Impact Assessment - Stinkwort (Dittrichia graveolens) 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, strongly aromatic herb, commonly 30 to 60 cm high.” Does not present a physical barrier. However, because of the strong odour produced by the plant, it may be a minor nuisance to humans.	P & C (2001)	ML
2. Reduce tourism?	“Because of the aromatic oil produced by glandular hairs on most parts of the plant, stinkwort is one of the strongest smelling of all weeds.” Because of the strong odour, some recreational activities may be affected.	P & C (2001)	MH
3. Injurious to people?	“…some people are allergic to the oil and develop severe dermatitis.” Toxic properties are present for most of the year.	P & C (2001)	MH
4. Damage to cultural sites?	During summer, 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?	“In summer, infested paddocks appear to be carrying a dense growth of lush green fodder.” This suggests that plant density is high. As it is a summer annual with a taproot and numerous laterals, it is unlikely to increase soil erosion.	P & C (2001)	L
8. Reduce biomass?	“In summer, infested paddocks appear to be carrying a dense growth of lush green fodder.” Biomass may increase slightly.	P & C (2001)	L
9. Change fire regime?	No data available on changes to fire risk; assume to be no change.		L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (E); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. Prefers open, unshaded areas. Occurs in medium to large populations. Major displacement of annual grasses/forbs.	P & C (2001) Carr et al (1992)	MH
(b) medium value EVC	EVC=Coastal dune scrub (D); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. Impact as in 10(a) above.	P & C (2001) Carr et al (1992)	MH
(c) low value EVC	EVC=Lowland forest (LC); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential. Impact similar to 10(a) above, however, population density limited due to overstorey cover.	P & C (2001) Carr et al (1992)	ML
11. Impact on structure?	In Victoria, it is widely distributed in medium to large populations in dry coastal vegetation, mallee shrubland, lowland grassland & grassy woodland, and dry sclerophyll forest & woodland. “In summer, infested paddocks appear to be carrying a dense growth of lush green fodder.” Potential to affect ground covers/grasses seriously.	Carr et al (1992) P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	In Victoria, it is widely distributed in medium to large populations in dry coastal vegetation, mallee shrubland, lowland grassland & grassy woodland, and dry sclerophyll forest & woodland. “…grazing animals find it disagreeable, eating the plant only when it is very young.” Likely to have a minor effect reducing fodder for fauna species.	Carr et al (1992) P & C (2001)	ML
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“Sheep eat the flower heads at times and serious losses have been attributed to the plant.” It occurs in a broad range of vegetation communities in Victoria; potential to harm 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?	Not known to provide harbor.		L
Agriculture
19. Impact yield?	“Sheep eat the flower heads at times and serious losses have been attributed to the plant.” Serious impact on quantity.	P & C (2001)	H
20. Impact quality?	“The oil also taints meat and milk of animals forced to graze the plant.” Minor impact.	P & C (2001)	ML
21. Affect land value?	“…it is no longer an important agricultural weed because of a general increase in soil fertility.” Unlikely to affect land prices.	P & C (2001)	L
22. Change land use?	“…it is no longer an important agricultural weed because of a general increase in soil fertility.” Change in land use is not required.	P & C (2001)	L
23. Increase harvest costs?	Not known to affect harvest costs.		L
24. Disease host/vector?	None evident.		L