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

The conservation of biodiversity in landscape mosaics requires an understan ding of the impacts of human land use within mosaic elements and an evaluat ion of the biological uniqueness of different elements. We address these is sues by examining patterns of ant diversity in three semiarid rangeland lan dscapes used predominantly for grazing. These landscapes lie along a region al gradient from shortgrass steppe through a transitional zone to desert gr assland, along which climate and ant species composition vary. Within each landscape, we compared the effects of grazing and natural variation in soil s and vegetation on ant diversity and community composition. Grazing had li ttle effect on ant richness, diversity, or composition at the transitional zone or the desert grassland site, but ungrazed areas at the shortgrass ste ppe site had a higher overall richness and favored the abundance of some sp ecies. Some samples of saltbush (Atriplex canescens) shrubland were similar to ungrazed samples in richness and species composition. In both the trans itional zone and the desert grassland, creosotebush (Larrea tridentata)-dom inated habitats harbored comparatively species-rich and distinct ant commun ities. In addition, mesquite (Prosopis glandulosa) coppice dunes at the des ert 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 shor tgrass steppe site, whereas soil texture and associated differences in shru b density best explained differences in composition at the transitional and desert grassland sites. The characterization of habitats based upon vegeta tion classification did not adequately reflect environmental variation that was important to ants in shortgrass steppe, but reflected important soil t extural variation in the transitional and desert grassland sites. These res ults suggest that ant conservation in these semiarid rangelands should emph asize patterns of variation in soil properties. The results add to a growin g consensus that a variety of variables determined by the responses of seve ral focal taxa may be needed to characterize biodiversity patterns.