Terrestrial food-web theory has been developed largely by examining species interactions in primary producer food webs. However, the decomposer subsystem can have strong influences on aboveground communities and ecosystem functioning. Here I examine, at several spatial scales, the complexity of terrestrial food-web interactions by considering interactions between species in detritivore and primary-producer food webs. I focused on Spartina alterniflora marshes and interactions among the numerically dominant herbivore Prokelisia dolus, its major spider predator Pardosa littoralis, and several detritivores (Littorophiloscia vittata, Orchestia grillus, Melampus bidentatus and Littoraria irrorata). I found that predator-detritivore interactions have weak indirect effects on plant growth and decomposition (Chapter 1). Furthermore, by serving as alternative prey, detritivores can influence the strength of predator-herbivore interactions. However, the strength of predator-herbivore-detritivore interactions was species-specific and depended on habitat structure (leaf litter - Chapter 1) and detritivore identity (Chapter 2). Although detritivore species are often functionally redundant in soil communities, changes in detritivore species composition can have divergent influences on aboveground predator-herbivore interactions (Chapter 2). Whereas some detritivores (Littorophiloscia vittatta) promote herbivore and predator survival, other detritivores (Littoraria irroratta) reduce predator and herbivore densities. Moreover, the geographic distribution of detritivores influences the relative strength of predator-herbivore interactions across broader spatial scales (Chapter 3). I found a shift in the relative abundance of dominant detritivore, herbivore, and predator species across a 1660 km latitudinal gradient. Detritivorous Littoraria snails that abound on low-latitude marshes modify Spartina vegetation structure and create an unfavorable habitat for Pardosa spiders. Pardosa exert stronger predation pressure on Prokelisia planthoppers on high-latitude marshes where spiders are abundant. Changes in global carbon cycles may influence the strength of linkages between primary production and decomposition food webs. I examined how changes in the detritivore food chain influenced the growth of two plant species (Scirpus olneyi and Spartina patens) under elevated and ambient CO2 conditions. I found limited and species-specific support for the increased importance of the decomposition pathway under elevated CO2 conditions. Overall, detritivores modified predator-herbivore interactions, live plant growth, and decomposition. The strength of these interactions changed with the composition of the detritivore community, latitude, and atmospheric CO2 conditions.