The potential for new crops to become unwanted 'weeds' – how do we assess the risk?
Steve M. Csurhes
Policy Officer (Potential Pests)
Pest Management Strategy
Queensland Department of Natural Resources
Locked Bag 40, Coorparoo Delivery Centre
Weeds cost Australia $3 billion per annum and the number of weed species is currently increasing at the rate of at least eleven new weed species a year. More than 300 plant species are declared 'noxious' in Australia and many have had multi-million dollar impacts on the economy, as well as causing irreversible damage to native ecosystems.
Perhaps 1-2% of all new plant species introduced into Australia will themselves become weeds. Some of these species will have significant impacts on agriculture, native plant communities and human health.
Hence, there is a significant risk that non-indigenous plant species, once introduced into this country, will become unwanted pests sometime in the future. To date, approximately 5% of plant species introduced into Australia have naturalised, that is, have persisted and reproduced in the wild.
Additional plant species continue to be imported either accidentally, as contaminants of seed, or deliberately as garden ornamentals, timber species, cattle fodder or as new crops.
Predicting the risk
Most plant introductions do not become major pests. It is difficult to predict which plant species will become significant weeds. However, attempts have been made to identify the species’ attributes that confer weed risk. These have been discussed by Csurhes and Edwards (1998), as follows.
Forcella et al. (1986) and Zamora et al. (1989) suggested that an invading plant’s weed potential is indicated by the climatic characteristics of its native country, its history of spread in other countries and its similarities to recognised weeds.
Carr et al. (1992) listed criteria for assessing weed potential:
Noxious potential can often be inferred by association (Forcella 1985), since the families Poaceae and Asteraceae contain almost 40% of the world’s weeds of agriculture (Henry and Scott 1981, Radosevich and Holt 1984).
Seed and fruit morphological features that facilitate dispersal have been recognised as important indicators of potential invasiveness (Salisbury 1961, Howe and Smallwood 1982, Carr et al. 1992).
Generalised indicators of potential weediness have been considered by Hazard (1988) and incorporated into a scoring system designed to quantify the risk posed by the importation of nominated plant species into Australia.
Hazard’s indicators of potential weediness risk include:
Panetta (1993) suggested that a weed’s history as a pest elsewhere in the world (in similar climatic conditions) may be the most reliable basis for predicting weediness in Australia.
In 1997, the Australian Quarantine and Inspection Service (AQIS) implemented a ‘Weed Risk Assessment’ system (WRA). The WRA is used to assess all proposed plant imports for weed risk, based on a number of criteria including:
Based on these criteria, a candidate species is assigned a numerical ‘score’ and is either rejected, assessed in more detail or approved for import depending on its score. Quarantine law requires all new plant species proposed for import into Australia go through the WRA process.
The Queensland Department of Natural Resources has assessed the weed risk of several hundred non-indigenous plant species and has prepared a 'priority list' of potential weed species considered to pose the greatest threat.
These species have been listed under the Queensland Rural Lands Protection Act as ‘declared potential weeds’ and it is an offence to introduce, cultivate or sell these plants in Queensland.
Most of these plants are potentially useful species (for example, Prosopis are useful fodder trees, Miconia are attractive garden ornamentals and Equisetum are potential new medicinal crops).
Despite these benefits, the net impact of these plants on the economy is predicted to be negative. These species have well-documented histories as major weeds overseas in climates similar to that of Australia and as such, possess the key attribute considered to confer weed potential.
If they are cultivated, escape is inevitable and the resultant proliferation could cause multi-billion dollar damages.
As an example, the introduction of some species of Striga or Orobanche (parasitic plants) could result in contamination of Australian export grain and the loss of export markets, which would have a devastating impact on the economy.
While some new crop plants could become valuable assets to Australia, others could become troublesome liabilities.
Proponents of new crop plants are advised to check with Commonwealth quarantine authorities prior to importing new material and to check with relevant State pest management agencies prior to release of material stored in Australian genetic resource centres.
A thorough assessment of weed risk will help ensure that the proposed crop does not escape cultivation to impose substantial costs on other industries.
References and further reading
Carr, G.W., Yugovic, J.V. and Robinson, K.E. (1992). Environmental Weed Invasions in Victoria - conservation and management implications. (Department of Conservation and Environment and Ecological Horticulture, Victoria).
Csurhes, S.M. and Edwards, R. (1998). Potential environmental weeds in Australia: candidate species for preventative control. (Environment Australia, Canberra).
Forcella, F., Wood, J.T. and Dillon, S.P. (1986). Characteristics distinguishing invasive weeds within Echium (bugloss). Weed Research 26, 351-64.
Hazard, W.H.L. (1988). Introducing new crop, pasture and ornamental species into Australia - the risk of introducing new weeds. Australian Plant Introduction Review 19, 19-36.
Henry, R.D and Scott, A.R. (1981). Time of introduction of the alien component of the spontaneous Illinois vascular flora. American Midland Naturalist 106, 318-324.
Howe, H.F. and Smallwood, J. (1982). Ecology of seed dispersal. Annual Review of Ecology and Systematics 13, 201-228.
Panetta, F.D. (1993). A system of assessing proposed plant introductions for weed potential. Plant Protection Quarterly 8, 10-14.
Radosevich, S.R. and Holt, J.S. (1984). Weed Ecology. (John Wiley and Sons, New York).
Salisbury, E. (1961). Weeds and Aliens. (Collins Publishing, London).
Zamora, D.L. Thill, D.C. and Eplee, R.E. (1989). An eradication plan for plant invasions. Weed Technology 3, 2-12.