  Microorganisms live in environments that inevitably fluctuate between mild
and harsh conditions. As harsh conditions may cause extinctions, the rate at
which fluctuations occur can shape microbial communities and their diversity,
but we still lack an intuition on how. Here, we build a mathematical model
describing two microbial species living in an environment where substrate
supplies randomly switch between abundant and scarce. We then vary the rate of
switching as well as different properties of the interacting species, and
measure the probability of the weaker species driving the stronger one extinct.
We find that this probability increases with the strength of demographic noise
under harsh conditions and peaks at either low, high, or intermediate switching
rates depending on both species' ability to withstand the harsh environment.
This complex relationship shows why finding patterns between environmental
fluctuations and diversity has historically been difficult. In parameter ranges
where the fittest species was most likely to be excluded, however, the beta
diversity in larger communities also peaked. In sum, how environmental
fluctuations affect interactions between a few species pairs predicts their
effect on the beta diversity of the whole community.
