Priority effects determine how dispersal affects biodiversity in seasonal metacommunities

Background/Question/Methods

The arrival order of species frequently determines the outcome of their interactions. This ubiquitous phenomenon, called the priority effect, contributes to the local community structure, but little is known about how it influences biodiversity in spatial communities with dispersal. Priority effects can be generated by two different mechanisms, “numeric” or “trait-mediated”. Numeric priority effects arise from the positive frequency dependence of the early arriver and the outcome of competition is determined by relative frequencies. Trait-mediated priority effects arise from a change in interaction strength when relative arrival times shift, which is typical for most seasonal communities. Systems dominated by numeric priority effects should thus be more affected by dispersal rates, while systems with trait-mediated priority effects depend on the relative arrival times and should be less affected by dispersal. Here, we use a seasonal metacommunity model to explore how mechanisms of priority effects may interact with dispersal to determine biodiversity. We asked the following questions: (1) How will different mechanisms of priority effects change the relationship between dispersal and diversity patterns? (2) When will local and regional diversity be maintained under high dispersal? (3) How sensitive are patterns of biodiversity to initial heterogeneity among local communities?

Results/Conclusions

We found that priority effects play a key role in shaping local and regional biodiversity of seasonal metacommunities and how they are influenced by dispersal. Under spatial and temporal variations of species arrival times, metacommunities with numeric priority effects display dispersal-diversity relationships predicted by the classic theory: high dispersal homogenizes the metacommunity, decreasing alpha, beta, and gamma diversity, and alpha diversity is maximized at intermediate dispersal. In contrast, trait-mediated priority effects promote local and regional species coexistence, maintaining high biodiversity even under very high levels of dispersal. Furthermore, dispersal-diversity relationships of metacommunities with numeric priority effects are highly sensitive to initial conditions, while those with trait-mediated priority effects are surprisingly robust. These results expand our current theoretical framework of dispersal-diversity relationships and help to explain why in empirical studies beta diversity does not decrease with high dispersal as predicted by classic theory. Together, our model provides novel insights into how priority effects shape the dispersal-diversity relationships in nature and highlight the importance of considering the seasonal nature of local community assembly to predict regional patterns.