Habitat Use and Its Effect on Body Size Distribution in a Population of the Tree Lizard, Urosaurus ornatus

Abstract

The abiotic environment can play an important role in generating variation within and among populations of ectotherms (see Dunham et al., 1989). Among the more important of abiotic factors influencing ectotherm life histories and growth rate is temperature (Sinervo and Adolph, 1989,1994; Sinervo, 1990), which is often mediated by its potential effect on the duration of activity periods (Grant and Dunham, 1990). Individuals and/or populations living in different habitats or slopes (e.g., east vs. west-facing; north vs. south-facing) can potentially have different potential activity periods which, in turn, may influence an individual's ability to grow, as well as the body size distributions among habitats and slopes within a single population. In a population of Sceloporus virgatus along a creekbed in the Chiricahua Mountains of southeastern Arizona, G. Smith (1995) found that the mean body sizes of individuals differed between habitats (an open slide habitat vs. a closed woods habitat) and between slopes (north-facing vs. south facing). In addition, there were differences in growth rate, survivorship, and proportion of females reproducing in their first year (G. Smith, 1995). In this paper, I report observations on habitat use and body size in a population of Urosaurus ornatus occurring in the same creekbed as the S. virgatus studied by G. Smith (1995, 1996), where they have previously been shown to be potential competitors (D. Smith, 1981). Because of the differences in body size distributions and life history and demographic traits previously observed in S. virgatus, I expected to find parallel differences in U. ornatus. A population of Urosaurus ornatus was studied from 8 April to 22 July 1993 (concurrent with the studies on S. virgatus). The study site was located in the Coronado National Forest, 2.5 km SW of the Southwestern Research Station in the Chiricahua Mountains of southeastern Arizona. The site consisted of a 1.2 km