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

Abstract

The introduction and expansion of exotic species have caused dramatic changes in marine macroalgal assemblages globally. Mechanisms underlying the impacts of macroalgal invaders have been identified for only a few species over small portions of their introduced distribution, and over short time scales. Invasive macroalgae can impact native species through competition for different resources such as light, space or nutrients, as well as modification of abiotic stress and disturbances. We used field experiments to assess the impacts of the invasive macroalga Sargassum muticum (Yendo) Fensholt (Phaeophyceae: Fucales) on native macroalgal communities of the west coast of Vancouver Island, Canada. We manipulated S. muticum in the low intertidal zone to test its effects via both light and space competition at different densities over two consecutive growing seasons. We found non-linear, density-dependent effects of S. muticum on native macroalgal richness. At high cover, S. muticum excludes native species and reduces richness through light competition by shading smaller, understory macroalgae. Light competition was more important than space competition. Native richness was greater when S. muticum cover was reduced to 20% than when it was eliminated entirely, suggesting some facilitation of native species at low cover. The morphology of native macroalgae (canopy, understory, basal) did not predict their response to S. muticum removal, suggesting substantial heterogeneity within functional groups in the types of interactions with S. muticum. The biomass produced by S. muticum failed to compensate for the loss of native biomass due to competition with the exotic, suggesting a negative effect of S. muticum on community productivity. Our results suggest the effects of S. muticum on native macroalgae are largely negative when the invader is abundant, but neutral or positive when the invader is rare.