TOP > Geological and Biological History of the Amami Islands. Unique Showcase of Species Evolution on the Continental Island.

Geological and Biological History of the Amami Islands
Unique Showcase of Species Evolution on the Continental Island


1. Geography 2. .Geological history of the Amami Islands
3. .Unique Biota of the Amami Islands 4. Unique subtropical rainforests
5. Diversified Environment of the Amami Islands 6. Important endemic species of the Amami Islands
7. People and nature of the Amami Islands 8. International Recognition of Amami's Biodiversity

 

[Geography ]

The Amami Islands are located at the eastern edge of the Eurasian Continent. The Amami Islands extend 200 kilometers from north to south. They form part of the Nansei Islands, at the southwestern tip of the Japanese Archipelago. Made up of eight small inhabited islands, the Amami Islands have a total land area of 1,231 km2. The Kuroshio Current, one of the strongest warm ocean currents in the world, fl ows in the west of the Amamis, between the Eurasian Continent and the Amami Islands.

 

 

 

 

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[Natural History]

[ Geological history of the Amami Islands ]

The Amami Islands are included in the Nansei (Shoto) Islands Chains and are located on the border between geographically the Eurasian Plate and the Philippine Sea Plate. The Nansei Islands were created as a result of the Okinawa Trough formation and tectonic shifting some 15 million years ago. The sea level has changed greatly over the past 1.7 million years, largely because of climatic variation, and the deposited Ryukyu limestone has built up along with the formation of the island chain's immense coral reefs. Due to repeated separation and reattachment to the Eurasian continent, two types of islands ultimately came into being: 'high islands', rich with hills and rivers, and 'low islands', formed of limestone that developed into coral reefs, and feature abundant groundwater.

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[ Unique Biota of the Amami Islands ]

About 15 million years ago, the Amami Islands were part of the Eurasian continent, resulting in an overlap in the composition of terrestrial species. When the islands were isolated from the Eurasia mainland by the sea, the island biota also became separated from the continent and pursued an independent evolutionary course. Therefore, many species of fl ora and fauna found in the Nansei Islands, as well as the Amami Islands, are relicts of those found on the Eurasian continent. The Kuroshio Current served as a barrier separating the coastal area of the island chain from neighboring continents and islands. This indicates that the biota unique to the coastal areas of the Amami Islands have evolved independently.
The Nansei Islands have been likened to a "unique living museum" as they showcase the evolution of species on a continental island.

1500NO15 million years ago:

The present area of the Amami Islands once comprised the
eastern tip of a landmass extending from the southern part of Japan to Southeast Asia. It is thought that ancestral species of the Amami rabbit and Anderson's crocodile newt, as well as the Southeast Asian Ryukyu long-haired rat and Amami spiny rat were once found on the area.

 

 

 

 

 

 



 

Two million years ago :

The Okinawa trough began to sink greatly and the Inland sea (the present East China Sea) was terminated. This phenomenon gave rise to the Amami Islands. It appears that the flora and fauna expanding to the eastern tip of the continent before this era remained on the limited areas (high lands) and became isolated from the continent.

 

 

 

 

 

 

 

 

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[ Unique Subtropical Rainforests ]

In general, subtropical zone are found between the warmth indexes of 180 and 240. Subtropical zone occur in those areas which are located between a southern latitude of 20 to 30 and a northern latitude of 20 to 30 on the high-latitude side of the tropical zone. Thesubtropical zone of the world is comparable to the mid-latitude dry zone and rarely has rainforests. Most areas of the subtropical zone are arid or covered in grassland.
Although the Amami Islands belong to a subtropical zone in terms of the warmth index, the islands have subtropical rainforests. This particular situation exists in only a few of the world's subtropical areas, such as in the high mountains of Central America, where monsoons bring heavy rainfall amounting to more than 2000mm a year. Forests similar to the cloud forests that usually appear at high altitudes exceeding 1000m in the tropical zone also exist in the mountaintop areas of Amami Oshima and Tokunoshima, namely Mt.Yuwandake (694m) and Mt.Inokawadake (645m).

*Warmth index: In general, monthly average of over 5C is needed for plant growth. The Index puts basis on 5C and it counts temperatures over 5C and the Index is an annual total of monthly averages.

Distribution of temperature environment and rainfall
Plants of the World Vol.13, Asahihyakka
Modified from Ecological geography for plants (1977)
Pictorial by Mitsuru Hotta


 

 

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[ Diversified Environment on the Amami Islands ]

The islands are divided into two types according to their geological nature: high islands, which are somewhat mountainous, and low islands, which generally consist of raised coral reefs. Subtropical rainforests and mangrove forests develop on high islands, while built-up coral terraces and limestone karst landscapes can be found over large parts of the low islands. Subtropical coastal vegetation develops on all of the low islands, and coral reefs grow in the surrounding sea.
Although Amami Islands are a group of small islands, it possesses a variety of environmental factors that form different types of subtropical landscapes. Furthermore, these differ from island to island, resulting in various types of natural environments that provide unique habitats to a number of endemic or threatened species.

High islands : Hilly island formed primarily of pre-Tertiary deposits and rock.
 



[ Tokunoshima(Is.) (247,76km2) ]
Living fossils

    [ Amami Oshima(Is.) (712,33km2) ]
Extensive subtropical rainforests
 
Subtropical rainforests

Rainforests, which are rarely found in subtropical zones, develop due to the area's high rainfall associated with frequent monsoons.
The rainforests in Amami Is. provide habitats for many endemic species.
 
Mangrove forests

A mangrove forest exists at the estuary of Sumiyou River on Amami Oshima. Although the forests are not large in scale, species diversity is considered high and a number of threatened species can be found.

 

 

 

Low islands : Flat island formed of limestone that was built up through the movement of coral reefs in the Quarternary Era.

 
  [ Yoron (Is.)@(20.47km2) ]
Coral island surrounded by barrier reefs
[ Okinoerabu (Is.)@(93.65km2) ]
Karst island with many caves
[ Kikaijima(Is.)@(56.90km2) ]
Rising above sea level at a speed rarely observed in other regions of the world
 
Limestone karst

Many limestone caves have developed on the low islands, originating from raised coral reefs. Of note among these, Okinoerabu Island has about 150 caves in an area comprising less than only 100 km2 .
 
Raised coral reef terrace

Hyakunodai terrace on Kikaijima rises 2mm a year in average, making it one of the fastest rising landmasses in the world. The terrace appears to have been at sea level around 120,000 to 130,000 years ago. It has become an important subject in the world, especially in regard to climate change in light of recent scientific research.

  Coastal Environment
 
Tropical coastal forests

Tropical plants, including Cycas revoluta, Pandanus tectorius, Scaevola furtescens, Ficus microcarpa and Ficus superba var. japonica, are often found on the coastal area. These plants are intimately associated with the lifestyle of the indigenous Amami people.
 
Coral reefs

While coral reefs are found on all of the Amami Islands, the better - established reefs can be found on those islands that are more directly influenced by the Kuroshio Current. About 220 species of corals exist through the entire islands area.

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[Important endemic species of the Amami Islands ]

[The relict endemic species contributing to the conservation value of the Amami Islands]

The islands are endowed with many endemic species that are not found anywhere else in the world. In particular, there are a number of relict species that have survived only in these islands, while other related species have become extinct in most parts of the Eurasian Continent and the Japanese mainland. This occurred due to the past geographic isolation of the islands.
Among the relict endemic species are internationally recognized endangered species that are included in the IUCN* Red List of Threatened Species. For instance, no closely related species of the Amami rabbit (Pentalagus furnessi ), which is the single species of a single genus, are known. No closely-related species of Lidth's jay (Garrulus lidthi ) has been found in either Japan main land or Taiwan, only a closely related species does exist in the Himalayan mountains, which are far from the islands.

*IUCN = International Union for Conservation of Nature and Natural Resources

[Subspecies that show continuing signs of evolution]

Even now, among species common to Japan main land and the Nansei Islands, differentiation within species is taking place. This ongoing process continues to lead to the presence of subspecies endemic to the Amami Islands. For example, while the Japanese little horseshoe bat (Rhinolophus cornutus) is commonly found on the main Japanese archipelago and the Nansei Islands, those found in the Nansei Islands have diversifi ed into an endemic subspecies, Orii's least horseshoe bat (R. c. orii). Another example of a species that has differentiated in the Nansei Islands is Kuroiwa's ground gecko (Goniurosaurus kuroiwae kuroiwae), which is a relict endemic species in the Nansei Islands. This species has diversifi ed into fi ve subspecies within the Nansei Islands, including the Banded ground gecko (G. k. splendens), which is endemic to Tokunoshima.

 



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[ The people and nature of the Amami Islands]

As seen on the front cover of this brochure, the life of Amami islanders is closely associated with animals and plants. Islanders have been dependent on nature since early times. Natural resources are not only vital for their day-to-day life in regard to clothing, food and housing, but are also important for maintaining traditional local culture and industry.

  • Oshima - tsumugi (hand-spun silk fabric)
    • "Doro - zome" literally "mud - dye" is the established dyeing method for "Oshima- tsumugi," a traditional fabric made in the Amami Islands. People prepare chips from the "Techigi (in local name. Rhaphiolepis indica var. umbellatam)" wood, which they hew in the upland area. They make it into a liquid by boiling the chips.








      Silk threads are dyed with the liquid from the boiled 'Techigi' chips, while mud is used to wash the dyed silk threads. The silk threads then
      become a deep black. A small pond used for 'Doro - zome' dyeing is called a 'Doro - ta' which serves as a habitat for aquatic creatures.
    • After the dyeing process is complete, the dyed silk threads are weaved into traditional patterns and the fabric is used to produce kimono called 'Oshima-tsumugi.' The traditional patterns include sea turtles and cycads designed in the Amami nature motif.



  • Limestone caves
    • On the low islands, where limestone caves are abundant, people once utilized the water that gathered on the fl oor of the caves. The caves and human relations are an important subject in folklore. They also provide habitats for many cave-dwelling creatures.







  • Coral
    • The Amami Islands are hit by many typhoons every year. Stone walls made of coral are strong enough to resist typhoons. There are artisans who have mastered the skill of stonewalling.




  • Cycad
    • Since early times, islanders have made use of cycad, cycas revoluta, a plant listed in the IUCN Red List. They produce miso (soy bean paste) and mochi (rice cakes) from cycad fruit. Cycads are also planted for protection against the wind.

 

 

 

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[ International Recognition of Amami Islands' Biodiversity ]

The Amami Islands have been internationally recognized as an important area for the preservation of the biodiversity, since the presence of coral reefs that support a great number of taxa and other ecosystems provide habitats for
internationally endangered species as well as endemic species.
The Nansei Islands including the Amami Islands are considered globally significant in terms of conservation efforts for migratory species. Sea turtles and migratory birds use the islands to breed and also as a stopover point.

  • Example,international recongnitions of the biodiversity

    1. Conservation International gBiodiversity Hotspotsh
      Conservation International has identified the Japanese Archipelago as an ecological hot spot. In particular, the Amami Islands are considered important as habitats for two threatened endemic genus, the Amami rabbit and the Ryukyu long-haired rat (Diplothrix legata).
      The Japanese Archipelago is also known to have a relatively high diversity of amphibians and reptiles. Endemism of these animal groups of the Nansei Islands, including the Amami Islands, is particularly high: approximately 85% of reptiles and approximately 77% of amphibian, are endemic to the islands. Especially Kuroiwa's ground gecko has been designated as an endangered species.
    2. Birdlife International "Important Bird Areas"
      Birdlife International has identified three endemic Amami Islands species as being threatened with extinction, and therefore in need of protection. The islands have been designated as the habitat, either periodic or constant, for Lidth's jay, White's thrush (Zoothera dauma major) and the Amami woodcock (Scolopax mira). The islands have also been named as an area where eight restricted - range species live in relatively large numbers. They are the Elegant scops (Otus elegans), the Japanese wood pigeon (Columba janthina janthina), the Whistling green pigeon (Sphenurus formosae permagnus), the Ashy minivet (Pericrocotus divaricatus tegimae) and the Ryukyu robin (Erithacus komadori komadori), as well as the three species mentioned above.
    3. WWF "Ecoregion Global 200"
      The forest ecosystem and the coastal ecosystem of the Nansei Islands, including the Amami Islands, have been designated as WWF Global Ecoregions.
      1. Forest ecosystem (Ecoregion 32)
        Subtropical forests and endemic plants and birds living in the forests are recognized as unique instances of the endemism found in these subtropical islands. WWF's local species of interest include the Amami rabbit, which is a relict endemic species that is only found in this area of the world.

      2. Coastal ecosystem (Ecoregion 217)
        The islands are recognized as an extremely isolated region that has facilitated unique speciation. It is acknowledged that a wide variety of coral reefs in this region support a number of animal populations, including seabirds, marine mammals and endemic fi sh species.
    4. Conservation International designated as "Top 10 Reef Hotspots Fact Sheet". The coral reefs that span from the Nansei Islands through to Taiwan and the coast of southern China are considered to support one of the world's richest accumulations of reef species that are found nowhere else.

 

 

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