The Distribution of Ant Colonies in a Semiarid Landscape: Implications for Community and Ecosystem Processes

Abstract

We studied the spatial distribution of harvester ants (Pogonomyrmex occidentalis) across multiple scales in the shortgrass-steppe of northeastern Colorado, USA. Using low-level aerial photographs (1: 2000 scale), we mapped the locations of ant colonies in five areas (65-130 ha) that differed in topography and grazing intensity by domestic cattle. Ant colonies were also mapped in field plots to verify data obtained from aerial photographs. Densities of ant colonies were analyzed by subdividing each of the areas by soil map units and 1-ha quadrats. We performed simulations to examine dispersion patterns at different scales using the 1st-10th nearest-neighbor distances (NND). Overall ant densities ranged from 6 to 16 colonies/ha among the five study areas. Local densities varied more widely among 1-ha quadrats (0-50 colonies/ha) and among soils (0-18 colonies/ha). Variability in colony density was greatest in areas with rolling topography and lowest under heavy grazing. Dispersion patterns obtained from the distribution of 1st NND showed regular and clumped departures from random at different parts of the distribution. Over broader scales, analyses of the 1st-10th mean NND showed a regular or random dispersion at the 1st or 2nd NND and a clumped pattern at the 3rd NND and beyond. Across the 1st-10th NND, quantitative patterns of dispersion differed among the five study areas. Ant distributions are likely influenced both by fine-scale factors such as neighborhood interactions and by broad-scale factors such as soils, topography, and grazing. Whether due to natural factors or land use, broad-scale variability in the distribution of ant populations may have important consequences to behavioral, community and ecosystem processes.