Weeds are often defined as plants that are out of place. Most of
the plants that cause problems in Australia have been imported, but
even native plants can become weeds if they proliferate away from
their natural habitat. Imported plants may not be weeds in their
country of origin, but when introduced into Australia they are
often free of the diseases and insects that would have controlled
them.
These weeds can then out-compete native plants for essential
resources including space, light, water and nutrients. Weed
invasion is one of the most serious environmental issues facing
Australia's tropical savannas. Weeds choke rivers, smother
grasslands and take over national parks. They change the landscape,
often forever.
Plants are regarded as weeds if they:
- outcompete other plants, crowding them out and using up
available water, nutrients and light (e.g. rubbervine, giant
sensitive plant); and/or
- alter the conditions in the ecosystem making it unsuitable for
the native plant species or destroying native animal habitat (e.g.
athel pine and giant sensitive plant).
The most important factors that encourage weeds are disturbance
of the natural environment, increased nutrient levels and the
absence of predators. These can occur singly, or in combination
with occasional events such as floods.
In the tropical savannas, disturbance of the natural environment
includes alteration of the natural fire regime, over-grazing,
extensive tree-clearing and changes in water availability.
In the 1970s a barbed invader exploded onto the northern
Australian landscape, and is now considered to be one of the
greatest threats to Kakadu National Park. Giant sensitive
plant (Mimosa pigra) forms dense thickets which are
devastating the beautiful wetlands of Kakadu - many of the
waterbirds which make the Adelaide River floodplain so famous have
now become scarce.
Rubbervine is another example of a weed that
conservationists consider to be important because of its impact on
native ecosystems. It outcompetes native species, forming dense
thickets and increasing soil erosion. Rubbervine costs millions of
dollars every year in control measures and also results in lost
pastoral production in savannas.
Other important weeds affecting pastoral production are the
so-called prickle bushes. These include:
- prickly acacia (Acacia nilotica)
- Parkinsonia (Parkinsonia aculeata) - its floating
seed makes it one of the most troublesome weeds of the
seasonally-flooding Channel Country
- mesquite (Prosopis spp.) - currently in localised
pockets across Australia.
These bushes form dense thickets which out-compete native
vegetation and use up valuable soil water. They also restrict
access to water by stock and make mustering difficult. Because
native grass species don't grow under these dense thickets, loss of
ground cover results in increase soil erosion and the spread of
deserts.
Many weeds were originally introduced as potential pasture
species. Of the 463 exotic grasses and legumes which were
introduced as pasture species, only 21 are now considered useful,
including buffel grass (Cenchrus ciliaris), which is widely
used as a pasture plant.
The tropical savannas, which are areas of dense grass and
scattered trees with a seasonal rainfall, are particularly
vulnerable to invasion by weeds mainly because they:
- are prone to disturbance and are subject to inappropriate land
use, such as overgrazing and lack of fire management, which makes
it easier for weeds to out-compete the native species
- climatic variation: rainfall may vary dramatically from season
to season as well as year to year
- cover vast areas with low human population, making weed control
in savannas expensive and difficult.
Weeds have an enormous impact on Australia's savannas, and the
problem is growing rapidly every year.
The control and management of weeds requires an integrated
approach, combining preventative, chemical, mechanical and
biological means.
However, preventing new weed introductions, as well as
preventing the spread of existing weeds into new areas, is the most
cost-effective way to fight weed invasion.
Weed control and management also requires a strategic approach,
including local, regional and national strategies. The National
Weeds Strategy has been developed as a mechanism to reduce the
impact of weeds on the sustainability of Australia's productive
capacity and natural ecosystems.
Successful weed management requires a coordinated effort from
local communities, and all levels of government.
There is a long history of planned plant introduction to the
savannas of northern Australia, with the aim of increasing pasture
productivity. Of the 463 grasses and legumes introduced in this way
to the tropical savannas, 60 per cent are now listed as weeds of
agriculture, conservation or both. In 1994 Mark Londsdale reviewed
the consequences of this program in a paper called Inviting
trouble1. 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.
For example, para grass (Brachiara mutica) is a
semi-aquatic grass species which was introduced as a pasture plant
in shallow, permanently-inundated areas. It has now spread through
the wetlands and streams of northern Australia and is destroying
the habitat of waterfowl such as the magpie goose (Anseranas
semipalmata). Infestations reduce the ability of the magpie
goose to feed in open water and are destroying its breeding
habitat.
Of these introduced plant species, some of the few which have
become useful pasture species are considered serious weeds as well.
Buffel grass is considered to be an excellent fodder species and is
widely used as a pasture plant. At the same time, it is also a
major concern in the arid inland of Australia. Buffel grass
displaces native grasses and forms dense swards in moist habitats
such as along river banks or in alluvial pans. It also alters the
fire regime.
Extensive clearing of woodland for use as pasture also aids the
spread of weeds. Of the original vegetation, only islands or
corridors are left. These disturbed areas are easily invaded by
weeds and the original vegetation is slowly overwhelmed.
