Impact Assessment - Illyrian thistle (Onopordum illyricum) 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?	“Dense patches pose a physical barrier to humans.” The spiny nature of the plant is an annoyance to humans.	P & C (2001)	MH
2. Reduce tourism?	“…occurs on neglected sites and pastures of reasonable fertility.” Not a significant weed of recreational areas. However, where it does occur in dense patches, some recreation activities may be affected	P & C (2001)	ML
3. Injurious to people?	Dense spines are present for much of the year.	P & C (2001)	MH
4. Damage to cultural sites?	An erect herb to 2 m high; Infestations uncommon, but plants can be locally abundant at infested sites; dense infestations would have a moderate negative visual impact.	P & C (2001) CDFA1	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?	“Even when plants die, they usually remain standing for a long period of time and, by keeping the area bare of other vegetation, can maintain a niche for recruitment of the next generation.” Not an extensive root system; bare patches of soil may be subjected to erosion.	Sindel (1997)2	ML
8. Reduce biomass?	An annual that grows best on fertile soils. Direct replacement of biomass.	P & C (2001)	ML
9. Change fire regime?	Stems can persist into the next season with spiny phyllaries and receptacles attached. In dense patches, this dry matter may introduce a minor change to the frequency of fire risk.	CDFA	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (E); CMA=Port Phillip; Bioreg=Central Victorian Uplands; VH CLIMATE potential. “It grows best on high-nitrogen soils…rather than unimproved native vegetation.” Would have a minor effect on grasses/forbs.	P & C (2001)	ML
(b) medium value EVC	EVC=Heathy woodland (D); CMA=Corangamite; Bioreg=Central Victorian Uplands; VH CLIMATE potential. “It grows best on high-nitrogen soils…rather than unimproved native vegetation.” Would have a minor effect on grasses/forbs.	P & C (2001)	ML
(c) low value EVC	EVC=Heathy woodland (D); CMA=Corangamite; Bioreg=Otway Plain; H CLIMATE potential. “It grows best on high-nitrogen soils…rather than unimproved native vegetation.” Would have a minor effect on grasses/forbs.	P & C (2001)	ML
11. Impact on structure?	“[Thistles] will often grow above the pasture canopy and dominate the grazing landscape throughout much of the year” “…an effective invader and a strong competitor to establish in grassland communities.” However, “It grows best on high-nitrogen soils…rather than unimproved native vegetation.” Would have a minor effect on the lower stratum in natural ecosystems.	Sindel (1997) P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“Illyrian thistle…is not grazed by stock because of the dense spines.” Its presence in natural ecosystems would reduce forage for native fauna.	P & C (2001)	ML
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“If it is eaten, the spines cause mechanical damage, particularly around the mouths and eyes of animals.”	P & C (2001)	H
Pest Animal
17. Food source to pests?	No known as a food source to pests.		L
18. Provides harbour?	Not known to provide harbour.		L
Agriculture
19. Impact yield?	“Illyrian thistle is competitive in pastures and is not grazed by stock because of the dense spines.” “Plants can be locally abundant at infested sites.” Likely to reduce carrying capacity and have a major impact on the yield.	P & C (2001)	MH
20. Impact quality?	“Plants contribute to vegetable fault in wool.”	P & C (2001)	ML
21. Affect land value?	“Seeds below 4–5 cm may remain viable for many years.” “…an infestation of Onopordum illyricum was able to flare again after 6 years control during which time no seeds were allowed back into the soil seed bank.” With the necessity for ongoing control measures and the associated cost, the value of land may decrease.	Sindel (1997)	M
22. Change land use?	In pasture situations, some degree of control can be achieved by grazing goats and cattle. “Goats graze flowering plants…and also eat smaller plants and rosettes.” No serious impact on agricultural return.	P & C (2001)	M
23. Increase harvest costs?	Not a weed of cropping.		L
24. Disease host/vector?	None evident		L