Christmas Island Red crabs and the Yellow crazy Ants

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Red crabs Gecarcoidea natalis

“It takes the red crabs almost two months to make their way from the forest to the sea and back again. But that doesn't stop the island's human inhabitants needing to get from A to B. The result? The world's most careful drivers. 

“The red crab is by far the most obvious of the 14 species of land crabs found on Christmas Island. Millions of these land crabs live over the island.

They are a big crab with an adult body shell (or carapace) measuring up to 116mm across. The carapace is round shouldered and encloses their lungs and gills. Males are larger than females but females have a much broader abdomen and usually have smaller claws than males.

Where do they live?
Although most common in the moist environment of the rainforest, red crabs live in a variety of habitats including coastal shore terraces, and even domestic gardens. They dig burrows in soil or live in deep crevices in rock outcrops. For most of the year, a crab will settle in one place, living in their burrow.

The crabs' burrows have a single entrance tunnel which leads to a single chamber. Only one crab lives in a burrow and (except for the breeding season) red crabs are solitary and do not tolerate intruders into their burrows.

Red crabs are diurnal (active during the day) and almost inactive at night despite lower temperatures and higher humidity. Sensitivity of crabs to moisture, combined with the seasonal climate on Christmas Island, create a distinct seasonal pattern of activity. Crabs retreat into the humid interior of their burrows during the dry season. They plug the burrow entrance with a loose wad of leaves to maintain a high humidity level, and effectively disappear from view for up to two to three months of the year.

What do they eat?
Red crabs diet consists mainly of fallen leaves, fruits, flowers and seedlings. They are not solely vegetarian however and will eat other dead crabs, birds, the introduced giant African snail and palatable human refuse if the opportunity presents itself.

Red crabs and forest ecology
Red crabs are important in the Christmas Island rainforest ecosystem. Their droppings scattered over the forest floor act as fertilizer. Their burrowing turns and aerates the soil and they are a major determinant of the unique structure and composition of the Christmas Island forest by their selective browsing on seeds and seedlings”(Christmas Island National Park service).

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These red crabs are vulnerable to the rather aggressive,"Yellow Crazy Ants". 

The exotic invasive yellow crazy ant (Anoplolepis gracilipes), was accidentally introduced to Christmas Island between 1915 and 1934. They are thought to have come with produce from either Malaysia or Singapore. They didn't appear in the records until about 1930. The ants have no 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.

Crazy ants are recognised by their pale yellow body colour, unusually long legs and antennae. The name "crazy ant" 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, 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 crazy ant has a significant destructive impact on the island's ecosystem, killing and displacing crabs on the forest floor. The super-colonies also devastate crab numbers migrating to the coast. This has seen a rapid depletion of land crab numbers which are vital to Christmas Island's biodiversity. They are a keystone species in the forest ecology by digging burrows, turning over the soil, and fertilising it with their droppings.

Seedlings that were previously eaten by crabs started to grow, and as a result, changed the structure of the forest. Weeds also spread into the rainforest because there are no 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.

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 crab migrations many crabs move through areas infested with ants and are killed. Studies show the ant has displaced an estimated 15-20 million crabs by occupying their burrows, killing and eating resident crabs, and using their burrows as nest sites.

Although 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 a land crab 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 overwhelms the crabs, and they are usually blinded 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. 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, 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 crabs encourages seedling growth and weed invasion into the forest.