The biological exuberance of the equatorial region is staggering to behold. Walking through a temperate forest (as one might find in many areas of northern Japan, the northern United States or across much of central Europe), it is commonplace to have a clear view for hundreds of meters -- even to the horizon in hilly country. The temperate forest is almost cathedral-like: calming and peaceful, with pillared trunks and the chorus of birdsong.

By contrast, the equatorial forest is a frenetic mass of competitive vegetation, impenetrable to light and teeming with animal life.

I made my first visit to the Peruvian Amazon in 1982. There I encountered tiny, gaudily dangerous poison-arrow frogs, shrieking macaws, prehensile-tailed capuchin monkeys, termites galore, an astonishing diversity of ants and a host of other unrecognizable insects.

The birds I was pursuing were elusive: brightly colored, yet astonishingly difficult to find and to see. There were innumerable species, yet few individuals of each.

The hummingbirds were the most tantalizing -- so colorful in book illustrations, while their metallic plumage appeared black in real life. Brightly colored only in sunlight, hummingbirds vanished so quickly that a sighting was virtually impossible.

But it was the density and diversity of plant matter that most astounded me. There were trees of all sizes: Towering giants stood tall, breaking through the canopy composed of hundreds of lofty tree species. Down below, masses of saplings awaited a shaft of light.

Each of these supported a plethora of saprophytic fungi, lichens, epiphytic "air-plants," together creating yet more nooks and crevices, more homes for insect life.

How can anyone who hasn't experienced it firsthand visualize such a density of plant matter? Imagine a square meter of land in an equatorial forest: In the space between the ground and the canopy, the total leaf surface area amounts to some 8 sq. meters. (In the temperate region, that figure is 6 sq. meters at best, and typically less.)

More significantly, leaves in the tropics contain more biomass, such that the dry weight of leaves from 1 hectare of tropical forest weighs in at about 1 ton, whereas those from a temperate forest amount to just half a ton. All that additional material provides extra feeding area for animal life.

In the 20 years since I made my first visit to the Peruvian Amazon, I have returned to the South American continent several times, on each occasion reveling in its astonishing biodiversity.

Just a month ago I was there again, on an expedition that went from Belem in Brazil to Port of Spain in Trinidad, by way of the Amazon, Surinam, Essequibo and Orinoco rivers.

The trip was boat-based, as we were searching for river dolphins. Nonetheless, during brief visits ashore and sightings from the small boats, I counted almost 300 species of birds, including 10 species of woodpeckers and four species of woodcreepers.

Even that number represents only a fraction of the 42 woodpecker species and 21 species of woodcreeper that inhabit the region. The neotropical woodpeckers and woodcreepers are a wonderful example of divergent evolution, demonstrating how niche specialization permits a large number of species to coexist in one region.

Contrast that situation with Japan, and you can begin to grasp the difference between northern temperate forests and neotropical forests.

Here, there are just nine species of woodpecker and but a single species of treecreeper. Despite the great extent of the Japanese archipelago, Hokkaido is home to only seven of the nine species of woodpecker, while Honshu has five, Kyushu just three and Okinawa a mere two species.

Japan's smaller islands tend to have just two species, one large and one small. Okinawa boasts the rare and endangered Pryer's or Okinawan woodpecker (noguchi-gera; see photo); the smaller is the widespread Japanese pygmy woodpecker (ko-gera). Amami-oshima, likewise, has two species, but here the niche of the larger species is occupied by Owston's white-backed woodpecker.

Honshu, with its greater range of habitats, supports the pygmy, the white-backed and, in addition, the Japanese green woodpecker, the great spotted woodpecker and, in the north, a relict population of black woodpeckers.

Hokkaido, for reasons uncertain, is blessed with seven species: It adds the uncommon and attractively pied lesser spotted woodpecker (see photo), the little-known northern three-toed woodpecker, and in place of Honshu's Japanese green woodpecker, the grey-headed woodpecker.

All five of the more common of these species occur in the 2,000 hectares of the Nopporo Forest Park in Hokkaido -- an remarkable example of niche specialization. By feeding and nesting in different micro-habitats, these species can all coexist in a single forest.

My visit to the Peruvian Amazon was not the only significant event of 1982. It was in April of that year that I embarked on what has become a 20-year odyssey -- on April 2, 1982, the first installment of what was to become Wild Watch was published in these pages.

Back then, columns were bashed out on a typewriter, copies were made using carbon paper and manuscripts were submitted by post. I had no stock of photographs, and so many of the early articles were accompanied by decidedly amateur sketches that embarrass me now. Today, my columns are submitted via e-mail -- and are sometimes written in the field on a pocket computer.

Support from you, the readers, has been wonderful over these two decades. Some of you have written to say how you have collected every article; how you have used them to teach classes; how you have enjoyed them as a vicarious walk in the wild; or how they have stimulated a latent interest in the natural world.

One thing that has not changed is the way in which "issues" provoke a flood of reader responses. If I write about habitat loss, whaling or the extermination of bears, about hunting, culling, animals in captivity or (as I did recently) the presence of aliens and our treatment of them, a flurry of replies is guaranteed.

These responses are tremendous feedback and are greatly valued. They let me know your interests and opinions and stimulate me to ever further thought about the natural world.

Some of you are newly interested in the natural world, others, like me, are professionals in the field. Some of you are first-time readers, others are regular followers -- and some even remember those early articles of 1982!

I, in turn, strive not only to address these different readerships, but also to cover as wide a range of subjects as possible: from insects to ice, from migration to manatees, from butterflies to birds, from forest forms to population norms. It is a constant challenge.

Some topics are more emotive than others. When I wrote recently about animal death in the ice of Hokkaido's shoreline, some of you believed me to be too callous toward feral dogs; and when I wrote about possums and their devastating environmental impact, others among you felt I was far too lenient on the creatures.

Death is a perennially awkward issue to raise in articles, especially personal experiences of confronting or avoiding it. But one confident, nature-loving Kiwi wrote to me of the alien possum I encountered in New Zealand's South Island, saying: "You should have killed the little s**t, Mark. With whatever was handy; using your foot if nothing else was on hand. Don't let the false cuteness of a wee baby possum fool you. They grow up to be annoying, soul-destroying, nature-raping monsters . . ."

What do I learn from my own experience and your input? The bottom line -- one that I repeat endlessly to my students -- is that there are no environmental problems. The environment itself has no problems, it merely changes with time. What we call environmental problems are all human problems visited on nature. It is we humans, with our avaricious behavior, who are responsible.

One recent comment that I treasure came, coincidentally, from another Kiwi, who wrote to acknowledge New Zealand's saddening loss of native wildlife to introduced aliens. "Thank you for your articles, which avoid the pat and predictable," the message read. "They always make rewarding reading."

As I embark on my 21st year of writing Wild Watch, I take that and many other positive comments to heart. I will strive to avoid the pat and predictable, and endeavor to make each piece rewarding reading. I hope that you will continue to read this column and provide the valuable feedback that helps to keep the wild in Wild Watch.