Bulletin

Issue: August 2004

Research in Action

Red crabs vs yellow ants: Round2?

It's a common story in the fight against exotic, invasive species.

Foreign species invade and wreak havoc on local native biota, there is a flurry of research and control activity resulting in a major win against the pest - and then things go off the boil in the warm afterglow of success. The problem is perceived as 'fixed', and securing long-term funding for follow-up monitoring, control, research and development becomes problematic.

This is the situation facing La Trobe University lecturer in Botany, Dr Peter Green, following his initial success in helping to control the invasion of the exotic yellow crazy ant, Anoplolepis gracilipes, on Christmas Island.

Thanks to Dr Green and his colleagues, who have conducted research on the ecology of the island for 15 years, a control program launched in 2002 eliminated all known crazy ant infestations which threatened the ecology. A major factor was that the ants were destroying the island's massive population of red land crabs.

Christmas Island red land crabs are internationally famous for their annual breeding migrations, when millions march to the sea to breed. Less well known is their pivotal role in the ecosystem. While conducting his PhD research over four years on the island between 1988 and 1991, Dr Green conducted a series of experiments to determine their influence on seedling germination and survival, and on nutrient cycling.

He excluded red land crabs from experimental plots, finding that 'carpets' of young seedlings quickly established, and that a thick layer of leaf litter built up and persisted. Conversely, few seedlings established where crabs were common on unfenced control plots, and there was little build-up of leaf litter. He concluded that red land crabs were a 'keystone' species in Christmas Island rainforest, where the normal condition is to have bare forest floors.

In 1997 and 1998, Dr Green and his colleagues from Monash University discovered tens of thousands of dead red land crabs at several locations where crazy yellow ants had formed 'super colonies'. Millions of ants swarmed over the crab bodies.

'It was obvious what had happened. Yellow crazy ants squirt fine jets of formic acid to subdue their prey. Individual ants can only squirt a tiny amount, but the massed densities of crazy ants in super colonies were obviously sufficient to kill red crabs, hundreds of times their size. At one site - dubbed 'The Valley of Death' - the ants annihilated an entire breeding migration of red crabs.'

The exotic yellow crazy ant, a golden coloured six millimetre long native of West Africa, arrived accidentally on Christmas Island decades before, probably concealed in cargo. Their numbers increased dramatically during the mid 1990s and they began to form super colonies, ranging from less than one hectare to more than 700 hectares. At one point, super colonies had infested about 30 per cent of the island's forested areas.

'We found up to 2,000 ants per square metre on the ground, with as many foraging in tree canopies where they collect honey dew excreted from lac scale insects, Tachardina aurantiaca, and a variety of soft scale insects. The ants have a close mutualistic relationship with scale insects, collecting their honeydew from which they obtain their carbohydrate requirements,' Dr Green said.

'In the early stages of super colony formation, yellow crazy ants source much of their protein from land crabs. The impact of this invasion has been catastrophic for the red crabs as they are completely wiped out where ant super colonies form'.

The fallout from these localised extinctions mirrored Dr Green's research from years earlier, when he excluded crabs from small experimental plots. 'I saw my experiment recapitulated at a landscape scale. In super colonies, there is a build-up of leaf litter on the ground, and a massive recruitment of seedlings and saplings in the understorey. Also, there has been some canopy dieback due the effects of sap-sucking scale insects, which reach outbreak densities,' he said.

'We calculated that by 2002, yellow crazy ants had killed 20 million of the island's 60 million red land crabs, presenting the possibility that eventually this significant part of Australia's natural heritage might be lost altogether.'

With colleagues from Monash University and Parks Australia North, Dr Green drew up a 'shock and awe' campaign to control the crazy ant invasion.

Using GPS and GIS technology, and a lot of leg power, the researchers surveyed the entire island for super colonies and mapped their boundaries. A helicopter then spread granular bait over 2,500 hectares of infested forest.

'The aerial baiting campaign was extra-ordinarily successful,' Dr Green said. 'We reduced ant activity by an average of 99.4 per cent across all super colonies. Essentially, we wiped out all known super colonies, with very few 'non-target impacts' on native species.

'However, new super colonies are beginning to appear in areas not baited, and there are early signs of recovery in some baited areas. We are now looking at more environmentally-acceptable baits and are also looking to co-operate with colleagues in the US Department of Agriculture in Florida, who are starting to research biological agents to control scale insects.

'We think that we can achieve long-term control over the yellow crazy ant by controlling scale insect. Without this source of carbohydrate, we think that the likelihood of super colony formation will be greatly reduced.

'We would like to piggy back on this R&D program - but as yellow crazy ant numbers on Christmas Island are comparatively low at this point in time, it is difficult to attract funds to participate in this research.

'We have nominated the invasion of Christmas Island rainforest by the yellow crazy ant as a 'Key Threatening Process' under the Federal Environment Protection Biodiversity Conservation Act.

'We hope this nomination will be confirmed by the Minister for the Environment and Heritage so that funds for continuing monitoring, control and research can be found,' Dr Green added.

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