RESPONSE OF A LIZARD SPECIES TO THE SENESCENCE OF A DOMINANT PLANT INTHE SONORAN DESERT

Animal species often respond to changes in the plant community during ecological succession. I discovered such a response in the distributio n of a lizard (Urosaurus ornatus, tree lizard) associated with differe nt stages of senescence of saguaro cactus (Carnegiea gigantea). From 1 986 through 1995, I periodically assessed the distribution and numbers of tree lizards in the mesquite (Prosopis juliflora), pale verde (Cir cidium microphylum), and saguaro association in the Saguaro National P ark, Arizona. Many of the lizards use saguaro cactus as vantage points for territory defense, feeding and oviposition. Two study plots conta ined 137 sites which consisted of living, dead standing; and dead pron e saguaro cactus. Plots were surveyed for tree lizards in 1986, 1987, 1988, 1992, and 1995. In each year, a site was counted as used by liza rds if a resident lizard (captured during two or more census periods) was present. During the study, only 3/91 (3.3%) living cactus were eve r used by lizards. In contrast, 73% (27/37) of dead standing saguaro a nd 12.58 (4/32) of dead prone cactus were used in at least one year of the study. Living saguaro comprised an average of 63% of all sites, d ead standing was 19%, and dead prone was 18%. An average of 79% of all lizard records were from the use of dead standing cactus. Only 7% of lizard records were from living saguaro. Analysis of the pattern of us e of the classes of cactus showed a highly significant saguaro effect, no significant differences in use among years, and no significant sag uaro class by year interaction. Saguaro cactus provide opportunities f or exploitation of a distinct microhabitat resource (primarily dead st anding cactus) by tree Lizards. It appears that dead saguaro are quick ly recognized and exploited by tree lizards, which allows an expansion of the territory and home range boundaries of individual lizards.