Crazy AntsCrazy Ants

Crazy Ants

The exotic invasive yellow crazy ant (Anoplolepis gracilipes) arrived on Christmas Island more than 70 years ago, and is now widespread throughout rainforest on the island (Orchard et al. 2002). These ants have the ability to form multi-queened 'super-colonies', in which the ants occur at very high densities. Supercolony formation has apparently been a relatively recent phenomenon; the first supercolony was discovered in 1989, with further dramatic increases in supercolony formation probably beginning around the mid-1990s.

At supercolony densities, this invasive, alien ant is having a devastating impact on the island's ecosystems. Red crabs, robber crabs, blue crabs and most other ground dwelling animals such as reptiles have been and are continuing to be severely impacted, sometimes to the point of local extinction in heavily infested areas. By eliminating local populations of red crabs, the ants are also having a marked effect on forest composition and structure, and litter dynamics in infested areas. Further, the feeding activities of these ants and their mutualistic scale insects can stress large trees to the point of death, and appear to be causing canopy dieback in areas of supercolony formation (Orchard et al. 2002). In addition it should be noted that groundings of birds in crazy ant supercolonies would most probably result in the death of the bird. In the recent Risk Watch List compiled for the Christmas Island National Park and Conservancy, the crazy ant invasion was rated as an Extreme Risk to biodiversity and conservation values, with catastrophic consequences of failure to implement effective control measures.

Parks Australia North field workers and assistants conducted an island-wide ant survey during the months May-August, 2001. This survey was designed by CAMBI (Centre for the Analysis and Management of Biological Invasions, Monash University). Of 972 sites surveyed, 741 surveyed points fell in natural forest. Crazy ants were recorded at 46.7% (346/741) of sites in natural forest. Super-colony densities were recorded at 22.7% (168/741) of sites in natural forest. Using these survey points as a representative sample of the forest, it was estimated that 2,379 hectares of the estimated 10,492 hectares of forest on the island was infested with crazy ants at super-colony densities (Orchard et al. 2002).

Crazy Ants are not evenly distributed throughout natural forest but are more commonly found on terrace forests and less commonly found in deep-soil tall-closed forest, which make up most of the natural forests on the island plateau. Deep-soil tall-closed forest made up 31% of census sites in natural forest but only 6.5% of supercolony records (Orchard et al. 2002). Christmas Island Frigatebirds are especially at risk from Crazy Ants because of this concentration of supercolonies on terrace forests where all Christmas Island Frigatebirds nesting habitat occurs. The distribution of Crazy Ant colonies in relation to Christmas Island Frigatebird nesting areas is shown in Figure 1.