Beyond the compensatory continuum: environmental resource levels and plant tolerance of herbivory

The impact that herbivore damage has on plant fitness depends on the environmental conditions in which the plant is growing. It has long been assumed that a plant's tolerance of herbivory should be greater in low‐stress, resource‐rich environments, and this assumption has been formalized in what has become known as the compensatory continuum hypothesis (CCH). Despite the widespread acceptance of the CCH, recently accumulating results from numerous studies relating resource levels to tolerance of herbivory show that the relationship is not as simple as this hypothesis predicts. In fact, plants have often been found to be less tolerant of a given amount of herbivory in resource‐rich environments than they are of the same amount of herbivory in resource‐poor environments. In this article, we present a new model, the limiting resource model (LRM), that can more reliably explain the range of observed effects of resource levels on tolerance. The main advance of this model is that it specifically considers which factors are limiting plant fitness and which resources are affected by particular herbivores. The LRM is presented in a general and flexible form as a flowchart with seven pathways to three potential outcomes: greater tolerance, equal tolerance, or lower tolerance in low‐ vs high‐resource environments. We then review a specific example to illustrate each of these seven pathways with a study from the published literature. We finish with a description of four general criteria to direct the design of future studies to test the predictions of the LRM.