Ant Mound Influence on Vegetation and Soils in a Semiarid Mountain Ecosystem

Abstract

-We examined vegetation patterns and soil properties associated with western harvester (Pogonomyrmex occidentalis) mounds in a pinyon-juniper community and a ponderosa pine community near Los Alamos, New Mexico. Plant-clearing habits of the ants exerted a minor influence on total plant cover. Denuded zones (discs) around the mounds in the pinyon-juniper and ponderosa pine sites represented 1.2% and 1.0% of the total surface area and reduced plant cover 0.35% and 0.49%, respectively. Vegetation near the perimeter of the cleared discs had decreased species richness and lowered percent cover compared with adjacent reference areas (sampled 3.0 m from discs). Comparisons of plant species occurrences around the discs revealed that (1) most species, including dominant understory plants, were evenly dispersed in relation to nest discs; (2) two species-one in each site-were significantly associated with areas near discs; and (3) six species-four in the pinyon-juniper site and two in the ponderosa pine site-were significantly associated with reference areas. Analysis of vegetation on abandoned mounds suggested that seed predation by the ants influences patterns of plant species occurrence adjacent to nests. Active mounds in the pinyon-juniper and ponderosa pine sites occurred at densities of 17/ha and 14/ha and had an average mass of 38 kg and 48 kg, respectively. Particle size analysis indicated that the ants construct mounds primarily of gravel and sand fractions. Mound soils at both sites had elevated concentrations of NO3, P and K, increased conductivity and lower water content compared with disc and reference soils. Disc soils in both sites had lower organic matter content but were otherwise similar to reference soils. Activities of P. occidentalis cause localized accumulations of nutrients that are unavailable to plants until mound abandonment. Colony influences on vegetation patterns beyond the denuded discs and lingering effects of abandoned mounds contribute to plant community heterogeneity.