Islands can serve as model systems for understanding how biological invasions affect
community structure and ecosystem function. Here we show invasion by the alien crazy
ant Anoplolepis gracilipes causes a rapid, catastrophic shift in the rain forest ecosystem of a
tropical oceanic island, affecting at least three trophic levels. In invaded areas, crazy ants
extirpate the red land crab, the dominant endemic consumer on the forest floor. In doing
so, crazy ants indirectly release seedling recruitment, enhance species richness of seedlings,
and slow litter breakdown. In the forest canopy, new associations between this invasive ant
and honeydew-secreting scale insects accelerate and diversify impacts. Sustained high
densities of foraging ants on canopy trees result in high population densities of hostgeneralist
scale insects and growth of sooty moulds, leading to canopy dieback and even
deaths of canopy trees. The indirect fallout from the displacement of a native keystone
species by an ant invader, itself abetted by introduced/cryptogenic mutualists, produces
synergism in impacts to precipitate invasional meltdown in this system.