From the leaf to the community: Distinct dimensions of phytochemical diversity shape insect–plant interactions within and among individual plants

Abstract

Plant secondary chemistry is known to be an important driver of plant–insect community structure across ecological scales. Recently, the concept of phytochemical diversity (PD) has been introduced to help describe variation in plant secondary chemistry and explain how this variation affects community structure. Previous studies show that PD among individuals and species results in phytochemical mosaics, known as the phytochemical landscape. However, plant traits can vary at finer scales, such as within individuals, and even a single host plant may be perceived as an entire phytochemical landscape by an interacting insect. Using the neotropical shrub Piper amalago, we tested and compared how herbivory, caterpillar biodiversity and plant–herbivore network structure are affected by the compositional (number and concentration of compounds) and structural (diversity of distinct chemical structures) dimensions of PD. We analysed variation among individual plants and among-plant height strata within individual plants. This allowed us to decompose PD within and among plant individuals and analyse how variation at both scales affects the plant–herbivore network. We found that both within and among plants greater structural diversity decreased herbivore feeding damage. Furthermore, each dimension of PD has different effects on herbivore biodiversity and network structure depending on the scale of biological organization. Within plants, the compositional dimension, specifically low concentrations of compounds tentatively identified as Piper amides, increased herbivore biodiversity. This dimension also increased the capacity of strata within plants to mediate ecological cascades through direct and indirect effects on herbivore abundance in the plant–herbivore network. In contrast, a greater structural diversity among plants decreased herbivore biodiversity and the capacity of plants to affect all other herbivores and plants directly and indirectly in the network. Synthesis. Based on our results we expand the concept of the phytochemical landscape to multiple scales of biological organization and provide evidence that PD may be maintained by how its multiple dimensions have distinct roles across scales of biological organization.