A massive tsunami promoted gene flow and increased genetic diversity in a near threatened plant species.

The magnitude and frequency of disturbances affect species diversity and spatial distributions, but the direct effects of large-scale disturbances on genetic diversity are poorly understood. On March 11, 2011, the Great Tohoku Earthquake in Japan caused a massive tsunami that resulted in substantial alteration of community compositions. Populations of a near-threatened tidal marsh Carex rugulosa inhabiting brackish sandbars was also affected. We found four out of six remnant C. rugulosa populations along the Pacific Ocean had become completely extinct. Newly emergent post-tsunami populations, however, had higher allelic numbers than pre-tsunami populations, indicating higher genetic diversity after the tsunami. In addition, genetic differentiation (Fst) between post-tsunami populations was significantly lower than that of pre-tsunami populations. We therefore conclude that the tsunami enhanced gene flow. Seeds of many Carex species persist for long periods in soil, which suggests that seed banks are important genetic resources for post-disturbance recovery of genetic diversity. When its brackish sandbar habitat is no longer subject to disturbance and changes to the land, C. rugulosa is outcompeted by terrestrial plant competitors and eliminated. Disturbance is a driving force for the recovery and maintenance of populations of species such as C. rugulosa-even after near-complete eradication.