of the Northern Territory University argues that one of the most
profound threats to the tropical savannas is developing under our
noses yet little research is being carried out on the issue.
Gamba grass produces robust tussocks that can reach a height of 4
metres, producing fuel loads for fires four to 10 times that of
native savanna grasses. By changing fire regimes, the grass may be
irrevocably transforming the landscapes of northern Australia.
Photo: Sam Setterfield
Over the past 50 years agronomists have undertaken a massive
evolutionary experiment in northern Australia by introducing more
than 450 grasses and legumes in the hope of discovering species
that improve cattle production. Species from distant parts of the
world have been brought into contact with north Australian savanna
ecosystems in a flash of evolutionary time.
In 1994 Mark Londsdale reviewed the consequences of this program in
a paper called
. The reason for this provocative title was that the agronomy
project has backfired badly.
He showed that of the 463 species that had been introduced between
1947 and 1985 only four species were found to be useful and had not
become weeds while 60 species had become listed as weeds and 17
were listed as weeds despite being identified as being useful. Even
useful exotic pasture plants come with a considerable risk that
they will become environmental weeds.
Playing with fire
There are few ecosystems in northern Australia that are weed-free,
so a reasonable person may accept the escape of a pasture plant as
the price paid for the economic utilisation of north Australian
savannas. However, this sanguine view overlooks the dramatic,
irreversible ecological consequences that some exotic grass species
can have on fire regimes. These consequences have been ably
reviewed by the highly respected north American scientists Carla
D'Antonio and Peter Vitousek
They concluded that of all the ecological changes caused by
invading grasses the most significant is due to the interaction
between grass and fire. Grasslands are highly flammable given the
abundance of quick-drying and well-aerated fine fuels. Further,
perennial grasses can recover rapidly from fire because of
The real danger comes from exotic fire-loving grasses that can
trigger fires that are more intense, frequent and widespread. The
cycle would unfold as follows: firstly the more intense fire feeds
off the exotic grass and destroys many of the woody plants, which
in turn increases the dominance of the following year's exotic
grasses that suppress more native herbs and grasses. Eventually
this grass-fire cycle can convert a diverse habitat with many
different species and forms of plants to a grassland dominated by a
few exotic grasses with little capacity to be recolonised by native
Such changes can also cause drier microclimates, further adding
momentum to the grass-fire cycle. Furthermore, because some
elements, such as carbon and nitrogen, are volatilised and lost in
smoke while other nutrients, such as phosphorus, are made more
chemically mobile and thus susceptible to leaching, nutrient cycles
are disrupted with the consequent decline in overall stored
nutrients for plants. These changes further reinforce the
grass-fire cycle because the fire-loving grasses thrive on the
temporary increased availability of soil nutrients.
Gamba grass threat
D'Antonio and Vitousek's model is relevant to understanding the
future of northern Australian savannas. A number of exotic grass
species are aggressively invading native vegetation here with
consequent changes in fire regimes. Examples include parra grass (
) on freshwater floodplains, mission grass (
) and gamba grass (
) in eucalypt savannas.
Since the early 1990s there has been a growing consensus among
biologists and managers that gamba grass poses by far the biggest
. Why? The primary reason concerns the structure and size of this
plant and how this influences wildfires. Gamba grass has robust
tussocks that reach a height of 4 metres.
High densities of this grass produce standing crops of between 10
to 20 tonnes per hectare, four to 10 times more fuel than native
tropical grasses4. Unlike the other invasive exotic grasses there
are no equivalents to gamba grass in northern Australian savannas
The lifecycle of gamba grass is also different to native grass
species. It flowers in the early dry season and continues to grow
well into the mid-dry season, presumably reflecting extremely
efficient use of water and root systems that can exploit moisture
in the subsoil. This leads to a dangerous combination: great height
and above-ground biomass in the late dry season. This creates fires
for which there is no previous parallel. Indeed gamba can behave
more like a short flammable tree than a grass; flames of these
fires can reach the leafy canopy of eucalypts and defoliate or kill
them. Gamba grass fires can be so intense that the efforts of fire
fighters to contain blazes are thwarted.
The Bushfire Council of the NT considers the grass a major threat
to public safety and in conjunction with other government
departments has initiated a campaign urging landowners in the rural
areas outside Darwin to control gamba grass by destroying
infestations on their properties
Gamba grass has the potential to invade all tall tropical grass
savannas in northern Australia given its broad ecological
toleranceiv. It is able to disperse and establish itself tin
uncleared native vegetation from nearby areas such as pastoral
lands and unintentional infestations on roadsides and other areas
of disturbance. Because gamba grass jumps property boundaries and
hitches rides it is merely a matter of time before it is able to
realise its geographic potential.
In sum, we have unwittingly pitted fire-tolerant northern
Australian eucalypt trees against an extremely fire-adapted west
African grass in a potentially fierce evolutionary struggle.
Garry Cook of CSIRO Wildlife & Ecology, is of the view that
gamba grass and its associated intense, late dry-season fires have
the potential to radically transform north Australian eucalyptus
savannas, by causing the loss of biodiversity dependent upon a
continuous tree and shrub layer6. Eucalypt savannas could become
fire-promoting weed infestations.
Existing data on the effect of invasive grasses in northern
Australia is remarkably sparse given their scope for fundamental
and irreparable ecological change. Without more research it is
difficult to know how best to tackle the problem of invading
grasses generally, and gamba grass specifically.
Equally, it is difficult to know if a species actually has the
capacity to destroy the tall-grass savanna ecosystem in northern
Australia as has been suggested by Gary Cook
Given the current wait-and-see approach of land managers we will
have an answer to this great evolutionary experiment in the next
Dr David Bowman is Principal Research Fellow Northern Territory
Click here to view the reply
Cut and dried: the issue is to manage disturbance
1. Londsdale W.M. (1994) Inviting Trouble: introduced pasture
species in Northern Australia, Australian Journal of Ecology (19),
2. D'Antonio, C.M. & Vitousek, P.M. (1992) Biological invasions
by exotic grasses, the grass/fire cycle, and global change, Annual
Review of Ecology and Systematics, (23), pp 63-87.
3. Cook, G. (1991) Gamba grass: impending doom for Top End
Savannas? CSIRO TERC Newsletter 91 (12), p 2.
4. Barrow, P. (1995) The ecology and management of gamba grass
(Adropogon gayanus Kunth.) Final report to the Australian Nature
Conservation Agency, NT DPIF, Unpublished report.
5. Russell Anderson, CEO Bushfires Council of the NT, personal
communication, May 1999.
6. Cook, G. (1997) A tale of two savannas: the effects of fire on
vegetation patterns in West Africa and the Northern Territory.
Bushfire '97: Proceedings of the Australian Bushfire Conference
8-10 July 1997. CSIRO, Winellie and NTUPrint, Darwin, pp 45-50.