Contrasting trophic cascades generated by a community of generalist predators

We examined whether intraguild predation (IGP) weakens the ability of two taxa of generalist predators, carabid beetles and lycosid spiders, to control herbivore densities in gardens of cucumber and squash. We also measured fruit production to determine whether IGP weakens the ability of the predators to impact plants indirectly through trophic cascades. These predators did impact herbivore densities, but their impact on different herbivores varied dramatically. As the herbivore community changed through the season, so too did the predators' indirect effect on fruit production. In spring cucumber gardens, lycosids caused fruit production to increase by reducing densities of striped cucumber beetles, which are important early-season herbivores. Carabids also weakly reduced cucumber beetle densities, and the entire carabid-lycosid guild enhanced cucumber yield. However, the carabid-lycosid guild's impact changed dramatically later in the season. In summer squash gardens, carabids increased fruit production by reducing densities of the squash bug, the major late-season herbivore. In marked contrast, lycosid spiders strongly reduced squash yield, possibly by IGP on important hemipteran predators of the squash bug. The late-season effects of carabids and lycosids counteracted one another, so that the complete carabid beetle-lycosid spider guild had no impact on squash fruit production. Thus, the complex trophic connections between generalist predators and the rest of the community made the predators' impact relatively fluid, changing even within a single season. Results of pairing predators with herbivores or other predators in laboratory microcosms supported hypothesized trophic interactions in both spring and summer food webs.