal parts could 

receive dispersing flat-tailed horned 

lizards from the large interior part, 

which may help further reduce the 

likelihood of deleterious effects 

associated with ‘‘small populations.’’ 

Moreover, Mexico Federal Highway 8 

may also be permeable, suggesting that 

the southernmost portion of the 

Southeastern Population (Parts SE–9, 

SE–11, SE–12, and SE–13 combined) 


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may also be connected with the 

extremely large Gran Desierto part (Part 

SE–5). 

Lastly, the portion of the Southeastern 

Population in the United States is 

divided into four parts, including one 

large part (see below) and three smaller 

parts, the latter including one north of 

Interstate 8 and two west of the new 

Yuma Area Service Highway. The part 

north of Interstate 8 is 1,018 ha (2,516 

ac) (Part SE–2; Table 5) and may be at 

risk of deleterious effects associated 

with small populations. We expect the 

multi-lane Interstate 8 to be nearly a 

complete barrier along this stretch of the 

road and, as mentioned above, the 

evidence suggests that the population 

there may be exhibiting inbreeding or 

genetic drift (Culver and Dee 2008, p. 2). 

The two small parts west of the Yuma 

Area Service Highway are 8,804 ha 

(21,755 ac) (Part SE–3; Table 5) and 

1,364 ha (3,371 ac) (Part SE–4; Table 5); 

both may have small populations that 

could be at risk from the deleterious 

effects of small population size. The 

large part in Arizona (56,736 ha 

(140,198 ac)) (Part SE–1; Table 5) is 

mostly composed of the Yuma Desert 

Management Area and is large enough 

to avoid deleterious effects from small 

population size. Culver and Dee (2008, 

pp. 1–14) also sampled the Yuma Desert 

Management Area and did not report 

any evidence of inbreeding or genetic 

drift in flat-tailed horned lizards from 

this large part, in contrast to the small, 

isolated part (Part SE–2) north of 

Interstate 8. 

In sum, for the Southeastern 

Population, assuming the identified 

potential barriers are complete barriers 

(which is not likely, see above, although 

we do not know how permeable they 

may be), and assuming the most 

conservative density of 0.3 flat-tailed 

horned lizards per ha (0.1 per ac), we 

calculate that about 97 percent of the 

area is in large enough blocks that the 

populations of flat-tailed horned lizards 

therein are not likely to be affected by 

threats associated with small 

populations. However, much of the 

dune areas of the Gran Desierto de Altar 

are likely to have few, if any, flat-tailed 

horned lizards. Nevertheless, given the 

limited amount of manmade 

development within large areas of the 

Southeastern Population and the fact 

that about 97 percent of the area 

contains large blocks of flat-tailed 

horned lizard habitat, the Southeastern 

Population is not substantially 

composed of ‘‘small populations.’’ 

Therefore, we conclude the flat-tailed 

horned lizards in the Southeastern 

Population are not substantially 

threatened now or in the foreseeable 

future by effects associated with barriers 

that subdivide populations or the 

deleterious effects that may follow. 

For the Western, Eastern, and 

Southeastern Population areas 

combined, about 91 percent of the 

1,585,157 ha (3,917,008.25 ac) area is in 

large enough blocks that the populations 

of flat-tailed horned lizards therein are 

not likely to be affected by threats 

associated with small populations. As 

mentioned above, the part that is 

primarily composed of the Gran 

Desierto de Altar is very large; it makes 

up about 45 percent of the total area of 

the three populations combined and is 

larger than the Western and Eastern 

Population areas combined. Without the 

Gran Desierto part, about 84 percent of 

the total area is in parts that are likely 

to contain populations large enough to 

avoid deleterious effects associated with 

small populations. Thus, despite not 

having complete population data for the 

species throughout its range, through 

this analysis of size of the habitat areas, 

and application of conservative 

estimates (the smallest density value 

within the estimated range, and the 

largest population size value below 

which we are considering (for our 

analysis of this species) a ‘‘small 

population’’), we conclude that the flattailed horned lizard populations are not 

small and the species is not habitatlimited in the United States or Mexico. 

In conclusion, this evaluation 

suggests that despite the presence of 

multiple barriers that potentially divide 

the Western, Eastern, and Southeastern 

Population areas into smaller parts, 

most of the areas within the current 

distribution outside of the greater 

Coachella Valley are in parts large 

enough to support populations of flattailed horned lizards that are large 

enough to avoid deleterious effects 

associated with small populations. 

Therefore, the implied meaning of 

fragmentation is not a significant threat 

to the flat-tailed horned lizard 

throughout its range or within the 

Western, Eastern, and Southeastern 

Population areas. 

Coachella Valley Population 

The Coachella Valley Population 

differs from the other three in that it has 

been highly affected by past agricultural 

development and recent (and 

continuing) urban development (see 

Factor A). As mentioned previously, the 

only areas with recent detections of flattailed horned lizards are within the 

Thousand Palms and the Dos Palmas 

reserves. The precise amount of habitat 

that is occupied is not known, but based 

on an analysis of habitats within the 

Coachella Valley MSHCP plan, the 

Thousand Palms and Dos Palmas 

reserves are anticipated to be 1,707 ha 

(4,219 ac) and 2,078 ha (5,134 ac), 

respectively (Table 2). Of these, 94 

percent of the Thousand Palms reserve 

is already in protected status, while 34 

percent of the Dos Palmas reserve is 

protected (Table 2). Using the 

conservative estimated density of 0.3 

adult flat-tailed horned lizards per ha 

(0.1 per ac), neither of these reserves— 

presently or even at their anticipated 

size—is large enough to support a ‘‘large 

population.’’ Thus, these two small, 

fully isolated occurrences may be more 

likely to experience deleterious effects 

associated with small population sizes. 

In our evaluation of the monitoring and 

management of flat-tailed horned lizard 

populations and habitat expected under 

the Coachella Valley MSCHP (USFWS 

2008, Appendix A, p. 322), we stated: 

‘‘The proposed Plan provides reasonably 

competent direction for monitoring and 

adaptive management, but not all details 

can be anticipated beforehand and 

much would depend on how the 

monitoring and adaptive management is 

implemented. We assume the 

implementation of the monitoring and 

adaptive management plan would 

strictly adhere to the guidance in the 

Plan. The extra pressures of edge effects 

and invasive species may be buffered by 

management to prevent pressures that 

would push a naturally low population 

to extinction. Populations are expected 

to increase in numbers again if 

anthropogenic factors are effectively 

managed.’’ Additionally, as noted above, 

even small populations in small habitat 

areas may be viable in the long term; 

however, for the purposes of this 

analysis (to be conservative) we are 

assuming they are not. Therefore, we 

conclude the continued existence of the 

Coachella Valley Population is likely to 

face significant threats within the 

foreseeable future. 

Summary for Barriers and Small 

Populations 

Past assessments identified 

‘‘fragmentation’’ as a threat to the flattailed horned lizard. Fragmentation, as 

a term used in conservation biology, is 

ambiguous. To address the implied 

meaning of the term, we assessed 

potential barriers and the resulting flattailed horned lizard population sizes 

throughout the species’ range. 

Barriers prevent movement of 

individuals and, thus, restrict or prevent 

gene flow. As such, barriers subdivide 

larger populations into smaller ones. For 

vertebrate species, populations of more 

than about 7,000 individuals are not 

likely to be affected by deleterious 

intrinsic and extrinsic forces and factors 


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over the long term. Not all potential 

barriers are complete barriers and some 

potential barriers may be 

‘‘semipermeable.’’ Movement of 1 to 10 

individuals per generation across a 

semipermeable barrier is likely enough 

to maintain connectivity between 

populations. 

The populations of flat-tailed horned 

lizards in the Western, Eastern, and 

Southeastern Population areas are 

potentially divided by artificial 

manmade barriers. Flat-tailed horned 

lizards are difficult to detect, and 

population estimates are limited to a 

few, well-surveyed areas. Density 

estimates of adult flat-tailed horned 

lizards range from as low as 0.3 

individuals per ha (0.1 per ac) to as 

much as 4.4 individuals per ha (1.8 per 

ac), depending on the analysis used (see 

Background section). Our evaluation of 

the range of the species suggests that the 

Western, Eastern, and Southeastern 

Population areas were divided by 

manmade barriers into 12, 9, and 13 

‘‘parts,’’ respectively. Using the lowest 

(most conservative) estimates of 0.3 

adult flat-tailed horned lizards per ha 

(0.1 per ac), we calculated that the 

Western, Eastern, and Southeastern 

Population areas had about 83 percent, 

73 percent, and 97 percent of the areas 

(respectively) in parts likely to support 

populations that are large enough to 

avoid deleterious effects associated with 

small populations. For those values, we 

assumed all identified potential barriers 

were complete barriers; however, the 

circumstance for each individual part 

varies, and some of the potential 

barriers we identified are likely to not 

be complete barriers. As such, some of 

the parts we identified as separate may 

contain populations of flat-tailed horned 

lizards that are actually connected with 

neighboring populations. Thus, we 

believe these percentages are 

conservative because we used the 

conservative density estimates and the 

parts, as analyzed, may not actually 

contain separate populations of flattailed horned lizards. 

Additionally, the Coachella Valley 

Population area has numerous barriers 

and the remaining flat-tailed horned 

lizards are restricted to two small areas. 

The populations of flat-tailed horned 

lizards in these areas are likely to be 

affected by threats associated with small 

population size. 

We again note that we have very little 

specific data regarding whether or to 

what degree populations of flat-tailed 

horned lizards are actually being 

affected by threats associated with small 

population size. Even for the flat-tailed 

horned lizard population in Part SE–2, 

which may be exhibiting genetic 

deterioration because of isolation and 

small population size, we do not have 

direct information on the status of that 

population. Thus, based on information 

from the scientific literature on the 

potential effects of small population 

size, for the purposes of this threats 

assessment, we have assumed these 

‘‘small’’ populations of flat-tailed horned 

lizards are being substantially affected 

by threats associated with small 

population size or are likely to be 

substantially affected by threats 

associated with small population size in 

the foreseeable future. 

Even so, our evaluation suggests that 

despite the presence of multiple barriers 

that potentially divide the Western, 

Eastern, and Southeastern Population 

areas into smaller parts, most of the area 

within the current distribution outside 

of the greater Coachella Valley are in 

parts large enough to support 

populations of flat-tailed horned lizards 

larger than 7,000 individuals, meaning 

they are not habitat-limited and are not 

likely to suffer from threats associated 

with small populations now or in the 

foreseeable future. As such, the implied 

meaning of term ‘‘fragmentation’’ is not 

a threat to the flat-tailed horned lizard 

throughout its range. 

Edge Effects 

Another effect associated with 

fragmentation and barriers is that there 

are more habitat edges. When two 

ecosystems are separated by an abrupt 

transition (an ‘‘edge’’), there may be an 

interaction between two adjacent 

ecosystems, known as an edge effect 

(Murcia 1995, p. 58). As noted 

previously, predation of flat-tailed 

horned lizards may be greater adjacent 

to urban and agricultural areas (Barrows 

et al. 2006, p. 486), and may extend 

several hundred meters (yards) from the 

neighboring developed area (Young and 

Young 2005, p. 7). Additionally, 

invasive, nonnative plants may also 

occur at higher densities along road 

edges (Gelbard and Belnap 2003, p. 

420); however, native plant growth may 

also increase along roads (Lightfoot and 

Whitford 1991, p. 310). Increased plant 

growth may lead to increased seeds, 

which may benefit harvester ants, the 

primary food of the flat-tailed horned 

lizard. 

Additionally, the invasive, nonnative 

Argentine ant (Linepithema humile) has 

been found to be a problem for coastal 

horned lizards (Phrynosoma coronatum) 

in habitat edges (Suarez et al. 1998, p. 

2041; Suarez and Case 2002, p. 291). 

However, Argentine ants do not tolerate 

hot, arid conditions (Holway et al. 2002, 

p. 1610) and are not known to be a 

problem away from habitat edges in flattailed horned lizard habitat (Barrows et 

al. 2006, p. 492); thus, we expect the 

effect of Argentine ants to be limited to 

areas adjacent to edges that have water 

sources. 

Although edge effects may result in 

increased mortality of flat-tailed horned 

lizards, primarily resulting from 

increased levels of predation, the area 

affected is within several hundred 

meters (yards) of the edge. As discussed 

in the ‘‘Barriers and Small Populations’’ 

section, much of the area occupied by 

the flat-tailed horned lizard is in large 

areas (or ‘‘parts’’). In such areas or parts, 

the ratio of linear edge compared to the 

areal size of the part is small, meaning 

large parts have larger ‘‘interior’’ areas 

that are not affected by edge effects. As 

such, the populations of flat-tailed 

horned lizards in large areas or parts are 

less likely to be substantially affected by 

edge effects. Conversely, smaller parts 

have a smaller percentage of their area 

that is likely to be affected by edge 

effects. As such, flat-tailed horned lizard 

populations in the small parts are more 

likely to be substantially affected by 

edge effects. 

Because ‘‘parts’’ are created by 

infrastructural elements associated with 

urban and agricultural development, the 

small ‘‘parts’’ are more likely near urban 

and agricultural areas. Moreover, 

because edge effects are most 

pronounced near urban and agricultural 

development, the flat-tailed horned 

lizards in small parts are the most likely 

to be substantially affected by edge 

effects. Thus, edge effects are an added 

threat faced by flat-tailed horned lizard 

populations in the small parts. As such, 

edge effects are not additional threats to 

the flat-tailed horned lizard, but instead 

are part of the threats faced by flat-tailed 

horned lizard populations in small 

parts. Therefore, like small population 

size, we do not believe edge effects are 

a significant threat to the flat-tailed 

horned lizard now or in the foreseeable 

future. 

Pesticide Spraying 

Past assessments identified the 

spraying of pesticides as part of the 

California Department of Food and 

Agriculture’s Curly Top Virus Control 

Program as a threat to the flat-tailed 

horned lizard, mainly in the East Mesa, 

West Mesa, and Yuha Desert (58 FR 

62627; FTHLICC 2007, p. 20). As 

described in the program’s 

environmental assessment (BLM 2007b, 

p. 8), beet curly top virus is a disease 

of commercially important crops, and 

also backyard vegetable and flower 

gardens. The only known vector of beet 

curly top virus is an insect known as the 

sugar beet leafhopper (Circulifer 


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tenellus). The Curly Top Virus Control 

Program includes aerial and groundbased spraying of malathion, which is 

the only product registered in California 

for the control of sugar beet leafhopper 

on rangeland (BLM 2007b, p. 15). The 

areas to be sprayed (treated) are 

prioritized; treatment priorities are 

given to areas subject to perennial virus 

infection, areas sustaining significant 

infection from the previous year, and 

areas with the highest current sugar beet 

leafhopper populations (BLM 2007b, p. 

8). 

Available information in the scientific 

literature regarding the effects of 

malathion, a broad-spectrum 

insecticide, on lizard species are 

equivocal, with some suggesting that 

malathion has substantial deleterious 

effects on lizards (such as O ̈ zelmas and 

Akay 1995, pp. 730–737; Khan 2003, pp. 

821–825; Khan 2005, pp. 77–81), and 

others suggesting the effects are less 

pronounced (such as Holem et al. 2006, 

pp. 111–116; Holem et al. 2008, pp. 92– 

98). We are not aware of any studies 

examining the effects of malathion on 

horned lizard species. 

Flat-tailed horned lizards are 

insectivorous, primarily feeding on 

harvester ants. If the food source for the 

flat-tailed horned lizard is substantially 

affected by the spraying of malathion, 

the flat-tailed horned lizard could be 

affected. To address this concern, 

implementation of the Curly Top Virus 

Control Program in the Imperial Valley 

in 1991 included monitoring of 

harvester ant colonies. Results showed 

malathion killed worker ants on the 

surface at the time of the spraying, 

negatively affecting ant colonies 

temporarily; however, it also showed 

that the colonies, with the queen and 

other workers below ground, rapidly 

recovered (Peterson in litt. 1991, p. 10; 

see also BLM 2007b, p. 75). Although 

that monitoring was cursory, the 

information suggests that spraying is not 

likely to substantially affect the primary 

food source of the flat-tailed horned 

lizard now or in the foreseeable future. 

Even if flat-tailed horned lizards or 

harvester ants are affected by malathion, 

the Curly Top Virus Control Program 

includes measures to limit its impact. 

The threat from pesticide spraying has 

been reduced by avoidance and 

minimization measures incorporated in 

the program since the publication of the 

1993 proposed rule to list the flat-tailed 

horned lizard, including the following 

(BLM 2007b, p. 33): 

(1) No malathion treatments shall 

occur in designated flat-tailed horned 

lizard Management Areas as set forth in 

the Flat-tailed Horned Lizard Rangewide Management Strategy. 

(2) Application of malathion within 

the geographic range of the [flat-tailed 

horned lizard] will consist of no more 

than a single treatment per given area 

per year. 

(3) All application [within flat-tailed 

horned lizard habitat] will be aerial. No 

spraying from off-road vehicles or use of 

off-road vehicles on other than 

designated roads will be used within 

[flat-tailed horned lizard] habitat. 

Beyond the avoidance and 

minimization measures incorporated 

into the Curly Top Virus Control 

Program, aerial spraying is conducted 

infrequently in the Imperial Valley— 

aerial treatments have been necessary 

only twice in the 9 years prior to the 

2007 environmental assessment (BLM 

2007b, p. 9). Additionally, the State’s 

program administrator for the Curly Top 

Virus Control Program indicated that 

although the program will continue in 

the region, the frequency of aerial 

treatments in the foreseeable future is 

anticipated to decrease; instead, 

treatments are more likely to be 

implemented via ground-based spraying 

in areas near agriculture outside of flattailed horned lizard habitat (R. Clark, 

California Department of Food and 

Agriculture, pers. comm., 2010). 

Because of the avoidance and 

minimization measures incorporated 

into the Curly Top Virus Control 

Program, and because of the likely 

limited effects to the flat-tailed horned 

lizard and its food source at the levels 

that the program is expected to be 

implemented, we conclude that 

implementation of the Curly Top Virus 

Control Program is not a threat to the 

flat-tailed horned lizard. 

Vehicle Activity 

Flat-tailed horned lizards may be 

directly affected by vehicle activity. The 

assessments in the 1993 and 2003 

documents (58 FR 62624 and 68 FR 331, 

respectively) identified impacts from 

vehicles as a threat to the species, 

especially OHV activity. Impacts of 

vehicle activity on flat-tailed horned 

lizard habitat are addressed in Factor A, 

above. Additionally, individual flattailed horned lizards may be killed— 

crushed—by vehicle activity. As 

discussed above, because flat-tailed 

horned lizards are unlikely to flee from 

oncoming traffic, when flat-tailed 

horned lizards are on paved roadways 

they are likely to be killed by any 

vehicle activity. Additionally, flat-tailed 

horned lizards may be killed by vehicles 

operating off paved roads, including 

vehicle activity on established dirt or 

gravel roads and trails, or vehicle 

activity off established roads and trails 

(OHV activity as defined in Factor A) 

(Muth and Fisher 1992, p. 33). Vehicle 

drivers may not see or recognize flattailed horned lizards because their 

cryptic coloration makes them difficult 

to spot or they may be interpreted as 

rocks. Moreover, the species’ propensity 

to freeze rather than flee makes them 

particularly susceptible. Impacts from 

vehicles are more likely when the 

lizards are on or near the surface; 

hibernating flat-tailed horned lizards are 

generally buried deep enough that they 

are not crushed by vehicles driving over 

them (Grant and Doherty 2009, p. 511). 

Additionally, most of the OHV activity 

in the region occurs during the cooler 

times of the year (Wone 1992, pp. 4–5), 

suggesting that fewer flat-tailed horned 

lizards would be on the surface during 

peak times of OHV activity. 

Moreover, the density of flat-tailed 

horned lizards is apparently naturally 

low. Even at the highest estimated 

density of 4.4 adult flat-tailed horned 

lizards per hectare (1.8 per acre) (see 

Background), which is equivalent to 

0.00044 individuals per square meter 

(0.00004 per square foot), the chances of 

a flat-tailed horned lizard being run over 

by a vehicle is low, even in areas of high 

OHV activity (for example, see Nicola 

and Lovich 2000, pp. 208–212). 

Nevertheless, mortality of flat-tailed 

horned lizards resulting from OHV 

activity has been documented, even in 

areas of low OHV use. For example, in 

an area closed to OHV traffic, 2 of the 

42 radio-tagged flat-tailed horned 

lizards were killed by illegal OHV 

activity, and 1 was killed by a vehicle 

on a paved road (Muth and Fisher 1992, 

pp. 18 and 33). However, in 

comparison, in that same study, 16 of 

the 42 radio-tagged flat-tailed horned 

lizards were depredated over the same 

period (Muth and Fisher 1992, p. 33). 

In the past, OHV activity along the 

United States-Mexico boundary (border) 

from Border Patrol activity and other 

border-related OHV traffic has been 

specifically identified as a threat. 

Border-related OHV activity is part of 

our definition of OHV activity and is 

covered above. Moreover, since 2008, 

the U.S. Customs and Border Protection 

constructed the ‘‘border fence,’’ which is 

a vehicle and, in some areas, pedestrian 

barrier, plus associated infrastructure, in 

certain areas between the United States 

and Mexico. Although some areas of the 

border are not fenced, the areas of flattailed horned lizard habitat along the 

border are fenced (USCBP 2008a, p. 1– 

5; USCBP 2008b, p. 2–4; Rorabaugh 

2010, p. 181). Evidence suggests the 

border fence has reduced illegal crossborder traffic and associated OHV 

activity (Rorabaugh 2010, p. 190), 

thereby reducing the amount of 


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potential impacts to flat-tailed horned 

lizards along the border from illegal 

trans-border OHV activity and 

subsequent law-enforcement OHV 

activity by the Border Patrol. 

Because the flat-tailed horned lizard 

occurs naturally in low densities, roads 

are generally widely separated, and 

OHV activity is only intense in a few 

areas, the chances that a flat-tailed 

horned lizard being crushed by vehicle 

activity is low over the majority of the 

species’ range; therefore, we conclude 

that vehicle activity is not a substantial 

threat to the species throughout its 

range, nor do we expect it to become a 

significant threat in the foreseeable 

future. 

Drought and Climate Change 

The assessments in the 1993 and 2003 

documents (58 FR 62624 and 68 FR 331, 

respectively) included drought as a 

potential threat to the flat-tailed horned 

lizard. Additionally, changes in weather 

patterns associated with global climate 

change, particularly the timing and 

amount of rainfall in this arid region, 

are a potential threat to the species. We 

examine both below. 

Prolonged periods of atypically low 

rainfall (drought) may potentially affect 

flat-tailed horned lizard by affecting its 

food chain (see Background section). 

Plants produce fewer seeds during 

periods of low rain, leading to a 

reduction in the number of foraging ants 

(Tevis 1958, p. 698), which reduces the 

amount of food available for flat-tailed 

horned lizards. However, harvester ant 

colonies do appear to survive prolonged 

periods of drought (Tevis 1958, p. 701; 

Whitford et al. 1999, p. 165), indicating 

that flat-tailed horned lizards will have 

some food available. Depressed flattailed horned lizard populations 

associated with reduced abundance of 

ants are known to have rebounded after 

ant populations returned, even in small 

populations of flat-tailed horned lizards 

(Barrows and Allen 2009, p. 314). Thus, 

we do not expect droughts to 

permanently affect large populations of 

flat-tailed horned lizards, although 

droughts may contribute to the 

extirpation of small populations. 

Because about 91 percent of the area 

occupied by flat-tailed horned lizards 

are in areas large enough to support 

large populations (see ‘‘Barriers and 

Small Populations’’ section above), and 

because evidence shows that even small 

populations of flat-tailed horned lizards 

have survived periods of drought (see 

above), this suggests that it is not likely 

that all of the 9 percent of the ‘‘small 

population’’ area would be affected by 

drought. Therefore, we do not anticipate 

drought to be a significant threat to the 

species throughout its range. 

Current climate change predictions 

for terrestrial areas in the Northern 

Hemisphere indicate warmer air 

temperatures, more intense 

precipitation events, and increased 

summer continental drying (Field et al. 

1999, pp. 1–63; Cayan et al. 2006, pp. 

1–47; Meehl et al. 2007, pp. 747–843). 

Assessments for the Sonoran Desert are 

few, but the region is expected to warm 

(IPCC 2007, p. 887). Indeed, since about 

the 1970s, the region appears to have 

experienced ‘‘widespread warming 

trends in winter and spring, decreased 

frequency of freezing temperatures, 

lengthening of the freeze-free season, 

and increased minimum temperatures 

per winter year’’ (Weiss and Overpeck 

2005, p. 2065). Further, if summertime 

temperatures increase in the already 

typically hot Sonoran Desert, 

temperatures may exceed the ability for 

many animals, including the flat-tailed 

horned lizard, to survive. For example, 

Sinervo et al. (2010, p. 895) suggest that 

Phrynosomatid lizards (the family to 

which flat-tailed horned lizards belong) 

are susceptible to increased risk of 

extinction because of their intolerance 

to an increase in environmental 

temperatures. Increased temperatures 

would result in longer periods of time 

when the flat-tailed horned lizard 

would be forced to seek cooler 

microclimates (shade, burrows), leaving 

less time available in the day for feeding 

or other necessary activities (see also 

Huey et al. 2010, pp. 832–833). 

However, we are not aware of any 

information indicating that the flattailed horned lizard is being 

substantially affected by a reduced 

frequency of cold temperatures or 

increased frequency of high 

temperatures, or that it will be 

substantially affected in the foreseeable 

future. 

Additionally, precipitation may 

become more variable (Weiss and 

Overpeck 2005, p. 2065). Increased 

severity, frequency, or duration of 

droughts may exceed the resiliency of 

the flat-tailed horned lizard, or the 

species in the food chain upon which it 

depends. In contrast, models suggest 

that the frequency and intensity of El 

Nin ̃ o-Southern Oscillation events may 

increase as a result of global climate 

change (Field et al. 1999, p. 10), which 

may lead to increased rainfall in some 

portions of the species’ range. Although 

typically considered a benefit, increased 

rainfall may negatively affect harvester 

ant abundance and thus negatively 

affect flat-tailed horned lizards, at least 

in some areas (Barrows and Allen 2009, 

p. 312). Also, increased rainfall may 

disproportionately promote growth of 

nonnative, invasive plant species, 

which can increase the prevalence of 

wildland fire and be a physical 

hindrance to flat-tailed horned lizard 

locomotion (see ‘‘Invasive, Nonnative 

Plants’’ section in the Factor A 

discussion, above). 

Thus, the effects associated with 

global climate change may affect the 

flat-tailed horned lizard, but at this 

time, the level of uncertainty in climate 

predictions is high. Moreover, we do not 

know whether such a change would 

substantially affect the flat-tailed horned 

lizard. While we recognize that climate 

change is an important issue with 

potential effects on species and their 

habitats, we lack adequate information 

to make accurate predictions regarding 

its effects to the flat-tailed horned 

lizard. We do not have any evidence to 

suggest that the flat-tailed horned lizard 

is being substantially affected by climate 

change at this time, or will be within the 

foreseeable future. Therefore, the effects 

of climate change are not a significant 

threat at this time. 

Summary of Factor E Threats 

For Factor E, we assess the natural 

and manmade threats that affect the 

status of the species. Small populations 

may be disproportionately affected by 

extrinsic and intrinsic factors that 

reduce population size. Given that 

historical agricultural and urban 

development destroyed large swaths of 

potential flat-tailed horned lizard 

habitat, we assessed whether the 

remaining populations are large enough 

to likely avoid the deleterious effects 

associated with small populations. 

Within the Coachella Valley Population 

area, where habitat destruction has 

continued (see Factor A), flat-tailed 

horned lizards are now found only in 

two small locations and may be more 

likely to be affected by the deleterious 

effects associated with small 

populations. Using conservative 

estimates of flat-tailed horned lizard 

density in combination with the size of 

the Western, Eastern, and Southeastern 

Populations areas (as a whole), we 

conclude that each is large enough to 

support populations that are not likely 

to be affected by the deleterious effects 

associated with small populations. 

However, the Western, Eastern, and 

Southeastern Populations areas have 

within them potential manmade barriers 

(canals, roads, railways) that may 

further act as complete barriers or 

semipermeable barriers that subdivide 

the populations into smaller 

subpopulations. Thus, we assessed 

whether the areas created by these 

potential barriers were large enough to 


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likely support populations 

(subpopulations) that were likely greater 

than 7,000 adult individuals. Using the 

most conservative flat-tailed horned 

lizard density estimate of 0.3 individual 

adults per hectare (0.1 per acre), which 

is the lowest value in the range of 

estimates that extends to 4.4 individuals 

per hectare (1.8 per ac), and assuming 

(1) all potential barriers are complete 

barriers, which is unlikely because some 

barriers likely allow some movement of 

individuals (see above) and only 1 to 10 

individuals per generation are needed to 

maintain population connectivity; and 

(2) 7,000 adults is the threshold above 

which a population is large enough to 

likely avoid the deleterious effects 

associated with small populations, 

which is at the high end of the range of 

estimated population thresholds, we 

concluded that about 83 percent, 73 

percent, and 97 percent of the Western, 

Eastern, and Southeastern Population 

areas (respectively), and about 91 

percent of the area overall, are in large 

enough blocks that the populations of 

flat-tailed horned lizards within them 

are not likely to be affected by threats 

associated with small populations. 

Thus, the vast majority of the current 

distribution of the flat-tailed horned 

lizard occurs in blocks of habitat large 

enough to support populations greater 

than 7,000 adults; therefore, small 

population size is not a threat to the flattailed horned lizard and the species is 

not habitat-limited. 

Pesticide spraying associated with the 

Curly Top Virus Control Program is not 

a threat to the flat-tailed horned lizard 

because of the small area within the 

range of the species over which it is 

likely to occur, the avoidance and 

minimization measures built into the 

program, and the likely limited effects 

of spraying on the flat-tailed horned 

lizard and its harvester ant food source. 

Additionally, vehicle activity—on 

paved roads, non-paved roads, and offroad—is not a substantial threat to the 

species because the chances of a flattailed horned lizard being crushed by 

vehicle activity are low over the 

majority of the species’ range. Drought 

is also not likely to be a substantial 

threat to the species throughout its 

range. Climate change could potentially 

affect flat-tailed horned lizards, but the 

future effects of climate change are 

uncertain. Moreover, no substantial 

effects of climate change to the flattailed horned lizard are known at this 

time. Therefore, the effects of climate 

change are not a significant threat at this 

time. 

We do not consider the potential 

threats analyzed above to be substantial 

threats to the flat-tailed horned lizard, 

either individually or in combination. 

Therefore, based on our review of the 

best available scientific and commercial 

information we find the flat-tailed 

horned lizard is not threatened by 

natural or manmade factors affecting its 

continued existence, either now or in 

the foreseeable future. 

Conservation Efforts 

Before we may determine whether a 

species should be listed as endangered 

or threatened, section 4(b)(1)(A) of the 

Act requires that we take into account 

those efforts, if any, being made by any 

State or foreign nation, or any political 

subdivision of a State or foreign nation, 

to protect the flat-tailed horned lizard. 

Of particular note is the Interagency 

Conservation Agreement between and 

among participating State and Federal 

agencies implementing the Rangewide 

Management Strategy, which is 

discussed in detail in the Background 

section. Other conservation efforts 

include regulatory mechanisms, which 

are discussed under Factor D in the 

Summary of Factors Affecting the 

Species section. 

On April 3, 2008, the Secretary of the 

U.S. Department of Homeland Security 

(DHS), pursuant to his authority under 

section 102(c) of the Illegal Immigration 

Reform and Immigrant Responsibility 

Act (8 U.S.C. 1103 note) (IIRIRA), 

exercised his authority to waive certain 

environmental and other laws in order 

to ensure the expeditious construction 

of tactical infrastructure along the 

United States-Mexico Border (i.e., the 

‘‘border fence’’) (73 FR 18293). As such, 

activities associated with construction 

and operation of the border fence are 

exempt from regulatory mechanisms 

described in Factor D. These activities 

also do not need to comply with the 

avoidance, minimization, or mitigation 

measures described in the Rangewide 

Management Strategy. However, the 

Secretary committed DHS to continue to 

protect valuable natural and cultural 

resources (USCBP 2008a, p. ES–1). As a 

result, the U.S. Customs and Border 

Protection prepared Environmental 

Stewardship Plans for the portions of 

the United States-Mexico border that 

fall within the current distribution of 

the flat-tailed horned lizard (USCBP 

2008a, 2008b, entire documents). United 

States Customs and Border Protection 

has expressed an intent to work in a 

collaborative manner with local 

government, State and Federal land 

managers, and the interested public to 

identify environmentally sensitive 

resources and develop appropriate best 

management practices to avoid or 

minimize adverse impacts resulting 

from the installation of tactical 

infrastructure (USCBP 2008b, p. ES–1), 

including certain conservation measures 

from the Rangewide Management 

Strategy that will be implemented to the 

fullest extent applicable and practicable. 

Thus, implementation of the 

Environmental Stewardship Plans is 

best considered a conservation effort for 

the species. 

Finding 

The flat-tailed horned lizard 

monitoring data on which we relied in 

this document are more robust than the 

data we relied on in our 1993 proposed 

rule (58 FR 62624) and our earlier 

withdrawal documents (62 FR 37852, 68 

FR 331, and 71 FR 36745), thus enabling 

us to conclude with increased 

confidence that flat-tailed horned lizard 

populations in the Management Areas 

are not low in abundance or declining. 

Although no comparable historical 

abundance data exist, our analysis 

suggests that occupancy of flat-tailed 

horned lizards within survey areas is 

relatively high. Density estimates 

obtained through the new survey 

methodology are roughly in the same 

range provided by previous estimates, 

suggesting no marked declines in 

density since the late 1990s. Although 

additional surveys are needed before the 

recently collected data can provide 

long-term trend information, the shortterm data do not currently indicate 

declines. Because of data limitations, we 

cannot extrapolate the data rangewide; 

however, for the Management Areas 

surveyed (see Population Dynamics 

under the Background section), the best 

available scientific information suggests 

that population levels are not low and 

not declining. In other words, 

recognizing that the areas surveyed 

compose only a fraction of the overall 

range of the flat-tailed horned lizard, it 

is our interpretation that the available 

population data (alone and without 

considering potential threats) do not 

support a conclusion that the species is 

in danger of extinction. Additionally, 

despite the lack of long-term trend data, 

the general agreement of the recent data 

with the older data from the available 

scientific literature lead to our 

interpretation that the available 

population data (alone and without 

considering potential threats) do not 

support a conclusion that the species is 

likely to become endangered within the 

foreseeable future. 

Although our past assessments 

suggest that historical loss of habitat 

resulted in artificial barriers, except for 

the Coachella Valley Population, the 

information currently available indicate 

otherwise. We conclude that the 

manmade barriers resulting from 


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historical agricultural and urban 

development merely expanded preexisting natural barriers. This 

conclusion is based on genetic data that 

show separation of the Western, Eastern, 

and Southeastern Populations occurred 

prior to the development of the region 

more than a century ago. Genetic data 

also suggest that flat-tailed horned 

lizards in the Coachella Valley had 

limited connection with the Western 

Population; thus, the historical 

agricultural development northwest of 

the Salton Sea, along with the continued 

development in that region, has created 

an artificial barrier at this location. As 

such, the treatment of flat-tailed horned 

lizards in the Coachella Valley as a 

separate population is more an artifact 

of manmade activities than of natural 

divisions within the flat-tailed horned 

lizard population as a whole. 

Moreover, we determined herein that 

the Western, Eastern, and Southeastern 

Population areas (each as a whole) are 

not threatened by the factors associated 

with small population size and are not 

habitat-limited. Thus, ramifications of 

historical habitat loss are not likely to 

constitute a significant threat to the 

species within the foreseeable future in 

these populations. Additionally, 

because the majority of the Western, 

Eastern, and Southeastern Population 

areas are not subdivided by other 

barriers (such as canals, roads, railways, 

or border infrastructure), it is unlikely 

these areas would be substantially 

affected by the intrinsic and extrinsic 

factors, including edge effects, that may 

negatively affect small populations. 

In the Coachella Valley, the precise 

amount of habitat that is occupied is not 

known, but based on an analysis of 

habitats within the Coachella Valley 

MSHCP plan, the Thousand Palms and 

Dos Palmas reserves are anticipated to 

be 1,707 ha (4,219 ac) and 2,078 ha 

(5,134 ac), respectively (see Table 2). Of 

these, 94 percent of the Thousand Palms 

reserve is already in protected status, 

while 34 percent of the Dos Palmas 

reserve is protected (Table 2). These two 

small areas are unlikely to support flattailed horned lizard populations large 

enough to escape from being 

substantially affected by the intrinsic 

and extrinsic factors, including edge 

effects, that may negatively affect small 

populations. However, even if the 

Coachella Valley Population may be 

threatened by the effects of barriers and 

the intrinsic and extrinsic factors that 

may negatively affect small populations, 

the 3,785-ha (9,353-ac) Coachella Valley 

Population area makes up only about 

0.2 percent of the roughly 1,585,000 ha 

(3,916,600 ac) of the rest of the species’ 

range and about 0.8 percent compared 

to the 467,000 ha (1,154,000 ac) of the 

U.S. portion of that range, and the 

threats to the Coachella Valley 

population do not substantially threaten 

the species as a whole. 

Therefore, the effects to the species 

associated with the implied meaning of 

fragmentation—that is, the division of 

the species’ populations into smaller 

populations by the introduction of 

manmade barriers and the subsequent 

deleterious effects that may be 

associated with small population size— 

are not likely to constitute a substantial 

threat to the species now or within the 

foreseeable future. 

Within the United States, most of the 

area occupied by the species is under 

Federal or State control and overseen by 

agencies that are signatories to the 

Interagency Conservation Agreement 

and associated Rangewide Management 

Strategy. Although the Interagency 

Conservation Agreement is voluntary, 

several signatories—including the BLM, 

which is a major landowner within the 

U.S. portion of the range of the flattailed horned lizard—have incorporated 

aspects of the Rangewide Management 

Strategy into their planning documents, 

thus making them less voluntary 

because those plans implement existing 

regulatory mechanisms. Implementation 

of this strategy resulted in creation of 

five Management Areas that, as of 2009, 

total 185,653 ha (458,759 ac) of higher 

quality flat-tailed horned lizard habitat 

(Table 1). Management objectives also 

provide avoidance and minimization 

measures to reduce impacts from 

permitted projects and limit the 

development area within each 

Management Area to 1 percent. 

Additionally, implementation of the 

Rangewide Management Strategy calls 

for monitoring, management, land 

acquisition, and research; further, it 

promotes coordination with 

governmental and non-governmental 

groups in Mexico to provide 

conservation benefit for the species in 

that country. The tasks identified by the 

Rangewide Management Strategy have 

been consistently implemented by 

signatory agencies per the Rangewide 

Management Strategy’s schedule. Thus, 

we conclude the conservation efforts 

implemented by signatories of the 

Interagency Conservation Agreement 

and associated Rangewide Management 

Strategy reduce the impact of existing 

threats in the United States and promote 

actions that benefit the flat-tailed 

horned lizard throughout its range, 

including Mexico. 

Threats to flat-tailed horned lizards 

associated with development activities 

are reduced or limited by the 

Interagency Conservation Agreement on 

signatory lands, particularly within 

Management Areas. Additionally, 

threats to the species and its habitat in 

areas outside of the Management Areas 

are likely restricted by the limited 

amount of water available in this arid 

region and remoteness of much of the 

habitat, especially in Mexico. Less 

remote areas, such as the Coachella 

Valley, Borrego Springs, Yuma, San Luis 

de Colorado, and Puerto Pen ̃ asco areas, 

are more likely to have urban or 

agricultural development; however, 

impacts in these areas are anticipated to 

be small relative to the amount of 

available habitat throughout the species’ 

current distribution. 

Development associated with new 

energy facilities is likely to be reduced 

or limited by continued implementation 

of the Rangewide Management Strategy. 

Although few energy development 

projects have been fully permitted to 

date, we anticipate more will be 

proposed in the foreseeable future. 

Within the range of the flat-tailed 

horned lizard, we expect development 

within the Western Population between 

Interstate 8 and the existing railway 

(Part W–5) to reduce the already limited 

connectivity across Interstate 8, 

although South Fork Coyote Wash is 

expected to continue to be a potential 

corridor for flat-tailed horned lizard 

movement. We conclude the remaining 

habitat in the Western Population area 

(i.e., north of the railway and south of 

Interstate 8, including areas designated 

as Management Areas) is large enough 

to support flat-tailed horned lizard 

populations. Also, we expect the total 

acreage of potential development for 

renewable energy facilities to be small 

compared to the overall range of the 

species, including on private land. 

Additionally, on lands managed by 

signatory agencies to the Interagency 

Conservation Agreement, we expect the 

impacts to flat-tailed horned lizard 

habitat (whether inside or outside of 

designated Management Areas) will be 

further reduced because of the 

avoidance, minimization, and 

compensation measures of the 

Rangewide Management Strategy. 

Additionally, invasive, nonnative 

plants; vehicle activity, including OHV 

use near the United States-Mexico 

border and elsewhere; and pesticide 

spraying are not likely substantial 

threats to the species throughout its 

range. Predation is not likely a 

substantial threat in and of itself, but 

because several species that prey upon 

flat-tailed horned lizards likely occur in 

higher numbers near manmade areas, 

predation may contribute to the 

deleterious effects (as an ‘‘edge effect’’) 

associated with urban and agricultural 


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development and increase the level of 

impermeability of some semipermeable 

barriers. However, we do not expect 

increased levels of predation to 

substantially affect the species where it 

occurs in large ‘‘parts,’’ which is a 

majority of its range overall and within 

the Western, Eastern, and Southeastern 

Populations. Drought and climate 

change have the potential to affect flattailed horned lizards, but the magnitude 

of this threat, although unclear because 

of the high level of uncertainty 

associated with climate predictions, do 

not appear to be significant now or 

within the foreseeable future. 

Finally, we acknowledge we lack 

complete population data for the species 

throughout its range. However, through 

our analysis of size of the habitat areas, 

and application of conservative 

estimates (smallest density value within 

the estimated range, and largest 

population size value below which a 

population may be considered ‘‘small’’), 

we conclude that the flat-tailed horned 

lizard populations are not small and the 

species is not habitat-limited in the 

United States or Mexico at this time, nor 

do we expect the species to suffer from 

the deleterious effects of small 

population size in the foreseeable 

future. 

As required by the Act, we considered 

the species’ status relative to one or 

more of the five factors described in 

section 4(a)(1) of the Act, and the 

standards for listing as endangered or 

threatened throughout all of its range, 

and we considered the conservation 

efforts being made by any State or 

foreign nation. We have carefully 

assessed the best scientific and 

commercial data available regarding the 

past, present, and reasonably 

anticipated future threats faced by this 

species. Our analysis of the information 

pertaining to the five threat factors did 

not identify threats of imminence, 

intensity, or magnitude, either 

individually or in combination, to the 

extent that the species requires the 

protection of the Act throughout its 

range. Further, there is no information 

to suggest that the flat-tailed horned 

lizard population is declining or is in 

danger of becoming an endangered 

species in the foreseeable future. 

Therefore, we conclude that the species 

is not in danger of extinction or likely 

to become so within the foreseeable 

future and is not in need of the 

protections afforded by the Act at this 

time. 

Distinct Population Segment 

Under section 3(16) of the Act, a 

‘‘species’’ is defined as including not 

only the full, taxonomically defined 

species (i.e., the species as a whole, 

including any and all taxonomically 

defined subspecies) but also any 

(individual) subspecies and any distinct 

population segment (DPS) of a 

vertebrate species (16 U.S.C. 1532). On 

February 7, 1996, we, along with the 

National Marine Fisheries Service 

(National Oceanic and Atmospheric 

Administration—Fisheries), finalized a 

joint policy that addresses the 

recognition of DPSs of vertebrate species 

for potential listing actions (DPS policy) 

(61 FR 4722). The policy was developed 

(1) to implement the measures 

prescribed by the Act and Congressional 

guidance, (2) to allow for a more refined 

application of the Act to better reflect 

the biological needs of the taxon being 

considered, and (3) to avoid the 

inclusion of entities that do not require 

protective measures of the Act. As noted 

in the policy (61 FR 4725), 

Congressional guidance indicates that 

the authority to list DPSs is to be used 

‘‘sparingly.’’ 

As mentioned previously, we 

proposed to list the flat-tailed horned 

lizard—the entire species throughout its 

range—as a threatened species under 

the Act in 1993 (58 FR 62624). Since 

then, we conducted several additional 

analyses on the status of the species. 

From the 1993 proposed rule through 

the 2006 withdrawal document (71 FR 

36745), we noted the disjunct 

distribution of the species. Our 2003 

withdrawal document in particular 

explicitly addressed threats over four 

disjunct populations of the flat-tailed 

horned lizard that we identified in the 

United States, including: (1) The 

Coachella Valley in California, (2) the 

area west of the Salton Sea and Imperial 

Valley in California, (3) the area east of 

the Salton Sea and Imperial Valley in 

California, and (4) the Yuma Desert area 

in Arizona (68 FR 331). Additionally, 

we addressed separately the populations 

in Mexico. 

Also in our 2003 withdrawal 

document, we conducted a brief 

evaluation of a potential DPS for the 

Coachella Valley population (and only 

that population) in a response to a 

public comment (68 FR 336). We 

alluded to the population possibly being 

discrete (because it was disjunct), but 

we concluded that it was not significant 

within the meaning of the DPS policy 

because: (1) It was not ‘‘genetically, 

behaviorally, or ecologically unique’’; (2) 

it was not a ‘‘large population’’ (not 

necessarily as defined in the present 

document); and (3) it did not contribute 

‘‘individuals to other geographic areas 

through emigration.’’ Our response 

concluded, ‘‘If additional information 

becomes available that indicates the 

Coachella Valley population is 

biologically or ecologically significant 

pursuant to the [DPS policy], we may 

reconsider the status of the Coachella 

Valley population for the purpose of 

listing under the Act’’ (68 FR 336). 

Since then, additional information 

has become available on the genetic 

structure of flat-tailed horned lizard 

populations. Genetic data could, as 

indicated by the DPS policy (61 FR 

4725), inform our analysis of 

discreteness or significance. Therefore, 

in light of this new information and our 

past DPS analysis, we believe it is 

appropriate to evaluate potential DPSs 

of the flat-tailed horned lizard. 

The 1996 DPS policy specifies that we 

should address two elements prior to 

determining a population segment’s 

conservation status in relation to the 

Act’s standard for listing (61 FR 4725). 

These include: (1) The population 

segment’s discreteness from the 

remainder of the species to which it 

belongs, and (2) the population 

segment’s significance to the species to 

which it belongs. If we determine that 

a population segment meets the 

discreteness and significance standards, 

then we evaluate the level of threat to 

that population segment based on the 

five listing factors established by section 

4(a) of the Act to determine whether 

listing the DPS as either endangered or 

threatened is warranted. 

As described in Description of 

Specific ‘‘Populations’’ in the 

Background section above, the 

distribution of the flat-tailed horned 

lizard may be divided into four, 

physically (geographically) separated 

populations. Below, we evaluate these 

populations as potential distinct 

vertebrate population segments under 

our DPS policy. 

Discreteness 

Our DPS policy states that a vertebrate 

population segment may be considered 

discrete if it satisfies either of the 

following two conditions (61 FR 4725): 

(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 may 

provide evidence of this separation; or 

(2) It is delimited by international 

governmental boundaries within which 

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. 


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First Condition for Discreteness 

As noted at various points in the 

Background section, each of the four 

described populations—the Coachella 

Valley, Western, Eastern, and 

Southeastern Populations—are 

geographically separated from each 

other by natural barriers, manmade 

barriers, or both. The four populations 

of flat-tailed horned lizards are 

markedly separated from each other as 

a consequence of physical factors and 

each may be readily circumscribed and 

distinguished from the others. 

Therefore, the four populations of flattailed horned lizards meet the first 

condition for discreteness under our 

DPS policy. 

Additionally, the Coachella Valley 

Population, although more extensive in 

the recent past, now consists of two 

isolated occurrences, the Thousand 

Palms and Dos Palmas subareas. In the 

Summary of Factors Affecting the 

Species section, we considered the 

Thousand Palms and Dos Palmas 

subareas together as the Coachella 

Valley Population because both had the 

potential to share similar threats due to 

proximity, and both were covered by the 

Coachella Valley MSHCP. However, as 

noted, the genetic affinities of the Dos 

Palmas subareas are not known. Thus, 

combining the Thousand Palms and Dos 

Palmas subareas into the Coachella 

Valley Population was a grouping of 

convenience, adequate for evaluating 

threats, but not necessarily for assessing 

the population segments as potential 

DPSs. Thus, we consider the Thousand 

Palms and Dos Palmas subareas 

separately in our assessment of 

significance for the Coachella Valley 

Population. These two occurrences are 

markedly separated from each other and 

from the other populations of flat-tailed 

horned lizards as a consequence of 

physical factors (geographical 

separation); therefore, each meets the 

first condition for discreteness under 

our DPS policy. 

Second Condition for Discreteness 

The Western, Eastern, and 

Southeastern Populations extend across 

the international border with Mexico; as 

a result, each of these three populations 

could potentially be further divided into 

separate population segments under the 

policy’s second condition for 

discreteness. 

Application of the second condition 

for discreteness (61 FR 4725) with 

respect to the flat-tailed horned lizard 

tests for significant differences in: (1) 

The control of exploitation, (2) the 

management of habitat, (3) the 

conservation status, or (4) the regulatory 

mechanisms between the United States 

and Mexico. Below, we present a brief 

synopsis of these four categories, 

combining the last two. Please refer to 

the Summary of Factors Affecting the 

Species and Findings sections of this 

document for additional details. 

• Control of exploitation: We have no 

information suggesting that the flattailed horned lizard is significantly 

exploited on either side of the border 

(see the discussion under Factor B). 

• Management of habitat: 

Management of flat-tailed horned lizard 

habitat is essentially the same in the 

United States and in Mexico, although 

the underlying mechanisms differ. For 

example, in the United States large areas 

are protected as Management Areas 

through implementation of the 

Rangewide Management Strategy, and 

in Mexico large areas are protected as 

National Parks and Biosphere Reserves 

(see the discussion under Factor A). 

• Conservation status and regulatory 

mechanisms: In terms of actual 

designations of listing under the two 

countries’ respective species-protection 

laws, the conservation status differs 

between the United States and Mexico. 

In the United States, as a result of this 

withdrawal, the species is not listed; in 

Mexico, it is listed as a threatened 

species under the Official Mexican 

Norm NOM–059–ECOL–2001 

(SEMARNAT 2002, p. 134). However, in 

the United States, existing conservation 

efforts and regulatory mechanisms 

reduce the magnitude of potential 

threats to the species to a point where 

protections afforded by the Act are not 

necessary (see the discussion under 

Factor D and the Findings and 

Conservation Efforts sections). 

We conclude the second condition is 

not satisfied because no significant 

differences exist with respect to the flattailed horned lizard across the 

international boundary between the 

United States and Mexico. As such, the 

Western, Eastern, and Southeastern 

Populations described above are 

discrete in themselves and not with 

respect to the international boundary 

between the United States and Mexico. 

Conclusion for Discreteness per 1996 

DPS Policy 

We conclude that each of the four 

population segments analyzed (Western, 

Eastern, Southeastern, and Coachella 

Valley) meets the discreteness element 

of the 1996 DPS policy because each can 

be considered markedly separated from 

the other flat-tailed horned lizard 

populations as a consequence of 

physical factors (first condition for 

discreteness). Within the Coachella 

Valley Population, flat-tailed horned 

lizards in the Thousand Palms and Dos 

Palmas subareas also meet the 

discreteness element of the 1996 DPS 

policy under the first condition for 

discreteness. None of the population 

segments that cross the United StatesMexico boundary meet the second 

condition for discreteness. 

Significance 

If a population segment is considered 

discrete under one or more of the 

conditions described in our 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. Because 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 does provide four possible 

reasons why a discrete population may 

be significant. 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 four conditions (61 FR 4725): 

(1) Persistence of the discrete 

population segment in an ecological 

setting unusual or unique for the taxon, 

(2) Evidence that loss of the discrete 

population segment would result in a 

significant gap in the range of the 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 criteria to be 

considered significant. Furthermore, the 

list of criteria is not exhaustive; other 

criteria may be used as appropriate. 

Below, we assess whether the four 

discrete populations defined above are 

significant per our DPS policy. 

First Condition—Persistence of the 

discrete population segment in an 

ecological setting unusual or unique for 

the taxon. 

None of the four primary populations 

of flat-tailed horned lizard occurs in an 

ecological setting unusual or unique for 

the species. Although the ecological 


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setting varies across and within the 

range of the four populations, important 

ecological characteristics are similar 

among the four populations (see 

Background section). Climatic 

conditions across the range of the four 

populations are characterized by hot 

summer temperatures, mild winter 

temperatures, and little rainfall. Across 

the four populations, flat-tailed horned 

lizards are associated with creosotewhite bursage plant associations in 

areas characterized as sandy flats or 

valleys (see Setting and Habitat in the 

Background section). 

The ecological setting for the 

Coachella Valley Population as a whole, 

or the Thousand Palms and Dos Palmas 

subareas separately, are not markedly 

unusual or unique. The arenaceous 

(sandy) soils that support flat-tailed 

horned lizards in the Coachella Valley 

are derived from the surrounding areas 

and are compositionally different from 

those deposited by the Colorado River 

(van de Kamp 1973, p. 827), which is 

the source for much of the sand over a 

large portion of the range of the species 

(see Setting and Habitat in the 

Background section). However, the 

range of the flat-tailed horned lizard 

includes other areas where soils are 

derived from sedimentation from the 

surrounding areas, particularly the 

western edge of the Western Population 

where it meets lower extremities of the 

Peninsular Range (see Setting and 

Habitat in the Background section). 

Thus, evidence indicates this difference 

in substrate does not translate into an 

ecological setting unusual or unique for 

the flat-tailed horned lizard. We 

conclude that none of the four 

population segments meets the first 

significance condition. 

Second Condition—Evidence that loss 

of the discrete population segment 

would result in a significant gap in the 

range of a taxon. 

Loss of the Western, Eastern, or 

Southeastern population segment would 

result in a significant gap in the range 

of the species because each of these 

population segments represents a 

relatively large portion of the total range 

of the species (Table 6). In contrast, the 

range of the Coachella Valley 

Population as a whole, or the separate 

Thousand Palms or Dos Palmas 

subareas, is very small relative to the 

total range of the species. The range of 

the Coachella Valley Population 

represents only 0.24 percent of the total 

range of the species (0.80 percent of the 

U.S. portion of the range) (Table 6). The 

range of the Thousand Palms population 

represents only 0.11 percent of the total 

range of the species, and the range of the 

Dos Palmas population represents only 

0.13 percent of the species’ total range 

(Table 6). Loss of the Coachella Valley 

population segment would not result in 

a significant gap in the range of the 

species. We conclude that the Western, 

Eastern, and Southeastern population 

segments meet the second significance 

condition, but the Coachella Valley 

population segment does not. 

TABLE 6—SIZE (AREA) OF THE POPULATIONS UNDER CONSIDERATION TO BE POTENTIAL DISTINCT VERTEBRATE POPULATION SEGMENTS UNDER THE ACT. THE THOUSAND PALMS AND DOS PALMAS OCCURRENCES ARE SUBSETS OF THE

COACHELLA VALLEY POPULATION (SEE DESCRIPTION OF SPECIFIC ‘‘POPULATIONS’’ IN THE BACKGROUND SECTION

FOR DETAILS). 

Population 

Total range of species U.S. portion of range only 

Size (area) 1 Percent of 

total Size (area) 1 Percent of total 

Western ...................... 341,989 ha (845,073 ac) .............................. 21.52 253,020 ha (625,226 ac) .............................. 53.74 

Eastern ....................... 169,617 ha (419,133 ac) .............................. 10.67 146,121 ha (361,073 ac) .............................. 31.03 

Southeastern .............. 1,073,551 ha (2,652,802 ac) ........................ 67.56 67,922 ha (167,839 ac) ................................ 14.43 

Coachella Valley ........ 3,785 ha (9,353 ac) ...................................... 0.24 3,785 ha (9,353 ac) ...................................... 0.80 

(Thousand Palms subarea).1,707 ha (4,219 ac) ...................................... 0.11 1,707 ha (4,218 ac) ...................................... 0.36 

(Dos Palmas subarea) 2,078 ha (5,134 ac) ...................................... 0.13 2,078 ha (5,135 ac) ...................................... 0.44 

Total .................... 1,588,942 ha (3,926,361 ac) ........................ 100.00 470,848 ha (1,163,491 ac) ........................... 100.00 

1Area values are estimated through a GIS-based assessment. Despite the level of precision presented, area values are approximate; however, we believe they are accurate enough to draw the conclusions presented. 

Third Condition—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. 

Populations of the flat-tailed horned 

lizard have not been introduced outside 

the species’ historic range, so none of 

the four population segments meets the 

third significance condition. 

Fourth Condition—Evidence that the 

discrete population segment differs 

markedly from other populations of the 

species in its genetic characteristics. 

As described in Populations and 

Genetics in the Background section, the 

Western, Eastern, and Southeastern 

Populations are genetically cohesive 

populations within themselves but are 

significantly genetically differentiated 

from each other (Mulcahy et al. 2006, 

pp. 1807–1826; Culver and Dee 2008, 

pp. 1–14). Thus, the evidence indicates 

that the Western, Eastern, and 

Southeastern Populations of flat-tailed 

horned lizards differ markedly from 

each other in their genetic 

characteristics. 

However, evidence shows that the 

Thousand Palms subarea (occurrence) 

within the Coachella Valley Population 

is not markedly different from the 

Western Population in its genetic 

characteristics, although the Thousand 

Palms occurrence within the Coachella 

Valley Population, like the Western 

Population, is genetically significantly 

different from the Eastern and 

Southeastern Populations. Although 

haplotypes unique to flat-tailed horned 

lizards from the Thousand Palms 

occurrence within the Coachella Valley 

Population have been found, genetic 

differences between these lizards and 

Western Population lizards were not 

statistically significant (Mulcahy et al. 

2006, p. 1811 and p. 1817). Although 

Coachella Valley flat-tailed horned 

lizards are currently markedly separated 

geographically from other flat-tailed 

horned lizard populations as a result of 

isolation due to past agricultural and 

urban development, genetics 

information suggests that the flat-tailed 

horned lizards in the Thousand Palms 

occurrence were historically not 

separated from Western Population flattailed horned lizards (Mulcahy et al. 

2006, p. 1821). Thus, the evidence 


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indicates that the population of flattailed horned lizards in the Thousand 

Palm occurrence within the Coachella 

Valley Population does not differ 

markedly from the Western Population 

in its genetic characteristics. 

We are not aware of any genetic 

information on the Dos Palmas subarea 

(occurrence). [We believe the map 

shown by Culver and Dee (2008, Figure 

1, p. 14) to be in error because they used 

the same samples for the Coachella 

Valley Population that Mulcahy et al. 

(2006) used (Culver and Dee 2008, p. 4), 

which indicated that genetic samples of 

flat-tailed horned lizards were collected 

from the Thousand Palms subarea 

(Mulcahy et al. 2006, p. 1826 and Figure 

3, p. 1809) (see also Mendelson et al. 

2004, p. 5)]. Although the genetic 

affinities of the Dos Palmas occurrence 

are unknown, it is likely this occurrence 

was historically connected with the 

Western Population through a 

connection to the north or west (when 

the Salton Basin was dry) or possibly 

the Eastern Population through a 

connection to the south along the 

eastern side of the Salton Trough when 

Lake Cahuilla was not full. Thus, the 

evidence suggests that the population of 

flat-tailed horned lizards in the Dos 

Palmas occurrence within the Coachella 

Valley Population is unlikely to differ 

markedly from the Western Population 

or Eastern Population in its genetic 

characteristics. Therefore, we conclude 

the Coachella Valley Population does 

not differ markedly from other 

populations of the species in its genetic 

characteristics. 

We believe the best scientific and 

commercial information available 

indicates that the Western, Eastern, and 

Southeastern Populations meet the 

fourth condition for significance, but 

that the best scientific and commercial 

information available do not support a 

determination that the Coachella Valley 

Population (and the Thousand Palms 

and Dos Palmas subareas, individually) 

meet the fourth condition for 

significance. We did not identify 

additional criteria for determining 

significance beyond the four identified 

in the 1996 DPS policy. 

Conclusion for Significance Element of 

1996 DPS Policy 

We conclude that the Western, 

Eastern, and Southeastern Populations 

of flat-tailed horned lizards meet the 

significance element of the 1996 DPS 

policy, but that the Coachella Valley 

Population does not. Loss of the 

Western, Eastern, or Southeastern 

Population would result in a significant 

gap in the range of the species (second 

significance condition), and information 

indicates that each of these three 

population segments differs markedly in 

genetic characteristics from the other 

populations of flat-tailed horned lizards 

(fourth significance condition). In 

considering the importance of the 

Coachella Valley Population (the 

Thousand Palms and the Dos Palmas 

occurrences together) or the Thousand 

Palms and the Dos Palmas occurrences 

separately to the species as a whole, we 

determined that neither the Coachella 

Valley Population, the Thousand Palms 

occurrence, nor the Dos Palmas 

occurrence met any of the four 

significance conditions identified in the 

1996 DPS policy, and we did not 

identify other considerations that would 

lead us to conclude that the respective 

population segments met the 

significance element of the policy, 

especially given Congressional guidance 

that the authority to list DPSs be used 

‘‘sparingly’’ while encouraging the 

conservation of genetic diversity. 

Conservation Status of DPSs 

As stated by our DPS policy (61 FR 

4725), if a population segment is 

discrete and significant (i.e., it is a 

distinct population segment), its 

evaluation for endangered or threatened 

status will be based on the Act’s 

definitions of those terms and a review 

of the factors enumerated in section 

4(a). It may be appropriate to assign 

different classifications to different 

DPSs of the same vertebrate taxon. 

Above, we determined the Western, 

Eastern, and Southeastern Populations 

are discrete and significant, and thus, 

each is a distinct vertebrate population 

segment. We thus evaluate the 

conservation status of each of these 

three distinct population segments. We 

do not further separately evaluate the 

conservation status of the Coachella 

Valley Population or the two 

occurrences of flat-tailed horned lizards 

because we determined that these 

population segments do not meet the 

significance element of the 1996 DPS 

policy, and thus none are considered a 

distinct population segment under the 

Act and our DPS policy. For the 

remainder of the DPS analysis, we 

consider the Coachella Valley 

Population, which includes the 

Thousand Palms occurrence and the 

Dos Palmas occurrence, to be part of the 

Western DPS. Although it is possible 

that the Dos Palmas occurrence may 

more properly be placed in the Eastern 

DPS, for the purposes of our evaluation 

for endangered or threatened status, we 

are considering it to be within the 

Western DPS. 

In our analysis of section 4(a) threats, 

we evaluated whether potential threats 

were significant at the scale of flat-tailed 

horned lizard across its entire range, as 

well as whether any of the threats were 

significant at the scale of the four major 

populations (see Summary of Factors 

Affecting the Species section). 

For Factor A, we identified and 

evaluated habitat threats from 

agricultural development, urban 

development, energy development, 

invasive and nonnative plants, OHVs, 

and military training activities. This 

analysis led us to conclude that none of 

these potential habitat threats, either 

individually or cumulatively, is 

significant enough to cause the flattailed horned lizard to be in danger of 

extinction now or likely to become so 

within the foreseeable future throughout 

all of its range. We also conclude based 

on the results of this same analysis 

presented in Summary of Factors 

Affecting the Species that none of these 

potential habitat threats is significant 

enough to cause the Eastern, Western, or 

Southeastern distinct population 

segments of flat-tailed horned lizard to 

be in danger of extinction now or likely 

to become so within the foreseeable 

future throughout all of their respective 

ranges. 

For Factor B, we concluded that 

potential threats associated with 

overutilization due to collection for the 

pet trade and scientific and educational 

purposes are not significant threats to 

flat-tailed horned lizards now or within 

the foreseeable future across its range. 

We also conclude, based on this same 

analysis presented in Summary of 

Factors Affecting the Species, that 

potential overutilization threats are not 

significant enough to cause the Eastern, 

Western, or Southeastern distinct 

population segments of flat-tailed 

horned lizard to be in danger of 

extinction now or likely to become so 

within the foreseeable future throughout 

all of their respective ranges. 

For Factor C, we concluded that 

potential threats associated with disease 

or predation were not significant threats 

to flat-tailed horned lizards now or 

within the foreseeable future across its 

range. We also conclude based on this 

same analysis presented in Summary of 

Factors Affecting the Species that 

potential disease or predation threats 

are not significant enough to cause the 

Eastern, Western, or Southeastern 

distinct population segments of flattailed horned lizard to be in danger of 

extinction now or likely to become so 

within the foreseeable future throughout 

all of their respective ranges. 

For Factor D, we concluded that 

existing regulatory mechanisms are not 

inadequate and do not threaten the flattailed horned lizard throughout all or a 


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significant portion of its range either 

now or within the foreseeable future. 

We also conclude based on this same 

analysis of the best available 

information presented in Summary of 

Factors Affecting the Species that any 

potential threats associated with 

inadequate existing regulatory 

mechanisms are not significant enough 

to cause the Eastern, Western, or 

Southeastern distinct population 

segments of flat-tailed horned lizard to 

be in danger of extinction now or likely 

to become so within the foreseeable 

future throughout all or a significant 

portion of their respective ranges. 

For Factor E, we identified and 

evaluated threats from other natural or 

manmade factors including barriers and 

small populations, edge effects, 

pesticide spraying, vehicle activity, 

drought, and climate change. This 

analysis led us to conclude that none of 

these potential threats, either 

individually or cumulatively, is 

significant enough to cause the flattailed horned lizard to be in danger of 

extinction now or likely to become so 

within the foreseeable future throughout 

all of its range. We also conclude, based 

on this same analysis of the best 

available information presented in 

Summary of Factors Affecting the 

Species, that none of these potential 

threats is significant enough to cause the 

Eastern, Western, or Southeastern 

distinct population segments of flattailed horned lizard to be in danger of 

extinction now or likely to become so 

within the foreseeable future throughout 

all of their respective ranges. 

Conclusion for Conservation Status 

Element of 1996 DPS Policy 

In our analysis of the species as a 

whole as detailed in Summary of 

Factors Affecting the Species section, 

we noted potential threats from 

development, invasive species, military 

training, vehicle (including OHV) 

activity, barriers and small populations, 

edge effects, pesticide spraying, and 

climate change. Additionally, we 

identified regulatory mechanisms and 

conservation efforts that reduced certain 

threats in certain areas. We determined 

that none of the potential threats, either 

individually or cumulatively, 

significantly affected the species 

throughout its range. In that analysis, 

we also addressed (where appropriate) 

separate flat-tailed horned lizard 

populations, including the Western, 

Eastern, and Southeastern Populations 

that we have determined, per the 

analyses in this section, are DPSs. 

Although all of the identified potential 

threats occur to a greater or lesser degree 

in each of the three DPSs, and although 

the regulatory mechanisms and 

conservation efforts differ within and 

among DPSs, we found no one threat to 

be unique to any one DPS, nor did we 

find a threat that occurred with 

markedly greater magnitude in any one 

DPS. We therefore conclude that the 

Western, Eastern, and Southeastern 

distinct population segments of flattailed horned lizard also are not likely 

to be in danger of extinction now or 

likely to become so within the 

foreseeable future throughout all of their 

respective ranges. 

Significant Portion of the Range 

Having determined that neither the 

flat-tailed horned lizard nor the 

identified distinct population segments 

of flat-tailed horned lizard meet the 

definition of an endangered or 

threatened species, we must next 

consider whether there are any 

significant portions of the range where 

the flat-tailed horned lizard is in danger 

of extinction or is likely to become 

endangered in the foreseeable future. 

We considered whether any portion of 

the flat-tailed horned lizard’s range 

warrants further consideration. Our 

consideration of areas that may 

constitute significant portions of the 

species’ range focuses on areas where 

the geographic concentration of threats 

may be greater relative to the entire 

range. We consider whether there are 

any significant portions of the range of 

the flat-tailed horned lizard (the species 

as a whole) or of the identified DPSs 

that are in danger of extinction or are 

likely to become endangered in the 

foreseeable future. 

Decisions by Ninth Circuit Court of 

Appeals in Defenders of Wildlife v. 

Norton, 258 F.3d 1136 (2001) and 

Tucson Herpetological Society v. 

Salazar, 566 F.3d 870 (2009) found that 

the Act requires the Service, in 

determining whether a species is 

endangered or threatened throughout a 

significant portion of its range, to 

consider whether lost historical range of 

a species (as opposed to its current 

range) constitutes a significant portion 

of the range of that species. While this 

is not our interpretation of the statute, 

we first address the lost historical range 

before addressing the current range. 

Lost Historical Range 

As shown in Figure 1, the current 

range of the flat-tailed horned lizard 

consists of three, large, separate 

population areas (the Western, Eastern, 

and Southeastern Populations), plus 

two, small, isolated occurrences that, 

together, compose the Coachella Valley 

Population (see the Description of 

Specific ‘‘Populations’’ section, above). 

In our past assessments of the species, 

following the lead of the information 

then available to us, we concluded or 

implied that the historical range of the 

flat-tailed horned lizard was mostly 

without substantial discontinuities and 

that modern discontinuities in the 

species’ range were the result of 

manmade changes, primarily habitat 

loss through agricultural development 

and the creation of the Salton Sea (for 

example, see the Factor A analyses at 58 

FR 62625–62626, 62 FR 37857, and 68 

FR 341; also Rado 1981, pp. 1–21; 

Hodges 1997, pp. 1–23). This 

characterization of the range of the 

species suggested to the reader that the 

conversion from habitat to non-habitat 

of the large swath of land between the 

Coachella Valley, Western, Eastern, and 

Southeastern Populations is what 

created those now-separate populations 

and that prior to the manmade changes 

all of the now-lost interstitial areas used 

to be occupied flat-tailed horned lizard 

habitat. However, the best currently 

available information indicates that 

such a conclusion is incorrect. 

In our 2006 analyses (71 FR 36750– 

36751), we determined that the area of 

the historical lakebed of the former Lake 

Cahuilla (see Background section), 

which occupied most of the areas now 

under agriculture in the southern half, 

or so, of the Coachella Valley and most 

of the area now under agriculture in the 

Imperial Valley (for example, see Patten 

et al. 2003, p. 3), was frequently 

unavailable (through historical and prehistorical time) and likely contained 

little quality habitat for the flat-tailed 

horned lizard. The 2006 analysis then 

addressed the now-developed areas 

outside of the historical lakebed, 

including remaining portions of the 

Coachella Valley and Mexicali Valley, 

and the San Luis Valley. However, as 

detailed in the Background and further 

discussed in the ‘‘Barriers and Small 

Populations’’ section of Factor E, above, 

the available information now leads us 

to conclude that the Western, Eastern, 

and Southeastern Populations have long 

been separated from each other by 

natural barriers south of the Lake 

Cahuilla lakebed that pre-date any 

manmade changes. Specimen data show 

that large amounts of this now-lost area 

was formerly occupied by the species 

(see, for example, Funk 1981, p. 281.1), 

but as described in the Setting and 

Habitat section, above, the evidence 

also shows that, in addition to the 

historical lakebed of the former Lake 

Cahuilla, some unknown amount of the 

area in the Mexicali Valley and the San 

Luis Valley, was also frequently affected 

by the deltaic meandering and avulsive 


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flooding of the Colorado River. These 

hydrologically active areas likely 

contained little quality habitat for the 

flat-tailed horned lizard and formed 

natural barriers to movement of flattailed horned lizards thereby allowing 

genetic differentiation among the 

Western, Eastern, and Southeastern 

Populations (see the Populations and 

Genetics section, above). Thus, as we 

found for the Lake Cahuilla lakebed in 

our 2006 analyses (71 FR 36750–36751), 

we have also determined that these 

additional areas should not be 

considered part of the species’ historical 

habitat. 

Therefore, we consider the flat-tailed 

horned lizard’s historical habitat to be 

(1) habitat outside the area of the former 

Lake Cahuilla and (2) the habitat outside 

the areas historically subject to periodic 

flooding by the Colorado River. Because 

we do not know the real extent of the 

non-habitat areas that created the 

natural barriers separating the 

populations, we cannot reasonably 

estimate (quantify) the size of the areas 

that do constitute the lost historical 

habitat for each of the separate 

populations. As a result, the remainder 

of this analysis qualitatively considers 

the species’ lost historical habitat. 

Because the habitat needs of the flattailed horned lizard are met within the 

home range of each flat-tailed horned 

lizard individual, the areas of former 

habitat within the lost historical range 

did not provide any special or unique 

features or meet any life-history needs 

that present-day flat-tailed horned 

lizards need to survive. In other words, 

there is no evidence in the available 

information to indicate that the habitat 

within the lost historical range provided 

special features for the flat-tailed horned 

lizard such as key breeding grounds, lek 

sites, or migratory pathways, which are 

examples of special habitat features 

other species need to survive. Had the 

habitat within the lost historical range 

provided any special or unique features 

or met any particular life-history needs 

of the flat-tailed horned lizard—in other 

words, had the habitat in the lost 

historical range been significant to the 

species—the loss of these habitat areas 

would have been detectable in further 

contraction in the range of the species 

or each DPS over the past 100 or so 

years (more than 25 flat-tailed horned 

lizard generations, as described in our 

2006 analysis (71 FR 36751)), the time 

since most of the historical habitat was 

lost. Since the areas of historical habitat 

were converted to agriculture early in 

the 20th century, the distribution of the 

flat-tailed horned lizard has remained 

about the same, except in areas of 

continuing urban expansion where such 

reductions in range are attributable to 

continued habitat loss (see Factor A). 

(Although adequate sample sizes to 

determine population trends have been 

difficult to obtain in the flat-tailed 

horned lizard, the distribution of the 

species, and thus its range, is based on 

where the species was and is detected— 

presence-absence data—which is much 

more easily obtained.) Moreover, the 

agricultural and urban development of 

the now-lost historical range did not 

create any new barriers that separated 

the Western, Eastern, and Southeastern 

Populations (DPSs) but merely 

expanded upon pre-existing, natural 

barriers (see Background section). 

Therefore, the historical loss of habitat 

has not resulted in substantial presentday ramifications to the species; in other 

words, the lost historical range is not 

biologically significant to the flat-tailed 

horned lizard and does not contribute 

meaningfully to the viability of the 

species overall or to the viability of each 

DPS. 

Moreover, as described under Factor 

A, we do not expect additional 

significant conversion of flat-tailed 

horned lizard habitat to agriculture in 

the future in the Imperial Valley and 

elsewhere along the Colorado River 

given: (1) The existing limitations on the 

availability of water for irrigation, and 

(2) the water transfer agreement with 

San Diego that requires some fields to 

remain fallow (unirrigated); therefore, 

agricultural use has even decreased in 

this area (IID 2006). 

The past agricultural and urban 

development that created the swath of 

now-lost historical habitat in the United 

States and Mexico removed the 

biological features that provided habitat 

for the flat-tailed horned lizard in these 

areas. Much of this habitat has been 

permanently lost due to urbanization, 

flooding of the Salton Sea, or both. 

Although habitat lost due to agricultural 

uses could potentially be restored in 

certain cases in the future, most 

agricultural fields are isolated from 

existing flat-tailed horned lizard 

populations by major irrigation canals, 

such as the Coachella Canal, Highline 

Canal, and All-American Canal, as well 

as, depending on the site’s location, one 

or more smaller canals and drains. 

Therefore, we do not anticipate any 

significant amount of previously lost 

habitat will likely become suitable as 

habitat for the flat-tailed horned lizard 

in the foreseeable future. 

In sum, we believe the lost historical 

habitat does not represent a significant 

portion of the range of the flat-tailed 

horned lizard because the habitat was 

lost decades ago and the species has not 

experienced a continuing range 

contraction due to the loss of this 

habitat. Most of the lost habitat was lost 

early in the 20th century and that lost 

habitat was not significant enough to 

lead to substantial extirpation of the 

species within intact habitat (which 

would be detectable through a reduction 

of the species’ distribution). The 

historically lost habitat did not provide 

any special or unique features or meet 

any life-history needs of the flat-tailed 

horned lizard that made those areas any 

more significant than any other habitat. 

The habitat within the lost historical 

range was not continuous and contained 

natural barriers that separated the 

Western, Eastern, and Southeastern 

Populations, which means the historical 

loss of habitat did not create any new 

barriers within the lost historical range. 

We do not expect the agricultural 

development that created the large 

‘‘swath’’ of lost habitat to continue to 

expand substantially, nor do we expect 

significant amounts of land that are 

currently under agriculture to become 

flat-tailed horned lizard habitat within 

the foreseeable future. Therefore, the 

lost historical range is not a significant 

portion of the range for the flat-tailed 

horned lizard. 

Current Range 

We use the concepts of resiliency, 

redundancy, and representation (see 

below) as the basic tenets for 

determining whether a portion of a 

species’ range is significant to that 

species. A portion of a taxonomic 

species’ or DPS’s range is significant if 

it is part of the current range of the 

species or DPS and it contributes 

substantially to the representation, 

resiliency, or redundancy of the species 

or DPS. The contribution must be at a 

level such that its loss would result in 

a significant decrease in the viability of 

the species or DPS. 

We chose to identify any portions of 

the range of the species that warrant 

further consideration as the first step in 

determining whether a taxonomic 

species or DPS is endangered or 

threatened in a significant portion of its 

range. The range of a species or DPS can 

theoretically be divided into portions in 

an infinite number of ways. However, 

there is no purpose to analyzing 

portions of the range that are not 

reasonably likely to be significant and 

endangered or threatened. To identify 

only those portions that warrant further 

consideration, we should, under the 

framework we chose for this evaluation, 

determine whether there is substantial 

information indicating that (i) the 

portions may be significant and (ii) the 

species or DPS may be in danger of 

extinction there or likely to become so 


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within the foreseeable future. In 

practice, we believe a key part of this 

analysis is whether the threats are 

geographically concentrated in some 

way. If the threats to the species are 

essentially uniform throughout its 

range, no portion is likely to warrant 

further consideration. Moreover, if any 

concentration of threats applies only to 

portions of the range that are not 

significant to the viability of the species, 

such portions will not warrant further 

consideration. 

Under this framework, if we identify 

any portions that warrant further 

consideration, we then determine 

whether in fact the species or DPS is 

endangered or threatened in any 

significant portion of its range. 

Depending on the biology of the species, 

the range of the species or DPS, and the 

threats the species or DPS faces, it may 

be more efficient for us to address the 

significance question first, or the status 

question first. Thus, if we determine 

that a portion of the range is not 

significant, we need not determine 

whether the species is endangered or 

threatened there; if we determine that 

the species or DPS is not endangered or 

threatened in a portion of its range, we 

need not determine if that portion is 

significant. 

The terms resiliency, redundancy, and 

representation are intended to be 

indicators of the conservation value of 

portions of the range. Resiliency of a 

species allows the species to recover 

from periodic or occasional disturbance. 

A species or its members within a DPS 

will likely be more resilient if large 

populations exist in high-quality habitat 

that is distributed throughout the range 

of the species or DPS in such a way as 

to capture the environmental variability 

found within the range of the species or 

DPS. It is likely that the larger the size 

of a population, the more it will 

contribute to the viability of the species 

overall. Thus, a portion of the range of 

a species may make a meaningful 

contribution to the resiliency of the 

species or DPS if the area is relatively 

large and contains particularly highquality habitat or if its location or 

characteristics make it less susceptible 

to certain threats than other portions of 

the range. When evaluating whether or 

how a portion of the range contributes 

to resiliency of the species, it may help 

to evaluate the historical value of the 

portion and how frequently the portion 

is used by the species or DPS. In 

addition, the portion may contribute to 

resiliency for other reasons—for 

instance, it may contain an important 

concentration of certain types of habitat 

that are necessary for members of a 

species or DPS to carry out their lifehistory functions, such as breeding, 

feeding, migration, dispersal, or 

wintering. 

Redundancy of populations may be 

needed to provide a margin of safety for 

the species or DPS to withstand 

catastrophic events. This does not mean 

that any portion that provides 

redundancy is a significant portion of 

the range of a species. The idea is to 

conserve enough areas of the range such 

that random perturbations in the system 

act on only a few populations. 

Therefore, each area must be examined 

based on whether that area provides an 

increment of redundancy that is 

important to the viability of the species. 

Adequate representation insures that 

the species’ adaptive capabilities are 

conserved. Specifically, the portion 

should be evaluated to see how it 

contributes to the genetic diversity of 

the species or DPS. The loss of 

genetically based diversity may 

substantially reduce the ability of the 

species or DPS to respond and adapt to 

future environmental changes. A 

peripheral population may contribute 

meaningfully to representation if there 

is evidence that it provides genetic 

diversity due to its location on the 

margin of the species’ habitat 

requirements. 

Applying the process described above 

for determining whether the flat-tailed 

horned lizard or any of the identified 

DPSs are likely to become endangered 

throughout a significant portion of their 

respective ranges, under this framework 

we next address whether any portions of 

the range of the flat-tailed horned lizard 

or the identified DPSs warrant further 

consideration. Based on past approaches 

and other treatments in the literature, 

the flat-tailed horned lizard may be 

divided into four ‘‘populations.’’ As 

detailed above, we conducted our 

analysis of threats to the species based, 

in part, upon those populations. 

Moreover, we determined that the 

Western Population (including the 

Coachella Valley Population), the 

Eastern Population, and the 

Southeastern Population were DPSs 

under the Act per our DPS policy. We 

found that the species as a whole is not 

in danger of extinction or likely to 

become endangered within the 

foreseeable future throughout all of its 

range. We also found that the three 

DPSs are not in danger of extinction or 

likely to become endangered within the 

foreseeable future throughout all of their 

respective ranges. Because we 

determined that the DPSs (each as a 

whole) are not endangered or threatened 

within those portions of the species’ 

range, we need not determine if the 

Western, Eastern, or Southeastern DPSs 

(each as a whole) are ‘‘significant.’’ 

We found that the Coachella Valley 

Population was faced with substantial 

threats. Also, we noted certain barriercreated ‘‘parts’’ within the ranges of the 

Western, Eastern, and Southeastern 

Populations were small enough that the 

flat-tailed horned lizards therein were 

more likely to suffer from threats 

associated with small populations (see 

‘‘Barriers and Small Populations’’ under 

Factor E) or were facing or likely to face 

other threats. 

An important consideration in 

determining what portions of the 

species’ or distinct population 

segments’ ranges may be appropriate to 

consider for this analysis is the fact that 

there are no specific life-history traits of 

the flat-tailed horned lizard that make 

any one portion of its range significantly 

more important to the survival of the 

species than any other. The flat-tailed 

horned lizard is a small animal with 

limited abilities to move long distances, 

and the habitat features necessary for 

activities like breeding, feeding, and 

sheltering, may be found within or very 

close to the home range of each 

individual flat-tailed horned lizard. 

Moreover, a flat-tailed horned lizard’s 

home range size (perhaps as much as 10 

ha (25 ac)), although large compared to 

other horned lizard species, is very 

small compared to the overall range of 

the species (1.6 million ha (3.9 million 

ac)). In other words, this species does 

not need any particular portion of its 

range outside the general home-range 

area of each individual to meet any life 

history needs, such as particular 

breeding grounds, lek sites, or migratory 

pathways. As such, the ‘‘parts’’ 

identified in Factor E are appropriate 

subjects to address as potential 

significant portions of the species’ 

range. 

Thus, because the portions of the 

species’ range that compose the 

Coachella Valley Population and the 

portions of the species’ range that are 

formed by the small ‘‘parts’’ of the other 

three populations may face substantial 

threats, we next determine whether 

these portions of the species’ range are 

‘‘significant.’’ As described above, we 

need not assess whether the portions of 

the species’ range that are not facing 

substantial threats are ‘‘significant.’’ 

Coachella Valley Population Area 

As discussed previously, the 

Coachella Valley Population, which is 

peripheral to the population-as-a-whole 

of the species, now consists of two small 

occurrences, Thousand Palms and Dos 

Palmas. These two occurrences are 

small in area and, thus, likely have 


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small populations of flat-tailed horned 

lizards (see ‘‘Barriers and Small 

Populations’’). As such, the populations 

of flat-tailed horned lizards that 

comprise these occurrences may not be 

large enough to avoid deleterious effects 

associated with small population size 

(see ‘‘Barriers and Small Populations’’). 

This suggests that the respective 

portions of the flat-tailed horned lizard’s 

range in these two occurrences may face 

substantive threats and have the 

potential to be endangered or 

threatened; thus, we should evaluate 

whether the portions of the species’ 

range are significant portions of the 

species’ range. To do so, we assess (1) 

Whether the population of flat-tailed 

horned lizards in each occurrence 

contributes meaningfully to the 

resiliency, redundancy, and 

representation of the entire species; (2) 

whether the Thousand Palms 

occurrence contributes meaningfully to 

the resiliency, redundancy, and 

representation of the Western DPS; and 

(3) whether the Dos Palmas occurrence 

contributes meaningfully to the 

resiliency, redundancy, and 

representation of the Western DPS or 

Eastern DPS. 

Resiliency—Resiliency of a species, as 

described in greater detail above, allows 

the species to recover from periodic or 

occasional disturbance. The size of the 

flat-tailed horned lizard population at 

the Thousand Palms and Dos Palmas 

occurrences (each separately or the two 

combined) is likely small because the 

amount of available habitat within each 

of these occurrence areas are small. 

Small populations are less resilient than 

large populations. Additionally, neither 

occurrence nor the two combined 

contains an important concentration of 

certain types of habitat that are 

necessary for flat-tailed horned lizards 

to carry out their life-history functions 

because each flat-tailed horned lizard 

has the habitat types it needs within its 

home range. Although the sands in the 

Coachella Valley are largely derived 

from local sediments (as opposed to 

being derived from the Colorado River, 

as are much of the sands within the 

range of the species), flat-tailed horned 

lizards occur in a number of areas with 

locally derived sediment (see 

Background). 

Additionally, there is nothing in the 

available information to indicate that 

the location or characteristics of these 

occurrences (separately or combined) 

makes them significantly less 

susceptible to certain threats than other 

portions of the species’ range. Moreover, 

there is no indication that these 

occurrences have provided value to the 

species historically. The ebbing and 

flowing of Lake Cahuilla through 

historical time has meant these two 

occurrences have likely been 

periodically disconnected from each 

other and from the Western DPS (or, for 

Dos Palmas, possibly the Eastern DPS). 

Even prior to any natural or manmade 

reductions in the geographical or 

numerical extent of these populations, 

they were outposts of the main 

population and did not contribute 

meaningfully to the viability of the 

larger Western Population (or, 

potentially for the Dos Palmas 

occurrence, the Eastern Population). 

Thus, the flat-tailed horned lizard 

populations in the Thousand Palms and 

Dos Palmas occurrences (each 

separately or the two combined) do not 

contribute meaningfully to the 

resiliency of the entire species, the 

Western DPS, or the Eastern DPS. 

Redundancy—Redundancy, as 

described in greater detail above, 

provides a margin of safety for the 

species or DPS to withstand 

catastrophic events. As discussed in the 

‘‘Barriers and Small Populations’’ 

section under Factor E, the respective 

populations of flat-tailed horned lizards 

in the Thousand Palms and Dos Palmas 

occurrences, or the two combined, is 

more likely to be significantly affected 

by deleterious effects associated with 

small population size, including 

catastrophic events, than areas with 

larger populations (see the ‘‘Other Small 

‘Parts’ of the Three DPSs’’ section, 

below). As such, the Coachella Valley 

occurrences do not provide a significant 

margin of safety for the species. 

Additionally, as discussed under 

Resiliency, above, the population of flattailed horned lizards in each of these 

occurrences is likely small because the 

amount of available habitat within each 

part is small. Similarly, the entire 

population of the flat-tailed horned 

lizard rangewide and the respective 

populations of flat-tailed horned lizards 

within each DPS are each relatively 

large compared to the respective 

populations of flat-tailed horned lizards 

in the Thousand Palms or Dos Palmas 

occurrences, or the two combined, 

because the amount of available habitat 

throughout the species’ range and 

within each DPS is relatively large 

compared to the Coachella Valley 

occurrences. As such, the Coachella 

Valley occurrences, or the two 

combined, provide an unsubstantial 

increment of redundancy. Thus, the 

Thousand Palms and Dos Palmas 

occurrences separately, or the two 

combined, do not contribute 

meaningfully to the redundancy of the 

entire species, the Western DPS, or the 

Eastern DPS. 

Representation—Representation, as 

described in greater detail above, 

ensures that the species’ adaptive 

capabilities are maintained. The genetic 

differences between the Thousand 

Palms occurrence and the Western 

Population are not statistically 

significant, despite having some unique 

haplotypes (see Populations and 

Genetics in the Background section). 

Thus, the Thousand Palms occurrence 

does not contribute meaningfully to the 

maintenance of the adaptive capabilities 

of the flat-tailed horned lizard 

rangewide or the Western DPS. 

Although the genetic affinities of the 

Dos Palmas occurrence are unknown, it 

is likely this occurrence was historically 

connected with the Western Population 

through a connection to the north or 

west (when the Salton Basin was dry) or 

possibly the Eastern Population through 

a connection to the south along the 

eastern side of the Salton Trough when 

Lake Cahuilla was not full. Thus, the 

Dos Palmas occurrence likely does not 

contribute meaningfully to the 

maintenance of the adaptive capabilities 

of the flat-tailed horned lizard. 

Therefore, neither the Thousand Palms 

occurrence, the Dos Palmas occurrence, 

nor the two occurrences combined (that 

is, the Coachella Valley Population) 

contributes meaningfully to the 

representation of the entire species, the 

Western DPS, or the Eastern DPS. 

Therefore, in sum, we do not expect 

the Coachella Valley Population as a 

whole, or the Thousand Palms and Dos 

Palmas occurrences separately, to 

contribute substantially to the 

resiliency, redundancy, or 

representation of the species, the 

Western DPS, or the Eastern DPS. As a 

result of this information, we believe 

neither the Coachella Valley Population 

(the Thousand Palms and Dos Palmas 

occurrences combined), nor the 

Thousand Palms and Dos Palmas 

occurrences separately, constitute a 

significant portion of the range of the 

entire species, the Western DPS, or the 

Eastern DPS. 

Other Small ‘‘Parts’’ of the Three DPSs 

In our analysis in the ‘‘Barriers and 

Small Populations’’ section, we 

identified certain portions, or ‘‘parts,’’ of 

the Western, Eastern, and Southeastern 

Population areas. In the Distinct 

Population Segment section, we 

determined these three Populations to 

be DPSs. We now evaluate whether any 

of these parts constitute a significant 

portion of the range of the flat-tailed 

horned lizard (the species as a whole) or 

the three DPSs. However, there is no 


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purpose to analyzing portions of a 

species’ range that are not reasonably 

likely to be both significant portions of 

that species’ range and endangered or 

threatened. We have chosen in this 

section to first assess whether the flattailed horned lizard is reasonably likely 

to be endangered or threatened within 

each part. 

For the reasons discussed in the 

Summary of Factors Affecting the 

Species section (note that the 

discussions go beyond the simple yesno results presented in Tables 3 through 

5), we believe the populations of flattailed horned lizard in the respective 

following parts (portions of the species’ 

range) do not face significant threats: 

W–1, W–3, W–5, W–7, W–9, W–11, W– 

12, E–3, E–5, E–9, SE–1, SE–5, SE–8, 

SE–9, and SE–13 (Figures 3 through 7). 

Although the specifics vary to some 

extent from part to part, none of these 

parts faces or is likely to face in the 

foreseeable future significant threats 

associated with: 

(1) Small population size, because the 

parts are large in size (area) or, for parts 

W–7, W–9, and W–11, likely have 

higher densities of flat-tailed horned 

lizards than the most conservative 

estimate (see the Barriers and Small 

Populations section) and, therefore, 

likely support large populations of flattailed horned lizards; 

(2) Significant loss of habitat from 

development, because what impacts 

may occur are expected to be small 

relative to the size of the parts because 

they are (i) remote; (ii) are receiving and 

are expected to continue receiving 

avoidance, minimization, and 

mitigation measures associated with the 

Rangewide Management Strategy 

(including those aspects that have been 

incorporated into agency plans that 

implement regulatory mechanisms) in 

the United States, or in Mexico, 

protections from biosphere reserves and 

listing under the Official Mexican 

Norm; or (iii) some combination thereof; 

and 

(3) Climate change; nonnative, 

invasive species; or other range-wide 

threats identified in the five-factor 

analysis, because none of these potential 

threats are significantly concentrated in 

any one part. 

As a result, the flat-tailed horned 

lizard is not reasonably likely to be 

endangered or threatened within the 

parts listed above. Thus, these parts do 

not warrant further consideration in this 

section. 

The remaining parts, W–2, W–4, W– 

6, W–8, W–10, E–1, E–2, E–4, E–6, E– 

7, E–8, SE–2, SE–3, SE–4, SE–6, SE–7, 

SE–10, SE–11, and SE–12 (Figures 3 

through 7), are either small in area and, 

thus, likely have small populations of 

flat-tailed horned lizards or, in the case 

of parts E–1 and E–4, which are larger 

in area, likely have small populations of 

flat-tailed horned lizards because they 

primarily contain areas of deep, actively 

shifting sands of the Algodones Dunes 

that are likely rarely used by flat-tailed 

horned lizards (see ‘‘Barriers and Small 

Populations’’). As such, the populations 

of flat-tailed horned lizards in these 

parts may not be large enough to avoid 

deleterious effects associated with small 

population size (see ‘‘Barriers and Small 

Populations’’). This suggests that the 

respective portions of the flat-tailed 

horned lizard’s range in the latter group 

of parts may face substantive threats and 

have the potential to be endangered or 

threatened; thus, we should evaluate 

whether the portions of the species’ 

range are significant portions of the 

species’ range. To do so, we assess 

whether the population of flat-tailed 

horned lizards in each part contributes 

meaningfully to the resiliency, 

redundancy, and representation of the 

species as a whole or to each DPS. 

Resiliency—Resiliency of a species, as 

described in greater detail above, allows 

the species to recover from periodic or 

occasional disturbance. The respective 

populations of flat-tailed horned lizards 

in parts W–2, W–4, W–6, W–8, W–10, 

E–1, E–2, E–4, E–6, E–7, E–8, SE–2, SE– 

3, SE–4, SE–6, SE–7, SE–10, SE–11, and 

SE–12 are likely small because the 

amount of available habitat within each 

part is small, including the relatively 

large (in area) parts E–1 and E–4 that 

primarily consist of the deep, actively 

shifting sands of the Algodones Dunes 

that are likely rarely used by flat-tailed 

horned lizards (see discussions in the 

‘‘Barriers and Small Populations’’ 

section under Factor E). Small 

populations are less resilient than large 

populations. Additionally, no one part 

contains an important concentration of 

certain types of habitat that are 

necessary for flat-tailed horned lizards 

to carry out their life-history functions 

because each flat-tailed horned lizard 

has the habitat types it needs within its 

home range. Moreover, there is nothing 

in the available information to indicate 

that the location or characteristics of 

part W–2, W–4, W–6, W–8, W–10, E–1, 

E–2, E–4, E–6, E–7, E–8, SE–2, SE–3, 

SE–4, SE–6, SE–7, SE–10, SE–11, or SE– 

12 makes it significantly less susceptible 

to certain threats than other portions of 

the species’ range. Thus, none of the 

flat-tailed horned lizard populations in 

the remaining parts contribute 

meaningfully to the resiliency of the 

species as a whole or to each DPS. 

Redundancy—Redundancy, as 

described in greater detail above, 

provides a margin of safety for the 

species or DPS to withstand 

catastrophic events. As discussed in the 

‘‘Barriers and Small Populations’’ 

section under Factor E, the respective 

populations of flat-tailed horned lizards 

in parts W–2, W–4, W–6, W–8, W–10, 

E–1, E–2, E–4, E–6, E–7, E–8, SE–2, SE– 

3, SE–4, SE–6, SE–7, SE–10, SE–11, and 

SE–12 are more likely to be significantly 

affected by deleterious effects associated 

with small population size, including 

catastrophic events, than the respective 

populations of flat-tailed horned lizards 

in parts W–1, W–3, W–5, W–7, W–9, W– 

11, W–12, E–3, E–5, E–9, SE–1, SE–5, 

SE–8, SE–9, and SE–13. As such, the 

former group of parts do not provide a 

significant margin of safety for the 

species. Additionally, as discussed 

under Resiliency, above, the population 

of flat-tailed horned lizards in each of 

these respective parts is likely small 

because the amount of available habitat 

within each part is small, including the 

relatively large (in area) parts E–1 and 

E–4 that primarily consist of the deep, 

actively shifting sands of the Algodones 

Dunes that are likely rarely used by flattailed horned lizards (see discussions in 

the ‘‘Barriers and Small Populations’’ 

section under Factor E). Similarly, the 

entire population of flat-tailed horned 

lizards and the population within each 

DPS are each likely relatively large 

compared to the respective populations 

of flat-tailed horned lizards in parts W– 

2, W–4, W–6, W–8, W–10, E–1, E–2, E– 

4, E–6, E–7, E–8, SE–2, SE–3, SE–4, SE– 

6, SE–7, SE–10, SE–11, and SE–12 

because the amount of available habitat 

throughout the species’ range and 

within each DPS is relatively large 

compared to the parts under 

consideration here (see Tables 3 through 

5). As such, parts W–2, W–4, W–6, W– 

8, W–10, E–1, E–2, E–4, E–6, E–7, E–8, 

SE–2, SE–3, SE–4, SE–6, SE–7, SE–10, 

SE–11, and SE–12 provide an 

unsubstantial increment of redundancy. 

Thus, none of the flat-tailed horned 

lizard populations in the remaining 

parts provide a significant level of 

redundancy for the species as a whole 

or to each DPS. 

Representation—Representation, as 

described in greater detail above, 

ensures that the species’ adaptive 

capabilities are maintained. The 

scientific information on the genetics of 

flat-tailed horned lizard populations 

indicates that the Western, Eastern, and 

Southeastern Populations (DPSs) are 

significantly different from each other 

(see Populations and Genetics); thus, 

the representation of the species is 

provided by the three Populations. 

Although we do not have genetic data 


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from every ‘‘part,’’ the available 

information suggests the genetic 

diversity is fairly uniform and does not 

differ significantly within each of the 

three DPSs. As such, no one part within 

the respective DPSs contributes 

meaningfully to the representation of 

the species as a whole or to each DPS. 

Moreover, as discussed in the 

Populations and Genetics section, one 

part, Part SE–2, shows evidence 

suggesting the genetic variability of the 

flat-tailed horned lizard population in 

that part has declined as a consequence 

of being small and isolated by a 

manmade barrier. This suggests that the 

species’ adaptive capabilities in this 

part have declined. That is, the ability 

of the flat-tailed horned lizard 

population to provide adequate 

representation has been reduced in Part 

SE–2. It is possible the representation of 

the other parts with small populations 

and with complete barriers has been or 

may become similarly reduced. 

Therefore, it is unlikely that parts W–2, 

W–4, W–6, W–8, W–10, E–1, E–2, E–4, 

E–6, E–7, E–8, SE–2, SE–3, SE–4, SE–6, 

SE–7, SE–10, SE–11, and SE–12 

contribute significantly to the species’ 

adaptive capabilities, and thus, the 

respective parts do not contribute 

meaningfully to the representation of 

the species as a whole or to each DPS. 

In sum, we found that none of the 

‘‘parts’’ identified in the ‘‘Barriers and 

Small Populations’’ section constituted 

significant portions of the range of the 

flat-tailed horned lizard. For the reasons 

discussed in the Summary of Factors 

Affecting the Species section (note that 

the discussions go beyond the simple 

yes-no results presented in Tables 3 

through 5), we determined that the 

portions of range of the flat-tailed 

horned lizard in parts W–1, W–3, W–5, 

W–7, W–9, W–11, W–12, E–3, E–5, E– 

9, SE–1, SE–5, SE–8, SE–9, and SE–13 

are not reasonably likely to be 

endangered or threatened; thus, we did 

not need to determine whether the 

portions of the range that these parts 

represented are significant portions. We 

determined that the flat-tailed horned 

lizards in the remaining parts, parts W– 

2, W–4, W–6, W–8, W–10, E–1, E–2, E– 

4, E–6, E–7, E–8, SE–2, SE–3, SE–4, SE– 

6, SE–7, SE–10, SE–11, and SE–12, may 

face substantive threats and have the 

potential to be endangered or 

threatened. As such, we assessed 

whether any of the portions of the 

species’ range within the parts in this 

latter group is a significant portion of 

the species’ range overall or of the 

ranges of each DPS. We found that the 

portions of the species’ range within the 

respective parts in this latter group 

likely contained small populations of 

flat-tailed horned lizards that did not 

contribute meaningfully to the species’ 

resiliency, redundancy, or 

representation of the species as a whole 

or of each DPS. We determined, 

therefore, the portions of the flat-tailed 

horned lizard’s range in parts W–2, W– 

4, W–6, W–8, W–10, E–1, E–2, E–4, E– 

6, E–7, E–8, SE–2, SE–3, SE–4, SE–6, 

SE–7, SE–10, SE–11, and SE–12 are not 

significant portions of the range of the 

species as a whole or of each DPS. 

Summary of Significant Portion of the 

Range 

In summary, we examined whether 

the lost historical range of the species, 

the current range of the species in the 

Coachella Valley Population, or the 

current range of the species in the other 

respective ‘‘parts’’ of the Western, 

Eastern, and Southeastern DPSs 

constituted significant portions of the 

species’ or distinct population 

segments’ respective ranges under the 

Act. We determined the lost historical 

habitat does not represent a significant 

portion of the range of the flat-tailed 

horned lizard because the habitat was 

lost decades ago and, despite the 

amount of time that has since 

transpired, the species has not 

experienced a continuing range 

contraction due to the past loss of 

habitat. Additionally, the historically 

lost habitat did not provide any special 

or unique features or meet any lifehistory needs of the flat-tailed horned 

lizards that made those areas any more 

significant than any other habitat. 

Moreover, the lost historical range was 

not continuous and contained natural 

barriers that separated the Western, 

Eastern, and Southeastern Populations. 

We also determined that neither the 

Coachella Valley Population as a whole 

nor the Thousand Palms and Dos 

Palmas occurrences separately 

contribute substantially to the 

resiliency, redundancy, or 

representation of the entire species, the 

Western DPS, or the Eastern DPS. 

Therefore, we conclude that neither the 

Coachella Valley Population as a whole 

nor the Thousand Palms and Dos 

Palmas occurrences separately 

constitute a significant portion of the 

range of the entire species, the Western 

DPS, or the Eastern DPS. 

Lastly, we determined that none of 

the ‘‘parts’’ identified in the ‘‘Barriers 

and Small Populations’’ section 

represented a significant portion of the 

range of the flat-tailed horned lizard. We 

found that the flat-tailed horned lizards 

in Parts W–1, W–3, W–5, W–7, W–9, W– 

11, W–12, E–3, E–5, E–9, SE–1, SE–5, 

SE–8, SE–9, and SE–13 were not 

reasonably likely to be endangered or 

threatened; thus, we did not need to 

determine whether the portions of the 

range that these parts represented are 

significant portions. We determined that 

the flat-tailed horned lizards in parts 

W–2, W–4, W–6, W–8, W–10, E–1, E–2, 

E–4, E–6, E–7, E–8, SE–2, SE–3, SE–4, 

SE–6, SE–7, SE–10, SE–11, and SE–12 

may face substantive threats and have 

the potential to be endangered or 

threatened, meaning that we needed, 

under our framework, to assess whether 

the flat-tailed horned lizards in these 

parts constituted significant portions of 

the species’ range. We found that the 

portions of the species’ range within the 

respective parts in this latter group 

likely contained small populations of 

flat-tailed horned lizards that did not 

contribute meaningfully to the 

resiliency, redundancy, or 

representation of the species as a whole 

or of each DPS. Thus, we determined 

the portions of the range of this latter 

group of parts are not significant 

portions of the range of the species as 

a whole or of each DPS. Therefore, no 

portion of the range of the flat-tailed 

horned lizard is a ‘‘significant portion of 

[the species’] range’’ under the Act. 

Conclusion 

Threats to the flat-tailed horned lizard 

rangewide or within the three identified 

DPSs have been reduced, managed, or 

eliminated, or found to be less 

substantial than originally thought. 

Additionally, implementation of the 

Interagency Conservation Agreement 

and associated Rangewide Management 

Strategy, including those aspects of it 

that have been incorporated into 

documents that implement existing 

regulatory mechanisms, is an important 

conservation effort that reduces threats 

in the United States and benefits the 

species throughout its range and within 

the identified DPSs. Therefore, we 

conclude that none of the existing or 

potential threats are likely to cause the 

flat-tailed horned lizard as an entire 

species or as any one of the Western, 

Eastern, or Southeastern DPSs to be in 

danger of extinction or likely to become 

so within the foreseeable future 

throughout all or a significant portion of 

its range. 

Withdrawal of Proposal To List FlatTailed Horned Lizard 

Based on the information discussed 

above, we withdraw our November 29, 

1993 (58 FR 62624), proposal to list the 

flat-tailed horned lizard (Phrynosoma 

mcallii) as a threatened species under 

the Act. 


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Peer Review 

As described in our 2003 withdrawal 

(68 FR 340) and in accordance with our 

July 1, 1994, Interagency Cooperative 

Policy for Peer Review in Endangered 

Species Act Activities (59 FR 34270), we 

solicited six individuals with scientific 

expertise on flat-tailed horned lizard, its 

habitat, and the geographic region in 

which the species occurs to provide 

their expert opinion and to review and 

interpret available information on the 

species’ status and threats. Peer 

reviewer comments and our responses 

to those comments were included in our 

2003 withdrawal (68 FR 340) and are 

hereby included in this document by 

reference. 

Summary of Comments and 

Recommendations 

Public Comments 

All public and peer review comments 

we received during public comment 

periods and public hearings prior to our 

March 2, 2010, Federal Register 

announcement on the reinstatement of 

the 1993 proposed rule and notice of 

public hearings are included in this 

document by reference (see Previous 

Federal Action section for dates, times, 

and locations of prior comment periods 

and hearings). 

Since the proposed rule was 

reinstated on March 2, 2010 (75 FR 

9377), there has been one public 

comment period and four public 

hearings. During the 60-day comment 

period from March 2 to May 3, 2010, for 

the reinstated proposed rule, we 

received a total of 24 comment letters in 

response to our request for new 

information: 2 from Federal agencies 

(duplicate letter from 2 submitters), 4 

from State or local agencies and 

governments, and 18 from organizations 

or individuals. During the public 

hearings on March 23, 2010, in Palm 

Desert, California, and March 24, 2010, 

in Yuma, Arizona, we received a total of 

4 comments: 1 written comment and 3 

oral comments. Two of these comments 

were from local government 

representatives and the remaining two 

from organizations or individuals. All 

comments received were reviewed for 

substantive issues and new information 

regarding the 1993 proposed rule to list 

the flat-tailed horned lizard as a 

threatened species, and we address 

those comments below. 

Comments From Federal Agencies 

Comment 1: The U.S. Navy does not 

support the listing of flat-tailed horned 

lizard as a threatened species because: 

(1) Listing or designation of critical 

habitat would encroach on the ability to 

perform military readiness activities at 

NAF El Centro; (2) the species is not 

threatened throughout a significant 

portion of its range; (3) conservation 

should be implemented through the 

existing Interagency Conservation 

Agreement, the Rangewide Management 

Strategy, and the updated NAF El 

Centro INRMP; and (4) conservation 

should be implemented through a 

continued working partnership with 

other State and Federal agencies, 

including the U.S. Navy. 

Our Response: Based on the rationales 

provided in this document, we agree 

with the U.S. Navy that the species does 

not warrant listing under the Act. 

Additionally, we agree that the 

Interagency Conservation Agreement 

and associated Rangewide Management 

Strategy make important contributions 

to reducing threats to the flat-tailed 

horned lizard and its habitat through 

efforts contributed by the Service, BLM, 

BOR, U.S. Marine Corps, U.S. Navy, 

Arizona Game and Fish Department, 

CDFG, and CDPR. Although many of 

these efforts are voluntary, conservation 

actions are formally incorporated into 

planning documents of participating 

agencies (such as the NAF El Centro 

INRMP and BLM’s California Desert 

Conservation Area Plan). We appreciate 

the U.S. Navy’s support of this longterm partnership and commitment to 

conservation of sensitive species, 

including the flat-tailed horned lizard, 

and their habitats through its 

participation in the Interagency 

Conservation Agreement and 

implementation of the NAF El Centro 

INRMP. For additional information on 

the Interagency Conservation Agreement 

and the associated Rangewide 

Management Strategy and the U.S. 

Navy’s conservation actions, please see 

Management and Populations under the 

Background section and Sikes Act under 

Factor D. 

Comments From State Agencies 

Comment 2: The Arizona Department 

of Transportation believes the flat-tailed 

horned lizard Interagency Conservation 

Agreement is an adequate regulatory 

mechanism that provides strong 

protection for the species on signatory 

lands. Much of the remaining habitat in 

southwestern Arizona is managed by 

agencies that are signatories to the 

Interagency Conservation Agreement. 

For example, project proponents for the 

construction of Arizona State Route 195 

(Yuma Area Service Highway) used the 

Rangewide Management Strategy to 

avoid and mitigate impacts to the flattailed horned lizard and its habitat. 

Additionally, the Arizona Department of 

Transportation believes the flat-tailed 

horned lizard Interagency Conservation 

Agreement is a viable mechanism for 

the long-term conservation of the 

species in the absence of listing under 

the Act. 

Our Response: We agree the 

Interagency Conservation Agreement 

and associated Rangewide Management 

Strategy is a viable conservation effort to 

promote the long-term conservation of 

flat-tailed horned lizard. The avoidance, 

minimization, and mitigation measures 

incorporated into the Yuma Area 

Service Highway project reduced 

impacts to the flat-tailed horned lizard 

and is an example of how the 

Rangewide Management Strategy can 

reduce impacts to the species associated 

with development (see Factor A). 

Comment 3: The CDPR expressed a 

concern that listing flat-tailed horned 

lizard as a threatened species would 

restrict CDPR’s ability to manage 

recreational activities and park 

operations at Ocotillo Wells State 

Vehicular Recreation Area (SVRA), and 

that listing the species under the Act 

may cause OHV use to move to off-site 

areas with little or no management 

control. The CDPR also stated that 

listing the species may potentially 

reduce the number of visitors, resulting 

in a negative economic impact on the 

region. Further, they believe that 

recreational OHV use does not 

conclusively show adverse effects to the 

species. 

Our Response: Although OHV activity 

has the potential to crush flat-tailed 

horned lizards (see Factor E) and impact 

the species’ habitat (Factor A), we 

determined it is not currently a 

substantial threat to the species 

throughout its range. We agree that OHV 

activity in designated and managed 

open or limited-use areas is preferable 

to unmanaged OHV activity elsewhere. 

We acknowledge CDPR’s contributions 

to the Rangewide Management Strategy 

through monitoring and management at 

Ocotillo Wells SVRA, and we encourage 

CDPR’s continued participation in the 

Interagency Conservation Agreement. 

Comment 4: The CDPR states that 

long-term studies of flat-tailed horned 

lizard are needed because annual 

climatic conditions can result in 

variability in population sizes. They 

believe that long-term studies and an 

adaptive monitoring program are 

warranted prior to listing the species 

under the Act. 

Our Response: We agree that more 

information on the effects of weather 

and climate on the flat-tailed horned 

lizard and its habitat would be helpful; 

however, we are required to make a 

determination based on the best 

available scientific and commercial 


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information. We determined the flattailed horned lizard does not require 

protection under the Act. CDPR’s 

contributions to the Rangewide 

Management Strategy have included 

funding studies to increase the 

knowledge of the species, and we 

encourage CDPR’s continued 

participation, including contributing to 

developing and implementing long-term 

studies and adaptive management 

programs. 

Comments Related to Biology, Ecology, 

or Climate Change 

Comment 5: One commenter believes 

flat-tailed horned lizard populations 

will take longer to ‘‘* * * rebound to 

stable wild populations than other 

classes of animals.’’ The commenter 

believes listing flat-tailed horned lizard 

as a threatened species under the Act is 

warranted because of low clutch 

survival rates from breeding to maturity 

due to impacts from predators and 

human activities. 

Our Response: The commenter did 

not provide any information regarding 

the class of animals to which he or she 

was referring in comparison to the flattailed horned lizard, or any information 

to substantiate the claim that wild 

populations of flat-tailed horned lizards 

are not stable. With regards to the 

commenter’s concerns about ‘‘low clutch 

survival rates from breeding to maturity 

due to impacts from predators and 

human activities,’’ flat-tailed horned 

lizards are known to produce relatively 

small clutches of eggs (N = 31; mean 

clutch size = 4.7; range = 3 to 7) 

(Howard 1974, p. 111) compared to 

most other horned lizards (Sherbrook 

2003, p. 139), and predation has been 

identified as a potential threat to the 

flat-tailed horned lizard (FTHLICC 

2003a, pp. 16–17). However, available 

information indicates predation does 

not appear to be excessively high 

throughout its range, although it is 

likely higher than natural levels near 

developed areas. Such results suggest 

that higher levels of predation of flattailed horned lizards observed in some 

areas is an ‘‘edge effect,’’ but much of the 

species’ distribution is away from 

habitat edges (see Factor C, Disease or 

Predation section). 

Comment 6: One commenter states 

that climate change will become more of 

an issue as ant population numbers 

decline because flat-tailed horned lizard 

populations will subsequently decline. 

Our Response: Flat-tailed horned 

lizards do feed primarily on harvester 

ants; however, what effects climate 

change may have on harvester ant 

populations is unclear. Although 

populations of harvester ants decline 

during periods of both drought and 

increased rain, they rebound as do 

populations of flat-tailed horned lizards 

(Tevis 1958, p. 701; Barrows and Allen 

2009, p. 311). Harvester ants are also 

capable of surviving extremes in 

temperature (Tevis 1958, p. 704). The 

effects that global climate change may 

have on localized climate in areas 

inhabited by flat-tailed horned lizards 

and harvester ants is unclear, and we 

are not aware of any evidence indicating 

that harvester ant populations will 

decline in the foreseeable future. 

Comment 7: One commenter stated a 

belief that the Service’s final 

determinations in the past have been 

correct and the flat-tailed horned lizard 

should not be listed as threatened under 

the Act. The commenter further stated 

that there are more flat-tailed horned 

lizards known today compared to 20 

years ago, and (with respect to climate 

change) there has been adequate rainfall 

to produce forage in the desert for this 

species to flourish. 

Our Response: As discussed in the 

Background section, the number of flattailed horned lizards is difficult to 

estimate. We do not have acceptably 

accurate data to show any trend, either 

increasing or decreasing, in flat-tailed 

horned lizard populations. Rainfall 

varies from year to year in the Colorado 

Desert (Shreve and Wiggins 1964, pp. 

18–20). We determine if a species needs 

protection under the Act based on 

analysis of the species’ status relative to 

one or more of the five factors described 

in section 4(a)(1) of the Act, and the 

standards for listing as endangered or 

threatened (see Summary of Factors 

Affecting the Species section). We 

determined the species is not in need of 

the protections afforded by the Act at 

this time. 

Comment 8: One commenter provided 

information resulting from research they 

conducted on flat-tailed horned lizard 

habitat loss in the Coachella Valley. The 

commenter believes that the reasons 

that the flat-tailed horned lizard was not 

listed in the past are because there was 

not enough known about this species’ 

biology and distribution, and the largest 

share of the species’ distribution was on 

Federal (BLM, DOD) lands such that the 

species could be managed without 

listing. The commenter’s opinion is that 

neither of the above reasons is 

applicable today. The commenter also 

believes the Coachella Valley has been 

underrepresented in past assessments 

and that construction of the border 

fence, OHV activity, and development 

of energy facilities pose threats to the 

species. 

Our Response: Our determination of 

whether to list a species is based on our 

assessment of the five listing factors 

described in the Act and the standards 

for listing as endangered or threatened. 

A determination is made using the best 

scientific and commercial information 

available. In the Summary of Factors 

Affecting the Species section, we 

address the potential threats that may be 

affecting the species, including those 

identified by the commenter. 

Additionally, we have also addressed 

the Coachella Valley Population in 

detail. 

Comment 9: One commenter opposed 

to the listing of the flat-tailed horned 

lizard believes that before this species 

should be considered for listing, 

researchers should conduct monitoring 

of the full desert ecosystem, as declines 

for this species may be a result of 

natural processes. 

Our Response: Our determination of 

whether to list a species as endangered 

or threatened is based on our 

assessment of the five listing factors 

described in the Act using the best 

available scientific and commercial 

information. These include potential 

threats from natural and manmade 

sources. Although anecdotal evidence 

suggests that flat-tailed horned lizard 

populations are smaller now than 

compared to the past (for example, 

Luckenbach and Bury 1983, p. 278), we 

do not have data to suggest a positive or 

negative trend (see Population 

Dynamics in the Background section). 

Comments Related to Threats 

Comment 10: Four commenters 

support listing the flat-tailed horned 

lizard as a threatened species, and one 

commenter supports listing as an 

endangered species with designated 

critical habitat. These commenters 

believe listing is warranted due to a 

number of threats, including: 

Recreation; OHV use (such as in the 

Yuha Desert, Coachella Valley, West 

and East Mesas, near Algodones Dunes, 

and near Yuma, Arizona); construction 

of the border fence and border patrol 

traffic; development (including 

renewable energy projects such as SES 

Solar Two Project or Ocotillo Express 

Wind Project); power lines (Sunrise 

Powerlink); road/highway development 

(Yuma Area Service Highway, El Golfo 

to Rocky Point Highway); other 

miscellaneous development (such as 

Travertine Point, Drop 2 Reservoir, All 

American Canal, Coyote Wells Specific 

Plan Project, Reynolds Atlas RV Storage 

Facility); nonnative plant invasions; 

predation; and climate change. In 

general, the commenters believe these 

threats will continue, resulting in more 

habitat lost than gained. Further, the 

commenter that asserts the species 


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should be listed as endangered states 

that Federal protection is necessary to 

ensure the survival of the species and 

eventual recovery, and ultimately 

reduce the costs of recovery. 

Our Response: Although we 

acknowledge losses of habitat can and 

do occur through natural and manmade 

processes, the determination to list a 

species is made by looking at the five 

factors described in section 4(a)(1) of the 

Act and the status of the species relative 

to the standards for listing as 

endangered or threatened. This 

determination is made solely on the 

basis of the best scientific and 

commercial information available, and 

takes into account regulatory 

mechanisms that many benefit the 

species and those efforts, if any, being 

made by any State or foreign nation, or 

any political subdivision of a State or 

foreign nation, to protect the species 

through habitat protection or other 

conservation practices. As described in 

the Summary of Factors Affecting the 

Species section, we assessed the 

potential threats to the species using the 

five factors. We also assessed the 

existing efforts and measures that 

benefit the species or its habitat that 

may potentially reduce threats. We 

determined that threats to the flat-tailed 

horned lizard throughout its range, 

including recreational OHV activity; 

various types of development; invasive, 

nonnative plants; predation; and climate 

change, are not of a magnitude that it is 

likely to become endangered in the 

foreseeable future. Specifically, the 

identified development projects are not 

a significant threat to the species 

throughout its range or the respective 

DPSs identified in the Distinct 

Population Segment section, above, 

because the projects (1) are subject to 

the avoidance, minimization, and 

compensation measures of the 

Rangewide Management Strategy (in the 

United States only); (2) are relatively 

small compared to the range of the 

species or DPSs; (3) do not result in 

complete barriers to flat-tailed horned 

lizard movement; (4) do not result in the 

elimination of large ‘‘parts’’ where the 

deleterious effects associated with small 

population size are likely to 

substantially affect the population; (4) 

or a combination of these, as detailed in 

the Summary of Factors Affecting the 

Species section. 

Comment 11: One commenter 

believes that urban development is 

conflicting with flat-tailed horned lizard 

survival. 

Our Response: As described in the 

Urban Development section under 

Factor A, urban development within the 

range of the flat-tailed horned lizard is 

largely occurring within areas that were 

previously developed for agriculture 

and is not resulting in additional habitat 

loss because the prior agricultural 

conversion had already made the land 

unavailable for the species. Urban 

development in flat-tailed horned lizard 

habitat is occurring, but in a limited 

area compared to the large area 

occupied by the species. Additionally, 

large areas of the species’ range are 

under some level of protection where 

urban development is prevented or 

restricted, including Management Areas 

created through implementation of the 

Rangewide Management Strategy, CDPR 

lands, BLM wilderness, Coachella 

Valley MSHCP reserves, and portions of 

two biosphere reserves in Mexico. 

Moreover, where urban development 

may occur, its impact is further reduced 

(through avoidance, minimization, and 

mitigation) by the measures that benefit 

the flat-tailed horned lizard (such as the 

Rangewide Management Strategy, 

Coachella Valley MSHCP, and Mexican 

Federal listing). Thus, we concluded 

that urban development is not a 

substantial threat to the species. 

Comments Related to the Rangewide 

Management Strategy 

Comment 12: Four commenters state 

that the Rangewide Management 

Strategy currently in place is working to 

the benefit of the species, and there is 

no need to list the flat-tailed horned 

lizard as a federally threatened species. 

Two of these commenters further agree 

with the 2008 Annual Progress Report 

which states that the Interagency 

Conservation Agreement and Rangewide 

Management Strategy continue to 

provide an effective management focus 

to conserve flat-tailed horned lizard 

habitat throughout its range. Two 

commenters also expressed concern that 

listing the species could undermine the 

Interagency Conservation Agreement 

and questioned the efficacy of listing the 

flat-tailed horned lizard prior to 

completion of the surveys called for by 

the Rangewide Management Strategy. 

Our Response: We agree with the 

commenters that the Rangewide 

Management Strategy is providing 

important conservation benefits to the 

flat-tailed horned lizard and its habitat 

in the United States. Although many of 

these efforts are voluntary, conservation 

actions are formally incorporated into 

planning documents of participating 

agencies (such as BLM’s California 

Desert Conservation Area Plan). 

Moreover, most of the measures 

outlined in the Rangewide Management 

Strategy are being successfully 

implemented (FTHLICC 1998, pp. 1–11; 

FTHLICC 1999, pp. 1–13; FTHLICC 

2001, pp. 1–24; FTHLICC 2003b, pp. 1– 

32; FTHLICC 2004, pp. 1–33; FTHLICC 

2005, pp. 1–37; FTHLICC 2006, pp. 1– 

34; FTHLICC 2007, pp. 1–33; FTHLICC 

2008a, pp. 1–35; FTHLICC 2009, pp. 1– 

38; FTHLICC 2010, pp. 1–33). Most of 

the benefits to the species occur within 

the United States. Although 

implementation of the Rangewide 

Management Strategy is also 

contributing to the conservation of the 

species in Mexico by promoting 

partnerships with local organizations in 

that country that are implementing 

programs that benefit the species, the 

benefits associated with the avoidance, 

minimization, and mitigation measures 

called for by the Rangewide 

Management Strategy are not in effect in 

Mexico. As such, the benefits afforded 

the species through implementation of 

the Rangewide Management Strategy, 

important though they may be, are 

limited. We appreciate the commenters’ 

support of the Interagency Conservation 

Agreement that is benefitting the flattailed horned lizard and its habitat. 

Please see our response to Comment 1 

and Management and Populations 

under the Background section for more 

information regarding the Rangewide 

Management Strategy. 

Regarding the commenters’ concern 

over the possibility that we may make 

a determination to list the species 

without complete flat-tailed horned 

lizard survey information, we note that 

we are required to make a final listing 

determination. Our determination of 

whether to list a species as endangered 

or threatened is based on our 

assessment of the five listing factors 

described in the Act using the best 

scientific and commercial information 

available. Although we agree population 

trend data would help us better 

understand the current status of the 

species, we must meet our obligations 

under the Act by examining the threats 

to the species. This analysis is presented 

in the Summary of Factors Affecting the 

Species section. We conclude that the 

species is not in need of the protections 

afforded by the Act at this time. 

Additionally, because we are not listing 

the species, the question of the potential 

effects of listing on the implementation 

of the Interagency Conservation 

Agreement is moot. 

Comment 13: Three commenters 

asserted that implementation of the 

Rangewide Management Strategy, 

including the designation of 

Management Areas, is not working to 

recover the species. The commenters 

stated that mitigation lands are 

insufficient to make up for losses of 

habitat, especially from threats such as 

OHV use and large-scale renewable 


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energy projects. Two of the commenters 

stated the strategy is inadequate and not 

rangewide. A fourth commenter stated 

that the Service has relied heavily on 

the Rangewide Management Strategy to 

prevent the flat-tailed horned lizard’s 

listing in the past. 

Our Response: With regard to the 

commenters’ concerns that mitigation 

lands may be insufficient to recover the 

species, we concluded that none of the 

existing or potential threats are likely to 

cause the flat-tailed horned lizard as an 

entire species or as any one of the 

Western, Eastern, or Southeastern DPSs 

to be in danger of extinction or likely to 

become so within the foreseeable future 

throughout all or a significant portion of 

its range; thus, the species does not 

need to be ‘‘recovered.’’ Implementation 

of the Rangewide Management Strategy, 

including the mitigation (compensation) 

by the signatory agencies is providing 

for the consolidation of the existing 

Management Areas by purchasing 

private inholdings within the 

Management Areas. Moreover, 

implementation of the avoidance and 

minimization measures included in the 

Rangewide Management Strategy is 

reducing certain potential future threats, 

including development of energy 

generation facilities and associated 

infrastructure on signatory lands. 

With regard to the commenters’ 

concerns that the Rangewide 

Management Strategy is not rangewide, 

the purpose of this strategy is to provide 

a framework for conserving sufficient 

habitat to maintain several viable 

populations of the flat-tailed horned 

lizard throughout the range of the 

species in the United States. Five 

Management Areas were designed to 

identify large areas of public land in the 

United States where flat-tailed horned 

lizards have been found, and to include 

most flat-tailed horned lizard habitat 

identified as key areas in previous 

studies (Turner et al. 1980, pp. 1–47; 

Turner and Medica 1982, pp. 815–823; 

Rorabaugh et al. 1987, pp. 103–109; 

FTHLICC 1997, p. 35). Furthermore, the 

Management Areas were delineated to 

include areas as large as possible, while 

avoiding extensive, existing and 

predicted management conflicts (such 

as OHV open areas). The Management 

Areas are meant to be the core areas for 

maintaining self-sustaining populations 

of flat-tailed horned lizards in the 

United States (FTHLICC 2003a, p. 47). 

Although this strategy does not include 

Mexico, implementation of the 

Rangewide Management Strategy 

includes coordination with partners in 

Mexico to promote efforts to benefit the 

species in that country (FTHLICC 2009, 

p. 14). Additionally, approximately 60 

percent of the habitat in Sonora 

(Mexico) lies within two Mexican 

Federal natural protected areas where 

impacts from development and other 

activities is limited (see Management 

and Populations in the Background 

section for further discussion). 

Regarding the use of the Rangewide 

Management Strategy in our past listing 

determinations (withdrawals), we did 

not rely solely on the Rangewide 

Management Strategy in our decisions, 

nor do we do so in this determination. 

As we state in our response to Comment 

12, the evidence indicates that 

implementation of the Rangewide 

Management Strategy is providing 

important conservation benefits to the 

flat-tailed horned lizard and its habitat; 

however, that is but one aspect we 

consider. Our determination to list a 

species is made by looking at the five 

factors described in section 4(a)(1) of the 

Act and the status of the species relative 

the standards for listing as endangered 

or threatened. This determination is 

made solely on the basis of the best 

scientific and commercial information 

available, and takes into account those 

efforts, if any, being made by any State 

or foreign nation, or any political 

subdivision of a State or foreign nation, 

to protect the species through habitat 

protection or other conservation 

practices. Our assessment of the effects 

of the five listing factors on the flattailed horned lizard is presented in the 

Summary of Factors Affecting the 

Species section. Our assessment of those 

efforts being made to protect the species 

through habitat protection or other 

conservation practices is presented in 

the Conservation Efforts section (see 

also Management and Populations 

under the Background section)—which, 

in this case, included the Rangewide 

Management Strategy. Thus, we have 

considered but have not relied solely 

upon the Rangewide Management 

Strategy in our determination. 

Comment 14: One commenter states 

that the Rangewide Management 

Strategy does not discuss impacts of the 

border fence (which they believe 

isolates populations) and proposed solar 

energy projects. Specifically, this 

commenter and a second commenter 

believe that the border fence in the 

Yuha Management Area and the 

proposed Tessera Solar North America 

Project (also known as the Imperial 

Valley Solar Project) will result in 

isolated populations of the species and 

fragmented habitat. Further, the second 

commenter believes this project will 

result in impacts to the flat-tailed 

horned lizard and its habitat from 

construction and maintenance, 

vibrations from vehicle traffic, changes 

in topography, destruction of vegetation 

that is a food source for harvester ants, 

and increased dust deposition on 

vegetation. Additionally, the first 

commenter believes the Service should 

analyze the impacts of the border fence 

and proposed solar projects on the 

viability of flat-tailed horned lizard 

populations and cumulative impacts of 

habitat loss. 

Our Response: As discussed in our 

Factor A and E analyses (Summary of 

Factors Affecting the Species section), 

we acknowledge that the border fence 

and solar (energy generation) projects 

may result in the loss or degradation of 

flat-tailed horned lizard habitat and 

potentially serve as barriers, isolating 

populations of flat-tailed horned lizards. 

Although not extensively discussed by 

the Rangewide Management Strategy, 

private development of solar and other 

energy generation facilities on lands 

controlled by signatory agencies is still 

subject to the avoidance, minimization, 

and mitigation measures called for by 

the Rangewide Management Strategy. 

For example, the project proponent for 

the Imperial Valley Solar Project 

designed the project to avoid and 

minimize impacts to flat-tailed horned 

lizard Management Areas and is 

providing funds to acquire off-site 

habitat areas as compensation for 

unavoidable impacts, all per the 

specifications of the Rangewide 

Management Strategy (BLM 2009, pp. 4– 

7 to 4–10). Because of the prevalence of 

Federal and State lands in the U.S. 

portion of the range of the flat-tailed 

horned lizard and because most of this 

land is managed by signatories to the 

Interagency Conservation Agreement 

implementing the Rangewide 

Management Strategy, we expect that 

the vast majority of proposed energy 

development projects that are likely to 

affect flat-tailed horned lizard habitat 

will be subject to the avoidance, 

minimization, and compensation 

measures incorporated into the 

Rangewide Management Strategy (see 

Energy Generation and Facility 

Development section). 

Such projects may also serve as 

barriers to flat-tailed horned lizard 

movement. Many of the proposed and 

anticipated projects are likely to occur 

in the Western Population area. As 

described in the ‘‘Barriers and Small 

Populations’’ section under Factor E, the 

parts of the Western Population north 

and south are large enough to likely not 

be substantially affected by the threats 

associated with small population size. 

Moreover, Interstate 8, which runs along 

the southern edge of the Imperial Valley 

Solar Project and many of the other 

proposed or anticipated energy 


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generating projects in the area, is 

already likely to be a substantial barrier 

to flat-tailed horned lizards within the 

area of the Imperial Valley Solar project. 

Development of renewable energy is 

not without impacts, but 

implementation of the Rangewide 

Management Strategy, either under the 

voluntary Interagency Conservation 

Agreement or as it is incorporated into 

existing regulatory mechanisms, is 

anticipated to reduce the direct and 

indirect effects, including habitat loss 

and isolation of populations. We do not 

believe vibrations of vehicle traffic, 

changes in topography, destruction of 

vegetation that is a food source for 

harvester ants, and dust on vegetation 

will be any more substantial than the 

actual loss or degradation of flat-tailed 

horned lizard habitat, the effects of 

which we anticipate to be reduced by 

avoidance, minimization, and 

mitigation measures of the Rangewide 

Management Strategy. Moreover, the 

cumulative effects of habitat loss are 

reduced through implementation of the 

Rangewide Management Strategy by the 

creation and maintenance of large 

blocks of flat-tailed horned lizard 

habitat, including the establishment of 

Management Areas, the 1 percent cap on 

impacts, the avoidance and 

minimization measures directed by the 

Rangewide Management Strategy, and 

the consolidation of the respective 

Management Area through the purchase 

of private inholdings with monies 

acquired from compensation for 

unavoidable impacts from development 

activities. 

Regarding the concerns raised by the 

commenter about the border fence, we 

also acknowledge in our Factor E 

analysis that tactical infrastructure 

(such as fencing, lighting, and access 

and patrol roads) along portions of the 

border fence area has the potential to 

serve as a barrier for flat-tailed horned 

lizard movement. However, installed 

fencing has been constructed to allow 

movement of small animals (USCBP 

2008a, pp. 1–4 to 1–6 and Appendix B; 

USCBP 2008b, pp. 2–5 and 8–9); thus, 

we do not anticipate the fence itself to 

completely hinder flat-tailed horned 

lizard movement (see ‘‘Barriers and 

Small Populations’’ under Factor E). 

Additionally, with respect to the Yuha 

Desert Management Area, this area was 

selected for management protections of 

flat-tailed horned lizards because it is 

likely to support high densities of 

lizards (i.e., 0.7 individuals per ha (0.3 

per ac), which is a conservative 

estimate). Moreover, as mentioned 

above, the border fence is likely a 

semipermeable barrier for small species 

such as flat-tailed horned lizard, 

allowing some connectivity between the 

Yuha Desert Management Area and the 

areas of habitat in Mexico. 

Comment 15: One commenter 

believes the Rangewide Management 

Strategy does not provide enough 

protection because the document 

acknowledges that it is unknown 

whether the lands set aside are 

sufficient, and that the Ocotillo Wells 

State Vehicular Recreation Area is not 

being managed adequately. A second 

commenter stated that they believe BLM 

is understaffed and underfunded, which 

has led to its inability to reduce impacts 

on flat-tailed horned lizard Management 

Areas. 

Our Response: As described in the 

‘‘Barriers and Small Populations’’ 

section under Factor E, we evaluated the 

size of the parts formed as a result of 

potential barriers. We calculated the 

Western, Eastern, and Southeastern 

Population areas, as defined herein and 

based upon the current distribution map 

presented in the revised Rangewide 

Management Strategy (FTHLICC 2003a, 

p. 5), are 341,989 ha (845,073 ac), 

169,617 ha (419,133 ac), and 1,073,551 

ha (2,652,802 ac), respectively. Within 

those three Population areas combined, 

we found about 91 percent of the area, 

despite containing potential barriers, is 

in large enough blocks that the 

populations of flat-tailed horned lizards 

are not likely to be affected by threats 

associated with small populations. 

Although the Rangewide Management 

Strategy is an important conservation 

effort that provides substantial benefit to 

the flat-tailed horned lizard and its 

habitat, especially within the United 

States, the status of the species does not 

depend solely upon the lands set aside 

through implementation of the 

Rangewide Management Strategy. 

Similarly, the status of the flat-tailed 

horned lizard does not depend solely on 

management that may or may not be 

adequate on Ocotillo Wells SVRA; 

however, management activities that 

reduce threats to the species make 

important contributions to the status of 

the species at a local or regional level. 

Moreover, for implementation of the 

Interagency Conservation Agreement to 

be successful, each signatory agency 

should implement its share of the 

Rangewide Management Strategy (see 

also the Management and Populations 

in the Background section, and the 

Description of Specific Populations 

section for further discussion). 

Comment 16: One commenter stated 

that flat-tailed horned lizards should not 

be listed as a threatened species because 

there has been sufficient management in 

place over the past 10 years. The 

commenter believes management efforts 

should be implemented to eradicate 

Argentine ants (Linepithema humile), 

which displace the main food source 

(harvester ants) for flat-tailed horned 

lizards. 

Our Response: We agree with the 

commenter that there is sufficient 

management and conservation occurring 

for flat-tailed horned lizards (see the 

Finding section and Management and 

Populations under the Background 

section of this document for discussion 

of the long-term management of this 

species). We will continue to work with 

our partners to implement management 

actions to benefit this species. 

With regard to the commenter’s 

concern about Argentine ants, the 

evidence indicates that Argentine ants 

are not a threat to flat-tailed horned 

lizards. Argentine ants do not tolerate 

hot, dry conditions (Holway et al. 2002, 

p. 1610). The range of the flat-tailed 

horned lizard is hot and dry (see 

Background section), suggesting that 

Argentine ants do not invade flat-tailed 

horned lizard habitat to any substantial 

degree (Barrows et al. 2006, p. 492); 

thus, they do not substantially affect the 

primary food of the species throughout 

most of the species’ range. Therefore, we 

do not believe eradication of Argentine 

ants in flat-tailed horned lizard habitat 

is a warranted management action to 

benefit the species. 

Comment 17: One commenter states 

that the management strategy is 

sufficient for flat-tailed horned lizard 

and therefore listing this species is not 

warranted. Specifically, the commenter 

described the following management 

actions that are benefiting the species: 

(1) Since 1997, the Imperial Irrigation 

District has paid $10,000 to offset 

potential project impacts to habitat; (2) 

although border patrol and unpermitted 

OHV use continue to impact the species, 

there are no significant trends in lizard 

encounter rates in Yuha Desert, East 

Mesa, or West Mesa from 1979 to 2001; 

(3) agricultural land development is no 

longer occurring; (4) urbanization is not 

occurring in Yuha Desert, East Mesa, or 

West Mesa; and (5) the Mexican 

Government is providing protections to 

flat-tailed horned lizards. 

Our Response: As described in our 

analysis above, we agree with the 

commenter’s statements in general. The 

mitigation (compensation or off-setting) 

measures associated with the 

Rangewide Management Strategy are 

important to consolidating the 

Management Areas under the control of 

signatory agencies. We agree monitoring 

data indicate that flat-tailed horned 

lizard populations in the surveyed 

Management Areas are not low and are 

not declining. We also agree that 


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14268 Federal Register / Vol. 76, No. 50 / Tuesday, March 15, 2011 / Proposed Rules 

agricultural and urban development are 

not significant threats to the species, as 

discussed under Factor A, and that the 

protections afforded to the species by 

Mexican laws are not inadequate. 

References Cited 

A complete list of all references cited 

in this document is available on the 

Internet at http://www.regulations.gov. 

Additionally, a complete list of all 

references cited, as well as others, is 

available upon request from the 

Carlsbad Fish and Wildlife Office (see 

ADDRESSES). 

Author 

The primary authors of this document 

are staff members at the Carlsbad Fish 

and Wildlife Office (see ADDRESSES

above). 

Authority: The authority for this action is 

section 4(b)(6)(B)(ii) of the Endangered 

Species Act of 1973, as amended (16 U.S.C. 

1531 et seq.). 

Dated: February 25, 2011. 

Daniel M. Ashe, 

Acting Director, Fish and Wildlife Service. 

[FR Doc. 2011–5411 Filed 3–14–11; 8:45 am] 

BILLING CODE 4310–55–P 


jlentini on DSKJ8SOYB1PROD with PROPOSALS3