Throughout the Earth’s long history, the sea level has changed again and again, varying by as much as 200 meters overall. With this in mind, as you travel around the Oki Islands, you may notice things you have never noticed before.

Naumann’s Elephant Tusk and Tooth

One example is the Naumann’s elephant tusk and tooth on display here. From these fossils that were found in the Oki Islands, we know that the Naumann’s elephant was once in the Oki Islands, about 20,000 years ago. But how did elephants come here to the islands? The answer is in the Earth’s history of rising and falling temperature, and the related changes in sea level. During the last glacial period about 20,000 years ago, the level of the Sea of Japan was about 130 meters lower than it is today. The current depth of the sea between the Oki Islands and mainland Shimane Peninsula is only about 70 meters. This means that during the glacial period, the Oki Islands were not actually islands but a peninsula, connected to Japan’s main island of Honshu. Of course, if Oki was a peninsula, the Naumann’s elephant could easily cross over on foot.

The fact that the Earth’s overall temperature has grown colder and warmer repeatedly throughout history means that the islands have also been repeatedly connected to the mainland and separated from it in the past, alternating between islands and peninsula. This environment has brought about the unusual plant distribution in the Oki Islands.

Unusual Plant Distribution

One strange thing about the Oki Islands is that they have only 33 endemic species. Compared to other remote islands around Japan, this is relatively few—Yakushima for example has about 130 endemic species, and the Ogasawara or Bonin Islands have 660 endemic species. The Oki Islands have previously been overlooked because of this, but in recent years, the distribution of plants in the Oki Islands has been attracting attention.
In June, the Rugosa rose can be found blooming on one of the coasts on Dōgo Island. On the same coast, if you turn around, you can find the Nagoran orchid blooming as well. The Rugosa rose is a plant typically found in Hokkaido, and the Nagoran orchid is usually found in Okinawa. These two plants from very different northern and southern climates coexist here in the Oki Islands.

In addition, plants usually found on mountains at elevations from 1,000 to 1,500 meters can be found on the coast here, and plants typically found in continental environments coexist with the islands’ other plants.

Refugium for the Japanese Cedar

How did the Oki Islands' unusual distribution of plants come to be the way it is? We can actually learn a lot about this process through studying the Japanese cedar. Please move to the exhibition panel titled "Refugium for the Japanese Cedar."

The Japanese cedar is a species of cypress endemic to Japan, and is widely distributed across the country, except for Hokkaido. However, the Japanese cedars in different parts of the country—on the Sea of Japan side, on the Pacific Ocean side, and on the southern island of Kyushu—each have different characteristics. During the last glacial period about 20,000 years ago, these Japanese cedars faced the threat of extinction due to the drop in temperature. Through research, it has been found that the Japanese cedars on the Sea of Japan side survived in the Oki Islands, the trees on the Pacific side survived in the Izu Peninsula, and the ones in Kyushu survived on Yakushima Island.

But how did the Japanese cedars on the Sea of Japan side survive the glacial period in the Oki Islands? It is thought that the trees migrated to Oki 20,000 years ago, when the sea level was low and the islands were connected to Shimane Peninsula by dry land. The area of Oki was warmer than the mainland of Japan, allowing the trees to survive the colder periods. In the same way these trees migrated, other plants migrated as well. During the glacial period, many plants were trapped here on these small islands, which is why we see such a wide variety of plants coexisting on the islands today.