Notes on the mathematical basis of the UK Non-Native Organism Risk Assessment Scheme

In 2004, the UK Government’s Department for Environment, Food and Rural Affairs (DEFRA) commissioned research with the objective of developing a scheme for assessing the risks posed to species, habitats and ecosystems in the UK by nonnative organisms. The outcome was the UK Non-Native Organism Risk Assessment Scheme . A comprehensive overview of the risk assessment methodology and its application is provided by Baker et al. (2008). Unfortunately, the mathematical basis of the procedure for summarising risks and uncertainties deployed in the Risk Assessment Scheme, as outlined in Baker et al. (2008, section 3.5) and described in more detail in two further papers (Holt, 2006; Holt et al., 2006), is problematic. The problems are discussed in the notes that follow.