Interference by wood ant influences size selection and retrieval rate of prey by Formica fusca

Summary Dominance relations between the territorial wood ant Formica polyctena and the submissive F. fusca generate testable predictions on the mechanisms of their coexistence. Here I tested the influence of interference competition by the dominant F. polyctena on the foraging of F. fusca. In the presence of F. polyctena, the activity and the retrieval rate of items of F. fusca decreased significantly. When F. fusca were given a choice between small chironomids and flies a hundred times heavier they selected nearly always flies in the absence of wood ants; when disturbed F. fusca took proportionately more chironomids. In nature, irrespective of distance from the wood-ant mound the size distribution of potentially available food items was the same. F. fusca collected smaller items close to the wood-ant mound, where the density of interfering wood ants was high, than far from it. Also, F. polyctena carried larger protein items from the outskirts of its territory than from the center. The items of F. fusca were on average smaller than those of F. polyctena although the item-size overlap was substantial. When encountering a F. polyctena, those F. fusca workers carrying a fly always lost their booty to the dominant but always managed to bring the chironomids to the nest. F. fusca and F. polyctena were equally efficient in detecting single chironomids placed on the surface of ground, but the presence of either species decreased the discovery rate of the other. This implies mutual exploitation competition between the species for the locally most abundant protein resource. F. polyctena found single flies faster than F. fusca. In an earlier study I showed that close to the wood-ant mound the colony size and production of sexual offspring of F. fusca were reduced. I suggest that the suppressed colony success of F. fusca is attributable to the diminished size and decreased retrieval rate of prey items close to the woodant mound abounding with interfering wood-ant foragers.