|Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the List of Key Threatening Processes under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)
1. Name and description of the threatening process
Nominated name: ‘Loss of biodiversity and ecosystem integrity following invasion by the Yellow Crazy Ant (Anoplolepis gracilipes) on Christmas Island, Indian Ocean.’
The Yellow Crazy Ant, in addition to having been introduced to Christmas Island, has also been introduced to Arnhem Land, Northern Territory. Consideration was given to expanding the threatening process to cover mainland Australia rather than restricting the process to Christmas Island. However, while there has been much research leading to strong evidence to support the listing of the threatening process on Christmas Island, there has been relatively little research in Arnhem Land. Also, the threatening process described is specific to Christmas Island, an ocean island with a high number of endemic species, a highly impoverished native ant fauna, and an ecosystem that is greatly influenced by the large number of endemic Red Land Crabs that are present. A specific component of the described threatening process is the reduction in numbers of the Red Land Crabs that is occurring as a result of the invasion of Yellow Crazy Ants, and the subsequent impact this will have on the island’s ecosystem.
Arnhem Land does not have such a unique ground-dwelling, keystone species dominating its ecosystem, and has an extremely diverse native ant fauna. Also, it is not clear to what extent Yellow Crazy Ant supercolonies form. In their review on invasive ants of the world, Holway et al (2002) state that species invading oceanic islands with few or no native ants may exhibit patterns of invasion different from those observed in regions with indigenous ants. The review summarized the reports of Yellow Crazy Ants impacting on other species of fauna from around the world, and impacts have only been reported from islands. Outside of Australia, there appears to be few published observations of Yellow Crazy Ants impacting on native fauna away from islands. For these reasons, the threatening process that was considered was restricted to Christmas Island.
The Yellow Crazy Ant and its characteristics
The Yellow Crazy Ant (Anoplolepis gracilipes) is an ant that lives in colonies and is noted for its frenetic activity when disturbed (hence its common name, Yellow Crazy Ant). It is one of the five ‘tramp ant’ species that are well known for being invasive and ecologically damaging, the others being the Red Imported Fire Ant (Solenopsis invicta), the Big-headed Ant (Pheidole megacephala), the Little Fire Ant (Wasmannia auropunctata), and the Argentine Ant (Linepithema humile). All these species are very aggressive and competitive so that they are able to dominate food sources, are easily dispersed by human activities, and have the capacity to form ‘supercolonies’ with multiple queens that allows rapid and extensive colonisation.
Appearance: In Yellow Crazy Ants, workers have long, slender, yellow-brownish bodies, approximately 4-5 mm long. Queens are larger and much more robust than workers, averaging about 10 mm in length.
Defence and subduing prey: The Yellow Crazy Ant lacks a sting, but it sprays formic acid from the tip of its abdomen as a defence mechanism and to subdue prey. In areas of high ant density, the movement of an animal or human can disturb the ants, whereby the ants instinctively spray formic acid as a form of defence. Spraying can cause skin burns and eye irritation in humans.
Geographical range and habitats: The native range of the Yellow Crazy Ant is not known explicitly, although authors have speculated its origin as West Africa, India or China (Wilson & Taylor 1967). It has been widely introduced across the subtropics and tropics, including East Africa (two countries), South and Southeast Asia (nine countries), Australasia (two countries), and the Indo-Pacific Islands (20 island groups). It is reported to have an elevational limit of 1000-1200 m (Van der Goot 1916, but see Wetterer 1998). It is capable of invading both disturbed and undisturbed tropical/subtropical habitats, including urban areas, rural villages, plantations, coastal strand, grassland, savanna, woodland, and rainforest.
Colony organization and reproduction: The Yellow Crazy Ant is polygynous (multi-queened) and aggression among workers is not reported. Colonies can exist with relatively low densities of ants or can occur with extremely high densities of ants that are called ‘supercolonies’. The species can form diffuse supercolonies, sometimes extending continuously over large areas (up to 750 ha). Most dispersal and foundation of new colonies appear to occur through colony budding, i.e. when mated queens and accompanying workers leave the nest to establish new nests. The mating flights have not been reported in the literature and the importance of aerial dispersal by winged queens is unclear. Single nests of Yellow Crazy Ant can contain upwards of 1000 queens and tens of thousands of workers. The life cycle of Yellow Crazy Ant has been estimated to be 76-84 days (Rao & Verresh 1991).
Nest sites: Yellow Crazy Ants have generalized nesting habits, nesting on the ground in soil, in and below woody debris, below rocks, at the base of trees, underneath leaf litter on the forest floor, and in animal burrows (e.g. O’Dowd et al 1999). They also nest on vegetation - in tree hollows and at the base of epiphytes. When supercolonies form, it is often difficult, if not impossible, to identify single nests (Way 1953). Researchers have found that in ant-infested areas on Christmas Island, the density of nest entrances on the forest floor ranges from 2.8 to 16.0/m2, and virtually every canopy tree has many nest entrances encircling its base.
Nutrition: The Yellow Crazy Ant has a broad diet. Initially described as a scavenger, it has been called subsequently a "scavenging predator". It preys on a variety of litter and canopy fauna, from small isopods, myriapods, earthworms, mollusks, arachnids and insects to large land crabs, birds, mammals and reptiles (Lewis et al 1976; Haines et al 1994; O’Dowd et al 1999). In addition to these protein-rich foods, the Yellow Crazy Ant obtains carbohydrates and amino acids from both plant nectar and from honeydew excreted by Homoptera (aphids and scale insects), which the Yellow Crazy Ant tends on stems and leaves of a wide variety of tree and shrub species. Honeydew is particularly important in its diet (e.g. O’Dowd et al 1999, 2001).
Foraging activity: In ant-infested areas on Christmas Island, Yellow Crazy Ants in supercolonies forage night and day across every available surface on the forest floor and in the canopy. The density of foraging workers on the forest floor can average over 1000 individuals/m2. Also, Yellow Crazy Ants forage intensively in the canopy on honeydew from a variety of sap-sucking scale insects with which the ants have a mutually beneficial relationship. In supercolonies, columns of foraging Yellow Crazy Ants continuously stream up and down the trunks of most large trees.
The Yellow Crazy Ant as an Invasive species: The Yellow Crazy Ant has accidentally been introduced across the world’s tropics and subtropics as a result of commercial activities. Because it has very generalized nesting habits, it is readily transported through a variety of pathways to new locations. For example, it can be accidentally transported during the transport of timber, soil, horticultural products, machinery and road vehicles and packaging material (e.g. Stanaway et al 2001).
The Yellow Crazy Ant possesses a number of traits that, when combined, increase its potential to become a threatening, invasive species. For example, with its general omnivorous feeding behaviour (eating both animals and plants) and versatile nesting habits, it has a high chance of locating suitable food and nesting sites within any area that it invades. With such traits, the Yellow Crazy Ant can build up to high densities and form supercolonies, and become the most common consumer over large areas on the ground and in the forest canopy. Yellow Crazy Ants can have large direct and indirect impacts on the ecosystems in which supercolonies form. These impacts have been well documented on Christmas Island (see below), and described to various extents in disturbed forest in the Seychelles and Hawaii (Gillespie & Reimer 1993; Haines et al 1994; Feare 1998, 1999), riparian corridors in east Arnhem Land (Young et al 2001), plantations in East Africa, Papua New Guinea and the Solomon Islands (Way 1953; Greenslade 1972; Young 1996), and agricultural fields and villages in southern India (Verresh 1987).
Invasion in Australia: Yellow Crazy Ants have become established on Christmas Island and in east Arnhem Land, Northern Territory. They have also been reported or intercepted near ports in Brisbane, Townsville, Cairns, Darwin and Perth but have subsequently been eradicated. The nearest source areas for Yellow Crazy Ants occur directly to the north and north east of Australia, in Indonesia, Papua New-Guinea, the Solomon Islands and Vanuatu. These incursions demonstrate the ease with which this ant can translocate through human commerce and trade.
Christmas Island and the characteristics of its ecosystem
Christmas Island is located in the Indian Ocean, at latitude 10°25' south and longitude 105°40' east. It is isolated, being approximately 2600 km north-west of Perth and 360 km south of the western head of Java.
Christmas Island is the summit of an otherwise submarine mountain. The island rises steeply to a central plateau up to 361 meters high and is dominated by rainforest. The island's 80 km coastline is an almost continuous sea cliff, with the island’s port located at Flying Fish Cove. The climate is tropical, with a wet season between November and April. Extensive phosphate deposits have been mined on the island for many years (DOTARS 2004).
The island covers approximately 135 km2, of which about 85 km2 (63%) is the Christmas Island National Park. Christmas Island supports a wide range of unique and unusual species and habitats, and until the invasion of Yellow Crazy Ants, it was considered that much of the natural ecosystem was intact. The National Park is managed by Parks Australia North - a division of the Department of the Environment and Heritage.
Christmas Island's ecology is considered to be unique due to its endemic flora and fauna and the presence of large populations of land crabs and seabirds. The natural vegetation on Christmas Island includes large areas of rainforest. There are approximately 411 plant species and approximately 18 of these are endemic to Christmas Island (Flora of Australia - Vol. 50, 1993). Approximately 126 of these species are not known to occur anywhere else in Australia and its Territories, and 28 species are currently considered rare or threatened (Dept of the Environment & Heritage 2004).
The inventory of the invertebrate fauna of Christmas Island is incomplete. There a number of ant species on the island and, though it has been reported that there are no native ant species on Christmas Island (Taylor in Lawrence 1990), there is some debate as to whether some of the species are native or not. The terrestrial fauna of Christmas Island is dominated by land crabs (which depend on the ocean for their larval development) and in particular the Geocarcoidea natalis (Red Land Crab). This crab is the dominant consumer on the forest floor, and plays a major role in determining the structure and function of the rainforest on Christmas Island. The diversity and abundance of land crabs are striking features of the invertebrate fauna, not matched on any other island in the world.
Most of the native terrestrial vertebrates, including all the land birds and three of the sea birds, are endemic. The island is a focal point for breeding seabirds in the area. BirdLife International has listed Christmas Island as an “Endemic Bird Area”. These areas harbour a high concentration of endemic bird species, and are regarded as being of the highest priority for the global conservation of bird biodiversity.
Invasion of Yellow Crazy Ants on Christmas Island: Yellow Crazy Ants were probably accidentally introduced to Christmas Island between 1915 and 1934 (O’Dowd et al 1999) with the species now widespread throughout rainforest and settled areas on the island. Supercolony formation on the island has been a relatively recent phenomenon; the first being discovered in 1989. This colony remained isolated and eventually declined (Dept of the Environment & Heritage 2004). In the mid 1990s more colonies were recorded and by the late 1990s supercolonies of Yellow Crazy Ants had formed in a number of areas and were spreading, with up to 10 fold increases in colony area in a year predicted (O’Dowd et al 1999). One expert commented that by September 2002, supercolonies had grown to cover 3000 ha in just 7 years.
Before recent abatement programs, Yellow Crazy Ant supercolonies had formed in 24.4% of island’s rainforest (Orchard et al 2002), with researchers reporting densities as high as 79 million workers per hectare on the forest floor. 90% of known Yellow Crazy Ant supercolonies occurred within the Christmas Island National Park boundaries, or about 25 km2, about 30% of the park, had been infested with Yellow Crazy Ants.
Management of Yellow Crazy Ants on Christmas Island: The possible impact of Yellow Crazy Ants on Christmas Island’s unique habitat was apparent in the late 1990s. Since then, there has been a concerted effort by the Australian Government’s Parks Australia North and experts to manage the Yellow Crazy Ant problem. An outline of the threat abatement work that has occurred to date is presented below when Threat Abatement Planning is discussed. While the program has been successful in destroying Yellow Crazy Ant supercolonies in most of the infested areas, smaller numbers of supercolonies are forming in other areas. It is too early to evaluate how well the previously infested areas are recovering and what the damaging long-term effects of the supercolonies have been. Also, at this stage, threat abatement is aimed at suppressing Yellow Crazy Ant supercolonies, and it may not be possible to completely eradicate Yellow Crazy Ants from Christmas Island.
The process - Overview
The loss of biodiversity and ecosystem integrity following invasion by the Yellow Crazy Ant results from its rapid increase to high populations or supercolonies, its rapid spread, its association with outbreaks of scale insects, and its broad foraging range and generalist feeding habits. When the Yellow Crazy Ant invades an ecosystem, it can directly impact on a range of fauna and flora within the ecosystem, which can in turn lead to changes in the structure of the habitat and alterations to the ecosystem processes. On Christmas Island, the combined impact of these is likely to lead to considerable losses of biodiversity.
There are a number of ways Yellow Crazy Ants are impacting on the Christmas Island ecosystem, and these can be divided into three main areas:
the direct impact caused by direct predation1 and competition with, other species;
the impact caused by the removal of the keystone forest species; and
the impact caused by the mutualistic relationship between Yellow Crazy Ants and scale insects.
The impact caused by direct predation and competition with other species
Yellow Crazy Ants in supercolonies are known to directly kill or compete with native species. The Yellow Crazy Ant is known to directly kill invertebrates, reptiles, hatchling birds, small mammals and other newborn animals. Yellow Crazy Ants also displace and interfere with local species, using resources such as tree hollows, and restricting access to food sources. On Christmas Island, Yellow Crazy Ant have directly affected, and inversely impacted on, a number of species. The impact on individual species is discussed under each assessment criteria.
The potential loss of biodiversity though direct means, such as predation and competition, may be considerable, but, the direct impact that the Yellow Crazy Ant has had on the Red Land Crab, a keystone species (a species that has a critical role in the structure of the biological community) on Christmas Island, is likely to lead to a further loss of biodiversity on the Island (O’Dowd et al 2003). As supercolonies form and spread across Christmas Island, the Red Land Crab, the dominant endemic consumer, is killed and displaced from ant-invaded forest. Furthermore, during the annual migration of the Red Land Crab, large numbers are killed in transit when migratory pathways intercept ant supercolonies. This depletes crab populations in extensive areas that are not yet directly invaded by the Yellow Crazy Ant (O’Dowd et al 2001, 2003).
The impact caused by the removal of the keystone forest species
Alteration of ecosystem processes: The Red Land Crab is the major seed, seedling and litter consumer in the rainforest of Christmas Island. Their removal or reduction in numbers caused by Yellow Crazy Ants results in alterations in the rates of seedling recruitment and litter breakdown, alterations in the recruitment dynamics of rainforest trees and almost certainly changed patterns of nutrient availability. This leads to a rapid shift in forest structure and composition or a “state change” in the rainforest ecosystem.
This affect has been recently documented (e.g. O’Dowd & Lake 1989, 1990, 1991; O’Dowd et al 1999, 2001, 2003; Green et al 1997; 1999). Experiments conducted at a small spatial scale over the last decade show that removal of the Red Land Crabs leads to alterations in seedling recruitment, seedling species composition, litter breakdown, and the density of litter invertebrates. Elimination of the Red Land Crab by Yellow Crazy Ants copies these effects but at a landscape scale. As a consequence, forest processes in Yellow Crazy Ant supercolonies are indirectly affected: on average, litter cover has been found to be double, seedling densities 30-fold higher, seedling species richness 3.5-fold higher, and forest understorey structure markedly different in ant-invaded areas. Species composition of seedlings also changes in Yellow Crazy Ant supercolonies.
Thus, the direct impact of Yellow Crazy Ants on the Red Land Crab, the dominant native consumer, has long-term implications for forest structure and species composition. In turn, these changes will affect, to varying extents, the way other species of fauna are able to use the ant-invaded forests.
The impact of Yellow Crazy Ants on fauna, other than the Red Land Crab, may also lead to other indirect impacts on the ecosystem. For example Yellow Crazy Ants may impact directly on seed dispersing endemic birds such as the Christmas Island White-eye, Christmas Island Thrush and Christmas Island Emerald Dove, leading to changes in the ways seeds are dispersed on the island.
Secondary invasions: Endemic Red Land Crabs provide “biotic resistance” to a wide range of potential alien invaders on Christmas Island (Lake & O’Dowd 1991). For example, crabs will eat the introduced Giant African Snail (Achatina fulica) that enter their habitat, thereby preventing the snails from becoming established. They also eat weeds. Therefore, the crabs’ elimination from the rainforest by the Yellow Crazy Ant may pave the way for invasion by the Giant African Snail and a variety of environmental weeds (e.g. Carica papaya, Capsicum frutescens, Muntingia calabura) (e.g. Green et al 2001). It is unknown whether deletion of crabs facilitates rainforest invasion by feral mammals, including cats (Felis cattus), Black Rats (Rattus rattus), and House Mice (Mus musculus), or facilitates exploitation of primary rainforest by the Wolf Snake (Lycodon capucinus), a species introduced to Christmas Island in about 1987 and which, until recently, has been largely confined to disturbed habitats, including secondary growth. Secondary invasion by these species is a possibility as a result of the Red Land Crabs being removed from parts of the forest.
The impact caused by the mutualistic relationship between Yellow Crazy Ants and scale insects
Mutualism with scale insects: Yellow Crazy Ants get much of their food requirements from scale insects. Scale insects feed on sap of trees and release honeydew, a sugary liquid, onto the plant. On Christmas Island, Yellow Crazy Ants eat honeydew, and in return protect the scale insects from predators and parasites – it is sometimes considered that the ants are ‘farming’ the scale insects. This relationship is called mutualism. The honeydew not eaten by ants stays on the trees and encourages the growth of sooty mould (Capnodiaceae) over the leaves and stems, reducing the health and vigour of the plant. Mutualism between the Yellow Crazy Ant and introduced or cryptogenic (origin not known) scale insects can result in scale insect outbreaks. Therefore, in areas invaded by Yellow Crazy Ants, there are high densities of Yellow Crazy Ants streaming up and down trunks of trees foraging on honeydew in canopy trees, population outbreaks of scale insects, and honeydew-dependent sooty moulds on canopy stems and leaves. Population outbreaks of the cryptogenic lac insect Tachardina aurantiaca (Kerridae) and introduced Coccus celatus (Coccidae), in particular, are associated with Yellow Crazy Ant supercolonies on Christmas Island. These insects are associated with at least 21 and 10 tree species, respectively, including most canopy dominants (O’Dowd et al 2001, 2003).