The dense Vegetation of the tropical rain forest can be a barrier preventing living organisms from moving easily. This can be seen In the way the rain forest limits human movement in these areas. For example, each valley in New Guinea, an island known for its large tropical rain forests, has its own tribe-each with its own language manner of dress, and child-rearing tendencies. This diversification is so great that New Guineans have developed about 1000 different languages, in contrast to the 50 that occur throughout all of Europe. Often, people born into these isolated tribes never venture farther than 10 miles from their birthplace.

It is not unreasonable to surmise that the same cause of the great diversity of tribes and languages in the rain forests of New Guinea-the extreme hindrance to movement caused by ecological congestion-may also have contributed to the great diversity in plants and animals in rain forests worldwide. In open continental regions, over grasslands or dry stretches, birds or insects may travel many miles before they come upon a new likely residence. But beneath the rain forest canopy (the uppermost layer in a forest) potential for dispersal becomes drastically affected, and no bird animal, or insect can move very far before encountering a wealth of edible opportunities and available habitats. Jungle populations that become separated from each other by just hundreds or even tens of miles may be as effectively and permanently isolated from each other as if they were on opposite sides of the globe. Indeed, this is one of the evolutionary reasons for the very bright colors and loud calls of many rain forest plants and animals. Consider the vocalizations and vibrancy of parrots and toucans (tropical birds with typically brightly colored feathers) or consider the howler monkey, the worlds loudest land animal. These noisy or highly visible shows help the animals discover the whereabouts of other members of the same species within the lavish display of green.

Thus, for many jungle organisms, new species can begin to occur at very short distances, and like a wildfire, this great diversity would continue to spread and fuel itself, for the different plants and animals would provide great variations in the survival benefits and drawbacks within all the small neighboring micro-territories. These extreme isolating effects of course would only occur below or at the level of the canopy, which is that upper, oceanic layer of leaves situated some 45-50 meters above the ground that forms the unbroken living roof of the rain forest.

But there is another less crowded layer, the emergent layer made up of the occasional enormous tree that grows 60-70 meters high, like the giant kapok, or silk cotton, tree, which often breaks through the darkness of the canopy and extends upward into the open sunshine. In South America, these more sparsely situated leafy treetops have become the nesting sites of the harpy eagle, the largest eagle in the world, With a wing span reaching 2 meters. The harpy eagle uses its advantageous position and view to prey upon he monkeys rustling through the meadow of canopy leaves below a hunting system that Is similar to that of North American birds of prey that sit in trees looking down for animals moving about the fields and grasslands In the rain forest, though, this system begins at 50 meters off the ground.

Unlike the more than 40 species of toucans that inhabit the Amazon rain forest, the range of the harpy eagle extends from Central America and all across the South American jungles; yet it has not diversified. Clearly, the freedom of movement offered above the canopy has allowed a continuous flow of genes and has not encouraged diversification of the species. The giant kapok trees, in which these eagles so often build their nests, are similarly unconfined by their neighboring trees and have retained their powers of dispersal. Their seeds, surrounded by fluffy cotton fibers float through the air for great distances above the canopy. Like the harpy eagle, the range of the kapok tree extends across the rain forest With remarkably little genetic differentiation. Indeed. even the Andes, the longest continental mountain range in the world, have not been able to block the spread of the kapok, and within the pas 15 million years, its floating puffs of seeds have even managed to cross the Atlantic between South America and Africa.