Evidence for impacts of non-indigenous macroalgae: a meta-analysis of experimental field studies

Document Type

Journal Article


Wiley-Blackwell Publishing


Faculty of Computing, Health and Science


School of Natural Sciences / Centre for Marine Ecosystems Research




Thomsen, M. S., Wernberg, T., Tuya, F., & Silliman, B. R. (2009). Evidence for Impacts of Nonindigenous Macroalgae: A Meta‐Analysis of Experimental Field Studies1. Journal of Phycology, 45(4), 812-819. Available here


Invasions by nonindigenous macroalgal species (NIMS) potentially cause severe impacts on native species. We conducted a meta-analysis of 18 field-based manipulative experiments to quantify the direction and magnitude of impacts (Hedges effect size d, hereafter ES). We found significant small-to-medium negative effects on “macrophyte abundance” (cover, biomass of native taxa; EScumulative = −0.30) and medium-to-large negative effects on “macrophyte assemblages” (richness, diversity, total abundance; EScumulative = −0.70). In contrast, EScumulative were not significant for “macrophyte processes” (growth, mortality; EScumulative = −0.39), “animal abundance” (densities; EScumulative = −0.13), or “animal assemblages” (richness, diversity; EScumulative = 0.75). The nonsignificant effect sizes were characterized by low sample sizes and should be interpreted with caution. Three study-specific effect sizes were particularly large (cumulative are likely biased toward larger effects because only the most conspicuous NIMS have been tested and because nonsignificant results are less likely to be published. To better understand the impacts of NIMS, more manipulative experiments are needed, testing more species and under contrasting environmental conditions. Future studies should include procedural control treatments and report the abundance of the NIMS to avoid ambiguous interpretations. In conclusion, current experimental evidence shows that NIMS have, on average, small-to-large negative impacts on native plant species and assemblages. It is possible that these effects can result in severe consequences when accumulated over long time periods and large spatial scales.





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