The Argentine ant (Linepithema humile) is a small, dark brown insect that has become a globally recognized invasive species. Originating from Northern Argentina, this ant is found across urban, natural, and agricultural habitats. Its unusual social organization allows it to form massive aggregations known as supercolonies. This structure enables cooperative behavior over vast distances, distinguishing its societal scale from typical ant colonies.
The Supercolony Phenomenon
Argentine ants exhibit unicoloniality, meaning geographically separate nests do not show aggression. Unlike most ant species that defend territories against non-nestmates, Argentine ants from different nests within a supercolony mingle and cooperate peacefully. This lack of intraspecific aggression defines their supercolony structure, allowing for exceptionally high population densities.
Genetic studies reveal low genetic variation within supercolonies, which contributes to their cooperation. This genetic homogeneity means individuals from distant nests recognize each other as nestmates, fostering an expansive network. The sheer scale of these supercolonies is remarkable; for instance, one in Europe spans over 6,000 kilometers, and another in California extends for more than 900 kilometers.
This cooperative framework enables the ants to pool resources and labor across numerous interconnected nests, forming a single, vast society. The absence of internal conflict frees up energy that would otherwise be spent on territorial defense, redirecting it towards foraging and reproduction. This allows Argentine ant populations to achieve enormous numbers of workers, enabling them to dominate environments.
Global Spread and Ecological Impact
Native to the Paraná River basin in South America, the Argentine ant has spread globally via human transport. It has established populations on every continent except Antarctica, thriving particularly in regions with Mediterranean climates or mild winters. This widespread distribution has led to its classification as one of the world’s 100 worst invasive species.
The presence of Argentine ants has significant ecological consequences, displacing native ant species. In invaded areas, native ant populations can be reduced by as much as 90%, altering the natural ant community structure. This displacement disrupts local food webs, as native ants often play roles in seed dispersal and pollination. For example, their invasion has been linked to declines in populations of the coast horned lizard in Southern California, which preys on native harvester ants.
Beyond displacing native ants, Argentine ants impact broader biodiversity. They can prey on native insects and compete with native pollinators, leading to reduced pollinator abundance and even affecting seed size in some native plants. Their mutualistic relationship with plant-feeding insects, like aphids, from which they collect honeydew, can worsen plant pest problems in agricultural and natural settings. These ants are considered significant pests in urban, agricultural, and natural environments worldwide.
Biological Traits of Dominance
The success of Argentine ants in forming supercolonies and dominating new environments is attributed to several biological and behavioral traits. One notable characteristic is their polygynous nature, meaning each colony can contain multiple queens, sometimes hundreds. This contrasts with many other ant species that typically have only one queen per colony.
The presence of numerous queens contributes to a high reproductive rate, allowing colonies to grow rapidly and produce vast numbers of workers. Queens can lay dozens of eggs per day, and new queens can be produced from existing eggs and larvae within the colony. Argentine ants also employ a propagation method called budding, where one or more queens, accompanied by a group of workers, leave an established nest to form a new colony nearby. This allows for quick expansion into new territories without the risks associated with traditional mating flights.
Argentine ants are highly adaptable, capable of nesting in diverse habitats, including moist soil, under rocks, in wood debris, and even within structures. They are omnivorous and opportunistic feeders, consuming a wide range of food sources from sweets like honeydew and nectar to insects and carrion. Their efficient foraging strategies involve recruiting large numbers of workers to food sources, enabling them to outcompete other insects. This combination of rapid reproduction, adaptable nesting, and cooperative foraging makes them highly effective invaders.