Chaotic flow: The physics of species coexistence

Hydrodynamical phenomena play a keystone role in the population dynamics of passively advected species such as phytoplankton and replicating macromole cules. Recent developments in the field of chaotic advection in hydrodynami cal flows encourage us to revisit the population dynamics of species compet ing for the same resource in an open aquatic system. If this aquatic enviro nment is homogeneous and well-mixed then classical studies predict competit ive exclusion of all but the most perfectly adapted species. In fact, this homogeneity is very rare, and the species of the community (at least on an ecological observation time scale) are in nonequilibrium coexistence. We ar gue that a peculiar small-scale, spatial heterogeneity generated by chaotic advection can lead to coexistence. In open flows this imperfect mixing let s the populations accumulate along fractal filaments, where competition is governed by an "advantage of rarity" principle. The possibility of this gen eric coexistence sheds light on the enrichment of phytoplankton and the inf ormation integration in early macromolecule evolution.