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1. There is growing evidence that granivorous ants compete for seeds in desert ecosystems. But ants that diverge in diet may still interact strongly if existing colonies deter foundress queens. Granivores (seed eaters) and omnivores (ants that scavenge and feed on.plant exudates) are common components of desert ecosystems.

2. The results of a 15-year granivore reduction experiment to explore the responses of two common omnivorous ants, Forelius cf pruinosus and Dorymyrmex insana, in a desert grassland were analysed. The food competition hypothesis predicts that granivore removal will have no net effect on omnivores. The spatial competition hypothesis predicts compensatory increases in omnivores on granivore removal plots.

3. A grain-based gut poison significantly reduced the densities of three of the four common granivorous ants. Densities of the large-colony (10(5)-worker) omnivore Forelius did not increase with granivore removal. In contrast, and consistent with the hypothesis of competition for space, densities of the small-colony omnivore (10(2)-10(3) worker) Dorymyrmex increased on granivore removal plots; however, this effect was only observed in years of high Dorymyrmex abundance.