Alteration of ecosystem processes: High densities of scale insects damage trees, and high levels of sooty mould cover can reduce photosynthesis in these trees. Outbreaks of sap-sucking scale insects associated with Yellow Crazy Ants are known to stress trees, leading to decreased seed production, and high mortality in some canopy species. For example Inocarpus fagifer (Tahitian Chestnut), one of the trees that dominates the forest, has been found to be heavily infested and infected by lac scale insects which has contributed to deaths of these trees and lower recruitment of seedlings (see Criteria 1 below).
As forest trees dieback, light gaps are created in the forest canopy. Light gaps, along with removal of crabs, encourages seedling growth and weed invasion into the forest.
Summary of process
Yellow Crazy Ants are generalist predators as well as competitiors that directly impact on species, and their introduction to an area has general implications for food web structure, and specific implications for a wide variety of endemic invertebrate and vertebrate species. Their killing and displacement of the Red Land Crab and direct mutualism with scale insects is in turn causing a rapid state transformation in the forest ecosystem structure and composition on Christmas Island.
2. How judged by TSSC in relation to the Environment Protection and Biodiversity Conservation Act 1999 criteria
Section 188(4) of the Environment Protection and Biodiversity Conservation Act 1999 states:
A threatening process is eligible to be treated as a key threatening process if:
a) it could cause a native species or an ecological community to become eligible for listing in any category, other than conservation dependent; or
b) it could cause a listed threatened species or a listed threatened ecological community to become eligible to be listed in another category representing a higher degree of endangerment; or
c) it adversely affects 2 or more listed threatened species (other than conservation dependent species) or 2 or more listed threatened ecological communities.
Research on the impact of the Yellow Crazy Ant on the biota of Christmas Island indicates that a diverse range of taxa – from litter invertebrates to canopy vertebrates to canopy trees - are likely to be adversely affected by the Yellow Crazy Ant. The following is a discussion on the key species to support the listing of this process as a key threatening process.
A) Could the threatening process cause a native species or an ecological community to become eligible for listing in any category, other than conservation dependant?
A list of native species and ecological communities which could become eligible for listing due to the threatening process is at Attachment (i).
Impacts on native plants
Inocarpus fagifer (Tahitian Chestnut) is one of the native species that dominates the rainforest canopy on Christmas Island. In one study, both seedlings and trees of the Tahitian Chestnut were found to be heavily infested with, and especially affected by, the lac scale insect Tachardina aurantiaca in areas of supercolonies, and a higher proportion of deaths of Tahitian Chestnut trees was occurring. At sites free of Yellow Crazy Ants, seedlings of the Tahitian Chestnut comprised 24% of all seedlings, but in Yellow Crazy Ant supercolonies, they comprised just 0.1% of all seedlings. As a consequence of the lac scale outbreaks caused by Yellow Crazy Ants, the Tahitian Chestnut is suffering from canopy dieback, high tree mortality, reduced fecundity and high seedling mortality (O’Dowd et al 2001; also unpublished data). This species could become eligible to be listed as vulnerable because of this threatening process.
Impacts on native mammals
There are three endemic mammals on Christmas Island - Pteropus melanotus natalis (Christmas Island Flying Fox), Crocidura attenuata trichura (Christmas Island Shrew) and (Christmas Island Pipistrelle (Pipistrellus murrayi), a bat. The latter two are threatened species and there is some evidence that Yellow Crazy Ants are impacting on them (see assessment under Criterion 2). One expert states that there is evidence that the Christmas Island Pipistrelle has declined significantly, though the reason for decline is not known, and that Yellow Crazy Ants may be directly interfering with bats or young at roosts. In addition, Yellow Crazy Ants may compete for food, and may be causing the decline of rainforest plants that are important to this bat. It has also been reported that supercolonies have formed in areas where three of the five larger camps of this bat reported by Tidemann (1985) are located. This threatening process could cause the Christmas Island Flying Fox to become eligible for listing as a threatened species under the EPBC Act.
Impacts on native birds
Assessing the impacts of Yellow Crazy Ants on the birds of Christmas Island is difficult. There are a number of factors that suggest Yellow Crazy Ants are having a detrimental affect on native bird species in areas where ant supercolonies occur, such that they could become eligible for listing as threatened (other than as conservation dependent).
Direct impacts: There are a few observations in supercolonies of Yellow Crazy Ants swarming over dying birds that have for some reason been grounded. These species include Turdus poliocephalus erythropleurus (Christmas Island Thrush), Zosterops natalis (Christmas Island White-eye), Papasula abbotti (Abbott’s Booby), Phaethon lepturus fulvus (Christmas Island White-tailed Tropicbird), Accipter fasciatus natalis (Christmas Island Goshawk), Ducula whartoni (Christmas Island Imperial-Pigeon), and Chalcophaps indica natalis (Emerald Dove). About 25 birds in total have been found over 3 years, and grounded birds were rarely found in forest that was not affected by Yellow Crazy Ants. These observations suggest that Yellow Crazy Ant may attack and kill a range of native bird species on Christmas Island, but the lack of bird carcasses found in intact forest may also be explained by the presence of scavenging land crabs eating the carcasses. Therefore, it is not clear if the Yellow Crazy Ants kill healthy birds on Christmas Island. On Bird Island in the Seychelles, invasion by Yellow Crazy Ants led to about 60,000 pairs of Sterna fuscata (Sooty Tern), which nest in colonies on the ground, being displaced from their nesting sites and also caused the death of Gygis alba (White Tern) chicks (Feare 1999). Yellow Crazy Ants may also take over nesting sites of known tree-hollow nesting birds.
Indirect impacts: In the long-term, if control measures were to cease, fail or become less effective in the future, then it is highly possible that the ecosystem changes brought about by Yellow Crazy Ants would have a profound effect on terrestrial birds species. As well as issues such as how suitable the changed habitat would be for the existing birds species, there is the issue of secondary invasion of the forest by Black Rats and cats. Terrestrial birds can be susceptible to extinction on small islands, and it is often related to the introduction of cats and rats. On Christmas Island, introduced rats and cats seem to have difficulties maintaining feral populations away from the settlement. Nowhere do rats and cats appear to be numerous in the forest, although they do occur throughout the island. Rugged terrain, lack of surface water in years of low rainfall and lack of dependable food supply, coupled with competition with Red Crabs may be the reason for the lack of proliferation of these feral species. Through the elimination of Red Crabs, Yellow Crazy Ants may facilitate a proliferation of Black Rats and cats, which could further impact upon bird species.
Species studies: There has only been one study (Davis 2002) that has investigated the impact of the Yellow Crazy Ant on native Christmas Island birds that are not listed as threatened. This was a short-term study from September 2001 to January 2002 on the Christmas Island White-eye, Christmas Island Thrush, Christmas Island Emerald Dove and Christmas Island Imperial-Pigeon (Davis 2002; Davis et al 2002). The study provides some data to support the claim that non-threatened native birds are being adversely affected by Yellow Crazy Ant. In areas where Yellow Crazy Ant supercolonies were established, altered behaviours, including altered foraging patterns, were noted in the Christmas Island Thrush, Christmas Island Emerald Dove and Christmas Island White-eye. The study indicated that the Yellow Crazy Ant has the potential to directly cause population declines in at least the Christmas Island Thrush and Christmas Island Emerald Dove, as well as to indirectly alter the structure, function and integrity of the island’s ecosystems through negative impacts on seed dispersion. There was little evidence that the Christmas Island Imperial Pigeon was impacted upon by Yellow Crazy Ants and the degree of impact on Christmas Island White-eye was uncertain (Davis 2002). The following details the impacts of Yellow Crazy Ants on the dove and thrush.
Chalcophaps indica natalis (Christmas Island Emerald Dove): This subspecies of Emerald Dove is endemic to Christmas Island, and there is reasonable evidence that this threatening process could cause it to become threatened if abatement actions are not undertaken or are unsuccessful.
The Yellow Crazy Ants affect Emerald Doves in a number of ways. Direct attacks on injured Emerald Doves have been observed in supercolony areas, though it was unclear whether attack by Yellow Crazy Ants was the initial cause of injury, as healthy birds appeared to be able to dislodge the ants before serious injury occurred. A report that Yellow Crazy Ants were responsible for killing Emerald Dove nestlings (Garnett & Crowley 2000) has not been verified.
There is evidence that the Yellow Crazy Ant may be reducing population numbers of the Emerald Dove by altering the forest habitat in areas where supercolonies occur and where Red Crabs have been eliminated so that it is less suitable for doves. Davis (2002) found that in areas where supercolonies occurred, there was a 5-10 fold decline in abundance of Emerald Doves, compared to uninvaded forest. This may have been because Emerald Dove’s normally feed on fruit on the ground, where the Yellow Crazy Ants occur at their highest densities. Within supercolonies, the doves would be exposed to competition for food and aggressive attacks by Yellow Crazy Ants. In addition, in areas of forest where Yellow Crazy Ants have eliminated Red Crabs, it was found that the seeds that were previously consumed by the Red Crabs were germinating, and the forest structure had altered, with dense layers of seedlings covering the forest floor. This makes foraging on the ground more difficult for the Emerald Dove and may result in a possible long-term alteration of forest structure and composition.
The study also suggests that though Emerald Doves are attempting to nest in ant-invaded areas, many attempts are failing. It is thought that nesting Emerald Doves may be particularly vulnerable to ant predation as their nests are often low in the vegetation where the threat of interference by ants is high (Davis 2002). While healthy adult Emerald Doves are unlikely to suffer from direct predation by Yellow Crazy Ants, it is possible that their reproductive success could be reduced by Yellow Crazy Ant’s harassing nesting adults (e.g. adults incubating eggs), and predating on nestlings and juvenile birds. Therefore, areas occupied by Yellow Crazy Ant supercolonies are likely to provide much poorer breeding habitat compared to unoccupied areas.
The Emerald Dove occurs throughout most habitats on the island, and as an endemic species has a restricted geographic distribution. A number of impacts on this species have been demonstrated or observed in areas where Yellow Crazy Ant supercolonies occur. As Yellow Crazy Ant control programs are currently being implemented effectively, significant decreases in abundance are unlikely in the future. However, if control measures were to cease, fail or become less effective in the future, this could cause the Emerald Dove to become eligible for listing as threatened.
Turdus poliocephalus erythropleurus (Christmas Island Thrush): This subspecies of Island Thrush is endemic to Christmas Island, and common on most habitats on the island (Davis 2002). It normally feeds on or near the ground. Davis (2002) found that Yellow Crazy Ants had no significant impact on the reproductive success of this species’ nests, but that the number of juveniles and the availability of nest sites appeared lower in ant-invaded forest compared to uninvaded forest. The study also found that some behaviours of the Island Thrush altered in areas where ant supercolonies were present – the Thrushes appeared to adjust foraging behaviour in ant-invaded areas to minimise direct encounters with ants. Healthy adults appeared to be able to avoid direct predation, and to remove ants from their bodies if needed.
However, the elimination of Red Crabs in large areas of Christmas Island by the Yellow Crazy Ant may facilitate a secondary invasion, or increase in abundance, of the Black Rat, which may also impact on the Island Thrush. Black Rats have been implicated in the extinction of Island Thrushes elsewhere – for example, the Turdus poliocephalus vinitinctus (Vinous-tinted Thrush) on Lord Howe Island.
Like the Emerald Dove, this ground-frequenting endemic bird has a restricted distribution and has been observed to be impacted upon by Yellow Crazy Ants. If control measures were to cease, fail or become less effective in the future, this could cause the Christmas Island Thrush to become eligible for listing as threatened.
Other species: There is some anecdotal evidence available that indicates the impact Yellow Crazy Ants are having on other species of birds. For example, the Christmas Island White-tailed Tropicbird primarily nests in forest trees, and there is potential for high densities of foraging Yellow Crazy Ants to cause chick mortality and nest abandonment. However, there is little information on the impact supercolonies have on this species.
Impacts on native reptiles
Outside Australia, Yellow Crazy Ants are known to have had a detrimental affect on some reptiles. In the Seychelles, on Mahé Island, searches for the endemic skink Mabuya seychellensis found 10-fold fewer skinks in Yellow Crazy Ant-infested areas (Haines et al 1994). Similarly, on Bird Island, no M. seychellensis were found in areas where Yellow Crazy Ant supercolonies occurred (Feare 1999). Yellow Crazy Ants were seen carrying an about 5 cm Ramphotyphlops brahminus (burrowing blindsnake) to their nest (Feare 1998).
There is some evidence that Yellow Crazy Ants are impacting on reptiles on Christmas Island. The distribution of three native reptiles, Cyrtodactylus sadlieri (Christmas Island Giant Gecko), the Cryptoblepharus egeriae (Blue-tailed Skink) and the Emoia nativitatis (Forest Skink), overlap with that of Yellow Crazy Ant supercolonies, and these species are located in microhabitats where Yellow Crazy Ants forage intensively. Hatchlings, juveniles and adults of these three species are likely to be attacked and eaten by the Yellow Crazy Ant.
In 1998, in areas free of supercolonies, the Christmas Island Giant Gecko appeared widespread, abundant and secure (Cogger & Sadlier 1999). Searches for this species have found that this gecko still inhabits ant-infested forests but at lower numbers than in forests free of supercolonies (Cogger & Sadlier 1999; Stork et al 2002), though this has not been substantiated. Though the Blue-tailed Skink has contracted recently in range, its contraction appears to be associated with threats other than those posed by the Yellow Crazy Ants (Cogger & Sadlier 1999). The Forest Skink, as its name suggests, is primarily a forest species. This species is unlikely to survive in areas of ant supercolonies, and the impact of Yellow Crazy Ants on this species is potentially catastrophic (Cogger & Sadlier 1999).
While there is evidence that Yellow Crazy Ants are impacting on some reptile species of Christmas Island, there is little data on the degree of impact. The information available suggests that the endemic Christmas Island Giant Gecko and Forest Skink may be particularly vulnerable to the Yellow Crazy Ant invasion, and that the Forest Skink, with its preference for forest habitat, has the potential to become threatened. In the long-term, it is likely that if the Yellow Crazy Ant were allowed to spread unabated, then the ecosystem changes are likely to have a profound effect on terrestrial reptiles.
Impacts on invertebrates
Land Crabs: The terrestrial fauna of Christmas Island is dominated by land crabs and, in particular, by the Red Land Crab. Red Land Crabs are the dominant consumers on the forest floor, and play a major role in determining the structure and function of the rainforest on Christmas Island. One of the most significant impacts of Yellow Crazy Ants on Christmas Island occurs as a result of the Yellow Crazy Ants’ direct predation and competition with the Red Land Crab. Yellow Crazy Ants have also been reported to attack and kill land crabs in the Seychelles and in the Marshall Islands (Feare 1999, National Biodiversity Team of the Republic of the Marshall Islands 2000).
Gecarcoidea natalis (Red Land Crab): The Red Land Crab is by far the most obvious of the 14 species of land crabs found on Christmas Island. These large bright red crabs occur in the millions. They live in a variety of habitats, including coastal shore terraces, but are most common in the moist environment of the rainforest. For most of the year, these crabs live near their burrows, and during the dry season, they stay in their burrows for 2-3 months. Their diet consists mainly of fallen leaves, fruits, flowers and seedlings, but they also eat other dead crabs, and the introduced Giant African Snail. Before the build up of Yellow Crazy Ants numbers, the Red Land Crab had virtually no competition from other species for their food resources, due to their high numbers and dominance of the forest floor (National Parks 2004).
Once a year, most adult Red Land Crabs undergo a synchronized migration from the forest to the coast, to mate and release fertilized eggs into the sea. Masses of crabs gather into broad "streams" as they move toward the coast following traditional routes. After breeding, the adults return inland, and then, after about a month in the ocean, young crabs return to the shore and begin their “march” inland. Here young crabs seem to disappear and are rarely seen, living in rocky outcrops and under fallen tree branches and debris on the forest floor for the first three years of their life (National Parks 2004).
Yellow Crazy Ants are known to directly impact on the Red Land Crab. In areas of Yellow Crazy Ant supercolonies, the movement of the Red Land Crab disturbs the ants, and an alarm response ripples through the ant workers that results in the spraying of formic acid repeatedly and indiscriminately. These crabs are frequently blinded by hundreds of workers spraying acid on or near their eyestalks. Red Land Crabs under attack show signs of stress, frothing at the mouth, and exhibiting extreme lethargy. Death usually results in 24-48 hours. As the supercolonies of Yellow Crazy Ants spread across Christmas Island, ants attack Red Land Crabs on the ground and in burrows and the ants occupy the crabs’ burrows and use them for their nest sites (O’Dowd et al 1999, 2001). As well as being killed round their burrows, Red Land Crabs are killed in transit, when migratory pathways intercept ant supercolonies. This depletes crab populations in extensive areas not yet directly invaded by the Yellow Crazy Ant (O’Dowd et al 2001). Therefore, once a supercolony forms, Yellow Crazy Ants eliminate or displace all the resident Red Land Crabs from that area, and any crabs that subsequently migrate through that area.
Since 1995, it is estimated that Yellow Crazy Ants have killed 10-15 million red crabs, about one quarter to one-third of the total island-wide population (P.T. Green in O’Dowd et al 2003), a catastrophic decline in this keystone species on Christmas Island. The direct impacts of Yellow Crazy Ants on both resident and migrating red crabs are most evident in the western third of the island where comparatively few red crabs remain. The Yellow Crazy Ant could cause the Red Land Crab to become eligible for listing as a threatened species.
Other crab species: The Red Land Crab is only one of three major land crab species in the rainforest proper on Christmas Island. All these species migrate to the sea to spawn. In Yellow Crazy Ant supercolonies, Birgus latro (Robber Crabs) and Geograpsus grayi (Little Nippers) are also killed. The Christmas Island National Park is a major refuge for the Robber Crab, once widespread throughout the Indo–Pacific region but now in rapid decline across its range. Yellow Crazy Ants also attack and kill the Cardisoma hirtipes (Blue Crab). Though a widespread species, the Blue Crab occurs in its blue form only on the Christmas Island.
Although Yellow Crazy Ants are killing these other species of crabs, there does not appear to be much data to indicate that they will become eligible for listing as threatened as a result of the impact from Yellow Crazy Ants. However, in the long-term and if the threat of Yellow Crazy Ant were allowed to spread unabated, then the ecosystem changes could have a profound effect on land crabs.
Other Invertebrates: Yellow Crazy Ants have been observed feeding on a wide range of invertebrates including cockroaches, mantids, Orthoptera, caterpillars, fly larvae, beetles, spiders and other species of ants. Yellow Crazy Ants are renowned for the rapidity in which they forage on insect prey (e.g. Townes 1946; Haines 1994 et al; Feare 1999). The few overseas studies that have investigated the impact of the Yellow Crazy Ant on invertebrates suggest that it is a serious threat to invertebrate biodiversity. In Hawaii, the Yellow Crazy Ant is implicated in the loss of diversity of endemic forest spiders (Gillespie & Reimer 1993) and riparian insects (Hardy 1979). In the Solomon Islands, East Africa and India, Yellow Crazy Ants can displace other ant species, including Oecophylla smaragdina and Pheidole megacephala (Way 1953; Greenslade 1971, 1972; Soans & Soans 1971). In the Seychelles, Yellow Crazy Ants have been found to affect litter invertebrate communities (Hill et al 2003).
The list of native invertebrates (other than land crabs) presented at Attachment (i) is a subset of endemic invertebrates described by Lawrence (1990) that occurs in rainforest microhabitats on Christmas Island that is heavily used by Yellow Crazy Ants. It is considered to be a conservative list. Knowledge of the endemic invertebrate fauna of Christmas Island is not complete, nor is knowledge of their conservation status. A group of invertebrates that is relatively well known is the Coleoptera (beetles) and this group contains a high number of endemics (Lawrence 1990), suggesting that invertebrate fauna more generally may include a high number of endemics.
Though the nomination claims that it is likely that the Yellow Crazy Ant, either directly or indirectly, adversely affects many litter-inhabiting and arboreal invertebrates on Christmas Island, there have been only a few limited studies. In one study, the densities of litter-inhabiting invertebrates were found to be lower in ant-infested forest than intact forest (Davis et al 2002), and in another, Yellow Crazy Ants did not appear to affect the density of canopy arthropods. Anecdotal observations suggest that Yellow Crazy Ants may adversely affect invertebrates. For example, one expert has observed Yellow Crazy Ants on Christmas Island directly attacking, by spraying of formic acid and dismembering, a number of species including earthworms, praying mantises, stick insects, cicadas and cockroaches. Another expert recorded stridulating katydids less often in the rainforest canopy in ant-infested forest than in other areas (G. Richards in Schulz & Lumsden 2004). In addition, the endemic Oxypleura calypso (Christmas Island Cicada), which undergoes its final moult on the trunks of trees, would be exposed to foraging columns of Yellow Crazy Ants, and a group of endemic orthopterans (an order of cricket-type insects that includes Clitumnus stilpnoides (Christmas Island Stick Insect) and Psyra pomona (tettigoniid) would encounter Yellow Crazy Ants on foliage.
At this stage, there is not much data available to indicate whether this threatening process will cause native invertebrates (other than land crabs) on Christmas Island to become eligible for listing as threatened. However, in the long-term and if the Yellow Crazy Ant were allowed to spread unabated, then the ecosystem changes could have a profound effect on many of the Christmas Island native invertebrates.
Impacts on Christmas Island ecological communities: