The Eastern Collared Lizard (Crotaphytus collaris) has experienced extensive population declines over the past half century in the Ozark Mountains. Previous research suggests that glade habitat degradation through woody vegetation encroachment is an important factor resulting in population declines. In this dissertation I used information on time-energy budgets to investigate the link between habitat degradation and shifts in life history traits likely resulting in population declines of Eastern Collared Lizards (Crotaphytus collaris) in the Ozarks. In chapter one, I addressed the influence of dense woody vegetation encroachment on age-specific growth, body size, body condition and reproduction of C. collaris in northern Arkansas. Results from chapter one suggested that populations in encroached glades (defined by dense woody vegetation encroachment) experienced reduced body growth rates, delayed age of maturity, reduced clutch sizes and a nearly 50% decline in annual population fecundity rates. In chapter two, I investigated the role of behavior (postural adjustment) on the thermal ecology estimates of surface-active lizards using thermal data of C. collaris in one of my study populations. I used the data in chapter two to determine the most suitable methodologies to investigate the thermal ecology of C. collaris in northern Arkansas. In chapter three, I investigated the thermal sensitivity of digestion in C. collaris. Results in chapter three suggested that C. collaris digestion performance is affected by temperature, primarily through effects on passage times and voluntary feeding rates. In chapter four, I used results from the previous three chapters to erect several mechanistic hypotheses aimed at identifying the link between glade encroachment and reduced reproductive rates in C. collaris. Data in chapter four suggest C. collaris in encroached glades experience reduced environmental heat loads, shorter activity-times and less time-at-temperature suitable for digestive processing. The reduction in time-at-temperature for digestive processing resulted in lower energy available for growth and reproduction (4̃1%), which led to a near 50% decline in energy devoted to reproduction (# of eggs). Results from this dissertation provide a compelling explanation to the mechanisms causing a shift in life history traits that appear to play an important role in population declines of C. collaris in the Ozarks.