Landscape Genetic Method to Explain Intra- and Inter-Island Propagule Transport of Mangrove Species in Okinawa Islands

Understanding migration systems is integral to designing Marine Protected Areas (MPAs) that effectively maintain genetic diversity and population connectivity in the face of environmental change. In island systems, many mangrove populations are fringe habitats in coastal areas, their population size is small, discrete and vulnerable to destruction. Despite their ecological importance, the rate of habitat loss has been high, and migration system, including their dispersal range in island systems, is understudied. This study investigated the population connectivity and migration system of Rhizophora stylosa, a mangrove species common throughout the Okinawa islands in the northern West Pacific region and a representative species of the family Rhizophoracea, which is abundant throughout the Indo-West Pacific. We used landscape genetic method to demonstrate how differences in habitat landscape affect the migration patterns among islands. To further investigate connectivity among isolated populations and their dispersal patterns, we performed nuclear microsatellite analysis for R. stylosa samples collected from 14 coastal areas spanning four islands and representing a spatial scale of over 500 km. The genetic results were compared with physical oceanographic data derived from the Regional Ocean Modeling System (ROMS) and surface drifting buoy experiments to evaluate how well genetic and oceanographically simulated connectivity align. Our genetic results suggest that R. stylosa propagule dispersal is primarily limited within local population, and our oceanographic data for local hydrodynamics agrees. Additionally, both genetic and oceanographic data suggest rare but consistent inter-population and inter-island interactions with consistent migration-source populations in the region. A loss of source populations may, therefore, cause reduced regional genetic diversity, thus highlighting the importance of identifying such key populations for conservation.