In their native Argentina and Uruguay, colonies of Linepithema humile are mutually hostile and occupy modest territories. Introduced populations behave differently. Because the founding stock passed through a genetic bottleneck, the cuticular chemical cues by which ants recognise strangers became uniform across huge areas, so ants from distant nests treat each other as nestmates.
The result is the European main supercolony, extending along the Mediterranean coast from northwestern Italy to northwestern Spain and Portugal, estimated at around 6,000 kilometres. Comparable structures exist in California and in Japan, and reciprocal aggression tests have shown ants from these three regions are non-aggressive towards one another, suggesting a related lineage spread by trade.
The consequence is competitive. Energy that native ants spend on territorial fighting is redirected into foraging and reproduction, and Argentine ants overwhelm local species through sheer numerical dominance at food sources. Displacement of native ants in turn affects seed-dispersing plants and lizards that specialise on native ant prey, so the effects run well past the ants themselves.