Alternative stable states in microbial communities induced by environmental fluctuations

Environmental fluctuations are pervasive in ecosystems, from daily and yearly variations in temperature to gut microbes receiving influxes of resources. In experiments with soil microbes, we have observed that a fluctuating environment causes a two-species community to exhibit alternative stable states that depend on the initial abundances of each species. Environmental disturbances were administered via daily dilutions of bacteria competitions. The magnitude of the daily dilution factor determines the fraction of cells discarded and therefore the depth of disturbance to the environment. We found the outcome of a competition with a fluctuating dilution factor to be the same as that with a constant dilution factor equal to the average of the fluctuating dilution factors. This outcome is in line with a prediction of the Lotka-Volterra competition model with an added global mortality rate: an environment with a fluctuating death rate equilibrates to the same fixed point as one with the time-averaged added mortality rate. We also observed fluctuation-induced alternate stable states in a more complex community with three species. These results suggest that, at least in a simple ecosystem, a fluctuating environment and a constant environment are in fact equivalent.