Prioritization and thresholds for managing biological invasions in urban ecosystems

Urban areas are hubs for invasive alien (non-native) species (IAS) which can cause major problems in and around urban areas. Urban conservation practitioners face complex decisions about which IAS require management, where and when these management interventions are necessary, and how to implement them effectively. While researchers increasingly advocate the assignment of critical thresholds informing IAS management decisions, little attention has been given to the development of criteria for such thresholds or related practical application protocols in the context of urban environmental management. We review approaches that have been applied to manage IAS in urban areas and evaluate which thresholds are considered and applied before, during, and after management actions. Our literature search revealed 75 publications, with clear geographic bias. Less than half of all studies had implications for the prioritization of IAS management in urban areas and only 31% of these directly assessed such priorities. Only 8% of studies referenced a threshold or decision trigger when proposing management approaches for IAS in urban areas. This suggests that decisions to manage IAS in urban areas are often made on an ad hoc basis, without considering objective and transparent criteria, and/or are prompted by external factors (such as funding availability) that are not recorded in the formal literature. There is a need for IAS management in urban areas to be evidence-based and informed by well-tested measures and transparent decision triggers. Resources should be directed towards integrating evidence-based thresholds and tailored prioritization schemes into urban management frameworks to support decisions about what, where, and when IAS management is required.