Detritivore community influences on primary producer food-webs

Although it is widely recognized that primary production and decomposition are necessarily linked, there has been little investigation into the nature of interactions between primary producer and decomposer-based food webs.  In the salt marsh system, predatory spiders such as Pardosa littoralis aggregate in areas rich in leaf litter-detritus.  Therefore detritivores living in leaf litter have the potential to influence primary producer food webs by at least three mechanisms: 1) serving as an alternative food resource for predatory spiders, 2) engineering the structure of detritus the preferred spider habitat, and 3) mineralizing litter nitrogen as a nutrient resource for live plant growth.  In a combination of field surveys and factorial mesocosm experiments we examined how changes in detritivore community composition impact the relative influence of each mechanism on predator-herbivore interactions.  Detritivores varied in both their vulnerability to predation and the voracity in which they consumed leaf litter.  Therefore decomposition rates were enhanced when detritivore communities were dominated by voracious, unsusceptible detritivores such as Littoraria irrorata.  Decomposition was reduced in the presence of spiders, and detritivore communities dominated by vulnerable, or moderate consumers such as Orchestia grillus.  Detritivores predominately influenced herbivore suppression by serving as alternative prey rather than influencing host plant quality.  These results emphasize that the strength and consistency of interactions in the primary producer food web can be influenced by the composition of detritivore communities.