Impact Assessment - Variegated thistle (Silybum marianum) 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 or biennial herb, commonly 90 to 180 cm high. Stem leaves spiny, stiff and often reflexed; flower heads surrounded by large stiff reflexed bracts ending in sharp spines. Variegated thistle is very competitive in pastures and, when well established, eliminates most other plants.” The size of the plant, its spiny nature and its possible population density would be highly annoying to humans.	P & C (2001)	MH
2. Reduce tourism?	In dense patches, some recreational activities may be affected.		MH
3. Injurious to people?	“The spines in the leaves, stems and flower bracts can injure the delicate parts of animals and provide sites for infection.” Potentially harmful to humans; spines present for most of the year.	P & C (2001)	MH
4. Damage to cultural sites?	Dense infestations may create a negative visual impact.		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?	“Clumps of variegated thistle leave the soil bare at the end of summer.” Moderate probability of soil erosion.	P & C (2001)	ML
8. Reduce biomass?	“A weed of cultivated land, roadsides, neglected areas and, most importantly, grazing land.” Invader generally replaces biomass.	P & C (2001)	ML
9. Change fire regime?	“Clumps of variegated thistle leave the soil bare at the end of summer.” Little fuel remains to establish or support fire. No change to fire regime.	P & C (2001)	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (E); CMA=Port Phillip; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. Is very competitive in open situations such as pasture, roadsides and neglected areas. When well established it eliminates most other vegetation. Major displacement of grasses/forbs.	P & C (2001)	MH
(b) medium value EVC	EVC=Lowland forest (D); CMA=Corangamite; Bioreg=Otway Plain; VH CLIMATE potential. Impact similar to 10(a) above, though forest canopy may restrict population density.	P & C (2001)	MH
(c) low value EVC	EVC=Lowland forest (LC); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. Impact similar to 10(b) above.	P & C (2001)	MH
11. Impact on structure?	“Variegated thistle is very competitive in pastures and, when well established, eliminates most other plants.” Occurs in open situations. Likely to have a major impact on grasses and forbs.	P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“Variegated thistle is very competitive in pastures and, when well established, eliminates most other plants.” Potential to significantly decrease useful fodder for fauna species.	P & C (2001)	ML
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“The spines in the leaves, stems and flower bracts can injure the delicate parts of animals and provide sites for infection.” Similar threat to fauna species.	P & C (2001)	MH
Pest Animal
17. Food source to pests?	Not known as a food source to pest animals.		L
18. Provides harbor?	“Seedlings prefer disturbed soil on which to establish so sheep camps, rabbit warrens and cultivated fire breaks are suitable sites. Dense clumps of variegated thistle provide harbour for vermin animals, particularly rabbits.” Capacity to provide permanent harbor	P & C (2001)	H
Agriculture
19. Impact yield?	“Variegated thistle is very competitive in pastures and, when well established, eliminates most other plants.” Level of impact not documented. Assume serious impact on carrying capacity.	P & C (2001)	H
20. Impact quality?	Not a serious weed of cropping. “Contamination of hay is also important [as a means of seed dispersal] because plants are flowering when hay is cut.” Possible major impact on hay quality.	P & C (2001)	MH
21. Affect land value?	“In Victoria, an estimated 4.8 million hectares is infested.” Like Cirsium vulgare (spear thistle), which infests 9.7 million hectares in Victoria, Sylibum marianum, may be, “so well established in much of southern Australia that it is accepted as a permanent part of the vegetation.” Little impact on land value.	P & C (2001) (see entry for Cirsium vulgare p209	L
22. Change land use?	Predominantly a weed of cultivated land on fertile soils, control can be effected by, “encouraging pasture species able to utilise the raised fertility and compete with the weed species.” Little change to land use.	P & C (2001)	L
23. Increase harvest costs?	Not known to affect harvest costs.		L
24. Disease host/vector?	None evident.		L

Impact Assessment Record - Variegated thistle (PDF - 32KB)
Impact Assessment Record - Variegated thistle (DOC - 60KB)
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