This research article examines the species-specific strain richness (SR) of the human gut microbiome, finding that SR varies significantly across species and is lower in the gut than in other environments. The study demonstrates that SR is transferable via fecal microbiota transplantation (FMT) and can be temporarily increased through supraphysiologic administration of strains. Importantly, the researchers show that SR predicts microbial addition or replacement in FMT, influencing engraftment outcomes. Factors such as species prevalence, genome size, and metabolic diversity influence SR, while host health status appears to have minimal impact.