(a) Rank abundance curve with different thresholds of mean relative abundance, (b) relative abundance of the different classes of rarity compared to the total community at a given threshold of relative abundance and (c) taxonomic composition of the classes of rarity. The ‘CRT’ and ‘PER’ denote ‘conditionally rare taxa’ and ‘permanently rare taxa’, respectively. All the other rare ones were assigned as ‘RARE’. ‘Doubleton’ means the OTUs with two sequences occurring in one or two samples, and the ‘Absolute singleton’ denotes the OTUs with sequence amount of 1 and only occurring in one sample.