State of the Environment Report 2007
Many terms are used to describe introduced and native plants (Table B4.1). Weeds are plants that are not considered native to WA, and have the capacity to impact upon environmental, social or economic values. They may include herbs, grasses, shrubs, trees, vines and aquatic plants. Weeds have been introduced deliberately for agricultural benefits (for crops, pastures or seed), soil stabilisation (for erosion control), for ornamental purposes (e.g. garden plants, fish tanks) or accidentally in products such as animal feed or pot plant soil (Hussey et al., 1997). Many plants introduced into Australia over the past 200 years have become weeds.
Plants that become weeds can often propagate themselves in more than one way and typically produce large numbers of seeds, assisting their spread. Seeds spread into natural environments, including waterways, by wind, water, people, vehicles, machinery, birds and other animals. Weeds can rapidly invade natural sites where the soil has been disturbed, where there has been clearing, or where fire regimes have changed. Weeds also thrive in nutrient rich soils or where fertilizers have been added.
Weeds threaten the survival of many native flora and fauna because they grow faster and out-compete native plants for available nutrients, water, space and sunlight. They can smother and replace native plants including those used by animals for habitat, and are commonly resilient to pests or diseases. Consequently, weeds have the potential to dominate and simplify natural ecosystems. They may also lead to significant changes to ecosystems and fire regimes. Weeds also cause economic losses in agriculture as they reduce yields, contaminate crops, poison stock, reduce livestock carrying capacity, downgrade wool or taint milk (Hussey et al., 1997).
A total of 1233 weed species (defined as being able to reproduce without human involvement) have been recorded in WA, 55% of which are considered primarily environmental weeds (Keighery & Longman, 2004). At present, there are only 92 formally declared weed species under State legislation (which means they are subject to restrictions on movement or sale and control must be undertaken; Department of Agriculture and Food, 2007a). Weeds are present across most of WA (Figure B4.1). The Swan Coastal Plain has the highest number of weeds identified - over 800 species. In general, most South West bioregions have over 300 weed species identified (Figure B4.1). This could be associated with densely populated areas and highly disturbed environments (cleared and fragmented native vegetation), but could also be attributed to greater survey efforts. Parts of the Goldfields, Mid West, Pilbara and Kimberley have between 100-200 weed species identified. In comparison, there are very low numbers of weeds identified in the central deserts.
![Figure B4.1: Number of weed species found per terrestrial bioregion. [Data source: Keighery & Longman (2004). Presentation: EPA. Note: Each IBRA region is labelled with the total number of weeds and the percentage of the total number which as classed as environmental weeds (in brackets).]](/site/files/images/Figure-B4.1_Large.jpg)
While the total number of weed species is a good measure of overall condition, the environmental effects of these species vary considerably. Some environmental weed species are recognised as being very significant in various regions in terms of having current or potential impacts on WA biodiversity (Tables B4.2 and B4.3). This is based on factors such as proven ability to spread in other places with similar climatic conditions, ability to alter ecosystems and threaten native species, and absence of appropriate management action.
The list of Weeds of National Significance details the top 20 weed species Australia-wide based on invasiveness and impact, potential and current spread, and current primary industry, environmental and socioeconomic impacts (Australian Weeds Committee, 2004). A total of eleven of these occur in WA and all have the potential to expand their range. A further seven weed species have the potential to spread from other states to WA.
![Table B4.2: Top five worst terrestrial environmental weed species by geographic region. [Data source: Department of Conservation and Land Management - worst environmental weeds (unpublished data) [ver. 2006]; Global Invasive Species Database (2007); Australian Weeds Committee (2004), Department of Agriculture and Food (2007a.).]](/site/files/images/Table-B4.2_Large.jpg)
![Table B4.3: Top five worst aquatic or wetland environmental weed species by geographic region. [Data source: Department of Conservation and Land Management - worst environmental weeds (unpublished data) [ver. 2006]; Global Invasive Species Database (2007); Australian Weeds Committee (2004), Department of Agriculture and Food (2007a.).]](/site/files/images/Table-B4.3_Large.jpg)
In addition to the weeds listed in Tables B4.2 and B4.3, another seven Weeds of National Significance are present in WA. These are alligator weed (Alternanthera philoxeroides), bitou bush/boneseed (Chrysanthemoides monilifera), blackberry (Rubus fruticosus agg), gorse (Ulex europaeus), lantana (Lantana camara), parkinsonia (Parkinsonia aculeata) and rubber vine (Cryptostegia grandiflora). Gorse and lantana are also recognised on the international 100 worst invasive species list along with three other species that are present in WA - black wattle (Acacia mearnsii), giant reed (Arundo donax) and maritime pine (Pinus pinaster).
Monitoring trends in weed species distribution is a difficult and time consuming task and few examples are available. One example of weed spread over time comes from surveys conducted in the Ashburton River catchment for 1978 and 2001-02 (Payne et al., 2004). Buffel and Birdwood grasses (Cenchrus ciliaris and C. setigerus) distribution was determined over the two periods (Figures B4.2 and B4.3). The weeds had spread considerably and increased in dominance at many sites. Pastoralists reported a rapid expansion after a flood in 1997 which deposited silt and sand over the catchment's extensive alluvial plains.
![Figure B4.2: The extent of vegetation dominated by Cenchrus spp. (buffel and Birdwood grasses) in 1978. [Data source: Payne, et al. (2004). Presentation: Payne, et al. (2004).]](/site/files/images/Figure-B4.2_Large.jpg)
![Figure B4.3: The extent of vegetation dominated by Cenchrus spp. (buffel and Birdwood grass) in 2002. [Data source: Payne, et al, (2004). Presentation: Payne, et al. (2004).]](/site/files/images/Figure-B4.3_Large.jpg)
Plants that become successful weeds are usually those that have adapted elsewhere to a similar climate. For example, the South West is vulnerable to plants that have adapted to other Mediterranean climates - South Africa, California, South America and Mediterranean countries. The Kimberley is vulnerable to plants from tropical areas of the world. The desert areas of central and northern WA have acted as a barrier to many weeds from eastern Australia.
Weeds often dominate new locations because they lack natural competitors, herbivores and pathogens that would regulate them in their native environment. Many weed species introduced for agricultural and pastoral production have become very significant environmental weeds, notably buffel grass in the rangelands. Of 463 exotic pasture species introduced to northern Australia, less than 5% became useful, and less than 1% of those are useful without also being a weed. About 10% of species became weeds with no recorded use (Lonsdale, 1994).
The gardening industry is by far the largest importer of exotic plants, accounting for 94% of new species brought into Australia (Groves et al., 2005). About two-thirds of the weeds now established in Australia originated from gardens. Many garden plants known to be weeds continue to be imported and sold in nurseries. Controlling the spread of weeds has centred on restricting the sale of many species known to become weeds, vigilance in detecting new garden escapees and national coordination of regulation and resourcing (Groves et al., 2005). Transport networks also present many opportunities for the introduction of new species, with seeds being accidentally dispersed by vehicles, aircraft, trains or ships. Cuttings or seeds can also be found in imported goods, cargo, mail and luggage and require vigilant quarantine and inspection procedures.
Quarantine and inspection services play an important role in preventing incursions into WA. This may include preventing the entry of weed species into WA and the early detection and eradication of weeds. If new weeds are discovered before they are become established, eradication is possible. Once weeds become well established, control methods must be used to limit further spread and to protect natural resources. Examples of control methods include:
Many potential weeds are known to be invasive in other parts of the world, Australia and within WA, and represent a significant risk to local environments if they become established. For example, the most serious potential terrestrial and aquatic weed threats from a biodiversity perspective (Department of Conservation and Land Management, unpublished data, 2006) to WA are:
Quarantine: The role of the Australian Quarantine and Inspection Service is to prevent entry of introduced plants (and animals and pathogens) to Australia in incoming luggage, cargo and mail, or associated imported goods. The Western Australian Quarantine and Inspection Service (run by the Department of Agriculture and Food) aims to prevent weed incursions from other parts of Australia. As of March 2007, 1364 taxa had been assessed and judged a quarantine risk to WA (in addition to the 92 declared weeds) and were prohibited entry to the State. These weeds are featured in a WA quarantined species list. Over 14 400 taxa have had a formal weed risk assessment and can be imported into WA. Any species not on the list must undergo a weed assessment prior to importation (Department of Agriculture and Food, 2007a & 2007b).
Legislation: The Noxious Weeds Regulations 1973 and Agriculture and Related Resources Protection Act 1976 provide for plants or seeds to be prevented from entering WA (for trade, sale or transit to other states); require eradication across affected areas, control of weed populations, and quarantine of existing populations; and require the control of weeds on public land or land under control of local government. There are currently 92 declared weeds in WA. The Department of Agriculture and Food is responsible for overseeing legislation requiring the management of declared weeds.
Strategies: The National Weed Strategy was released in 1997 to prevent the introduction of new weed species around Australia and strengthen action against those already established. The strategy focused on weeds of national significance, which were selected according to invasiveness, current location and potential to spread, and impacts on primary industry, the environment and social values (ARMCANZ, et al., 1997). This was reviewed in 2002 and found to have been highly successful with 15 strategies completed, 10 strategies with significant work undertaken (and ongoing work required) and only one strategy requiring major work for completion (National Weeds Strategy Executive Committee, 2002).
The Australian Weed Strategy was released in 2006 and replaced the National Weed Strategy. It addresses the prevention of new weeds, abatement of existing weeds and the enhancement of capacity to combat such problems. It is a national framework to guide and complement state, territory, regional and local government strategies and industry initiatives that are translated into on-ground weed management actions (Australian Weeds Committee, 2006).
The Weed Plan for Western Australia (also known as the State Weed Plan) was released in 2001 to coordinate effective weed management across the State (State Weed Plan Steering Group, 2001). The Western Australian Weeds Committee has been set up as a result of the plan. An Environmental Weed Strategy for Western Australia was released in 1999 (Department of Conservation and Land Management, 1999b) to address and record environmental weeds specifically. Many actions contained in the strategy were not commenced until the Saving Our Species suite of programs commenced in 2006.
Natural Heritage Trust/National Action Plan for Salinity and Water Quality (NHT/NAP): These are two Commonwealth Government programs that aim to ensure environmental (on-ground) improvements occur via a targeted strategic approach at the regional level. All regional NRM groups have identified weeds as a problem and implemented a range of actions to control and manage weeds.
On-ground actions: Many government agencies (including the Department of Agriculture and Food, Department of Environment and Conservation, Main Roads, Westrail and local governments) have programs to remove weeds on land under their jurisdiction. Individual landholders are responsible for controlling weeds on their land (including declared plants). Many community groups, most notably the Environmental Weeds Action Network and its associated clubs and societies, conduct removal and management of weeds.
The Saving Our Species program began in 2006 and the weed eradication and control component builds on the Environmental Weed Strategy for Western Australia (Department of Conservation and Land Management, 1999b). Forty weed species are being targeted in the initial 18 projects to eradicate entire weed populations at a local scale where possible. Targets under the program include: cactus on pastoral land north of Carnarvon; asparagus fern in Denmark, Albany and Margaret River; cape tulip and watsonia in parts of the Perth metropolitan area; athel pine (tamarisk) at Lake Boonderoo; various weed species on the south coast; neem in Kununurra; prickly acacia in the Durack River area in the Kimberley; date palms in Pilbara wetlands; and various weeds in the Shark Bay World Heritage Property. Projects targeting South African grass species in high conservation value areas in Perth are also a priority.
Biodiversity Conservation Strategy: The Department of Environment and Conservation has released A 100-year Biodiversity Conservation Strategy for Western Australia: Blueprint to the Bicentenary in 2029. Public submissions were requested and a final biodiversity conservation strategy is in preparation (Department of Environment and Conservation 2006a).
Research: The Department of Agriculture and Food conducts research on a wide range of weed species and houses the largest group of weed researchers in the State, whose primary focus is on agricultural weeds. The Cooperative Research Centre for Australian Weed Management is working to reduce risks posed by current and new weed incursions through programs in research, education and information delivery across Australia. Major research efforts have contributed to the search for biocontrol insects to control weeds such as blackberry, bitou bush/boneseed, parthenium weed and Paterson's curse.
Weeds have significant impacts on WA's environment, society and economy. It has been estimated that the cost to Australian agriculture alone is over $4 billion a year (Australian Weeds Committee, 2006) and may consume as much as 20% of production costs (State Weed Plan Steering Group, 2001). Aquatic weeds can also foul water supplies and clog irrigation and drainage systems, requiring extensive maintenance works. Weeds can impact human health if they produce toxic substances, irritants or allergens and may lead to a loss of recreational and aesthetic values. The cost of weeds to the environment is incalculable. Loss of biodiversity (including extinctions and permanent changes to ecosystems) will continue with further invasion and spread of weeds. Weed control requires consistent and sustained effort over time. Environmental weeds are generally not well controlled compared to agricultural or pastoral weeds. Many areas of WA have significant economic and workforce limitations that make ongoing control very difficult. The spread of weeds can also contribute to changed fire regimes and landscape-level change to ecosystems.
5.16 Implement the Environmental Weed Strategy for Western Australia. While the strategy was launched in 1999, very limited progress has been made to date.
5.17 Prevent the entry of exotic plants to Western Australia, and restrict the sale of potential environmental weeds throughout the State.
5.18 Expand current programs for weed surveillance and control.
