The Argentine ant, Linepithema humile (formerly Iridomyrmex humilis), is a dark ant native to northern Argentina, Uruguay, Paraguay, and southern Brazil. It is an invasive species that has been established in many Mediterranean climate areas,inadvertently introduced by humans to many places, including South Africa, New Zealand, Japan, Easter Island, Australia, Hawaii, Europe, and the United States.
The ants are ranked among the world’s 100 worst animal invaders.In its introduced range, the Argentine ant often displaces most or all native ants. This can, in turn, imperil other species in the ecosystem, such as native plants that depend on native ants for seed dispersal, or lizards that depend on native ants for food. For example, the recent severe decline in coastal horned lizards in southern California is closely tied to Argentine ants displacing native ant species on which the lizards feed.
Argentine ants sometimes tend aphid colonies, and their protection of this plant pest can cause problems in agricultural areas by protecting plant pests from predators and parasitoids. In return for this protection the ants receive a food as an excretion, known as honeydew. Thus, when Argentine ants invade an agricultural area, the population densities of these plant parasites increase and so too does the damage they cause to crops.
They have been extraordinarily successful, in part, because different nests of the introduced Argentine ants seldom attack or compete with each other, unlike most other species of ant. In their introduced range, their genetic makeup is so uniform that individuals from one nest can mingle in a neighboring nest without being attacked. Thus, in most of their introduced range they form supercolonies. “Some ants have an extraordinary social organization, called unicoloniality, whereby individuals mix freely among physically separated nests. This type of social organization is not only a key attribute responsible for the ecological domination of these ants, but also an evolutionary paradox and a potential problem for kin selection theory because relatedness between nest mates is effectively zero.” In contrast, native populations are more genetically diverse, genetically differentiated (among colonies and across space), and form colonies that are much smaller than the supercolonies that dominate the introduced range. Argentine ants in their native South America also co-exist with many other species of ants, and do not attain the high population densities that characterize introduced populations.