Direct and apparent compensation in plant–herbivore interactions

The potential positive effects of herbivores on plants have been the subject of debates for decades. While traditionally, herbivory was considered to have a negative impact on plants, some studies also reported possible mutualism between plants and herbivores. Plant defences, and in particular tolerance and resistance, seem to play an important role in shaping plant–herbivore interactions. The aim of this study is to show how a direct plant compensation mechanism translates into apparent compensation, i.e. the long-term biomass response to herbivory, in simple plant–herbivore models. A special emphasis is then put on how it interacts with resistance mechanisms. A qualitative study of the proposed models shows that they can exhibit different plant–herbivore patterns, including neutral, antagonistic (negative apparent compensation) and mutualistic (positive apparent compensation) interactions. Moreover, it is shown that density dependence plays a crucial role since, for a given system, the realized plant–herbivore pattern critically depends on the initial plant and herbivore levels. Our study shows the importance of direct compensation for the presence of plant–herbivore mutualism, a finding which we show, has significant implications both in ecosystems ecology and in agricultural pest management.