Impact Assessment - Caltrop (Tribulus terrestris) 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?	“A prostrate summer-growing annual herb.” Even in dense infestations, its presence does not hinder human access.	P & C (2001)	L
2. Reduce tourism?	Its presence is likely to affect recreational activities. The fruit is a woody burr with sharp rigid spines to about 6 mm long. It splits into five segments each with two spines. The spines on the fruit and segments can easily penetrate human flesh causing discomfort. Infestations may limit recreational activities.	P & C (2001)	MH
3. Injurious to people?	The fruit and resultant segments present rigid spines to about 6 mm long. Fruit are formed continuously through the summer and autumn. In India, plants have produced up to 1,000 fruit.	P & C (2001)	MH
4. Damage to cultural sites?	In dense infestations in NSW the plant is known to provide cover of up to 45 cm deep for several months of the year. Also, in shaded areas or when competing with taller plants the stems will grow erect. The physical structure of site is not affected, but dense infestations would seriously affect the aesthetics of an area.	P & C (2001)	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?	“It occurs mostly on dry sandy soils where there is little competition.” Its presence may help to reduce soil erosion marginally during its growth and flowering period.	P & C (2001)	L
8. Reduce biomass?	Grows best in disturbed areas with minimal vegetation. Invader replaces biomass.	P & C (2001)	ML
9. Change fire regime?	A summer annual it adds little if any fuel load. Dry matter would present little or negligible effect on fire risk.	P & C (2001)	L
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (E); CMA=North Central; Bioreg=Victorian Riverina; VH CLIMATE potential “…caltrop is most frequently found in the north of the State…where it occurs in railway yards, nature strips, parks and waste areas in almost every town.” “Prefers dry sandy soils where there is little competition.” Allelopathic. Major displacement of grasses or ground covers.	P & C (2001)	MH
(b) medium value EVC	It is not likely to occur in medium value EVCs.		L
(c) low value EVC	It is not likely to occur in low value EVCs		L
11. Impact on structure?	Grows best in disturbed areas with minimal other vegetation. It is reported to have allelopathic properties which inhibit the growth of grass seedlings, and its extensive root system can compete effectively for moisture and nutrients. In dense infestations it can dominate the ground flora.	P & C (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	The areas where T. terrestris inhabit (railway yards, nature strips and waste areas) would not be a significant food source for grazing fauna. Its presence may have a minor effect in reducing food source.	P & C (2001)	ML
15. Benefits fauna?	No known benefits.		H
16. Injurious to fauna?	“Spines on the fruit damage the feet of animals.” When eaten, the fruit can damage the mouth and possibly the lining of the stomach and intestines. It is toxic to sheep and may present similar properties to native fauna.	P & C (2001)	H
Pest Animal
17. Food source to pests?	Not known as a food source to pests.		L
18. Provides harbor?	A prostrate annual herb with spiny fruit, it is unlikely to provide habitat for pest animals.	P & C (2001)	L
Agriculture
19. Impact yield?	A significant pest in crops, it is able to extract soil moisture from great depth. This allows it to offer severe competition under very dry conditions. In pasture situations, in the absence of other suitable fodder, animals will graze new growth of T. terrestris, which may cause stock losses. The level of impact is not documented but losses in New South Wales and Western Australia have been reported.	Holm et al 1 P & C (2001)	MH
20. Impact quality?	It contaminates harvested product, particularly in the dried fruit industry, and is a vegetable fault in wool. However, presence of the seed does not affect wool processing.	P & C (2001) AWTA Ltd2	MH
21. Affect land value?	The weed causes some agricultural loss, but it can be readily controlled with suitable mechanical and chemical controls. Its presence may affect land value somewhat.	Holm et al	M
22. Change land use?	The potential threat to grazing animals presented by the weed may dictate a temporary change to land use. In NSW grows exceptionally well on farms with a system of winter cereal crops and sheep grazing. A period of continuous cropping may be required until the weed is controlled.	P & C (2001)	M
23. Increase harvest costs?	Seeds and seed segments are a nuisance causing painful injury to fruit pickers and shearers. Continuous interruption (to remove the offending spine) may increase the time to harvest.	P & C (2001)	M
24. Disease host/vector?	None evident		L