SPATIAL VARIATION IN RATES OF SEED REMOVAL BY HARVESTER ANTS (POGONOMYRMEX OCCIDENTALIS) IN A SHRUB-STEPPE ECOSYSTEM

Spatial variability in the intensity of plant-animal interactions can have important implications for plant community structure. We examined factors influencing spatial patterns of seed removal by the western h arvester ant (Pogonamyrmex occidentalis) using three seed removal expe riments. In the first two experiments, which assessed the effect of fo raging trails on colony foraging patterns, seed removal rates from bot h low- and high-density seed patches were higher near foraging trails than away from them. Observations suggest that differences in removal rates are due in part to a higher probability of discovery for patches located near trails. In both experiments, there was significant varia tion in seed removal patterns among individual ant colonies. The third experiment examined patterns of seed removal at a range of spatial sc ales. Rates of seed removal were not affected by the distance of seeds from the nest nor by the surface location (undershrub or intershrub s pace) in which seeds were placed. At che largest spatial scale conside red (areas encompassing multiple ant colonies), the risk of seed preda tion varied between years. Though seed removal; was monitored for up t o 27 days in this experiment, >30% of seeds taken were removed within 1 day, suggesting that colony foraging areas are a mosaic of frequentl y and less frequently visited areas in which the risk of seed predatio n is highly variable.