Andersen Woltz, V. L. | Search bioRxiv

bioRxiv2019

Emergence of alternative stable states in microbial communities in a fluctuating environment

The effect of environmental fluctuations is a major question in ecology. While it is widely accepted that fluctuations and other types of disturbances can increase biodiversity, we have only a limited understanding of the circumstances in which other types of outcomes can occur in a fluctuating environment. Here we explore this question with laboratory microcosms, using cocultures of two bacterial species, P. putida and P. veronii. At low dilution rates we observe competitive exclusion of P. veronii, whereas at high dilution rates we observe competitive exclusion of P. putida. When the dilution rate alternates between high and low, we do not observe coexistence between the species, but rather alternative stable states, in which only one species survives and initial species fractions determine the identity of the surviving species. The Lotka-Volterra model with a fluctuating mortality rate predicts that this outcome is independent of the timing of the fluctuations, and that the time-averaged mortality would also lead to alternative stable states, a prediction that we confirm experimentally. Other pairs of species can coexist in a fluctuating environment, and again consistent with the model we observe coexistence in the time-averaged dilution rate. We find a similar time-averaging result holds in a three-species community, highlighting that simple linear models can in some cases provide powerful insight into how communities will respond to environmental fluctuations.

ecology↗

bioRxiv2018

Mortality causes universal changes in microbial community composition

All organisms are sensitive to the abiotic environment, and a deteriorating environment can lead to extinction. However, survival in a multispecies community also depends upon inter-species interactions, and some species may even be favored by a harsh environment that impairs competitors. A deteriorating environment can thus cause surprising transitions in community composition. Here, we combine theory and laboratory microcosms to develop a predictive understanding of how simple multispecies communities change under added mortality, a parameter that represents environmental harshness. In order to explain changes in a multispecies microbial system across a mortality gradient, we examine its members pairwise interactions. We find that increasing mortality favors the faster grower, confirming a prediction of simple models. Furthermore, if the slower grower outcompetes the faster grower in environments with low or no added mortality, the competitive outcome can reverse as mortality increases. We find that this tradeoff between growth rate and competitive ability is indeed prevalent in our system, allowing for striking pairwise outcome changes that propagate up to multispecies communities. These results argue that a bottom-up approach can provide insight into how communities will change under stress.

ecology↗