Coastal wetlands are ecosystems that provide goods and services critical to our way of life. As dependent as we are upon these features, our understanding of their responses to global factors such as climate, sea-level rise, and eutrophication is limited. The transitional community where tropical mangrove meets temperate salt marsh is an ideal setting to test hypotheses about vegetative shifts caused by environmental changes. Black mangrove ( Avicennia germinans) is at its northernmost boundary within the Gulf of Mexico where it commingles with smooth cordgrass (Spartina alterniflora ). The purpose of this study was to examine what role grazing by the marsh periwinkle (Littoraria irrorata) plays in the competitive interactions between smooth cordgrass and black mangrove. This study centers on coastal Louisiana where black mangroves have been expanding for the past fifteen years. Our results indicate that environmental stressors such as nutrient deficiency or salinity have a greater impact on mangrove-marsh competition than snail grazing in healthy cordgrass stands, but that grazing effects are important in stands already stressed by environmental factors. These findings are consistent with recent observations that mangrove expansion increased following large-scale dieback of salt marsh in Louisiana due to drought-related stress.