Mutual avoidance at resources by the native winter ant and the invasive Argentine ant

Local interactions determine species coexistence. The native winter ant (Prenolepis imparis) has been successful in resisting and sometimes displacing the invasive Argentine ant (Linepithema humile) in northern California. We asked how human-impacted and natural habitats differed in the presence at food baits, over 120 min, of P. imparis, 8 other native species, and L. humile. In the natural habitats, where L. humile was not present, all native species except Formica moki and Temnothorax sp. persisted at food baits at least 50% of the time once they arrived. Prenolepis imparis was equally or more likely to persist at food baits than any other native species. In the human impacted habitats, P. imparis and L. humile exhibited mutual avoidance of one another; whichever species arrived at the bait first was likely to persist at that bait. Stable isotope analysis showed that the dietary preferences of L. humile converge with those of P. imparis in human-impacted habitats. These results suggest that the capacity of P. imparis to exclude L. humile at food resources contributes to its occurrence in invaded areas and its spread into areas once occupied by L. humile.