Impact Assessment - Gazania (Gazania linearis) in Victoria

Back | Table | Feedback

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?	As a groundcover to 30 cm tall (Faithfull, 2006), unlikely to restrict human access at all.	L	MH
2. Reduce tourism?	As a groundcover to 30 cm tall (Faithfull, 2006), unlikely to be noticed by the average visitor.	L	MH
3. Injurious to people?	Gazania species in general noted as a harbour for White Italian Snail, that can harbour a parasite that is harmful to human health (Enviroweeds, 2002). If the snails are eaten, humans can become infected with a flatworm that causes gastroenteritis, but the parasite cannot be contracted unless the snails are eaten (Horstman, 2002). No further injury to people has been found documented and this disease is not directly related to Gazanias.	L	MH
4. Damage to cultural sites?	As a clump-forming groundcover to 30 cm tall (Faithfull, 2006), unlikely to cause structural or visual damage.	L	MH
Abiotic
5. Impact flow?	Invades streambanks (Webb et al, 1988), but is only recorded as growing in terrestrial environments (Faithfull, 2006). Unlikely to impact flow.	L	MH
6. Impact water quality?	Invades streambanks (Webb et al, 1988), but is described as perennial (Batten & Bokelmann, 1966), rhizomatous (Webb et al, 1988) and as growing in terrestrial environments (Faithfull, 2006). Likely to bind soil near streambanks, reducing erosion, and consequently turbidity.	L	MH
7. Increase soil erosion?	Described as perennial (Batten & Bokelmann, 1966), rhizomatous (Webb et al, 1988) and as growing in sandy soils (Faithfull, 2006). Likely to bind soil, reducing erosion.	L	MH
8. Reduce biomass?	Prevents any regeneration of coastal native species (Pittwater Council, 2007). This article implies that G. linearis even suppresses the regeneration of tree species, such as coastal wattle. Major reduction in biomass, even replacing permanent woody vegetation.	H	M
9. Change fire regime?	Fire resistant (Batten & Bokelmann, 1966) and able to naturalise in grasslands (Faithfull, 2006) and prevent regeneration of native species (Pittwater Council, 2007). Likely to reduce the incidence and intensity of fire.	H	M
Community Habitat
10. Impact on composition (a) high value EVC	EVC= Ridged Plains Mallee (E); CMA=Mallee; Bioreg= Murray Mallee; CLIMATE potential=VH. Only grow to about 30 cm (Faithfull, 2006), however, it prevents any regeneration of coastal native species (Pittwater Council, 2007). This article implies that G. linearis even suppresses the regeneration of tree species, such as coastal wattle. Patches of Gazania are “usually devoid of other species” (Cordingley & Petherick, 2005). Major displacement of some dominant species. Able to form monocultures, but only in discreet patches.	MH	M
(b) medium value EVC	EVC= Woorinen Sands Mallee (D); CMA=Mallee; Bioreg= Murray Mallee; CLIMATE potential=VH. Only grow to about 30 cm (Faithfull, 2006), however, it prevents any regeneration of coastal native species (Pittwater Council, 2007). This article implies that G. linearis even suppresses the regeneration of tree species, such as coastal wattle. Patches of Gazania are “usually devoid of other species” (Cordingley & Petherick, 2005).	MH	M
(c) low value EVC	EVC= Loamy Sands Mallee (LC); CMA=Mallee; Bioreg= Murray Mallee; CLIMATE potential=VH. Only grow to about 30 cm (Faithfull, 2006), however, it prevents any regeneration of coastal native species (Pittwater Council, 2007). This article implies that G. linearis even suppresses the regeneration of tree species, such as coastal wattle. Patches of Gazania are “usually devoid of other species” (Cordingley & Petherick, 2005).	MH	M
11. Impact on structure?	Grow to about 30 cm (Faithfull, 2006). “Naturalised and known to be a minor problem [in natural ecosystems] warranting control at 4 or more locations” within Australia (Groves et al, 2003). Gazania species in general displace native vegetation and directly threaten a nationally threatened daisy on Eyre Peninsula” (DeLaine & Stokes, 2006). Has naturalised in an area of native grassland that was once in good condition, now no longer considered significant due to the Gazania infestation (Faithfull, 2006). Prevents any regeneration of coastal native species (Pittwater Council, 2007). This article implies that G. linearis even suppresses the regeneration of tree species, such as coastal wattle. Patches of Gazania are “usually devoid of other species” (Cordingley & Petherick, 2005). Major effect on all layers	H	M
12. Effect on threatened flora?	Despite evidence that Gazania species in general threaten a nationally threatened daisy on the Eyre Peninsula (DeLaine & Stokes, 2006), no evidence was found that G. linearis affects threatened flora in Victoria. However, this species appears to have a major effect on coastal native species (Pittwater Council, 2007) and is extremely competitive with ground flora in the Murray Darling basin (J. Virtue, 2007, pers. Comm., 23rd March).	MH	L
Fauna
13. Effect on threatened fauna?	No evidence was found that G. linearis affects threatened fauna in Victoria. However, this species appears to have a major effect on coastal native species (Pittwater Council, 2007) and is extremely competitive with ground flora in the Murray Darling basin (J. Virtue, 2007, pers. Comm., 23rd March.), which may have flow on effects for threatened fauna.	MH	L
14. Effect on non-threatened fauna?	This species is capable of displacing native vegetation (DeLaine & Stokes, 2006) and degrading grassland (Faithfull, 2006). It may reduce the availability of food for some fauna species.	ML	M
15. Benefits fauna?	No evidence that this species benefits indigenous fauna. G. rigens is eaten (heavily damaged) by rabbits (Metzger & Weisberg, 2001), however, no evidence was found about the palatability or toxicity of the plant.	M	L
16. Injurious to fauna?	Taxonomic descriptions of G. linearis do not indicate that it has either spines or burrs (Webb et al, 1988). No evidence suggests that this species injures indigenous fauna. No evidence found about the palatability or toxicity of the plant, though.	M	L
Pest Animal
17. Food source to pests?	G. rigens is eaten (heavily damaged) by rabbits (Metzger & Weisberg, 2001), however, no evidence was found about the palatability of this plant.	M	L
18. Provides harbor?	Clump-forming species growing to 30 cm high (Faithfull, 2006). Not recorded as harbouring pest species, but it may be capable of harbouring minor pest species such as rodents.	ML	M
Agriculture
19. Impact yield?	“Naturalised and known to be a minor problem [in agricultural ecosystems] warranting control at 4 or more locations” within Victoria (Groves et al, 2003). Naturalised populations in the Southern wheatbelt of WA appear to be on roadsides and wasteland around settlements, rather than on agricultural land (Hussey et al, 1997). Not considered an agricultural weed in South Australia (D. Cooke, 2007, pers comm., 23rd March), but form carpets in citrus orchards, competing with trees for nutrients and water (J. Virtue, 2007, pers. Comm., 23rd March). Considered an environmental weed in Victoria (Enviroweeds, 2002), but no further information about its seriousness as an agricultural weed was found. Likely to have a minor impact on yield.	ML	M
20. Impact quality?	A low-growing plant that appears to only affect horticultural crops (see Q.19). Unlikely to impact quality.	L	M
21. Affect land value?	A low-growing plant that appears to only affect horticultural crops (see Q.19). Unlikely to impact land value.	L	M
22. Change land use?	A low-growing plant that appears to only affect horticultural crops (see Q.19). Unlikely to change land use.	L	M
23. Increase harvest costs?	High rates of herbicide, with an effective penetrant are required to control gazania in horticultural crops (J. Virtue, 2007, pers. Comm., 23rd March). May increase harvest costs slightly, but as this is still a normal control method, unlikely to be a major cost increase.
24. Disease host/vector?	Gazania species in general noted as a harbour for White Italian Snail, a crop and pasture contaminant (Enviroweeds, 2002). White Italian Snail is “probably the most well-known pest of Australian agricultural crops” that can destroy crops and contaminate grain (Rudman, 2003). It is also a host for parasitic nematodes of sheep and cattle (Garrison, 1993). A widespread pest that has major consequences, but is unlikely to increase the incidence of the pest, given that it is so common anyway.