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Assessment of plant invasiveness is done by evaluating biological and ecological characteristics such as germination requirements, growth rate, competitive ability, reproduction methods and dispersal mechanisms. Assessment of plant impacts, however, is determined by the extent to which a plant affects a land manager’s environmental, economic and social resources.
The relative importance of these resources varies depending upon the value people place on them and, as such, the assessment process is subjective. For example, a farmer is likely to place a higher emphasis on the impact of a plant on production (economic resource) than its impact on areas of natural vegetation occurring on the farm. Conversely, a Landcare or Friends group would value environmental or social resources more than economic resources.
Recognising that the value of resources vary between different land tenures, plant impact assessments allow a prioritisation of resources by land managers. Assessments can apply at a local, regional or state level, and the relative values of each resource identified may differ at each level.
The impact assessment method used in the Victorian Pest Plant Prioritisation Process uses three broad resource categories: social, environmental and agricultural, each with a number of related attributes. For example, social resources include such attributes as how the plant affects human access for recreation, or if it creates a health risk due to toxicity or by producing allergens.
Each resource attribute, or criterion, is assessed relative to a list of intensity ratings. Depending upon information found in relation to each criterion, a rating of Low, Medium Low, Medium High or High is assigned. Descriptions of the impact criteria and intensity ratings used in this process can be viewed here. |
Question | Comments | Rating | Confidence |
Recreation | |||
1. Restrict human access? | Stems from 1 to 1.5 m long (Parsons & Cuthbertson 2001). ‘..commonly found as a scrambling form extending from the edge of waterways and forming dense tangled mats in open water’ (CRC for Australian Weed Management 2003). Impedes individual access. | ML | MH |
2. Reduce tourism? | ‘It impedes water flow, navigation and recreation’ (Parsons & Cuthbertson 2001). Some recreational uses affected. | MH | MH |
3. Injurious to people? | Plant not noted to be toxic or injurious to people (PIER 2001). | L | MH |
4. Damage to cultural sites? | Semi-aquatic plant. Negligible effect on aesthetics of site. | L | MH |
Abiotic | |||
5. Impact flow? | ‘Dense infestations impede water flow and growth of native plant species’ (Weber 2003). ‘.. effects of flooding are worse because infestations block drainage channels’ (CRC for Australian Weed Management 2003). Major impact on surface or subsurface flow. | MH | MH |
6. Impact water quality? | ‘Water quality may decline if large amounts of Senegal tea plant die off and rot under water’ (CRC for Australian Weed Management 2003). Noticeable but minor effects in either dissolved oxygen or light levels. | ML | M |
7. Increase soil erosion? | ‘Numerous, finely fibrous adventitious roots developing from the nodes’. Plants are dormant during winter. (Parsons & Cuthbertson 2001). Moderate probability of large scale soil movement. | ML | MH |
8. Reduce biomass? | ‘.. can rapidly cover water bodies with a floating mat, excluding other plants’ (CRC for Australian Weed Management 2003). Plants are dormant during winter (Parsons & Cuthbertson 2001). Biomass may increase at certain times of year. | L | MH |
9. Change fire regime? | Semi-aquatic. Negligible effect on fire risk. | L | MH |
Community Habitat | |||
10. Impact on composition (a) high value EVC | Basin=Tambo River- Nicholson river (ISC=excellent); CMA=East Gippsland; CLIMATE=VH; Can rapidly cover water bodies with a floating mat, excluding other plants’ (CRC for Australian Weed Management 2003). Displaces all species within a layer. | H | M |
(b) medium value EVC | Aquatic species. All Victorian water bodies considered to comprise high value EVCs only (Weiss pers. coms.)i. | L | H |
(c) low value EVC | Aquatic species. All Victorian water bodies considered to comprise high value EVCs only (Weiss pers. coms.)ii. | L | H |
11. Impact on structure? | ‘.. can rapidly cover water bodies with a floating mat, excluding other plants’ (CRC for Australian Weed Management 2003). Major effect on less than 60% of floral strata. | MH | M |
12. Effect on threatened flora? | This species is not documented as posing an additional risk to threatened flora. | MH | L |
Fauna | |||
13. Effect on threatened fauna? | This species is not documented as posing an additional risk to threatened fauna. | MH | L |
14. Effect on non-threatened fauna? | ‘.. can rapidly cover water bodies with a floating mat, excluding other plants and the animals that rely on them’ (CRC for Australian Weed management 2003). Minor reduction in habitat / food / shelter. | ML | MH |
15. Benefits fauna? | Weed not known to benefit or facilitate the establishment of indigenous fauna. | H | M |
16. Injurious to fauna? | Plant is not toxic and does not have burrs or spines which affect indigenous fauna. | H | M |
Pest Animal | |||
17. Food source to pests? | Not documented to be a food source to pests. | L | M |
18. Provides harbour? | Plant not documented to provide harbour for serious pests. | L | M |
Agriculture | |||
19. Impact yield? | Not a weed of agriculture. | L | M |
20. Impact quality? | Not a weed of agriculture. | L | M |
21. Affect land value? | Weed not documented to affect land value. | L | M |
22. Change land use? | Weed not known to cause a change in priority of land use. | L | M |
23. Increase harvest costs? | Not a weed of agriculture. | L | M |
24. Disease host/vector? | Weed not known to be an alternative host or vector for diseases of agriculture. | L | M |