Impact Assessment - Star thistle (Centaurea calcitrapa) 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 bushy annual or biennial herb, commonly 40 to 80 cm high. Flower heads…surrounded by numerous bracts each ending in a rigid, sharp, white or yellowish spine 1.5. to 3 cm long and at the base of which are 2 to 6 shorter spines.” It is found in medium to large populations in lowland grassland & grassy woodland and dry sclerophyll forest & woodland. The spiny nature of the plant may hinder individual access.	P & C (2001) Carr et al (1992)	ML
2. Reduce tourism?	The spiny nature of the plant may restrict some recreational activities.		MH
3. Injurious to people?	“…once the heads are formed, their spines injure grazing animals.” Potential to injure humans. Spiny heads present for much of the year.	Carr et al (1992)	MH
4. Damage to cultural sites?	Large populations 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?	“…is most commonly confined to roadsides, channel banks and neglected areas.” Not likely to increase soil erosion.	P & C (2001)	L
8. Reduce biomass?	“…is most commonly confined to roadsides, channel banks and neglected areas.” Biomass may increase in those areas where it is commonly found.		L
9. Change fire regime?	Plants die in autumn. However, there is no information regarding what hard matter is left that may contribute to fire. Assume little change.		L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassy woodland (E); CMA=North Central; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. In Victoria, it is present in medium to large populations. Potential to displace species in lower stratum. Occurs in open, unshaded situations. Overstorey cover may restrict population density.	Carr et al (1992) P & C (2001)	MH
(b) medium value EVC	EVC=Box Ironbark forest (D); CMA=North Central; Bioreg=Goldfields; VH CLIMATE potential. Impact as in 10(a) above.	Carr et al (1992) P & C (2001)	MH
(c) low value EVC	EVC=Lowland forest (LC); CMA=Corangamite; Bioreg=Victorian Volcanic Plain; H CLIMATE potential. Impact similar to 10(a) above, however, CLIMATE distribution potential is HIGH only.	Carr et al (1992) P & C (2001)	ML
11. Impact on structure?	“It is confined to roadsides, channel banks and neglected areas.” Occurs in open, unshaded situations. In Victoria, it is present in medium to large populations. In natural ecosystems likely to have a minor effect on species in the lower stratum.	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?	“…is most commonly confined to roadsides, channel banks and neglected areas.” Minimal impact on fauna species.	P & C (2001)	L
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“…once the heads are formed, their spines injure grazing animals.” Potential to injure fauna species where they co-occur.	P & C (2001)	ML
Pest Animal
17. Food source to pests?	Not known as a food source to pests.		L
18. Provides harbor?	“The spines also make clumps of the plant an effective harbour for rabbits.”	P & C (2001)	MH
Agriculture
19. Impact yield?	“It occurs but is not important …in the Victorian Wimmera and southern Mallee. Star thistle competes with crops and pastures.” Level of competition not documented; assume minimal"	P & C (2001)	ML
20. Impact quality?	“Seeds…are spread to some extent by contaminating agricultural produce.” Minor impact on quality.	P & C (2001)	ML
21. Affect land value?	“It occurs but is not important …in the Victorian Wimmera and southern Mallee.” Unlikely to affect land value.	P & C (2001)	L
22. Change land use?	“It occurs but is not important …in the Victorian Wimmera and southern Mallee.” Not a serious weed of agriculture. Change in land use not required.	P & C (2001)	L
23. Increase harvest costs?	Not known to affect harvest costs.		L
24. Disease host/vector?	None evident.		L