Temporally varying larval settlement, competition, and coexistence in a sessile invertebrate community

Storage effect theory describes conditions under which recruitment fluctuation can promote the coexistence of multiple species competing for a single resource. In communities of benthic marine animals, larval settlement is often highly variable, and it has long been suspected that settlement variability promotes coexistence via the storage effect, but no empirical studies have provided quantitative support for this. Here we tested for a necessary component of the storage effect using the bryozoan Watersipora subtorquata, a poor competitor during recruitment. We found that from 2008 to 2009, per capita settlement of W. subtorquata was uncorrelated with the strength of interspecific competition during recruitment; this implies that the storage effect has the potential to promote the persistence of this species. We also found a difference between years in patterns of within-year fluctuation. In 2008, settlement and competition were uncorrelated, while in 2009, settlement and competition were strongly positively correlated, minimizing any role for the storage effect within 2009. The difference between years is due to reduced settlement, in 2009, of a dominant competitor whose settlement phenology differs from that of Watersipora. Our results add to supporting evidence for the storage effect from terrestrial plants and zooplankton, while suggesting that the strength of this mechanism may itself be temporally variable.