Impact Assessment - Perennial ragweed (Ambrosia psilostachya) 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?	Erect robust perennial 30 cm to 150 cm high, forming dense colonies. With plants at full height, dense growth may be a nuisance to pedestrians.	P & C (2001)	ML
2. Reduce tourism?	“…usually found as a weed along roadsides, in uncultivated fields, vacant lots and waste places.” Presence of the weed would be obvious but unlikely to restrict recreational activities.	P & C (2001)	ML
3. Injurious to people?	“Copious pollen from the small flower heads of perennial ragweed causes hay fever in many people.” Plants flower for about 3 months of the year.	P & C (2001)	ML
4. Damage to cultural sites?	Dense patches would seriously affect the aesthetics of an area. Moderate visual effect.	P & C (2001)	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?	The plant develops, “…an extensive, interwoven mass of creeping perennial roots in the upper 30 cm of soil, with occasional branches extending downwards to 1 metre.” Not likely to affect soil erosion.	P & C (2001)	L
8. Reduce biomass?	“…usually found as a weed along roadsides, in uncultivated fields, vacant lots and waste places.” Biomass likely to increase.	P & C (2001)	L
9. Change fire regime?	Not data available on infestation density or		L
Community Habitat
10. Impact on composition (a) high value EVC	Highest potential distribution is in Mallee & Wimmera CMAs. Murray Mallee bioregions not yet mapped for EVCs, thus no data available. It is a strongly competitive plant occurring on semi-arid grasslands. Displaces grass/forb species.	P & C (2001)	MH
(b) medium value EVC	Potential distribution limited to small areas in the Mallee and Wimmera CMAs. Occurs on semi-arid grasslands. Unlikely to affect medium value EVCs.		L
(c) low value EVC	As in 10(b) above.		L
11. Impact on structure?	“Perennial ragweed is a strongly competitive plant, often growing densely to the detriment of crop and pasture. Extracts from aerial growth are allelopathic, inhibiting germination and early seedling growth of several plant species.” Occurs on open situations; likely to have serious effect on the lower stratum.	P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“It is not grazed by stock, dense infestations thus reducing pasture productivity considerably.” Would lead to a reduction in available fodder for native fauna.	P & C (2001)	ML
15. Benefits fauna?	In the United States, “Western ragweed is used for food and nesting material, and as a habitat component by small mammals and non-game birds…and is important food (seeds and foliage) on activity sites for upland gamebirds.” However, there are no known benefits to native fauna.	Pavek (1992)1	H
16. Injurious to fauna?	“The foliage and stems contain cinnamic acid and sesquiterpene lactones that deter herbivory. However, western ragweed is not considered a poisonous plant.”	Pavek (1992)	L
Pest Animal
17. Food source to pests?	Potential food source for pest birds (see Q15 above).		ML
18. Provides harbour?	In the United States, the plant offers poor environmental protection to many species. Similar effect in Victoria.	Pavek (1992)	L
Agriculture
19. Impact yield?	“Perennial ragweed is a strongly competitive plant, often growing densely to the detriment of crop and pasture. Extracts from aerial growth are allelopathic, inhibiting germination and early seedling growth of several plant species. It is not grazed by stock, dense infestations thus reducing pasture productivity considerably.” Likely to have a serious effect on agricultural yield.	P & C (2001)	MH
20. Impact quality?	Not known to affect the quality of agricultural produce.		L
21. Affect land value?	Dense infestations may affect the value of grazing properties due to the considerable reduction in productivity and the cost of chemical control.	P & C (2001)	M
22. Change land use?	“At present, perennial ragweed occurs in rapidly spreading patches on the North Western Slopes and Plains and the North Coast [of NSW].” As stock do not graze the plant, if left untreated it would seriously impact the usefulness of grazing land.	P & C (2001)	H
23. Increase harvest costs?	No evidence of affect on harvesting.		L
24. Disease host/vector?	None evident.		L