Connectivity modelling informs metapopulation structure and conservation priorities for a reef-building species

Aim: In coastal marine systems, biogenic reef-building species have great importance for conservation as they provide habitat for a wide range of species, promoting biodiversity, ecosystem functioning and services. Biogenic reef persistence and recovery from perturbations depend on recolonization by new recruits. Characterizing larval dispersal among distant reefs is key to understanding how connectivity shapes metapopulation structure and determines network coherence; all of which are of critical importance for effective conservation. Location: Northeast Atlantic coast and western English Channel, France. Methods: We used a biophysical transport model to simulate larval dispersal of the reef-building polychaete Sabellaria alveolata. We combined dispersal modelling and network analysis into a framework aiming to identify key reef areas and critical dispersal pathways, whose presence in the network is vital to its overall coherence. We evaluated changes in dispersal pathways constrained by different connectivity thresholds, i.e., minimum dispersal rate for the presence of a connection. We tested scenarios of sequential loss of reefs: randomly, by habitat quality (a score for reef status and occupancy in an area) or by betweenness centrality metric (BC; quantifying the proportion of shortest paths connecting all areas that are passing through any given area). Results: We found that the network of S. alveolata reefs forms two main regional clusters, the Atlantic coast and the English Channel, which are connected only through weak sporadic dispersal events. Within each cluster, the network is characterized by relatively high connectivity among neighbouring areas with reefs, maintained even under higher connectivity thresholds. Simulating scenarios of sequential loss of reefs further identified high centrality reefs, those with highest BC values, key to network coherence. Main conclusions: Effective conservation of this important reef habitat requires a network of protected areas designed to sustain a combination of locally important source reefs, and those that act as stepping stones connecting distant reefs.