Behavioural plasticity associated with propagule size, resources, and the invasion success of the Argentine ant Linepithema humile.
The number of individuals involved in an invasion event, or 'propagule size', has a strong theoretical basis for influencing invasion success. However, rarely has propagule size been experimentally manipulated to examine changes in invader behaviour, and propagule longevity and success. We manipulated propagule size of the invasive Argentine ant Linepithema humile in laboratory and field studies. Laboratory experiments involved L. humile propagules containing two queens and 10, 100, 200 or 1000 workers. Propagules were introduced into arenas containing colonies of queens and 200 workers of the competing native ant Monomorium antarcticum. The effects of food availability were investigated via treatments of only one central resource, or 10 separated resources. Field studies used similar colony sizes of L. humile, which were introduced into novel environments near an invasion front. In laboratory studies, small propagules of L. humile were quickly annihilated. Only the larger propagule size survived and killed the native ant colony in some replicates. Aggression was largely independent of food availability, but the behaviour of L. humile changed substantially with propagule size. In larger propagules, aggressive behaviour was significantly more frequent, while L. humile were much more likely to avoid conflict in smaller propagules. In field studies, however, propagule size did not influence colony persistence. Linepithema humile colonies persisted for up to 2 months, even in small propagules of 10 workers. Factors such as temperature or competitor abundance had no effect, although some colonies were decimated by M. antarcticum. Synthesis and applications. Although propagule size has been correlated with invasion success in a wide variety of taxa, our results indicate that it will have limited predictive power with species displaying behavioural plasticity. We recommend that aspects of animal behaviour be given much more consideration in attempts to model invasion success. Secondly, areas of high biodiversity are thought to offer biotic resistance to invasion via the abundance of predators and competitors. Invasive pests such as L. humile appear to modify their behaviour according to local conditions, and establishment was not related to resource availability. We cannot necessarily rely on high levels of native biodiversity to repel invasions.