As we travel south through the broad swath of continental Asia, we move along two contrasting gradients. First, land area declines as we approach the tropics from the Arctic. Second, and in direct contrast, species diversity increases enormously, as do elements that are uniquely Asian.

The Asian tropics teem with a rich variety of wildlife, much of which has never been documented. Habitat loss and environmental degradation have pushed many creatures, however, to the brink of extinction, including the Asian elephant and orangutan (below).

Along with light, fresh water is crucial to the survival of life on land. Asia's biomes range from the extremely hostile, such as the Taklamakan's and the Gobi's virtually water-free expanses of sandy and stony wastes, to the positively prolific.

Key elements in defining the habitats that occur in Asia are the topography and climate. While these combine to leave the immense desert heart of Asia one of the driest places on Earth, where only the hardiest species survive, southeast of the great Himalayan barrier the reverse applies.

In some regions of the world, the transition is a gentle one, from temperate to subtropical and then to tropical, but Asia's makeup does not allow this. As it nears the equator, the great continent narrows and ultimately breaks up into a scattering of islands, across which transitions may be disjointed and abrupt.

In Southeast Asia, we find majestic forests, tropical and lush, dripping from seasonal rainfall all year round and teeming with life.

Here, among one of the oldest ecosystems on the planet, we find the continent's richest terrestrial biodiversity, the greatest concentration of truly Asian plants and animals.

Orangutan

Whereas one may walk across the canopy of trees in the far north, in the tropics one walks, antlike, far beneath the crowns of immense, towering trees. These shadowy, cathedral-like forests harbor an enormous diversity of life: gibbons that rush through the foliage 30 meters above one's head, bizarre flying snakes and flying frogs, gemlike sunbirds fueled by nectar, and tigers and orangutans -- the icons of Asia.

In every direction are species after species, some living by literally piggy-backing on others, creating a three-dimensional maze of life, from the microscopic to the elephantine. Whereas the Arctic abounds in numbers of a few species, in the tropics ecological niches are so diverse that species are abundant, though there are fewer representatives of each. The extent of their diversity is still only partly known: Even during the 1990s, new large mammals and birds were discovered in Laos and Vietnam.

The Asian one-horned rhino, which once ranged throughout Southeast Asia, is now confined to the Himalayan foothills.

A mere newcomer compared with the Himalayan peaks and the Southeast Asian rain forests, but ancient in human terms, is the Jomon-Sugi of Japan's southern island of Yakushima.

This venerable Japanese cedar tree has been estimated to be 7,200 years old, making it possibly the oldest living creature on the planet. Its life began from a seed that fell when we humans were still struggling to survive as hunters and gatherers.

During its lifetime, civilizations have come and gone, and the human population of Asia has risen from a thin scattering to a dense covering. The passing seasons have been like heartbeats for this ancient tree, yet the status and distribution of species on the continent have changed dramatically.

Asia may be vast, and the enormous scale of its biodiversity may seem inexhaustible, but it also supports the highest concentration of humans in the world. As the light of the first dawn of the new millennium spread across Asia, it spread across a region burgeoning with life, particularly human life.

More than a third of the Earth's rapidly increasing 6-billion human population lives here in Asia. In parts of India, China, Japan and Indonesia, the human density is already more than 200 people per square kilometer.

Thus, from supporting an amazing diversity of life, Asia now faces the greatest human and biodiversity crisis on Earth. We humans are like a blazing forest fire out of control, consuming and converting biodiversity as we spread.

If the Jomon Sugi survives another century, let alone another millennium, it will in all probability look out across an Asia without tigers or orangutans, lacking many of its other primates and missing thousands of other species of plants and animals. It will see a wave of extinctions resulting from massive habitat loss and environmental degradation.

For some species, conservation efforts are probably already too late; for others, perhaps not so. The people of Asia hold the choice in their hands. The future of wild Asia is theirs.