Foundation species canopies affect understory beta diversity differently depending on species mobility.

Beta diversity measures the spatial variation in species composition. Because it influences several community attributes, studies are increasingly investigating its drivers. Spatial environmental heterogeneity is a major determinant of beta diversity, but canopy-forming foundation species can locally modify environmental properties. We used intertidal communities dominated by the canopy-forming alga Mazzaella laminarioides as a model system to examine how a foundation species affects spatial environmental heterogeneity and the resulting beta diversity. Since canopies were found to reduce the spatial variation of temperature and desiccation during low tides, we hypothesized that canopies would decrease understory beta diversity, which we tested through a field experiment that contrasted canopy removal with presence treatments over 32 months. The beta diversity of sessile species was always lower under canopies, but canopies never affected the beta diversity of mobile species. The observed responses for sessile species may result from their abundance being more dependent on spatial abiotic variation than for mobile species, which can occur in stressful areas while temporarily foraging or in transit to other areas. These responses may likely apply to other systems exhibiting canopy-forming foundation species hosting sessile and mobile species assemblages.