Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Introduction of novel biota into native habitats and communities (continued)
Weed infestation [L Indicator 4.1]
There are about 30 000 exotic vascular plant species in Australia. Of these, 2000 have become naturalised, representing 15% of the total number of vascular plant species in Australia (Groves 1986), but many of these occur in extensively altered environments where conservation of native habitats or species is not a primary objective. Weed competition has probably been responsible for the extinction of four native plant species (Gentiana baeuerlenii, Hydatella leptogyne, Hypsela sessiliflora, Trachymene scapigera) and remains a continuing threat to another 57 species (Leigh and Briggs 1992).
The rate of spread of some 'declared' weeds has been well documented for long periods (see parthenium weed). However, those weeds that are environmental threats are not so well documented. Many of the earliest state legislation on natural resource management in Australia relate to the control of weeds, pests and diseases that threatened major economic activities of the day.
The rate of spread of weeds is very variable. Modelling of weed spread has been undertaken for weeds whose biology and life-cycle is sufficiently well documented. The current distribution of the weeds of national significance is documented, and forms a baseline for the published future encroachment estimates, based on climatic prediction alone.
While many of the earlier introductions came from Europe, the current distribution of centres of origin is much more varied (Figure 48). The implications are significant in developing suitable agents for biological control, which traditionally uses pests that are natural predators of the weed in its region of origin (Groves 1998).
Figure 48: Means of introduction and regions of origin of plant species naturalised in Australia between 1971 and 1995.
Source: Groves (1998)
Despite extensive international research, it remains difficult to predict with certainty which species will invade different types of ecosystems. The presence of the right conditions for invasion (an 'invasion window') may be the result of unusual seasons, a reduction in the competition from native species (such as a disease or pest invasion), or a more general disturbance of the habitat. Disturbance has been documented as the most probable cause of many invasions of weeds into reserves. Reserve fragmentation through road and track creation are considered to have been the cause of invasion of many weeds in many national parks (Scott 2000). The prediction of weed spread and distribution has relied heavily on computer models that combine knowledge of the species' biology with climatic factors. One well known model, CLIMATE, has been used to develop the maps of expected distribution and spread of the weeds of national significance, shown in Table 24.
Disturbances that may influence the spread of weeds include fire, change in river and stream flows, land clearing, creation of new roads and trackways.
Declared and noxious weeds are the responsibility of state and territory regulatory agencies, that are charged under the provisions of various Acts, to control such pests plants. A list of pest plants, and the relevant authorities is provided in the National Weeds Strategy website.
Georgiana Malloy was one the first settlers of Augusta in south-western Western Australia in the 1830s. She became a keen and skilled botanist revelling in the extraordinarily rich flora of her new home. She corresponded with, and sent specimens and seeds of Australian flora to a gentleman botanist, Captain Mangles, in return for which he supplied her with flowers from England (Hasluck 1955).
As well as taking seeds of food species such as fruit trees, vegetables, potatoes and other essential crops to Australia, she had thistle seeds sent from her home in Scotland, and brought a number of flowering species from the Cape in South Africa on the journey out. Among the plants mentioned in her writings that came to Australia, as well as thistles, are South African watsonia, yucca, oleander, Cape gooseberry, English columbine, foxgloves and poppies. Many of her imported garden favourites such as larkspur, roses, pinks and hibiscus have not had the same propensity to 'jump the fence', however, as her admired South African imports, which were so well adapted to another Gondwanan fragment.
In 1955 parthenium weed (Parthenium hysterophorus), a native of the Caribbean, was first identified in northern Queensland. Between 1955 and 1979 it spread through 170 000 km 2 of north-eastern Queensland, with lighter infestations gradually extending southward. It reached the border with New South Wales in 1979 and spread into that state. It had reached nearly to the Victorian border by 1989, and local infestations have been found in the Roper River area of the Northern Territory and the Kimberleys.
Parthenium plants grow to about 1.5 m and each plant can carry over 10 000 small seeds that are easily spread in hay, grain, running water, and soil attached to tyres, boots or hooves. The pollen causes severe allergic reactions in humans and domestic stock. It is a declared weed species that must be reported to state and territory regulatory authorities for control because it degrades the quality of Australian grain and hay, which are then rejected for export. As a result it is controlled wherever it is identified by roadsides, but in wet region and irrigation areas it is almost beyond control.
It has covered many pastoral properties, and is capable of spreading rapidly on disturbed bare soil. It can tolerate long dry periods and is not confined to subtropical environments, although it is not likely to spread into winter-rainfall regions.
Parthenium weed spread in a typical Queensland environment.
Source: Queensland Department of Natural Resources
Weeds continue to have a severe impact on Australian indigenous plant communities. The extent to which weed populations affect ecosystem functions and processes, however, is much less understood than are the effects on biodiversity through habitat alteration. Many weed scientists consider the scope of environmental weed problems to be poorly quantified and probably underestimated (Vranjic et al. 2000). This contrasts with weed management systems in agricultural, horticultural and forestry systems. Environmental weed research and management is hampered by severe lack of resources and clearly identifiable economic benefits. The diversity of species and habitat types affected also mitigate against uniformly prescribed management options.
In northern Australia, major weed invasions appear to be predominantly of a single weed species spreading across large areas, whereas in southern Australia remnant vegetation patches are often surrounded by many infection sources and have been invaded by numerous species (Humphries et al. 1991).
Paterson's curse near Lithgow, New South Wales.
Where weed species have become the major plant species (e.g. Acacia nilotica ) across large areas they have a competitive advantage that leads to the suppression of much of the indigenous flora. (The impacts of environmental weeds are discussed in the Biodiversity Theme Report.) Water and nutrient cycling changes may also occur, although these have seldom been documented. Legumes, like acacia and broom, are classic 'colonising' species, which enter newly cleared areas because of their ability to fix nitrogen. Where soil nitrogen is inherently low, as it is in many arid regions of Australia, such invaders have a long-term advantage. In many smaller reserves and parks in southern Australia, the understorey has changed from native sclerophyll shrubs to legume shrubs, which increase soil nitrogen level to an extent where, after fire, annual exotic grasses invade. These significantly affect soil organic matter and nutrient levels, then inhibit the ability of many specialised native species to reinvade after subsequent fires.
Studies in central Australia and the Western Division of New South Wales indicate that between 5 and 20% of the flowering plants of these regions are exotics (Grice, 2000). They are predominantly forbs (herbaceous annuals and perennials, including grasses). A proportion (about 85 of 520 exotics) are declared noxious weeds but at least 50 indigenous species are also considered to be weeds, either on the basis of their toxicity to livestock, or because they have spread and dominate certain pastoral habitats. Woody weeds, which may be either exotic or indigenous in origin, shade out understorey herbs and grasses valued for grazing. Definition of weediness here is highly related to presumed preferred land use, rather than to ecosystem function.
In northern Australia exotic woody species such as prickly acacia (Acacia nilotica), mimosa (Mimosa pigra), rubber vine (Cryptostegia grandiflora) and parkinsonia (Parkinsonia aculeata) have significantly altered the relatively open woodland stands that were present at the time of European settlement. All of these are weeds of national significance.
Weeds of national significance include prickly acacia and rubber vine.
Introduced grasses and pasture legumes that are regarded as valuable forage additions, such as buffel grass (Cenchrus ciliaris) and Stylothanthes spp., have been the subject of persistent research efforts and deliberate introduction into many regions. There is now considerable debate between pastoral and biodiversity interests as to whether these species are valuable fodder or weeds.
Despite the very significant efforts to control weeds, integrated weed management strategies for weeds of national significance are in their infancy. In extensive land use areas, such as the pastoral rangelands, efforts to control weeds are severely limited by the immensity of the areas covered, the extremely small human population density, and the low economic value of the land.