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What Is A Rainforest?

Rainforests are found throughout the world, wherever the annual rainfall is more than 2,000 millimeters (80 in.) and evenly spread throughout the year. Rainforests grow in temperate zones (such as the Pacific Northwest) as well as in tropical regions. Tropical rainforests are found in a thick belt extending around the earth on either side of the Equator. The constant heat and humidity in a tropical rainforest allow trees and plants to grow year-round. Vast columns of hot, humid air rise and then condense as rain, resulting in annual rainfall of between 2,000 to 10,160 millimeters (80-400 in.): annual temperatures average over 27°C (80°F).

What distinguishes the Amazon from other tropical rainforests is the sheer variety. Biologically, it is the richest and most diverse region in the world, containing about 20% of all higher plant species, roughly the same proportion of bird species, and around 10% of the world's mammals. Each type of tree may support more than 400 insect species. The Amazon is estimated to contain 2,000 species of fish -- 10 times the number found in European rivers -- and countless varieties of reptiles. Why the Amazon should be so diverse is a question that mystifies scientists still.

Not only is it the most diverse, but the Amazon is also the largest tropical rainforest in the world, covering 3.7 million sq. km (2.3 million sq. miles) over nine countries, including Peru, Colombia, Venezuela, and Bolivia. The largest chunk, about 60%, falls within Brazil's borders.

The lifeline of the forest is the Amazon River, the second longest in the world and the largest in terms of water flow and drainage area. The river has over 1,000 tributaries, some of them sizable rivers in their own right. In March and April, the water levels in these rivers rise and flood the banks and the surrounding forest for months. A seasonally flooded forest is called varzea (on the Rio Solimões) or igapó (on the Rio Negro), while the upland forest that stays above water is called terra firme.

The annual flooding of the forest is one of the key factors in maintaining this ecosystem. As the river covers the forest floor, sometimes leaving just the treetops above water, the fish also expand their terrain. These fish play a vital role in the fertilization of the forest. Many Amazonian fish are fruit eaters that feast on the nuts and seeds of the trees in the wet season. The fish feces deposit the seeds in the fertile ground of the forest, and when the waters recede, new growth occurs.

The difference in water levels between wet and dry season can be as much as 12m (40 ft.). Many of the smaller side channels -- called igarapé -- and creeks that are perfectly navigable in March and April are completely dry by November.

The rainforest itself has a very defined structure. The understory is what you see when you hike through the forest. Shielded by the tall trees and overhead layers, the understory can be surprisingly dark. Plants, fungi, and animals found here thrive in the dark, humid conditions. The canopy is where a lot of the action is; birds, butterflies, and monkeys are usually up here, 18 to 27m (60-90 ft.) above the understory. With more sunlight and less humidity, the rainforest at this height is green and lush. The emergent layer refers to those trees, often upwards of 36m (120 ft.), that send their crowns above the canopy to find the sunlight. Being higher also means these trees are more exposed to storms and rain. Wide buttress roots support these trees. This layering of the forest contributes to the biodiversity; each layer has its own flora and fauna that thrive in those particular conditions. Cutting through the forest opens up the understory to bright, unexpected light. The dark, humid environment vanishes, other plants and trees take over the space, and the ecological balance is thrown out of whack.

Viewing Wildlife -- Wildlife viewing in the Amazon is a subtle event. It's the occasional flutter of a giant blue butterfly, the glimpse of a startled bird, or the splash as a caiman swims away from your canoe. With a keen eye you may spy snoozing bats tucked away underneath the tree branches, or a large, perfectly camouflaged iguana draped over the branch of a tree. At night a flashlight reveals the many small frogs that dot the flooded forest; a searchlight shows the hiding places of the caiman as they lie in wait, only their eyes and snout above water. What isn't subtle is the noise. It's everywhere: the tortured squawks of macaws, the high-pitched screams of monkeys high in the trees. And in the "still" of the night, the decibel levels generated by chirping cicadas and "ribbiting" frogs rivals that of a construction site.

Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.