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Weeds of the tropical savannas

What are weeds?

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.

Weeds in the tropical savannas

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.

Management options

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.

Planned plant introduction: production vs conservation

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.