Australian Quarantine Review Secretariat Australian Quarantine a shared responsibility

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5 WEEDS

The report on weeds was compiled by a team convened by the Cooperative Research Centre for Weed Management Systems, which is based at the Waite Campus of the University of Adelaide. The team was convened by Dr Richard Groves of CSIRO Division of Plant Industry and the Cooperative Research Centre.

The report considered 'weeds' in its broadest sense, including terrestrial and aquatic plants that are regarded as weeds from both the agricultural and environmental aspects. It included weeds in all Australian ecosystems, from temperate to tropical.

5.1 Number and Rate of Incursions

The commissioned report concluded that at least 290 exotic species of plants have become naturalised in Australia over the past 25 years, and that there is a trend for an increasing number of naturalisations when considered in five-year periods during the study period. However, because plants may be present for some time before they are recognised as weeds, many species that have been recorded as naturalised during the study period were probably introduced before 1970. Invasive plants rarely spread as rapidly as pathogens or even insect pests, and most are localised and may remain so for many years. Thus the apparent increase in the rate of naturalisation may not necessarily bear a direct relationship to the number of species introduced and established during the past 25 years.

Species that have become naturalised represent a wide range of plant families — with the Asteraceae, Fabaceae, Iridaceae, Poaceae, Cyperaceae and Salicaceae being the main families represented. Asteraceae, Fabaceae and Poaceae are among the most commonly represented families in the weed flora of most regions, but the Iridaceae and the Salicaceae are significant additions.
The report noted that although most early weeds in Australia came from Europe (particularly northern Europe) there has been a gradual shift in the countries of origin of Australian weeds. This change is most clearly reflected in the weed flora of two Australian States — that of Queensland, in which weeds of American origin have become proportionally more significant recently, and that of South Australia, in which plants originating in the Mediterranean region have become the major group of weeds. For the 87% of cases where the country of origin is known, the report concluded that major sources of plants naturalised in Australia since 1971 were the Americas (25%), Europe (24%) and Africa (23%).
The data available suggest that there may be a trend towards an increasing rate of establishment of weeds over the past 25 years (see Table 8).
Table 8: Number of plants naturalised in Australia between 1971 and 1995

Period	Number naturalised
1971–75	39
1976–80	37
1981–85	70
1986–90	59
1991–95	85
Total	290

5.2 Probable Means of Introduction

Although it is often considered that most weeds are introduced accidentally (and usually as contaminants of imported crop or pasture seed), the report concluded that — based on the data available — most weeds have been introduced deliberately as plants for horticultural purposes. Only when they 'escape' from home or botanic gardens do they become naturalised and a problem as weeds.

The report ascribed a means of introduction for all but about 20% of all species naturalised over the past 25 years. Of those species for which information on means of introduction was available, most had been introduced deliberately, with 65% of the total having been introduced as ornamental plants for horticulture and only 7% for agricultural purposes. The number of species introduced as seed contaminants was low (2%). Few plant species introduced to Australia resulted from natural migration or invasion. For the South Australia flora, Kloot (1984) claimed that about 10% of the total number of plant species could have arrived by a process of long-distance dispersal. Such 'cosmopolitan' species (now regarded as 'native') may have arrived naturally in Australia in prehistoric times and over millennia. Such a group of species seem to be associated particularly with aquatic environments — seashores or wetlands — and could have been brought here by migratory birds. The report found no documented evidence of examples of this means of introduction, and concluded that it is probably not of significance over the past 25 years.
The report's findings are consistent with other studies undertaken in Australia. For example, Kloot (1987) concluded that of 904 naturalised plant species in South Australia, 515 were intentionally introduced, 214 were unintentionally introduced, and no information was available for 175 species. Of the intentionally introduced species, 359 (about 40% of the total number of naturalised species) were ornamental species introduced for horticulture. Other categories of intentionally introduced species included those introduced as fodder plants, for culinary purposes, as hedges, and for medicinal purposes. Among the unintentionally introduced categories were those 'attached to stock', as contaminated seed, ballast plants, contaminated footwear, and contaminated fodder. Similarly, Carr (1993) estimated that as much as 65–70% of the 1221 naturalised introduced taxa in Victoria had been deliberately introduced. Within this group, Carr drew attention to the predominance of environmental weeds currently available in the nursery trade, often as highly popular garden plants. Several submissions to the Review also noted work on the weediness of species imported as pasture plants for grazing livestock in Australia. For example, Lonsdale (1994) reviewed exotic pasture species introduced into northern Australia between 1947 and 1985. Of the 474 species introduced during this period, only 21 (4%) were subsequently recommended for use, but 71 (15%) became recognised weeds and only 4 (0.8%) did not become weeds in any location and could be regarded as unconditionally useful.

5.3 Cost of Weed Incursions

The economic cost of weeds comprise both direct and indirect costs. Most weeds also have some perceived positive values or benefits that are occasionally assessable (e.g. their value in herbal medicine or their contribution to the honey industry). Although there have been some previous attempts to estimate the costs of certain major individual weeds, there has been only one attempt to assess the overall cost of weeds to the Australian community. Combellack (1989) estimated that the total annual cost of weeds was more than $3000 million, including crop and pasture weeds but not the so-called environmental weeds. The indirect costs of the latter group are more difficult to estimate and little economic information is available for them.

The report concluded that it is an almost impossible task to assign even direct costs to most of the recently naturalised species, and noted that in 98% of all cases information is not available for the cost of individual species to Australia. The report considered three instances of species where some costs to Australia are available:
· Chromolaena odorata
The most economically significant recent naturalisation is probably the weedy form of Chromolaena odorata (Siam weed). Since it was inadvertently discovered near Tully in northern coastal Queensland in 1994, direct control costs to governments have amounted to about $460 000. This weedy form of Chromolaena odorata is thought to have been introduced accidentally in the early 1970s as a contaminant of pasture seed. The plant is a major weed that hitherto has not been known to occur in Australia, although it occurs in Indonesia, the Philippines and Papua New Guinea. Before its recent discovery in northern Queensland, Australia funded (through the Australian Centre for International Agricultural Research) a four-year research program on biological control of this weed in South-East Asia at a cost of just under $500 000.
· Kochia scoparia
The species Kochia scoparia is a variable taxon. One form of the species, K. scoparia var. trichophylla, has been grown as an ornamental plant in Australia for many years and apparently has never become weedy. However, in 1990 the weedy form of this species , K. scoparia var. scoparia, was deliberately introduced from the United States to Western Australia as a forage plant suitable for salinised soils. By early 1992, the weedy form had begun spreading from sites where it had been sown and efforts commenced to eradicate it from Western Australia. The eradication program has incurred a direct cost of $50 000 and still has one year to run. More recently, a second incursion of the weedy form has been reported in northern Tasmania after introduction as an impurity in carrot seed.
· Parasitic weeds
Costs of incursions of certain parasitic weeds may be considerable. For example, Cuscuta campestris continues to be repeatedly introduced as a contaminant of seed of various spices, especially sweet basil. Seed of C. campestris may be present in spice seed at a level below that likely to be detected using International Seed Testing Association's protocols. Seed of C. campestris has been found at least three times in individual seed imports to South Australia over the past 25 years (in 1981, 1988 and 1990). Although the latter two incursions were detected rapidly and involved only trivial costs (less than $1000), the 1981 incursion has already resulted in direct costs (for eradication, compensation paid to the landowner, surveys and ongoing inspections) of at least $600 000 to date, and the program continues, with the species last found near Keith in 1993.

5.4 Discussion and Conclusions

The commissioned report examined changes in the rate of naturalisation of plant species in Australia over the past 25 years, and concluded that:

· at least 290 species of plants are known to have naturalised since 1971;
· the apparent annual rate of naturalisation has increased for the period 1981–95;
· those species that have naturalised originate equally in Africa, the Americas and Europe;
· most recently naturalised species are still only locally distributed;
· most of the recently naturalised species have been introduced deliberately and usually legally;
· the direct costs of these recent incursions are known in only few instances; and
· the environmental costs of these recent incursions are unknown and little studied.
The report notes the existence of weedy and non-weedy genotypes in some species such as Chromolaena odorata and Kochia scoparia. The proportion of new introductions that are different genotypes of the same species already in Australia is unknown — they comprise another (and significant) component of the total number of introductions. The report concludes that the number of species known to have naturalised is probably only a small proportion of the total number of introductions over the past 25 years.

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