Agricultural intensification has led to a systematic erosion of the biodiversity in arable ecosystems. Despite this, densities of plant species in the seedbank of arable fields are still sufficient to provide the potential for recovery of the arable flora and associated fauna. Identification of management practices to achieve this, while minimising negative effects of weed competition on crop yield, requires a mechanistic understanding of functional diversity in arable systems. However, a review of the ecological, physiological and genetic characteristics of 66 representative UK arable weed species revealed major gaps in this knowledge for even the most common species. Even less is known about the degree of variation between individuals within these species that contributes to overall levels of functional diversity. Classification of organisms into functional groups on the basis of species-level taxonomy is inadequate to describe the functional diversity of a system, since variability in a particular physiological trait can be as great between individuals within a species as between species. We therefore propose an individual based approach to examine the functional attributes of arable plants that affect resource acquisition, partitioning and energy transfer through the food web. Capsella (shepherd's purse) is proposed as a key species that is widespread, ecologically important, and physiologically and genetically diverse. Current understanding of Capsella systematics is therefore reviewed and a methodological approach is described that establishes a foundation for studies of biodiversity and function in arable systems.