Avian Community Dynamics in Desert Grasslands: Observational Scale and Hierarchial Structure

Avian communities living in a complex environment were studied by censuses of singing males, using several levels of resolution which focussed on different properties of the community. Within each level of resolution, various community parameters in two grassland habitat types were calculated over the entire breeding season, and for census intervals within the breeding season. The community level of resolution focussed on community—wide patterns, such as total density and total biomass, in order to reveal constraints operating on all species in each habitat. An intermediate level of resolution focussed on relative properties of species such as relative abundances, species richness, and dominance concentration. At the individual species level of resolution, densities and identities of species were the properties upon which the analysis focussed. These techniques allowed characterization of responses of the avian communities to complex changes in the system in which they were embedded. Long—term averages in monthly rainfall on the sites used for this study indicated a bimodal distribution of rainfall. The first peak occurred in December and the second in July and August. During the two years of study, summer rainfall was similar, but winter rainfall in 1983 was °2—3 times as great as in 1982. Avian communities in habitats with mesquite trees (mesquite savannah) responded differently than communities in grassland habitats, and these responses could be seen at different levels of resolution. Peak densities and biomass occurred during May—June in mesquite savannah habitats, while grassland communities achieved highest densities and biomass during July and August. Average density per individual showed little seasonal pattern, but was lower for mesquite savannah communities than for grassland communities. Species richness increased in mesquite savannah habitats in 1983, but decreased in grassland habitats that year. Patterns of evenness and dominance concentration suggested that in 1983, rare species of large body size were added to the community in mesquite savannah, while in grassland habitats, species already present during 1982 increased in density. A community ordination revealed that mesquite savannah habitats were dominated numerically by small—bodied insectivorous species during the early part of the breeding season but became similar to grassland communities in July and August. Grassland communities were dominated by large—bodied emberizine finches. These characteristics of the avian communities corresponded closely to the phenology of important primary producers. Mesquite produces leaves high in nutritional value and produces flowers during April—June; thereafter older leaves decline in nutrient content and flowers disappear. These changes in mesquite productivity undoubtedly produce corresponding changes in insect communities exploitable by small—bodied insectivores. Perennial grasses concentrate aboveground productivity to July—August and insect communities associated with grasses increased in biomass during this period, thus increasing resources available for breeding. These results suggest that avian community responses to complex changes in their environments can be identified and related to outside inputs into the environment, such as rainfall. Identification of community responses is facilitated by emphasis on several observational scales. Communities appear individualistic to observers because (1) adaptational units of species are probably much larger than study areas; (2) species ability to adapt to local conditions might be diluted by gene flow among populations; and (3) if most evolutionary change occurs during speciation, then species adaptations will reflect a relatively narrow range of selection pressures that operated briefly on the species during the past.