Coupling unstable agents in biological control.

It has long been a goal of farm policy to manage production in such a way that expensive off-farm inputs and negative environmental consequences can be simultaneously minimized. One generalized philosophy that has gained currency in recent years is autonomous pest control, in which complex ecological interactions are encouraged to maintain the ecosystem in a state of permanence with the pest below economic thresholds. Early experience with biological control was hampered significantly by the inherent instability of many of the control agents, suggesting that pursuit of the autonomous strategy could be difficult. Here we show that combining two unstable two-dimensional systems (pest-predator and pest-pathogen) produces a stable three-dimensional system (pest-predator-pathogen) that is robust to perturbations in initial conditions. Contrary to expectations, the inclusion of negative interactions, which are arguably a necessary consequence of increased complexity, can stabilize unstable conditions and rescue biological control of simpler, ineffective pest management systems.