Australia: The Land Where Time Began

A biography of the Australian continent 

Ants                       Photo Gallery

Worldwide, ants are usually at their most varied in the equatorial latitudes. New Guinea has 172 ant species, one of the richest ant faunas, but in Australia 100 species can be found in an area 20 m by 40 m.

Ants of the arid zone

The arid zone soils are of many types, after having been sorted by the wind and streams for millions of years and now form a mosaic of soil types. It is believed that the ant diversity is largely related to this diversity of soil types, and because of this there are many more ant species in the arid zone than would otherwise be expected. The very presence of ants in a particular soil alters it, feedback loops can operate, changing the whole soil profile. Often different soil types are associated with different plant communities, and this in turn leads to the possibility of a particular ant fauna being associated with it. This complicated mix of soil type, vegetation type, microclimate type, etc. all add to the number of niches that can be occupied by ant species.

Another factor in the abundance of ants and ant species in the arid country is the vast numbers of termites, particularly in mulga country, where the termites feed on wood, grass and animal dung. There are at least 60 species of termites in the Mulga country. One of the termites species in the mulga land is Drepanotermes perniger.

Meat ants, Iridomyrmex purpureus

The 10-mm-long, iridescent blue or green meat ant, Iridomyrmex purpureus, is the emperor of the ant empire of the arid lands. It controls every other arthropod that lives above the soil surface, whatever their size. No other ant can compete, head to head, with the large, inquisitive and aggressive meat ants. Because meat ant colonies must be some distance from each other, they are not tolerated closer, it allows other ant species to find nesting sites among the patchwork of meat ant nests, so long as they have evolved a way of avoiding much contact with the meat ants, whether it be time, temperature range, speed or mimicry.

All workers of this species are the same size, their roles in the colony being determined by their age, the youngest being restricted to nest duties such as cleaning, care of the young and the queen while the older workers forage. Some forage individually for undiscovered food, but the majority follow the trails to known food and back to the nest. The workers also take part in territorial disputes on the boundaries of their territory.

The colonies of this species is based on 1 or more nests, the area around each entrance being mounded and covered with gravel or twigs. Each entrance leads to a separate system of galleries, the workers moving between them during the day but returning to their home nest at night. The largest colony of Iridomyrmex ever found consisted of 87 nests with 1688 entrances. Colonies of this size are unsustainable in the long term as they exhaust the food resources of the area.

The spider Storena mimics the iridescent colour of the Iridomyrmesx. It uses this camouflage to prey on Irodomyrmex.

Melophorus

One such species that dodges contact with the meat ants is Melophorus. It is active at midday when the air temperature reaches 40 C and above and at the soil surface is about 65 C, and the meat ants are inactive in their nests. This species is a lone forager that moves about at high speed in the heat. Melophorus has diversified further into a dark species that forages when the temperature is about 45 C and a red-headed species that forages when the temperature is about 50 C. Because the Melophorus genera has found a time slot when it can successfully avoid Iridomyrmex, it has been able to diversify into species that exploit different food resources. There are seed harvesters, nectar feeders, which includes the honey ant, termite predators, and other predators and scavengers. One species,  even occupies unused chambers in the nests of Iridomyrmex, avoiding their hosts by emerging only when the meat ants have become inactive in the hottest part of the day.

Componotus

Another Australian genus, Componotus. The species of this genus usually avoid Iridomyrmex by foraging at a different time or by being a different size to Iridomyrmex. Componotus species often nest between the territories of Iridomyrmex colonies. Some of the species that nest in Iriomyrmex territory mimic Iridomyrmex, foraging at the same time as Iridomyrmex, but their smaller minor soldiers avoid contact with Iridomyrmex foragers and are very fast movers. Their larger major soldiers blocking the entrance to their nests with their heads that just fit the entrance. The smooth heavily armoured head with mandibles at the ready is apparently difficult to dislodge, even for Iridomyrmex.

Polyrachis

This genus from the mulga country constructs elaborate nest entrances that cope with the sheet flooding that occurs in the area. Species of this genus construct their nest entrances in a number of different ways. Some build a solid earth turret or cone with a thick layer of mulga phyllodes placed around the base. Other species build the earthen turret and cover it with phyllodes as well as incorporating the phyllodes into the structure and around the base. These structures both keep out the flood and the phyllodes prevent them from being eroded by the water flowing around them. Simple but effective.

In most ant nests activity levels drop during winter, with few if any eggs or larvae.

Dinosaur ants

Ants as Bioindicators

In the wetter parts of Australia there are up 50 species per Ha, and in the interior, where most of the country is semi-arid or seasonally arid, the number of species can reach as high as 150 per Ha.

They are very important to functioning ecosystems. Among the functions they carry out are perturbation, nutrient recycling, predation, storing seeds, increasing water penetration by tunnelling.

The construction of their nests increases the bioturbation of the soils.

Sources & Further reading