Ecological communities comprise a wide diversity of species, and explaining how this diversity can be maintained is not an easy endeavour. Early ecologists often wondered about how so many species were able to coexist on apparently limited sets of resources, and explanations generally involved the partitioning of different dimensions of species niches, in order to avoid the exclusion of lesser competitors. These ecological mechanisms can be analysed quantitatively in idealized conditions, e.g. for the coexistence of two species of which very detailed ecological information is available. However, the study of communities with diversity of species, trophic guilds, interaction types, and spatial heterogeneity demands novel approaches for understanding their persistence. In this talk I will summarise my research on these questions, in which I combined novel ecological theory with focused observational data from annual grassland communities. I will show that the structure of biotic interactions is a fundamental piece of the puzzle for understanding how diversity is maintained in natural communities.