Mapping the assembly of protist communities in microcosms

Theoretical models of community assembly predict a variety of possible assembly phenomena, but evidence for them in natural systems is hard to obtain because of problems of temporal scale and replication. We used laboratory experiments to investigate the potential assembly pathways in a small pool of protist species. Mapping the paths of assembly involved two steps: testing the ability of every possible species combination to persist, and testing each persistent community for its capacity to be invaded and changed by every other species from the pool. We then compared the properties of assembly in the experimental system with those predicted by theoretical models.

We found that the number of community states and assembly paths was much smaller than theoretically possible, but that the system nonetheless had quite a complex range of assembly behaviors. There were many alternative paths to most communities, but overall, the species pool had just one state to which the paths eventually led. This absorbing state involved an oscillation between two communities, driven in part by a catalytic species that could persist in neither of them. Catalytic species have the property of invasion, changing the resident community, and then going extinct. In the species pool used here, only the predatory species acted as catalysts. Several communities capable of long-term persistence were unreachable by sequential assembly. One community had the property that it could not be reassembled from the species it contained (a “Humpty-Dumpty” community).

These results are for the most part in keeping with those from numerical simulations of community assembly, although there are certain discrepancies and new phenomena. The results also highlight the potential importance of long-term transient dynamics on assembly in natural systems.