Before so many people, Japans open wet habitats were mostly floodplains, with rivers that flooded and shifted around the way rivers do. Those rivers are now dammed, channelized, the floodplains are cities and fields, much the same as in the Eastern US. Rice paddies became the wetland for frogs, fireflies, dragonflies,wading birds. If the paddies go to forest, those species don’t return to floodplains, because the floodplains no longer exist.