Impact Assessment - Parthenium weed (Parthenium hysterophorus) 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 30 cm to 150 cm high. “…an aggressive coloniser of fallows, wasteland, roadsides, and overgrazed pastures.” Dense infestations may impede individual access, though not seriously.	P & C (2001)	ML
2. Reduce tourism?	Weeds would be obvious, but recreational activities would not be greatly affected.		ML
3. Injurious to people?	“Regular contact with the plant, usually over a prolonged period, produces an acute form of dermatitis, and asthma in humans.” Potentially harmful for most of the year.	P & C (2001)	MH
4. Damage to cultural sites?	Dense patches would 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?	“Parthenium weed becomes established in disturbed, degraded or bare soil.” Vigorous growth (“In good conditions Parthenium weed can germinate, flower and set seed within four weeks.”) would provide some protection against erosion.	Trounce (1998)1	L
8. Reduce biomass?	“Parthenium weed becomes established in disturbed, degraded or bare soil.” Likely to increase biomass.	Trounce (1998)	L
9. Change fire regime?	Although the woody stems remain after the plant dies, there would be insufficient fuel to support fire. Low risk of fire.	Bob Trounce pers. comms2	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (); CMA=North Central; Bioreg=Victorian Riverina; H to VH CLIMATE potential. It is not a problem in undisturbed vegetation. However, native pastures may not be able to compete with parthenium weed, and dense infestations can eliminate most other species.	Trounce (1998) P & C (2001)	MH
(b) medium value EVC	Does not appear likely to invade medium value EVCs in Victoria.		L
(c) low value EVC	Does not appear likely to invade low value EVCs in Victoria.		L
11. Impact on structure?	“Parthenium weed is a vigorous species that colonises weak pastures with sparse ground cover.” “Most seeds germinate at the one time, producing a mat of seedlings. The young rosettes, with their leaves closely appressed to the soil prevent the emergence of other species leading to pure stands of parthenium weed.” Open situations only; affects grasses and forbs.	NRM3 P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	As an aggressive coloniser it would impact on the food source of native species.	P & C (2001)	ML
15. Benefits fauna?	No benefits		H
16. Injurious to fauna?	Causes contact dermatitis and asthma in humans. “Cattle may also be affected, and more importantly, pass the toxic principle into their milk.” Potential also to be harmful to fauna.	P & C (2001)	ML
Pest Animal
17. Food source to pests?	No evidence of pest species using the plant as a food source.		L
18. Provides harbour?	Not known to provide harbour.		L
Agriculture
19. Impact yield?	“The weed is a menace to agriculture because it competes with pastures and reduces carrying capacity.” “Some grain sorghum crops are infested with sufficient parthenium weed plants to suppress yield.” Because of its aggressive nature impact on yield likely to be of major significance.	Trounce (1998) P & C (2001)	MH
20. Impact quality?	Meat from animals who graze the plant is badly tainted, “and unfit for the table unless the animals are grazed on parthenium-free paddocks for at least a month prior to slaughter.” Major impact on quality.	P & C (2001)	MH
21. Affect land value?	“If allowed to spread, the weed will become a serious and persistent problem in NSW, resulting in a downgrading of property values.”	Trounce (1998)	H
22. Change land use?	“Parthenium weed will become dominant if pastures are overgrazed and not managed properly. Carrying capacity will be reduced and the area will cease to be economically viable.” This could lead to a serious loss of income.	Trounce (1998)	H
23. Increase harvest costs?	Not know to affect harvesting activities.		L
24. Disease host/vector?	None evident.		L