LIFE-HISTORY AND DEMOGRAPHIC VARIATION IN THE LIZARD SCELOPORUS-GRACIOSUS - A LONG-TERM STUDY

An 11-yr study of life history and demographic variation in the sagebr ush lizard Sceloporus graciosus was carried out on two study areas (Ra ttlesnake Ridge and Ponderosa Flat) in the Kolob Mesa Section of Zion National Park, Utah. Two primary objectives of this mark-recapture stu dy were to: (1) quantify variation in age structure, age and size at m aturity, age-specific survivorship and fecundity, and individual growt h rates, and (2) conduct a series of density reduction experiments des igned to elucidate the effects of density on growth rates and survival of posthatchling lizards. In addition, we examined the relationships of variation in population density and deviation from long-term averag e precipitation and temperature to variation in individual growth, rep roduction, and demography. At both sites the active season was almost- equal-to 160 d, extending from early April to mid-September. Reproduct ion occurred during a 50-d period between mid-May and early July. Mean clutch size was 3.7 eggs and most females produced their first clutch in the 2nd yr of life (their third active season) at an age of almost -equal-to 22-24 mo and a minimum snout-vent length of almost-equal-to 50 mm. Most mature females produced two clutches of eggs per year, and there was no statistically significant variation in either mean clutc h size or body-size-adjusted clutch size among the 11 yr of study. Clu tch size was significantly correlated with body size. Relative clutch mass averaged 0.247 and was not significantly correlated with body siz e. Since hatchlings first appeared in early to mid-August, their first growing season was almost-equal-to 2 mo long. There was no significan t sexual dimorphism in growth rate or body size in either population. There was great variation in estimates of egg-yearling survival among years. Egg-yearling survival probability varied from 0.12 to 0.59 with a mean of 0.28. At Ponderosa Flat, the survival of yearling males (0. 38) was significantly lower than that of yearling females (0.47). Surv ival of yearling males (0.45) and females (0.43) at Rattlesnake Ridge was not significantly different. There were no other significant diffe rences in the survival of males and females (XBAR = 0.56 for both sexe s) within any age class in any year of the study. However, the surviva l of yearlings was significantly lower than that of older lizards in b oth populations. Mean posthatchling survival over all years was 0.45, and there was significant heterogeneity in posthatchling survival amon g years. Average annual survival of immigrants (0.32) was significantl y lower than that of residents (0.44). There was a significant negativ e linear relatonship between yearling body size in late June and total density of posthatchling lizards. A stepwise linear regression model revealed significant effects of both rainfall (and presumably resource availability) and population density on the growth of yearlings. This model explained 78% of the annual variation in yearling growth. Rank correlation analysis revealed that survivorship of hatchlings was nega tively correlated with density of conspecific lizards. The negative co rrelation implies direct density dependence of hatchling mortality rat es and is a potentially important mechanism of population regulation. Removals of almost all yearling and older age lizards from the study s ites resulted in significant increases in growth rates of hatchlings i n the year of the removal and yearlings during the following year. Fou r results from this study combine to suggest substantial resource limi tation of S. graciosus on the Kolob Mesa. (1) Snout-vent lengths attai ned by yearling lizards were positively correlated with deviations fro m long-term mean rainfall values. (2) Body sizes attained by yearlings were greatest in the years following density reductions. (3) Body siz e attained by yearlings was negatively correlated with density of cons pecifics. And (4), in a year in which a density reduction followed a w arm, wet spring, more yearling females reached maturity than in all ot her years of the study combined. Finally, we compared data from the Ko lob Mesa populations to those from other populations of S. graciosus.