Eco-evolutionary dynamics lead to functionally robust and redundant communities
Microbial communities are functionally stable and taxonomically variable: species abundances fluctuate over time and space, while the functional composition is robust and reproducible. These observations imply functional redundancy: the same function is performed by many species, so that one may assemble communities with different species but the same functional composition. The clarity of this observation does not parallel with a theoretical understanding of its origin. Here we study the eco-evolutionary dynamics of communities interacting through competition and cross-feeding. We show that the eco-evolutionary trajectories rapidly converge to a "functional attractor", characterized by a functional composition uniquely determined by environmental conditions. The taxonomic composition instead follows non-reproducible dynamics, constrained by the conservation of the functional composition. Our framework provides a deep theoretical foundation to the empirical observations of functional robustness and redundancy.