Drift

Low-abundance species are more likely to become extinct from a community, especially after disturbance, even if they may have been better adapted to that community than other taxa that happened to be present at higher abundances.
Gilbert & Levine 2017 Proc R Soc B Biol Sci. 284(1855). doi:10.1098/rspb.2017.0507

Drift can play a particularly prominent role in reuniting communities after a major disruption such as a course of antibiotics.
Zaura et al. 2015 MBio. 6(6). doi:10.1128/mBio.01693-15
Dethlefsen & Relman 2011 Proc Natl Acad Sci U S A. 108(Suppl 1): 4554-61…