Biological invasions have well-known direct effects on native ecosystems but may also unleash forces with complex, unexpected consequences. These ecological surprises may be especially common in simple systems, like islands, following introduction of 'megainvaders,' like tramp ants.
In the September issue of Ecology Letters, O'Dowd, Green, and Lake show that impacts of invasion by the crazy ant Anoplolepis gracilipes ramify through the food web in rainforest on Christmas Island, totally reconfiguring this ecosystem in just 1-2 years.
On the forest floor, crazy ant supercolonies extirpate the dominant native omnivore, which indirectly increases seedling recruitment but slows litter decomposition. In the forest canopy, new ant-Homoptera partnerships accelerate, exacerbate, and diversify impacts. Sustained high densities of ants are associated with outbreaks of host-generalist scale insects and honeydew-dependent sooty moulds, leading to canopy dieback and even tree deaths.
The indirect fallout from the displacement of a native keystone species by an ant invader, itself abetted by introduced mutualists, precipitates invasional 'meltdown' in this island ecosystem. Even in simple systems, unforeseen effects and novel associations following introduction of a single alien species can make forecasting of impacts an elusive goal.
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