Impact Assessment - Erect prickly pear (Opuntia stricta) 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 about 1 metre high. “Patches of Opuntia spp. grow densely forming an impenetrable barrier, hence their use as live fences in some areas.” However, O. stricta, while widespread, is not densely established in Australia. 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 or 2 sharp spines about 2 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?	Stands of O. stricta 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. stricta occurs in small populations on lowland grassland and woody grassland, dry sclerophyll forest and woodland, and riparian 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. stricta grows densely forming impenetrable barriers. Stands can hinder growth of smaller shrubs and ground flora. Major impact on lower and mid strata.	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. stricta was an aggressive invader of agricultural land in Queensland and NSW in the early 20th century. “Spread was helped, to some degree, in the 1902 drought when plants were cut and fed out as fodder.” Spread due to natural means is not documented. However, “Before the introduction of biological control agents it was the most serious weed in Australia and capable of growing in most parts of the continent.” Impact is now limited due to biological control with Cactoblastis cactorum. However, the effectiveness of control is sometimes reduced in colder climates. In those areas dense patches would limit carrying capacity.	P & C (2001)	ML
20. Impact quality?	Effect on quality of produce unknown. Not a weed of cropping or cultivated areas. Animals avoid the plants.	P & C (2001)	L
21. Affect land value?	Biological control has reduced the potential for serious infestation. Land prices would not be affected.	P & C (2001)	L
22. Change land use?	Biological control has reduced the potential for serious infestation. Land can still be used for intended purpose without loss.	P & C (2001)	L
23. Increase harvest costs?	No impact on harvest costs.		L
24. Disease host/vector?	None evident		L

Impact Assessment Record - Erect prickly pear (PDF - 61KB)
Impact Assessment Record - Erect prickly pear (DOC - 33KB)
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