Impact Assessment - Golden thistle (Scolymus hispanicus) 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, much-branched perennial herb to 90 cm high. It is armed with stout spines on rigid leaves and dense infestations become almost impenetrable to grazing stock. Humans too, would also have difficulty in walking through an infestation; the spines would be a significant nuisance.	P & C (2001)	ML
2. Reduce tourism?	Infestations are known to occur along riverbanks and dense infestations may restrict access to water-based recreational activities.	P & C (2001)	ML
3. Injurious to people?	The stems can remain on the plant after the aerial parts die off in autumn. Spines would be present for much of the year.	P & C (2001)	MH
4. Damage to cultural sites?	The root system is not vigorous and would not affect the structure. Dense patches would have a moderate impact on the visual aesthetic 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?	Although the plant has a robust taproot the aerial parts of the plant die back in summer leaving areas of soil exposed. Moderate probability of large-scale soil movement.	P & C (2001)	ML
8. Reduce biomass?	Not documented as dominating species, it replaces existing biomass.		ML
9. Change fire regime?	Dry flower stems break off and are carried by the wind to collect against any obstruction. Large collections of this dry matter may create a minor change in the frequency of fire risk.	P & C (2001)	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Creekline grassy woodland (E); CMA=North Central; Bioreg=Victorian Riverina; VH CLIMATE potential. “…much of the present Victorian infestation is along the Loddon River downstream [of Smeaton].” New rosettes can grow to 60 cm in diameter, which would restrict growth of other plants. Not recorded in Carr et al (1992). Minor impact in grassland/riparian situations.	P & C (2001) Carr et al (1992)	ML
(b) medium value EVC	EVC=Grassy dry forest (E); CMA=North Central; Bioreg=Goldfields; VH CLIMATE potential. Impact similar to 10(a) above, however, growth under open forest canopy may restrict population density.	P & C (2001) Carr et al (1992)	ML
(c) low value EVC	EVC=Heathy dry forest (E); CMA=North Central; Bioreg=Goldfields; VH CLIMATE potential. Impact as in 10(b) above.	P & C (2001) Carr et al (1992)	ML
11. Impact on structure?	Does not appear to significantly affect the structure of an invaded site. Where it does occur in pastures, it is weedy because animals will not graze the plant. Clumps may prevent the regrowth of desirable forage species.	P & C (2001)	ML
12. Effect on threatened flora?12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	Grazing animals do not eat the aerial growth because of the stout spines on the rigid leaves, and the spiny nature of the plant discourages animals from grazing near it. Likely to have similar effect on native fauna. Some reduction in habitat.	P & C (2001)	ML
15. Benefits fauna?	No documented benefits.		H
16. Injurious to fauna?	The stout spines present for much of the year may injure animals.	P & C (2001)	MH
Pest Animal
17. Food source to pests?	Not documented.		L
18. Provides harbor?	“Clumps of golden thistle provide effective harbor for rabbits.”	P & C (2001)	H
Agriculture
19. Impact yield?	Stock do not eat the aerial growth and, “avoid grazing close to plants [thus] valuable pasture is neglected.” Dense infestations become almost impenetrable and are likely to reduce carrying capacity.	P & C (2001)	MH
20. Impact quality?	It occurs in vineyards and orchards in part of its native range and does not appear to impact on the quality of harvest.	P & C (2001)	L
21. Affect land value?	The plant is effectively controlled by mechanical or chemical methods. Until the plant is controlled, grazing areas may be of limited use. May reduce land value slightly	P & C (2001)	M
22. Change land use?	Left untreated, the plant would restrict grazing on permanent pasture. It can be controlled by cultivation, so land may have to be used for cropping activity.		M
23. Increase harvest costs?	No know effect on harvesting costs. Assume limited impact.		L
24. Disease host/vector?	None evident.		L

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