The proposed research will continue long-term manipulations of rodents and ants and standardized censuses of rodents, ants and plants to provide data on responses of individuals, populations and communities both to experimental perturbations and to background temporal and spatial variation in the environment. The research will test several hypotheses: 1) Population abundances of rodents are strongly influenced by extreme climatic events. While increased precipitation generally affects animal populations in arid environments positively, the research will examine how both winter and summer high rainfall events can result in high rodent mortality; 2) Population dynamics are synchronized by climate. Population changes are often driven by changes in food abundance and so species feeding on different foods often exhibit different population dynamics. The research will test whether the population dynamics of seed-eating and omnivorous ants are synchronized by large-scale climatic fluctuation, such as El Nino events; 3) Climate and livestock grazing interact to control the desert-grassland transition. The research will compare vegetation changes that have occurred both inside and outside the grazing exclusion fence to evaluate hypothesized mechanisms of desertification and grassland restoration; 4) Biotic resistance and productivity affect species invasion. The research will test the proposed contrasting effects of community diversity and productivity to explain the abundance of a recent invading plant species at the site. Studying this system will provide general insights, useful to ecologists, conservation biologists and ranchers working in arid grassland habitats, into how biotic and abiotic factors affect the abundance of animal and plant populations.
