Trait plasticity is more important than genetic variation in determining species richness of associated communities

1. Intraspecific variation can be an important driver of ecological interactions in species-rich communities. Predicting the effects of intraspecific variation in different environments, however, remains a major challenge. This is because we often do not quantify both the effects of functional traits on associated communities and the extent to which trait variation is due to genetics (genotype effects) vs. plasticity (environment effects). As a consequence, the relative importance of trait plasticity vs. genetic variation in structuring associated communities remains unclear. 2. We sought to fill this gap by conducting common garden experiments with the plant Salix hookeriana across biotic (ant-aphid interactions) and abiotic (wind exposure) environmental gradients in a coastal dune ecosystem. In each experiment, we simultaneously measured plant traits and species richness of associated above- and below-ground communities. We then used statistical models to quantify the relative importance of trait plasticity vs. genetic variation in structuring communities. 3. Our major finding was that trait plasticity was more important than genetic variation in determining the number of species in associated communities. This result was consistent across different environmental contexts (experimental manipulations of ant-aphid interactions and wind exposure), multiple years, and for aboveground arthropods and root microbes. This occurred because the traits that had the largest effect on species richness were also the most plastic. 4. Synthesis. These results indicate that trait plasticity can be a dominant driver of above- and below-ground biodiversity.