The red crab and Christmas Island have a huge, yet small, pest on their hands.
Christmas Island is an Australian territory in the Indian Ocean that lies south of Java, Indonesia and has a modest human population of approximately 2,000. Most of the 135-sq-km island is covered by a national park that offers rainforest hikes to wetlands and waterfalls. The humidity provided by the tropical climate of Christmas Island allows for a wide variety of flora and fauna. The forests are dominated by several tree species that are accompanied by ferns, orchids, and vines. Native wildlife includes nesting seabirds and the red crab, the island’s most famous inhabitant known for its migrations that paint whole streets and beaches in red. Unfortunately, the red crab and therefore Christmas Island have a huge yet small pest on their hands.
The Tiny Menace of Christmas Island
A. gracilipes, an invasive species of ant known as the yellow crazy ant has had a devastating effect on the ecosystem, depleting red crab populations alarmingly fast and greatly disturbing the ecology of the island.
The yellow crazy ant has evolved some highly advanced behaviors such as the farming of other insects. They farm and tend to scale insects that live as parasites on plants. This mutualistic relationship benefits the scale insects by increasing their dispersal
rates and protecting them from potential predators. The scale insects in turn produce excess honeydew for the ants to consume. However, this may be considered more of a parasitic relationship as the yellow crazy ant kills the scale insect by laying eggs inside it.
Although the main food supply for the crazy ants is the honey dew secreted by the scale insects, they also eat grains, seeds, arthropods, decaying matter, and small invertebrates. The yellow crazy ant has a very large venom reservoir that is specialized in the production of formic acid. The ants defend themselves and attack the invertebrates they eat by spraying the acid. The ants attack the red crab in large numbers while the crab is left helplessly defenseless.
The ants spray formic acid into the eyes and joints
of the crab. In defense, the crab tries to flush out the acid with stored water, but instead dies of dehydration. Yellow crazy ants live with multiple queens in super-colonies. This makes the red crab conservation efforts of control programs much harder. Scientists suspect that the invasive species was brought over to the island by ships hundreds of years ago.
The Ecological Benefit of the Red Crab
Approximately 80-120 million Red crabs occupy Christmas Island, dominating its forest floor. Red crabs are endemic and have great ecological importance to Christmas Island. This species lives in burrows and cultivates the forest floor when digging its home. Red crabs till the soil by aerating and replenishing it while simultaneously recycling and removing debris. They eat leaf litter, seeds, and seedlings. The consumption of seeds and seedlings create open understory in the rainforest. After consumption the crabs excrete these as nutrient-rich deposits.
Red crabs prevent invasion by secondary invader species such as the giant African land snail, a renowned pest of the tropics. Red crabs control the giant African land snail by consuming them in areas of the island that are not invaded by the yellow crazy ant.
Healthy areas of the forest on Christmas island support large numbers of red crab, which in turn clean the leaves of the forest floor, suppress seedling growth, and till the soil. In a yellow crazy ant super-colony red crabs are not supported, which causes large amounts of leaf litter. This negatively impacts microclimates and nutrient cycling. The absence of the Red crab also causes large numbers of seedlings, which form understory thickets. Areas removed of red crabs are now occupied by giant African land snails.
For nearly two decades, the Christmas Island National Park in collaboration with La Trobe University and an advisory panel of experts (the Crazy Ant Scientific Advisory Panel – CASAP) has implemented an annual yellow crazy ant control program to minimize the ecological impacts of the yellow crazy ant, especially on red crabs by using insecticides to wipe out super‐colonies once they have formed. The program uses an ant bait called AntOff, which is a protein based granular product with Fipronil as the active ingredient. The chemical bait is applied in two ways, either through broadcast by hand or aerially using a helicopter. Although chemical baiting has proved to be somewhat successful, it is not sustainable. Super‐colonies are difficult to find and can only be baited safely once the ants have reached very high densities – and already killed the resident red crab population. Additionally, applying the ant bait is expensive. Other methods of control are still being researched such as the toxins hydromethylnon and indoxacarb as well as growth regulators such as s‐methoprene and pyriproxyfen. More research is needed to come up with alternative methods such as biological control. Research is being conducted to determine if direct biological control is an option by means of predators, parasites or pathogens. Indirect biological control is also being considered. Super‐colonies of yellow crazy ants occur in association with large numbers of honeydew‐producing scale insects, ensuring a plentiful supply of the carbohydrates. A biological control agent could be used against the scale insects as a way of reducing the food supply of yellow crazy ants.
The red crab is essential to the health and survival of Christmas Island. If the yellow crazy ant succeeds in eradicating the red crab, this could lead to ecosystem imbalance, the extinction of essential plant and animal species that rely on the ecosystem benefits of the red crab, and therefore the downfall of the island’s environment.
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