Results/Conclusions The 11 protist species displayed a clear competition/disturbance tolerance trade-off: when grown in isolation, weak competitors persisted at high levels of disturbance while the best competitors persisted only at low levels of disturbance. However, in the 11-species mixtures, competition increased species extinction rates, especially at high levels of disturbance where the most tolerant species disappeared in the early stage of the experiment. Competition increased the probability of species extinction by reducing the population size of all co-occurring species that increased their vulnerability to extinction over the entire disturbance gradient. Consequently, species diversity declined monotonically with disturbance despite the presence of the competition-disturbance tolerance trade-off. These results demonstrated a predominant role of competition in regulating community structure at high disturbance levels and challenge the common belief among ecologists that the importance of competition declines with disturbance. The important structuring role of competition also provides an explanation for the non-hump-shaped disturbance-diversity relationships commonly reported in empirical studies.