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).
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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