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 vari ability. We examined the influence of the western harvester ant, Pogon omyrmex occidentalis, on the spatial heterogeneity of soil seeds in th e 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 sp atial scale sampled - the microhabitat (undershub vs. 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. Fir st, seed numbers in 1989 were higher in interspace areas away from for aging trails than in interspace areas near them, indicating that trail s restrict colony search effort. Second, and contrary to our expectati ons, seed numbers in 1990 were higher near harvester ant nests than in surrounding areas. This difference was attributable to higher densiti es of the alien annual grass Bromus tectorum and suggests that harvest er ants indirectly facilitate B. tectorum near their nests. (C) 1996 A cademic Press Limited