Do invasive ants respond more strongly to carbohydrate availability than co-occurring non-invasive ants? A test along an active Anoplolepis gracilipes invasion front

Invasions by non-native insects can have important ecological impacts, particularly on island ecosystems. However, the factors that promote the success of invaders relative to co-occurring non-invasive species remain unresolved. For invasive ants, access to carbohydrate resources via interactions with both extrafloral nectary-bearing plants and honeydew-excreting insects may accelerate the invasion process. A first step towards testing this hypothesis is to determine whether invasive ants respond to variation in the availability of carbohydrate resources, and whether this response differs from that of co-occurring, non-invasive ants. We investigated the effect of carbohydrate subsidies on the short-term foraging and hemipteran-tending behaviours of the invasive ant Anoplolepis gracilipes (Formicidae) and co-occurring ant species on an extrafloral nectary-bearing plant by experimentally manipulating carbohydrate levels and tracking ant recruitment. We conducted experiments in 2 years at two sites: one site was invaded by A. gracilipes prior to 2007 and the other became invaded during the course of our study, allowing pre- (2007) and post-invasion (2009) comparisons. Short-term increases in carbohydrate availability increased the density of A. gracilipes workers on plants by as much as 400% and reduced tending of honeydew-excreting insects by this species by up to 89%, with similar responses across years. In contrast, ants at the uninvaded site in 2007 showed a weak and non-significant forager recruitment response. Across all sites, A. gracilipes workers were the only ants that responded to carbohydrate manipulations in 2009. Furthermore, ant-carbohydrate dynamics at a site newly invaded by A. gracilipes quickly diverged from dynamics at uninvaded sites and converged on those of the site with an established invasion. These findings suggest that carbohydrate resources may be particularly important for A. gracilipes invasions, and underscore the importance of species interactions, particularly putative mutualisms, in facilitating exotic species invasions.