Impact Assessment - Bathurst burr (Xanthium strumarium) 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 annual herb usually to about 1 metre high. Thick burr patches in pastures eliminate almost all other species, and may be so dense (up to 30 plants per square metre) that sheep are denied access to watering points.” Similarly, because of the prickly nature of the plant, patches would restrict human access.	P & C (2001)	MH
2. Reduce tourism?	“In Victoria …it exists on flood-prone areas along the [Murray River] and its tributaries. Xanthium spp. is spread widely along waterways.” Dense patches would affect some recreational activities such as boating or fishing in these situations.	P & C (2001)	MH
3. Injurious to people?	The fruit is a “woody ellipsoid burr 1.2 to 2.5 cm long.” “The seed and seedlings are poisonous to animals…both species cause contact dermatitis in humans and the pollen may cause hay fever.”	P & C (2001)	H
4. Damage to cultural sites?	Dense patches would create a negative visual impact and seriously affect the aesthetics of an area.		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?	A competitive annual, “thick burr patches in pastures eliminate almost all other species.” Potential to leave areas of soil exposed to erosion. “In Australia, it thrives on the banks of rivers and creeks.” In these situations there may be off-site implications associated with soil erosion.	P & C (2001) Groves et al (1995)	MH
8. Reduce biomass?	“…these plants [are] strong competitors in both pasture and summer crops.” Direct replacement of biomass by invader.	P & C (2001)	ML
9. Change fire regime?	Plants die in autumn, but remain standing. Dense patches may increase the intensity of fire risk.	P & C (2001)	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Creekline grassy woodland (E); CMA=North Central; Bioreg=Victorian Riverina; VH CLIMATE potential It occurs along flood-prone areas of the Murray River and its tributaries. “Thick burr patches in pasture eliminate almost all other species.” Prefers unshaded situations. Major impact on grasses/forbs.	P & C (2001)	MH
(b) medium value EVC	EVC=Riverine grassy woodland (D); CMA=Goulburn Broken; Bioreg=Murray Fans; VH CLIMATE potential Impact as in 10(a) above.	P & C (2001)	MH
(c) low value EVC	EVC=Riparian forest (LC); CMA=West Gippsland; Bioreg=Highlands – Southern Fall; L CLIMATE potential Prefers unshaded situations. Likely CLIMATE potential and forest situation would limit impact.		ML
11. Impact on structure?	“Noogoora burr can form very dense stands which are completely dominant and cover entire paddocks. Some forms prefer open communities and die out if shaded or crowded.” Major impact on lower stratum.	Groves et al (1995)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	In natural environments it is most likely to establish dense infestations in disturbed areas. It also tends to colonise in medium to large populations. In those situations, the presence of the weed would have a significant impact on the habitat of non-threatened fauna. Spiny nature of plant would deter grazing.	Carr et al (1992) Muyt (2001) P & C (2001)	ML
15. Benefits fauna?	“Small birds feed on the seeds.” Bird species not mentioned. Limited food source.	P & C (2001)	MH
16. Injurious to fauna?	The plant has spines for most of the year. Potential for injury.	P & C (2001)	H
Pest Animal
17. Food source to pests?	Not known as a food source to pests. “The seeds and seedlings of both species are poisonous.”	P & C (2001)	L
18. Provides harbor?	Not known to provide harbor.		L
Agriculture
19. Impact yield?	“These plants [are] strong competitors both in pasture and summer crops such as maize, cotton and soybeans.” Its presence in pasture seriously reduces carrying capacity.	P & C (2001)	MH
20. Impact quality?	Burrs contaminate wool reducing its value. “In 1978–79, between 7% and 8% of the wool sold in eastern Australia was contaminated with Noogoora burr.”	P & C (2001)	MH
21. Affect land value?	The plant can be effectively controlled, but management programs can extend for more than six years to ensure the seed bank is exhausted. The cost of implementing such a program, plus the potential loss of income from reduced yields and quality, would likely have a negative impact on land value.		M
22. Change land use?	With an appropriate control program, land use would not need to change significantly. “Neither species is particularly important to cattle producers.” If the weed is left untreated, changing from sheep to cattle may be required.	P & C (2001)	M
23. Increase harvest costs?	The burrs “…damage carding machines.” The cost of processing contaminated wool increases. “…travelling stock must be inspected and any burrs removed.”	P & C (2001)	M
24. Disease host/vector?	“In North America, Noogoora burr is a host of two Sclerotinia pathogens which cause important diseases in soybeans. It is not known if it plays a similar role in Australia.” Potential host a threat to agriculture.	P & C (2001)	H