Density dependent effects of an exotic marine macroalga on native community diversity

► We studied effects of exotic S. muticum on native macroalgae. ► We tested effects via both light and space competition at different exotic densities. ► Effects on native species richness are non-linear and change with exotic density. ► At high cover, S. muticum shaded understory macroalgae, reducing native richness. ► Richness was greater with low exotic cover then none, suggesting some facilitation.