Habitat Heterogeneity and Avian Community Structure in North American Grasslands

Abstract

In a variety of environments, patchiness of habitat structure appears related to faunal diversity. I examined the relations between vegetational heterogeneity and several attributes of avian communities over a range of North American grassland and shrub steppe situations. Habitat heterogeneity was measured by assessing the variability in vertical density of vegetation within clustered point samples, and an index to express this horizontal patchiness was derived. Heterogeneity was directly related to the areal coverage of bare ground and woody vegetation and to patchiness in litter distribution of sample plots, and inversely related to grass coverage and litter depth. Heterogeneity generally increased with decreasing annual precipitation and primary production, tall grass-prairie sites being the least and western shrub steppe the most heterogeneous. Analysis of breeding bird population censuses from these plots revealed no apparent relationships between habitat heterogeneity and the diversity of breeding avifaunas or the extent of spatial (territorial) overlap between species. The density of breeding bird populations decreased slightly witth increasing vegetation patchiness; the standing crop biomass of the avian community, however, decreased markedly as heterogeneity increased. This pattern appeared to result from a replacement of largesized species by medium-sized species in the transition from tall grass to short grass areas and an increasing dominance of small-sized species in the western Palouse and shrub steppe sites. INTRODUCTION Many workers have recently drawn attention to the relation between habitat structure and faunal diversity. In deciduous forests in eastern North America, for example, the diversity of breeding bird species in a community increases with increasing patchiness or vertical layering of the vegetation (MacArthur and MacArthur, 1961; MacArthur, 1965), and a correlation between increasing habitat complexity and diversity in breeding bird faunas seems most general (e.g., Karr and Roth, 1971; Cody, 1970; Orians, 1969). Diversity in lizard faunas also may be related to habitat complexity (Pianka, 1967; Schoener and Schoener, 1971), and the structure of some small mammal communities may be closely tied to structural heterogeneity of the environment (Rosenzweig and Winakur, 1969). These observations and others have led to the development of a rather extensive body of theory of habitat heterogeneity (e.g., Levins, 1968; MacArthur and Levins, 1964, 1967; MacArthur and Pianka, 1966), although a good deal of this theory remains untested. In forest communities a good share of the increase in bird species diversity with increasing patchiness in the vertical distribution of foliage may be due to an increased vertical layering of the bird species. Spatial overlap between species, as projected on a single horizontal plane, may thus be extensive. In structurally simple habitats such as