Taxonomic, phylogenetic and functional β‐diversity of stream algae is driven by the number of added nutrients and herbivory

A growing body of mostly observational research has examined how β‐diversity and its turnover and nestedness components respond to environmental gradients across taxonomic, phylogenetic and functional dimensions. To our knowledge, this is the first manipulative investigation to assess how two major environmental influences—nutrient enrichment and herbivory—control β‐diversity and its components across dimensions in phototrophs. We used algal data from field and laboratory experiments, manipulating herbivory and/or the number of added nutrients (NAN), which ranged from zero to three (N, P and Fe) or four (N, P, Fe and Mn). We compared control/nutrient treatment communities with control communities and non‐grazed with grazed communities in terms of taxonomic, phylogenetic and functional diversity. Taxonomic and phylogenetic β‐diversity (βSor) was partitioned into turnover (βSim) and nestedness (βNes) components. We proposed a novel partitioning approach for functional β‐diversity, outperforming the conventional approach, which calculates βSor, βSim and βNes. Instead, we used Bray–Curtis distances derived from the number of taxa within morpho‐functional groups and calculated overall functional β‐diversity (βBC) and its balance (βBal) and gradient (βGra) components. We developed three hypotheses predicting that (i) βNes and βGra would rise at higher NAN because of increased taxonomic richness, and phylogenetic and functional diversity (hypothesis 1); (ii) grazing would either reduce (hypothesis 2a) or elevate βNes and βGra (hypothesis 2b) depending on the balance between extinction of grazer‐sensitive taxa versus establishment of grazer‐resistant taxa; and (iii) the relative importance of βNes and βGra would depend on NAN and dimension (hypothesis 3). Our results supported Hypotheses 1, 2b and 3. Synthesis. Across dimensions, enrichment with multiple nutrients elevated biodiversity, the nestedness and gradient components of β‐diversity and often, the overall β‐diversity. Herbivory contributed to this increase by promoting grazer‐resistant but competitively inferior taxa. Thus, streams with higher levels of both micro‐ and macronutrients and unimpacted herbivorous fauna may represent biodiversity hotspots and targets for conservation. The relative importance of βNes and βGra increased with NAN and at the functional dimension, βGra was the dominant component. Therefore, shorter environmental gradients may promote turnover, while longer gradients, colonization/extinction, the latter becoming particularly prominent at the functional dimension.