Using stable isotope analysis to infer trophic position, we evaluated the dietary inputs of invasive Argentine ants relative to other ants in native and introduced populations. We were specifically interested in understanding the trophic ecology of Argentine ants (a) in their native range relative to other ants they compete and interact with, and (b) in introduced populations relative to the native ant species they displace. Our results for three native sites in northern Argentina suggest that the ant community spans many trophic levels ranging from nectar/honeydew feeders to highly carnivorous army ants. In the native range, Argentine ants have an omnivorous diet, and they coexist with many other competitors at similar trophic positions. Ant communities in southern California also span multiple trophic levels but have fewer species relative to communities within the Argentine ant’s native range. However, in communities invaded by Argentine ants, ant species that feed at higher trophic levels are displaced rapidly, or have an isotopic signature indicating they feed at lower trophic levels post invasion. Our data suggest that Argentine ants may fundamentally alter the trophic structure of the communities they invade.