The presence of multiple predator species in an ecosystem can have positive, negative, or neutral effects on lower trophic levels, depending on whether the interactions between predators are synergistic (i.e. predators complement each other's ability to consume prey) or antagonistic (i.e predators interfere with each other's ability to consume prey). For example, several of the predators in this community engage in intra-guild predation-i.e. predators at the same trophic level feeding on each other, with potential consequences for primary production.
To investigate these consequences, the scientists designed an experiment in which they manipulated the presence and diversity of the arthropods on Spartina plants, and measured the Prokelisia density (planthoppers per square meter) and smooth cordgrass production (Spartina aboveground biomass). They included the following four treatments:
1. Plants control, with just Spartina (all arthropods excluded)
2. Herbivores only, with Spartina and Prokelisa (all predators excluded)
3. Low predator diversity, with Spartina, Prokelisa, and Tytthus bugs only
4. High predator diversity, with Spartina, Prokelisa, and all predators present
