Background/Question/Methods   Genetic diversity is a form of biodiversity that is predicted to affect population success via two ecological mechanisms. Complementarity effects occur when increased diversity decreases intra-specific competition and increases resource partitioning while sampling effects occur when increased diversity increases the chance a population contains a highly functioning or productive genotype. Our objective was to examine the relative importance of these two mechanisms in driving positive genetic diversity effects in invasive species populations in different environments. We directly manipulated the genetic diversity of populations of an annual invasive grass (Avena barbata) and grew populations in different soil resource environments to test the following hypotheses: 1) Increasing genetic diversity increases population success and 2) resource environment will influence the relative strengths of the ecological mechanisms (complementarity and sampling) that promote the population success through genetic diversity. Populations of 80 individuals were assembled to contain 1, 2, 4 or 8 genotypes and were grown under spatially homogenous (high or low) and spatially heterogeneous (high and low) soil resource environments. Survivorship, biomass, seed production, and effects on native species growth were measured to assess population success. The relative strengths of ecological mechanisms contributing to genetic diversity effects were assessed using an additive partition model.

Results/Conclusions   We found significant positive genetic diversity effects on population seed production (a direct measure of population fitness) indicating that increases in genetic diversity could impact the abundance and spread of invasive species via increased propagule pressure. Complementarity effects were generally positive and significantly increased with increasing genetic diversity in all environments. Sampling effects were generally negative, small in magnitude, and did not significantly change with increasing genetic diversity. Complementarity effects did not differ between homogeneous and heterogeneous resource environments indicating that genotypes did not partition resources spatially. However, increased variation in flowering time among genotypes within a population positively correlated with seed production suggesting that genotypes may partition resources temporally through differential timing of flowering and seed production. Increasing genetic diversity had no effect on survivorship or biomass of Avena or native species, indicating genetic diversity may not impact competition between invasive populations and native communities. These results suggest that temporal trait differences between individual genotypes allow for increased resource partitioning in genetically diverse populations, which may increase invasion success via increased propagule pressure.