Effects of Fragmentation and Invasion on Native Ant Communities in Coastal Southern California

We investigated the roles of habitat fragmentation and the invasion of an exotic species on the structure of ground-foraging ant communities in 40 scrub habitat fragments in coastal southern California. In particular, we asked: how do fragment age, fragment size, amount of urban edge, percentage of native vegetation, degree of isolation, and the relative abundance of an exotic species, the Argentine ant (Linepithema humile) affect native ants? Within these fragments, Argentine ants were more abundant near developed edges and in areas dominated by exotic vegetation. The number of native ground-foraging ant species at any point declined from an average of >7 to <2 species in the presence of the Argentine ant. Among fragments, a stepwise multiple regression revealed that the abundance of Argentine ants, the size of the fragment, and the number of years since it was isolated from larger continuous areas of scrub habitat best predict the number of remaining native ant species. The Argentine ant was found in every fragment surveyed as well as around the edges of larger unfragmented areas. Fragments had fewer native ant species than similar-sized plots within large unfragmented areas, and fragments with Argentine ant-free refugia had more native ant species than those without refugia. The relative vulnerability of native ants to habitat fragmentation and the subsequent presence of Argentine ants vary among species. The most sensitive species include army ants (Neivamyrmex spp.) and harvester ants (genera Messor and Pogonomyrmex), both of which are important to ecosystem-level processes. Our surveys suggest that the Argentine ant is widespread in fragmented coastal scrub habitats in southern California and strongly affects native ant communities.