One hectare of rainforest can hold 42,000 insect species and 313 tree species
Tropical rainforests shelter somewhere between 40 and 75 percent of all species on Earth, including two-thirds of flowering plants, on ground whose soils are surprisingly poor. The heavy rain that sustains them washes nutrients away. Only 2 percent of sunlight reaches the forest floor, so life crowds upward into the canopy.
True rainforests grow mostly within 10 degrees of the equator, where every month is warmer than 18 °C and gets at least 60 millimetres of rain. Annual totals are never below about 1,680 millimetres, usually fall between 1,750 and 3,000, and can exceed 10 metres. Most survive on fragments of Gondwana, the Mesozoic supercontinent, whose breakup scattered rainforest from Madagascar and New Guinea to the Amazon, with smaller patches surviving in Australia. Their exact origins are unclear because the fossil record is incomplete.
The forest is organised in storeys. Emergent giants rise to 45 to 55 metres, occasionally 70 to 80, and must withstand heat and wind above the rest; this layer exists only in tropical rainforest. Below lies the canopy, typically 30 to 45 metres, where epiphytes such as orchids, bromeliads and lichens cling to branches and harvest water and debris. Canopy arthropods alone may number up to 20 million species. The understory receives about 5 percent of the light, so its plants rarely pass 3 metres and often grow oversized leaves.
On the dim floor, vegetation is sparse away from rivers and clearings, leaving room for large animals like okapi, tapirs and western lowland gorillas. Dead matter vanishes fast in the warm damp, broken down by fungi. One hectare may hold as many as 1,500 kinds of higher plant, with its 807 trees belonging to 313 separate species, and many millions of species may still be undiscovered. More than a quarter of natural medicines have come from these forests, earning them the nickname of the world's largest pharmacy.
They also store carbon and help regulate climate, and indigenous peoples have lived in them for millennia. Heavy logging and farm clearance through the 20th century have fragmented and shrunk them rapidly, and fast human-driven habitat loss is suspected to be a major cause of extinctions. International programmes such as REDD and REDD+ aim to slow deforestation, yet losses from clearing and climate change continue.
Source: Tropical rainforest
