Impact Assessment - Soursob (Oxalis pes–caprae) 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 perennial herb, commonly 15 to 35 cm high.” Its presence would not restrict human access.	P & C (2001)	L
2. Reduce tourism?	Dense infestations are obvious, particularly during the flowering stage when areas are covered by a dense carpet of yellow flowers. Recreational activities not affected, but aesthetic of site would be.	P & C (2001)	ML
3. Injurious to people?	Although the plant is considered toxic to animals (it contains oxalic acid), “its flowering stems are occasionally chewed and sucked by children for the sour taste.” Toxicity is a problem of prolonged consumption. Consider mildly toxic.	Blood (2001)	ML
4. Damage to cultural sites?	Dense infestations would create a moderate negative visual impact.		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?	Because of its potential for dense growth, “death of top growth in late spring – early summer leaves large patches of bare soil prone to erosion..” High probability of large scale soil movement.	Blood (2001)	MH
8. Reduce biomass?	“Infestations of 3000 plants per sq m not uncommon.” Biomass may increase.	Blood (2001)	L
9. Change fire regime?	When aerial growth dies, “it does not leave any hard trash.” Not likely to affect fire regime.	P & C (2001)	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassy woodland (E); CMA=West Gippsland; Bioreg=Gippsland Plain; VH CLIMATE potential. “Soursob has a severe impact on indigenous ground-flora with dense infestations eradicating most smaller plants.” It grows in a broad range of vegetation. Major displacement of grasses/forbs.	Muyt (2001) Carr et al (1992)	MH
(b) medium value EVC	EVC=Lowland forest (D); CMA=Corangamite; Bioreg=Otway Plain; VH CLIMATE potential. Impact as in 10(a) above.	Muyt (2001) Carr et al (1992)	MH
(c) low value EVC	EVC=Heathy woodland (LC); CMA=West Gippsland; Bioreg=Gippsland Plain; VH CLIMATE potential. Impact as in 10(a) above.	Muyt (2001) Carr et al (1992)	MH
11. Impact on structure?	“Soursob has a severe impact on indigenous ground-flora with dense infestations eradicating most smaller plants.” “Initial growth low and spreading, quickly covers ground, preventing germination of other seedlings and smothering surrounding plants.” Major effect on ground flora.	Muyt (2001) Blood (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“Soursob has a severe impact on indigenous ground-flora with dense infestations eradicating most smaller plants.” It occurs in medium to large populations in a number of different vegetative communities including; dry coastal vegetation, heathland & woody heathland, lowland grassland & grassy woodland; dry sclerophyll forest & woodland, riparian vegetation, and rock outcrop vegetation. Additionally, “it is not usually eaten by livestock because of its high oxalic acid content which makes the plant sour.” Such infestations would have a serious impact on the habitat of fauna.	Muyt (2001) Carr et al (1992) Blood (2001)	MH
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“Prolonged consumption by livestock can cause chronic kidney damage and death.” Potential to affect fauna species, however, the plant is not usually eaten because of the high oxalic acid content.	Blood (2001)	ML
Pest Animal
17. Food source to pests?	Not known as a food source to pest animals.		L
18. Provides harbour?	Not known to provide harbour.		L
Agriculture
19. Impact yield?	“Yield losses in wheat of up to 50% have been recorded in South Australia but soursob appears to be less competitive in Victoria. Although primarily a weed affecting cultivated areas, soursob is also important in pastures where it may replace almost all desirable plants from the time of the autumn rains to well into the spring.” Potentially serious reduction in carrying capacity.	P & C (2001)	MH
20. Impact quality?	Not known to affect the quality of produce.		L
21. Affect land value?	Although a number of control methods are available, the serious impact on agricultural yield due to the presence of the plant may reduce land value.	P & C (2001)	M
22. Change land use?	Cultivation is a recommended method of control. In cropping situations, land use would not need to change.	P & C (2001)	L
23. Increase harvest costs?	Not known to affect harvest costs.		L
24. Disease host/vector?	“Oxalis pes-caprae is not known to host diseases that affect crops. However, Lovett Doust et al. (1985) reported that O. stricta L., O. corniculata L., O. dillenii ssp. dillenii Jacq. and O. dillenii Jacq. ssp. filipes (Small) Eiten host the maize rusts Puccinia sorghi Schw. and P. polysora Underw., sorghum rust P. purpurea Che. and Andropogon rust P. andropogonis Schw.” Potential host?	Panetta et al. (1998)	H

Impact Assessment Record - Soursob (PDF - 33KB)
Impact Assessment Record - Soursob (DOC - 61KB)
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