The tropical forests are found in a broad belt encircling the world at the Equator, broken only by oceans and mountains. Their distribution coincides with the band of low-pressure areas that occurs where rising tropical air is replaced by moist air flowing in from the north and south to form a system of converging winds.
The rain forest is the floral product of great heat and copious moisture. At all times the average temperature must be between about 21 degrees Celsius and 32 degrees Celsius and the annual rainfall in excess of 150 centimeters. As the sun is roughly overhead throughout the year, the climatic conditions have a constancy found in no other habitat.
Tropical forests are often associated with great rivers, which carry away the copious rainfall. Such rivers are found in the South American island continent, the African subcontinent and the subcontinent of Australia.
Despite the constant fall of discarded leaves the soils of the rain forests are very thin. The conditions are so favorable for decomposition that humus does not have a chance to form. The tropical rain washes the clay minerals out of the soil, preventing important nutrients such as nitrates, phosphates, potassium, sodium and calcium from being retained as they are in temperate soils. The only nutrients found in tropical soils are contained in the decomposing plants themselves.
There are many variations on the basic form of tropical forest resulting both from climatic and local environmental differences. Gallery forest is found where the forest comes to an abrupt halt, as at the edge of a broad river. Here the branches and leaves form a dense wall of vegetation reaching to the ground, to take advantage of light coming in from the side. Less luxuriant monsoon forests exist in regions where there is a distinct dry season. They are found at the edge of continental areas, where the prevailing winds blow from the dry interior at one particular time of year, and are typical of the Indian peninsula and parts of the Australian subcontinent. Mangrove forest is found in saline swamp areas along muddy shorelines and the mouths of rivers.
There are no dominant species of trees in the tropical forest as there are in other forest habitats. This is because there are no seasons and therefore the insect population does not fluctuate; the insects that feed on a particular species of tree are always present and will destroy the seeds and seedlings of that tree if they are sown nearby. Therefore the only seeds that flourish are those that are transported some distance away from their parent and its permanent insect population. In this way stands of particular tree species are prevented from forming.
The area of tropical forest has increased considerably since the rule of humanity. In the past a great deal of damage was done to the habitat by mankind's agricultural practices. Primitive societies cut down areas of trees and farmed the clearings for a few years until the thin soil became exhausted, compelling them to move on to another area. In the cleared areas the original forest did not immediately reestablish itself and it was many thousands of years after humanity's extinction before the tropical forest belt returned to anything like its natural condition.
The tropical forest is one of the most luxuriant habitats on earth. The high rainfall and stable climate mean that there is a perpetual growing season and there are therefore no periods in which there is nothing to eat. The copious vegetation, thrusting upwards to reach the light, although continuous, is arranged very roughly in horizontal layers. Most photosynthesis takes place at the very top, in the canopy layer, where the tops of the trees branch out to form an almost continuous blanket of greenery and flowers. Beneath this the sunlight is more diffused and the habitat consists of the trunks of the taller trees and the crowns of those that do not quite reach the canopy. The forest floor is the gloomy domain of shrubs and herbs, which sprawl out to make the best use of the little light that filters down.
Although the tremendous variety of plant species supports an equal diversity of animal species, the number of individuals in each is comparatively small. This situation is exactly the opposite of that found in harsh environments such as the tundra, where, because few life forms can adapt to the conditions of the region, there are many fewer species of either plants or animals but correspondingly more individuals in each. As a result the animal population of the tropical forests remains stable and there are no cyclical plagues of either predator or prey species.
Birds of prey such as eagles and hawks are the important predators of the treetops, as they are in any other habitat. The tree-living prey animals of these regions must be swift enough to elude them and also to escape from tree-climbing predators coming up from below. The mammals that accomplish this best are the primates - the monkeys, apes and lemurs.
Compared with the canopy layer the floor of the tropical rainforest is a dark, humid place. Little light penetrates through from the treetops, and although there are many shrubs and herbs they nowhere present a thick, impenetrable barrier. Despite a steady fall of dead leaves from above, the soil cover is very shallow. A large number of birds inhabit these regions.
Beyond the mountains of the Far East - the most extensive and the highest chain in the world, greater even than the Himalayas at their zenith 50 million years ago - lies the great Australian subcontinent.
The conditions in this area today - lush tropical forests occupying vast river basins - make it difficult to believe that a mere 100 million years ago this landmass was part of the Antarctic continent. When at this time Australia split off and began drifting northwards, the Age of Mammals was well under way and the continent already had its own mammal population. These mammals were nearly all marsupials - mammals that nursed their young in a pouch on their abdomen - and because of Australia's long history of isolation have largely remained so. In the rest of the world, however, the marsupials were gradually superseded by the placentals - mammals not giving birth until their young are more fully developed.
By the reign of humanity, Australia had reached the desert and tropical grassland latitudes, where the conditions provided the evolutionary impetus for the development of running and burrowing animals such as the kangaroo, Macropus spp., and the wombat, Vombatidae. After man the continent continued its drift northwards until, sometime in the last ten million years, it collided with the mainland, throwing up the great barrier mountains that exist today. Although some diffusion of animals has taken place between Australia and the rest of the Northern Continent, the mountains have kept this cross traffic to a minimum and the subcontinent still has a predominantly marsupial fauna - albeit one adapted to the tropical forests.
As in previous ages the Australian marsupials have developed forms that are superficially very similar to those of placental mammals existing in similar environments in other parts of the world.
The floor of the great rainforest of the Australian subcontinent is home for a number of marsupial mammals.