Tokara Archipelago, cannot be seen south of the Amami Islands, and the same species and allied species inhabit the Far East, the Korean Peninsula, and northern China. Meanwhile, species distributed south of the Amami Islands cannot be seen north of the Tokara Archipelago, including in Honshu, and most of their allied species are distributed from the southeast portion of China through Southeast Asia (Figure 2-2-3).

Organisms that have been geographically isolated are separated into other species after many years have passed. The 5 species of Okinawa tip-nosed frog, which are distributed in islands south of the Amami Islands on the Ryukyu Archipelago, were isolated approximately 8.7 million years ago. They were separated from their continental ancestors because the sea level rose after the last glacial age, and then after approximately 7 million years they separated into 5 species such as the Amami tip-nosed frog (Rana amamiensis) and Utsunomiya's tip-nosed frog (Rana utsunomiyaorum) and so on, which are endemic species of the Ryukyu Archipelag. In addition, in the Ornate narrow-mouthed frog (Microhyla ornate), which is an endemic species of the Ryukyu Archipelago, extremely large genetic differences have been identified on each of the islands in the Ryukyu Archipelago. This is thought that the Ornate narrow-mouthed frog is separating into different species on each island in the Ryukyu Archipelago.

A characteristic of the amphibians in Japan is that many of them are endemic species. This is because amphibians, which inhabit wetland areas, have become geographically isolated on individual islands or separated due to Japan’s mountainous landscape. Of the 64 species and subspecies of amphibians that inhabit Japan, approximately 80% are endemic to Japan. This is the eleventh highest ratio in the world (Figure 2-2-4).

## (3) The Vast Interaction between Forests and the Sea

Nutrients flow from forests into the sea through rivers. In the sea, phytoplankton and seaweed grow by using those nutrients. This phytoplankton and seaweed becomes food for zooplankton, small fish and shellfish, and these small living organisms are consumed by bigger fish. The fish and shellfish are used as food by birds and humans. In this way, it is possible to think that ocean and land are connected ecosystems. This ocean-land interaction is sometimes expressed by the phrase “the forest is the ocean’s sweetheart.”

This kind of interaction is not only the forests and the sea, but also at the borders of ocean and land there are sandy beaches and rocky shores, and a wide variety of organisms inhabit the area. Tidal flats and seaweed beds are particularly rich in organisms and have high water purification capacity. The amount of production of organisms in such coastal areas is extremely abundant, and the juvenile fish are grown in these areas.

A large-scale example of the ocean-land interaction is Amur River and the ocean. Nutrients flow out of the Amur River into Sea of Okhotsk and are carried by the dense and cold flow of seawater that occurs when the sea ice is formed in the northern area of the Sea of Okhotsk. This nutrients nurture rich offshore fishery in the southern area of the Sea of Okhotsk and in the Pacific Ocean.

The Amur River (Chinese name is Hēilóng Jiāng River)

Figure 2-2-3 Distribution of Frogs in Regions on Both Sides of the Tokara Strait

Photographs: Japanese tree frog photograph provided by Tomohiko Shimada. All others by the Ministry of the Environment