Assembly history dynamics of a pitcher-plant protozoan community in experimental microcosms

Background/Question/Methods

Immigration history is increasingly recognized as a fundamental determinant of community structure. Theory predicts that history drives community assembly through differences in the density of residents and species that arrive later (density effects), and differences that are unrelated to density but due purely to the sequence in which species arrive (sequence effects). Density effects represent predictable dynamics that depend on the initial abundances of competitors, and the tendency for initial differences either to propagate or disappear. Density effects are a history-free process firmly anchored in population dynamic principles, but cause what appears to be historically contingent community structure. Sequence effects represent, in contrast, a purely historical, density-free process that relies on the order of immigration events per se. Previous studies have manipulated the sequence of species introduction, varied the period of time between successive introductions, and looked for patterns that demonstrate historical effects on community assembly. Because these experiments have correlated timing with density, the underlying mechanisms leading to historically contingent community structure remain unclear. Our experiment independently varied sequence effects and density effects in a factorial design to follow community assembly in a three-species protozoan community in experimental microcosms.

Results/Conclusions

Our factorial experimental analysis showed that a density effect, but not a sequence effect, exerted a strong influence on the ultimate community structure and population persistence times.  The timing of immigration of an inferior competitor Poterioochromonas did not alter its population persistence time, but Poterioochromonas persisted only when introduced at high density. A weak sequence effect occurred between Colpoda steinii and Eimeriidae gen. sp., even though no density effect was found. As a whole, density and timing interacted in a potentially complex way to determine the ultimate community structure. This study suggests that distinguishing density and sequence effects is crucial for further mechanistic lines of inquiry beyond understanding the importance of immigration history in community assembly.