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Colonies of the Mediterranean harvester ant Messor barbarus develop trunk trails for the foraging of seeds. Less palatable parts of the seeds collected by workers are accumulated near nest entrance holes, forming conspicuous chaff piles. Both trunk trails and chaff piles are annual structures which represent estimates of the colonial harvesting activities and crop production of each year, respectively. A population of colonies of this ant species was studied in a grassland ecosystem of Central Spain. The plant community of this ecosystem also has prominent annual density characteristics and it can thus be analyzed simultaneously with the described features of ant colonies. There was a significant association between presence of trunk trails and presence of chaff mounds in the colonies. Both trunk trail length and territory resources (defined by plant density) per colony had a positive relationship with annual crop production (estimated by mound volume). Two basic features thus determine annual crop production in the Messor barbarus colonies: foraging worker force and location of the nest within the annual plant density distribution.