When and why top-down regulation fails in arthropod predator-prey systems

The existence of top-down regulation in predator-prey models is a result of the functional and numerical responses included in the model. Examples are given of arthropod predator-prey systems where functional and numerical responses may be irrelevant for the system dynamics, or relevant for only a part of the population. It is argued that there are advantages in using a patch dynamics approach, when describing the feeding and oviposition behaviour of adult predators. This leads to predictions that in arthropod predator-prey systems, in which the predator has a long generation time relative to that of its prey, predator reproduction should be correlated with the age of a prey patch rather than the number of prey present, and top-down regulation does not occur. The predictions are tested against data for various species of aphids, coccids, mites, mosquitoes and their predators and the effectiveness of different species in biological control.