Dynamic Phenotypic Clustering in Noisy Ecosystems

In natural ecosystems, hundreds of species typically share the same environment and are connected by a dense network of interactions such as predation or competition for resources. Much is known about how fixed ecological niches can determine species abundances in such systems, but far less attention has been paid to patterns of abundances in randomly varying environments. Here, we study this question in a simple model of competition between many species in a patchy ecosystem with randomly fluctuating environmental conditions. Paradoxically, we find that introducing noise can actually induce ordered patterns of abundance-fluctuations, leading to a distinct periodic variation in the correlations between species as a function of the phenotypic distance between them; here, difference in growth rate. This is further accompanied by the formation of discrete, dynamic clusters of abundant species along this otherwise continuous phenotypic axis. These ordered patterns depend on the collective behavior of many species; they disappear when only individual or pairs of species are considered in isolation. We show that they arise from a balance between the tendency of shared environmental noise to synchronize species abundances and the tendency for competition among species to make them fluctuate out of step. Our results demonstrate that in highly interconnected ecosystems, noise can act as an ordering force, dynamically generating ecological patterns even in environments lacking explicit niches. Author Summary: In natural ecosystems, hundreds of species with different characteristics typically live side by side, some competing for the same foods and some preying on others. A central question in ecology is how the abundance of a given species in such an ecosystem depends on its particular characteristics (its phenotype). Clearly, fixed environments can favor certain phenotypes (thick fur in a cold climate), but what happens when environmental conditions fluctuate randomly as e.g. the weather does? We investigated this question using a simple mathematical model of an ecosystem with many competing species. We found that, paradoxically, randomness in the environment can lead to the appearance of ordered clusters of abundant species with similar phenotypes, with the species adopting intermediate phenotypes being much less abundant (a mountains-and-valleys pattern). The clusters move around so that different phenotypes are favored at different times. We found that these effects arise from the tension between the tendency of noise to level out difference in abundances and the tendency of competition to create larger abundance differences.