The dynamics of bottom-up and top-down control in a New England salt marsh

Traditionally, salt marsh ecosystems were thought to be controlled exclusively by bottom-up processes. Recently, this paradigm has shifted to include top-down control as an additional primary factor regulating salt-marsh community structure. The most recent research on consumer impacts in southern US marshes has shown that top-down forces often interact with biotic and abiotic factors, such as secondary fungal infection in grazer-induced wounds, soil nutrients and climatic variation, to influence ecosystem structure. In a more northern salt marsh, located in New England, we examined the separate and interactive effects of nutrient availability, insect herbivory and secondary fungal infection, on growth of the foundation species, Spartina alterniflora. We used a factorial design with two levels of nutrients (control and addition) insects (control and removal) and fungi (control and removal). Nutrient addition increased plant biomass by 131% in the absence of herbivores. When insect consumers were allowed access to fertilized plots, biomass was reduced by nearly 45% when compared with treatments with nutrients and insecticide. In contrast, insect herbivores did not affect plant biomass in unfertilized control treatments. These differences suggest that consumer effects are triggered under high nutrient levels only. We also found that secondary fungal infections in grazer-induced wounds, in contrast to lower latitude marshes, did not significantly impact primary production. Our results suggest that while New England salt marshes may typically be under bottom-up control, eutrophication can trigger dual control with inclusion of top-down regulation. However, unlike lower latitude marshes, consumer control of plant growth in northern US salt marshes is not dependent on herbivores facilitating fungal infections that then control grass growth, suggesting that the intensity of disease mediated top-down control by small grazers may be regulated by climate and/or grazer identity that co-vary with latitude.