Ecologists have long recognized that simplified agricultural systems are more prone than natural systems to large-scale disease and pest outbreaks. Incorporating the complexity of natural systems in agriculture is one suggested solution. In biological control, natural enemies are released or otherwise supported to control pest outbreaks. When multiple natural enemies are involved, questions arise as to how complex interactions will combine to influence overall biological control. Predator-prey theory is a clear starting point for addressing these questions. Pests are prey species that grow without limit, becoming economic threats to agriculture. Thus, in contrast to traditional Lotka-Volterra predator-prey models, it is reasonable to model pest systems such that predators rather than prey are constrained to some carrying capacity. Here, I synthesize recent theoretical work using Lotka-Volterra predator-prey equations where predators experience more constraint to their populations than prey to describe how autonomous biological control, where interactions between multiple natural enemies combine to keep pest species below economic thresholds, may be achieved.
Results/Conclusions
When the stabilizing effects of intraspecific competition on pest species are removed, negative interactions between competing control agents may help rather than hinder biological control. Competition and intraguild predation between control agents keeps pest populations large enough to sustain both enemies, but small enough to prevent either agent from satiating and becoming ineffective. Though pest populations are consistently held below economic thresholds, temporal dynamics are complex. Chaos results from control switching between the two natural enemies. Dominance is ephemeral since neither enemy is capable of controlling the pest alone. These interactions are hysteretic; which enemy is dominant and the consequent effect on pest population dynamics depends on initial conditions. Incorporating a carrying capacity for predators in Lotka-Volterra models can create the possibility of hysteresis. Double hysteretic loops and hidden stable states arise from the non-linearities of the predator carrying capacity and complex interactions between agents. Hidden stable states imply that effects of biological control programs on ecosystems can be permanent. Theoretically, complex hysteretic patterns may be leveraged to contain pests in low-abundance stable states.