Ant biodiversity in semiarid landscape mosaics: the consequences of grazing vs. natural heterogeneity

The conservation of biodiversity in landscape mosaics requires an understanding of the impacts of human land use within mosaic elements and an evaluation of the biological uniqueness of different elements. We address these issues by examining patterns of ant diversity in three semiarid rangeland landscapes used predominantly for grazing. These landscapes lie along a regional gradient from shortgrass steppe through a transitional zone to desert grassland, along which climate and ant species composition vary. Within each landscape, we compared the effects of grazing and natural variation in soils and vegetation on ant diversity and community composition. Grazing had little effect on ant richness, diversity, or composition at the transitional zone or the desert grassland site, but ungrazed areas at the shortgrass steppe site had a higher overall richness and favored the abundance of some species. Some samples of saltbush (Atriplex canescens) shrubland were similar to ungrazed samples in richness and species composition. In both the transitional zone and the desert grassland, creosotebush (Larrea tridentata)‐dominated habitats harbored comparatively species‐rich and distinct ant communities. In addition, mesquite (Prosopis glandulosa) coppice dunes at the desert grassland site favored the abundance of several species that were rare across the site. Canonical correspondence analysis revealed that variation in soil strength and texture best explained community variation at the shortgrass steppe site, whereas soil texture and associated differences in shrub density best explained differences in composition at the transitional and desert grassland sites. The characterization of habitats based upon vegetation classification did not adequately reflect environmental variation that was important to ants in shortgrass steppe, but reflected important soil textural variation in the transitional and desert grassland sites. These results suggest that ant conservation in these semiarid rangelands should emphasize patterns of variation in soil properties. The results add to a growing consensus that a variety of variables determined by the responses of several focal taxa may be needed to characterize biodiversity patterns.