Diversity maintenance: new concepts and theory for communities as multiple-scale entities

Background/Question/Methods

Current approaches to community assembly, species coexistence, and the maintenance of biodiversity focus on the coexistence of species on small areas of habitat, which are assumed to support self-sustaining biological communities. The local communities on these areas are assumed to be assembled from the regional set of species according to local environmental conditions and interactions between species.  However, coexistence in local communities has long been controversial.  Moreover, local communities are not natural ecological entities in most cases, but are defined more by convenience for study.  In general, local communities are affected by immigration on ecological time scales, and so do not satisfy the requirements for most theories of coexistence and diversity maintenance, which assume that communities are closed to immigration. A resolution of these issues comes from scale transition theory and the techniques for quantifying species coexistence that have been developed in that framework. Scale transition theory allows contributions to species coexistence to be integrated over space and time.  For a given unit of space, contributions to species coexistence are measured by how well species recover from perturbations to low density without the benefit of immigration from other spatial units.

Results/Conclusions

For any given unit of space on a given scale, scale transition theory reveals how much diversity can be explained by processes taking place within that unit of space compared with contributions from other spatial units or other scales. We use scale transition theory to show how the joint contributions of multiple mechanisms of coexistence can be studied. These mechanisms include point mechanisms, such as resource partitioning, spatial mechanisms such as the spatial storage effect and fitness-density covariance, and temporal mechanisms such as the temporal storage effect.  We show how the contributions of each mechanism on any scale can be partitioned from a measure of total mechanism strength, how they integrate over scales, and how they yield new mechanisms on higher scales by their interactions with spatial and temporal variation.  This multi-scale community theory removes the artificial distinction between the local community and the species pool. Instead, it shows how the species pool and the local community are mutually dependent, with scale-dependent properties. The measures of mechanism strength from scale transition theory can be operationalized for field and experimental studies, and provide rigorous approaches to the study of coexistence in natural communities as multiple scale entities.