Islands of diversity and divergence

by Mark Brazil

Although the islands of New Zealand, which I wrote about last time, are fascinating, we don’t need to travel so far to find isolated islands supporting interesting biodiversity. Japan’s own southern archipelago, straggling from Kyushu toward Taiwan, known as the Nansei Shoto, is so rich in both flora and fauna, and such a fine example of the effects of isolation, that the islands have quite rightly been called the Asian Galapagos.

The lowering of sea levels in the past connected the Japanese islands to the Eurasian continent in three places. In the north, Hokkaido was connected to Sakhalin and to the continent; in the south, Kyushu was connected to the Korean Peninsula. The Nansei Shoto was connected to Taiwan, and thence to the continent. These land bridges allowed mammals to cross over to what later became islands as the sea levels rose again.

Repeated sea-level changes have enabled different animals to cross these land bridges during different periods, and the result is that Japan now supports nearly 100 species of mammals, some of which are widespread across Asia, such as the Asiatic black bear and the badger, while others are unique to these islands, such as the Japanese dormouse.

The depth of the Tsugaru Channel separating Hokkaido from Honshu and the prolonged connection of Hokkaido to Sakhalin and Siberia have left Hokkaido inhabited by mammals that differ very little from those found elsewhere in eastern Siberia. Honshu, Kyushu and the Nansei Shoto, however, each support mammal species found nowhere else.

Even among the islands of the Nansei Shoto, not all share the same species. Connection and isolation have played their part in the Nansei Shoto too. The Kerama and Tokara straits effectively divide the archipelago in three parts. In times past, the northern section had a connection with Kyushu, and the southern section retained a connection via Taiwan to the continent. At other times, all the islands have been connected, and at still other times all have been isolated.

Japanese squirrels, red fox, tanuki and Japanese serow, found throughout the main islands of Japan, do not occur in the Nansei Shoto. There, the largest mammal is a rather small sub-species of wild boar, which I have been lucky enough to encounter in northern Okinawa. The largest predator is the Iriomote wildcat, but that is highly elusive, and I have only encountered its scent so far.

There are other species in the islands, though. The genetics of the Japanese field mouse, Ryukyu spiny rat, Ryukyu longhaired rat, Amami black rabbit and Iriomote wildcat have been much studied, and reveal much about the histories of the islands and their wildlife.

Modern genetics, by studying the divergence found in mitochondrial and nuclear genes, makes it possible to infer approximate rates of change among mammals, and thus to measure the extent of divergence between groups of species or even within a species. Even a few years ago, such relationships or differences could only be guessed at. Today, however, by comparing Japanese species with closely related continental counterparts, scientists have been able to measure the extent to which they have differentiated.

The Japanese field mouse belongs to a group of species that is widespread in the Temperate Zone. It also occurs on all four of Japan’s main islands, and in the northern part of the Nansei Shoto, but it is entirely absent from the central and southern islands of that chain. Nonetheless, though the mice of the northern Nansei Shoto have been isolated for some time from those in Kyushu, they still share many similarities and are not very distinct from those of other areas.

At the other end of the Nansei Shoto, in the extreme south, lives the Iriomote wildcat, which was only discovered as recently as 1965. Research has shown that although it is distinctive, it is closely related to the widespread wild cat of Asia, Felis bengalensis, and that there has been a relatively recent exchange of genetic material between the continental and island populations. Although unique, then, the Iriomote wildcat is a relatively recently derived species.

So if the animals of the northern and southern islands of the Nansei Shoto are still closely related to those of the main islands of Japan, or of southern Asia, from which they have only recently diverged, what of the animals living on the central islands of Okinawa and Amami?

Here, the situation is very different indeed. The Ryukyu spiny rat, Ryukyu longhaired Rat and Amami black rabbit are very distinctive mammals. Research has shown their levels of endemism to be so high that they not only belong to distinct species, but to distinct genera (the next highest category in which species are grouped).

The spiny rat (Tokudaia osimensis) of Okinawa, Tokunoshima and Amami Oshima is particularly special; its lineage is as distinct as those of the various groups of mice, rats, voles and field mice.

The Ryukyu longhaired rat (Diplothrix legata) has been found to be closely related to better-known species of the genus Rattus. Though found on the widely separated islands of Okinawa and Amami Oshima, the Ryukyu long-haired rat populations differ only slightly — a consequence of relatively recent gene flow between them, perhaps as recently as the latter half of the Pleistocene.

On the other hand, the degree of divergence between the populations of spiny rat on the closely situated islands of Tokunoshima and Amami Oshima is very high.

It seems, then, that although populations of Ryukyu longhaired rats mixed across the land bridges formed between Okinawa, Tokunoshima and Amami Oshima during periods of low sea levels, spiny rat populations did not experience the same gene flow. What was it that kept the spiny rat population separate, despite the islands that they lived on having been joined together?

It seems that such a major divergence between the populations of Tokunoshima and Amami Oshima had already taken place, that even when land bridges reconnected their ranges they were unable to breed together. Although, to the untrained eye, animals from the Tokunoshima and Amami Oshima populations look identical, an examination of their chromosomes reveals an enormous difference. Those from Tokunoshima have just 25 chromosomes, but those from Amami Oshima have 45.

Such a massive difference would have made it impossible for individuals from the two populations to breed, and so they would have maintained their differences, even while the longhaired rats were blending theirs.

As genetic research has already revealed such a major difference between the geographically close populations of spiny rats on Amami Oshima and Tokunoshima, it is more than likely that future research will prove the population on Okinawa to be equally distinct. In fact, before long, all three populations may be elevated to separate species on the basis of their genetic differences.