1. The flow of energy and nutrients across ecosystem boundaries can have significant community-wide effects, but the role of productivity of the recipient habitat in mediating these effects remains unclear. This is especially true when organisms moving across ecosystem boundaries serve simultaneously as predators and prey.
2. In this study, the effects of odonates, primarily Enallagma civile (Hagen), on a salt marsh system were examined. Cages were used to exclude odonate predators, but not other arthropods, from experimental plots of the sea oxeye daisy, Borrichia frutescens (L.) in high and low productivity areas. The effects were assessed on the in situ arthropod community and the host plant.
3. There were strong direct effects of predation on the herbivores Pissonotus quadripustulatus Van Duzee and Asphondylia borrichiae Rossi and Strong, with higher densities where damselflies were excluded. Damselflies also served as prey for web-building spiders. This resulted in lower spider densities inside cages, and a positive indirect effect on grasshopper densities.
4. Direct and indirect effects of odonates were greater in the high productivity area, resulting in a trophic cascade, with greater damage and reduced flowering and density of the host plant inside cages.
5. The results of this study support the subsidy hypothesis and show that theoretical models of trophic dynamics, which were developed to explain exchanges within ecosystems, may have predictive and explanatory value for exchanges across ecosystems as well.