Is unicoloniality not as important as we thought in the invasive success of Argentine ants?
Heller, N. E. (2004) Colony structure in introduced and native populations of the invasive Argentine ant, Linepithema humile. Insectes Sociaux 51(4): 378 - 386.
Summary. The Argentine ant, Linepithema humile, severely decreases the abundance and diversity of native ant fauna in areas where it invades, but coexists with a more diverse assemblage of ants in its native range. The greater ecological dominance of L. humile in the introduced range may be associated with differences in colony structure and population density in the introduced range relative to the native range. In this study, I compared aspects of L. humile colony structure, including density, the spatial pattern of nests and trails, and patterns of intraspecific aggression in parts of the introduced and native ranges. I also compared the number of ant species coexisting with L. humile. Introduced and native populations did not differ significantly in nest density, ant density, nest size, and nearest-neighbor distances. In three of the four study populations in the native range and all of the study populations in the introduced range, colonies were organized into supercolonies: they consisted of multiple, interconnected nests that were dense and spatially clumped, and aggression among conspecifics was rare. In one population in the native range, colonies were organized differently: they occupied single nest sites, nests were sparse and randomly dispersed, and ants from neighboring nests were aggressive toward each other. Species richness was significantly higher in the native range than in the introduced range, even in areas where L. humile formed dense supercolonies. The results suggest that differences in species coexistence between ranges may due to factors other than L. humile colony structure. One likely factor is the superior competitive ability of other ant species in the native range.
After the series of astute papers a few years ago by Tsustui, Suarez, Giraud, Keller, and others documenting a genetic bottleneck at introduction correllating with a decrease in intraspecific aggression and an increase in the extent of unicoloniality in the Argentine ant's introduced range, the idea spread that the enormous success of the Argentine ant in eliminating native species was due to this change in colony structure. So, the Argentine ant in its native range must not be unicolonial, right?
Apparently the story is more complicated. Heller's central finding- that Argentine ants can frequently be unicolonial in the native range and that nests can be just as dense while coexisting with numerous other ant species- shows that there is something beyond mere Argentine ant unicoloniality in the spread of the species around the world. Heller thinks that native species in Argentina are simply tougher than native species in California. I am sympathetic to this idea, but what we need to flesh this out is some careful ecological experimentation in the native range.
(disclaimer: I helped collect some of the data in Heller's study, so I am not an entirely unbiased observer...)