Impact Assessment - Thorn Apple, common (Datura stramonium) 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?	Erect annual herb, commonly 30 to 60 cm high. Unlikely to affect human access.	P & C (2001)	L
2. Reduce tourism?	In Victoria, it occurs in medium to large populations in riparian situations. Dense infestations would have a major effect on aesthetics. The leaves produce an unpleasant odour when crushed and the fruit is armed with spines; some recreational activities may be affected.	Carr et al (1992) P & C (2001)	MH
3. Injurious to people?	“All parts of the plant, particularly the seeds, are poisonous.” The seed capsule bears approximately 100 slender spines.	P & C (2001)	H
4. Damage to cultural sites?	Dense patches likely to create a negative visual effect.		ML
Abiotic
5. Impact flow?	Terrestrial species.	P & C (2001)	L
6. Impact water quality?	Terrestrial species.	P & C (2001)	L
7. Increase soil erosion?	“Dense infestations occur in pastures and on river flats, producing a complete ground cover. As an annual, bare ground left when plant dies would be subject to erosion.	P & C (2001)	L
8. Reduce biomass?	“Thornapples now occur…in Victoria in the irrigated cropping areas close to Melbourne, at Orbost, along the Murray River, and I the tobacco crops of the north east.” In these situations the weed would replace biomass.	P & C (2001)	ML
9. Change fire regime?	“The plants die in the cooler autumn weather, but dead plants bearing capsules may remain standing through winter into spring.” Potential for a minor change in the frequency of fire risk within dense patches.	P & C (2001)	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Riparian woodland (E); CMA=North Central; Bioreg=Victorian Riverina; VH CLIMATE potential. Compete strongly with summer species for moisture and nutrient. In Victoria, it occurs in medium to large populations. Major displacement of annual grasses/ground covers.	P & C (2001) Carr et al (1992)	MH
(b) medium value EVC	EVC=Riverine grassy woodland (D); CMA=Murray Goulburn; Bioreg=Murray Fans; VH CLIMATE potential. Impact as in 10(a) above.	P & C (2001) Carr et al (1992)	MH
(c) low value EVC	Does not occur in any low value EVC in Victoria.		L
11. Impact on structure?	“Dense infestations occur in pastures and on river flats, producing a complete ground cover. D. stramonium has an allelopathic effect on other species,…and it is probable that this property contributes greatly to its competitiveness with many crops.” Likely to have a major impact on <60% of the lower stratum.	P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	In Victoria, it is widespread in medium to large populations in riparian vegetation. Likely to reduce available fodder for fauna species in those areas.	Carr et al (1992)	ML
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“All parts of the plant, particularly the seeds, are poisonous.”	P & C (2001)	H
Pest Animal
17. Food source to pests?	Not known as a food source to pests.		L
18. Provides harbor?	Not known to provide harbor for pest animals.		L
Agriculture
19. Impact yield?	“Thornapples…are of most importance in Queensland and northern New South Wales where they compete with many crops. Mung beans and soybeans are seriously affected and yield losses of up to 90% have been recorded.” Serious impact on quantity.	P & C (2001)	H
20. Impact quality?	“Thornapple seeds are difficult to separate from seeds of grain sorghum…and contaminated samples are downgraded or rejected.” Produce rejected for sale.	P & C (2001)	H
21. Affect land value?	Primarily a weed of cropping, control can be achieved by repeated cultivations. However, as H. stramonium plants can produce up to 30,000 seeds and seeds remain viable for more than 30 years, control is a long-term exercise. With the serious impact this plant can have on agricultural production, land values may be greatly affected.	P & C (2001)	M
22. Change land use?	Normal farm practices in cropping situations may be sufficient to control the plant, and a change in land use does not appear necessary. Grazing land may be temporarily unavailable while control efforts are undertaken.		M
23. Increase harvest costs?	“The plants being large and coarse also interfere with harvesting equipment.” Greater time and labour required to conduct harvest.	P & C (2001)	M
24. Disease host/vector?	“They [Datura spp.] act as alternative hosts of insect pests and diseases of solanaceous crops such as tomatoes, tobacco, and potatoes but the significance of this is not known in Australia.” Potential impact?	P & C (2001)	H

Impact Assessment Record - Thorn apple, common (PDF - 32KB)
Impact Assessment Record - Thorn apple, common (DOC - 60KB)
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