|Wiens, J. - THE NATURE CONSERVANCY|
Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 3, 2000
Publication Date: August 1, 2001
Citation: BESTELMEYER, B.T., WIENS, J.A. ANT BIODIVERSITY IN SEMIARID LANDSCAPE MOSAICS: THE CONSEQUENCES OF GRAZINGVS. NATURAL HETEROGENEITY. ECOLOGICAL APPLICATIONS. 2001. V. 11(4). P. 1121-1140. Interpretive Summary: This paper describes the responses of ant communities to grazing and changes in soils in three semiarid grasslands. Because of their commonness and diversity, ants have been used as indicators of the health of ecosystems. We asked whether cattle grazing had a negative impact on ants in grasslands that differed in the responses of plants to grazing pressure, ,ranging from a minor response in shortgrass steppe of Colorado to a severe response in the desert grasslands of New Mexico. We also asked whether natural differences in soils had important effects on changes in ant communities across space; and, if so, how did these effects compare with those of cattle grazing. We found that cattle grazing had little impact on a variety of ant community measures. Two ants in the shortgrass steppe were favored in ungrazed settings because of differences in soils and woody plants. Ants in the highly modified desert grassland sites were largely unaffected by grazing. On the other hand, natural variation in soils (or historical desertification) caused important differences in the ant communities at all sites. This result emphasizes that conservation of ant diversity in rangelands can be accomplished by preserving areas (in a grazed or ungrazed, but at least semi-natural, condition) with differences in soils. It also indicates that contemporary grazing may not negatively impact some diverse animal groups in some settings.
Technical Abstract: Conservation of biodiversity in landscape mosaics requires us to understand the impacts of human land use within mosaic elements and an evaluation of the biological uniqueness of different elements. We examined patterns of ant diversity in three semiarid rangeland landscapes used predominantly for grazing. 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. Also, mesquite (Prosopis glandulosa) coppice dunes at the desert grassland site favored the abundance of several rare species. Canonical correspondence analysis revealed that variation in soil hardness 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. While habitat characterization based on vegetation classification did not adequately reflect environmental variation important to ants in shortgrass steppe, it reflected important soil textural variation in the transitional and desert grassland sites. This suggests that ant conservation in these semiarid rangelands should emphasize patterns of variation in soil properties.