3.2 Introduction
In Australia, as elsewhere in the world, invasive plants pose a serious threat to the conservation of native flora and fauna and ecosystem processes, as well as threatening agricultural industries, infrastructure and in some instances human health. Even in the Northern Territory, which boasts large areas of intact native vegetation, declared weeds and invasive plants are identified, together with feral animals and altered fire regime as a major threat to biodiversity and rural primary industries. With increasing development in the Northern Territory, and the potential range expansion of many weeds as a result of predicted climate change, it is likely that the problems associated with invasive plant species will increase.
In 2008 there were 119 declared weeds in the Northern Territory, but the list of declared species is currently under review. This review has been prompted by concerns that some species that should be declared are currently not listed and others that are currently declared may not warrant listing. In addition, there are many weeds found elsewhere in Australia and overseas that have not been recorded in the Northern Territory but have the potential to become established here. Despite this large and increasing problem, there will always be limited resources to tackle invasive plants. An objective and defensible method of assessing weed risk is needed to identify and restrict the entry of new weeds into the Northern Territory, and to assist land managers to prioritise management actions for those weeds already present.
Weed risk management (WRM) systems are a set of decision support tools that allow an evidence-based and strategic approach to the management of invasive species. WRM systems are based on an objective assessment of the likelihood and magnitude of risks posed by a species, and the feasibility of control options should the species become established. At a national level, the development of value of a weed risk management systems has been progressed via the development of the National Post-Border Weed Risk Management Protocol (Virtue et al 2006). In developing a WRM system for the NT the national protocol was to guide the design, creation and implementation of the NT WRM system. and in the Northern Territory the need for such a system has been highlighted in the Natural Resource Management Strategy (Landcare Council of the Northern Territory 2005).
3.3 A WRM system for the Northern Territory
The Northern Territory WRM system has been developed through collaboration between Charles Darwin University, the Department of Natural Resources, Environment, The Arts and Sport, the Department of Regional Development, Primary Industries, Fisheries and Resources, the Australian Quarantine Inspection Service and other stakeholder groups. It has been developed to provide decision support tools that are consistent with recognised Australian standards for the management of invasive plants. Development of the NT WRM system has been guided by the National Post-Border Weed Risk Management Protocol (HB 294: 2006 Standards Australia/Standards New Zealand/CRC Australian Weed Management 2006) and the input of key stakeholders within the NT and weed risk experts from other jurisdictions.
The NT WRM system is a comprised of a series of linked steps which are described below and represented in Figure 1.
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Which weeds? Determines candidate species for weed risk analysis. This involves collating existing weeds (declared and undeclared) lists and a review of potential weed species. In addition we sought suggestions for candidate species from the key stakeholders involved in the construction of the NT WRM system.
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Risk posed? Assesses the comparative risk of weed candidates using a WRA tool that scores and categorises weeds according to key risk indicators: Invasiveness, Impact and Potential Distribution.
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Feasibility of control? The feasibility of control for each candidate species is evaluated using a system that scores and categorises three control related criteria: current distribution, control costs and duration of control (ie to eradication or maintenance).
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How to respond? The consideration of weed risk versus feasibility of control is done using a management action matrix and provides an indication of the recommended management actions for a given species. These management actions might include: preventing entry, eradication, containment and improving targeted control techniques (Tables 1 & 2).
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Management response. The management response represents the transition from the strategic planning stage of the WRM to the on-ground application of management responses. This stage may, for declared weeds, involve the drafting of a statutory weed management plan, which provides detailed information about the management actions required, the recommended timing and techniques for control and where in the landscape different types of control need to be pursued and supported by statute (the Weeds Management Act). This last stage is informed by, but is outside the WRM system.
During the development stage a continuous process of consultation with stakeholders and ongoing monitoring and review allowed the refinement of the assessment process and recommended responses to mitigate the weed risks identified. As with any decision support tool dependent on the quality of the data used, periodic review of the WRM system performance will be a standard procedure. It should also be noted that additional decision making or analysis eg benefit cost analyses or detailed survey may be required before a final management response can be made.
Figure 1 Overview of the NT weed risk management process showing the main elements
of the WRM process
3.3.1 Development of the NT WRM system
The development of the NT WRM system involved a series of meetings of key stakeholder groups (pastoral producers, conservation, indigenous organisation representatives, Department of Defence and horticulture), during which attendees were invited to review and compare WRM systems from South Australia, Victoria, Queensland and from the Australian Quarantine Inspection Service and select the most appropriate (if any) for use in the NT. The workshop attendees considered the strengths and weaknesses of each system, based on information and resource requirements and major applications within its state of origin. A draft framework was established for the NT that included a number of guiding principles relating to ensuring transparency and accountability and ongoing stakeholder engagement. A WRM Technical Working Group was established to guide the development and testing of the NT WRM system.
The NT WRM system consists of a two-stage risk-assessment process: (i) an assessment of the comparative risk a species poses (weed risk assessment WRA) and (ii) an assessment of the likelihood of management intervention success (feasibility of control). Both weed risk and feasibility of control are assessed using a list of questions about the species biology, invasiveness and negative impacts, current and potential future distribution and costs / complexity of control measure required for a given species. For some species; that offer economic benefits as well as potential environmental, social or cultural costs, a benefit cost analysis can also be undertaken before a final management recommendation is made. A comparison of the weed risk versus the FOC enables a species to be categorised and prioritised for management actions using a weed risk management matrix (Figure 1).
One of the key steps in the development of the NT WRM system was modifying some elements of the WRA process from the South Australian model to a system that better suits the NT environment and land use systems. This involved changing the questions on the three criteria for assessing weed risk (invasiveness, impact and potential distribution), deleting questions where they were not appropriate and adding questions that were more appropriate for application in the NT. The primary reason for these changes was the difference in the extent of land modification and the types of land use in South Australia and the NT. Some of the key changes that were made included rewording of the questions to take into account the need to protect native vegetation, to take into consideration indigenous as well as western values and to give appropriate consideration to fire (and particularly the grass-fire cycle) as a key ecosystem driver in the NT. In addition, the South Australian model assesses candidate species differently for individual land use types but it was decided that the NT WRM system will assess candidate species in respect to one land use type, namely the broader landscape with its relatively intact native vegetation(Setterfield et al 2006). Questions that could not be reliably or consistently answered for candidate species in the NT were removed from the WRA model. The final scores for comparative weed risk and feasibility of control is derived by multiplying the scores for each of the component criteria (ie invasiveness, impact, potential distribution), as per the South Australian system (Virtue et al 2005).
After each candidate species is assessed using the WRA model and the FOC model, it is assigned to categories (eg low, medium, high, very high) and can then be placed within a cell in a management action matrix that is used to identify priority weed risks (Table 1) and make broad management recommendations. Those species that fall within the high and very high weed risk categories are identified as priority species. Based on a combination of weed risk and feasibility of control categories assigned suitable management actions can be recommended (Table 2).
Table 1 Categorisation of priority species based on risk and FOC (the weed risk management Matrix)
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Feasibility of control
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Low
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Medium
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High
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Very high
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Weed risk
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Low
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Medium
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|
|
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High
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Priority species
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Very high
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Table 2 Management action matrix showing comparison of weed risk and feasibility of control (Note: high feasibility of control = high likelihood of success)
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Feasibility of control
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Low
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Medium
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High
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Very high
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Weed risk
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Low
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Assist interested parties
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Assist interested parties
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Assist interested parties
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Monitor or assist interested parties
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Medium
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Improve
general weed management#
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Improve
general weed management
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Targeted control
Improve general weed management
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Targeted control Monitor
Protect priority sites
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High
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Targeted control
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Targeted control
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Protect priority sites
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Prevent entry Contain regional spread
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Very high
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Targeted control (incl biocontrol) protect priority sites
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Targeted control (incl biocontrol) Protect priority sites
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Prevent entry Contain regional spread protect priority sites
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Prevent entry Regional eradication protect priority sites
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# eg improve vehicle hygiene, reduce disturbance
3.3.2 Application of the NT WRM system
In November 2007, the WRM Technical Working Group with assistance from professional weed contractors, tested the WRA and FOC models on 80 candidate species, consisting of a variety of growth forms (grass, herb, shrub, tree), from variety of habitats (aquatic/terrestrial); with different status (declared /un-declared); plus species which identified as representing a range of potentially very high through to low risk. This testing was undertaken to assess the ability of the WRM system to assign these weeds to defensible categories of risk and FOC. The prediction of the WRM Technical Group was that most (but possibly not all) of the declared weed species would be assessed as a high/very high weed risk.
Somewhat unexpectedly, of the 80 species that were assessed using the WRA and FOC models, 40 were ranked in the high/very high risk category and 40 in the low/medium risk category (Table 3). Management recommendations are also being developed for these 80 species based on the results of this assessment. Of those species assigned to the high/very high risk category a large proportion were found to be grassy species and many were undeclared species in the NT. Of those species ranked in the low/medium risk categories, four were species that are currently declared in the NT.
Based on the WRM principles, used to guide the construction and application of the NT WRM system, species ranked as high/very high risk should be nominated for declaration in the NT. A total of 15 of the 80 assessed species are candidates for declaration. These are: Acacia mangium; Andropogon gayanus gamba grass; Cenchrus ciliaris buffel grass; Leucaena leucocephala coffee bush; Sporobolus pyramidalis and S. natalensis giant rats tail grass; Megathyrsus maximus guinea grass; Pennisetum pedicellatum annual mission grass; Urochloa mutica para grass; Dichanthium annulatum sheda grass; Hyparennhia rufa thatch grass; Neptunia plena and N. oleracea water mimosa; Schinus terebinthifolius Brazilian pepper; and Azadirachta indica neem tree).
A total of 13 species (Barleria prionitis, Tribulus terrestris and T. cistoides caltrop; Senna alata candle bush; Dalbergia sissoo; Alternanthera pungens khaki weed; Datura ferox longspine thornapple; Leonurus leonotis lion’s tail; Hyptis capitata knob weed; Cenchrus echinatus Mossman river grass; Carthamus lanatus saffron thistle; Acanthospermum hispidum starr burr; and Argemone ochroleuca Mexican poppy) are currently declared but should be reviewed given their scores for comparative weed risk. The distribution of these species needs to be further considered to determine if they are a problem sub-regionally and therefore should remain as declared species. This reflects the fact that the NT WRM system assesses feasibility of control being assesses at the regional scale.
Table 3 Comparative weed risk of 80 species assessed using the WRA and FOC models (as of November 2007) * NT Weed Declaration Categories: Class A – to be eradicated; Class B – growth and spread to be controlled; Class C – Not to be introduced into the NT; ‘-’ no weed declaration status in the NT; WONS (weed of national significance).
Common name
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Botanical name
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Current NT declaration status*
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Very high risk species
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Athel pine
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Tamarix aphylla
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B and C (WONS)
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Bellyache bush
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Jatropha gossypifolia
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B and C
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Brazilian pepper
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Schinus terebinthifolius
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-
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Buffel grass
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Cenchrus ciliaris
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-
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Cabomba
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Cabomba spp
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A and C (WONS)
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Chinee apple
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Ziziphus mauritiana
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A and C
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Coffee bush
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Leucaena leucocephala
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-
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Gamba grass
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Andropogon gayanus
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A/C and B/C as of Nov 2008
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Grader grass
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Themeda quadrivalvis
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B and C
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Guinea grass
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Megathyrus maximus
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-
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Lantana (common)
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Lantana camara
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B and C (WONS)
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Limnocharis
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Limnocharis flava
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C
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Mequite
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Prosopis sp
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A nd C
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Mimosa
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Mimosa pigra
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B and C (WONS)
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Mission grass – annual
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Pennisetum pedicellatum
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-
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Mission grass – perennial
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Pennisetum polystachion
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B and C
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Neem
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Azadirachta indica
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-
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Olive hymenachne
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Hymenachne amplexicaulis
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B and C (WONS)
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Para grass
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Urochloa mutica
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-
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Parkinsonia
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Parkinsonia aculeata
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B and C (WONS)
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Parthenium
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Parthenium hysterophorus
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A and C (WONS)
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Pond apple
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Annona glabra
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A and C (WONS)
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Prickly acacia
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Acacia niloltica
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A and C (WONS)
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Rubber vine
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Cryptostegia spp
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A and C (WONS)
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Salvinia
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Salvinia molesta
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B and C (WONS)
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Sheda grass
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Dicanthium annulatum
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-
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Siam weed
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Chromolaena odorata
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C
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Sicklepod
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Senna obtusifolia
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B and C
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High risk species
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|
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Acacia mangium
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Acacia mangium
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-
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Castor oil plant
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Ricinis communis
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B and C
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Devils claw
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Martynia annua
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A and C
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Giant rats tail grass
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Sporobolus pyramidalis & Sporobolus natalensis
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-
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Hyptis
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Hyptis suaveloens
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B and C
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Kosters curse
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Clidemia hirta
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C
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Mikania
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Mikania microcantha
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C
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Noogoora burr
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Xanthium occidentale
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B and C
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Rubber bush
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Calotropis procera
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B and C
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Sida
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Sida acuta
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B and C
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Thatch grass
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Hyparennia rufa
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-
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Water mimosa
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Neptunia plena & Neptunia oleracea
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-
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Medium risk species
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|
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African mahogany
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Khaya senegalensis
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-
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Coral vine
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Antigon leptopus
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-
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Knob weed
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Hyptis capitata
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B and C
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Kudzu
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Pueraria Montana var lobata
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-
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Lions tail
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Leonotis nepetifolia
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B and C
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Longspine thornapple
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Datura ferox
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A and C
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Mexican poppy
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Argemone ochroleuca
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B and C
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Miconia
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Miconia calvescens
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-
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Mossman River grass
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Cenchrus echinatus
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B and C
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Mother of millions
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Bryophyllum spp
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-
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Singapore daisy
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Sphagneticola trilobata
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-
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Tully grass
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Urochloa humidicola
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-
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Low risk species
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|
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African tulip tree
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Spathodea campanulata
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-
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Bahia grass
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Paspalum notatum
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-
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Barleria
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Barleria prionitis
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A and C
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Caltrop (T cistoides)
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Tribulus cistoides
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B and C
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Caltrop (T terrestris)
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Tribulus terrestris
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B and C
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Candle bush
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Senna alata
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B and C
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Cavalcade
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Centrosema pascuorum
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-
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Crotalaria/rattlepod
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Crotalaria gorensis
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-
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Dalbergia
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Dalbergia sisso
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A and C
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Finger grass
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Digitaria milanjiana
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-
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Fishtail palm
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Caryota mitis
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-
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Golden rain tree
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Cassia fistula
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-
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Khaki weed
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Alternanthera pungens
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B and C
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Lippia
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Phyla canescens
|
_
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Molasses grass
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Melinis minutiflora
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-
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Mother-in-laws-tongue
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Sanseviera trifasciata
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-
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Neurada
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Neurada procumbens
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-
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Pannical joint vetch
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Aeschnomene paniculata
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-
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Poinciana
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Delonix regia
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-
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Ruby dock
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Acetosa vesicaria
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-
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Sabi grass
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Urochloa mosambicensis
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-
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Saffron thistle
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Carthamus lanatus
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B and C
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Siamese cassia
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Cassia siamea
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-
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Spider flower (fringed and prickly)
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Cleome rutidosperma & Cleome aculeata
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-
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Starr burr
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Acanthospermum hispidum
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B and C
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Tipuana
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Tipuana tipu
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-
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Vetiver grass
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Vetiveria zizanioides
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-
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Yellow oleander
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Cascabela peruviana
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-
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Once the assessment of these 80 species was completed to the satisfaction of the WRM Technical Working Group, broad management recommendations were assigned to each of the 80 species based on their positioning in the weed risk management matrix (see Table 5 for preliminary Darwin Region risk management matrix).
Table 5 Preliminary Darwin region weed risk management matrix
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Feasibility of control
|
|
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Low–Medium
Medium
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High
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Very high
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Weed Risk
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Low
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1 Assist interested parties
Cavalcade, Crotalaria, Finger grass, Sabi grass
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2 Assist interested parties
Bahia grass, Barleria, Caltrop (T. cistoides), Caltrop (T. terrestris), Candle bush, Khaki weed, Pannicle joint vetch, Poinciana
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3 Monitor/assist interested parties
African tulip tree, Dalbergia, Fishtail palm, Golden rain tree, Molasses grass, Mother-in-laws-tongu, Siamese cassia, Spider flower (fringed & prickly),Tipuana, Vetiver grass, Yellow oleander
|
Medium
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4 Improve general weed management
African mahogany, Knob weed, Mossman River grass, Tully grass
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5 Targeted control/ Improved general weed management
Lions tail, Singapore daisy
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6 Targeted control/Monitor/ Protect priority sites
Mother of millions
|
High
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7 Targeted control
Coral vine, Giant rat’s tail grass, Hyptis, Noogoora burr, Rubber bush, Sida
|
8 Protect priority sites
Castor oil plant, Devils claw, Water mimosa
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9 Prevent entry/contain regional spread
Acacia mangium, Thatch grass
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Very high
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10 Targeted control/ protect priority sites
Buffel grass,Cabomba, Coffee bush, Gamba grass, Grader grass, Guinea grass, Mimosa, Mission Grass (annual), Mission Grass (perennial), Olive hymenachne, para grass, Salvinia, Sicklepod
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11 Prevent entry/ contain regional spread/ protect priority sites
Bellyache bush, Lantana (common), Neem
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12 Prevent entry/ Regional eradication/ protect priority sites
Brazilian pepper, Chinee apple, Parkinsonia, Parthenium#, Pond Apple#
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# FOC scores for these species are under review at the time of writing
The information derived from the NT WRM system is being used to undertake a review of the NT declared weeds list and associated management plans, including nominating high/very high risk species for declaration. The development of management plans for priority species will be guided by the results of this assessment process. Recommendations will be provided to regional weed managers and to NRM managers to facilitate co-ordinated implementation of the WRM process. The results of these assessments will also allow the identification of opportunities for cross-jurisdictional co-operation where weed risk priorities are aligned.
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