An invasion revisited:: the African big-headed ant (Pheidole megacephala) in northern Australia

Long-term studies provide the best information for invasion ecology as multi-temporal sampling can illuminate invasion dynamics, such as population changes and rate of spread. In 1996, Hoffmann et al. surveyed an infestation of African big-headed ant, Pheidole megacephala, in semi-natural rainforest of northern Australia. Here, we re-survey this infestation 9 years later to assess the dynamics of this invasion against the key finding of the initial study. Importantly, we re-sample a site sampled prior to invasion to demonstrate the causal link between an infestation and changes in a native invertebrate community. We found the area infested had almost doubled, and P. megacephala biomass in infested sites was up to 18 times greater than that of native ants in uninfested sites. In the two sites with the youngest infestations in 1996, P. megacephala abundance had increased more than 20-fold than that measured in 1996. Native ant abundance and species richness in infested sites were notably lower in 2005, with only one native ant specimen found in the most recently infested site. The abundance of other macro-invertebrates was the lowest in the three oldest infested sites. Coleoptera and Orthoptera were less abundant in infested sites. This study supports the findings of the first 'snapshot' study, and has shown that the area infested has almost doubled, populations are increasing, and ecological impacts remain severe. It reinforces that P. megacephala represents a serious ecological threat, and we argue that there is a greater need for management of this ant globally.