Niche specialization is widespread throughout the digestive tract even among adjacent body habitats. (a) Circular cladogram based on the RDP Taxonomy  reporting taxa significantly more abundant in supragingival (red) and subgingival plaque (green) and demonstrating the extensive specialization even at these highly related sites. At the class level, Actinobacteria, Bacilli, Gamma-proteobacteria, Beta-proteobacteria, and Flavobacteria are characteristic of the supragingival plaque, whereas Fusobacteria, Clostridia, Epsilon-proteobacteria, Spirochaetes, Bacteroidia, and unclassified Bacteroidetes are biomarkers for the subgingival plaque. (b) Circular cladogram comparing the digestive tract (red, GI) with non-mucosal body habitats (green, NON-GI: comprising samples from the anterior nares, and from the bilateral skin sites, antecubital fossae, and retroauricular creases). Only a few clades are detected as differentially present and abundant throughout the entire digestive tract, as the high degree of specialization and community variability at each body site prevents any individual community member from being representative of all ten body habitats. BM, buccal mucosa; TD, tongue dorsum; SupP, supraginval.