There are a number of disparate models predicting variation in plant chemical defences between species, and within a single species over space and time. These can give conflicting predictions. Here, we review a number of these theories, before assessing their power to predict the spatial-temporal variation of thapsigargins between and within populations of the deadly carrot (Thapsia garganica). By utilising multiple models simultaneously, we will highlight gaps in their predictions and evaluate the performance of each.Thapsigargins are potent anti-herbivore compounds that occur in limited richness across the different plant tissues of T. garganica, and therefore represent an ideal system for exploring these models. Thapsia garganica plants were collected from six locations on the island of Ibiza, Spain, and the thapsigargins quantified within reproductive, vegetative and belowground tissues. The effects of sampling time, location, mammalian herbivory, soil nutrition, and changing root-associated fungal communities were analysed on the concentrations of thapsigargins within these in situ observations, with the results compared to our model predictions.The models performed well in predicting the general defence strategy of T. garganica, and the aboveground distribution of thapsigargins, but failed to predict the considerable proportion of defences found belowground. Models predicting variation over environmental gradients gave conflicting and less specific predictions, with intraspecific variation remaining less understood.Here we found that multiple models predicting the general defence strategy of plant species could likely be integrated into a single model, while also finding a clear need to better incorporate belowground defences into models of plant chemical defences. We found constitutive and induced thapsigargins differed in their regulation, and suggest models predicting intraspecific defences consider them separately. Finally, we suggest that in situ studies be supplemented with experiments in controlled environments to identify specific environmental parameters that regulate variation in defences within species.