Factors determining the spatial variability of seed densities in a shrub-steppe ecosystem: the role of harvester ants

The seed banks of deserts typically show a high degree of spatial variability. We examined the influence of the western harvester ant,Pogonomyrmex occidentalis, on the spatial heterogeneity of soil seeds in the semi-arid shrub–steppe of south-western Wyoming. Seeds were sampled in the foraging areas of three ant colonies over 2 years, primarily to assess the distribution of seeds with respect to harvester ant nests and foraging trails. In 1989, seed densities varied at the smallest spatial scale sampled — the microhabitat (undershubvs. interspace). In 1990, seed densities varied at the largest spatial scale sampled — the colony foraging area. Seed abundance varied temporally in both years. Harvester ants affected the patterns of seed density in two ways. First, seed numbers in 1989 were higher in interspace areas away from foraging trails than in interspace areas near them, indicating that trails restrict colony search effort. Second, and contrary to our expectations, seed numbers in 1990 were higher near harvester ant nests than in surrounding areas. This difference was attributable to higher densities of the alien annual grassBromus tectorumand suggests that harvester ants indirectly facilitateB. tectorumnear their nests.