Community Assembly in Time-Varying Environments*

We study community assembly in time-varying environments such as growth-dilution or circadian cycles and find conditions under which the competitive exclusion principle may be violated. We find self-organized resource sharing results in "dynamic niches". For example, a fast-growing species might modify the environment (e.g., pollution) in a way that eventually limits its own growth; if a slower-growing pollution-resistant species can sufficiently grow in the modified environment, such species could co-exist in growth-dilution cycles of specific timescales. Such communities feature species with different instantaneous growth rates but are nevertheless stabilized by repeatedly going through unstable transients in composition. We abstract general conditions under which physiological interactions conspire with environmental variations to create such dynamic niches. We further quantify the robustness and susceptibility of such dynamic coexistence. The dynamic niches described here disappear in any limit of a constant environment such as a chemostat and thus cannot be predicted using any effective averaged environment. Thus, our work suggests distinct ecological principles for coexistence in fluctuating environments.