Impacts of Single and Multiple Co-Existing Invasive Species on Subtropical Native Ant Communities

Biological invasions pose a significant threat to ecosystem stability by altering the taxonomic and functional diversity of native communities. It is still uncertain, however, whether multiple invasive species have varying effects on native communities, or whether their interactions in a co-invasion scenario are antagonistic or facilitative. To address this gap, this study investigated 24 sampling sites in Hong Kong, encompassing single invasion, co-invasion, and non-invaded control scenarios across the dry and wet seasons. We systematically explored how the functional traits and invasion intensity of four invasive ant species (Anoplolepis gracilipes, Paratrechina longicornis, Pheidole megacephala, and Solenopsis invicta) shape the structure and function of native ant communities. In addition, we evaluated the ecological effects of co-invasion scenarios to determine how interactions between invaders affect communities. The results revealed that, for some invasive species, increased invasion intensity significantly reduced local species diversity and resulted in selective replacement and systematic loss of local species. The invasion intensity of three invasive species had significantly negative effects on functional diversity indices (e.g., RaoQ and FDiv), while in areas invaded by P. megacephala, the near absence of native species (present in ~4% of the pitfall traps) prevented analyses. Comparisons across non-invaded, single invasion, and co-invasion scenarios revealed distinct patterns: single invasions caused pronounced reductions in both functional and taxonomic diversity, whereas co-invasion scenarios exhibited more limited changes. Further analyses demonstrated that under specific circumstances, antagonistic interactions between co-invasive species could mitigate the negative effects on α-diversity and preserve ecosystem functions through functional substitution. Nevertheless, this dynamic equilibrium is fragile and unsustainable, underscoring the need to prioritize preventive and control strategies in invasive species management to safeguard ecosystem resilience.