Weed risk assessment: a way forward or a waste of time?

1. Many alien weeds pose significant environmental and/or economic threats across the globe, and methods to assess the potential risk of species introductions are key components in the management of plant invasions. Three broad approaches have been adopted in weed risk assessment: quantitative statistical models, semi-quantitative scoring and qualitative expert assessment. Yet, the effectiveness of these different approaches is rarely evaluated. By bringing together perspectives drawn from statistics, complexity theory, bioeconomics and cognitive psychology, this review presents the first interdisciplinary appraisal of whether weed risk assessment is a valuable tool in the management of plant invasions. 2. Problems in obtaining an objective measure of the hazards posed by weeds, challenges of predicting complex hierarchical and nonlinear systems, difficulties in quantifying uncertainty and variability, as well as cognitive biases in expert judgement, all limit the utility of current risk assessment approaches. The accuracy of weed risk assessment protocols is usually insufficient, given inherent low base-rates even when the costs and benefits of decisions are taken into account, and implies that the predictive value of weed risk assessment is questionable. 3. Current practices could be improved to address consistent hazard identification, encompass a hierarchy of spatio-temporal scales, incorporate uncertainty, generate realistic base-rates, and train risk assessors to limit cognitive biases. However, such refinements may still fail to predict weed risks any better than a knowledge of prior invasion history and quality of climate match. 4. Alternative approaches include scenario planning that seeks qualitative inputs regarding hypothetical events to facilitate long-range planning using multiple alternatives each explicit in their treatment of uncertainty. This represents a change from prevention towards adaptive management where the difficulty in prediction is acknowledged and investment targets early detection, mitigation and management. 5. Synthesis and applications. Scenario planning may be particularly suitable for weeds as they can be rapidly surveyed and have sufficiently long lag phases between naturalisation and invasion that early detection is often feasible. If integrated with assessments of ecosystem vulnerability to invasion and interventions to improve ecosystem resilience, it would deliver a robust post-border approach to invasive plant management. This approach would address threats from new introductions as well as 'sleeper weeds' already present in a region.