Crazy ant invasion on Christmas Island

Yellow crazy ants (Anoplolepis gracilipes) are probably native to Africa, but now have a pan-tropical distribution. They are highly invasive and have been listed by the IUCN as one of the world's worst 100 invasive species. They have been present on Christmas Island for decades, but only became a problem during the 1990s when they began forming expansive (10s to 100s of ha), multiqueened 'supercolonies'. The density of ants in supercolonies is astonishing, up to 2000 ants per square meter.

Anoplolepis has invaded rainforest on Christmas Island and have built up huge population densities in association with lac scale insects (Tachardina aurantiaca). Supercolonies of yellow crazy ants extirpate local populations of red land crabs, which eat the seeds and seedlings of rainforest trees. In the absence of red crabs, large swathes of rainforest on the island are being transformed.

Supercolonies have transformed the rainforest on Christmas Island. Normally, the feeding activities of the island's famous red land crabs (Gecarcoidea natalis) keep seedling abundance and diversity in check. But at supercolony densities, crazy ants extirpate local crab populations. Anoplolepis doesn't bite or sting, but subdues its prey with tiny squirts of formic acid. Because there are so many ants, there is sufficient formic acid to kill these large land crabs. The impact of the red crab population has been catastrophic - it is estimated that yellow crazy ants have killed 20 million red crabs - around one third of the entire population - over the last decade.

The deletion of crabs has deregulated seedling recruitment, and led to a transformation of the understory over thousands of hectares.

Uninvaded forest showing the understory of rainforest with an intact population of red crabs when crazy ants form supercolonies. The ground is clear of litter, and there are few seedlings because red crabs eat most seeds and seedlings.

Invaded forest showing the understory just a few years after invasion by yellow crazy ants, and the subsequent deletion of the local crab population. Leaf litter persists, and there is abundant seedling recruitment.