Food web complexity is thought to weaken the strength of terrestrial trophic cascades in which strong impacts of natural enemies on herbivores cascade to influence primary production indirectly. Predator diversity can enhance food web complexity because predators may feed on each other and on shared prey. In such cases, theory suggests that the impact of predation on herbivores relaxes and cascading effects on basal resources are dampened. Despite this view, no empirical studies have explicitly investigated the role of predator diversity in mediating primary productivity in a natural terrestrial system. Here we compare, in a coastal marsh community, impacts of arthropod predators on herbivores and plant productivity between a simple food web with a single predator species and a complex food web with a diverse predator assemblage. We show that enhancing predator diversity dampens enemy effects on herbivores and weakens trophic cascades. Consequently, changes in diversity at higher trophic levels can significantly alter ecosystem function in natural systems.

Classical ecological theory suggests that the coexistence of consumer species is fostered by resource-use differences, leading to greater resource use in communities with more species. However, explicit empirical support for this idea is lacking, because resource use by species is generally confounded with other species-specific attributes. We overcame this obstacle by co-opting behavioral plasticity in food choice among a group of animal consumers, allowing us to manipulate patterns of resource use while controlling for the effects of species identity and diversity. Within an aphid-parasitoid-radish community, we created a fully factorial manipulation of consumer resource-use breadth (specialist versus generalist) and species diversity (one versus three species) and found that resource exploitation improved with greater specialist, but not generalist, diversity. Therefore, resource partitioning, and not diversity per se, fostered greater overall resource consumption in our multispecies consumer communities.

Single trophic-level studies of the relationship between biodiversity and ecosystem functioning highlight the importance of mechanisms such as resource partitioning, facilitation, and sampling effect. In a multi-trophic context, trophic interactions such as intraguild predation may also be an important mediator of this relationship. Using a saltmarsh food web, we investigated the interactive effects of predator species richness (one to three species) and trophic composition (strict predators, intraguild predators, or a mixture of the two) on ecosystem functions such as prey suppression and primary production via trophic cascades. We found that the trophic composition of the predator assemblage determined the impact of increasing predator species richness on the occurrence of trophic cascades. In addition, increasing the proportion of intraguild predator species present diminished herbivore suppression and reduced primary productivity. Therefore, trophic composition of the predator assemblage can play an important role in determining the nature of the relationship between predator diversity and ecosystem function.

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Species diversity at lower trophic levels generally improves ecosystem functioning. However, the impact of greater predator diversity on herbivore regulation is uncertain because predator species both compete with and prey on each other. In a large-scale field experiment we examined the relationship between predator species diversity and the suppression of two herbivores, green peach and cabbage aphids, on collard plants. We show that, for both aphid species, the strength of herbivore suppression increased with higher predator biodiversity. Greater resource exploitation by predators in diverse communities generally led to improved predator survivorship and reproduction. Herbivore population size was negatively correlated with plant biomass, providing evidence that greater aphid suppression leads to improved plant growth. Our study suggests a harmonious relationship between predator conservation and herbivore control, and a relatively weak role for predator interference, within this community.

Multiple-predator limitation of prey populations may be mediated by both predator-predator interactions and vegetation structure. Antagonistic interactions among predators, such as intraguild predation, can diminish the collective impact of natural enemies on prey population size. However, structurally complex vegetation may moderate such interactions by providing a refuge for predators, thereby enhancing prey suppression. Specifically, we examined the combined impact of two salt-marsh-inhabiting invertebrate predators, the mirid Tytthus vagus and the wolf spider Pardosa littoralis, on suppression of their shared prey, the planthopper Prokelisia dolus, in simple (thatch-free) and complex (thatch-rich) vegetation.In structurally simple habitats in the laboratory, the predators interacted antagonistically, due to the intraguild predation of mirids by spiders, and predation pressure on the planthopper population was relaxed. However, structurally complex habitats dampened this antagonistic interaction by providing a refuge for mirids from spider predation, thereby increasing the combined effectiveness of these predators in suppressing planthopper populations. Consistent with our laboratory results, we found enhanced co-occurrence of these predators in complex habitats in the field, where mirids are apparently at lower risk from spider predation and outbreaks of planthoppers are less likely. In contrast, in simple habitats, mirids were relatively less abundant, a finding consistent with the expectation of increased intraguild predation from spiders. Therefore, in this salt marsh system, complex vegetation diminished the occurrence of intraguild predation between mirids and spiders and increased overall enemy impact on their shared herbivore prey, demonstrating for the first time that plants can mediate enemy effects on insect herbivores by influencing predator-predator interactions.

Multiple-predator limitation of prey populations may be mediated by both predator-predator interactions and vegetation structure. Antagonistic interactions among predators, such as intraguild predation, can diminish the collective impact of natural enemies on prey population size. However, structurally complex vegetation may moderate such interactions by providing a refuge for predators, thereby enhancing prey suppression. Specifically, we examined the combined impact of two salt-marsh-inhabiting invertebrate predators, the mirid Tytthus vagus and the wolf spider Pardosa littoralis, on suppression of their shared prey, the planthopper Prokelisia dolus, in simple (thatch-free) and complex (thatch-rich) vegetation.In structurally simple habitats in the laboratory, the predators interacted antagonistically, due to the intraguild predation of mirids by spiders, and predation pressure on the planthopper population was relaxed. However, structurally complex habitats dampened this antagonistic interaction by providing a refuge for mirids from spider predation, thereby increasing the combined effectiveness of these predators in suppressing planthopper populations. Consistent with our laboratory results, we found enhanced co-occurrence of these predators in complex habitats in the field, where mirids are apparently at lower risk from spider predation and outbreaks of planthoppers are less likely. In contrast, in simple habitats, mirids were relatively less abundant, a finding consistent with the expectation of increased intraguild predation from spiders. Therefore, in this salt marsh system, complex vegetation diminished the occurrence of intraguild predation between mirids and spiders and increased overall enemy impact on their shared herbivore prey, demonstrating for the first time that plants can mediate enemy effects on insect herbivores by influencing predator-predator interactions.