The role of plant genotypic diversity and defense variability on arthropod community structure differs across trophic levels

Background/Question/Methods A growing number of studies show that increased levels of plant genotypic diversity lead to greater arthropod species abundance, richness, and diversity. Since variability in plant defense has long been known to impact herbivore populations, and these effects may cascade to higher trophic levels, we tested the hypothesis that variability in defense is an important component of plant patch-level genotypic diversity leading to altered arthropod community structure. In a common garden experiment using Solanum dulcamara (bittersweet nightshade), we non-destructively surveyed the arthropod community that recruited to patches of plants nine times over the growing season. Treatments consisted of half-sibling monocultures and two forms of genotypic diversity: a diversity treatment with high genotypic variability in both the defensive enzyme peroxidase and tortoise beetle bioassay resistance (diverse-diverse), and another treatment with low variability in each of these traits (diverse-similar). We also measured plant biomass since genotype complementarity in resource utilization and subsequent facilitation is an alternative mechanism that can lead to altered arthropod communities. Results/Conclusions Overall, our results mirror previous studies since plot level species richness was higher in diverse patches compared to monocultures when the two genotypic diversity treatments were pooled. When diversity treatments were separated, our results varied by trophic level. We found that herbivore richness and abundance were higher in diverse-similar patches than diverse-diverse patches and monocultures. The abundance pattern among treatments was largely driven by two herbivores: a specialist flea beetle (Psylliodes affinis) and a generalist spittlebug (Philaenus spumarius). This pattern of herbivore richness and abundance cannot be explained by the more individuals hypothesis since plant biomass was equivalent among treatments. In contrast to herbivore abundance, predator abundance was higher in diverse-diverse patches than monocultures, while diverse-similar patches were intermediate. Thus, the role of genotypic diversity and defense variability on community structure differed across trophic levels. We conclude that the bottom-up influence of plant genotypic diversity on arthropod community structure can affect trophic levels differently depending on variability in plant defense traits.