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3. Criteria for Weeds of National Significance
3.2. ‘Potential for spread’ criterion 3.3. 'Socioeconomic and environmental values' criterion
3.3.2. Environmental values 3.3.3. Social values The intent of the criteria is to mirror as far as possible and in an objective way the decision-making processes of managers and policy-makers when they consider the issues that make a weed "important". While many factors can be considered when making decisions on priorities, our assessment of WONS is based on four major criteria:
The spread of weeds across a range of land uses or ecosystems is important in the context of socioeconomic and environmental values, but this aspect has the potential to heavily bias the other criteria. As such, land use is generally considered an adjunct rather than a major determinant under the first three criteria. The exception to this is where poor management and inappropriate land uses contribute significantly to a weed’s development, since a weed’s invasiveness, impact and distribution are assessed under best practice situations. In addition, for many land uses, particular weeds may be considered as either an established component or accepted part of the system and are therefore not nationally significant. Several factors and attributes for which data are not available are not included as part of the criteria. They include:
These aspects are considered less important in priority setting, or should be adequately covered in strategy documents developed for individual weeds that receive WONS status.
These criteria concentrate on the biogeography of weed species, principally the key biological factors that affect the ability of a plant to naturalise over a wide geographic area. The Executive Committee of the National Weeds Strategy commissioned a review of techniques based on these criteria being used to predict ‘weediness’ under Australian conditions (Virtue et al. 1998). appendix 1 describes the attributes used to assess weeds by these two criteria. Three reference panels, covering tropical, subtropical and temperate weeds, were selected to rank weeds on the invasiveness and impacts criteria. Each had six or seven members drawn from recognised scientific and policy experts in weeds and weed risk assessment (see Acknowledgments). Members of the panels ranked weeds on each of the attributes, on a scale of one to six (or seven if the "don’t know" category is included). 3.2 ‘Potential for spread’ criterionThe two attributes considered under this criterion were the current and potential distribution of a weed.
Current distribution was determined by asking State and Territory primary industry agencies to provide information on the distribution and density of weed species in areas under their jurisdiction. Maps and pro-forma spreadsheets were provided to facilitate data collection and ensure consistency. The reference panels also helped here: they were consulted about the known and potential distributions for nominated weed species. appendix 2 gives the guidelines followed in preparing distribution data for mapping. The maps included an indication of weed density on a four-point scale — absent, low, medium or high. Any digitised data showing the distribution of a weed were also used. Where States and Territories provided no distribution data for a species, the distributions developed by Parsons and Cuthbertson (1992) and other sources were used. 3.2.2 Potential distributionCurrent distribution information and, where necessary, world distribution data were used to predict the potential distributions of weeds. The computer-based climate modeling program, CLIMATE, was used for predicting the potential distribution and relative abundance of a weed species. These distributions identify only where the climate is suitable for growth of a particular weed. With the exception of riparian and water weeds, no other limitations, such as soil type or land availability, were used to restrict potential plant distribution. Riparian and water weeds grow only where there is an adequate supply of water in a dam, swamp, lake, drainage line, stream or river. For these weeds a potential distribution is generated using the CLIMATE model, but ignoring rainfall. This results in greater areas of infestation being predicted for these species than would be the case if rainfall were included in the analysis. To offset this, only parts of Australia with perennial streams, rivers and lakes were counted using the overlay shown in Figure 2. This map was compiled by Virtue et al. (1998) from AUSLIG TOPO 10 m Geodata – Perennial Streams and Lakes, by combining the data into a 0.5 ´ 0.5 degree grid. This map does not distinguish between fresh and saltwater lakes. In order to treat agricultural, forestry and environmental weeds equally, this criterion was examined in terms of three separate attributes:
This attribute considered economic data for agricultural and forestry weeds. There are no readily available data at a national level that would allow a dollar value to be easily derived. The nominating State(s) were asked to provide estimated dollar values for the current annual costs of controlling weed species. appendix 3 describes the methodology employed. Evidence to support the prime data and method of calculation was also required. This allowed comparisons to be made between the figures supplied and the adjustments to be undertaken to achieve a level of consistency on a national scale. Figure 2. The aquatic land use grid map for Australia devised by Virtue et al. (1998)
3.3.2 Environmental values This attribute captures the conservation value of ecosystems and the potential effect of a weed species on biodiversity. There were no suitable environmental indicators for this category that could be applied on a national basis. A measure could be achieved only by taking a few pertinent environmental indicators and combining them into a ranking. Several indicators that are supported to some degree by national data-sets were developed. Others were supplied by the States and Territories according to guidelines provided (see appendix 4). These were combined into a measure of weed species environmental impact. When combined, the two groups below (biodiversity and conservation indicators), which are comprised of four indices give a measure of the severity of ecosystem degradation that may result from invasion by a particular weed species.
Weeds — be they terrestrial, aquatic or marine — can have a significant effect on plant and animal species diversity. Again, there was a lack of national data-sets describing the impact that invasive plants are having on biodiversity. Consequently, only two measures considered relevant to the criterion could be identified for compilation of national data-sets: the number of threatened species (that includes critically endangered, endangered and vulnerable species as defined by the International Union for the Conservation of Nature (IUCN)), and the number of threatened conservation areas. Threatened plant species are generally more numerous in highly developed areas such as south-eastern Australia. This is a result of extensive land clearing. Conversely, threatened communities tend to be more uniformly spread and, in the case of wetlands, more prevalent, in northern Australia. In order to readjust the imbalance caused by the higher numbers of threatened plant species in cleared areas, the numbers of designated special conservation areas threatened by a weed species were also counted and combined with the threatened plant species to give a biodiversity indicator.
The IUCN Red List (1994) defines a number of plant categories that can be combined into a threatened group. The data for Australia has been published by Briggs and Leigh (1995), and are held and revised by Australian herbaria. 3.3.2.1.2 Number of threatened conservation areasSpecial conservation areas included World and Natural Heritage areas, Ramsar Treaty Wetlands and Significant Wetlands of Australia, special conservation areas and areas identified under the Regional Forest Agreements. These areas have been included only where biodiversity significance has been identified through an agreed rational and scientifically credible process. No area of land has been counted more than once for this indicator. The ratio of the threatened sites to the overall total provides a measure of the impact that a weed is having.
Australia has been divided into 80 biogeographic regions known as IBRA (Interim Biogeographic Regionalisation of Australia) regions. This classification system is the only continent-wide ecosystem regionalisation agreed upon by all States and Territories. The regions are considered of equal value despite varying in area between 2,372 sq km and 423,751 sq km. The smaller regions occur within 300 km of the coastline, have a relatively high rainfall during the growing season and, in many cases, are mountainous. Of the largest regions, most are in arid or semi-arid areas. The use of IBRA regions (Thackway and Cresswell 1995) more closely approximates plant species richness under threat from a weed. It also avoids any dominance of Australian dryland areas that would arise from using only the total land area infested. The number of IBRA regions infested by a weed species indicates the weed’s ability to invade many ecosystems. The count was made from the weed distribution maps. The greater the number of regions infested, in combination with monoculture potential, the more likely is the weed to have a severe environmental impact. 3.3.2.2.2 Monoculture potentialMonoculture potential, expressed as infestations perhaps exceeding 100 sq m, represents the degree to which a weed species invades an ecosystem eliminating indigenous species. A pure stand of a particular weed species that crowds out all other species will gain a high score, while a low density, less competitive weed will gain a low score. Generally, weed species will strongly compete for light or water or both, and may suppress native vegetation in the establishment, juvenile or mature phases of growth. This indicator integrates all these factors. A rating for this indicator was gained from the reference panels that assessed the ‘invasiveness’ and ‘impacts’ criteria. Weeds can have a range of positive or negative social values that are not covered in the other criteria. These include: No objective data were available that adequately described the social impacts of weeds. Because of the diversity of weed species considered, every plant created a unique set of concerns and these were often not comparable. The reference panels were asked to provide additional information on this attribute. The range of social impacts was then compared, balancing positive and negative effects, which resulted in a four-step rating, ranging from nil to high.
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