Meltdown in an Island Forest

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Science  03 Oct 2003:
Vol. 302, Issue 5642, pp. 21
DOI: 10.1126/science.302.5642.21b

Examples of the harmful effects of alien species introductions continue to flood into the ecological literature, but few introduced species are as destructive as ants. O'Dowd et al. report a particularly devastating instance of an invasive ant whose effects cascade through an entire tropical island ecosystem, ultimately leading to the death of rainforest canopy trees. The yellow crazy ant, Anoplolepis gracilipes, was first introduced to Christmas Island in the Indian Ocean about 70 years ago. The ants persisted for decades at low density before their population began to explode in the late 1980s, forming supercolonies infesting one-fifth of the island. The ants eliminate the red land crab, a keystone consumer in the forest floor ecosystem. The elimination of the crab results in vastly increased tree seedling density and decreased litter decomposition on the forest floor. Meanwhile, in the forest canopy, the presence of ants promotes mutualistic population growth of honeydew-secreting scale insects, in turn resulting in increased growth of honeydew-dependent fungi that cause dieback and death of canopy trees. This invasional “meltdown” over a 2-year period shows how an invader can alter the trophic dynamics of an entire ecosystem through direct effects at multiple levels in the food web. — AMS

Ecol. Lett. 6, 812 (2003).

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