While comparatively few amphibian species have been described on the North East Asian mainland in the last decades, several species have been the subject of taxonomical debates in relation to the Yellow sea. Here, we sampled Dryophytes sp. treefrogs from the Republic of Korea, the Democratic People's Republic of Korea and the People's Republic of China to clarify the status of this clade around the Yellow sea and determine the impact of sea level change on treefrogs' phylogenetic relationships. Based on genetics, call properties, adult morphology, tadpole morphology and niche modelling, we determined the segregated status species of D. suweonensis and D. immaculatus. We then proceeded to describe a new treefrog species, D. flaviventris sp. nov., from the central lowlands of the Republic of Korea. The new species is geographically segregated from D. suweonensis by the Chilgap mountain range and known to occur only in the area of Buyeo, Nonsan and Iksan in the Republic of Korea. While the Yellow sea is the principal element to the current isolation of the three clades, the paleorivers of the Yellow sea basin are likely to have been the major factor for the divergences within this clade. We recommend conducting rapid conservation assessments as these species are present on very narrow and declining ranges.

Niche differentiation enables ecologically similar species to coexist by lessening competition over food and/or shelters and may be critical for reproductive isolation between closely related species in close proximity. Because no extra traits need to evolve, spatial and temporal differentiation may readily take place to complement other isolating mechanisms. Two closely related treefrog species occur together in Korea: the endangered Hyla suweonensis and the widespread Hyla japonica. Advertisement calls are differentiated, but it is unclear whether call difference is sufficient for reproductive isolation. We tracked individuals of both species to study fine-scale differentiation in microhabitat use in the diel cycle of the breeding season using a harmonic direction finder. tracking male movement patterns of both species revealed spatial and temporal differentiation in microhabitat use for calling and resting during the breeding season. Males of both H. suweonensis and H. japonica occurred in all 5 microhabitats identified in this study: rice paddy, ground, buried, grass and bush. Both treefrog species showed general similarities in calling from rice paddies and resting in grass and bush. However, H. suweonensis moved into rice paddies and produced advertisement calls 3 h earlier than H. japonica. These differences likely minimize contact between the species and provide an additional isolating mechanism. In addition, the activity of H. suweonensis may be contributing to the decline of this species, as resting in grass would increase dangers from predatory birds and habitat disturbance.

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