An extraordinary number of species live in tropical rainforests. Over the past decade the estimate for the number of insect species thought to inhabit the planet has risen from 2 million to more than 30 million, as a result of intensive research focused on the tropics.
The explanations for such species richness are numerous and in many cases they are not mutually exclusive. General theories hypothesize that the tropics themselves have experienced a relatively constant climate over millennia, meaning that flora and fauna have not been adaptively restricted by physical conditions, causing them to compete more vigorously with each other. This competition over countless generations has resulted in more specialized adaptations to reduce or overcome 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. A constant environment may also have resulted in less extinction compared to those in 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 rainforest vegetation may appear, its presence is not an indication of great soil fertility; on the contrary, tropical forest soils are among the poorest on Earth. This fact can be explained by considering the timescale during which plants have been actively competing for nutrients in this environment. Rainforests have been around for approximately 125 million years. Therefore, the length of time that contemporary tracts of forests have been present can be measured in millions of years, during which time plants have been experiencing greater and greater competition for soil nutrients, meaning that their adaptations for obtaining those nutrients have been honed to such an extent that today the nutrients locked into a dead leaf on the forest floor can be recycled directly into plant life without ever becoming part of the mineral soil. Around 95% of all nutrients in the forest are locked up in living matter. Recycling of nutrients from the dead is extremely fast and efficient.
The lack of nutrients and poor soil structure common to tropical forest ecosystems becomes extremely debilitating once forest cover has been removed, and this explains why farming of most tropical forest soils is non-sustainable and leaves permanent scars on land which cannot be colonized effectively by the forest again.