A Method of Counting Some Diurnal Ground Lizards of the Genera Holbrookia and Cnemidophorus with Results from the Big Bend National Park

Abstract

The Big Bend region lies in a great U-shaped bend in the course of the Rio Grande. The Big Bend National Park, at the southernmost tip of the Big Bend region and bordered on two sides by the Rio Grande, is included wholly within the Big Bend Biotic District of the Chihuahuan Biotic Province. The climate ranges from arid to semi-arid, and great variations of temperature are recorded within relatively short distances. The total annual rainfall is less than 10 inches at lower elevations but approaches or may even exceed 20 inches in the mountains. Most occurs during late summer and early fall. Quantitative studies on the herpetofauna were restricted to the diurnal ground lizards, Holbrookia texana, Cnemidophorus tigris, and C. sacki, on the northern slope of the Chisos Mountains, the highest range in the park. Trapping of lizards was carried on, and observations were made on marked individuals during the summer of 1956. A quadrat method was developed whereby all lizards are counted within a plot of known area for a specific number of days. These daily counts are averaged to arrive at an index number recorded as lizards per acre. Six quadrats, placed along a transect at elevations of approximately 2,800, 3,100, 3,400, 4,650, and 5,250 feet, were operated for two consecutive years. Lizard numbers and vegetation density were recorded and compared from elevation to elevation and from year to year. Only one quadrat (4,200 feet) varied significantly and showed an increase in lizard numbers from the first year to the second. A highly significant difference correlated with elevation was present. The greatest numbers of lizards were recorded at the 4,200-foot level, the least at the 2,800- and the 5,250-foot levels. The density of grasses and forbs was lowest at the 2,800-foot level and highest at 5,250 feet. No change in ratio between the different species of lizards occurring together was apparent during the years of operation.