Top-down and bottom-up regulation of community assembly

Background/Question/Methods

Assembly history is known to affect community structure in a context dependent manner.  Factors such as predation, disturbance, and ecosystem size have all been shown to impact the importance of assembly history in determining community properties.  While many studies have examined the impacts of single factors on the role of assembly history in determining community structure, few have investigated the role of multiple factors simultaneously, despite the fact that natural communities are always exposed to the influence of a host of factors.  In light of this, we conducted a protist microcosm experiment to assess how both top down and bottom up forces combine to influence the effects of assembly history on the structure of assembled communities.  Our experiment consisted of protist microcosms with varying levels of productivity (low and high) and predation (absent or present), as well as with different assembly histories (including three different assembly histories and a control in which all three species were added simultaneously).  Population densities were estimated periodically in microcosms over the course of 50 days. 

Results/Conclusions

Factors significantly affecting community structure and population densities varied dependent on the species in question.  These differences were likely due to physiological differences between species as well as differences in tolerating low productivity and predation.  Two of the three species were able to establish strong priority effects that lasted all 50 days, while the third species established a priority effect which diminished over the course of the experiment.  The failure of the third species to maintain long-term priority effects was most likely due to its weak competitive ability.  Other factors such as apparent competition also appeared to have significant impacts on the final composition of communities.  Our results indicate that history, predation and productivity likely combine to regulate community assembly, with each force working in a species dependent manner.