DDR Experiments 2013

Ecologists have long sought to understand diversity-disturbance relationships (DDRs) in ecosystems. The Intermediate Disturbance Hypothesis (IDH) has been an important theoretical foundation for much of this work, and posits that species diversity is maximized at intermediate levels of disturbance. However, recent reviews have indicated that IDH is supported in fewer than half of all empirical studies, leading some ecologists to argue for the abandonment of IDH. In this study, we addressed this inconsistency between theory and observations by characterizing multidimensional DDRs in complex freshwater microbial communities under controlled laboratory conditions. We found that unimodal DDRs predicted by IDH were not consistently observed when comparing microbial communities across trophic levels, as suggested by prior modeling work. However, when we looked within the same trophic level, we observed DDRs consistent with IDH across ranges of both intensity and frequency of disturbance, independent of disturbance type. We did not observe U-shaped DDRs predicted by recent models. While our results largely support the original mechanistic understanding of IDH, we propose a state-bifurcation model, whereby maximum diversity is stochastically maintained as the system approaches a transition point between two absorbing states. Together, our results lend strong support to IDH and reveal a potential non-equilibrium coexistence mechanism. The paucity of field studies supporting IDH likely stem from sampling a small window of the entire DDR curve, or from confusion over when and where we would expect to observe IDH patterns.