Abstract
This chapter provides guidance on mapping risks posed by invasive alien species to support pest risk analysis (PRA), the process required to justify phytosanitary measures. Because pest risk mapping can be challenging and resource-intensive, the situations in which risk maps are particularly useful are highlighted. The procedures described focus on mapping areas where the pest can establish and potentially cause the greatest harm. In the first stage of risk mapping, the factors that might influence the potential distribution and impacts of an invasive alien species are identified and the data are assembled and mapped. In the second stage, the maps of each factor are combined using matrix rules to generate areas of potential establishment and highest risk. These general procedures are illustrated with two examples. Risk maps for the western corn rootworm, a maize pest that has invaded Europe, are based on the combination of maps of climatic suitability, the presence of sandy soils, the distribution of grain and forage maize and the value of these commodities in Europe. Uncertainty is estimated by varying the classification of climatic suitability to obtain the worst, best and most likely scenarios. Risk maps for the common water hyacinth, an invasive plant on the Iberian Peninsula, are based on maps of climatic suitability, the distribution of suitable wetland habitats and areas of conservation importance. The chapter concludes by summarizing some of the major challenges that remain to enhance the production of risk maps for PRA and their interpretation by risk managers.