Transient states


The concept called "alternative stable states" has long been the dominant framework for studying historical contingency in community assembly. According to this theoretical concept, there are more than one final stable state of species composition that communities may approach as they assemble, depending on immigration history. Once a community assumes a stable state, it cannot move to another state unless heavily disturbed. It is well recognized, however, that many real communities are in a transient, not stable, state, even long after the last disturbance event.

Using a computer simulation model of plant community assembly, we have recently found that the conditions under which community structure is historically contingent differ greatly between stable and transient states. Based on this finding, we have argued that, rather than studying determinants of final variation in the predicted stable states of species composition, it would be more informative to also investigate those of initial variation at early stages of community assembly and the rate at which the initial level of variation approaches the final level. These efforts should allow integration of two closely related, but traditionally separated fields: community-assembly research, which has focused on final states, and succession research, which has focused on temporal changes.

With the simulation model, we are now investigating conditions for both alternative stable states and alternative transient states in greater depth. For this work, we focus on plant-soil feedback (PSF) as a mechanism of historical contingency. Much research on PSF, which is currently receiving considerable interest, is directed toward explaining the rate of succession, but few studies consider the trajectory of succession as affected by PSF-induced historical contingency. In nature, the sign and magnitude of PSF is known to vary across plant species and between intra-specific (effects on conspecific plants) and inter-specific (effects on heterospecific plants) feedback. We are working to determine the types of PSF that increase the number of alternative transient states and alternative stable states.

One direction of this research is to theoretically extend and empirically test the new hypothesis that we recently developed regarding the maintenance of plant species diversity, namely that complex plant-soil interactions cause local plant communities to enter into a prolonged period of species turnover, resulting in transient, yet long-lasting maintenance of the high regional diversity that reflects variable history of species immigration early in succession.