Tidal ranges of inner Saemangeum were largely reduced after the final dyke enclosure, resulting in the former tidal flats becoming either permanently exposed, still influenced by changing tide, or permanently submerged. The effect of reduced tidal range on survival and population stability of tidal flat macrofauna were investigated at three shifted habitats after the dyke completion. At the exposed area, several species survived for 80 days after the dyke enclosure. However, within 120 days, all macrofauna died off due to the elevated temperature during summer. At the intertidal area, some species were maintained until 170 days after the dyke enclosure with a large decrease of the faunal abundance. Species of the submerged area were seen to be more tolerant of the changed environments. The opportunistic species, such as Sinocorophium sinensis, Theora fragilis and Pseudopolydora Kempi, were massively introduced into the submerged area after the dyke enclosure, in which the benthic ecosystem was severely disturbed.