Impact Assessment - Giant arrowhead (Sagittaria montevidensis) in Victoria

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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 Giant arrowhead

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 - 1026 KB)
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: Giant arrowhead
Scientific name: Sagittaria montevidensis

Question	Comments	Rating	Confidence
Recreation
1. Restrict human access?	An emergent aquatic perennial herb. Inhabits shallowly flooded (to 45 cm) or marshy areas associated with rivers and streams. In south eastern Australia it is commonly found in drainage ditches and permanent swamps associated with irrigation and drainage systems (Parson & Cuthberston, 1992). Would not impede human access to waterways.	L	MH
2. Reduce tourism?	Although most commonly found in drainage ditches and irrigation canals, it has also been found in a number of creeks in Victoria including Nine Mile Creek, Wunghnu and Bullock Creek near the junction of the Oven and Murray Rivers. In drain and irrigation channels it can form dense infestations that block the flow of water (Parson & Cuthberston, 1992). Its presence may affect water-based activities such as fishing or swimming.	MH	MH
3. Injurious to people?	No prickles, spines or burrs described (Aston, 1973). Sagittaria species are eaten by ducks and sometimes humans, suggesting that they are not toxic either (Parson & Cuthberston, 1992).	L	MH
4. Damage to cultural sites?	An aquatic species (Parson & Cuthberston, 1992). Not likely to impact on sites of cultural significance.	L	MH
Abiotic
5. Impact flow?	"Sagittaria forms extensive infestations in shallow waterways, seriously restricting water flow and increasing sedimentation, thus aggravating flooding." (BPRC, 2004). An emergent aquatic, firmly rooted. “Dense infestations block channels and drainage ditches” (Parsons & Cuthbertson, 1992). “Arrowhead can severely retard and completely choke water flow in irrigation channels” (Gunasekera & Krake, 2001). Serious impacts to both surface and subsurface water flow.	H	MH
6. Impact water quality?	“Decaying plant material contributes to water pollution and sedimentation and may also increase the risk of algal bloom and microbial activity” (Sagliocco & Bruzesse, 2005). Likely to cause noticeable but minor effects on dissolved O₂.	ML	MH
7. Increase soil erosion?	In New Zealand it, "forms extensive infestations in shallow waterways, seriously restricting water flow and increasing sedimentation, thus aggravating flooding" (BPRC, 2004). Increased chance of flooding would create the potential for large-scale soil movement with minor off-site implications.	MH	MH
8. Reduce biomass?	Where it occurs in drainage channels and irrigation canals (Parson & Cuthberston, 1992) little other vegetation would normally exist. Infestations of arrowhead would increase biomass.	ML	MH
9. Change fire regime?	Aquatic species (Flower et al, 1999). Fire not applicable in this ecosystem.	L	H
Community Habitat
10. Impact on composition (a) high value EVC	Basin=East Gippsland- Cann River (ISC=Excellent); CMA=East Gippsland; CLIMATE=VH. “S. calycina [syn. S. montevidensis ssp. calycina] is very abundant in suitable years, sometimes forming vast beds” (Kaul, 1985). Can displace native water plants which occupy the same habitat (ESC, 2005). Able to form monocultures displacing all species within a layer.	H	MH
(b) medium value EVC	Aquatic species. All Victorian water bodies considered to comprise high value EVCs only (Weiss pers. coms.)ⁱ.	L	H
(c) low value EVC	Aquatic species. All Victorian water bodies considered to comprise high value EVCs only (Weiss pers. coms.)ⁱⁱ.	L	H
11. Impact on structure?	“S. calycina [syn. S. montevidensis ssp. calycina] is very abundant in suitable years, sometimes forming vast beds” (Kaul, 1985). Can displace native water plants which occupy the same habitat (ESC, 2005). Able to form monocultures.	H	MH
12. Effect on threatened flora?	No information found.	MH	L
Fauna
13. Effect on threatened fauna?	No information found.	MH	L
14. Effect on non-threatened fauna?	“Has a negative impact on native species and on the integrity of natural waterways, such as the waterways of the Barmah-Millewa forest, where it has been recorded” (GMW). Large populations obstruct the movement of wildlife and fish (Sagliocco & Bruzzese, 2005). May reduce habitat and have a minor effect on native fauna.	ML	MH
15. Benefits fauna?	Spoonbills have been observed feeding on the plant (Parson & Cuthberston, 1992). The plant may provide an alternate source of food.	MH	MH
16. Injurious to fauna?	No prickles, spines or burrs described (Aston, 1973). Spoonbills and carp have been observed feeding on the plant (Parson & Cuthberston, 1992). Not harmful to fauna.	L	MH
Pest Animal
17. Food source to pests?	Carp (considered a serious aquatic pest) have been observed feeding on the plant (Parson & Cuthberston, 1992).	MH	MH
18. Provides harbor?	Carp (considered a serious aquatic pest) have been observed feeding on the plant Parson & Cuthberston (1992) and dense infestations (Parsons & Cuthbertson, 1992) may also provide harbor for them.	MH	MH
Agriculture
19. Impact yield?	“Grain yields were not affected by Californian arrowhead densities up to 200 plants/m² (Gibson et al, 2001). “Arrowhead reduced the yield of rice by up to 75% if completely unchecked” (Flower, Pratley & Slater, 1999). The ability of rice to out compete arrowhead depends on when it is sown after flooding (Flower et al, 2002). This species has the capacity to have a serious impact on rice yield.	H	H
20. Impact quality?	“Weed senesced well before rice maturity” (Gibson et al, 2001). Unlikely to impact on the quality of the produce.	L	H
21. Affect land value?	Whilst this species can “severely retard and completely choke water flow in irrigation channels” (Gunasekera & Krake, 2001), it is the responsibility of the water service provider to control the weed (GMW) so is unlikely to have an impact on land value.	L	MH
22. Change land use?	Land use may need to change from rice growing to another crop (see Q. 19), however, this is unlikely to cause a reduction in income.	L	MH
23. Increase harvest costs?	One study showed that draining rice bays for up to two weeks killed very young arrowhead seedlings, but didn’t kill more mature seed-grown plants (Flower et al, 1999). Controlling arrowhead this way may cause a minor increase to the costs of harvest.	M	H
24. Disease host/vector?	Harbours several important plant viruses (Parsons & Cuthbertson, 1992).	H	MH

* Sagittaria species include (those naturalised in Australia): S. montevidensis ssp. montevidensis, S. montevidensis ssp. calycina, S. platyphylla & S. graminea.

ⁱ Weiss, J. Project Leader, Pest Plant Impact Assessment. DPI, Frankston. Tel 9785 0111. (11/07/03)
ⁱⁱWeiss, J. Project Leader, Pest Plant Impact Assessment. DPI, Frankston. Tel 9785 0111. (11/07/03)

This table can also be viewed as a PDF document (printer friendly).

Impact Assessment Record - Arrowhead (PDF - 73 KB)
Impact Assessment Record - Arrowhead (DOC - 67 KB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

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