A computational approach to model the evolution of herbivory defense compounds in plants.
There is a long standing hypothesis that plants living at latitudes closer to the equator, where there is greater herbivore biodiversity, will produce more total herbivore defense compounds. Thus, there would be an expected latitudinal/biodiversity gradient of defense compound concentration. However, there are two problems: 1) It is not clear from a theoretical perspective why this hypothesis should hold, and 2) the limited data that has been collected to test this hypothesis indicate that the reverse gradient might be true, with species at higher latitudes (lower herbivore diversity) producing higher total concentrations of defense compounds. Here, we take a simulation-based approach to test the theoretical foundations of the hypothesis. In a simple model of plant/herbivore evolutionary interactions what is the expectation? We hypothesized that the reverse gradient might actually be the expectation. Our reasoning is that a tight one-to-one interaction (i.e. a specialist herbivore) may yield an evolutionary arms race scenario where the plant and herbivore are very tightly coupled. This cycle would tend to drive up overall production as the herbivore evolved some degree of resistance to the toxicity. However, in the case of high herbivore diversity, where there are differential effects of defense compounds on different herbivores (in some cases these could be positive and negative) there is no tight coupling between plant and any single herbivore. Instead, the high herbivore diversity may drive production of a greater diversity of defense compounds. So, rather than driving up total production, we might expect to see a greater number of compounds at more equatorial latitudes.