Genotypic Diversity Increases Net Primary Productivity within a Non-native Seaweed Population
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Species diversity and intraspecific genetic diversity of foundation species have positive effects on community and ecosystem functioning. There are few tests of whether non-native foundation species generate similar diversity-function effects despite the lower levels of genetic diversity in their non-native range, nor whether diversity-function effects might positively influence their invasion success. In southeastern US estuaries, a non-native red seaweed, <i>Gracilaria vermiculophylla<i> (hereafter <i>Gverm<i>) has become a locally-dominant species, facilitating native invertebrate and bacterial communities largely by introducing structure to mudflat habitats. Multiple <i>Gverm<i> individuals are anchored into patches at small spatial scales (approximately 100-200 cm<sup>2<sup>) by the native decorator worm <i>Diopatra cuprea<i>. A single <i>Diopatra cuprea <i>tubes anchors an average of 11 unique genotypes (as determined by microsatellites), and thus generates an opportunity for genetic-diversity effects to arise. In a springtime field survey, genetic diversity of a <i>Gverm<i> patch showed no correlation with <i>Gverm<i> biomass, epifaunal abundance and diversity, nor with bacterial or epifloral abundances. We manipulated <i>Gverm<i> genetic diversity in the field (monoculture versus a polyculture with eight multi-locus genotypes) at two tidal heights during the spring and summer. We detected a three-fold increase in net primary productivity in polyculture patches relative to monoculture patches in the mid-intertidal in the summer; there was no effect of diversity on growth rate in any other combination of tidal heights or season. The increase in net primary productivity did not translate to measurable effects on the numbers or communities of epibiota. Facilitation of a non-native seaweed by relatively low levels of genotypic diversity suggest more broadly that intraspecific diversity may play an underappreciated role in the success of some non-native foundation species.