Impact Assessment - Nodding thistle (Carduus nutans) 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 commonly 80 cm to 120 cm high. “Nodding thistle grows in dense patches (up to 10 mature plants per square metre).” High nuisance value, but unlikely to restrict access.	P & C (2001)	ML
2. Reduce tourism?	An erect annual herb commonly 80 cm to 120 cm high. “Nodding thistle grows in dense patches (up to 10 mature plants per square metre).” Some recreational activities such as bushwalking may be affected.	P & C (2001)	MH
3. Injurious to people?	“Dried fragments and spines may cause injury.” An annual, spines are present for much of the year.	Groves et al (1995)	MH
4. Damage to cultural sites?	Dense patches would create a negative visual effect and seriously affect the aesthetics of an area.		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 is one of the weeds which establishes well on bare ground occurring in pastures at the end of summer.” Dense growth habit provides soil cover when other species not present. Not likely to increase erosion.	P & C (2001)	L
8. Reduce biomass?	“It is one of the weeds which establishes well on bare ground occurring in pastures at the end of summer. Plants make strong autumn and winter growth, thereby competing at a time when pasture production is at its lowest.” Biomass may increase.	P & C (2001)	L
9. Change fire regime?	“…research on fire and musk thistle was limited by difficulties in locating a thistle-infested research site where fuel was sufficient to support a fire.” Dead material does not alter fire risk.	TNC1	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (E); CMA=Corangamite; Bioreg=Victorian Volcanic Plain; M CLIMATE potential. Occurs in open situations, but establishes best on bare ground. “…grows in dense patches (up to 10 mature plants per metre).” Medium potential for infestation. Minor impact on ground-flora.	P & C (2001)	ML
(b) medium value EVC	EVC=Grassy woodland (E); CMA=West Gippsland; Bioreg=Highlands – Southern Fall; M CLIMATE potential. Impact as in 10(a) above.	P & C (2001)	ML
(c) low value EVC	Does not appear likely to infest low value EVCs in Victoria.		L
11. Impact on structure?	“Little other biomass production [occurred] within one-third of the area occupied by rosettes…[and] a thistle plant has a detrimental effect on pasture growth beyond the perimeter of its rosette.” Dense patches would have a major impact on ground-flora, but not known as a weed in natural ecosystems (not recorded in Carr et al 1992). Limited impact.	Groves et al (1995)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“Nodding thistle grows in dense patches and is not readily grazed because of its spiny foliage. Its presence also discourages animals from grazing neighbouring pasture plants. Rosettes can occupy a considerable area of pasture.” The plant would reduce food source of native fauna.	P & C (2001)	ML
15. Benefits fauna?	No benefits.		H
16. Injurious to fauna?	“Dried fragments and spines may cause physical injury.”	Groves et al (1995)	MH
Pest Animal
17. Food source to pests?	Not known as a food source to pest animals.		L
18. Provides harbour?	“Dense patches also provide harbour for pest animals, particularly rabbits.”	P & C (2001)	MH
Agriculture
19. Impact yield?	Seriously reduces carrying capacity. “Calculated stands of 1000 plants m-2 reduced pasture production by 13–14%.”	P & C (2001) Groves et al (1995)	MH
20. Impact quality?	“At maturity, the spiny heads contribute to vegetable fault in wool.”	P & C (2001)	ML
21. Affect land value?	Control can be readily achieved by cultivation and establishing a “perennial pasture which provides ground cover in late summer to compete with thistle seedlings.” Cost of control may reduce the value of land slightly.	P & C (2001)	M
22. Change land use?	The plant can be controlled with suitable management techniques. Land use would not change.		L
23. Increase harvest costs?	Not a weed of cropping.		L
24. Disease host/vector?	None evident.		L