Impact Assessment - Soldier thistle (Cirsium acarna) 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?	“An erect annual herb, 50 to 100 cm high. Stems much branched with spiny wings. Stem leaves long and narrow with long (1 to 1.5 cm) yellow spines, margins with short spines. Many infestations are sparse.” The spiny nature of the plant would be somewhat of a nuisance to humans, however, as it occurs in sparse populations it would not restrict human access.	P & C (2001)	ML
2. Reduce tourism?	Because of the spiny nature of the plant it may have a minor impact on some recreational activities.		ML
3. Injurious to people?	“Its sharp spines injure sheep and dogs.” Potential to injure humans as well. Spines present for much of the year.	P & C (2001)	MH
4. Damage to cultural sites?	Although infestations are sparse, it does occur, “as a weed of roadsides, channel banks and neglected areas.” Presence of the plant would be noticeable and possibly create a negative visual impact.	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?	“It occurs as a weed of roadsides, channel banks, neglected areas and occasionally cereal crops and pastures. Many infestations are sparse.” Not likely to contribute to soil erosion.	P & C (2001)	L
8. Reduce biomass?	“It occurs as a weed of roadsides, channel banks, neglected areas and occasionally cereal crops and pastures. Many infestations are sparse.” Biomass replaced by invader.	P & C (2001)	ML
9. Change fire regime?	“Many infestations are sparse.” Not likely to affect the fire regime.	P & C (2001)	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (E); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. “Weed of roadsides, channel banks and neglected areas.” Competes with annual species. Not known as a weed in natural ecosystems. Seeds spread by wind. Limited impact on grasses/forbs.	P & C (2001) Carr et al (1992)	ML
(b) medium value EVC	EVC=Riverine grassy woodland (D); CMA=Goulburn Broken; Bioreg=Murray Fans; VH CLIMATE potential. Impact as in 10(a) above. Infestations may be restricted due to overstorey cover.	P & C (2001) Carr et al (1992)	ML
(c) low value EVC	EVC=Heathy woodland (LC); CMA=Glenelg Hopkins; Bioreg=Dundas Tablelands; VH CLIMATE potential. Impact as in 10(b) above.	P & C (2001) Carr et al (1992)	ML
11. Impact on structure?	In some areas of the upper southeast and Murray Mallee region of South Australia, “there are paddock-size patches.” Not known as a weed in natural ecosystems. Minor effect on the floral strata.	P & C (2001) Carr et al (1992)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“It occurs as a weed of roadsides, channel banks, neglected areas and occasionally cereal crops and pastures.” Due to its spiny nature, animals avoid grazing the plant. Minor reduction in food source for fauna species.	P & C (2001)	ML
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“Its sharp spines injure sheep and dogs.” Potential to harm fauna species. Spines present for much of the year.	P & C (2001)	MH
Pest Animal
17. Food source to pests?	Not known to be a food source to pest animals.		L
18. Provides harbour?	Not known to provide harbour. A annual herb and, “many infestations are sparse.”	P & C (2001)	L
Agriculture
19. Impact yield?	“…it was troublesome in the Victorian Mallee and Wimmera in the 1930s and 1940s but is now much less important because of effective control measures and increased soil fertility. When well established, soldier thistle competes with cereal crops…and because it is avoided by stock, the available grazing area is much reduced.” Possibly reduces yield by <20%.	P & C (2001)	MH
20. Impact quality?	Not known to affect the quality of harvest.		L
21. Affect land value?	“Because soldier thistle is an annual, cultivation before flowering gives effective control. Many infestations are sparse.” Not likely to affect the value of land.	P & C (2001)	L
22. Change land use?	“Because soldier thistle is an annual, cultivation before flowering gives effective control. Many infestations are sparse.” Current land use not likely to be affected.	P & C (2001)	L
23. Increase harvest costs?	“When well established, soldier thistle…can choke harvesting machinery.” Increase in both time and labor to harvest produce.	P & C (2001)	M
24. Disease host/vector?	None evident.		L