The common practice to assess the effect of biodiversity on ecosystem functioning is to compare the observed community functioning (e.g. total biomass of our two-species community) with some expectation based on monoculture functions. While biodiversity increases ecosystem functioning in various ways in different models, theory has clarified two broad sets of processes that underlie biodiversity effects on ecosystem functioning, namely selection and complementarity effects. In multitrophic communities, biodiversity can be characterized along two dimensions, namely horizontal diversity (e.g. species richness within a trophic level) and vertical diversity (e.g. number of trophic levels). The two biodiversity effects (i.e. complementarity effects and selection effects) could also shift in relative importance as the study area increases from local to regional scales.