Dominance of invasive species is often assumed to be due to their superior ability to acquire resources. However, there should be other factors that underlie dominance of those species. In particular, in the river environments, flood often disturbs habitats, but invasive species recover more rapidly than native species, suggesting that tolerance to disturbances is a critical factor for dominance in these environments.

We compared the micro habitats and the responses to flood in Veronica anagalis-aquatica (invasive) and V. undulate (native) in the Kinu River (eastern Japan) and the Yodo River and Yamato River (western Japan). In the fields, we investigated their micro habitats. In glasshouse, we compared seedlings responses to the water levels.

Their micro habitats, in terms of distance from river margin, water levels and grade scales, were similar. In the glasshouse experiments, the performance of V. anagalis-aquatica was overall greater, but in particular in eastern V. undulate showed greater performances in fluctuating water levels.

These results suggested that V. undulate may rather show greater performance in the habitats of fluctuating water level, and invasion success of V. anagalis-aquatica in the river environments may be caused by not only greater recovery from flood disturbance, but also reduced fluctuation of water levels in habitats.