The secondary invasion of giant African land snail has little impact on litter or seedling dynamics in rainforest

In the absence of empirical evidence, invasive species are often assumed to have negative impacts because of their conspicuously high abundance. The giant African land snail Achatina (Lissachatina) fulica is one such invader where its impact in natural ecosystems remains completely untested. On Christmas Island (Indian Ocean), A.fulica has become established across large tracts of rainforest following the impacts of invasive yellow crazy ant (Anoplolepis gracilipes) in mutualism with non-native scale insects. Yellow crazy ants facilitate the secondary invasion of A.fulica by extirpating native red land crabs (Gecarcoidea natalis) that are normally effective predators of A.fulica. We used a multifaceted approach to investigate some potential impacts of abundant A.fulica in invaded rainforest. Over the course of a wet season, diel activity transects showed that A.fulica consumed detrital material almost exclusively. However, stable isotope analysis did not confidently identify A.fulica as a predominantly detritivorous species. We found no statistically significant treatment effects of A.fulica exclusion on standing leaf litter and seedling recruitment processes during a 6-month manipulative field study. However, litter cover and biomass did remain slightly higher where A.fulica were excluded, albeit with overlapping confidence intervals with control plots. Our study constitutes the first empirical test for impact of A.fulica in a natural ecosystem and suggests that for Christmas Island rainforest, this species is not a damaging invader. Other studies will need to assess the impacts of A.fulica in other natural areas before these findings could be considered broadly applicable.