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

Pheidole megacephala is an exotic ant species that has severely affected na tive invertebrate biodiversity throughout the tropics. Its impacts have bee n documented extensively in relatively depauperate invertebrate communities , but not in species-rich habitats such as tropical rain forests. Here we d escribe the local distribution of P. megacephala and its impacts on native invertebrate assemblages in and around a rain forest patch at Howard Spring s, in Australia's monsoonal tropics. P. megacephala was found to be confine d to a single area of approximately 25 ha, with its distribution centered o n drainage lines and the rain forest. Significant but weak correlations wer e found between its abundance and vegetative canopy cover (positive) and di stance from the rain forest (negative). In the most heavily infested area w ithin the rain forest, the abundance of P. megacephala was 37-110 times tha t of total native ant abundance found within uninfested plots, as measured by pitfall traps. The abundance and richness of native ants and other inver tebrates were significantly reduced in litter samples, pitfall catches and foliage beats where P. megacephala was present, inversely relative to the a bundance of P. megacephala. Only two individuals of a single native ant spe cies were found within the most infested plot, with native ant richness bei ng reduced to about half in the least infested plot. The most persistent fu nctional groups of native ants in infested plots were Cryptic species, whic h forage primarily within soil and leaf litter, and Opportunists, which exh ibit highly generalised foraging behaviour. The highest abundance of P. meg acephala corresponded with a 42-85% decrease in the abundance of other nati ve invertebrates. Insect larvae were totally absent from foliage beats coll ected at the most heavily infested plot. P. megacephala was found overall t o be expanding its range, averaging 12 m range expansion in the dry season and contracting 7 m in the wet season. It is able to spread into surroundin g savanna habitats by occupying relatively sheltered microsites, such as be neath logs and at the bases of trees. However, it is unlikely to attain hig h population densities in open savanna habitats because of its relative int olerance of desiccation, and the prevalence of behaviourally dominant nativ e ant species. Howard Springs is currently the only rain forest patch in mo nsoonal Australia known to be infested by P. megacephala, but clearly this ant is a serious potential threat to the region's rain forest invertebrate fauna.