Background. In the marine environment, the introduction and spread of non-native seaweeds may cause major alterations to resident assemblages and biodiversity with implications for ecosystem functioning and consequent ability to provide goods and services to humans. We compared the impacts of non-native seaweeds on resident consumers (at higher trophic levels) to those observed on native primary producers (same trophic level). In addition, we assessed variations in the effects of non-native seaweeds on resident benthic assemblages according to the degree of existing human impact.

Methods. We searched for both experimental and observational studies on the effects of non-native seaweeds on resident assemblages. We summarized effects of non-native seaweeds on resident species (abundance, biomass, growth and survival) and communities (abundance, biomass, diversity and evenness) by calculating standardized mean differences between invaded and non-invaded means (Hedges’ g*), which were used to perform meta-analyses.

Results. Literature search resulted in the extraction of data from both experimental and observational studies (for a total of 122 papers) investigating the effects of 13 different non-native seaweeds on single species or communities. Most of the studies were performed in Europe (the Western Mediterranean hosted the largest number of studies), Australia and Atlantic North America. No data were available from Africa or Asia. The effects of non-native seaweeds on resident primary producer communities and species were generally negative and greater than those that emerged on consumers. Seaweeds caused a significant decrease in the abundance, biomass, diversity and evenness of primary producer communities, likely reflecting the negative effects on the growth and survival of individual species. Effects on consumers were less pronounced, with significant declines detected only for community biomass and for the survival of individual species. This resulted in a significant difference in the effect of non-native seaweeds between primary producers and consumers for abundance and diversity of communities. Asymmetry in funnel plots emerged for community abundance and diversity, as a consequence of some data with large residual values and high variances. Analyses repeated after removal of these data suggested that publication bias did not have a severe effect on results on community abundance, while those on community diversity should be consider with more caution. Of the non-native seaweeds investigated, Caulerpa racemosa generated the largest negative changes on primary producer communities, which were significantly different from effects exerted on consumers for abundance and diversity. Results suggested considerable variation in the effects of non-native seaweeds among habitats, but negative effects were particularly severe in the Western Mediterranean. The effects of non-native seaweeds on most of the response variables did not vary among areas characterized by a different degree of human impact. However, there was a trend for the effects of non-native species on community biomass and abundance and on species abundance to become less negative at heavily impacted sites. By contrast, the magnitude of negative effects of seaweeds on community evenness tended to increase with human impact levels.

Discussion. Our results suggest that effects of non-native seaweeds are generally negative on resident primary producers (seaweeds, plants), while more heterogeneous on consumers and dependent upon the receiving geographical region and habitat type. An ecological indicator for potential impacts of non-native seaweeds should rely on effects on resident primary producers in different habitats and regions. In addition, effects on some response variables showed a tendency to be more severe in relatively pristine environments, with possible impingements to the effectiveness of conservation strategies (i.e. Marine Protected Areas), that are generally implemented in areas little exposed to human impacts. Efforts for controlling the impacts of non-native seaweeds should be mostly directed to prevent their spread into these relatively pristine areas.

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