The Determination of Weeds of National Significance
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1. Introduction Weeds — plants growing where they are not wanted — cause high economic, social and environmental costs in many parts of Australia. Some weed species are so widespread and insidious that they have become of national significance. Identifying and ranking those species was the objective of the study reported here. Weeds are not a recent phenomenon or focus of attention within Australia. There are over 2700 plant species that have been documented as weeds in Australia (Lazarides et al. 1997) with approximately 10% of naturalised plant species becoming weeds that cause significant economic and environmental damage (Williamson and Fitter 1996). State and Territory governments have declared over 370 plant taxa noxious weeds (National Weed Strategy Web Site 1999), while around 25% of naturalised plant species are significant or potentially significant environmental weeds (Humphries et al. 1991). Weeds are one of the most commonly reported land-degradation problems on broad-acre and dairy farms (Mues et al. 1998). Establishing successful primary industries since European settlement of Australia has inadvertently assisted the introduction and spread of increasing numbers of weed species. In addition, the environmental consequences of introduced ‘agricultural’ plants have been largely ignored. Lonsdale (1994) found that only 5% of non-native pasture plants (21 of 463 species) introduced into northern Australia between 1947 and 1985 were useful to agriculture, while 13% were subsequently listed as weeds. Of the 5% useful plant species, 81% became weeds on non-grazing land, with less than 1% (4 of 463 species) proving to be beneficial to agriculture without untoward side-effects. Primary industries are not the sole culprits for Australia’s weed problems. Public demand for exotic species for ornamental and cultural purposes has also contributed. At least 30% of noxious weed species were originally introduced as garden plants (Panetta 1993). Most of Australia’s waterweeds are exotic species imported for use in garden ponds and aquaria (Csurhes and Edwards 1998). While the problem is widely recognised at all levels, the focus has generally been on short-term solutions. In many cases, attention has been directed at control or eradication of specific weeds in a particular area, without fully understanding the reasons for the continued spread of weeds. The results have generally been less than satisfactory, leading to ever rising concern. The scale of the weed problem is increasing, as are the effort and money expended on it. Which species are weeds of national significance (WONS)? Until the study reported here, no objective criteria existed to permit identification and ranking of those weed species against which nationally coordinated action programs would bring greatest benefits. The circumstances that make a weed a problem can vary with time and place and assessments can be subjective. Consequently, a high degree of precision in developing national significance criteria is neither attainable nor necessary. What is important is to be able to compare weeds by a system of rankings or relativities. The determination of WONS as described in this report, is a system to prioritise weed problems for national action as part of the second goal of the National Weeds Strategy. It is not a scientific process. Rather, it attempts to draw together meaningful indicators where few national data-sets exist. Techniques were required that would enable discrimination between, sometimes very similar, species. The process devised combines measurable indicators to form an objective, transparent and defensible ranking system for weeds. The criteria used to determine WONS had to:
allow ranking of weed species, preferably with a single index or score.
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