Impact Assessment - Stemless thistle (Onopordum acaulon) 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?	“A prostrate annual herb.” Would not hinder human access.	P & C (2001)	L
2. Reduce tourism?	“Leaves [are] very woolly, deeply lobed, many spines on margins. Each flower head is surrounded by numerous sharply spined bracts.” Spiny nature of plant may restrict some recreational activities where it occurs on channel banks.	P & C (2001)	MH
3. Injurious to people?	The sharp spines may cause minor injuries to humans. Spines present for much of the year.		MH
4. Damage to cultural sites?	Dense patches may create a negative visual effect.		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 commonly occurs in volunteer pastures, and on fallows, roadsides and channel banks.” Unlikely to increase soil erosion.	P & C (2001)	L
8. Reduce biomass?	“It commonly occurs in volunteer pastures, and on fallows, roadsides and channel banks.” Biomass replaced by invader.	P & C (2001)	ML
9. Change fire regime?	A prostrate annual. Would not affect the fire regime.	P & C (2001)	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassy woodland (E); CMA=North Central; Bioreg=Wimmera; VH CLIMATE potential. Large rosettes (up to 60 cm dia.). Can shade other species preventing germination. Major displacement of ground cover species and annual grasses. Currently occurs in medium to large populations.	APA & PCB Carr et al (1992)	MH
(b) medium value EVC	Does not appear likely that O. acaulon would occur in any medium value EVCs in Victoria.		L
(c) low value EVC	Does not appear likely that O. acaulon would occur in any low value EVCs in Victoria.		L
11. Impact on structure?	In Victoria, it occurs in medium to large populations. Major effect on ground covers/grasses.	Carr et al (1992)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	According to Carr et al (1992), the plant is widespread in Victoria occurring in medium to large populations in mallee shrubland and lowland grassland & grassy woodland. As stock do not graze the plant, its presence may reduce the amount of available fodder for fauna spp.	Carr et al (1992)	ML
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“When stock have been forced to eat the plant cases of impaction and suspected liver damage have occurred.” Potential risk 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?	A prostrate annual, it would not provide harbor.		L
Agriculture
19. Impact yield?	“Stemless thistle is rarely eaten by stock…and, therefore, the carrying capacity of heavily infested paddocks is reduced.” Level of impact not quantified, but as the weed is distributed widely in medium to large populations, assume at least a major impact on yield.	P & C (2001) Carr et al (1992)	MH
20. Impact quality?	Not known to affect the quality of produce.		L
21. Affect land value?	“Seeds survive in soil for several years making eradication difficult.” Land value may be slightly reduced in pastoral areas.	P & C (2001)	M
22. Change land use?	“It is not known how demanding stemless thistle is for soil nutrients and what effect heavy patches may have on subsequent crops. Lucerne competes well with stemless thistle and should be established wherever practical.” No data to conclude a change in land use is required.	P & C (2001)	L
23. Increase harvest costs?	Not known to affect harvesting costs.		L
24. Disease host/vector?	None evident.		L