Developing a hybrid weed risk assessment system for countries with open and porous borders: insights from Bhutan

Intentional introduction of alien plant species through increased global trade and movement of people worldwide has contributed to the current problem of invasion by alien plant species and their significant impacts to primary production, the environment, human health and society. Weed Risk Assessment (WRA) systems have been developed to help screen out those potentially invasive alien plant species prior to their introduction (i.e. pre-border) and manage those that pose a risk once they arrive (i.e. post-border Weed Risk Management (WRM)). Despite some developed countries (e.g. Australia and New Zealand) having successfully developed, tested, and implemented WRA systems, broad-scale application in developing countries that have open and porous borders is yet to occur. This is, in part, because the WRA approach developed is predicated on the assumption that effective management of the movement of goods and people across borders is both practical and achievable, and if not then a post-border WRM approach is the ‘best’ option. Here, we examine how pre-border WRA and post-border WRM systems can be implemented in developing countries with open and porous borders using insights from the land-locked country of Bhutan to highlight the issues and practicalities of implementing a risk assessment approach for alien plants. Firstly, we examine the limitations and benefits associated with implementing a WRA approach at the border in countries with open and porous borders. Secondly, we explore the limitations and benefits of managing the risks using a post-border WRM approach. This assessment revealed that whilst some aspects of a both pre- and post-border systems could be adopted in countries with open and porous borders, it was impractical to adopt the pre-existing systems in such instances. Thus, many countries may adopt a hybrid approach for which no guidance is currently available. To address this issue, we propose a formal hybrid weed risk model, using elements of both pre- and post-border systems based on the individual circumstances of the country. To account for such circumstances, we constructed a series of decision trees to help managers and policy makers determine the most appropriate hybrid model. These decision trees provide a standardised structure for identifying and justifying the elements used in a hybrid model for managing the risk from alien plants in countries with open and porous borders. The adoption of such a hybrid approach will help prevent and manage potential invasive alien plant species in these countries through a more formalised risk assessment approach.