Japan is not just an island; it is an archipelago.

Archipelagos are whole strings or clusters of islands, related to but clearly separated from one another. Japan's archipelago is particularly special because of its location off the seaboard of a major continent, and because within the main archipelago are other minor archipelagos, such as the string of islands stretching south from Tokyo to the Ogasawara islands, and south from Kyushu almost to Taiwan.

Living on an archipelago, we take islands for granted; yet, of course, most of the Earth's landmass is actually distributed as continents. Life on continents and life on islands differs enormously.

The significant differences between islands and continents have become less and less of an impediment to us humans. Now that we have airports at almost every major city, there is nothing to prevent our own movement -- except the cost. In the past, international travel was difficult for those who lived on islands. The physical barrier of the sea was considerable. That physical barrier still affects many other species.

As animals raise their young, the young disperse, moving out of the areas where they were raised to set up new territory. Most land animals disperse on foot, so barriers formed by rivers, lakes, mountain ranges, channels and oceans have had considerable impacts on the distribution of species. Anyone who has become entranced by those mischievous-looking imps, the marmosets and tamarins of the South American forests, will notice how the boundaries of their ranges tend to coincide with the major rivers running through the Amazon region.

Even among species that can fly, such barriers are still significant. Many forest birds, for example, are reluctant to fly far over water, and so their ranges also are often delimited by major rivers.

Such barriers are not, however, confined to the distant Amazon. There are barriers affecting species all around us; we just need to be able to notice them. What are highways for us are barriers for other species, and the urban sprawl filling valleys can prevent the movement of animals from one range of hills to the next.

The natural barriers to life that mountains and water represent are ways in which animal and plant populations become isolated. With isolation, populations separate, diverge and, ultimately, change.

In short, isolation is the essential key to evolution and speciation. Isolation can occur in many different ways, and on many different scales, but isolation on islands is perhaps the clearest and most easily recognized form of all.

That fact makes islands fascinating places for the naturalist, and Japan, because of its many islands, is an incredible storehouse of biodiversity. The economic bubble may have burst, but from a biodiversity perspective Japan is still a very rich country indeed.

The facts of life on islands tend to be simple. If you are not already there, how do you get there? Once you are there how do you survive? Some islands have been formed by the steady erosion and fracturing of larger areas of land, or by rivers changing course, or by sea-level changes flooding low-lying areas. In such cases, species that were already present before isolation took place may still remain.

But what of those catastrophic occasions when new islands are born, or when events such as the past spread of glaciers has scraped islands clean of life (as happened to most of the British Isles)? This kind of event has provided naturalists with a fascinating opportunity to look at how life colonizes and what happens to life in isolation. By studying islands such as Kauai in the Hawaiian Islands, which was formed about 5 million years ago, or Krakatoa in Indonesia, which was blown asunder in 1883, or Surtsey, which erupted to the surface off Iceland in 1963, naturalists have been able to piece together the process of colonization, and even measure it as it happens.

Plankton in the sea is familiar to most of us, yet there is plankton in the air too, "aeolian" plankton: spores, seeds, tiny spiders ballooning on lines of spun silk, insects caught up in storm winds and such like. If you have ever hiked high in the mountains and found a patch of snow, you may have been surprised to see just how many flecks of life and detritus there were scattered across it. Most of it has been blown there, just as it has to islands.

Then there are the species that can fly, and those that can swim. One by one, they find their way to the new land. At first, the influx of new species is high, but then the rate falters and slows. Not all the new colonists are successful, but those that occupy the new land and the ecological niches that it offers, making it harder for latecomers to colonize. Not all of the successful colonists survive long term. Life settles down to a dynamic equilibrium between immigration and extinction.

The smaller the island, the fewer species it has. The reverse is also true. Even during a brief visit to the Galapagos Islands last year I was able to observe this relationship between the size of the islands I visited and the number of land birds I observed. This pattern, or "area effect," has been studied and quantified by ecologists who have found that, on balance, biodiversity doubles with a 10-fold increase in island area. Therefore, if a whole series of islands lies approximately the same distance off the coast of a continent, then the larger island will have the greater number of species.

Then there is the "distance effect." For a naturalist, the journey from Tokyo via the Izu Islands to the Ogasawara Islands is more than just a sun-filled holiday; it is a marvelous opportunity to observe the distance effect at first hand. Simply put, the farther an island is from a continent or larger island, the fewer the species that will occur there. On Honshu, seeing 50 or more species of land bird in a day would be possible, but less likely in the Izu Islands, and even half that number would be impossible in the Ogasawaras.

Beyond the area effect and the distance effect, though, there is another marvelous effect: time. Given isolation and time, species that colonized may diverge from their founding stock, and speciation is in action. Visitors to the Izu Islands or the Yaeyama Islands will have been amazed at how different-looking even common birds such as great tits and varied tits are there. They are still recognizable, but the origins of many island species are lost in time, and on islands we now find many unique and fascinating species. Their isolation, and their rich biodiversity does however make islands and the life on them highly vulnerable to human intervention.

Familiar forms may take on unfamiliar sizes. From fossils, we know of dwarf mammoths on islands. On many islands birds have become entirely flightless. Others may become giants. Giant tortoises used to be widespread; now they remain only on tiny islands in the Indian and South Pacific oceans.

The most famous of these, of course, is the Galapagos giant tortoise, an archetypal island creature. Just how could such a creature have reached these islands, and what has happened to it since it arrived? I will try to answer those questions next time.

Write to Mark Brazil by e-mail at [email protected] or by snail mail care of The Japan Times.