The Distribution and Abundance of the Flat-Tailed Horned Lizard (Phrynosoma mcallii)

Abstract

The occurrence of the flat-tailed horned lizard (Phrynosoma mcallii) was investigated in 458 sections (each 2.59 km2) in Riverside, San Diego and Imperial counties, California, in 1979. In all sections, 40 P. mcaUii (0.09/man-hour) and 2,191 horned lizard scats (4.8/man-hour) were counted. When sections were grouped in 12 general areas, mean numbers of horned lizards ranged from 0.40/ hr (north of Benson Dry Lake) to 0.00/hr (northern East Mesa, Coyote Wells area). The density of P. mcallii in a plot in the Yuha Basin (40 km west of El Centro) was 6-8/ha. The flat-tailed horned lizard is similar to P. platyrhinos in terms of sex ratio (1:1), age at maturity (20 months), and ability to la more than one clutch of eggs in favorable years. Mean home ranges for five male an urfemale P. mcallii (estimated as convex polygons) were 0.13 ha and 0.05 ha, respectively. P. mcallii feeds almost exclusively (>95% of items) on ants. The most commonly consumed species are harvester ants (Veromessor pergandei, Pogonomyrmex californicus and P. magnacanthus). In 1980 the relative abundance of P. mcallii (L), density (P), diversity (A1) of perennial plants and relative abundance of harvester ants (A) were estimated in ten plots in southeastern California. The abundance of the lizard was positively correlated (r = 0.93) with perennial density, and a clear positive association between lizard and ant densities was exhibited in nine of the ten plots. A regression model accounted for 91% of observed variation in lizard density, and predicted relative abundance of P. mcallii as: L = 75.0P + 8.6A1 - 0.04A - 7.9 At present, P. mcalii is not an endangered species. However, the original geographic range of this lizard has been diminished, and the species is not as abundant in certain parts of its range as it once was. Such changes have probably been brought about by human activities. P. mcallii, like many other desert species, is not in immediate danger of extinction but stands in jeopardy because of continued alteration or destruction of its habitat.