Disturbance and the species composition of rain pool microbial communities

Ephemeral rain pools contain diverse assemblages of protists and small metazoans. Rain pool organisms survive periods of desiccation as dormant cysts or resting stages, and become active shortly after the pools fill with water. Non-equilibrium mechanisms related to the frequent drying of pools could promote the observed coexistence of over 15 species of ecologically similar microorganisms. We used dormant organisms from the substrate of a well-characterized ephemeral pool to create 20 microcosms that experienced two different disturbance regimes. Undisturbed microcosms held a constant volume of medium over the entire 30-d study period. Disturbed microcosms dried periodically, at approximately five-d intervals, and were then refilled with medium to initiate new rounds of community development. Daily monitoring of the presence or absence of active organisms provided information about the number and identity of species in disturbed and undisturbed microcosms. Undisturbed microcosms contained a greater number of active species, primarily because some predatory species only became active after at least five to ten d of uninterrupted community development. Species composition diverged rapidly after the first drying episode as communities in undisturbed and disturbed microcosms developed along different successional trajectories. If non-equilibrium mechanisms contribute to diversity in this system, their contribution must occur over more episodes of disturbance than we observed.