Disentangling coexistence mechanisms in competitive metacommunities

Background/Question/Methods

In a metacommunity, species distributions vary according to species traits, the distribution of environmental conditions and connectivity among localities. Landscape configuration can thus impact local and metacommunity scale coexistence mechanisms such as species sorting, patch dynamics, mass effect and neutral dynamics. These mechanisms seldom act in isolation, however, and the impact of landscape configuration on their relative importance remains poorly understood. We aimed to resolve how the importance of the four main coexistence mechanisms of the metacommunity theory might vary in different landscapes. We first present an integrated modeling framework to simulate metacommunity dynamics. The integrated model predicts species distribution with multiple simultaneously acting coexistence mechanisms. We then analyze the results of model by varying landscape configuration and controlling the size of the species pool. We developed a mechanistic statistical model allowing the partitioning of the contribution of each mechanism to species distribution and comparing their importances by likelihood ratio tests.

Results/Conclusions

We present simulations of the model in landscapes of various configurations each time using the same species pool as starting point. This approach allowed us to define which of mass effects, species sorting, competition-colonization trade-offs or neutral dynamics was favored in various landscape configurations. We found that species-sorting is highest in landscapes of low connectivity and with smooth environmental gradients. Alternatively, we found that neutral dynamics and the mass effect are stronger in highly heterogeneous landscapes. The competition-colonization trade-off dominates at low spatial heterogeneity and connectivity. Interestingly, we also found that the landscape influences the average relative importance of the different mechanisms as well as the variance within the landscape. In other words, some localities in a landscape could be assembled according to one mechanism and others by another one. Thus changes in landscape properties might lead to a shift in coexistence mechanisms and, by extension, to a shift in the species we expect to persist. This confirms the importance of considering landscape properties for conservation and management. Finally, our results provide theoretical support to the idea that natural communities are the results of multiple mechanisms acting at the same time and complete our understanding of spatial processes in metacommunities.