Soil fertilization does not alter plant architectural effects on arthropod communities

Abstract

Aims While a growing number of studies have demonstrated the importance of intraspecific differences within plant species on associated arthropod communities, little is known regarding the relative strength of these effects compared to environmental factors. In this study, we examined whether intraspecific plant differences and nutrient fertilization interact to shape the arthropod community of a dominant coastal shrub, Baccharis pilularis (coyote bush). Methods We overlaid a fertilization treatment on a 12-year-old common garden experiment planted with erect and prostrate architectural morphs of Baccharis in California, USA. To collect the associated arthropod community, we vacuum sampled the crown of each Baccharis and identified individuals to species or morphospecies. Important Findings We found that arthropod richness and abundance were 2-To 3-fold greater on prostrate Baccharis than on erect morphs, but observed no main effects of fertilizer addition on the overall arthropod communities. Predators responded as strongly as herbivores to plant morph, and both were unaffected by nutrient additions. Only the specialist stem galler, Gnorimoschema baccharisella, showed an interactive response to plant morph and fertilization. Nitrogen, phosphorous and potassium addition had opposite effects on the two morphs, increasing stem gall abundance by 50% on prostrate morphs, but reducing galling by 20% on erect morphs. The architectural complexity of prostrate morphs could be the driving mechanism of differences in arthropod assemblages. Overall, our results demonstrate that community- level consequences of intraspecific differences in plants are strong, rather than being context dependent, and are generally maintained under different resource environments. The growing number of studies showing strong genotype than nutrient effects on associated arthropod communities suggests that this might be a generalized pattern.