Variability in the effects of harvester ant (Pogonomyrmex spp.) disturbances on vegetation and soils in semiarid grasslands: Integrating patterns and processes

Patch disturbances created by two species of harvester ant, Pogonomyrmex occidentalis on a Colorado shortgrass steppe (SGS) and Pogonomyrmex rugosus on the Chihuahuan desert grassland (CDG) in southern New Mexico, were studied during 3 years to document the hierarchical effects of landscape and region on the harvester ant disturbance regime.; Based on active and abandoned nest densities, topographic position and grazing intensity both influenced colony dynamics each year at the SGS. The size of individual nest disturbances varied according to topographic position and grazing at the SGS as well. In general, results indicated that the P. occidentalis disturbance regime is negatively impacted by heavy grazing, as evident by lower nest densities, smaller nest size, and higher nest turnover on a heavily grazed pasture than a lightly grazed pasture. Within pastures, nest densities were greater on upland plains under heavy grazing and greater on slopes under light gazing. Topographic variation in harvester ant densities translated into landscape-level variation in colony abandonment, with greatest nest turnover occurring on slopes under light grazing. Therefore, upland plains habitat appears to be the preferred habitat type, supporting higher nest densities under heavy grazing and lower colony turnover under light grazing. In contrast, P. rugosus nest densities on the CDG were similar across grazing regime and topographic position. Nest size was larger on the lower bajada, which may have resulted in greater nest turnover at that topographic position. In general, grazing intensity had very different effects on the harvester ant disturbance regime at both sites, and topography had a similar role in affecting nest size and colony turnover.; Regardless of regional differences between sites, both species of harvester ants had significant effects on soil nutrients and arbuscular mycorrhizal (AM) fungi. These results suggest that P. occidentalis and P. rugosus have similar functional roles in creating soil heterogeneity on two different semiarid rangeland ecosystems. Nests of both species were enriched in NO₃-N, total P, and AM fungi compared to surrounding soil, but levels varied with topography and year.; Plant community dynamics on and off harvester ant disturbances at both study sites were strongly controlled by broad-scale variation in grazing intensity, topography, and yearly fluctuations in precipitation. (Abstract shortened by UMI.)...