BY IZAWA Masako
OKINAWA-ken (prefecture) has a total land area of 2,265 km2 and is made up of around 160 different islands. These islands cover a portion of the Ryukyu Archipelago, which runs for around 1,300 km from Kyushu to Taiwan. The Ryukyu Islands are known as the "Galapagos of the East" because of their unique flora and fauna.
The islands of Okinawa are surrounded by coral reefs and mangrove forests, growing in the saltwater marshes where rivers empty into the sea. The forested area of northern Okinawa island (the main island) is home to many species of animals and plants that can be found nowhere else in the world. These include the Okinawa woodpecker, which has no close relative anywhere else in the world; the Okinawa rail, a flightless water bird discovered in 1981; the large, five- to six-centimeter-long long-armed beetle; the Oritzuru violet, which is thought to be a relic of an extinct plant type; and many species of wild orchids. The mountain streams are also home to many frog species unique to Okinawa such as the Ishikawa's stream frog, the Holst's spine-thumbed frog, and the tapering-snouted frog. The Iriomote yamaneko (wild cat) attracted worldwide attention when it was discovered in the second half of the twentieth century on Iriomote-jima (island), which lies near the far southern tip of Okinawa-ken. The habu snake, while deadly poisonous, is also an important part of Okinawan evolutionary history, as we shall see later.
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| The Iriomote cat (Felis iriomotensis), a small feline approximately as large as the common housecat, lives only on Iriomote Island. The cats, numbering around 100 in total, have been designated a Special Natural Monument of Japan. |
The grounds for speciation
Why is this collection of tiny islands home to so many unique organisms?
First of all, Okinawa's climate is subtropical. Furthermore, while most
other subtropical climatic zones are arid, Okinawa receives over 2,000 millimeters
of precipitation per year, making it a wet, or humid, subtropical climate.
The abundant rainfall—due to the Black Current, monsoons, and the annual
typhoon season—and an average annual temperature between 22 and 24 degrees
Celsius has made Okinawa's flora and fauna flourish while also giving it
a range of organisms that differs greatly from that of other regions.
An even more important factor behind Okinawa's unique wildlife is the very fact that the prefecture consists of many small islands. Land-dwelling plants and animals are confined on the island on which they are found and each island becomes a major stage for speciation. The geological history of the Okinawan islands is also deeply tied to the wealth of unusual plants and animals found there. Over the millennia, Okinawan islands rose from the seafloor and sank again. Sea levels, too, have risen and fallen. At certain points, land bridges connected some islands to the Asian continent, while others were connected to Taiwan and other southern landmasses while still others were linked to the Japanese archipelago.
Whenever islands were connected to the Asian mainland or landmasses to the north or south, animals would move across the land bridges to the islands, bringing plant seeds with them. Sometime later, however, the islands would become cut off from the larger landmasses, separating the organisms from their relatives back on the mainland. The island organisms then began to evolve along a separate path from their mainland relatives. Through the slow process of evolution, they acquired the morphological and behavioral traits best-suited to their new habitat, slowly diverging from their ancestors who first crossed land bridges to the islands. Thus, over time each island developed plant and animal life unique to that island.
Evolutionary relics
Okinawa is also home to evolutionary throwbacks or relics—species that
crossed over to an island in distant evolutionary time and continued to
survive there even though their relatives on the mainland died out. The
Kikuzato's stream snake, which is found only on Kume-jima, and the Kuroiwa's
eyelid gecko, which lives on Okinawa and several other islands, are good
examples of relic species. While these organisms have no close relatives
in the near vicinity, relatives can be found far away in southern China
and Southeast Asia.
There are countless other endemic species in Okinawa. Of all the insect species found in Okinawa-ken and the Ryukyu archipelago, around 25% are endemic species such as the long-armed beetle. Around 45% of the mammals are endemic species, including the Iriomote cat and the Yanbaru whiskered bat discovered in 1996. The figures for amphibians (70-80%) and plants (20%) are just as surprisingly high.
Many endemic species are further divided into subspecies, which inhabit separate islands and differ slightly from one another. Different subspecies of the Ryukyu flying fox live on Okinawa, Daitojima, and the Yaeyama islands. There are also many subspecies of the familiar brown-eared bulbul that can be seen in gardens and yards and many subspecies of the clown-like Okinawan tree lizard so loved by children.
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| The Ryukyu flying
fox (Pteropus dasymallus) is a large bat with a wingspread of nearly
one meter. It lives in the forest, hanging from the trees when it sleeps
and surviving on tree fruit and leaves as well as flowers. It has a
characteristic collar of gold fur around its neck. |
Thus, the plants and animals differ from one island to the next. The poisonous habu snake is evidence of Okinawa's evolutionary history. Today the snake is only found on some of the Okinawan islands. The snake spread out over the islands from the mainland some 1 to 2 million years ago, but died out on many islands. One explanation for this is that sea levels rose, submerging some islands and killing off the snakes. The habu survived on islands such as Okinawa and Amami Oshima where the mountains were high enough to remain above water and was wiped out on islands that were covered with water. Another explanation is that the habu died out on those islands where the environment was not favorable, based on differences in the availability of food and presence of competitors, and survived where environmental conditions were favorable. A recent fossil discovery on the Miyako islands lends support to the second theory, but there are still many unsolved mysteries regarding the wildlife of Okinawa.
The threat of extinction
There are 45 times more plant species within a unit of area on the
Ryukyu Islands than on the Japanese mainland. Around 20% of terrestrial
mammal species found in Japan live in Okinawa, as do 35% of Japan's amphibian
species and around 70% of its terrestrial reptiles. Considering that Okinawa
only accounts for 1% of Japan's total area, the numbers are quite high.
There are around 600 bird species registered in Japan and around 400 of
these are found in Okinawa. Around 90% of Okinawa's birds are migratory
and the Ryukyu Archipelago is an important route for migrations between
the surrounding landmasses.
The Ryukyu Islands gave birth to this wealth of rare animals and plants, but island habitats also present a major environmental problem. Since islands are small in area, they can only support a limited amount of wildlife. Thus, for example, there are thought to be a mere 100 Iriomote cats in existence and fewer than 500 Okinawan woodpeckers. With numbers this low, extinction is always a real and present danger. Furthermore, environmental changes wrought by humans on island habitats have greater impacts that are felt within a shorter amount of time than on larger landmasses.
The unique organisms that have evolved on the islands often cannot compete with new plants or animals that are brought into the islands by man. The Okinawa rail, for instance, evolved on an island that had no carnivores so it did not have the ability to fly as a means of escape from predators. Thus, the Okinawa rail is easy prey for the mongooses and feral cats that have been introduced to the rail's traditional habitat by humans.
Not only are the plants and animals of Okinawa extremely interesting and rare, they are also perhaps the most threatened by extinction. We are entrusted with the mission of protecting this invaluable legacy for future generations
