Background/Question/Methods There is long-standing debate over what maintains high diversity in competitive communities, particularly in pelagic systems which are homogenized in space and have little potential for niche partitioning.  Environmental fluctuations have been suggested as one mechanism able to maintain communities in a non-equilibrium state, because they ensure that communities never reach their species-poor steady states.  Although high diversity can be maintained for vast numbers of generations, random extinctions can accumulate quickly – making high diversity an early transient phase.
Results/Conclusions Using a model of phytoplankton competition, which assumes that species only differ in their growth-tolerance at different temperatures (their thermal niche), I show that coexistence of a diverse community is not transient, but is favored by a ‘storage-effect’ type mechanism which operates when some niches are vacant.  Conditions which are deleterious for all (vacant niches) synchronize species with community fluctuations, weakening the effects of competition when a species becomes rare thereby shielding them from extinction and promoting coexistence of a diverse set of species.  Given the demonstrated predominance of synchrony over compensatory dynamics in competitive communities there may be a strong relationship between interspecific synchrony and species diversity.