One of the central goals of ecology is to identify the coexistence mechanisms that give rise to biodiversity in natural ecosystems. Seminal work has identified such mechanisms operating at both local and regional scales. For instance, facilitation has been shown to promote coexistence at local scales. At regional scales, environmental heterogeneity can promote coexistence by allowing species to segregate in space and monopolize their preferred habitat. However, little is known about how coexistence mechanisms operating at different spatial scales interact to control local and regional biodiversity. To address this issue, we developed a spatially-explicit metacommunity model that included local (recruitment facilitation), regional (spatial environmental heterogeneity) and tradeoff-based (competition-colonization) coexistence mechanisms in order to determine their joint influence on patterns of diversity, abundance and extinction risk across scales. Specifically, we extended the patch-dynamic metacommunity framework by simulating 20 species whose competitive abilities were negatively correlated with their colonization rates in order to ensure coexistence within a site. We then added recruitment facilitation, which controlled the degree to which dominant species could only colonize patches occupied by subordinate species. Finally, we implemented spatial environmental heterogeneity in the form of a linear gradient controlling recruitment success, with each species having a different optimum.
Results/Conclusions
Our simulations revealed how coexistence mechanisms interact across multiple scales. Specifically, we found that recruitment facilitation and regional heterogeneity interacted antagonistically to reduce biodiversity across scales. This antagonistic interaction emerged because increasing recruitment facilitation strengthens the local dependence of dominant species on subordinate species. However, species also depend on different environmental conditions. This regional environmental dependence is at odds with the local interspecific dependence, as a single site cannot simultaneously be optimal for both dominant and subordinate species. Hence, although recruitment facilitation and environmental heterogeneity promote coexistence independently and at different scales, they interact antagonistically when combined, reducing species diversity across the metacommunity. Our simulations also provided insights into species-specific responses to increased connectivity (dispersal ability) and interaction strength (facilitation level). In general, as dispersal increased, the burden of coexistence shifted from the dominant to the subordinate species. Similarly, as facilitation increased, dominant species experienced greater extinction risk regionally than subordinates. Our results suggest that classical approaches focusing on a single spatial scale may not yield fundamental insights about the processes that structure natural ecosystems. Instead, our results call for a shift from single- to multi-scale frameworks in order to better understand the dynamics of complex and interconnected ecosystems.