Species-composition distributions under competition, ecological drift and immigration

Understanding how species interactions, ecological drift, and immigration jointly shape community composition is a central goal of community ecology. These processes determine not only average community composition, but also how often different community states occur. Here, we ask how competition reshapes the neutral distribution of community composition generated by drift and immigration. We analyze a stochastic two-species competition model with immigration and derive a closed-form stationary distribution of relative species composition under near neutrality and weak immigration. From this distribution, we obtain explicit boundaries separating distinct unimodal, bimodal, and trimodal composition regimes. Species dominance can become common even when the corresponding deterministic model predicts stable coexistence, particularly in smaller communities, and balanced and dominance states can occur together in a trimodal distribution. Competitive asymmetry further biases composition toward the competitively favored species. Finite community size systematically shifts stochastic transitions away from their deterministic competition thresholds. Thus, competition, ecological drift, and immigration jointly determine not only how much community composition varies, but also which community states are most likely to be observed. Our results provide an analytical bridge between neutral and competition theory at the level of the full distribution of community composition.