Changes in Competing Populations of Whiptail Lizards (Cnemidophorus) in Southwestern Texas

Abstract

Population dynamics of whiptail lizards were studied at the Black Gap Wildlife Management Area, Brewster Co., Texas, in 1951 and 1952. Some of these studies were repeated in 1962. In the decade between the studies, Black Gap recovered from the effects of a severe drought. Changes in the lizard populations consisted of changes in population densities and in diet. Cnemidophorus tigris showed an increased density in the ecological association dominated by it in 1952 and also increased densities in the associations dominated by the other two species of the area in 1952. Cnemidophorus septemvittatus and C. inornatus which equalled tigris in numbers in 1952 are now quite rare.. Changes in diet consisted primarily of a higher percentage of caterpillars and a lower percentage of termites in 1962 than in 1952. Population dynamics of whiptail lizards (Cnemidophorus) were studied at four stations in the Chihuahuan Biotic Province (Blair, 1940, 1950; Dice, 1943) of southwestern Texas during the summers of 1951 and 1952. The results of these studies were reported in a series of papers (Milstead, 1957a, 1957b, 1958, 1959, 1961). In the years following 1952, vegetational, climatic and faunal changes were observed at two of the stations: The Stockton Plateau, Terrell County, and the Black Gap Wildlife Management Area, Brewster County. Changes were also noted at the other two stations, although the regions were visited less frequently. A few of the changes at all four stations have been reported previously (Milstead, 1960, 1961). In 1962 attempts were made to duplicate some of the whiptail studies at the two areas most closely observed. These efforts failed on the Stockton Plateau because of devastation of the land by livestock, but work at the Black Gap area was more rewarding. The studies that were repeated consisted of stomach analyses of collected specimens and estimates of density by collecting and trapping. The results of the 1962 studies were then compared with the results obtained a decade earlier. On visits to Black Gap between 1952 and 1962, it was noted that the area underwent several vegetational changes. The years 1949 to 1957 were drought years over much of the Southwestern United States. In 1952, Black Gap was arid with a sparse cover of desert vegetation. During the climax of the drought in 1957, vegetation was very sparse generally, and virtually absent in some areas. By 1959 the station had begun to recover. In 1961 and 1962, the land had reached a condition far more mesic than it had been a decade earlier. The 1 This work was supported as a part of project NSF G 15556 from the National Science Foundation. The cooperation of the Texas Game and Fish Commission in permitting this work to be done at the Black Gap Wildlife Management Area is gratefully acknowledged. Mr. Donald R. Patten served as research assistant in 1962. Thanks are also due Dr. Donald W. Tinkle of Texas Technological College for a number of helpful suggestions.