Interspecies interactions drive community-level selection in microbial coalescence

Whether communities behave as cohesive units or loose collections of
independent species remains a fundamental question in ecology. Here, we
study this question through community coalescence---the mixing of
previously isolated communities. Using bacterial microcosm experiments and
ecological modeling, we show that interspecific interaction strength
determines whether community-level selection occurs. At moderate to strong
interactions, community-level selection is frequent, with one parent
community outcompeting the other. At weak interactions, species respond
independently and symmetric mixtures predominate. Experiments tuning
interaction strength via nutrient concentration confirm these predictions.
We further identify two regimes underlying community-level selection: an
emergent regime where collective dynamics shape outcomes unpredictably,
and a species-driven regime where dominant species determine the winner.
Our findings reconcile conflicting observations by showing that
interaction strength governs when communities behave as units of
selection.