BOX 6-1 Western Australia Weed Risk-Assessment System

Some states in Australia have established their own weed-assessment system in addition to using the federal system developed by the Australian Quarantine and Inspection Service (AQIS). The system for Western Australia (WA) is perhaps the most elaborate of these state systems (Randall and Stuart 2000). In establishing their own systems, states in Australia have recognized that environments at one end of the Australian continent differ enormously from those elsewhere in the country, and a nonindigenous species could well be invasive in one state or region and be innocuous (or even fail to become established) in another.

The WA system was designed primarily to evaluate requests for the deliberate introduction of plants into the state, but its protocol could also be applied to accidental introductions. Unlike the AQIS system, the system for WA is not trait-based but relies extensively on the history of nonindigenous species in WA and elsewhere. Expert judgment (for example, “assessors decision points”) is used to determine the extent to which a species with a history of naturalization might pose a threat in WA. An essential component of the system is the large database of species and their ecology that has been compiled by R. Randall, plant profiler, for Western Australia Agriculture. In addition, the protocol uses the results from a climate-matching program (CLIMEX) in determining whether the species could persist anywhere in the state, on the basis of the similarity of climates in the native range and potential new ranges in WA. Although the system appears to operate as a basis of allowing or prohibiting any proposed species entry, a risk assessment is performed, in effect, through determination, with a full weed assessment, that a species is an important weed and poses an immediate or imminent threat.

The WA system includes an essential but often overlooked component of assessment—correct identification of the species. The assessment of nonindigenous species worldwide contains celebrated cases in which evaluation and control were delayed by the misidentification of species that proved harmful. For plants

that is true, the overall likelihood of introduction should also be low. The analysis of consequences is more robust, and justification, including relevant literature, is provided for each of the rankings. Furthermore, although the characteristics used to determine the likelihood of invading (as part of the consequences evaluation) might be challenged, it is clear what characters were used and how they were determined.

Quantitative scenario analysis (Kaplan 1993, Firko 1995a) has been used to produce quantitative assessments for the likelihood of establishment. The process involves identifying and enumerating all potential pathways by which establishment can occur. Probability distributions are used to model the likelihood of each link in the chain and combined to estimate the overall likelihood of a nonindigenous species becoming established (Figure 6-4). Managing the risk consists



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