Extremely stochastic connectivity of island mangroves

Studies of mangrove population connectivity have focused primarily on
global to regional scales and have suggested potential for long-distance
connectivity, with archipelagos serving as stepping stones for
trans-oceanic dispersal. However, the contribution of propagule dispersal
to connectivity is still largely unknown, especially at local-scale.
Identifying fine-scale propagule dispersal patterns unique to individual
island systems is important to understand their contribution to global
species distributions, and to select appropriate sizes and locations for
mangrove conservation in archipelagos. Using population genetic methods
and a release-recapture method employing GPS drifting buoys, we
investigated the spatiotemporal scale of propagule dispersal of Rhizophora
stylosa, one of the widely distributed mangrove species in the Indo-West
Pacific. This study sought to quantify intra- and inter-island
connectivity and to assess their contributions to oceanic scale dispersal
of R. stylosa from the Ryukyu Archipelago, which spans over 545 km in
southwestern Japan. Using 7 microsatellite markers, we tested 354 samples
collected from 16 fringing populations on 4 islands. We identified 3
genetic populations, indicating distinct genetic structures comprising 3
distinguishable bioregions (genetic clusters). The western end of the
archipelago receives relatively frequent migration (m > 0.1),
but is genetically isolated from other sites. Based on genetic migration
rates, we found that the central area of the archipelago serves as a
stepping stone for southwestward, but not northeastward dispersal. On the
other hand, with in-situ drifting buoys, we did not confirm prevailing
dispersal directionality within the archipelago, instead confirming local
eddies. Some buoys trapped in those eddies demonstrated potential for
successful beaching from another island. A large portion of buoys were
carried predominantly northeastward by the Kuroshio Current and drifted
away from the coastal areas into the Pacific, contrary to local
migrations. We found that the spatiotemporal scale of propagule dispersal
is limited by the distance between islands (< 200km), propagule
viability duration, and fecundity. Over all, recruitment does not occur
frequently enough to unify the genetic structure in the archipelago, and
the Ryukyu Archipelago is isolated in the center of the global mangrove
distribution.