Impact Assessment - Wheel cactus (Opuntia robusta) 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 shrub commonly 1 to 2 metres high. “Patches of Opuntia spp. grow densely forming an impenetrable barrier, hence their use as live fences in some areas.” Most likely to be a high nuisance to people by restricting direct access.	P & C (2001)	MH
2. Reduce tourism?	Although not occurring in dense populations, its presence would be obvious and, due to the spiny nature of the plant, it may affect some recreational activities.	P & C (2001)	MH
3. Injurious to people?	Stems (cladodes) are regularly patterned with aureoles that bear very fine barbed bristles. Some aureoles also have 1 to 5 sharp spines to 4 cm long. The barbed bristles are obnoxious because they readily penetrate human skin causing sever irritation and are difficult to remove. Spines and bristles are present all year.	P & C (2001)	H
4. Damage to cultural sites?	Dense stands of O. robusta would create a negative visual impact on cultural sites and seriously affect the aesthetics of an area. The root system is fibrous and shallow and unlikely to cause structural damage.	P & C (2001)	MH
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 root system, while shallow, is fibrous. In dense patches, aerial growth provides good ground cover. Not likely to contribute to soil erosion.	P & C (2001)	L
8. Reduce biomass?	In Victoria, O. robusta occurs in small populations on lowland grassland and woody grassland situations. The plant would likely increase biomass.	Carr et al (1992)	L
9. Change fire regime?	A succulent, it would have a small or negligible effect on fire risk. “Because of their high moisture content, plants are not easily burnt.”	P & C (2001)	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassy woodland (E); CMA=North Central; Bioreg=Victorian Riverina; VH CLIMATE potential. Like other Opuntia spp., O. robusta grow densely forming impenetrable barriers. Stands can hinder growth of smaller shrubs and ground flora. Major impact on lower stratum, minor impact on mid stratum.	P & C (2001) Muyt (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) Muyt (2001)	MH
(c) low value EVC	EVC=Riparian forest (LC); CMA=West Gippsland; Bioreg=Highlands - Southern Fall; H CLIMATE potential. Similar impact as in 10(a) above, however, effect lessened due to high CLIMATE potential only.		MH
11. Impact on structure?	“Large stands can hinder the growth and regeneration of indigenous plants, particularly smaller shrubs and ground-flora.” Minor effect on the lower and mid strata	Muyt (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“Opuntia spp. are not usually grazed by stock because of the stout spines.” In dense patches, the plant could hinder access to water and reduce available fodder for fauna.	P & C (2001)	ML
15. Benefits fauna?	“seed…is spread in the droppings of birds, foxes and other animals.” May provide limited food to desirable species.	P & C (2001)	MH
16. Injurious to fauna?	“Opuntia spp. are not usually grazed by stock because of the stout spines and bristles damage their tongues and lips.” Spines present all year.	P & C (2001)	H
Pest Animal
17. Food source to pests?	“seed…is spread in the droppings of birds, foxes and other animals. Opuntia spp. are hosts to fruit-fly.” Food source to at least one serious pest animal at a crucial time of year.	P & C (2001)	H
18. Provides harbour?	“Patches also provide effective harbour for pest animals such as rabbits.” Plants are long-lived, which would allow for permanent harbor.	P & C (2001)	H
Agriculture
19. Impact yield?	O. robusta has not demonstrated the same degree of invasiveness as O. stricta. However, it is weedy in the north central region of Victoria. No biological control agent exists for this species, so it appears to be naturally contained. Animals do not graze the plant and dense patches may limit grazing capacity.	P & C (2001)	ML
20. Impact quality?	Not a weed of cropping. Animals avoid the plant because of the spines.	P & C (2001)	L
21. Affect land value?	Population densities are small. Not likely to affect land value.	Carr et al (1992)	L
22. Change land use?	The impact on agriculture is limited; change in land use not required.		L
23. Increase harvest costs?	No impact on harvest costs.		L
24. Disease host/vector?	None evident		L

Impact Assessment Record - Wheel cactus (PDF - 32KB)
Impact Assessment Record - Wheel cactus (DOC - 61KB)
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