The decline in farmland biodiversity is a serious conservation issue in Great Britain. So, it is not surprising that the decision to allow commercial introduction of genetically modified herbicide tolerant (GMHT) crops has been delayed until after the completion of farm-scale evaluations of the effects of these crops on farmland biodiversity. Watkinson et al. propose a mathematical model that seeks to measure the impact of GMHT sugar beet crops on weed populations in British fields and the consequences for farmland birds that are dependent on the weed seeds for food. Their model ties together weed population dynamics, farmer decision-making, and regional impacts on farmland birds.