Slopes of the diversity spectrum for all of the world's coastal large marine ecosystems for which sufficient data exist. From this, it is possible both to relate the diversity spectrum to environmental parameters (e.g. temperature) and to make regional predictions of unmeasured quantities, such as the diversity of small-bodied species. In Focus:Reuman, D.C., Gislason, H, Barnes, C., Mélin, F. & Jennings, S. (2014) The marine diversity spectrum. Journal of Animal Ecology, 83, 963–979. How do we begin to extract order from the elegant chaos of natural ecosystems? In a landmark new paper published in this issue, Reuman et al. (2014) go back to first principles, combining a range of established body size- and species-centred ecological theories with empirically well-supported relationships to construct a model that enables them to predict key features using only remarkably simple biological and environmental measurements. They test this model using widely available data on the communities living in all of the world's coastal seas. Here, I discuss the key features of their model, and especially how the general patterns they document can lead to further, empirically driven tests of theory across multiple ecosystems.