Interactions between marsh plant species vary in direction and strength depending on environmental and consumer context

1. Positive interactions between species are predicted to increase in importance at extreme ends of environmental gradients where neighbours offer either stress amelioration in harsh environments or associational defence from consumers in benign environments. However, this model has never been tested with the same species across a single environmental gradient in light of consumers. 2. Estuarine marsh vegetation, at a landscape-scale from salt to tidal freshwater, experiences strong gradients in physical (salinity) stress, but consumer effects are rarely considered in these systems under strong edaphic control. Here, I present evidence that small mammals are important consumers in coastal marshes of New England, USA, reducing vegetative cover by maintaining open runways in all salinity marshes, but whose top-down influence on plant species increases in less saline marshes. 3. A seedling transplant experiment into plots with and without neighbouring matrix grasses, and with varying environmental stress (salt, brackish and oligohaline marshes) and consumer pressure (with and without mammal herbivory), showed that seedling-matrix interactions were positive in salt marshes, negative in brackish marshes, and varied in oligohaline marshes depending on species. While competitive interactions predominated in oligohaline marshes, one transplant species had less negative neighbour interactions in the presence of consumers, indicating an increasing positive component of the interaction due to associational defence. 4. Synthesis. These results suggest that positive interactions may occur more consistently in harsh physical environments than in stressful biotic environments because consumer pressure is likely to vary by species and over space and time, while environmental stress is generally applied more constantly. Results show that interactions between the same species can reverse in direction or change in strength depending on environmental and consumer context, cautioning that single-site studies may not accurately predict community changes in increasingly altered climatic and consumer environments.