A scoping literature review on the impacts of non-native species on the native terrestrial biodiversity of an oceanic island

Background Many non-native species have been introduced to oceanic islands, with a subset becoming invasive, which pose the greatest threat to native terrestrial biodiversity. Yet, existing information on their impacts, particularly at the island scale, has rarely been synthesised. Addressing this gap is essential for revealing neglected aspects and for prioritising conservation management to optimise the use of scarce resources. Here, we use one volcanic oceanic island as a model to characterise the knowledge landscape about the impacts of non-native species on native biodiversity. Specifically, we (1) inventory the studied mechanisms through which non-native species affect native biodiversity; (2) evaluate the extent to which studies assess impacts at the level of biological organisation and/or biotic interactions; (3) assess the severity of impacts of non-native taxa and (4) highlight research gaps requiring most attention at the island scale. Method Mauritius was used for a scoping review based on four databases (Dimensions, Google Scholar, ScienceDirect and SpringerLink) to systematically search and identify relevant studies on the impacts of non-native species on native terrestrial biodiversity. We used the Environmental Impact Classification for Alien Taxa (EICAT) framework and its extension, EICAT+ to categorise the impact mechanisms and magnitude of impact. We searched for all records from the inception of each database until November 2023. We included 62 publications in the final analysis, selected from a total of 1,760 articles screened following the PRISMA guidelines for scoping reviews. Results We recorded a total of 273 impact records between non-native and native species. Non-native species of plants and mammals were the most common, affecting a wide range of native taxonomic groups. Negative impacts predominated (65.2%), followed by positive impacts (34.1%), with few neutral impacts (0.7%). Competition and predation were the most studied negative impact mechanisms, typically impacting many native species while positive impacts of non-native species were mainly associated with the provision of trophic resources and typically impacted few species. The main impacts of non-native species on native species studied were at the level of biological organisation ( N = 169), while only 22 of the impact records investigated their effects on biotic interactions. Conclusion Limited attention has been given to indirect mechanisms and the impacts of non-native species on biotic interactions. Future studies should prioritise these areas, as indirect mechanisms may reveal cumulative and often insidious pathways through which non-native species accelerate biodiversity loss, especially on islands. Investigating their effects on biotic interactions is critical for detecting cascading impacts to inform more effective and comprehensive conservation strategies.