Meta-analysis on the ecological impacts of widely spread non-indigenous species in the Baltic Sea.

The introduction of non-indigenous species (NIS) is a major driver for global change in species biogeography, often associated with significant consequences for recipient ecosystems and services they provide for humans. Despite mandated by several high-level international legislative instruments, comprehensive quantitative evaluation on ecosystem impacts of marine NIS is scarce and lack a robust and data-driven assessment framework. The current study is aiming at fulfilling this gap, through quantitative assessment on the effects of the widespread NIS of the Baltic Sea on multiple ecosystem features and components including direct food-web effects. The outcomes of this study allowed identifying the most impacting widespread NIS, together with defining the processes underlying the most significant changes and outlined major sources of uncertainty. Lack and/or bias in the availability of evidence of impacts was recorded for several (both recent and early) introductions. Realizing a sophisticated, data and information-hungry framework for the evaluation of ecosystem impacts of NIS is not pragmatic for management purposes in the foreseeable future. Instead, simple approaches, such as application of common statistical parameters like absolute effect size, are more likely to result in tangible outcomes. As bearing no unit, effect sizes can be later easily aggregated across taxa, affected ecosystem features or spatial scales. The proposed approach enables performing systematic comparisons on the severity of impacts of different NIS along different study disciplines and ecosystems.