Tropical rainforests are among the most outstanding natural ecosystems on the planet, an awe inspiring tangle of life to some, an unfathomable puzzle with more questions than answers to others. They represent the pinnacle of life on Earth. Vegetative production (2 kg/m2/yr) is more than double that found in the most productive temperate forests, species diversity hits the roof and the whole system is considered the most important natural regulator of the Earth's climate, for which we are forever indebted.

Tropical rainforests, among which the Amazon is the largest single tract on Earth, span the globe between the tropics where rainfall exceeds an average of 1500 mm per year (6 feet) and where temperatures do not drop below an average of 17ºC throughout the year.
The ecosystem is entirely self-sufficient, requiring only the steady input of sunlight to maintain the plants, which in turn maintain the rest through the many interconnecting webs of interactions, checks and balances that are a constant feature of this twilight world.

Plants form the basic structure of the forest on which everything else depends. The vegetation is broadly arranged into strata (see diagram opposite), which though not readily observed or defined, certainly exist. The most distinct division observable is between the canopy - a strata exposed to the full effects of the sun and the wind, and the undergrowth, which is poorly illuminated in comparison, although it is more stable with respect to the environmental conditions of temperature, humidity, wind and so forth. The contrast between these microclimates can be striking and goes some way towards explaining the complexity of life forms, each adapting to a unique array of microclimates and the micro ecosystems within them.

Animals also have been affected by the forest's structure. In the various strata the availability of food, the opportunities for concealment and possible modes of locomotion are very different. For example, an animal living in the treetops can readily obtain large quantities of vegetative foods (flowers, leaves, fruit, etc.) but must have limbs adapted to climbing, swinging, jumping, gliding or flying from tree to tree. In contrast, the ground dwellers have little or no climbing ability and depend largely on food falling down from above.

Only recently has the treetop community of plants and animals been a focus of interest to biologists and it is proving to be exceedingly rich in life. More than half of all the forest's animals are now believed to be arboreal (living in the trees) of which the majority complete their entire lifecycles without even approaching the ground.

The sheer number of species that live in tropical rainforests exceeds the imagination. Over the last decade the number of insect species thought to inhabit the planet has risen from a mere 2 million to 30 million or more due to the intensive research now being aimed in the direction of the tropics.

The explanations for such species richness are numerous and are not mutually exclusive. General theories hypothesize that the tropics themselves have experienced a fairly constant climate over the millennia, so that the flora and fauna have not been adaptively restricted as much by physical conditions, enabling them to compete more vigorously with each other. This competition over a protracted length of time has resulted in more specialized adaptations to reduce or overcome the competition resulting in slight physical and/or temporal changes in plant and animal populations, culminating ultimately in the creation of separate species. A high degree of specialization by organisms in these environments has been found to be common. The more constant environment may also have resulted in less extinction compared to the harsher conditions at greater latitudes where the weak or poorly-adapted are quickly weeded out.

It is one of the paradoxes of tropical ecology that however luxuriant the rainforest vegetation may appear, its presence is not an indication of great soil fertility; on the contrary such soils are some of the poorest of all. This fact however can be explained when one considers the timescale during which plants have been actively competing for nutrients in this environment. Rainforests have been around for approximately 125 million years. The length of time that current tracts of forests have been present can be measured therefore in millions of years, during which time the plants have been experiencing greater and greater competition for soil nutrients, so that their adaptations for obtaining these nutrients have been steadily honed to such an extent that today the nutrients locked up in a dead leaf on the forest floor can be recycled directly into the plant without ever becoming part of the mineral soil. 95% of nutrients as a whole are locked up in the living matter. Recycling of nutrients from the dead is fast and extremely efficient.

This lack of nutrient and poor soil structure, endemic in the tropics, is extremely debilitating once the forest cover has been removed and explains why farming on most tropical forest soils is non-sustainable and leaves permanent scars on land which cannot be colonized effectively by the forest again, ultimately due to its own efficiency!