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Wednesday, Jan. 17, 2001
BIOMES FROM TUNDRA TO TROPICS
Asian environmental extremes
By MARK BRAZIL
Second of three parts
As if the greatest mountain range on earth were not monument enough to the scale of Asia, other ranges, such as the Tien Shan and the Altai, join ranks with the Himalayas to make Central Asia the roof of the world.
Yet uplifted peaks are not all that Asia offers by way of mountains. This is also one of the most volcanically active areas on earth. Asia's eastern skirt drapes across the Pacific's "Rim of Fire," so that all the way from Indonesia to Kamchatka there are dozens of ancient and modern peaks ready to vent gases, ash or lava at a moment's notice.
Volcanoes destroy; they also build. Volcanic peaks erupting from the ocean bed have created many an Asian island. In comparison, the island building capabilities of corals are slow, and limited to the warmer southern seas, but they are none the less significant. Islands, whether volcanic or coral, are crucibles of creation. Melding the ingredients of isolation, time, light and water, islands enable dispersing species to gain footholds in new habitats, and allow adaptive radiation to occur. Asia's islands are home to the bizarre, the beautiful and the rare, from the weirdly tusked babirusa of Sulawesi to the world's largest lizard, the Komodo dragon; from the flying frogs of Borneo to the largest salamander in the world, in Japan.
From the Kuril in the north to the Sunda in the south, Asia's archipelagos support many unique species of plants, insects, amphibians, reptiles, birds and mammals, each island a separate experiment for life.
From a natural history perspective, Asia's great natural environmental regions, or biomes, support rich diversity. Each biome differs fundamentally from the next, and it is this aspect of Asia that is so often misunderstood.
The differences between these biomes may be found in their scale and in the communities of species that live there, but these are natural environments dependent on long patterns of climate and more fickle weather, and so, though broadly defined, they merge and blend. What is more, they even move, advancing here, retreating there as climatic patterns shift and change.
It is in the north that Asia's landmass is greatest, thus its tundra and taiga zone is enormous. Asia becomes far more attenuated in the south, yet despite this interesting disparity, the image of Asia most readily brought to mind is often the dripping greenery of tropical forests, of gaudily fluttering butterflies, slowly brachiating orangutans and hauntingly howling gibbons, rather than of endless coniferous forests or the ice-coated tundra where the wolf and the polar bear hunt.
Size, relative to geographic situation and climate, has interesting implications for Asia's biodiversity. Despite its vastness, Asia's northern region supports the fewest species, and even among the species that live there, few are uniquely Asian. After all, Europe and Asia are contiguous, joined at the Urals, the last remnant of ancient scar tissue from where two even older continents became welded together. Until about 10,000 years ago Asia and North America were also connected, by the Bering land bridge. This Bering link was a biological bridge that allowed the dispersal and migration of genes and the species carrying them across from the Asian continent to the North American continent and vice versa.
Species that can survive at these latitudes, though few, are generally successful enough to have enormous ranges, and the numbers of individuals may be enormous. At high latitudes, where winter dominates the year and summer is over in a blink, the plants and animals of Asia share many similarities with those of Europe and North America.
Across the Asian tundra the summer growing season is so short that the few tree species growing here struggle to reach ankle height. Here one may walk, giant-like, across the forest canopy itself.
Slightly farther south, where the influence of winter is weaker, one enters the taiga, a region of swamps, pools and shady conifer forests that is even more enormous. Here, the ground is free of ice for much longer each summer, and many more species survive than on the tundra to the north. Nevertheless, many of the species that are successful here occur across enormous ranges stretching to Europe in the west, and still have close links to species in North America.
Almost imperceptibly the taiga blends into the temperate zone at its southern limit. Biodiversity continues to rise, and what is more, uniquely Asian species increase. The temperate region is also immense, stretching from Japan across China to the flanks of the Himalayas. After the relative monotony of the coniferous taiga forests, the seasonal forests of the east are a riot of autumn colors, a paint-box palette to rival any on earth, and they are home to a messenger of the gods, the Japanese macaque. Farther west, towering temperate bamboos hide an undeniable icon of Asia, the giant panda, in its dwindling mountainous forest home.
Farther west still, the heart of Asia is not forested, and for some of the year at least it throbs with heat. Straddling the continent is an immense band of aridity, a desert corridor as broad as the continental United States. At its western extreme, beside the Caspian Sea, lies the Karakum; in the center is the Taklamakan, while at the eastern extreme is the Gobi. Together they span 60 degrees of longitude.
Arctic winds, blowing south across the Asian tundra and taiga, have lost their moisture by the time they reach the heart of Asia, and the water-laden monsoon winds from the south are blocked by the high barrier of the Himalayas, leaving Asia's desert corridor virtually moistureless.
Here, where winter temperatures plummet to minus 40 C yet soar in summer to 45 C, only creatures with specialized physiology and behavior are able to survive. Bactrian camels wander enormous distances in search of remote water holes. Smaller mammals such as desert gerbils avoid the extremes of temperature by burrowing, and quench their thirst on the moisture locked in seeds and vegetation.
The harsh desert environment at the heart of Asia is surprisingly like that in alpine regions and the Arctic. Plants in all three habitats face the desiccating and abrasive effects of the wind and debilitating extremes of temperature. They are forced into special growth patterns to resist the wind, the extreme heat and cold, and they must be efficient at collecting or trapping moisture.
Many species are mat-forming and have thickened leaves and stems to prevent moisture loss and protect against wind-borne sand or ice crystals. Desert mammals and insects in particular do their best to take advantage of these plants, for they represent a vital source of moisture.
The desert corridor is not only a challenge to life itself, but also a significant barrier to the movement of species across Asia. Life here in the deserts is specialized and scarce, and biodiversity here is low compared with elsewhere on the continent.
I am planning a new series of international Wild Watch Ecotours beginning with Australia in September 2002. Those interested in joining may e-mail me at firstname.lastname@example.org, or via snail mail care of The Japan Times.