Ecological Archives E088-126-A2

Kerri M. Crawford, Gregory M. Crutsinger, and Nathan J. Sanders. 2007. Host-plant genotypic diversity mediates the distribution of an ecosystem engineer. Ecology 88:2114–2120.

Appendix B. Monte Carlo simulations to test for nonadditivity.

To test for nonadditive effects of genotypic diversity on gall abundance, we used Monte Carlo simulations using data from genotype monoculture plots to construct null genotype mixtures and their associated gall abundances. We compared the observed gall abundances to these null communities. Each null community consisted of 3, 6, or 12 genotypes sampled to match the exact identities corresponding to a particular plot combination (e.g., for a 3-genotype plot containing G3, G13, and G19, we sampled only from monoculture plots containing these three genotypes) (Crutsinger et al. 2006, Johnson et al. 2006). For each sampled genotype, the appropriate number of individual ramets (4, 2, or 1) was randomly sampled without replacement from a randomly selected replicate monoculture plot. This process was repeated 5,000 times for every mixed genotype plot. To calculate statistical differences between gall abundance in observed and null communities, we used a bootstrap approach. For each of 10,000 iterations, we sampled seven null communities and calculated mean number of galls at the plot-level. We measured P values as the fraction of iterations in which the null mean was equal to or exceeded the observed mean. We calculated 95% confidence intervals using the percentile method (2.5th and 97.5th percentiles). If the effects of genotypic diversity on gall abundance were additive, we would expect no difference between observed and predicted means (P > 0.05). All Monte Carlo simulations were coded in Microsoft Visual C++ 6.0 (Microsoft, Redmond, Washington, USA).

LITERATURE CITED

Crutsinger, G. M., M. D. Collins, J. A. Fordyce, Z. Gompert, C. C. Nice and N. J. Sanders 2006. Plant genotypic diversity predicts community structure and governs an ecosystem process. Science 313:966–968.

Johnson, M. T. J., M. J. Lajeunesse, and A. A. Agrawal. 2006. Additive and interactive effects of plant genotypic diversity on arthropod communities and plant fitness. Ecology Letters 9:24–34.



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