Characteristics of peripheral populations of parthenogenetic Cnemidophorus laredoensis A (Squamata:Teiidae), in southern Texas

From 1984-2004 the distributional ecology of the parthenogenetic Cnemi-dophorus laredoensis (= Aspidoscelis laredoensis) complex both north and south of the Rio Grande between Amistad Reservoir and the Gulf of Mexico was studied. Although dozens of sites inhabited by clonal complex A of C. laredoensis were discovered within a few km of the river (over a geographic range in parts of Webb, Zapata, Starr and Hidalgo counties, Texas, and Tamaulipas State, Mexico), the species was observed at only three sites in two Texas counties that were widely removed and apparently disjunct from the river-centered zone. In order to better understand what factors limit the distribution of C. laredoensis A, these three most distant sites from the Rio Grande (55.5 to 75.5 km) where this hybrid-derived species is in syntopy with maternal progenitor C. gularis (= Aspidoscelis gularis): Catarina, Dimmit County, and Encinal and Artesia Wells, La Salle County, Texas were studied. Each peripheral site was characterized by sandy substrate that is known to be one of the most important requirements for C laredoensis A. The relative amounts of the original thorn scrub vegetation favorable to C. gularis and chronically disturbed habitat favorable to C. laredoensis A at each site constituted the major determinant of the relative size of populations of the two species. The absence of C. laredoensis A north of these sites in Dimmit and La Salle counties is probably a result of ecological resistance to expansion consisting of unsuitable substrate and vegetation. There was no evidence that a low frequency of hybridization between normally parthenogenetic females of C. laredoensis A and mates of C. gularis or periodic collection of C. laredoensis A at Catarina and Artesia Wells measurably destabilized these populations.