Appendix 1. Test for additive vs. non-additive diversity effects on arthropod abundance. Diversity effects can operate through sampling effects or non-additive effects [1]. In the first case, the contributions of individual genotypes to the group mean is the same in mono- and polycultures, but polyculture group means are greater than monoculture group means because of a greater probability of including individual genotypes with extreme (high) trait values. In the second case, the contribution of individual genotypes to the group mean differs between mono- and polyculture treatments. To test for whether the observed diversity effects were due to non-additive effects, we followed the approach of Loreau & Hector [2] in which observed polyculture values are compared to expected polyculture values based upon genotype measurements in monoculture. With this approach, a deviation from expected values is due to non-additive diversity effects. To make this comparison, we followed the approach of Johnson et al. [3], in which a second dataset was created for diversity treatments in which the mean measurements (aphids, parasitoids and ants) from monocultures were substituted for each plant in polyculture. This was done specific to mono- and poly-cultures within ant exclusion and control. The comparison of these two datasets then allows for a test for significant nonadditive effect; if diversity effects are due to sampling effects alone, then these two data sets will not differ. In contrast, a significant effect of dataset indicates non-additive genetic diversity effects. Significant non-additive diversity effects were detected for both aphid and parasitoid abundance, both in the presence and absence of ants. In ant access treatments, there were significant non-additive diversity effects for ant abundance (Table S1). 1 Table S1. Summary of the linear mixed model for the effect of data set (observed vs. expected values) on arthropod abundance in ant-excluded and control (with ants) treatments. Data set (observed vs. expected values) DFnum DFden F P Aphid 1 109 22.91 <0.001 Parasitoids 1 109 6.07 0.015 Aphid 1 109 31.73 <0.001 Parasitoids 1 109 17.19 <0.001 Ants 1 109 36.07 <0.001 Ant-excluded Control Literature cited [1] Wardle, D.A. 1999. Is ‘sampling effect’ a problem for experiments investigating biodiversity-ecosystme function relationships? Oikos 87, 403-407. [2] Loreau, M. & Hector, A. 2001. Partitioning selection and complementarity in biodiversity experiments. Nature 412, 72-76. (doi:10.1038/35083573) [3] Johnson, M.T., Lajeunesse, M.J. & Agrawal, A.A. 2006 Additive and interactive effects of plant genotypic diversity on arthropod communities and plant fitness. Ecol. Lett. 9, 24-34. (doi:10.1111/j.1461-0248.2005.00833.x) 2 Appendix 2. ANOVA results for arthropod abundance and plant growth. Table S2. Summary of the linear mixed model for the effects of plant genetic diversity (mono-, and poly-cultures) and presence of mutualistic ants (two levels: presence or absence) on aphid abundance. The effect of the particular combination of Baccharis salicifolia genotypes nested in each diversity treatment was included in the model. Ant treatments lasted four weeks. Final height was used as a covariate. Significant P values (P<0.05) are typed in bold. DFnum DFden F P 0.268 Ant 1 6 1.49 Diversity 1 96 54.59 <0.001 Ant × Diversity 1 96 0.11 0.741 Combination(Diversity) 22 96 2.53 0.001 Ant × Combination(Diversity) 22 96 2.52 0.001 Plant sex 1 96 0.02 0.878 Final height 1 96 11.94 <0.001 3 Table S3. Summary of the linear mixed model for the effects of plant genetic diversity (mono-, and poly-cultures) on ant abundance. The effect of the particular combination of Baccharis salicifolia genotypes nested in each diversity treatment was included in the model. Final height was used as a covariate. To test whether plant genetic diversity effects on ant abundance are density-mediated or trait-mediated indirect effects through changes in aphid populations we also included aphid abundance as a covariate. Significant P values (P<0.05) are typed in bold. Without aphid abundance as covariate DFnum DFden F P Diversity 1 48 21.74 <0.001 Combination(Diversity) 22 48 2.25 0.009 Plant sex 1 48 0.18 0.671 Final height 1 48 10.19 0.002 With aphid abundance as covariate DFnum DFden F P Diversity 1 47 1.71 0.197 Combination(Diversity) 22 47 2.78 0.002 Plant sex 1 47 0.40 0.533 Aphid abundance 1 47 14.54 <0.001 Final height 1 47 7.03 0.011 4 Table S4. Summary of the linear mixed model for the effects of plant genetic diversity (mono-, and poly-cultures) and presence of mutualistic ants (two levels: presence or absence) on parasitoid abundance. The effect of the particular combination of Baccharis salicifolia genotypes nested in each diversity treatment was included in the model. Ant treatments lasted four weeks. Final height was used as a covariate. To test whether plant genetic diversity effects on parasitoid abundance are density-mediated or trait-mediated indirect effects through changes in aphid populations we also included aphid abundance as covariate. Significant P values (P<0.05) are typed in bold. Without aphid abundance as covariate DFnum DFden F P Ant 1 6 10.43 0.018 Diversity 1 96 14.55 <0.001 Ant × Diversity 1 96 0.20 0.659 Combination(Diversity) 22 96 1.74 0.035 Ant × Combination(Diversity) 22 96 1.56 0.073 Plant sex 1 96 0.16 0.688 Final height 1 96 25.19 <0.001 With aphid abundance as covariate DFnum DFden F P Ant 1 6 17.29 0.006 Diversity 1 95 0.29 0.588 Ant × Diversity 1 95 0.03 0.860 Combination(Diversity) 22 95 2.00 0.011 Ant × Combination(Diversity) 22 95 2.27 0.003 Plant sex 1 95 0.00 0.954 Aphid abundance 1 95 Final height 1 95 104.75 <0.001 7.90 0.006 5 Table S5. Summary of the linear mixed model for the effects of plant genetic diversity (mono-, and poly-cultures) and presence of mutualistic ants (two levels: presence or absence) on total height. The effect of the particular combination of Baccharis salicifolia genotypes nested in each diversity treatment was included in the model. Ant treatments lasted four weeks. Plant height (“Initial height”) before ant exclusion treatments was used as a covariate. Significant P values (P<0.05) are typed in bold. DFnum DFden F P Ant 1 6 3.52 0.110 Diversity 1 96 0.46 0.501 Ant × Diversity 1 96 0.02 0.892 Combination(Diversity) 22 96 0.60 0.918 Ant × Combination(Diversity) 22 96 1.18 0.283 Plant sex 1 96 0.26 0.611 Initial height 1 96 370.69 <0.001 6 Figure S1. Effect of genotypic variation monocultures (eight different genotypes of Baccharis salicifolia) on the abundance of ant-tended aphids, aphid-tending ants and aphid parasitoids. Each genotypic variation monoculture includes four plants of the same genotype. Least-square means ± SE (N = 16). We did not find significant differences between monoculture plots. 7 Appendix 3. Effects of ants on arthropod abundance and plant growth Figure S2. Effect of ant presence (vs. exclusion) on the abundance of ant-tended aphids, total stem height in cm and abundance of aphid parasitoids. Least-square means ± SE (N = 32). Different letters indicate significant differences (P <0.05) among ant treatments. 8