Positive interactions between species are predicted to peak at extreme ends of environmental gradients where neighbors offer stress amelioration or associational defense, but this model has never been tested across a single environmental and consumer gradient.  Estuarine marsh vegetation, at a landscape scale, experiences strong gradients in physical (salinity) stress.  Here, I present evidence that small mammals are important consumers in coastal marshes of New England, reducing primary production by maintaining open runways in all salinity marshes, but whose top-down influence on plant species interactions increases in less saline marshes.  Through a seedling transplant study, I found that seedling-matrix grass interactions were positive in salt marshes and negative in brackish marshes, while species interactions in oligohaline marshes varied depending on the species tested and the balance between strong competition and associational defense.  These results suggest that positive interactions may occur more consistently in harsh physical than stressful biotic environments.  Additionally, results warn that single-site studies are insufficient for understanding species interactions in increasingly altered climactic and consumer environments.