Ancient and rich environments
Tropical rainforests and cloud forests are among the most ancient and biologically rich ecosystems on Earth. Rainforests thriving on lowlands in the Tropics first emerged during the Cretaceous period (~140 million years ago), spreading widely across equatorial regions as warm and humid climates favoured dense, multi-layered vegetation.
Cloud forests, a type of montane rainforest, developed later, in mountainous areas where persistent cloud cover envelops forests at mid-to-high elevations (typically 1,000–3,000 meters above sea level).
Straddling the equator or the mountains between the tropics
Biogeographically, rainforests are confined to the narrow band between 23.5° North and 23.5° South (between the Tropic of Capricorn and the Tropic of Cancer) and concentrated around the Equator—especially in the Amazon Basin (South America), the Congo Basin (Africa), and Southeast Asia, including Indonesia, Papua New Guinea, and Malaysia. Given that most of Earth’s landmass lies north of this region, rainforests naturally occupy a limited area, covering less than 7% of Earth’s surface, but harbour over 50% of known terrestrial species and are by far the most biodiverse terrestrial biome on the planet.
Cloud forests are more fragmented, found in the Andes (South America), Central America, East Africa, and Southeast Asia, typically along mountainous belts influenced by oceanic moisture.
Although less extensive, cloud forests host unique biodiversity adapted to cool, humid, and shaded conditions and exhibit high endemism (especially in plant groups like orchids, ferns, and bromeliads, and animal groups such as amphibians and birds) due to their isolation by valleys and ridges. These natural barriers prevent species from migrating to other forest areas, as lowland forests below and steep cliffs above create further obstacles to the movement of organisms and thus to gene flow.
Important for biodiversity, but not only…
The vertical stratification and microhabitats in both forest types foster niche specialization and evolutionary diversification, making them critical to global biodiversity. Beyond their ecological and evolutive significance, tropical rainforests and cloud forests help regulate global weather, sustain regular rainfall, and buffer against floods, droughts, and erosion. They also store immense amounts of carbon while producing and releasing a significant share of the world’s oxygen.
Forests by the sea
Along sandy coastlines in between the Tropics rainforests give way to mangroves forests, adapted to grow in silt-rich, saline (brackish water) environments, primarily along river deltas, estuaries, and coastal lagoons. These forests have low tree diversity, consisting mainly of mangroves species, trees and shrubs adapted to saltwater conditions with specialized breathing roots (pneumatophores) that emerge from oxygen-poor mud, and feature a broken canopy. Despite their relative low species richness, mangroves play a huge ecological role, as they serve as spawning grounds and nurseries for many marine organisms; additionally, their tangle of roots is crucial in coastal protection by acting as natural barriers against storm surges and wave erosion.
How many species in tropical forests? And why?
The exact number of species inhabiting the world’s tropical rainforests remains still unknown, with estimates ranging from 3 to 50 million and making rainforests -unquestionably- the most biologically diverse terrestrial ecosystems. This remarkable biodiversity is not coincidental but the result of unique evolutionary, climatological and ecological conditions.
The warm, humid climate of tropical rainforests is a major driver of their exceptional biodiversity. In general, species diversity and ecosystem productivity increase with the amount of solar energy available, because sunlight, captured by canopy leaves through photosynthesis, is converted into simple sugars that fuel the entire ecosystem entering the complex food webs. Tropical rainforests have the highest mean net primary production (estimated as the amount of carbon fixed by plants) of any terrestrial ecosystem and the equatorial region’s high humidity further supports biodiversity by ensuring a consistent water supply.
Stable, favourable and complex
Even the short- and long-term stability of the tropical rainforest environment fosters biodiversity by allowing plants and animals to interact year-round without needing winter interruptions of their life cycle or adaptations for extreme cold or frost. With continuous sunlight and favourable temperatures providing a steady food supply during the year, rainforest organisms do not experience seasonal food shortages, unlike those in temperate ecosystems. Over millions of years, this stable environment has enabled species to exploit every available niche. Intense competition has led to equally extreme specialization, preventing any single species from dominating an ecosystem. Moreover, interdependence is a fundamental characteristic of rainforest ecosystems, where highly-specialized species rely on each other in diverse ways. The result is an intricate web of interactions between predator and prey, host and parasite, pollinator and plant, leading to an astonishing variety of adaptations—camouflage, mimicry, specialized feeding behaviours, and symbiotic relationships.
A vertical world
Rainforests have a distinct, emphasized vertical structure composed of multiple layers, including the canopy, understory, shrub layer, and forest floor. The canopy, interspersed with an abundance of epiphytes (plants that grow on trees rather than rooting in the soil), forms a dense ceiling of intertwined branches and leaves from closely spaced trees 30–40 meters above the ground, occasionally punctuated by towering emergent trees that extend even higher, eventually creating the overstory. Beneath this, the understory consists of multiple tiers of foliage and branches. The lowest section of the understory, called the shrub layer, lies about few above the forest floor and consists mainly of saplings and smaller plants.
Specialised inhabitants
The canopy system characteristic of tropical rainforests significantly enhances biodiversity by creating a multi-layered system of diverse microhabitats that provide food, shelter, and new opportunities for species interactions. It is estimated that 70-90% of rainforest life exists within the trees. Wildlife is often cryptic and elusive. Rainforests are not characterized by sheer abundance but rather by incredible diversity—many species exist in low densities and are specialized to specific habitats.
Because species are highly specialized, habitat destruction can quickly drive them to extinction by eliminating the precise conditions they require for survival.
