Diversity and invasion within a predator community: impacts on herbivore suppression.
Management strategies to enhance ecosystem services in agroecosystems often seek to increase predator biodiversity. Invaders commonly dominate predator communities in agricultural landscapes, however, and the impacts of exotic predators on native predators and ecosystem services are largely unknown. Exotic predators may complement native predator diversity and enhance herbivore suppression, or they may disrupt herbivore suppression through predator interference. We examined the impacts of a dominant exotic predator within the diverse predator community of a California vineyard. Interactions between the exotic wandering spider Cheiracanthium mildei, its closest native ecological homologue (the wandering spider Anyphaena pacifica) and a native spider displaying a different hunting mode (the cobweb-weaving spider Theridion melanurum) were tested in a manipulative field experiment, with the grape leafhopper Erythroneura elegantula acting as a shared herbivore prey source. We found that the exotic wandering spider suppressed both grape leafhoppers and other unmanipulated leafhopper species that were present in cages. Native spiders had little or no impacts on leafhopper suppression. Thus, C. mildei appears to drive predatory impacts in this system, although evidence also indicated that it is limited by intraspecific interactions. The superior predatory ability displayed by this spider may be shared by other invasive predators that dominate agroecosystems. Despite its effects on leafhoppers, C. mildei negatively impacted a range of spider species, including A. pacifica, T. melanurum, and other unmanipulated native spiders. These impacts are likely to have been mediated through intraguild predation. The wandering hunting mode of C. mildei may have facilitated its role as an intraguild predator. Synthesis and applications. Our results highlight the sometimes conflicting aims of biological control and conservation. Here, a dominant invasive predator increased ecosystem services but also caused declines in numbers of native predators. As disturbed conditions in agroecosystems are likely to put invasive predators at an adaptive advantage, efforts to conserve biodiversity in agricultural landscapes should focus on shifting the outcomes of species interactions to favour native species.