Substratum simplification reduces beta diversity of stream algal communities

Summary Reduced species richness with increased habitat simplification is a well‐known relationship in community ecology. However, habitat simplification can also lead to a reduction in beta diversity if the loss of species is not random. We tested the hypothesis that beta diversity of periphytic algae is lower among simple than among complex substrata. We conducted a field experiment using simple (smooth) and complex (rough) artificial substrata colonised by periphytic algae to calculate beta diversity among each substratum type. We initially estimated beta diversity using the Jaccard dissimilarity index and its turnover component. As species richness differed between substratum types, we also employed the Raup–Crick dissimilarity index that estimates beta diversity by resampling from the species pool. We also deconstructed the total dataset into three functional groups based on the position occupied by each species within the periphytic matrix (low‐profile, high‐profile and motile functional groups). Beta diversity estimated using both Jaccard dissimilarity and its turnover component was higher among simplified substrata for the all‐species dataset and for high‐profile and motile groups. However, after taking into account the differences in species richness between substratum types using the Raup–Crick index, beta diversity was higher among complex substrata than among simple ones for the total dataset and for the low‐profile group. We emphasise that differences in species richness must be considered for the quantification of beta diversity, because this might confound the dissimilarity identified and, consequently, lead to erroneous conclusions. The higher beta diversity among complex substrata might be the result of priority effects, in which early colonists constrain the establishment of later arriving species, causing each patch to harbour a distinct species composition. Furthermore, algae life strategies may be an important driver of beta diversity among simple and among complex substrata, as periphytic algae position in the biofilm may affect their susceptibility to shear stress. On the one hand, stochasticity in colonisation history on complex substrata may have driven high beta diversity for the low‐profile group among this type of substratum. On the other hand, the reduced set of high‐profile and motile species on simple habitats may have driven these species to more occasional and rare occurrence, increasing beta diversity among this type of substratum and resulting in similar beta diversity among both types of substrata. Priority effects should be most frequent on complex substrata. However, only a reduced set of species might survive on simple substrata, occupying most of the available patches and causing beta diversity reduction.