URLanisms often live in environments that  fluctuate between mild and harsh conditions. Although such fluctuations are bound to cause local extinctionsand affect species diversity, it is unknown howdiversity changes at different fluctuation rates and how this relates to changes in species interactions . Here, we use a mathematical model describing the dynamics of resources, toxins, and microbial species in a chemostat where resource supplies switch. Over most of the explored parameter space, speciescompeted, but the strength of competition peaked at either low, high  or intermediate switching rates depending on the species' sensitivity to toxins. Importantly , however, the strength of competition in species pairs was a good predictor for how community diversity changed over the switching rate . In sum, predicting the effect of environmental switching on competition and community diversity is difficult, as species' properties matter. This may explain contradicting results of earlier studies on the intermediate disturbance hypothesis .
URLanisms 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, 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: response to fluctuations depends on species' properties. In parameter ranges where the fittest species was most likely to be excluded , however, the beta diversity in larger communities also peaked . In sum, while we find no simple rules on how the frequency of fluctuations shapes the diversity of a community, we show that their effect on interactions between two representative species predicts how diversity in the whole community will change .
D
often
A
inevitably
R
Although such fluctuations are bound to cause local extinctionsand affect species diversity, it is unknown howdiversity changes at different fluctuation rates and how this relates to changes in species interactions
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
R
use
build
R
the dynamics of resources, toxins, and microbial species in a chemostat where resource supplies switch. Over most of the explored parameter space, speciescompeted, but the strength of competition peaked
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, and peaks
A
,
R
the species' sensitivity to toxins. Importantly
both species' ability to withstand the harsh environment. This complex relationship shows why finding patterns between environmental fluctuations and diversity has historically been difficult: response to fluctuations depends on species' properties. In parameter ranges where the fittest species was most likely to be excluded
R
strength of competition in species pairs was a good predictor for how community diversity changed over the switching rate
beta diversity in larger communities also peaked
R
predicting the effect of environmental switching on competition and community diversity is difficult, as species' properties matter. This may explain contradicting results of earlier studies on the intermediate disturbance hypothesis
while we find no simple rules on how the frequency of fluctuations shapes the diversity of a community, we show that their effect on interactions between two representative species predicts how diversity in the whole community will change