Researchers set up three experimental plots in a salt marsh to study how the density of periwinkle snails affects the econystem. each plot was the same size and had a different number of snails that was kept constant for two years. during those two years, the researchers calculated the average cordgrass biomass in each plot. one of the most common plants in atlantic salt marshes is cordgrass. periwinkle snails cling to the top of tall cordgrasses during high tide. the tall cordgrasses provide protection from the snail's predators, such as the blue crab. while the snails do not feed directly on the cordgrass, the snails can damage the cordgrass when they graze on algae and fungi that grow on the grasses. if the snail populations grow too large, they can decrease cordgrass growth