Drift, determinism, and biodiversity persistence in spatial networks: The varied role of connectivity

Background/Question/Methods: The well-studied case of a metapopulation surviving tenuously in a fragmented landscape highlights patch connectivity as a key promoter of biodiversity persistence.  However, not all systems fit this scenario; ecological communities of interacting species often inhabit patches, and these interactions can promote or inhibit diversity persistence in a patch network.  While research has investigated the influence of connectivity on diversity in such ‘metacommunities’, less is known about how the complex topologies and asymmetries of realistic patch networks control diversity maintenance.  Using a Pacific coral reef network parameterized with biophysical models of larval dispersal as a test case, we use theory to investigate the role of complex network structure in maintaining diversity under different assumptions of neutral and non-neutral competition. 

Results/Conclusions: We find that under competitive neutrality connectivity weakly depresses diversity when dispersal connections across patches are symmetric and depresses it dramatically when connections are asymmetric.  In non-neutral scenarios asymmetric connectivity disrupts competitive hierarchies and shifts the balance towards ecological drift, which can either promote or depress diversity maintenance depending on the strength of competition.  Moreover, ‘upstream’ patches with greater outward than inward connectivity are usually the most critical to preserve in a metapopulation, but are often the most depressive of diversity in a competitive metacommunity.  This raises questions about the generality of lessons for biodiversity conservation gleaned from the analysis of single-species metapopulations.