Just Add Termites: Graeme O'Neill Discovers the Restorative Powers of Termites, the Earthworms or Australia's Arid Zone
O'Neill, Graeme, Ecos
From a human perspective, termites are social insects with distinctly antisocial habits. They are ubiquitous in Australian landscapes, and some of the continent's 350-plus species stand their ground when humans colonise their territory. It's a moot point as to who is invading whom.
In suburbia, termites and their talent for `recycling' cost hundreds of millions of dollars a year, but in natural ecosystems their benefits are invaluable. They may even merit recognition as `keystone' species, a term coined by American biologist and author Professor Edward O Wilson to describe species whose loss can cause ecosystems to collapse, and other species to become extinct.
This may be the case in Australia's tropical north, where a project at Darwin's Tropical Savannas Cooperative Research Centre (CRC) is exploring the role of termites in maintaining ecosystem health and restoring land damaged by overgrazing or mining. Dr Garry Cook and Dr Tracy Dawes-Gromadzki of CSIRO Sustainable Ecosystems are part of the project team.
Cook says Australia's northern cattle-grazing industry relies heavily on the productivity of tropical savanna grasslands and woodlands. But heavy grazing in some areas has caused soil erosion and changes to dominant plant species, and a consequent reduction in water infiltration and plant growth.
Revegetating mine dumps is also a challenge in the tropics. Mine dumps are often low in nutrients, with thin soils commonly contaminated by heavy metals and acid drainage: a hostile environment for plants.
Vanguards of restoration
Cook says termites are the `earthworms' of Australia's arid and seasonally arid regions, and manipulating termite density and activity may speed the restoration of degraded areas, or make ecosystems more resilient to disturbance.
He says tropical termite groups that variously specialise in eating grasses, leaf litter, wood and other organic matter recycle large quantities of plant biomass into the soil. They also keep the soil porous with their tunnelling, allowing water to infiltrate the soil profile, rather than evaporate. (In the intense tropical heat, evaporation rates typically exceed precipitation by a factor of two to three.)
`In areas with little vegetative cover, such as during the early stages of mine-site rehabilitation, the density of macropores in the soil is low,' Cook says.
`Macropores have a disproportionately important role in letting water into the soil: without them, rainwater runs off and the soil stays dry.'
The combination of high evaporation and runoff rates in the tropics, and the absence of macropores, makes it difficult for plants to survive. Using termites to create macropores would create rapid infiltration, which is critical to maintaining ecological processes.
`In our early work we found that if we put out mulch, in the form of dried grasses, on degraded soil, termites moved in rapidly and began creating macropores with their workings, allowing water to infiltrate,' Cook says.
`We also found that within a few weeks, mulched areas were colonised by other invertebrates such as centipedes, millipedes, native cockroaches, earwigs and ants, which all help to rebuild soil processes.
`We're trying to distinguish the direct effects of mulching from those due to invertebrates, by establishing plots with a range of treatments. Some have mulch, others have been left bare, and we have treated both mulched and bare sites with a commercial termiticide that also kills off other invertebrates.
`In similar savanna ecosystems in Africa, research has shown that the presence of termites greatly amplifies the benefits of mulching: they're a keystone group.'
Cook received some unexpected results from the early phase of a mulching experiment designed to run from the end of the 2001 dry season and through until the end of the wet. He discovered that the termites prefer to move in and eat the mulch when it is dry. …