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 has invaded the north-east
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.
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
Click here to see a list of research findings on prickly
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
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
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.