Spatial patterns, density dependence, and demography in the harvester ant, Pogonomyrmex rugosus, in semi-arid grasslands

To evaluate the relationship between the local spacing patterns of ant colonies and the broad-scale heterogeneity typical of real landscapes, we examined the spacing of colonies of harvester ants (Pogonomyrmex rugosus) on 12 1-ha plots in semi-arid grasslands in New Mexico, U.S.A. We linked land use and topography to colony densities and local spacing patterns, tested the theoretical prediction that regularity should be positively correlated with colony density, and determined the spatial scale of density-dependent recruitment. In general, colonies were regularly spaced at a free scale. We caution that broad-scale aggregation of colonies can conceal free-scale regularity. The prediction of density-dependent spacing was supported only within a single topographic position, but not across the entire landscape. Recruitment of new colonies was independent of plot-level densities but depended on locations of established colonies within plots. Landscape heterogeneity can influence the fine-scale spacing of ant colonies, both directly and indirectly, complicating predictions regarding density-dependent spacing.