General impacts: Anoplolepis gracilipes High densities of the yellow crazy ant have the potential to devastate native 'keystone' species, resulting in a rapid alteration of ecosystem processes and negative effects on endemic species. The most notable example concerns the native forests of Christmas Island, in which populations of the yellow crazy ant have exploded in recent decades (at least 60 years after its initial introduction) (CBD 2003). On the island the ant is continuously active and realises a broad diet, tending honeydew-producing scale insects (Tachardina aurantiaca ) and causing their populations to explode (CBD 2003). This results in an increased level of carbohydrate-rich honeydew residues on tree surfaces and an increase in the growth of sooty moulds on trees and tree death (CBD 2003). Anoplolepis populations have also proven devastating to native land crabs, including the red land crab Gecarcoidea natalis , of which up to 20 million have died due to the formidable formic acid it possessed by the ant (O ’Dowd et al . 1999; Parks Australia 2005). Regulators of the natural island rainforest ecosystem, land crabs maintain high nutrient cycling rates in the forest ecosystem by degrading leaf litter, reducing new seedling recruitment and speeding up the microbial decomposition processes (CBD 2003; Parks Australia 2005). The loss of the crab, as well as other changes incited by the high densities of the ant, are presenting serious problems to native habitats and endemic species on the island, as well as indirectly threatening tourism and the aesthetic value of the island for the public. As well as significantly and quickly altering the natural ecosystem processes and the associated environment, the yellow crazy ant has reduced native bird, reptile and mammal life. The excellent competitive ability of the ant is thought to be due to the ant ’s high foraging intensity and its greater foraging ability (for example it is able to utilise food sources in difficult-to-reach locations such as the rain forest canopy) (O ’Dowd et al . 1999). The ant threatens many endemic and endangered species, especially on islands, and undermines any potential or actual tourism investments. For example, on Bird Island in the Seychelles the tourism sector has been threatened by the ants ’ displacement of about 60,000 pairs of sooty terns ( Sterna fuscata ), a main attraction of the island (Feare 1999, in CBD 2003). The ant has the potential to impart significant damage, or alternatively be advantageous to agricultural systems and plant species, depending on variables such as the crop, the geographical region and the types of pest and/or beneficial insects present. The yellow crazy ant generally benefits honeydew-producing, plant-damaging insects, which would be expected to increase in number, while other insects (both plant- commensals and plant-exploiters) may be preyed by the ant and decrease in number. The mutualism between A. gracilipes and honeydew-secreting Homoptera has also cause insect population explosions in native habitats, resulting in the growth of moulds on, and the dieback of, important canopy trees. Particularly in rural areas, A. gracilipes can become a severe household and field nuisance. When disturbed it sprays formic acid, a chemical that can cause burns and irritation when it comes in contact with the skin or the eyes. Invasion by the ant may facilitate secondary invasions. On Christmas Island the giant African landsnail and some woody weeds increased in invaded areas. In introduced regions A. gracilipes forms uniclonal colonies, which gives it a greater potential to reach high local densities and dominate habitats (Holway et al . 2002; Ness and Bronstein 2004; McGlynn 1999). “Uniclonal ” refers to the social structure of an ant colony, and is used to describe the cooperation of several ant nests (each with its own queen) to provide food, resources and care for individuals in all nests. This multi- nest (polydomous) and multi-queen (polygynous) structure increases the probability of colony survival, because the unit as a whole has access to a larger variety and number of resources at any time, and is able to use surplus resources more effectively. Uniclonal nests of A. gracilipes can contain upwards of 300 queens and between 2 500 and 36 000 workers and have been reported to extend over areas as large as 150 hectares. In contrast “multiclonal ” colonies are monodomous and monogynous (one nest, one queen) and consist of workers that attend to and provide for only one queen and her nest. Such colonies have less access to potential resources and may be less resilient (McGlynn 1999). In multiclonal colonies the workers defend one queen and exhibit high aggression, defend the nest from other colonies. These colonies may also aggressively and episodically raid the nests of other ant species. The competitive pressure in uniclonal colonies, on the other hand is lower with the workers cooperating as a “supercolony ” to recruit food items (Ness and Bronstein 2004; McGlynn 1999). .