State of the Environment

2006

Indicator: IW-39 Examples of significant wetland weeds

Data

Two examples have been selected for consideration as examples of potentially invasive wetland plants: mimosa pigra in the Northern Territory and some species of willow tree, in south eastern Australia.

Mimosa

Mimosa has few or no natural enemies in its introduced range and therefore can rapidly colonise suitable wetland habitat, reducing the diversity of flora and fauna, including a number of vulnerable and endangered species. It favours wetland habitats such as floodplains (land subject to seasonal inundation), freshwater ponds and swamps, particularly where these habitats have been disturbed by other human activity. Major exposure pathways are water, wind, vehicles, boats, stock, wildlife and deliberate movements of earth.

Mimosa is present in approximately 80 000 hectares of coastal floodplains in the Top End of the Northern Territory. The extent of area occupied varies with river system. Outside the Northern Territory, a relatively small occupation was found in 2001 near Proserpine on the central Queensland coast. It is estimated that between 4.2 and 4.6 million hectares of wetlands in northern Australia are potentially at risk from Mimosa.

Some potential environmental impacts of mimosa pigra include:

Rare or vulnerable fauna potentially threatened by mimosa invasion in the Northern Territory
Species Status
False water-rat Vulnerable
Yellow-rumped mannikin Vulnerable
Grass owl Vulnerable
Red goshawk Endangered

Source: D Walden, R van Dam, M Finlayson, M Storrs, J Lowry & D Kriticos, 2004, A risk assessment of the tropical wetland weed Mimosa pigra in northern Australia, Department of the Environment and Heritage, Canberra, viewed 16 Nov 2005, http://www.deh.gov.au/ssd/publications/ssr/177.html, Table 1, p. 16

Rare or vulnerable flora in the Northern Territory potentially threatened by mimosa invasion

Aldrovanda vesiculosa: A free-floating aquatic herb found on wet floodplains and billabongs

Lemna tenera: A partially submerged free-floating aquatic herb known only from backwater swamps and billabongs

Monochoria hastate: A perennial emergent aquatic herb that grows to approximately 1.2 metres

Goodenia quadrifida: An annual herb

Ptychosperma bleeseri: A slender clumping, feather-leaved palm

Nymphoides subacuta: An annual or perhaps perennial (where water persists) waterlily

Nymphoides sp. (Nathan River entity): A waterlily from mud flats

Nymphoides exiliflora: A small annual waterlily

Source: Walden, D, van Dam, R, Finlayson, M, Storrs, M, Lowry, J and Kriticos, D 2004, A risk assessment of the tropical wetland weed Mimosa pigra in northern Australia, Department of the Environment and Heritage, Canberra, viewed N/A, http://www.deh.gov.au/ssd/publications/ssr/177.html, Table 1, p. 16 and Table 2, p. 17.

Effective control of mimosa has been difficult because of the extent of area it now occupies and the inaccessible and remote terrain where it occurs. Control of the plant is expensive and is further hampered by regrowth of the seedbed over many years requiring continual follow-up control, and the thorny nature of the stems.

Eradicating mimosa from all parts of Australia is presently not feasible or cost effective. Scientists are using some of the many different insects and fungi that feed on or infect mimosa in its native habitat to control the plant in Australia. So far, 11 species of insects and two disease-causing fungi have been released as biological agents onto mimosa in the Northern Territory. Of the agents tested, tip and stem borers have been the most effective because they do not require flowers or seed for survival and can therefore feed on the plant even during the dry season. The combined effect of these biological agents is to reduce seed production and seedling survival.

Although biological control looks promising, the mimosa problem is huge and the insects are small, making this approach to control of mimosa a long-term strategy. For example, insects released 14 years ago are only now beginning to cause a noticeable effect on the plant, although the effect should increase as insect numbers continue to grow.

Source: Department of the Environment and Heritage 2004, Managing invasive species in Australia - success stories: Biological control of mimosa, viewed 14 Dec 2005, http://www.deh.gov.au/biodiversity/invasive/publications/mimosa-success.html

In Kakadu National Park a vigilant and systematic approach to mapping and eradicating mimosa at a cost of $500 000 a year has controlled the plant and work continues to prevent new outbreaks. As a result, Kakadu remains the most significant mimosa-free wetland region in the Northern Territory.

Modelling supports the prediction that Mimosa will not be a serious problem outside the 750 mm annual rainfall zone in northern Australia.

A National Strategic Plan has been developed using a coordinated and collaborative approach involving government and Indigenous agencies from the Northern Territory, Queensland and Western Australia, as well as the CSIRO. The strategic plan has four main programs: inform and educate stakeholders and the community, prevent Mimosa from spreading to and impacting on new areas, further develop the knowledge base and methods for effective and efficient management of Mimosa, and reduce the current adverse impacts of Mimosa infestations.

Willows

Although willows can provide stream bank stabilisation and food and habitat where native vegetation is absent, they also can cause environmental impacts on inland waters by changing the physical and chemical properties of the water, altering stream and riparian habitats and faunal food quality, and displacing of native vegetation.

There are several species of willows but none are native to Australia. Most willows are potentially invasive in south-eastern Australia and are included on the list of Weeds of National Importance. Three species are exempt from that list because they are relatively benign. In some areas efforts are being made to remove willows.

In Tasmania at least 30-40 threatened flora species and several invertebrates, including freshwater snails, caddisflys and burrowing crayfish, as well as the native galaxid fish, are thought to be threatened by willows invasion.

On the other hand, in some instances, certain species such as the Giant Freshwater Crayfish, have adapted to the new environment created by willows. Broad scale/short term willow removal could constitute more of a threat to these species.

If willow removal is not accompanied by suitable revegetation, especially if willows are being removed on a large scale, then streambed and bank destabilisation and destruction of habitat occur.

Preventative management, emphasising entire wetland plant communities, is being encouraged. This includes improving awareness that once native vegetation is removed following overgrazing or inappropriate fire regimes, areas become prone to colonisation by non-native species.

Further information on response actions to control willows can be found in Indicator IW-36 Willow removal .

What the data mean

Although little data are available on the actual impacts of these species, as distinct from the habitat modification which enables their colonisation of riparian environments, the two plant species examined as examples of wetland weeds have considerable potential for invasiveness. Revegetating concurrently with removing vegetation is critically important.

Data Limitations

Only two examples of potentially invasive introduced wetland species have been examined.

Issues for which this is an indicator and why

Inland Waters — Response of biota - Invasive species 

In the absence of detailed data on the distribution and actual impact of various wetlands weeds, examples of how particular weeds can affect the aquatic environment are worthy of consideration.

Other indicators for this issue:

Land — Direct pressure of human activities on the land - Species introduction and species change 

In the absence of detailed data on the distribution and actual impact of various wetlands weeds, examples of how particular weeds can affect the environment are worthy of consideration.

Other indicators for this issue:

Biodiversity — Pressures on biodiversity - Invasive species 

In the absence of detailed data on the distribution and actual impact of various wetlands weeds, examples of how particular weeds can affect aquatic and terrestrial biodiversity are worthy of consideration.

Other indicators for this issue:

Further Information

Key

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