Impact Assessment - Alligator weed (Alternanthera philoxeroides) 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?	Prostrate, semi-prostrate or ascending plant in both terrestrial and aquatic situations. Restricts access on land and can seriously affect access to waterways.	P & C (2001)	H
2. Reduce tourism?	“In water dense mats…extend for 15 m or more.” Major impact on water-based recreation and creates a dangerous hazard for swimming.	Muyt (2001) Ensbey (2001)1	H
3. Injurious to people?	Not toxic through physical contact. The plant has been cultivated and consumed in Australia in the mistaken belief it is the culinary herb A. sessilis, known commonly as Mukunu-wenna. The plant does contain calcium oxalate crystals.	Muyt (2001) P & C (2001)	ML
4. Damage to cultural sites?	An “aggressive, invasive species.” In terrestrial situations it develops extensive root systems and is very persistent. Infestation could seriously affect historic or cultural feature. In attempting to eradicate the weed extensive excavation may need to be undertaken.	Muyt (2001)	H
Abiotic
5. Impact flow?	“The prolific growth restricts flow, increases sedimentation, aggravates flooding and acts as a barrier.”	Groves et al. (1995)	H
6. Impact water quality?	“…the thick mats…restrict light penetration…and create anaerobic conditions.” “The mats restrict light, thereby reducing primary productivity and anoxic conditions may result.”	P & C (2001) Groves et al (1995)	H
7. Increase soil erosion?	Terrestrial plants have thickened, dense roots to depths of 0.5 m. Soil erosion not increased as a result of infestation.	Muyt (2001)	L
8. Reduce biomass?	In aquatic situations biomass is increased significantly. Little effect in terrestrial situations	Groves et al (1995)	L
9. Change fire regime?	In both aquatic and terrestrial situations the plant does not die back leaving dry matter to support fire.	Gunasekera (pers comm.) 2	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains Grassy Wetland (E); CMA=Port Phillip; Bioreg:= Central Victorian Uplands; VH CLIMATE potential. Although not known as an invasive weed in Victorian natural ecosystems, it has the potential to invade riparian vegetation, freshwater wetland (seasonal and permanent). “It has a low light requirement.” In aquatic situations it has the capacity to form monocultures, but less so on land.	Groves, et al. (1995) P & C (2001)	MH
(b) medium value EVC	EVC=Sedgy riparian woodland (D); CMA= Port Phillip; Bioreg:= Central Victorian Uplands; VH CLIMATE potential. Similarly invasive as above.	Groves, et al. (1995) P & C (2001)	MH
(c) low value EVC	EVC=Wet forest (LC); CMA= Port Phillip; Bioreg:= Central Victorian Uplands; VH CLIMATE potential. As in 10(a).	Groves, et al. (1995) P & C (2001)	MH
11. Impact on structure?	“Floating mats of A. philoxeroides are generally monocultures but other species occur close to the bank. In wet terrestrial situations, it may occur as the only emergent species but is more often associated with clover, grasses or common rush.”	Groves et al (1995)	MH
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	Due to its invasive habit in waterways and its potential to create anoxic conditions, it is likely to reduce the habitat and threaten aquatic fauna. “…affects fish and other organism habitat.”	Groves et al (1995) Blood (2001)	MH
15. Benefits fauna?	“Alligator weed provides no benefits to Australia.”	Groves et al (1995)	H
16. Injurious to fauna?	No evidence of harm to fauna although, “In New Zealand and Australia alligator weed causes photosensitisation of skin in light pigmented cattle resulting in cancerous lesions.”	Ensbey (2001)	H
Pest Animal
17. Food source to pests?	Not evident, but some herbivorous insects do feed on plant.	Groves et al (1995)	ML
18. Provides harbor?	May provide harbour for minor pest species (birds).	Groves et al (1995)	ML
Agriculture
19. Impact yield?	It is a strong competitor in rice fields and pastures prone to waterlogging. “Alligator weed has eliminated small crops and turf farming from parts of the Lower Hunter (NSW).”	P & C (2001) Ensbey (2001)	H
20. Impact quality?	“If present, land and associated production can be quarantined and sales restricted due to W1 weed status.”	Ensbey (2001)	H
21. Affect land value?	As above.	Ensbey (2001)	H
22. Change land use?	In terrestrial situations it is difficult to eradicate. The plant is resistant to herbicide treatment and mechanical removal is difficult due to the depth of the root system. Excavation of up to 2 m is recommended. Significant change to use is dictated. See also comment in Q21 above.	Groves et al (1995) Muyt (2001)	H
23. Increase harvest costs?	Occurrence of the weed in water supplies for irrigation and the need to control it can increase production costs.	Ensbey (2001)	M
24. Disease host/vector?	“Provides habitat for disease vectors such as mosquitoes.”	Blood (2001)	M