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Dryad

Interspecies interactions drive community-level selection in microbial coalescence

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Apr 28, 2026 version files 2.09 GB
Aug 18, 2026 version files 2.10 GB

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Abstract

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.