Impact of an introduced ant on native rain forest invertebrates: Pheidole megacephala in monsoonal Australia

Abstract

*[We describe the local distribution of P. megacephala & its impacts on native invertebrate assemblages in & around a rain forest patch at Howard Springs, in Australia's monsoonal tropics. P. megacephala was found to be confined to a single area of approximately 25-ha, with its distribution centered on drainage lines & the rain forest. Significant but weak correlations were found between its abundance and vegetative canopy cover (positive) and distance from the rain forest (negative). In the most heavily infested area within the rain forest, the abundance of P. megacephala was 37-110 times that of total native ant abundance found within uninfested plots, as measured by pitfall traps. The abundance & richness of native ants & other invertebrates were significantly reduced in litter samples, pitfall catches, & foliage beats where P. megacephala was present, inversely relative to the abundance of P. megacephala. Only 2 individuals of a single native ant species were found within the most infested plot, with native ant richness being reduced to about half in the least infested plot. The most persistent functional groups of native ants in infested plots were Cryptic species, which forage primarily within soil & leaf litter, & Opportunists, which exhibit highly generalized foraging behavior. The highest abundance of P. megacephala corresponded with a 42-85% decrease in the abundance of other native invertebrates. Insect larvae were totally absent from foliage beats collected at the most heavily infested plot. P. megacephala was found overall to be expanding its range, averaging 12-m range expansion in the dry season & contracting 7-m in the wet season. It is able to spread into surrounding savanna habitats by occupying relatively sheltered microsites, such as beneath logs & at the bases of trees. However, it is unlikely to attain high population densities in open savanna habitats, because of its relative intolerance of desiccation, & the prevalence of behaviorally dominant native ant species.]