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EVIDENCE FOR IMPACTS OF NONINDIGENOUS MACROALGAE: A META-ANALYSIS OF EXPERIMENTAL FIELD STUDIES1
Version of Record online: 26 AUG 2009
© 2009 Phycological Society of America
Journal of Phycology
Volume 45, Issue 4, pages 812–819, August 2009
How to Cite
Thomsen, M. S., Wernberg, T., Tuya, F. and Silliman, B. R. (2009), EVIDENCE FOR IMPACTS OF NONINDIGENOUS MACROALGAE: A META-ANALYSIS OF EXPERIMENTAL FIELD STUDIES. Journal of Phycology, 45: 812–819. doi: 10.1111/j.1529-8817.2009.00709.x
Received 15 June 2008. Accepted 16 March 2009.
- Issue online: 26 AUG 2009
- Version of Record online: 26 AUG 2009
- field experiments;
- nonindigenous macroalgal species
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 (<−2.0), showing that, in specific cases, impacts can be highly negative. From a conservation perspective, focus could be on such worst-case scenarios. Still, the reported EScumulative 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.