Assessing the success of a horizon scanning approach in predicting invasive non-native species

Despite increasing awareness of invasive non-native species (INNS) and enhanced biosecurity controls in many countries, INNS are still arriving and establishing in new destinations, remaining a globally acknowledged threat to native biodiversity. Preventing the introduction of INNS, as opposed to managing them once they have arrived, is recognised as the most effective approach to their management. Horizon scanning represents one of the key tools to identify high-risk INNS that are yet to arrive within a region and has been applied in many contexts around the world, but to date there have been no studies that systematically assess the effectiveness of this approach. Here, we revisit the horizon scan for Great Britain conducted in 2013 that assessed the likelihood of high-risk INNS arriving within the next ten years, establishing and having an impact on biodiversity and ecosystems. We assess the success of this exercise in predicting arrival of these species within the subsequent ten years. Ninety-two species were shortlisted in the 2013 horizon scan. Overall, 31 of the 92 species identified in the 2013 horizon scan have arrived by 2023. We found that 12 of the top 20 species had arrived within ten years. In predicting arrival, there was a significant effect of arrival previously within Great Britain, and the number of countries in Western Europe and Baltic countries in which an INNS was found prior to 2013. We conclude that horizon scanning provides a rapid, affordable, and successful mechanism to predict the arrival of high-risk INNS. We highlight the importance of citizen science, including biological recording, and of local expertise for detecting and documenting arrival of INNS and discuss knowledge gaps that could help inform and improve future horizon scanning. In addition, we also recommend regularly repeating horizon scanning exercises to support biosecurity and awareness raising for INNS.