Joana Raquel Silva Vicente, Ana Sofia Vaz, Mariona Roige, Marten Winter, David Clarke, Bernd Lenzner, Melodie A. McGeoch

Monitoring the progress parties have made toward meeting global biodiversity targets requires appropriate indicators. The recognition of Invasive alien species (IAS) as a biodiversity threat has led to the development of specific targets aiming at reducing their prevalence and impact. However, indicators for adequately monitoring and reporting on the status of biological invasions have been slow to emerge, with those that exist being arguably insufficient. We performed a systematic review of the peer-reviewed literature to assess the adequacy of existing IAS indicators against a range of policy-relevant and scientifically valid properties. We found that very few indicators have most of the desirable properties, and that existing indicators are unevenly spread across the components of the Driver-Pressure-State-Response and Theory of Change frameworks. We provide three possible reasons for this: i) inadequate attention paid to the requirements of an effective IAS indicator, (ii) insufficient data required to populate and inform policy-relevant, scientifically robust indicators, or (iii) deficient investment in the development and maintenance of IAS indicators. This review includes a gap analysis of where current inadequacies in IAS indicators exist, and provides a roadmap for the future development of indicators capable of measuring progress made toward mitigating and halting biological invasions.