Intra-guild predation relaxes natural enemy impacts on herbivore populations

1. To investigate the role of intra-guild predation in mediating the impact of the natural enemy complex on herbivore populations, a manipulative field experiment was conducted using uncaged plots (islets of Spartina cordgrass) on a North American salt marsh. The densities (moderate or low) of two invertebrate predators, the generalist wolf spider Pardosa littoralis and the specialist mirid bug Tytthus vagus, were manipulated in a 2 x 2 factorial design, and the resulting treatment effects on the population growth of their herbivorous prey, Prokelisia planthoppers, were assessed. 2. The abundance of wolf spiders on experimental islets was unaffected by the presence of mirid bugs, however the density of mirid bugs was influenced very negatively by the presence of the wolf spider. 3. The negative effect of the wolf spider on mirid bugs most probably resulted from the intra-guild predation of mirids by spiders because planthopper limitation by the wolf spider alone was significantly greater than when both predators were present. 4. As a result of intra-guild predation, planthopper population growth was positive in the presence of both predators, despite the fact that each predator alone promoted a decrease in planthopper population growth. 5. Notably, the occurrence of intra-guild predation diminished top-down impacts on planthopper populations in a relatively simple food web where strong top-down effects were expected. This result, however, was limited to habitats on the marsh with simply structured vegetation lacking leaf litter.