Impact Assessment - Spear thistle (Cirsium vulgare) 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 perennial herb, commonly 15 to 35 cm high.” Its presence would not restrict human access.	P & C (2001)	L
2. Reduce tourism?	Dense infestations are obvious, particularly during the flowering stage when areas are covered by a dense carpet of yellow flowers. Recreational activities not affected, but aesthetic of site would be.	P & C (2001)	ML
3. Injurious to people?	Although the plant is considered toxic to animals (it contains oxalic acid), “its flowering stems are occasionally chewed and sucked by children for the sour taste.” Toxicity is a problem of prolonged consumption. Consider mildly toxic.	Blood (2001)	ML
4. Damage to cultural sites?	Dense infestations would create a moderate 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?	“During winter an extensive root system, consisting of several branched fleshy storage roots, develops. Rosettes grow rapidly through spring, reaching a diameter of about 60 cm. Normally, rosettes persist through summer and continue growth into the second autumn and winter.” Not likely to contribute to soil erosion.	DPIWE (2002)1	L
8. Reduce biomass?	“As a weed it occurs…particularly [in] annual pastures and neglected areas.” Dense infestations may increase biomass.	P & C (2001)	L
9. Change fire regime?	“Dead plants often remain standing for one or two years.” In dense infestations this dry matter may lead to an increase in the frequency of fire risk.	P & C (2001)	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Dry valley forest (V); CMA=West Gippsland; Bioreg=Highlands – Southern Falls; VH CLIMATE potential. Although it grows best on exposed sites, in Victoria, it is known to occur in a wide range of vegetation communities including woodlands and forests. It occurs in medium to large populations. The rosettes are large suppressing germination of other species. Carr et al (1992) consider C. vulgare to be a serious threat to one or more vegetation formations in Victoria. Minor displacement of species in the lower stratum.	P & C (2001) Carr et al (1992)	ML
(b) medium value EVC	EVC=Grassy woodland (D); CMA=East Gippsland; Bioreg=East Gippsland Uplands; H to VH CLIMATE potential. Impact as in 10(a) above.	P & C (2001) Carr et al (1992)	ML
(c) low value EVC	EVC=Montane dry woodland (LC); CMA=East Gippsland; East Gippsland Uplands; VH CLIMATE potential. Impact as in 10(a) above.	P & C (2001) Carr et al (1992)	ML
11. Impact on structure?	In Victoria, Cirsium vulgare invades a wide range of vegetation communities including woodlands and forests, and occurs in medium to large populations. Likely to have a major impact on 20 - 60% of the floral strata.	Carr et al (1992)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	C. vulgare occurs in medium to large populations in a great number of vegetative communities. “Dense patches of spear thistle are common in much of southern Australia. The spiny nature of the plant deters animals from grazing” Infestations are likely to have minor reduction on habitat for fauna species.	Carr et al (1992) P & C (2001)	ML
15. Benefits fauna?	No benefits to fauna species.		H
16. Injurious to fauna?	No evidence of injury to fauna, however, animals avoid the plant due to its spiny nature, and dead plants can remain standing for one to two years. Potential to harm fauna species.	P & C (2001)	M
Pest Animal
17. Food source to pests?	Not documented as a food source to pests. However, the “fleshy roots of spear thistle have been prized in the past as a bait for rabbit poisoning,” and rabbits are known to use the plants for harbor. Rabbits may eat the roots of growing plants. Wild pigs love em	P & C (2001) Wood pers comm	MH
18. Provides harbour?	“…thick patches of the weed provide effective harbour for rabbits.” But only temporarily	P & C (2001)	MH
Agriculture
19. Impact yield?	“A large spear thistle rosette covers about one-third of a square metre and, because it is not readily grazed by stock, the carrying capacity of paddocks with dense patches of thistle is reduced considerably.”	P & C (2001)	MH
20. Impact quality?	“…contaminated hay is downgraded in quality and price. The plant is an important component of vegetable fault of wool.”	P & C (2001)	ML
21. Affect land value?	“Spear thistle is so well established in much of southern Australia this it is accepted as a permanent part of the vegetation.” As such, its presence is unlikely to affect land values greatly.	P & C (2001)	L
22. Change land use?	“The level of spear thistle infestations varies from year to year which reflects the changing opportunities for seedling establishment provided by climate and grazing pressure in the autumn.” Grazing activities may change somewhat to ensure minimal infestations to limit impact on agricultural return.	P & C (2001)	M
23. Increase harvest costs?	Shearing costs may increase as, “spines in wool also cause difficulties in shearing.”	P & C (2001)	M
24. Disease host/vector?	“There is veterinary evidence that the spiny leaves and bracts are responsible for transmitting virus diseases, including myxomatosis and scabby mouth, between animals.”	P & C (2001)	M