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Impact Assessment - Dense waterweed (Egeria densa) in Victoria

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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.

The following table provides information on the impact of Dense waterweed.

A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:

Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1 MB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

Common Name: Dense waterweed
Scientific name: Egeria densa

Question
Comments
Rating
Confidence
Recreation
1. Restrict human access?Can interfere with boating, had could cause drowning due to entanglement (Champion 1995). Therefore a high nuisance impeding people and vehicles.
MH
MH
2. Reduce tourism?Can interfere with activities such as fishing, swimming and boating (Parsons & Cuthbertson 2001). However evidence of major impact and visitor complaints not reported.
MH
MH
3. Injurious to people?Poses the risk of drowning, due to entanglement (Champion 1995). This risk would be greatest at the highest biomass levels reported in February to March, when the species forms a thick mat of intertwining stems below the surface (Parsons & Cuthbertson 2001).
MH
MH
4. Damage to cultural sites?Can interfere with infrastructure, including, irrigation works, hydro-electric outputs and water supplies (Parsons & Cuthbertson 2001).
MH
MH
Abiotic
5. Impact flow?An attached submerged species preferring slow moving water, reported to seriously retard flow (Parsons & Cuthbertson 2001).
MH
MH
6. Impact water quality?Can help to stabilise shallow systems, by reducing wave action and stabilising sediment. However, when the species builds up to high biomass levels, the system can collapse. Benthic anoxia leads to death of large proportions of the egeria. This can cause the wetland to flip and become turbid and dominated by phytoplankton (Champion 2002).
H
M
7. Increase soil erosion?Can act as a semi-permeable dam, slowing flow and increasing stream depth (Champion & Tanner 2000). However during times of high flow, can cause flooding and therefore increase the chance of erosion (DiTomaso & Healy 2003).
ML
MH
8. Reduce biomass?This species would either be a direct replacement or potentially increases biomass, as can form dense monospecfic stands to depths of 5 m (Coffey & Clayton 1988).
ML
MH
9. Change fire regime?Aquatic species; not present in vegetation exposed to fire.
L
H
Community Habitat
10. Impact on composition
(a) high value EVC
EVC= Shallow Freshwater Marsh (E); CMA= Corangamite; Bioreg= Otway Plain; VH CLIMATE potential.
Can form monospecific stands (Coffey & Clayton 1988). Reported displacing native submerged macrophytes (Roberts, Church & Cummins 1999)
H
MH
(b) medium value EVC
Aquatic species. All Victorian water bodies considered to comprise high value EVCs only (Weiss pers. com).
L
MH
(c) low value EVC
Aquatic species. All Victorian water bodies considered to comprise high value EVCs only (Weiss pers. com).
L
MH
11. Impact on structure?Can form monospecific stands (Coffey & Clayton 1988). Reported displacing native submerged macrophytes (Roberts, Church & Cummins 1999)
H
MH
12. Effect on threatened flora?Has been reported to displace native submerged macrophytes (Roberts, Church & Cummins 1999). However no specific information on threatened species.
MH
M
Fauna
13. Effect on threatened fauna?Reported to harbour different fish assemblages than those of native submerged macrophytes (Growns et al 2003). However no specific information on impact on threatened species.
MH
M
14. Effect on non-threatened fauna?Reported to harbour different fish assemblages than those of native submerged macrophytes (Growns et al 2003). Therefore decrease in populations of some species with an increase in population of other species.
MH
MH
15. Benefits fauna?Provides a major food source for black swan (Cygnus atratus) (Champion 2002). Fish assemblages and invertebrate species are reported living amongst it, and can act as protection from predators (Duggan et al 2001; Growns et al 2003). Therefore it provides an abundant food source for at least one species and it creates a habitat and shelters various species. However unknown specifically for native species.
ML
MH
16. Injurious to fauna?No evidence of this reported. However it is reported to have potential to drown people through entanglement, it may therefore be able to drown animal species.
M
L
Pest Animal
17. Food source to pests?There are exotic fish species reported to eat significant quantities of the species that they could be used for biocontrol measures, However they are not reported in Australia (Parsons & Cuthbertson 2001).
L
MH
18. Provides harbor?Provides habitat for a fish assemblage (Growns et al 2003). Therefore could provide harbour for exotic fish species.
ML
M
Agriculture
19. Impact yield?Aquatic species, no evidence of this reported.
L
M
20. Impact quality?Aquatic species, no evidence of this reported.
L
M
21. Affect land value?Aquatic species, no evidence of this reported.
L
M
22. Change land use?Aquatic species, no evidence of this reported.
L
M
23. Increase harvest costs?Can interfere with irrigation activities and block pumps, therefore increased maintenance costs (Parsons & Cuthbertson 2001).
M
MH
24. Disease host/vector?Aquatic species, no evidence of this reported.
L
M

This table can also be viewed as a PDF document (printer friendly).

Impact Assessment Record - Dense waterweed (PDF - 82KB)
Impact Assessment Record - Dense waterweed (DOC - 58KB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

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