Tropical rainforests are ecosystems defined by towering, broad-leaved evergreen trees that form a continuous, multi-layered canopy. They are distinguished by high annual rainfall and consistently warm temperatures, creating an environment that supports an unparalleled concentration of biological diversity. Their global distribution is tightly constrained by specific climatic and geographical factors, limiting their presence to a narrow band around the planet’s midsection.
Defining the Tropical Zone
The tropics are a narrow geographical band situated between the Tropic of Cancer at 23.5° North and the Tropic of Capricorn at 23.5° South latitude. Within this zone, the sun’s energy is intensely concentrated, resulting in mean monthly temperatures that consistently exceed 18°C (64°F) year-round. This heat, combined with a steady influx of moisture, drives the formation of the rainforest biome.
Rainforests are often found within 10 to 15 degrees latitude of the Equator, where the climate is equatorial and lacks a significant dry season. To qualify as a rainforest, every month must receive a minimum of 60 millimeters (2.4 inches) of precipitation, with annual totals often ranging between 1,750 and 3,000 millimeters (70 to 120 inches).
The Neotropical Realm
The Neotropical biogeographic realm contains the largest expanse of contiguous tropical rainforest on Earth, centered around the Amazon River Basin in South America. This forest extends through nine nations, with the majority located in Brazil, Peru, and Colombia. The Amazon basin is responsible for this realm having the most rainforest coverage globally.
This major rainforest area also extends into the wet lowlands of Central America, covering significant areas in countries like Costa Rica and Panama. Pockets of rainforest are also present on larger Caribbean islands, such as Puerto Rico and parts of Cuba. The continuous warm, wet climate driven by the Intertropical Convergence Zone (ITCZ) makes the Neotropics the world’s most extensive reservoir of tropical forest biodiversity.
The Afrotropical and Indo-Malayan Realms
In the Eastern Hemisphere, the world’s rainforests are primarily divided between the Afrotropical and Indo-Malayan realms. The Afrotropical realm’s major concentration is within the Congo Basin of Central Africa, representing the second-largest continuous tropical forest globally. This forest spans several nations, with the Democratic Republic of Congo and the Republic of Congo holding the largest portions, alongside coverage in Gabon and Cameroon.
The Indo-Malayan realm encompasses the rainforests of Southeast Asia, which are some of the oldest on the planet. The majority of this coverage is found across the vast archipelago of Indonesia, including the islands of Sumatra and Borneo (shared with Malaysia). Mainland Southeast Asia also hosts tropical forests, and the large island of New Guinea contains another extensive rainforest that bridges the boundary with the Australasian realms.
Significance of Geographical Placement
The concentration of rainforests in these isolated, equatorial zones results in highly specialized ecological outcomes. The sustained high temperatures and consistent moisture levels promote rapid evolutionary processes and continuous growth, leading to high species richness. Geographic isolation—separated by oceans, deserts, or mountain ranges—has led to high levels of endemism within these major rainforest blocks.
The Wallace Line, a biogeographical boundary in the Indo-Malayan archipelago, separates the unique fauna of Southeast Asia from the Australian region, illustrating how barriers drive species specialization. These geographically distinct forests also play a role in global climate regulation by cycling vast amounts of water and sequestering atmospheric carbon dioxide. Their size and location near the equator make them moderators of global weather patterns and temperature stability.