9707406 Brown This project will continue the experimental studies in the Chihuahuan Desert near Portal, Arizona. The 20-ha study site contains 24 fenced plots, each 0.25 ha in area. The investigators have maintained the removal of some or all species of rodents and/or ants from 10 plots continuously since 1977 and from an additional 10 plots since 1988. Standardized censuses of rodents, ants, and plants provide invaluable data on responses of individuals, populations, and communities both to experimental perturbations and to background temporal and spatial variation in the environment. Results to date demonstrate: 1) reorganization of the entire ecosystem, including large increases in woody vegetation and extinction, colonizations, or shifts in abundance of many animal species, apparently caused by a change in climate since the late 1970's; 2) complex seasonal and interannual dynamics of rodent, ant, and plant populations; 3) direct effects of seed predation by rodents, birds, and ants which substantially alter species composition of winter annual plants but leave overall population density and species diversity unchanged; 4) direct effects of competition by kangaroo rats on population densities and species diversity of other seed-eating rodent species; and 5) indirect effects of kangaroo rats on vegetation (summer active grasses), other rodents, birds, and lizards. This project will continue the experimental manipulations and standardized censuses of rodents, ants, and plants. In addition, the investigators will: 1) analyze the relationship between local abundance, spatial distribution, and temporal distribution of annual and woody plants, rodents, and ants; 2) investigate the relationships between species composition and ecosystem processes; and 3) assess the interacting influences of climate, livestock grazing, and fire on the transition between desert shrubland and arid grassland.
