When predators also feed on plants: Effects of competition and plant quality on omnivore-prey population dynamics

We consider an ecological system that consists of an omnivore that feeds on both herbivores and plants. The omnivore (an intraguild predator in the system) and the prey (an intraguild prey) also are engaged in asymmetric competitive interactions for a plant-related resource (feeding site). We present empirical data that show that the omnivorous bug Onus insidiosus (Say) displaces its thrips prey from preferred sites on plants. A companion study shows that plants vary greatly in their suitability for this omnivore. We therefore developed a heuristic model to study how such displacement and variation in plant suitability for the omnivore affect the dynamic properties of the system. The model predicts that an increase in plant suitability for the omnivore, which results in increased plant-feeding, has a destabilizing effect and may bring about chaos under some conditions. Likewise, the stability of the system decreases with an increase in competitive interactions. Thus, plant-feeding and competition have destabilizing effects on food webs that include plant-feeding omnivores. In view of these findings, we discuss mechanisms that may lead to the persistence of systems where intraguild predators compete with their prey for plant-related resources.