Impact Assessment - Prickly acacia (Acacia nilotica ssp. indica) 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.

Each resource attribute, or criterion, is assessed relative to a list of intensity ratings. Depending upon information found in relation to each criterion, a rating of Low, Medium Low, Medium High or High is assigned. Descriptions of the impact criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the impact of Prickly acacia.

A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:

Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1 MB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

Common Name: Prickly acacia
Scientific name: Acacia nilotica ssp. indica

Question	Comments	Rating	Confidence
Recreation
1. Restrict human access?	Shrub or small tree from 7 to 9 m high. ‘Because of its rapid growth in favourable seasons and the presence of long spines or prickles, dense thickets of prickly acacia, impenetrable to stock, develop. Thickets along thousands of kilometres of bore drains prevent access to water’ (Parsons & Cuthbertson 2001). Major impediment to access waterways.	H	MH
2. Reduce tourism?	Because of infestation by prickly acacia, “…the Mitchell grass downs are being converted into a thorny scrubland similar to the African thornveld” (Mackey 1998). Weeds presence is obvious to visitors.	H	MH
3. Injurious to people?	Young stems are armed with stout stipular spines 5 to 50 mm long, Spines may be absent on older stems. Dogs cannot be used for mustering purposes because of spines on the ground (Parsons & Cuthbertson 2001). Potential for injury throughout the year.	H	MH
4. Damage to cultural sites?	Shrub or small tree from 7 to 9 m high (Parsons & Cuthbertson 2001). Moderate visual effect.	ML	MH
Abiotic
5. Impact flow?	Terrestrial species (Parsons & Cuthbertson 2001).	L	MH
6. Impact water quality?	Terrestrial species (Parsons & Cuthbertson 2001).	L	MH
7. Increase soil erosion?	“…a moderate canopy cover of prickly acacia reduces grass cover markedly and changes the relative abundance of native plant species in favour of forbs and annual grasses” (Mackey 1998). “Exacerbates and accelerates soil erosion” (ARMCANZ 2001).	MH	MH
8. Reduce biomass?	“In Australia, it occurs as a weed, principally along streams and bore drains, in the semi-arid tussock and hummock grasslands” (Parsons & Cuthbertson 2001). Biomass would increase in these situations.	L	MH
9. Change fire regime?	“…prickly acacia does not seem susceptible to fire” (Mackey 1998). Suppression of grass species also reduces fire risk. Minor change to frequency and intensity of fires.	ML	MH
Community Habitat
10. Impact on composition (a) high value EVC	EVC= Plains Savannah (E); CMA=Mallee; Bioreg=Murray Mallee; Climate=VH. An infestation in Queensland has converted parts of the Mitchell grass downs into a thorny scrubland. “…a moderate canopy of prickly acacia reduces grass cover markedly” (Mackey 1998). Major displacement of some dominant species (grasses) within a strata (groundcover layer).	MH	MH
(b) medium value EVC	EVC= grassy dry forest (D); CMA=Goulburn Broken; Bioreg=Central Victorian Uplantds; Climate=VH. An infestation in Queensland has converted parts of the Mitchell grass downs into a thorny scrubland. “…a moderate canopy of prickly acacia reduces grass cover markedly” (Mackey 1998). Major displacement of some dominant species (grasses) within a strata (groundcover layer).	MH	MH
(c) low value EVC	EVC= grassy dry forest (D); CMA=Goulburn Broken; Bioreg=Central Victorian Uplantds; Climate=VH. An infestation in Queensland has converted parts of the Mitchell grass downs into a thorny scrubland. “…a moderate canopy of prickly acacia reduces grass cover markedly” (Mackey 1998). Major displacement of some dominant species (grasses) within a strata (groundcover layer).	MH	MH
11. Impact on structure?	An infestation in Queensland has converted parts of the Mitchell grass downs into a thorny scrubland. “…a moderate canopy of prickly acacia reduces grass cover markedly” (Mackey 1998). Serious impact on grasses.	MH	MH
12. Effect on threatened flora?	The potential for A. nilotica ssp. indica to establish and naturalise in Victoria is highly unlikely due to ecoclimatic limitations (Thorp & Lynch 2000). No impact on threatened flora in Victoria.	L	MH
Fauna
13. Effect on threatened fauna?	The potential for A. nilotica ssp. indica to establish and naturalise in Victoria is highly unlikely due to ecoclimatic limitations (Thorp & Lynch 2000). No impact on threatened fauna in Victoria.	L	MH
14. Effect on non-threatened fauna?	“Since even a moderate canopy cover of prickly acacia reduces grass cover markedly and changes the relative abundance of native plant species in favour of forbs and annual grasses, this, and the shift in structure toward a shrub community, is producing a dramatic effect on native fauna habitat and the overall ecology of the system” (Mackey 1998). Major impact on habitat.	MH	MH
15. Benefits fauna?	“Its leaf is very digestible and has a high protein content” (Mackey 1998). May provide alternative food source for larger native herbivores.	MH	MH
16. Injurious to fauna?	“Dogs cannot be used because of the many thorns on the ground” (Parsons & Cuthbertson 2001). Potential for injury to non-ungulate species.	MH	MH
Pest Animal
17. Food source to pests?	“…the Australian plague locust (Chortoicetes terminifera (Walker)) is reported to feed on it,” though without any known flow-on to impact on agricultural activities (Mackey 1998). Provides food for an environmental insect pest.	ML	MH
18. Provides harbour?	Not known to provide harbour for pest species.	L	MH
Agriculture
19. Impact yield?	“Dense infestations significantly reduce pasture production…and the access of stock to water. Under normal grazing pressure a 25–30% canopy cover of prickly acacia reduces pasture production by 50% compared with acacia-free pasture” (Mackey 1998). Significant reduction in carrying capacity leading to reduced yield.	H	MH
20. Impact quality?	Not known to affect the quality of produce	L	MH
21. Affect land value?	“Heavily infested land is almost worthless since reclamation costs are often close to, or exceed, the value of uninfested land” (Mackey 1998). Serious impact on land value.	H	MH
22. Change land use?	See comment in 21 above. Land may be abandoned for agricultural use.	H	MH
23. Increase harvest costs?	“Dense infestations of prickly acacia…increase mustering time and cost.” An increase in cost of more than 10 times the usual cost is reported (Mackey 1998). Increase in both time and labour to harvest produce.	H	MH
24. Disease host/vector?	“Prickly acacia is a host for a variety of organisms which attack it, but it does not appear to be a primary host for any other pests in Australia, though the Australian plague locust (Chortoicetes terminifera (Walker)) is reported to feed on it” (Mackey 1998).	L	MH

Impact Assessment Record - Prickly acacia (PDF - 72KB)
Impact Assessment Record - Prickly acacia (DOC - 68KB)
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