Ecological predictability emerges at the population level in phytoplankton communities

Predicting the composition and dynamics of ecological communities is challenging because complexity increases rapidly with species richness. A common strategy is to adopt a reductionist framework in which community dynamics are inferred from simpler components, such as population-level parameters or organismal traits. However, it remains unclear at which level of biological organization ecological predictability emerges. Here we experimentally test this reductionist cascade in marine phytoplankton communities. We first ask whether multispecies dynamics can be quantitatively predicted from demographic parameters measured in monocultures and species pairs. We then test whether these predictive parameters can themselves be inferred from organismal traits, focusing on cell size. We find that community composition is highly reproducible and can be accurately predicted from population-level parameters measured in simpler experimental settings. In contrast, these parameters do not show systematic relationships with cell size and cannot be predicted from this commonly used trait. These results demonstrate that ecological predictability emerges at the population level, where demographic parameters capture the combined effects of underlying biological processes, but resist further reduction to simple trait-based descriptions, suggesting that ecological interactions reshape organismal performance across levels of organisation.