Ecologists have long sought to understand conditions for the alternative stable states of community structure. Alternative stable states arise when final community structure (such as species diversity and composition) depends on the history of community assembly (such as the sequence of species arrival during community assembly) even given the same species pool and environmental conditions. Understanding such historical contingency is essential to explaining the diversity and resilience of communities. Indeed, alternative stable states are recently receiving considerable interest as a theoretical framework for ecological restoration. Of course, real communities are often in transient, not stable, states because disturbance keeps communities from reaching a stable state. Even so, the concept of alternative stable states can still be useful if predicted stable states explain observed transient states. It is poorly understood, however, whether this relationship between predicted stable states and observed transient states can be assumed. To evaluate this assumption, we conducted computer simulation using a simple model of plant community assembly.
Results/Conclusions
Our results show that the assumption can be easily violated. Predictions regarding the importance of community assembly history can be fundamentally different depending on whether the focus is transient or stable states. For example, for communities that have reached a stable state, beta diversity is higher in the presence of positive feedback that causes alternative stable states than in its absence. In contrast, for communities that are in a transient state, beta diversity can be higher in the absence of positive feedback than in its presence, if individual mortality rate is low in the former case than in the latter. Moreover, communities can be in these transient states for a long term, relative to the generation time of species involved in community assembly, the average cumulative number of colonization attempts made per species, the time required for populations to reach carrying capacity, and the time required for populations to undergo a complete turnover of individuals. Our results indicate that ecologists may need to shift their focus from alternative stable states to alternative transient states to understand the effect of historical contingency on the diversity and resilience of ecological communities.
