Litter decomposition is a central ecosystem function because dead plant biomass plays a critical role for animals, microorganisms, carbon storage, and the nitrogen cycle. In the face of global change, interactions between organisms that participate in litter decomposition are likely to change due to increased nutrient pollution and species loss. To understand how multiple global change factors may interact to alter litter decomposition, we manipulated a dominant detritivorous snail (reduction, ambient, addition) and nitrogen concentrations for two years in a coastal salt marsh. We chose to study this abundant detritivorous snail (Melampus bidentatus) as its population size is expected to decline with sea level rise and habitat loss. We asked the following questions: 1) How does nitrogen addition, the population density of Melampus, and their potential interaction affect rates of litter decomposition? 2) Does the impact of Melampus on litter decomposition vary throughout decomposition stages? 3) How does nutrient enrichment impact the dominant modulators of litter decomposition and litter chemistry? 4) Do fungal community changes correspond with shifts in litter decomposition? In our 36 plots, we measured litter decomposition, detritivore densities (Melampus, amphipods, isopods), fungal community diversity and relative abundance, and the nitrogen concentrations of soil and litter.
We found that nutrient enrichment and higher densities Melampus increased rates of litter decomposition. This is the first study to show that varying densities of Melampus are associated with changes in litter decomposition rates in a field setting. Notably, the impacts of all detritivores were dependent on the stage of litter decomposition as well as the nitrogen content of the litter. Using linear mixed models, we found that nitrogen impacted litter decomposition, but mainly in an indirect fashion; nitrogen increased detritivore densities which were the primary drivers of mass loss. Further, preliminary results suggest that both nitrogen and Melampus densities altered the fungal communities within decaying litter in unique ways. Considering the potential reduction of Melampus population densities, as is predicted in the face of sea level rise, litter decomposition dynamics might be impacted in areas with high densities of Melampus. Our study reveals that detritivores have palpable impacts on litter decomposition and litter chemistry, which may be altered due to species loss and nutrient pollution.