Coexistence and spatial heterogeneity: A theoretical analysis of metacommunity paradigms

Background/Question/Methods

The metacommunity framework provides an effective way to analyze differences in community dynamics at local and regional scales, with the two spatial scales linked through dispersal of individuals. Metacommunity theory identifies four paradigms: patch dynamics, species-sorting, mass-effects, and the neutral model. While the community dynamics of each paradigm are well studied, the fundamental differences in coexistence mechanisms between the four paradigms are not well documented. We analyzed the key differences in each paradigm according to Chessonian spatial coexistence mechanisms. We compared both deterministic and stochastic individual based models of the four paradigms, building our models from ecological first principles of birth, death, and migration. 

Results/Conclusions

While the four metacommunity paradigms are often portrayed as distinct, they instead lie on a multi-dimensional continuum. The (1) fitness level, (2) dispersal rate, and (3) relative differences in the strength of these two variables dictate which paradigm dominates for a given system. Due to our choice of Beverton-Holt growth, we were able to solve for the exact formula of the Chessonian spatial coexistence mechanisms: non-spatial fitness, nonlinear competitive variance, the storage effect, and fitness-density covariance. As expected, in the neutral paradigm, we found only the nonspatial mechanism at play. When compared to other paradigms, species-sorting followed by mass-effects, exhibit the greatest contribution to coexistence from increased fitness-density covariance and the storage effect. We found that the patch dynamics paradigm was the only one which had spatio-temporal as well as spatial coexistence mechanisms at play. We conclude that the four paradigms of metacommunity theory are not truly distinct from one another, but lie on a continuum that affects both the magnitude and the relative contribution from each of the coexistence mechanisms.