Magazine article Geographical

Taking the Pulse: The World's Dwindling Tropical Rainforests Represent Our Largest Reservoir of Biodiversity and Play a Vital Role in Maintaining Ecological Services

Magazine article Geographical

Taking the Pulse: The World's Dwindling Tropical Rainforests Represent Our Largest Reservoir of Biodiversity and Play a Vital Role in Maintaining Ecological Services

Article excerpt

The tropical forests originated almost 200 million years ago during the early days of the dinosaurs. They were huddled together in the giant supercontinent Pangaea, which was centred around the equator and covered by huge ferns and early forms of conifer trees.

At the same time as the modern forest was evolving, Pangaea was gradually breaking up. First Australia and Antarctica (which itself was once forested) broke away. Then the Americas began to move off. Later, Madagascar separated from Africa. At each stage, evolution started to take a different course. Thus the forest monkeys that evolved in the Old and New Worlds, while coming from the same ancient stock and living similar lives in similar environments, are anatomically quite distinct. Similarly, lemurs survived only in Madagascar, in part because the island had no large predators.

Since the days of Panagea, the planet's climate has cooled somewhat. But throughout the wet tropics, where it typically rains most of the year, rainforests remain the natural vegetation. They are the planet's largest reservoir of biological diversity, containing more than half of its plant and animal species. They also play a vital role in maintaining ecological services such as the water and carbon cycles, by storing carbon, conserving soils and generating rainfall.

The largest of these forests is in the Amazon Basin. The basin contains roughly two thirds of the world's surviving tropical rainforests, representing some 30 per cent of all the biological material on the land surface of the planet. The forest receives so much rain that the river running through it has five times the flow of the river with the next largest flow, the Congo, which itself runs through the world's second largest continuous tract of rainforest, in Central Africa.

The third great rainforest region straddles Southeast Asia from Myanmar through Malaysia and Indonesia and on to the islands of the South Pacific, including New Guinea and the Solomons. Smaller patches also survive in West Africa, Central America, northern Australia, the Indian subcontinent and on some tropical islands, but most of the forests in these areas have been cleared for farming.

The vast majority of rainforests are found in the basins of great rivers. Most are on dry land--creating dark and surprisingly vegetation-free "cathedrals", as Edward Wilson calls them, beneath the canopy, where "there is almost never a need to slash a path with a machete through tangled vegetation". But some occupy land that is seasonally inundated by the rivers, as in the Amazon upstream of Manaus, or sit on top of great peat swamps, as in Borneo and Sumatra.

Other rainforests form on coasts, as dense mangrove thickets. The biggest of these are the forests of the Sunderbans on the Ganges delta of India and Bangladesh--the famed domain of the Bengal tiger--and on New Guinea, the world's largest tropical island.

And finally there are distinctive cloud forests in the near-permanent clouds of tropical uplands such as the eastern slopes of the Andes, the highlands of Central Africa, parts of Central America and the remote interior of New Guinea. The trunks of the trees here are short and gnarled by comparison with the tall straight trees of the lowlands. …

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