Plants found outside their natural range are considered ‘introduced’, however, when its introduction causes environmental or economic harm to the region, it is regarded as an ‘invasive’ plant (National Geographic, 2023). An example of this is Acacia longifolia subsp. longifolia (Figure 1), a shrub native to the coast of Victoria, New South Wales and Queensland that was introduced to Western Australia where it is outcompeting bushland species around Albany, and in creeks and swamps (Identic, 2024).
Most plants regarded as invasive are also considered ‘weeds’. A weed is defined as a plant which requires action to reduce its impact (Environment and Invasives Committee, 2017). These actions can include hand removal, removing the seeds before they spread, covering the weed with mulch or applying herbicides such as glyphosate (Shire of Serpentine Jarrahdale, 2024). Currently, Australia has roughly 3,200 introduced plant species with around 500 considered weeds (Environment and Invasives Committee, 2017).
Impacts from Invasive Plants
When introduced to a region, invasive plants can impact the environment by out competing native plants and altering the native ecosystems, making them more susceptible to wildfires, grazing and other impacts (DBCA, 2023). These impacts are especially harmful for Threatened species and ecological communities as the invasive plant can add extra competition for resources and change local conditions.
Andropogon gayanus (gamba grass) (Figure 2), for example, is a grass native to tropical Africa that was introduced to the northern part of Australia and is commonly found in pastures and along road verges, as well as savannas and wetlands (Weeds Australia, 2024). In these regions, gamba grass adds extra competition for water, with its high water demand and shallow roots reducing deep water drainage required for native tree species to survive (Setterfield, Clifton, Hutley, Rossiter-Rachor, & Douglas, 2018). Gamba grass also has a higher demand for soil nitrogen which it uses efficiently to continue growing months after native grasses have stopped, which limits nitrogen availability in the next season (Rossiter-Rachor, et al., 2009).
Invasive plants can also modify and disrupt ecosystem processes by replacing native plants, excluding native animals, or providing resources for introduced animals (DBCA, 2023). Some invasive plants, such as Phytolacca americana (pokeweed) (Figure 3), do this through releasing or containing toxins. Pokeweed is a shrub native to eastern United States that was discovered in Western Australia’s Balingup area in 2018 (DPIRD, 2019a). The species contains high concentrations of toxins, including oxalic acid and phytolaccotoxin (United States Department of Agriculture, 2004), which can be toxic to humans and animals (native and domestic) (DPIRD, 2019a). However, these toxins do not impact bird species, which allows the species to spread if management measures are not implemented.
Additionally, invasive plants can alter the seasonality, intensity or frequency of fire regimes in regions (DBCA, 2023). For example, Passiflora foetida (stinking passionflower) (Figure 4) is a vine originally from South and Central America but was introduced to the northern part of Australia (Webber, Yeoh, & Scott, 2014). Stinking passionflower can spread over native trees and forms dense vine mats, up to 80cm thick, suffocating ground-dwelling native species. As a result, the region’s fuel load is increased, leading to a higher intensity and risk of canopy fires (DBCA, 2023).
Economic cost to the agricultural industry from invasive plants is estimated at $1.2 billion for Western Australia, and $4.3 billion across Australia each year (Australian Bureau of Agricultural and Resource Economics and Sciences, 2023). The high cost relates to management actions along with losses in crop yield and reductions in livestock carrying capacity. It is expected the economic cost for landscape amenity, nature conservation and tourism are comparable, although they have not been quantified (Environment and Invasives Committee, 2017).
Management across Australia
The Commonwealth Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) protects Australia’s biodiversity through actions such as listing processes as a ‘key threatening process’. Invasive plants can be considered a key threatening process if they lead to the conservation status of a native species to become listed as Threatened, or increase its level of threat, or cause an impact to at least two Threatened species or ecological communities. Under the EPBC Act, impacts from invasive plants and invasive animals are considered under the broad key threatening process, Novel Biota and their Impact on Biodiversity (DCCEEW, 2021a). Within this management category there are also specific key threatening processes for the invasive plants, introduced grasses (including gamba grass), and escaped garden plants (including aquatic plants) (DCCEEW, 2024).
When a key threatening process can be feasibly, efficiently and effectively managed through enacting a Threat Abatement Plan, one is implemented under the direction of the Minister for Environment and reviewed at least every five years (DCCEEW, 2021b). There is currently only one Threat Abatement Plan relating to invasive plants; it relates to the Invasion of Northern Australia by Gamba Grass and Other Introduced Grasses. This plan seeks to reduce ecosystem degradation, habitat loss and species decline due to the presence of gamba grass, Urochloa mutica, Hymenachne amplexicaulis, Cenchrus polystachios and Cenchrus pedicellatus (DCCEEW, 2022).
Across Australia, there are existing ongoing management measures for invasive plants, and thus, a broad, nationwide Threat Abatement Plan for invasive plants isn’t required (DCCEEW, 2021a). In 1997, Australia developed the National Weeds Strategy to manage weeds’ environmental impact (Environment and Invasives Committee, 2017). It was then revised in 2017 to become the Australian Weed Strategy to manage invasive plants considered non-native to Australia, while native species considered weeds are managed by the specific State or Territory. The aims of the Australian Weed Strategy are to prevent, detect and enact early intervention measures for the introduction of weeds, and minimise the impact from established weeds (Environment and Invasives Committee, 2017). Implementation of the Australian Weed Strategy is the responsibility of the Australian government, State and Territory governments, local governments, landholders, natural resource management bodies and community groups. All Australians have a role to play in managing invasive species according to the Australian Weed Strategy.
The Weeds of National Significance program is another management measure for invasive plants which commenced in 1999 under the National Weeds Strategy to provide a targeted management plan for Nationally significant weed species (Wild Matters, 2020; Environment and Invasives Committee, 2017). However, since 1999 the list of nationally significant invasive species has grown, which resulted in the Established Pests and Diseases of National Significance framework (EPDNS) (National Biosecurity Committee, 2016). Consequently, Weeds of National Significance became part of the National Established Weed Priorities framework (NEWP) to better align with the EPDNS framework (Weeds Working Group, 2023).
Under the NEWP framework, Weeds of National Significance are invasive plants which have a significant impact at a national scale with feasible management options and clear, demonstrable benefits from a nationally coordinated effort (National Biosecurity Committee, 2016). The nationally coordinated effort will aim to complete the actions in a Weed of National Significance’s targeted management plan within 3-5 years; any actions remaining are then transferred to the National Established Weed Action List to allow a focus on new Weeds of National Significance (Weeds Working Group, 2023). Currently, there are 32 invasive plants classed as Weeds of National Significance which can be found at https://weeds.org.au/lists/established/ (Weeds Australia, 2024).
Management in Western Australia
In Western Australia, invasive plants are managed according to whether they impact biodiversity or the agricultural industry (DBCA, 2023). Invasive plants which impact biodiversity are managed by the Department of Biodiversity, Conservation and Attractions (DBCA), while the Department of Primary Industries and Regional Development (DPIRD) manages invasive plants impacting agriculture.
DBCA has established a Weed Prioritisation Process to effectively manage invasive plants while maximising biodiversity and economic benefits (DBCA, 2023). The Weed Prioritisation Process classifies invasive plants from low priority to very high priority across the nine DBCA regions (Figure 5) based on their invasiveness, ecological impacts, potential and current distribution along with feasibility of control (DBCA, 2013). More information on the Weed Prioritisation Process and invasiveness rankings of weeds identified for each of the nine DBCA regions can be found on DBCA’s website.
Western Australian invasive plants with the highest priority are those with a high impact and are rapidly invasive, which can be feasibly managed and removed (DBCA, 2023). While those already widespread are given a lower priority due to the lack of feasible management options (DBCA, 2013). Lower priority invasive plants are instead managed by identifying high-value biodiversity assets which they may impact and determining areas for effective control (DBCA, 2023).
DPIRD, on the other hand, manages invasive plants according to Western Australia’s Biosecurity and Agriculture Management Act 2007 (BAM Act). The BAM Act is designed to ensure new pests, weeds and diseases are prevented entry, and pre-existing pests are managed (DPIRD, 2022). DPIRD provides landholders, growers and biosecurity groups with weed identification service, information on crop weeds, regulated/declared pests, weed control and herbicides (DPIRD, 2024). An invasive plant is considered a declared pest under the BAM Act if it has or may have an adverse impact on another organism or humans, part or all of the environment, or commercial activities present or planned for the region. The impact can be considered for invasive plants not currently in the region or at a harmless population size, not just for current invasive plants (BAM Act). When an invasive plant is considered a declared pest, landowners are required to report sightings of the species to DPIRD and start eradicating its presence (DPIRD, 2019b). There are currently 927 invasive plants listed as a declared pest on DPIRD’s Western Australian Organism List including gamba grass and pokeweed.
Additional information on Weeds and Invasive Plants relevant to WA can be obtained from:
- https://weeds.org.au/lists/established/
- https://www.dbca.wa.gov.au/management/threat-management/weeds
- https://www.agric.wa.gov.au/organisms
Integrate Sustainability understands the impact invasive plants can have on biodiversity; if your organisation needs to understand its obligations to manage and eradicate these species, please call us on 08 9468 0338 or email us at enquiries@integratesustainability.com.au.
References
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