Mechanisms of altered arthropod evenness in response to plant genotypic richness

Background/Question/Methods   Both theory and experiments show that diversity at one trophic level promotes diversity at other trophic levels. While patterns of community richness are typically used as measures of “diversity”, patterns of evenness have received far less attention. Here, we draw on two years of data where we manipulated genotypic richness of the common evening primrose (Oenothera biennis) in an old-field common garden. We established two treatments by arranging rings of eight plants with either low (one O. biennis genotype) or high (eight O. biennis genotypes) genotypic richness. In the first year we censused the arthropod community on every plant two times over the growing season. In the second year, we censused individuals of the dominant insect in the experiment, the omnivorous Mirid bug Plagiognathus politus four times, and assessed numerous plant traits that P. politus potentially responded to. We also conducted choice assays in the laboratory with P. politus in order to determine feeding preferences across plant treatments on each plant genotype.

Results/Conclusions   Our first year of data showed that arthropod evenness decreased with O. biennis genotypic richness. This effect was driven by a single dominant omnivorous arthropod on O. biennis, Plagiognathus politus. P. politus responded to O. biennis genotypic richness with non-additive increases in abundance such that its abundance in polycultures was 94% greater than expected. Using our second year of data, we show that O. biennis genotypic richness led to increases in plant biomass and floral abundance. P. politus preferentially associated with O. biennis flowers compared to other plant parts, and floral abundance was the strongest predictor of P. politus abundance of all plant traits measured. Furthermore, choice assays in the laboratory indicated that P. politus showed no preference between plant tissues from genotypically rich patches compared to monocultures. These results suggest that increases in floral abundance in response to plant genotypic richness led to decreases in arthropod community evenness. Overall, our results suggest that an often overlooked aspect of species interactions- intraspecific variation- can have profound effects in communities.