Soil fertilization does not alter plant architectural effects on arthropod communities

Abstract

AimsWhile a growing number of studies have demonstrated the importanceof intraspecific differences within plant species on associatedarthropod communities, little is known regarding the relativestrength of these effects compared to environmental factors. In thisstudy, we examined whether intraspecific plant differences andnutrient fertilization interact to shape the arthropod community of adominant coastal shrub, Baccharis pilularis (coyote bush).MethodsWe overlaid a fertilization treatment on a 12-year-old commongarden experiment planted with erect and prostrate architecturalmorphs of Baccharis in California, USA. To collect the associatedarthropod community, we vacuum sampled the crown of eachBaccharis and identified individuals to species or morphospecies.Important FindingsWe found that arthropod richness and abundance were 2- to3-fold greater on prostrate Baccharis than on erect morphs,but observed no main effects of fertilizer addition on the overallarthropod communities. Predators responded as strongly asherbivores to plant morph, and both were unaffected by nutrientadditions. Only the specialist stem galler, Gnorimoschema baccharisella,showed an interactive response to plant morph andfertilization. Nitrogen, phosphorous and potassium addition hadopposite effects on the two morphs, increasing stem gall abundanceby 50% on prostrate morphs, but reducing galling by20% on erect morphs. The architectural complexity of prostratemorphs could be the driving mechanism of differences in arthropodassemblages. Overall, our results demonstrate that community-level consequences of intraspecific differences in plants arestrong, rather than being context dependent, and are generallymaintained under different resource environments. The growingnumber of studies showing strong genotype than nutrient effectson associated arthropod communities suggests that this might bea generalized pattern.