Temporal patterns of rarity provide a more complete view of microbial diversity.

Recently, conditionally rare taxa (CRTs)--those taxa that are typically in very low abundance but occasionally achieve prevalence--were shown to contribute to patterns of microbial diversity because their collective dynamics explained a large proportion of temporal variability in microbial community structure. Here the benefits and challenges of characterizing the presence and interpreting the role of CRTs are further explored, along with questions about CRT ecology. We also introduce a conceptual model for thinking about microbial taxa as dynamic components along the dimensions of occurrence and abundance. Accounting for CRTs in interpretations of microbial ecological dynamics is essential if we are to understand community stability and ecoevolutionary interactions.