  Microbial ecosystems are remarkably diverse, stable, and often consist of a
balanced mixture of core and peripheral species. Here we propose a conceptual
model exhibiting all these emergent properties in quantitative agreement with
real ecosystem data, specifically species' abundance and prevalence
distributions. Resource competition and metabolic commensalism drive stochastic
ecosystem assembly in our model. We demonstrate that even when supplied with
just one resource, ecosystems can exhibit high diversity, increasing stability,
and partial reproducibility between samples.
