Interpreting expert-judged priorities of invasive alien plant species by ex post weed risk scoring: A study in Japan

Prioritization for the management of invasive alien plants, a serious threat to biodiversity, often relies on judgment by expert scientists. While comprehensive, expert judgment may result in minimal explanations as to why certain species are prioritized over others and consequently may pose challenges for practitioners in justifying targeted species in their projects. In such cases, ex post scoring of weed risk may help uncover the logic behind experts’ comprehensive judgment, since it would enable us to link the assigned priority of species to explicit scores on various aspects of weediness. To rationalize existing expert-judged priorities, we conducted weed risk scoring of listed invasive alien plants in Japan, using the modified criteria of the post-border weed risk assessment system developed in Victoria, Australia. We scored 45 alien plant species with known expert-judged priorities for management (higher or lower) based on the assessment questions on invasiveness and impact. We then analyzed the correlations between the priority category and the questions and characterized the higher-priority species based on the scores of correlated questions. The higher-priority species showed greater total scores than the lower-priority species. The priority category was significantly correlated with specific questions on lifeform, growth rates, reproductive traits and impacts on social, environmental and agricultural assets. The higher-priority species identified by Japanese experts were aquatics causing damage to water flow and quality, and species that have shown superior capabilities for growth and reproduction and are displacing native species. Our results demonstrate that ex post scoring of the invasiveness and impact of listed alien plant species is a useful approach for interpreting the logic behind experts’ prioritization. Practitioners could better justify their targeted species for control by linking assigned priorities to scores of important questions on weediness.