Ecological impacts of the little fire ant (Wasmannia auropunctata) in Tahiti

The little fire ant (Wasmannia auropunctata) is one of the worst invasive exotic species of the world. Its pantropical expansion is explosive; the ant has been present in Tahiti for more than 20 years. This study is the first to be carried out in Tahiti to assess its ecological impacts. The ecological richness of three invaded environments located in or close to the Key Biodiversity Areas (Maruapo, Papehue, Hopuetamai and Orofero Valleys) was compared with that of adjoining uninvaded sites, in a xerophytic site of ferns and a rainforest in the bottom of a valley for invertebrates and finally a canyon and its cliffs for avifauna. At the last site, one bank is infested and the other bank is free of W. auropunctata. Invertebrates and birds were examined respectively by pitfall traps and observation points. The effects of W. auropunctata were significantly negative at each study site, and species diversity in infested sites was one-third of that in uninfested sites. For arthropods, the phytophages, omnivores, detritivores, predators and other ants were the most affected. Significantly fewer birds also frequented the infested shoreline, particularly endemic birds such as the Tahiti monarch (81% non-infested shoreline observation) and the Society fruit-dove in comparison with the adjacent non-infested shoreline. Moreover, Tahiti monarchs (which are territorial) were established in three territories located at the bottom of the infested zone or close to it in 2016. These birds have now deserted the areas colonised by W. auropunctata. Our results suggest that W. auropunctata is a serious threat to Tahitian terrestrial biodiversity, and related ecosystem services.