A variety of mechanisms, both anthropogenic and natural, can lead to changes in the vegetation composition of a community. In semi- desert grasslands, for example, Schlesinger et al. (1990) suggested that overgrazing can result in desertification through a shift from primarily grasses to desert shrubs.
Changes in plant communities can also occur as a result of introduced species, artificial watering, plant removal for human use, disease, competition, and prolonged drought. In the future, global problems, such as the greenhouse effect, could possibly lead to climate changes (and therefore changes in vegetation) that we can only attempt to predict. Changes in plant composition have the potential to cause disturbances in both structure and function throughout the ecosystem. One element of the ecosystem that could possibly be affected is bird diversity and behavior. Several studies such as Dixon (1959), Raitt and Pimm (1976), Naranjo and Raitt (1993), MacAurthur et al. (1962), Vander Wall and MacMahon (1984) and Raitt and Maze (1968), all suggest that certain species of birds depend more, or entirely, on habitats containing specific functional groups or species of vegetation. According to Naranjo and Raitt (1993), habitat preferences in birds develop because of factors such as availability of food, nesting and perching locations, and cover. Birds may utilize some plant species more than other plant species. Vander Wall and MacMahon (1984) suggest that some particular plant species are ideally suited to particular bird species. For example, the northern mockingbird relies on tall plants for perching and singing sites. Other plant species provide cover for birds like the Gambel's Quail. Certain species of birds may also use a wider variety of different plants because of broad habitat requirements. In this study we examine how different types of plant types and growth affect bird abundance, bird species diversity, and bird activity in the Chihuahuan desert.
Ammonia volatilization was measured at three sites in the Chihuahuan Desert of southern New Mexico, U.S.A. In dry soils, ammonia volatilization ranged from 9 to 11 micrograms of nitrogen per square meter per day, but rates increased to 95 micrograms of nitrogen per square meter per day in a shrubland site after an experimental addition of water.
Ammonia volatilization also increased with experimental additions of NH4Cl and decreased with additions of sucrose. Competition by nitrifiers for available NH4+ had little effect on NH3 volatilization: N-Serve, added to inhibit nitrification, decreased NH3 volatilization in a grassland site and had little effect at other sites. We suggest that NH3 volatilization is controlled by the rate of mineralization of NH4+ from soil organic matter, and mineralization is stimulated by rainfall. Overall rates of NH3 volatilization from undisturbed desert ecosystems appear to be much lower than those reported for rangeland and agricultural soils.
The data set shows ammonia volatilization from grassland, cresotebush, and playa habitats in response to a variety of experimental treatments chosen to elucidate the processes controlling the volatilization under dry and post-rainfall conditions. Ammonia is collected in weak acid in scintillation vials placed inside PVC chambers in the field. The rate of ammonia volatilized per unit area ugN/m2/day) is found by mulitplying the concentration in the acid by 1250 to account for volume and area corrections.
Data for rabbits, birds, and lizards recorded from the LTER II animal transects. Data consists of species names, numbers of individuals, and distances observed from transects. Data is collected from each transect once every two weeks. See history file for exceptions.