Without a quantitative framework that directly pairs theory and experiment, it has been difficult to determine the source of disagreement between the many conflicting predictions and observations surrounding DDRs. Earlier assertions that disturbance weakens or interrupts competition ( Connell, 1978 Huston, 1979) have been refuted by both theory ( Chesson and Huntly, 1997 Fox, 2013) and empirical findings ( Violle et al., 2010) that harsher environments instead reinforce dependence on limiting factors. However, DDRs derived from observational studies of disparate ecosystems and disturbance regimes often have inconsistent results ( Mackey and Currie, 2001 Hughes et al., 2007). A famous example is the Intermediate Disturbance Hypothesis ( Connell, 1978 Huston, 1979), in which species diversity peaks at intermediate disturbance intensities ( Figure 1a). DDRs describe how community diversity depends on disturbance intensity, which we define as the mean mortality rate imposed by the disturbance over time. Various diversity-disturbance (DDR) relationships have been proposed that draw intuition from observations of natural ecosystems. These disturbances are of ecological interest for the impact they have on a community, for example, by bringing about mortality of organisms and a reduction of biomass of a community. Accordingly, there have been many efforts aimed at understanding the role of environmental disturbances, which are perturbations to the state of an environment. Without a predictive understanding of how ecosystems respond to perturbations, we are poorly prepared for environmental changes of anthropogenic origin, such as rising global temperatures ( Hoegh-Guldberg and Bruno, 2010), and unable to design effective and robust interventions in ecosystems, such as microbiomes of medical or agricultural importance ( Lemon et al., 2012 Widder et al., 2016). While it is well appreciated that environmental changes influence species diversity in all ecosystems, the exact nature of this critical relationship is unclear. Introductionīiodiversity is a cornerstone of ecosystem stability and function ( Tilman et al., 2014). Broadly, our combined experimental and modeling framework demonstrates how distinct features of an environmental disturbance can interact in complex ways to govern ecosystem assembly and offers strategies for reshaping the composition of microbiomes. All our results are well-captured by a Monod consumer resource model, which also explains how U-shaped DDRs emerge via a novel ‘niche flip’ mechanism. Adding fluctuations increased community diversity and erased the U-shape. We observed an unexpected U-shaped relationship between community diversity and disturbance intensity in the absence of fluctuations. Here, using a high-throughput programmable culture system, we subjected a soil-derived bacterial community to dilution disturbance profiles with different intensities (mean dilution rates), applied either constantly or with fluctuations of different frequencies. However, an inability to specify the characteristics of a disturbance experimentally has produced an inconsistent picture of diversity-disturbance relationships (DDRs). Environmental disturbances have long been theorized to play a significant role in shaping the diversity and composition of ecosystems.

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