Life History and Demographic Variation in the Lizard Sceloporus Graciosus: A Long‐Term Study

Abstract

An 11—yr study of life history and demographic variation in the sagebrush lizard Sceloporus graciosus was carried out on two study areas (Rattlesnake Ridge and Ponderosa Flat) in the Kolob Mesa Section of Zion National Park, Utah. Two primary objectives of this mark—recapture study were to: (1) quantify variation in age structure, age and size at maturity, age—specific survivorship and fecundity, and individual growth rates, and (2) conduct a series of density reduction experiments designed 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 average precipitation and temperature to variation in individual growth, reproduction, and demography. At both sites the active season was °160 d, extending from early April to mid—September. Reproduction 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 °22—24 mo and a minimum snout—vent length of °50 mm. Most mature females produced two clutches of eggs per year, and there was no statistically significant variation in either mean clutch size or body—size—adjusted clutch size among the 11 yr of study. Clutch size was significantly correlated with body size. Relative clutch mass averaged 0.247 and was not significantly correlated with body size. Since hatchlings first appeared in early to mid—August, their first growing season was °2 mo long. There was no significant 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). Survival of yearling males (0.45) and females (0.43) at Rattlesnake Ridge was not significantly different. There were no other significant differences in the survival of males and females (X = 0.56 for both sexes) within any age class in any year of the study. However, the survival of yearlings was significantly lower than that of older lizards in both populations. Mean posthatchling survival over all years was 0.45, and there was significant heterogeneity in posthatchling survival among years. Average annual survival of immigrants (0.32) was significantly lower than that of residents (0.44). There was a significant negative linear relationship 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 negatively correlated with density of conspecific lizards. The negative correlation implies direct density dependence of hatchling mortality rates and is a potentially important mechanism of population regulation. Removals of almost all yearling and older age lizards from the study sites resulted in significant increases in growth rates of hatchlings in the year of the removal and yearlings during the following year. Four results from this study combine to suggest substantial resource limitation of S. graciosus on the Kolob Mesa. (1) Snout—vent lengths attained by yearling lizards were positively correlated with deviations from long—term mean rainfall values. (2) Body sizes attained by yearlings were greatest in the years following density reductions. (3) Body size attained by yearlings was negatively correlated with density of conspecifics. And (4), in a year in which a density reduction followed a warm, wet spring, more yearling females reached maturity than in all other years of the study combined.