Contributing to the debate over the occurrence of terrestrial trophic cascades is the fact that predator diversity, a combination of species richness and functional-group composition, may mediate herbivore suppression and thus indirectly enhance primary productivity. Specifically, increasing predator species richness could promote trophic cascades if species interact additively or hinder cascades if predators are antagonistic. Additionally, the functional-group composition of the predator guild (% intraguild predators) may also influence cascades since strict predators (predators which feed only on herbivores) may have distinctly different impacts on food-web dynamics than intraguild predators. To determine the impacts of predator species richness and functional-group composition on the occurrence of trophic cascades we factorially manipulated both aspects of predator diversity in mesocosms containing Spartina cordgrass, Prokelisia planthoppers, and a diversity of invertebrate predators (spiders, a coccinellid, and a mirid). From a pool of 3 strict and 3 intraguild predator species, diversity was manipulated by crossing predator richness (1, 2 or 3 species) with functional-group composition (strict predators only, intraguild predators only, or a mixture of strict and intraguild predators). Overall, predator species richness had no effect on the herbivore population. However, increasing the richness of strict predators decreased planthopper population size and increased Spartina biomass. Alternatively, increasing the proportion of intraguild predators present (changing functional-group composition) diminished planthopper control and dampened cascading effects on plants, both across and within species richness treatments. Therefore, functional-group composition rather than species richness appears to be an important aspect of predator diversity dampening the strong cascading effects of predation.