Herbivores and detritivores are ecologically linked through their participation in nutrient cycling and shared consumption of plant tissue. Herbivores can influence the quality of plant litter available to detritivores by feeding on living plants and inducing plant defensive responses. We typically assume that detritivores respond to these herbivory-induced differences in litter quality in the same way that they respond to other differences in litter quality. Well-studied effects include differences in plant species and preliminary microbial litter decomposition. However, plant defensive compounds and herbivore-induced changes in litter chemistry might influence detritivore litter processing and nutrient recycling differently than other changes in litter quality. I tested the impact of herbivory history on detritivore litter consumption and the transfer of those nutrients back into plant biomass in old-field experimental mesocosms. I added 15N labeled leaf litter with a factorial legacy of grasshopper herbivory and nitrogen fertilization to mesocosms with different densities of a common detritivorous woodlouse. The field had been recently burned as part of regular management leaving it bare of older leaf litter. After a growing season, I quantified differences in plant, microbial, and woodlouse biomass and 15N accumulation across the litter history x woodlouse treatments.
Herbivory history and nitrogen fertilization significantly impacted litter quality. Herbivory increased leaf toughness and both herbivory and fertilization increased litter nitrogen content. These differences in litter quality interacted with woodlouse abundance to influence total plant biomass at the end of the growing season. Woodlice decreased plant biomass when litter had a history of herbivory, but had no effect on plant biomass when litter was not fed on by herbivores. The opposite trend was observed for microbial biomass. Woodlice decreased microbial biomass when litter had no past herbivory. The woodlice consumed fertilized plant litter more than unfertilized plant litter and the liberation of nutrients from fertilized litter resulted in a grass-dominated plant community. Despite this shift in the plant community, it was herbivory history and woodlice, not fertilization, which influenced total plant biomass. My results provide a novel demonstration of strong feedbacks between herbivores and detritivores under field conditions. They also suggest that differential detritivore litter processing might modify the longer-term effects of herbivores on plant litter quality and plant communities.