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Tropical rainforests cover only about six percent of the Earth's land surface, yet they are home to more than half of the world's known plant and animal species. A single hectare of forest in Borneo or the Amazon can contain more tree species than the whole of North America. Most of these forests grow in a warm, wet belt near the equator, where temperatures remain high throughout the year and annual rainfall typically exceeds two thousand millimetres. These stable conditions have allowed life to diversify over tens of millions of years, producing ecosystems of unmatched complexity.
Rainforests are organised into distinct vertical layers, each with its own community of species. The tallest trees, known as emergents, rise above a continuous canopy that intercepts most of the sunlight and houses the majority of forest life, including monkeys, birds, insects and countless epiphytes. Beneath the canopy lies the dim understory, where plants adapted to low light grow slowly while waiting for a gap to open above them. The forest floor receives as little as two percent of the sunlight that reaches the canopy, so few herbs grow there; instead, it is a world of fungi, insects and decomposing leaf litter.
Paradoxically, the luxuriant growth of rainforests is supported by some of the poorest soils on the planet. Heavy rainfall leaches nutrients downwards, beyond the reach of roots, and warm temperatures accelerate decomposition. The forest survives by recycling nutrients with extraordinary speed and efficiency. Fallen leaves and dead wood are broken down within weeks, and specialised fungi attached to tree roots, called mycorrhizae, pass the released minerals directly back to the trees. As a result, most of the forest's store of nutrients is held in living vegetation rather than in the soil, which is why land cleared for farming often becomes infertile within a few years.
Rainforests also regulate the climate far beyond their boundaries. Through photosynthesis they absorb vast quantities of carbon dioxide, and the Amazon alone is estimated to store around one hundred billion tonnes of carbon in its trees and soils. Forests return moisture to the atmosphere through evapotranspiration, generating clouds and rainfall. Winds carry this moisture inland in what South American scientists call flying rivers, airborne streams of water vapour that supply rain to agricultural regions thousands of kilometres from the forest.
Despite their value, rainforests are being destroyed at an alarming pace. An estimated ten million hectares of forest disappear each year, an area roughly the size of Portugal. The causes vary by region. In the Amazon, cattle ranching and soybean cultivation account for most clearing, while in Southeast Asia vast areas have been converted to oil palm plantations. Logging, mining, road building and small-scale farming open up further areas, and fires set to clear land can escape into standing forest, particularly in drought years.
The consequences extend far beyond the loss of trees. Habitat destruction is now the leading driver of species extinction, and biologists warn that countless rainforest species could vanish before they are even described by science. Researchers studying the Amazon have raised a further concern: if deforestation and warming pass a certain threshold, the forest may no longer generate enough rainfall to sustain itself and could shift towards drier savanna across much of the basin. Such a transformation would release enormous quantities of stored carbon and would be effectively irreversible on human timescales.
Rainforests are also home to millions of indigenous people whose knowledge of forest plants has already contributed to modern medicine; a large share of prescription drugs contain compounds first identified in rainforest species. Efforts to protect the remaining forest combine several strategies. Governments have created protected areas and recognised indigenous land rights, which studies show are among the most effective forms of conservation. International programmes such as REDD+ pay developing countries to keep forests standing, and consumers are increasingly offered certified products, from timber to chocolate, that are produced without deforestation.
Whether these measures will be enough remains uncertain. Deforestation rates have fallen at times in some countries, only to rise again when political priorities change. Ecologists emphasise that protection and restoration must proceed together, because even degraded forest retains value that tree plantations cannot replace. The coming decades, they argue, will determine whether the tropical rainforest continues to exist as a functioning ecosystem or survives only in scattered fragments.