Community assembly in the presence of disturbance: A microcosm experiment

Ecologists recognize that alternative stable states can arise from different histories of community assembly, but disagree on how disturbance affects the likelihood of alterative stable states. It is also unclear whether and how community assembly influences the relationship between disturbance and species diversity. Using microbial communities containing bacterivorous ciliated protists, we experimentally explored these questions by independently manipulating disturbance and community assembly history. Disturbance, in the form of density-independent mortality, had three levels: low (10% mortality), intermediate (50% mortality), and high (90% mortality). Five sequences of community assembly were created from a pool of ten protist species, plus a control in which communities were colonized by all species simultaneously. Overall, increasing disturbance intensity increased the similarity among communities subjected to different assembly sequences. However, community similarity under low disturbance increased over time, resulting in no significant difference in similarity between the three levels of disturbance toward the end of the experiment. Patterns of species diversity in the controls, but not in communities subjected to sequential assembly, supported the intermediate disturbance hypothesis. In the controls, the presence of species with competition-disturbance tolerance tradeoffs contributed to increased diversity at the intermediate level of disturbance. In communities subjected to sequential assembly, diversity always declined with increasing disturbance, as weak competitors tolerant of disturbance often failed to establish if not introduced early. These results support the idea that increasing disturbance tends to reduce the likelihood of alternative stable states and suggest that community assembly may have the potential to modify the disturbance-diversity relationship.