title: EVIDENCE FOR IMPACTS OF NONINDIGENOUS MACROALGAE: A META-ANALYSIS OF EXPERIMENTAL FIELD STUDIES(1).
abstract: 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.
authors.affiliation: Marine Department, National Environmental Research Institute, PO. Box 4000 Roskilde, Denmark Centre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia School of Plant Biology, University of Western Australia, Crawley 6009 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia CIIMAR, Rua dos Bragas, 289-4050-123, Porto, PortugalDepartment of Zoology, University of Florida, Gainesville, Florida 32609, USA.
authors.forename: Mads S
authors.initials: MS
authors.lastname: Thomsen
authors.affiliation: Marine Department, National Environmental Research Institute, PO. Box 4000 Roskilde, Denmark Centre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia School of Plant Biology, University of Western Australia, Crawley 6009 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia CIIMAR, Rua dos Bragas, 289-4050-123, Porto, PortugalDepartment of Zoology, University of Florida, Gainesville, Florida 32609, USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: Wernberg
authors.affiliation: Marine Department, National Environmental Research Institute, PO. Box 4000 Roskilde, Denmark Centre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia School of Plant Biology, University of Western Australia, Crawley 6009 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia CIIMAR, Rua dos Bragas, 289-4050-123, Porto, PortugalDepartment of Zoology, University of Florida, Gainesville, Florida 32609, USA.
authors.forename: Fernando
authors.initials: F
authors.lastname: Tuya
authors.affiliation: Marine Department, National Environmental Research Institute, PO. Box 4000 Roskilde, Denmark Centre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia School of Plant Biology, University of Western Australia, Crawley 6009 Western Australia, AustraliaCentre for Marine Ecosystems Research, Edith Cowan University, 6027 Western Australia, Australia CIIMAR, Rua dos Bragas, 289-4050-123, Porto, PortugalDepartment of Zoology, University of Florida, Gainesville, Florida 32609, USA.
authors.forename: Brian R
authors.initials: BR
authors.lastname: Silliman
keywords: field experiments; impacts; meta-analysis; nonindigenous macroalgal species