habitat selection and population structure. Since competition for resources can cause a decrease in fitness, increased population size should cause a decrease in body size and growth rates across the population. In these lizards there is population density-dependent growth for females of this species but not for males. The temperature quality of the environment that these lizards live in also determines body size and composition. Lizards that are raised and produced in crowded spaces and environments with poor resources and lower temperatures grow smaller than lizards in higher quality environments with better suited temperatures and good resources. It has been suggested that the abundance of rainfall that the ornate tree lizard experiences induce a multitude of changes in this lizard's behaviors. During a four-year study in the grapevine hills of Big Bend National Park in Texas, researchers observed the behavior of this lizard from 1974 to 1978. During 1974 and 1976, precipitation was abundant, however, during 1975 and 1977, there was little precipitation. During the dry years, there is a decrease in available prey and the ornate tree lizard's individual foraging success, growth rates, body masses, and prehibernation lipid levels were significantly lower. Population density was also reduced. These results have led researchers to propose that dry years are stressors which induces changes in diet, behaviors, and health of individuals. Since drought induces food scarcity, there is increased intra-specific competition for resources. Studies have demonstrated that Sceloporus merriami and the U. ornatus lizards also compete interspecifically for resources. Limited resources lowers population density of both lizard species. There is seasonal variation in the intensity of competition between these species which is highly correlated to food availability and the amount of rainfall. Where rainfall varies, the intensity of interspecific competition also varies. Therefore, the state of the environment directly affects competition and food availability for the ornate tree lizard. Geographic range Urosaurus ornatus is one of the most widespread and abundant of North American lizards. It inhabits a wide range of states throughout the United States, including California, Nevada, Utah, Colorado, Wyoming, Arizona, New Mexico, and Texas. It is arboreal, semi arboreal, or saxicolous throughout its wide living range and habitats. It can also be found in areas in Mexico including states such as Sonora, Sinaloa, Chihuahua, and Coahuila. There are large temperature ranges throughout the southwestern United States and these lizards adapt to the different climates that they live in.