Fluctuating environments affect the strength of species interactions and diversity in microbial communities similarly

Microorganisms live in environments that often fluctuate between mild and harsh conditions. Although such fluctuations are bound to cause local extinctions and thereby affect species diversity in microbial communities, it is still unknown (i) how species diversity changes over the rate of environmental fluctuations and (ii) how this relates to changes in species interactions. Here, we use a mathematical model to describe the dynamics of resources, toxins and species abundances in a chemostat where resource supplies switch between scarce and abundant. Over the majority of the explored parameter space, species compete with one another, but the strength of competition between species pairs changed over the switching rate in a pattern that depended on their sensitivity to toxins. When their toxin sensitivities were low, an effect of competition was highest at a low switching rate. At other toxin sensitivity values, competition was instead highest at intermediate or high switching rates. In communities of up to ten species, the strength of competition in species pairs was a good predictor for how community beta diversity changed over the environmental switching rate: diversity was lowest when competition was highest. This shows that an analysis of pairwise species interactions can be used to estimate how beta diversity changes over environmental switching rates. Our results also indicate that predicting how environmental switching affects communities is very difficult a priori, as it depends on the properties of its members, such as their tolerance to environmental toxicity. This may explain the contradicting results of some earlier studies on the intermediate disturbance hypothesis.