Impact Assessment - Scotch thistle (Onopordum acanthium) in Victoria (Nox)

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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.

Question	Comments	Reference	Rating
Recreation
1. Restrict human access?	Erect annual or biennial herb commonly 1 to 1.2 metres. Comprises one main stem with numerous branches and broad spiny wings. Leaves also spiny. Dense patches would impede individual access, and the spiny nature of the plant would be highly annoying.	P & C (2001)	MH
2. Reduce tourism?	Because of the spiny nature of the plant, dense patches occurring in recreational areas would restrict human activities.		MH
3. Injurious to people?	Spines can be present for most of the year. “Dead plants remain standing for many months.”	P & C (2001)	MH
4. Damage to cultural sites?	Roots comprise of a stout taproot; unlikely to affect structures. Dense patches would create a moderate negative visual effect particularly with dead plants remaining, seriously affecting the aesthetics of an area.	P & C (2001)	ML
Abiotic
5. Impact flow?	Terrestrial species.	P & C (2001)	L
6. Impact water quality?	Terrestrial species	P & C (2001)	L
7. Increase soil erosion?	“Seeds germinate at any time of year, hence infestations consist of plants of various ages and sizes.” Root system is a stout taproot. Not likely to affect soil erosion.	P & C (2001)	L
8. Reduce biomass?	“It occurs as a competitive weed of Australian pastures.” Plant replaces biomass.	P & C (2001)	ML
9. Change fire regime?	“Dead plants remain standing for many months.” Dense infestations may create a minor change to the frequency of fire risk.	P & C (2001)	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Grassy woodland (E); CMA=Port Phillip; Bioreg=Central Victorian Uplands; VH CLIMATE potential. Grows best on moderate to high fertility soils rather than unimproved native vegetation. Competes with pasture species. Likely to have a minor impact on grasses/forbs in natural ecosystems. Impact also limited by overstorey cover.	P & C (2001)	ML
(b) medium value EVC	EVC=Riverine grassy woodland (E); CMA=Goulburn Broken; Bioreg=Murray Fans; VH CLIMATE potential. Impact as in 10(a) above.	P & C (2001)	ML
(c) low value EVC	Does not appear likely to occur in any low value EVC in Victoria.		L
11. Impact on structure?	“…its relatively large leaves smother other plants. As it survives well into summer, there is little chance of the other species recovering.” However, it grows best on moderate to high fertility soils rather than unimproved native vegetation. Likely to have a minor effect on the floral strata.	P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“The weed is rejected by stock because of the dense spines.” Competitive nature and physical properties would reduce food source for native animals.	P & C (2001)	ML
15. Benefits fauna?	No known benefits to fauna.		H
16. Injurious to fauna?	“If it is eaten, the spines cause mechanical damage, particularly around the mouths and eyes of animals.” Spines present for much of the year.	P & C (2001)	MH
Pest Animal
17. Food source to pests?	Birds are known to eat the seed. Potential food source to pest birds.	P & C (2001)	ML
18. Provides harbour?	Not known to provide harbour.		L
Agriculture
19. Impact yield?	“Newly sown pastures are often overrun by a high germination of thistle seeds and, later, its relatively large leaves smother other plants. As it survives well into summer, there is little chance of the other species recovering.” Serious impact on forage production leading to reduced carrying capacity.	P & C (2001)	MH
20. Impact quality?	“…contributes to vegetable fault in wool.” Major impact on quality of produce.	P & C (2001)	MH
21. Affect land value?	No evidence of effect on land value. Reduction in forage production due to presence of the weed may reduce values slightly.		M
22. Change land use?	Goats are known to graze the flowering plants in summer and autumn. To control the plant while retaining some agricultural return, a change in land use may be dictated.		M
23. Increase harvest costs?	Not known to affect harvest costs.		L
24. Disease host/vector?	None evident.		L

Impact Assessment Record - Scotch thistle (PDF - 31KB)
Impact Assessment Record - Scotch thistle (DOC - 58KB)
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