Mapping cumulative impacts of invasive alien species on ecosystem services

Marine invasive alien species (IAS) play a transformative role in ecosystems, driving ecological changes and affecting ecosystem services (ES) and socio-economic systems. Each IAS uniquely influences ecosystems, causing negative or positive impacts of varying intensities across locations, which collectively result in cumulative impacts. To address these complexities, we developed the Cumulative IMPacts of IAS on Ecosystem Services (CIMPAL-ES) index. This index combines the spatial distribution of IAS and ecosystem services with the reported magnitude of impacts and the strength of supporting evidence, providing a quantitative score of cumulative impacts for a given area. We applied this method to two case studies, the Mediterranean Sea and the Aegean Sea, to evaluate its applicability across distinct spatial scales and datasets. In the Mediterranean, we analyzed 80 IAS using a classification of 14 ES. Documented positive and negative impacts were identified for 12 of these services and mapped on a 100 km² grid based on species ecology and affected area. For the Aegean Sea, we applied the same approach for 30 IAS on a finer 1 km² grid. The results revealed marked variability among IAS, with some exerting exclusively positive or negative effects, while others had mixed impacts, benefiting certain services while negatively affecting others. Food provision was the most impacted ES in the Mediterranean, displaying both positive and negative effects. In the Aegean Sea, lifecycle maintenance was the most negatively impacted service, while cognitive effects showed the highest positive impacts. The CIMPAL-ES index highlighted strong spatial heterogeneity, identifying hotspots of cumulative impacts. In the Mediterranean sensitivity analysis, mean and median impact weights were strongly correlated with the baseline spatial rankings and retained 85.3–91.7% of identified hotspots. These findings represent the best current estimate of spatial variation in IAS impacts on ecosystem services in the Mediterranean Sea. The proposed methodological framework provides a baseline for refinement as more precise data become available, offering actionable insights for management. By identifying hotspots of negative and positive impacts and highlighting key ecosystem services and species, this approach can support targeted and effective IAS management.