Rainforest Biome
A rainforest is a thick warm forest that gets a huge amount of rain, usually more than 2,000 millimetres a year. Rainforests are the most biodiverse biome on Earth: they cover only approx. 6% of Earth's land but contain more than half of all the species we have ever discovered. A single hectare of Amazon can hold over 450 species of tree, more than the whole of Britain combined. The constant warmth, water and sunlight let plants grow non-stop, and that abundance of plants feeds a riot of animal life.
- Rainfall2,000+ mm/yearOften over 3,000 mm
- % of Earth's Landapprox. 6%Down from approx. 14% a century ago
- % of Earth's Species50%+Live in rainforests
- Temperature20 to 27 °CWarm all year
- Layers4Emergent, canopy, understorey, floor
- BiggestAmazon5.5 million km², South America
How wet is the rainforest?
Yearly rainfall (mm) compared with other biomes.
Rainforests get the most rain of any biome on land. That constant moisture supports more life than anywhere else on Earth.
The four layers of the rainforest
A tropical rainforest is built like a four-storey building, and each storey has its own light, climate and wildlife.
- Emergent layer (top): a few giant trees up to 60 m tall poke above the rest. They are battered by wind and sun, and home to eagles, butterflies and monkeys that rarely come down.
- Canopy (30 to 45 m): the leafy ceiling of the forest, where approx. 60 to 90% of the animals live. The canopy is so thick that almost no light reaches the ground.
- Understorey: dim, hot and damp. Vines (lianas), ferns and small trees fight for the scraps of light that get through. Jaguars, frogs and many snakes hunt here.
- Forest floor: dark because only approx. 2% of sunlight reaches the bottom. Dead leaves rot quickly in the heat and humidity, recycling nutrients back into the soil. Tapirs, agoutis and giant ants live down here.
Tropical vs temperate rainforests
Most rainforests are tropical, in a band around the equator: the Amazon in South America, the Congo in central Africa, and the rainforests of Borneo and New Guinea in south-east Asia. A handful of cooler temperate rainforests sit further from the equator and have totally different wildlife: the Pacific Northwest of North America (giant Sitka spruce and Roosevelt elk), the Valdivian rainforest in Chile, parts of Tasmania, and New Zealand's South Island.
Temperate rainforests get just as much rain as the tropics (sometimes more) but are much cooler. Instead of a thick four-layer canopy and tropical bird-of-paradise, they feature moss-draped giant conifers, ferns, salmon-bearing rivers and animals like bears and wolves.
Where rainforests are found
Tropical rainforests grow in a belt around the equator, roughly between 10 degrees north and 10 degrees south, where the air is warm all year and rises into thick rain clouds almost every afternoon. The main rainforest regions are:
- Amazon (South America): the largest rainforest on Earth at 5.5 million km², spreading across nine countries.
- Congo Basin (central Africa): the second-largest, home to gorillas, bonobos and forest elephants.
- South-east Asian rainforests: Borneo, Sumatra, the Malay Peninsula, New Guinea. Home to orangutans, tigers and the Rafflesia, the world's biggest flower.
- Daintree (Queensland, Australia): one of the oldest rainforests on Earth at approx. 180 million years.
- Central American rainforests: stretching from southern Mexico down through Panama, full of resplendent quetzals and howler monkeys.
Plants of the rainforest
Rainforest plants compete fiercely for light. The tallest trees, called emergents, race upwards with smooth trunks and crowns of leaves only at the very top. Below them, lianas (woody vines) climb other trees to reach the sunlight without having to grow their own trunk. Epiphytes like orchids, bromeliads and ferns skip the soil entirely and grow on the branches of other trees, catching rainwater and nutrients in their leaves. Some bromeliads collect so much water in their leaf cups that frogs lay their eggs in them, completing their entire tadpole life cycle high in the canopy.
The rainforest gives us many things we eat or use every day: cocoa, coffee, bananas, vanilla, sugar, rubber, mahogany and many spices. Roughly 25% of all the medicines used in modern hospitals were first discovered in rainforest plants, and scientists think most rainforest species have not yet been identified.
Animals of the rainforest
Rainforests are home to more animal species than any other biome. The Amazon alone contains over 1,300 species of bird, 430 mammals, 400 reptiles and 3,000 fish. Iconic rainforest animals include jaguars, sloths, toucans, macaws, anacondas, poison dart frogs, leafcutter ants and (in Asia) orangutans and tigers. The Congo rainforest is home to forest elephants, lowland gorillas, bonobos and the okapi, a giraffe relative that looks like a horse crossed with a zebra.
Many rainforest creatures are specialists that live their whole lives in just one part of the forest. Sloths spend their lives upside down in the canopy, moving so slowly that algae grows in their fur and helps camouflage them. Poison dart frogs the size of a thumbnail carry enough toxin to kill ten adult humans. The harpy eagle, the most powerful eagle in the world, snatches monkeys and sloths from the very tops of trees with talons larger than a grizzly bear's claws.
People and threats
Indigenous peoples have lived sustainably in rainforests for tens of thousands of years. Tribes such as the Yanomami, Kayapo and Asha'ninka in the Amazon; the Mbuti, Bambuti and Baka pygmies in the Congo; and the Penan of Borneo have developed deep knowledge of rainforest plants and animals. Many rainforest medicines came to Western science only after local healers shared them.
The biggest threat to rainforests is deforestation: cutting and burning the forest to clear land for cattle ranching, soy farming, palm oil plantations, gold mining or roads. Roughly half of the world's original rainforests have already been destroyed, and an area the size of a football pitch is lost approximately every six seconds. This matters for the whole planet: rainforests store huge amounts of carbon, produce oxygen, regulate rainfall, and protect the most diverse ecosystems on Earth. Once gone, the thin tropical soil quickly becomes useless even for farming.
Deeper dive: rainforests, Gondwana and how they survived the dinosaurs
Today's rainforests are descendants of ancient forests that already covered the southern supercontinent Gondwana over 100 million years ago. When Gondwana broke up between 180 and 30 million years ago, its forests went with the pieces, drifting apart on the modern continents of South America, Africa, Antarctica, Australia and the Indian subcontinent. This is why you find related plant families (such as the southern beeches, Nothofagus) in the rainforests of Chile, New Zealand and Tasmania, but not in the Northern Hemisphere. Australia's Daintree Rainforest preserves plant lineages that go back to that Gondwanan world.
Rainforests also survived the asteroid impact that wiped out the non-bird dinosaurs 66 million years ago. The impact triggered global fires and a sudden cold snap, but tropical forests in places far from the impact crater (in present-day Mexico) regrew. Pollen records show that within a few million years, dense tropical forests returned, but they were different from before: dominated by flowering plants (angiosperms) rather than conifers, more closed-canopy, and with the same kind of layered structure we recognise in modern rainforests.
Rainforests are also engines of the planet's water and carbon cycles. A single big tree in the Amazon can release approx. 1,000 litres of water vapour into the air every day through tiny pores in its leaves, helping seed the rain clouds that water the rest of the forest. This is why losing rainforest is so dangerous: cut down enough trees and the forest stops making its own rain, and the climate of the whole region shifts, sometimes permanently.
For famous individual rainforests including the Amazon, the Congo and the Daintree, see the Rainforests section.