Weeds in the Mitchell Grasslands

Regional profile

The Mitchell grass region straddles the states of the Northern Territory and Queensland. The major land use is pastoralism, and very little of this area has been set aside for conservation or Aboriginal uses. The Mitchell grass plains are largely devoid of native trees, which are obviously inhibited by factors relating to the shrinking and expanding cracking clay soils. Invading shrubs such as prickly acacia (Acacia nilotica) and mesquite (Prosopis limensis) are able to tolerate these harsh conditions, and without competition from other trees, have been aggressively invading large areas.

Prickly acacia in the Mitchell grasslands

Prickly acacia has invaded the north-east Mitchell Grasslands

Prickly acacia

At present vast areas of the north-eastern sector of the region - on the Queensland side of the border - are suffering from a very serious invasion by prickly acacia (Acacia nilotica). This prickly shrub, originally from the Middle East, was introduced as a shade and fodder tree. During the early part of the 1900s, seeds were carried in saddlebags and distributed from horseback by graziers. The distribution of prickly acacia however did not become problematic until production in the area shifted from sheep to cattle. While sheep digest a majority of prickly acacia seeds consumed, cattle do not and so act as major vectors.

The extent of prickly acacia now stands at some 7 million hectares, 500,000 of which cannot be used for grazing as a result. The economic and ecological costs of this invasion have been high, with recent estimates that $5 million is lost per year in productivity, and a further $4 million spent on control. On areas which are very heavily infested, such as the headwaters of the Barcoo-Thomson River system, costs of reclamation have risen beyond the economic and productive value of the land.

Prickly Acacia Containment Line

In July 1999, a Prickly Acacia Containment Line was developed in central west Queensland an effort to help stop the spread of the weed. Inside the line are five 'islands' of core prickly acacia areas. These stretch from Barcaldine north to Hughenden and west to Winton and Julia Creek. Eradicating prickly acacia inside these areas is currently impractical or not economically feasible and efforts concentrate on researching biological control agents. Outside this line, the weed is eradicated with the support of Queensland Department of Natural Resources SWEEP program.

Ecological costs

The establishment of large woody weeds represents a significant change in vegetation structure, and as such, is likely to result in substantial changes in the species composition of both fauna and flora. In fact, the establishment of a tree/shrub layer will probably favour some animals, birds for example, and these may become more common in the landscape. Also, the increased tree density and changes in ground layer composition would be likely to disadvantage grass species endemic to the Mitchell grass region. The spread of prickly acacia, as well as other prickly bushes such as mesquite, along water ways could also reduce the value of these areas for waterbird nesting.

Without sufficient resources allocated to the control of this weed, some suggest that the future will see the Mitchell grasslands becoming a thorny scrubland akin to African thornveld. (Mackey 1996: 8). Click here to see a list of research findings on prickly acacia.

Barkly Tableland weeds

To the west of the state border which divides the region is the rich grazing area known as the Barkly Tablelands. Prickly acacia is largely under control in this part of the Mitchell grasslands, although producers must remain vigilant for any outbreaks. The major weeds in the Barkly are Parkinsonia, noogoora burr and mesquite (Prosopis limensis). Parkinsonia forms impenetrable thickets which severely limit stock movement and access. These may be several kilometres across. Significant progress is being made the use of biological agents in the control of this weed. (See NT DBIRD's Annual Report for some examples). Click here to see a list of research findings on Parkinsonia.

Noogoora burr has become widespread over several major river systems in the Barkly. Once established, this woody herb spreads rapidly and is difficult to eradicate. It will invade grazing land and outcompete native shrub and grass species. The burrs of this plant can cause injury and discomfort to stock. Young plants are toxic if grazed by cattle and horses. Physical, chemical and biological means can be employed in the control of this burr. Click here to see a list of research findings on noogoora burr.

Mesquite ( Prosopis limensis ) is now common across the Barkly tablelands. These prickly bushes affect grazing activities in ways similar to prickly acacia: limiting stock access to water, and making mustering and general maneuverability more difficult. They also complete very successfully with native grasses for light and water, and so lessen the carrying capacity of country. The distribution of these bushes is aided by grazing, as the seeds pass through cattle undigested. Click here to see a list of research findings on mesquite.