Ant activity and the development of nutrient-rich soil profiles

David Eldridge
University of NSW
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Continent: Australia
Country: Australia
City/Town: Mount Hope
UTM coordinates and datum: none


Climate Settig: Semi-Arid
Tectonic setting:
Type: Process


Rainfall is one of the most important factors in arid and semi-arid (dryland) environments; it supplements soil moisture storage, stimulates plant germination, and brings new life to dormant vegetation. Rainfall can also be a important trigger of animal activity, causing a flurry of activity as animals such as ants and rodents collect and store seeds, insects pollinate plants, and birds prepare for nesting.

Dryland soils are generally nutrient-poor and sometimes eroded. In the semi-arid woodlands of eastern Australia, more than a century of overgrazing by domestic animals has severely damaged surface soils and has lead to reductions in their chemical fertility and biological activity. This means that when it rains, water flow through the soil is restricted, often because the natural pores in the surface have been destroyed through compaction or the surface has a thick, impenetrable seal.

Although dryland soils might at first appear to be biologically barren, most are habitat for a wide variety of insects, particularly ants. One common ant on sandy soils is the funnel ant (Aphaenogaster), which tends to occupy sandy soils dominated by white cypress pine (Callitris glaucophylla). Funnel ants are very small, less than a few millimeters across, but they build relatively conspicuous nest entrances which they surround with mounds of soil. The mounds resemble small volcanos (Figure 1), and their large cavities lead down into huge underground colonies. No one knows exactly how big the colonies are nor how many ants they support, but there are thought to be many thousands of ants in any given colony. Funnel ants rarely come out during the middle of the day, though they can sometimes be seen in the early morning and late afternoon carrying sand grains out through the nest entrances. Instead, they spend most of the daylight hours underground, eking out a living by tending fungi that grow on the roots of the white cypress pines.

It is rather a mystery why ants build such large volcano-shaped entrances. Their shape may be a mechanism to help them prevent water from entering the nests when sheets of water (overland flow) moves down these gently sloping hillsides. The large nest entrances would certainly deflect shallow flows of water away from the entrances. It is rather strange, however, as the soils on which the nests are constructed are generally very deep and tend to have high rates of water infiltration.

After even a few millimeters of rainfall, the ants can be seen busily cleaning out their nests, even during the daytime. They pile up fresh soil outside the nest entrances, effectively 'rebuilding' their volcanos. If you happen to be our a few days after rain the first thing you will notice is that the surface takes on the appearance of a lunar landscape. The next thing you notice is all of the ants.

We have been tracking the nest building behavior of funnel ants for the past 15 years and now appreciate the importance of the nests for altering runoff and sediment movement. On gently sloping sandy soils, falls of rain of more than about 5 mm over the period of a day are sufficient to generate overland flow; the movement of a thin layer of water down the soil surface. This water brings with it a huge payload of pine needles as it moves in and around the pine trees and the nest entrances. Pine needles generally occur in large patches under the trees and in places can be many centimeters thick.

When the water moves the pine needles down the long low slopes they quickly become trapped against the sides of the volcano-like nest entrances (Figure 2). This just happens to coincide with the time that the ants are out feverishly working to clean out their nests, trying to get rid of wet soil that has fallen off the sides of the nest entrances. Once it stops raining, ant activity speeds up, and soon the litter around the nests is covered by nest soil (Figure 2). Unwittingly, the nest cleaning activities by ants are also helping to build up a fertile soil profile. Some water does however manage to penetrate the nest entrances. Our studies on these soils indicate that rates of water flow can be in excess of 300 mm per hour (Eldridge 1993, 1994).

If you take a thin slice of the soil through one of the nests you'll see layer upon layer of pine needles, like layers of cake. Because the pine needles are sandwiched between the soil they quickly begin to break down through the action of fungi. Look closely at the pine needles and you will see the signs of fungal breakdown; long, fluffy white filaments called hyphae. The ecosystem effects of ant activity combined with the movement of litter by overland flow, results in the area around the nests having substantially higher concentrations of carbon, nitrogen and other trace elements (Eldridge and Myers 1998).

It's pretty hard to imagine how long this has all been going on, but it's probably been for many hundreds of thousands of years. These tiny ants have a close association with the sandy pine soils. Apart from making them more fertile they also help to move the soil around, a process called bioturbation. As ants create new tunnels and abandon others, the locations of the volcano-like nest entrances change, about once every few months or so. In fact, the mass of soil moved during this process is so large (about 3.4 tonnes per hectare per year) that the ants are responsible for 'turning over' the entire soil profile down to a depth of 30 cm every 200 years or so (Eldridge and Pickard 1994). This surely is an amazing feat earning them the title 'ecological engineers'.

Associated References

  • Eldridge, D.J. (1993). Effect of ants on sandy soils in semi-arid eastern Australia. Local distribution of nest entrances and their effect on infiltration of water. Australian Journal of Soil Research 31, 509-18.
  • Eldridge, D.J. (1994). Nests of ants and termites influence infiltration in a semi-arid woodland. Pedobiologia 38, 481-492.
  • Eldridge, D.J. and Pickard, J. (1994). The effect of ants on sandy soils in semi-arid eastern Australia. II. Nest turnover and consequences for bioturbation. Australian Journal of Soil Research32, 323-333.
  • Eldridge, D.J. and Myers, C.A. (1998). Some evidence that the ants Aphaenogaster barbigula (Myrmicinae) enhance soil nutrients around their nest entrances in semi-arid eastern Australia. Australian Journal of Soil Research 36, 1009-1017.

Glossary words:
pores (soil)
ecological engineer