Impact Assessment - Mimosa, giant sensitive plant (Mimosa pigra) 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	Rating	Confidence
Recreation
1. Restrict human access?	“This aggressive prickly shrub forms impenetrable thickets 4 to 5 metres high, making infested areas inaccessible to man” (Parsons & Cuthbertson 2001). “Traditional hunting areas have been choked by mimosa, so that people can no longer hunt [in some Aboriginal communities in NT]’ (CRC for Weed Management 2005). Dense infestations would create a major impediment to humans.	H	MH
2. Reduce tourism?	Infestations of M. pigra create, “…perceptions of degraded natural landscapes; reduced wildlife attractions; reduced area and access for tourism activities; reduced access to fishing, hunting and scenic areas” (ARMCANZ 2001). Dense infestations would have serious implications for recreational activities.	H	MH
3. Injurious to people?	“Stems are armed with broad-base prickles to 7 mm long” (Groves et al 1995). Prickles exist throughout the year. Potential for minor injury.	MH	MH
4. Damage to cultural sites?	“…perceptions of degraded natural landscapes; reduced wildlife attractions; reduced area and access for tourism activities; reduced access to fishing, hunting and scenic areas” (ARMCANZ 2001). Infestations would create a moderate visual effect.	ML	MH
5. Impact flow?	Terrestrial species invading, “…sedgeland and grassland communities on open floodplains” (Groves et al 1995).	L	MH
6. Impact water quality?	Terrestrial species.	L	MH
7. Increase soil erosion?	“A large central taproot penetrate 1 – 2 m into the soil, together with a lateral root system that extends up to 3.5 m from the stem” (Groves et al 1995). Unlikely to contribute to soil erosion.	L	MH
8. Reduce biomass?	The plant invades,“…sedgeland and grassland communities on open floodplains,” and can grow to a height of 6 m (Groves et al 1995). Biomass would increase.	L	MH
9. Change fire regime?	Mimosa is fire resistant (Marko 1999). In dense infestations, it would decrease horizontal continuity, thereby decreasing fire frequency and extent (Brooks et al 2004). Minor change to frequency of fire.	ML	H
Community Habitat
10. Impact on composition (a) high value EVC	Climate modelling predicts that this species is not likely to establish as a weedy species in Victoria. No impact on EVCs in Victoria.	L	H
(b) medium value EVC	Climate modelling predicts that this species is not likely to establish as a weedy species in Victoria. No impact on EVCs in Victoria.	L	H
(c) low value EVC	Climate modelling predicts that this species is not likely to establish as a weedy species in Victoria. No impact on EVCs in Victoria.	L	H
11. Impact on structure?	“In Australia, a largely intact natural landscape has been completely altered. Some 450 km2 of floodplain and swamp forest have been covered by dense monospecific stands of M. pigra which have little understorey except for seedlings and suckers of M. pigra” (Groves et al 1995).	H	MH
12. Effect on threatened flora?	The potential for M. pigra to establish and naturalise in Victoria is highly unlikely due to ecoclimatic limitations. No impact on threatened flora in Victoria.	L	MH
Fauna
13. Effect on threatened fauna?	The potential for M. pigra to establish and naturalise in Victoria is highly unlikely due to ecoclimatic limitations. No impact on threatened fauna in Victoria.	L	MH
14. Effect on non-threatened fauna?	“Braithwaite et al. (1989) found that M. pigra thickets had fewer birds and lizards” (Groves et al 1995). Reduces habitat for fauna species.	MH	MH
15. Benefits fauna?	“…the rare marsupial mouse Sminthopsis virginiae (Tarragon) has become more abundant as a result of M. pigra. It probably shelters from predators in the dense thickets” (Groves et al 1995). Provides some assistance to desirable species.	MH	MH
16. Injurious to fauna?	Not documented as causing injury. Not toxic.	L	MH
Pest Animal
17. Food source to pests?	Not known as a food source to pest animals.	L	MH
18. Provides harbour?	Not known to provide harbour to pest animals.	L	MH
Agriculture
19. Impact yield?	“Occurring, as it does, in damp situations along riverbanks and billabongs, mimosa interferes with stock watering, [and] irrigation projects. It also moves out onto the drier floodplains away from rivers, where it smothers pastures, [and] reduces the available grazing area” (Parsons & Cuthbertson 2001). “The rate of population increase in river systems is rapid, with infestations able to double in area every 1.2 years” (Groves et al 1995). Major impact on carrying capacity.	MH	MH
20. Impact quality?	Not known to affect quality of produce.	L	MH
21. Affect land value?	Primarily a weed of natural ecosystems, in the Northern Territory, it does occur on grazing land (ARMCANZ 2001). As an aggressive shrub that can spread rapidly reducing grazing and water resources, land prices may be somewhat affected.	M	MH
22. Change land use?	“If mimosa continues to spread, large stands could develop very quickly in pastoral and agricultural areas of eastern Queensland and in the Ord River region of Western Australia. The cost of mimosa control under these circumstances would be exorbitant” (ARMCANZ 2001). Land use may be seriously affected.	H	MH
23. Increase harvest costs?	Infestations make stock management and mustering difficult. Increase in time and labour to harvest animals (ARMCANZ 2001).	M	MH
24. Disease host/vector?	None evident	L	MH