ABC Radio National's Science Show
Christmas Island's environment
Christmas Island is a place of natural beauty, where a unique ecosystem of plants and animals has developed over millions of years.
Listen to the ABC report
The exotic and invasive yellow crazy ant (Anoplolepis gracilipes), was accidentally introduced to Christmas Island between 1915 and 1930. They are thought to have come by ship with produce or building materials from either Malaysia or Singapore. They didn't appear in the records until about 1930. The ants have no effective natural predators on Christmas Island and thrive on the habitat and sources of food available. They have a high reproductive rate and can form multi-queened super-colonies in which ants occur at very high densities. In fact, some of the highest ant densities recorded on earth.
Yellow crazy ants are recognised by their pale yellow body colour, unusually long legs and antennae. The "crazy ant" descriptor was derived from their frantic movements and frequent changes in direction, especially when disturbed.
A single super-colony was discovered on a high terrace above the Grotto in 1989. This colony remained isolated and eventually declined. Super-colonies were again found from 1995 to 1997. Subsequent surveys indicated at least 10 separate infestations, ranging from several hectares to at least one square kilometre, and distributed throughout the island.
At the height of their population growth in 2002, the super-colonies affected some 2500 hectares of the island, or 25 per cent of the total forest area. Once a super-colony is established, it can expand rapidly, in some cases doubling in size in 12 months. To put this in context, the edge of a super-colony can expand at around three metres per day or around one kilometre per year.
Effects on Christmas Island ecology
The yellow crazy ant has a significant destructive impact on the island's ecosystem, killing and displacing red, blue and robber crabs on the forest floor. The super-colonies also devastate red crabs on their annual migration to the coast. This has seen a rapid depletion of land crab numbers which are vital to Christmas Island's biodiversity. In particular, red crabs as they are a keystone species in the forest ecology where they dig burrows, turn over the soil, and fertilise it with their droppings.
Following the removal of red crabs by yellow crazy ants, seedlings that were previously eaten by red crabs started to grow, and as a result, changed the structure of the forest. Weeds also spread into the rainforest in super-colony areas because there are fewer crabs to control them. One of the most noticeable changes in the forest is the increased numbers of stinging tree Dendrocnide peltata, which now flourish along many of the walking tracks and other areas that people frequently visit around the island. Giant African snails, another introduced pest on the island, are able to proliferate in the areas where red crabs have been removed. In healthy red crab areas the snails are eaten by the crabs and their population is kept in check.
Robber crabs, red crabs, and blue crabs are completely wiped out from infested areas. Populations of other ground and canopy dwelling animals, such as reptiles and other leaf litter fauna have also decreased.
During red crab migrations, many crabs traverse areas infested with ants on their way to the coast and are killed. Studies estimate that yellow crazy ants have killed tens of millions of crabs since the invasion began - around half the red crab population - by occupying their burrows, killing and eating resident crabs, and using their burrows as nest sites.
Although yellow crazy ants do not bite or sting, they spray formic acid as a defence mechanism and to subdue their prey. In areas of high ant density, the movement of land crabs disturbs the ants and as a result the ants instinctively spray formic acid as a form of defence. The high levels of formic acid at ground level eventually overwhelm the crabs, and they are usually blinded, paralysed then eventually killed. As the dead crabs decay, a bonus source of protein becomes available to the ants.
Ants in general require two main types of food: carbohydrate to provide energy for the foraging workers, and protein to enable the queens to produce eggs. Yellow crazy ants get much of their food requirements from scale insects. Scale insects are serious plant pests that feed on sap of trees and release honeydew, a sugary liquid. Ants eat honeydew, and in return protect the scale from their enemies and spread them among trees. This relationship is called a mutualism.
The honeydew not eaten by ants drips onto the trees and encourages the growth of sooty mould over the leaves and stems giving the plants an ugly, black appearance, and reducing the health and vigour of the plant.
In summary, yellow crazy ants kill the fauna, but encourage scale insects. Increased densities of scale insects causes forests trees to dieback, creating light gaps in the forest canopy. Light gaps and removal of red crabs encourages seedling growth and weeds and other secondary invaders into the forest.
Control of yellow crazy ants
Over recent decades yellow crazy ants have emerged as one of the worst 100 invasive species in the world. Communities from Hawaii to Madagascar are trying to eradicate them, but no one has found a permanent solution yet.
Christmas Island is a focal point for this international control effort. It is the only place where the ants are known to have formed supercolonies - ants in densities high enough to impact upon red crabs and other land crab species. These supercolonies spread further and cause more damage than single colonies, and they pose the single greatest known threat to the island's biodiversity.
Staff from Christmas Island National Park have worked constantly for more than a decade to keep super-colonies in check. With help from the Christmas Island Crazy Ant Scientific Advisory Panel and support from the Australian Government they are holding ground.
To reduce the impacts of crazy ants on red crabs and the island's ecosystems, the national park carried out major aerial baiting programs in 2002, 2009 and 2012. The first step in each round of aerial baiting is to conduct an extensive island-wide survey to work out exactly where the super-colonies were. For several months staff traverse the entire island to map the target area for aerial baiting. In the latest aerial baiting round they mapped more than 1,000 hectares of yellow crazy ant super-colonies.
The latest round of aerial baiting occurred from July to September 2012. A helicopter was used to precisely bait yellow crazy ant super colonies, which covered more than 1,000 hectares of the island. A very low concentration of Fipronil bait (a tenth of one per cent) was used to control the ants, and two insect growth regulators were trialed in small areas as potential alternative baits. On initial indications, the baiting appears to have been successful, but it takes several months of monitoring for national park staff to confirm that the targeted super-colonies have been wiped out. In previous aerial baiting rounds, ant densities have been reduced by 99 per cent at the super-colony sites, and the park is hopeful to see similar results this time around.
Park staff place a high emphasis on minimising any non-target impacts of baiting. Food lures are dropped from a helicopter to attract robber crabs away from areas that are about to be baited. This technique, combined with the low concentration Fipronil bait, has proved to be highly successful with extremely low numbers of robber crabs and no red crabs known to be killed by the baiting.
Research: the Crazy Ant Scientific Advisory Panel (CASAP)
The role of the Crazy Ant Scientific Advisory Panel is to provide the Australian Government with scientific and technical advice for the management of yellow crazy ants on Christmas Island.
The seven member panel contains experts and internationally recognised research scientists with high level experience in the fields of ecology and invasive ant management. Some of the members have spent many years working and researching yellow crazy ant issues on Christmas Island.
The national park is partnering with researchers to explore the possible use of biological methods of yellow crazy ant control.
Scientists think yellow crazy ant super-colonies may depend on the honey dew produced by the tiny scale insects that provide the ants with limitless sugar. If so, it may be possible to manage yellow crazy ants by controlling scale insects.
This is being studied through a four-year research collaboration between the Director of National Parks and Monash and LaTrobe Universities. It involves three key components:
- Investigating the role that honey-dew producing scale insects have in sustaining crazy ants.
- Identifying if there are natural enemies to control scale insects.
- Identify the best strategies to implement a biological control program if appropriate natural enemies are found.
The research project is based on the best available scientific evidence and advice (including from CASAP).
Any future release of a suitable biological control agent would only be considered if it was determined that its release would have no impact on humans or the island's native plant and animal species.
Find out more
- Crazy ant baiting and the effects of Fipronil
- Final report | Monitoring of the 2009 aerial baiting of yellow crazy ants (Anoplolepis gracilipes) on non-target invertebrate fauna on Christmas Island 2011
- First record of Pulvinaria urbicola (Hemiptera: Coccidae), a potentially damaging scale insect, on Christmas Island, Indian Ocean 2011
Park staff are keeping both the local Christmas Island and broader community informed as plans evolve and action takes place on this extremely important issue. Updates on the research and efforts are placed in local papers and on on the Parks Australia blog.