This theme explores the mechanisms that drive plant-plant (e.g. crop-crop, crop-weed) interactions as well as interactions between plants and other organisms within crop ecosystems, and how these interactions scale to crop-level aspects, such as yield, resource-use efficiency, crop interactions with insects and other non-plant organisms and climate feedbacks. Particular emphasis is placed on the role of intraspecific and interspecific interactions. The ecology and non-chemical control of weeds is a focal area of interest. In this way CSA studies crops ranging in both spatial and temporal diversity (e.g. intercropping, different rotation systems, multi-variety crops) as well as semi natural systems (e.g. agro forestry, non-timber forest products).