Global synthesis of effects of plant species diversity on trophic groups and interactions

Numerous studies have demonstrated that plant species diversity enhances ecosystem functioning in terrestrial ecosystems, including diversity effects on insects (herbivores, predators and parasitoids) and plants. However, the effects of increased plant diversity across trophic levels in different ecosystems and biomes have not yet been explored on a global scale. Through a global meta-analysis of 2,914 observations from 351 studies, we found that increased plant species richness reduced herbivore abundance and damage but increased predator and parasitoid abundance, predation, parasitism and overall plant performance. Moreover, increased predator/parasitoid performance was correlated with reduced herbivore abundance and enhanced plant performance. We conclude that increasing plant species diversity promotes beneficial trophic interactions between insects and plants, ultimately contributing to increased ecosystem services.

Methods
Studies were selected through a search on the Web of Science (last accessed in May 2019) using the boolean search string: [plant diversity OR plant richness OR mix crop* OR polyculture OR trap crop* OR ground cover OR vegetation OR intercrop* OR interplant*] AND [predat* OR herbivor* OR parasit* OR wasp* OR yield OR biomass* OR biological control OR pest control OR natural enem* OR pest]. Reference lists of selected studies were also checked for relevant studies. Means, standard errors (or standard deviations) and sample sizes of the selected variables could be extracted from tables, figures, the main text or supporting information. Data extraction from figures was conducted with Get Data Graph Digitizer 2.25.