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The spatial and temporal foraging dynamics of western harvester ant colonies, Pogonomyrmex occidentalis (Cresson), in a shrub-steppe ecosystem in southwestern Wyoming were studied. Ant foraging was spatially organized by permanent trunk trails that were formed by workers traveling in corridors between shrubs. An exponential decline was observed in forager flows with distance along trails, a pattern which changed when foragers were recruited to experimental seed patches. Trails also differed in seasonal forger activity and the seed species returned by ants. Soil surface temperature was a significant contributor to variation in forager departure rates in 70% of the 48 samples taken across months and colonies during the 2-yr study. Densities of soil seeds in the foraging areas of P. occidentalis colonies were significantly correlated with seasonal ant activity levels. The composition of forage returned by ants differed seasonally and according to daily changes in surface temperature.