Prey temporarily escape from predation in the presence of a second prey species

1. Indirect interactions between populations of different prey species mediated by a shared predator population are known to affect prey dynamics. 2. Depending on the temporal and spatial scale, these indirect interactions may result in positive (apparent mutualism), neutral or negative effects (apparent competition) of the prey on each other's densities. Although there is ample evidence for the latter, evidence for apparent mutualism is scarce. 3. The effectiveness of using one species of predator for biological control of more than one pest species depends on the occurrence of such positive or negative effects. 4. We used an experimental system consisting of the two prey species Western flower thrips (Franklineilla occidentalis Pergande) and greenhouse whitefly (Trialeurodes vaporariorum Westwood) and a shared predator, the phytoseiid mite Amblyseius swirskii Athias-Henriot. We released all three species on the same plant and studied their dynamics and distribution along rows of plants. 5. We expected that the more mobile prey species (thrips) would escape temporarily in the presence of the other prey species (whitefly) by dispersing from plants with the predator. The predator was expected to disperse slower in the presence of two prey species because of the higher availability of food. 6. Evidence was found for slower dispersal of predators and short-term escape of thrips from predation when whiteflies were present, thus confirming the occurrence of short-term apparent mutualism. 7. The apparent mutualism resulted in a cascade to the first trophic level: a higher proportion of fruits was damaged by thrips in the presence of whiteflies. 8. We conclude that apparent mutualism can be an important phenomenon in population dynamics, and can significantly affect biological control of pest species that share a natural enemy.