Tropical rainforests are the most prolific ecosystems on Earth. They contain more species, more biomass per square yard, and grow more quickly than any other land biome, and often on relatively shallow and fragile older soils drained of nutrients. And of the world's tropical rainforests, the Amazon is by far the most diverse, possibly thanks to the fact that it is also the most extensive. Although there may never be a final or precise count, the Amazon basin is widely thought to be home to an estimated 30% of all the world's species, including hundreds of thousands of insect species, 3,000 fish species, well over 1,000 bird species, and more than 300 mammal species.
Amazonian ecosystems are amazingly complex, often comprising long chains of symbiotic relationships between parasites and hosts, pollinators and flowers, birds, insects, amphibians, fish, mammals and trees, epiphytes and other flora. Trees compete in height to reach the sunlight, with the tallest trees actually needing relatively small leaves and canopies to photosynthesize the tropical sun's powerful rays. However, little sunlight falls on the forest floor leaving it often surprisingly free of vegetation. Although the Amazon has an amazing variety of species, densities of each species are usually correspondingly low; so although a square kilometer of rainforest might have thousands of tree species, it may contain only a single specimen of many of those species. This is in contrast to temperate latitudes where vast forests may only contain a handful of tree species and, in some cases at higher latitudes, just a single type of tree.