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Literature Review

Flat-tailed Horned Lizard

Phrynosoma mcallii

USFWS: No official status
CDFG: Protected species, California Species of Special Concern

Background

Distribution, Abundance and Trends. The flat-tailed horned lizard is often associated with sand flats and sand dunes, although it is rare on larger dunes. It also occurs far from blowsand on concreted silt and gravel substrates (Beauchamp et al. 1998; C. Barrows, pers. comm.; Muth and Fisher 1992). In their comparisons of habitat types, Turner et al. (1980) determined the "best" habitat consisted of hard packed sand or desert pavement overlain with fine blowsand. The most common perennial plants associated with habitat for this lizard are creosote bush, Larrea tridentata and white bursage, Ambrosia dumosa (Turner et al. 1980; Muth and Fisher 1992).

Within the Plan area, the flat-tailed horned lizard occurs at lower elevations of the valley, generally below approximately 800 ft. (Fisher, pers. comm.). This lizard is found in two protected areas created by the Coachella Valley Fringe-toed Lizard Habitat Conservation Plan: the Coachella Valley Preserve and the Whitewater River Floodplain Preserve. Another population is known from an unprotected area at the east end of the Indio Hills on the north side of the Coachella Canal. A potential habitat corridor exists currently between the east end of the Indio Hills and the Coachella Valley Preserve. The plan area represents the northernmost and westernmost limits of flat-tailed horned lizard geographic range. The populations in the Coachella Valley are isolated from all other flat-tailed horned lizard populations by agricultural, urban development and by the Salton Sea. As a group, the Coachella Valley population can be viewed as a distinct vertebrate population pursuant to the federal Endangered Species Act and as such is a good candidate for future listing action. To date, no analyses have been completed to determine if this distinct population differs genetically from the more southeastern populations. The historic range of this species included suitable habitat in southeastern California, southwestern Arizona, northwestern Sonora, Mexico, and northeastern Baja California, Mexico. In California, they occurred in the Lower Colorado River Basin and the Salton Basin (Coachella and Imperial Valleys) from Palm Springs south-southeast to the Mexican border; an area of about 3,462 square miles. Historically there were about 694 square miles in the Coachella Valley Plan area of Riverside County. Currently, less than 50% of the historic habitat in California remains (Turner et al. 1980).

The flat-tailed horned lizard lives in low elevation desert, below about 800 feet in elevation. Extremely high temperatures and low rainfall and humidity characterize this low desert. The flat-tailed horned lizard has a higher preferred body temperature than its congener the desert horned lizard, Phrynosoma platyrhinos (Brattstrom 1965). This enables this lizard to exploit a hotter environment, but at the same time may restrict it to that environment. Thus there is little overlap in the geographic ranges of the two horned lizards found in the Coachella Valley (P. mcallii and P. platyrhinos).

Like related species, flat-tailed horned lizards are ant eaters, they are myrmecophageous. Ants, especially harvester ants, comprise about 98% of their diet. The proportion of ants in the diet is substantially higher in the flat-tailed horned lizard than in any other horned lizard (Pianka and Parker 1975, Turner and Medica 1982).

The flat-tailed horned lizard is relatively active for a desert lizard. A majority (54%) of the day is spent in some kind of activity, including feeding, digging burrows, and running (Muth and Fisher 1992). They eat ants they encounter while moving. They dig burrows to escape hot midday temperatures, and for winter hibernation. Most of the remaining activity involved running to locate food, suitable burrow sites, and mates. The mean home range size is nearly 300,000 sq. ft. (over 6½ acres), a large portion of which is covered daily. When approached by a potential predator, a flat-tailed horned lizard usually stops running and flattens its body against the ground. It relies on cryptic coloration to avoid predation and will usually remain immobile until after the threat has passed. This behavior makes the species difficult to locate in the field; in blowsand habitats, they may be located by following tracks left in freshly deposited sand (C. Barrows, pers. comm.).

Adult flat-tailed horned lizards are obligatory hibernators (Mayhew 1965). They hibernate from mid November to mid February in shallow burrows, although at least some juveniles are active on warm days during the winter (C. Barrows, pers. comm.). Reproductive activity begins in the spring and the first clutch of eggs hatches in late July. A second cohort may hatch in September. One or both of these cohorts may be lacking if environmental conditions are severe. Females lay about five eggs per clutch, on average. Young grow quickly and reach sexual maturity by one year of age.

About 50% of all individuals survive from one year to the next, with most mortalities in mid summer. Population density estimates range from 0.5 (Muth and Fisher 1992) to 2.4 (Turner and Medica 1982) flat-tailed horned lizards per acre. The lower value may underestimate the true density, and the higher value may overestimate it. In addition, density may vary annually with changes in environmental conditions.

Threats and Limiting Factors. Threats to the species include increased mortality and loss of habitat. A Population Viability Analysis indicates that populations are particularly sensitive to changes in mortality rate and fecundity, and environmental variables such as drought and years with above-normal rainfall. A slight change in mortality or fecundity can lead to extinction (Rorabaugh et al., unpublished data). Threats to habitat within the Coachella Valley MSHCP include agricultural development, urban development, expansion of the Salton Sea, expansion of utility corridors, and off-highway vehicle use. Here, 84% of the historic habitat has been lost to urban and agricultural development (K. Nicol pers. comm.). This estimate is conservative because much of the remaining habitat is now discontinuous and fragmented. Roads are known to dramatically increase mortality of desert reptiles, including flat-tailed horned lizards, and may deplete the population for as much as one mile from the road edge. Another serious edge effect is predation by household pets that are allowed to wander into habitat from surrounding urban development. Non-native species including Saharan mustard (Brassica tournefortii) and Russian thistle (Salsola tragus) can may impact this species as well.

Special Considerations. Within the Plan area, other species of concern whose habitat overlaps with that of the flat-tailed horned lizard include the Coachella Valley milkvetch, Palm Springs pocket mouse, Palm Springs ground squirrel, Coachella giant sand treader cricket, Coachella Valley Jerusalem cricket, and burrowing owl.