Abstract

Species diversity in a habitat is often termed alpha diversity. As it influences various community properties, many studies have investigated its drivers. For instance, intertidal macroalgal canopies limit understory thermal stress during low tides and thus often increase alpha diversity. More recently, beta diversity has also become of interest. Beta diversity measures the change in species composition across space and is another important attribute of communities because it influences their multi functionality, productivity, and resilience. Using data from a field experiment done in Atlantic Canada, we tested the hypothesis that fucoid macroalgal canopies limit beta diversity in intertidal communities. This prediction stems from previous evidence that such canopies limit thermal variation across the substrate during low tides, an important consideration because spatial thermal changes influence spatial variability in species composition. To test our hypothesis, we compared two treatments (full canopy cover and canopy removal) created the year before on intertidal areas that were originally all fully covered by canopies. Separately for each treatment, we calculated beta diversity as the Bray-Curtis dissimilarity between nearby quadrats using species abundance data. Overall, fucoid macroalgal canopies significantly reduced beta diversity, showing that these foundation species can have opposing effects on alpha and beta diversity.