Many experimental studies have quantified how the effects of competition vary with habitat productivity, with the results often interpreted in terms of the ideas of Grime and Tilman. Unfortunately, these ideas are not relevant to many experiments, and so we develop an appropriate resource competition model and use this to explore the effects of habitat productivity on the intensity of competition. Several mechanisms influencing the productivity–competition intensity relationship are identified, and these mechanisms explored using two classic data sets. In both cases, there is good agreement between the model predictions and empirical patterns. Quantification of the mechanisms identified by the models will allow the development of a simple predictive theory linking measures of the intensity of competition with ecosystem-level properties.