Environmental fluctuations reshape an unexpected diversity-disturbance relationship in a microbial community

Environmental disturbances have long been theorized to shape the diversity and composition of ecosystems. However, fundamental limitations in our ability to specify the scale and features of a disturbance in the field and laboratory have produced an inconsistent picture of diversity-disturbance relationships (DDRs). Using a recently developed automated continuous culture system, we decomposed a dilution disturbance into intensity and fluctuation components, and tested their effects on diversity of a soil-derived bacterial community across hundreds of replicate cultures. We observed an unexpected U-shaped relationship between diversity and disturbance intensity in the absence of fluctuations, counter to classic intuition. Adding fluctuations erased the U-shape and increased community diversity across all disturbance intensities. All of these results are well-captured by a Monod consumer resource model, and can be explained by a novel niche-flip mechanism wherein tradeoffs between species growth parameters produce coexistence regimes that collapse at intermediate disturbance levels. Our results illustrate that compositional complexity of an ecosystem can be generated and predictably reshaped using temporal environmental patterns, and highlight how distinct features of disturbance can interact in complex ways to govern ecosystem assembly.